Geologic map of Mount Chiginagak volcano, Alaska

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Title: Geologic map of Mount Chiginagak volcano, Alaska
Abstract:
Mount Chiginagak is a hydrothermally active volcano on the Alaska Peninsula, approximately 170 km south-southwest of King Salmon, Alaska. This small stratovolcano, approximately 8 km in diameter, has erupted through Tertiary to Permian sedimentary and igneous rocks. The eruptive products of Chiginagak volcano record a history of chiefly andesite lava flows and associated block-and-ash flows. The oldest lavas exposed are Pleistocene in age and are found everywhere around the edifice except in the northeast sector, where Holocene lava flows dominate the landscape. Holocene activity has covered the northeast flank with rubbly-topped andesite lava flows that extend as far as 4.6 km from their source vent at the summit crater. The farthest-reaching volcanic deposits are on the southeast flank, where block-and-ash-flow, pyroclastic-flow, and lahar deposits extend down valley as far as 9 km from the summit. Limited exposure of deposits of a presumed plinian eruption of middle Pleistocene age indicate at least one episode of explosive activity in Chiginagak's past. This 1:25,000 scale geologic map and accompanying report document the age, geochemical, and spatial distribution of eruptive products of the Mount Chiginagak volcano. Map units are interpreted from field observations, satellite imagery interpretation, geochemical and geochronological analysis.
Supplemental_Information:
This digital data distribution package provides shapefiles and geodatabase that portray features presented in RI 2017-10. Field names and values are organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP). This metadata file lists the table names and field names used in the geodatabase. Table and field names in the shapefiles are truncated at 10 characters. The geodatabase and selected shapefiles provide the following features:
Basemap: Geodatabase feature dataset that provides topographic contours, hydrology, and geographic place names 
CorrelationOfMapUnits: Geodatabase feature dataset that depicts the map unit correlation chart; includes applicable unit designators, brackets, dividing lines, and text 
ContactsAndFaults: Line features that depict faults and geologic contacts 
DataSourcePolys: Polygons that delineate the data sources for various parts of the map 
DataSources: Non-spatial reference table that provides bibliographic references for the datasets or information used to generate or interpret geologic or geographic features 
DescriptionOfMapUnits: Non-spatial reference table that provides key information about each map unit. Additional information may be found in an accompanying report. 
GenericPoints: A point file that provides locations and labels for key geologic features or observations. 
GeochronPoints: Geochronology sample location points. 
GeologicLines: Dikes, coal seams, ash beds, glacial features, anticline and syncline hinge-surface traces, and other kinds of key beds that do not participate in map-unit topology 
MapUnitPolys: Polygons that record the distribution of mapped or interpreted geologic units 
OtherPolys: Polygons that record the location and extent of geologic or geographic features other than the geologic units 
RepurposedSymbols: Non-spatial reference table that documents cases in which FGDC Standard symbols are supplemented with custom symbols or with FGDC symbols repurposed for the map. 
Stations: Field observation locations and descriptions
  1. How should this data set be cited?

    Schaefer, J.R., Scott, W.E., and Layer, P.W., 2017, Geologic map of Mount Chiginagak volcano, Alaska: Report of Investigation RI 2017-10, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 32 p., 1 sheet, scale 1:25,000

  2. What geographic area does the data set cover?

    West_Bounding_Coordinate: -157.103210
    East_Bounding_Coordinate: -156.846548
    North_Bounding_Coordinate: 57.185706
    South_Bounding_Coordinate: 57.072259

  3. What does it look like?

  4. Does the data set describe conditions during a particular time period?

    Beginning_Date: 2004
    Ending_Date: 2017
    Currentness_Reference: ground condition

  5. What is the general form of this data set?

    Geospatial_Data_Presentation_Form: report and digital data

  6. How does the data set represent geographic features?

    1. How are geographic features stored in the data set?

      This is a vector data set.

    2. What coordinate system is used to represent geographic features?

      Grid_Coordinate_System_Name: Universal Transverse Mercator
      Universal_Transverse_Mercator:
      UTM_Zone_Number: 4
      Transverse_Mercator:
      Scale_Factor_at_Central_Meridian: 0.999600
      Longitude_of_Central_Meridian: -159
      Latitude_of_Projection_Origin: 0
      False_Easting: 500000.000000
      False_Northing: 0

      Planar coordinates are encoded using coordinate pair
      Abscissae (x-coordinates) are specified to the nearest .00000001
      Ordinates (y-coordinates) are specified to the nearest .00000001
      Planar coordinates are specified in Meters

      The horizontal datum used is North American Datum of 1983.
      The ellipsoid used is GRS 80.
      The semi-major axis of the ellipsoid used is 6378137.
      The flattening of the ellipsoid used is 1/298.257222101000025.

  7. How does the data set describe geographic features?

    Basemap
    Geodatabase feature dataset that provides topographic contours, hydrology, and geographic place names (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    CorrelationOfMapUnits
    Geodatabase feature dataset that depicts the map unit correlation chart; includes applicable unit designators, brackets, dividing lines, and text (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    ContactsAndFaults
    Line features that depict faults and geologic contacts (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    Type
    Type values specify the kind of features represented by a line, point, or polygon (Source: Alaska Division of Geological & Geophysical Surveys)

    Feature type(s): contact; gradational contact; map boundary

    IsConcealed
    This is a flag to identify contacts and faults covered by overlying map units. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    NThe feature IS NOT covered by an overlying map unit.
    YThe feature IS covered by an overlying map unit.

    ExistenceConfidence
    Indicates the degree of confidence and/or certainty in the interpretation of a a feature's existence. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    certainIndicates that the author has sufficient observational and/or other supporting data to be reasonably confident in the scientific credibility of the interpretation of the feature. The level of certainty is specific to the map scale.

    IdentityConfidence
    Indicates degree of confidence in the identification and classification of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    certainIndicates that the author has sufficient observational and/or other supporting data to be reasonably confident in the scientific credibility of the interpretation of the feature. The level of certainty is specific to the map scale.

    LocationConfidenceMeters
    Half-width in meters of positional uncertainty envelope; position is relative to other features in the data. A value of -9 indicates that this measure is not available. (Source: Alaska Division of Geological & Geophysical Surveys)

    Range of values
    Minimum:25
    Maximum:75
    Units:meters

    Symbol
    References the symbol value that was assigned to the point. (Source: Alaska Division of Geological & Geophysical Surveys)

    Formal codeset
    Codeset Name:FGDC Digital Cartographic Standard for Geologic Map Symbolization; Appendix A. Geologic map symbols, colors, and patterns, "REF NO" field
    Codeset Source:Federal Geographic Data Committee [prepared for the Federal Geographic Data Committee by the U.S. Geological Survey], 2006, FGDC Digital Cartographic Standard for Geologic Map Symbolization: Reston, Va., Federal Geographic Data Committee Document Number FGDC-STD-013-2006, 290 p., 2 plates.

    DataSourceID
    Provides information about the source of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    Foreign key to the DataSources_ID field in the DataSources table.

    Notes
    Free text for additional information about a feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    The notes indicate the source of the feature interpretation or provide more information about the processing or data acquisition procedure.

    DataSourcePolys
    Polygons that delineate the data sources for various parts of the map (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    Notes
    Free text for additional information about a feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    The notes indicate the source of the feature interpretation or provide more information about the processing or data acquisition procedure.

    DataSourceID
    Provides information about the source of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    Foreign key to the DataSources_ID field in the DataSources table.

    DataSources
    Non-spatial reference table that provides bibliographic references for the datasets or information used to generate or interpret geologic or geographic features (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    DataSources_ID
    Provides information about the source of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    Primary key in the DataSources table; Example values = DAS1, DAS2, DAS3,... Null values not permitted

    Source
    Plain-text short description that identifies the data source. (Source: Alaska Division of Geological & Geophysical Surveys)

    By convention, for DataSources_ID = DAS1, Source = 'This report'. Other values are standard bibliographic references. Null values not permitted.

    Notes
    Free text for additional information about a feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    The notes indicate the source of the feature interpretation or provide more information about the processing or data acquisition procedure.

    DescriptionOfMapUnits
    Non-spatial reference table that provides key information about each map unit. Additional information may be found in an accompanying report. (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    MapUnit
    Short ASCII string that indicates the map unit. Note: map units in parentheses, such as (Qcf), indicate combined map units consisting of bedrock overlain by thin or discontinuous deposits of the map unit. Map units with a question mark, such as (Qus?), indicate an uncertain identification. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    HPbvAndesite lava flow of Bear valley - Andesite (57.1 and 57.3 weight percent SiO2; two analyses lava flow that underlies block-and-ash-flow deposits (unit Hba along south margin of Bear valley. Plagioclase is the dominant mineral phase; 1-1.5 mm plagioclase phenocrysts with less abundant 0.5 mm clinopyroxene and orthopyroxene. Chemical composition of relatively high TiO2 and FeO and low K2O, Ba, and Sr poorly matches other units. Riehle (1990 interpreted this flow as Holocene, but, owing to poor exposure that limits assessment of degree of glacial erosion prior to burial by deposits of block-and-ash flows, we leave open the possibility of late Pleistocene age. Undated.
    HPlbHydrothermally altered lava flows and breccias - Slightly to intensely hydrothermally altered lava flows and lava breccias that form the uppermost edifice on the south flank and the highest point on the crater rim. This unit is younger and has more steeply dipping beds than the more severely altered lava flows and intervening breccias of unit Pla, also on the south flank. Some may represent near-vent equivalent of summit lava flows of unit Hls. Unsampled.
    HaAlluvium of active channels and floodplains - Chiefly cobble and pebble gravel and sand in upstream areas grading to sandy pebble gravel, sand, and sand and silt in downstream areas near Mother Goose Lake, Chiginagak Bay, and the upper Dog Salmon River.
    HbaDeposits of block-and-ash flows and lahars of Bear valley - Bouldery andesitic (58.1 and 59.8 weight percent SiO2; two analyses diamicts with sandy matrix; poorly sorted, non-stratified; commonly with prismatically jointed clasts; clasts dense to vesicular; minor amount of hydrothermally altered clasts. Limited exposures suggest diamicts comprise numerous flow units on order of 1-3 m thick. They partly fill the unnamed stream valley on the east flank (which we informally call Bear valley up to 8 km distant from the presumed source at vent 5567. Likely related to collapse of growing lava dome or active lava flows. If lava-dome extrusion at vent 5567 fed the block-and-ash flows, that dome is buried by later lava flows of unit Hlf. Stratigraphic relationship of block-and-ash-flow deposits with unit Hlf is obscured by erosion and deposition from neoglacial advance, but we infer that both originated from eruptions near flank vent 5567. Geomorphic relations show that fill of unit Hba was eroded by stream along north side of valley prior to emplacement of lobe of unit Hlf at site 05JRSCH051. Riehle (1990 believed that much of the fill in Bear valley was a lava flow (our unit Plbv, except along the north side of the valley, and was only thinly mantled by block-and-ash-flow deposits. However, we find substantial cover of deposits and only limited outcrop of the lava flow. Above about 1,000 ft elevation, surface is largely barren of vegetation and reworked by solifluction and other surface processes. At lower elevations, the unit is covered by a discontinuous mantle of sand, silt, and organic material that contains a few thin tephra layers. The amount of soil development (tens of centimeters of oxidation suggests unit is probably of early Holocene age. Overlies lava flow of Holocene or Pleistocene age (unit HPbv exposed locally along south margin of Bear valley. Undated.
    HcColluvium and alluvium - Colluvium and alluvium of valley-wall fans and landslide deposits; gravel and sand; very poorly sorted to poorly sorted; non-bedded to crudely bedded. Forms sheets and cones of talus and fans of alluvium and debris-flow deposits. Along brush-covered east side of Indecision Creek valley includes small areas of pre-Quaternary bedrock (unit JTu, block-and-ash-flow and related deposits of north flank (unit Pnba, and Pleistocene glacial deposits (units Pgt and Pgu too small to show at map scale.
    HdfPyroclastic-flow and lahar deposits along Chiginagak creek - Gray bouldery and cobbly diamicts with blocks of juvenile lava and clay-rich lahar deposits rich in hydrothermally altered debris that form small terraces along informally named Chiginagak creek southeast of the volcano. Along upper Chiginagak creek, gray, sandy-matrix diamicts overlie yellow to orange-brown, clayey, massive lahar deposits. We infer that the clayey lahar deposits (individual units on the order of 1 to 2 m thick were generated by debris avalanches that contained sufficient water to transform into a lahar and the overlying gray diamict originated in collapses of growing lava domes or flows. Geochemical analysis of a block from the gray diamict resembles that of lava of unit Hls. Upper gray diamicts up to a few meters thick; clay-rich lahar deposits more than 10 m thick.
    HdiDebris-flow deposits of upper Indecision Creek - Poorly sorted gravel, sand, and orange-yellow clay; orange-brown sandy to silty-clay matrix; larger clasts are faceted andesite boulders, and gravel- to sand-sized angular clasts of hydrothermally altered andesite and gray dense andesite; forms terraces several meters above active channel along upper Indecision Creek. Probably originated from debris-covered glacier upstream (unit dcg. Varied terrace top elevations represent at least two ages of debris-flow deposits.
    HgoGlacial outwash of late Neoglacial age - Cobble and pebble gravel and sand, locally bouldery; crudely to moderately bedded; poorly sorted. Forms fans and channels fronting modern glaciers and moraines of late Neoglacial age. Includes small areas of late Neoglacial till and underlying lava flows too small to show at map scale. Merges downstream to alluvium of active channels (unit Ha and floodplains.
    HgtGlacial till of late Neoglacial age - Bouldery gravel and sand; very poorly sorted; in some locations the till forms sharp-crested moraines up to 1 km beyond margins of existing glacial termini. These likely date from the past several centuries. Includes patches of underlying lava flows too small to show at map scale.
    HlfLava flows from vent 5567 on southeast flank - Rubbly-topped andesite (55.3-60.5 weight percent SiO2; eight analyses lava flows that form a high point about 1.4 km southeast of summit crater (x5567 ft on USGS Ugashik A-3 topographic map and a series of flow lobes that surround portions of Pleistocene lava flows (units Plse and Pl on the southeast and south flanks. Lava is porphyritic and contains variable proportions of 0.5-1.5 mm plagioclase, hypersthene, augite, and minor olivine and oxides. Northernmost lava flow underlies the southern margin of summit lava flows (unit Hls. Detterman and others (1987b map vent 5567 as a lava dome surrounded by lava flows, but we find that the vent lavas form flows that reach the base of east the flank. Two prismatically jointed blocks (57.4 and 58.1 weight percent SiO2 that are chemically similar to lava of this unit lie on deposits of block-and ash flows of Bear valley (unit Hba. We infer that they are blocks emplaced by late pyroclastic flows or rockfalls from active lava flows of vent 5567. Geomorphic relations at the head of north Bear valley drainage show that emplacement of lava flows of unit Hlf followed erosion of a valley through deposits of unit Hba and passage of pyroclastic flows and lahars of unit Hpd.
    HlsLava flows from summit vent - Rubbly-topped andesite (56.4-59.0 weight percent SiO2; seventeen analyses lava flows that form a nearly continuous apron on the north and east flanks and one flow on upper west flank. Lava is porphyritic and contains variable proportions of 0.5-1.5 mm plagioclase, hypersthene, clinopyroxene, and oxides. Flow surfaces are modified by glacial erosion and deposition within the limit of neoglacial advances, but are typically rubbly and blocky beyond. Southernmost flow of unit on east flank overlies lava flow from a vent on the southeast flank (unit Hlf. Analyses of summit flows define a geochemical trend that is slightly (about 0.1 weight percent higher in K2O than lava flows from southeast flank vent (unit Hlf.
    HpdDeposits of pyroclastic flows and lahars - Lahar and poorly sorted pyroclastic-flow deposits consisting of andesite blocks (25percent are vesicular in a loose, pink to gray, sandy matrix. Most vesicular blocks are less than 10 cm, and the dense, juvenile, prismatically jointed blocks (60.2 to 60.8 weight percent SiO2 are larger, up to 1 m. Terraced surfaces of Hpd lie along the northeast side of Bear valley, where they are inset below the upper surface of block-and-ash-flow deposits of Bear valley (unit Hba, as well as in lower Bear valley, approximately 1,600 m farther down valley than the toe of unit Hba. The deposit is discontinuously mantled by unmapped, fine-grained loessial deposits with interbedded tephras and buried soils. A small, isolated outcrop of a similar diamict of possibly equivalent age is found along a tributary of Volcano Creek on the lower north flank surrounded by unit Pgu, where approximately 2 m of soil and tephra overlie a 1-m-thick gray, sandy, polylithic, matrix-supported lahar with a base deposit consisting of a pink monolithic pyroclastic-flow deposit with a variably cemented matrix. An organic layer in this upper mantle has a radiocarbon age of 3,110 +/- 80 yBP.
    JTuBedrock, undifferentiated - Chiefly Upper Jurassic Naknek Formation sandstone and siltstone (units Jns and Jnst of Detterman and others, 1987a, as well as intrusive, hypabyssal, and extrusive igneous rocks of Miocene to Pliocene age and nonmarine sedimentary rocks of Eocene age. Includes discontinuous and unmapped mantle of alluvial, colluvial, and glacial deposits.
    PgtGlacial till - Bouldery gravel and sand; massive; very poorly sorted. Weathering profile consisting of a few tens of centimeters of oxidized till and mantling fine-grained sediment. Only mapped in one area on the northwest flank of volcano where it lies on Pleistocene block-and-ash-flow deposits (unit Pnba.
    PguGlacial deposits, undifferentiated - Undifferentiated deposits of till and glaciofluvial and glaciolacustrine deposits that bury Pleistocene block-and-ash-flow deposits (unit Pnba, Pleistocene lava flows, and bedrock (unit JTu on the north flank. Also includes discontinuous mantle of chiefly fine-grained loessial sediments with interbedded tephra and buried soils.
    PlAndesite lava flows of south, west, and northwest flanks - Porphyritic andesite (57.2-61.8 weight percent SiO2; eighteen analyses lava flows containing abundant 1-1.5 mm plagioclase; smaller and less abundant hypersthene, and augite to 0.5 mm; abundant oxide growth in the pyroxenes; common fine-grained mafic enclaves. Includes some thick (up to 350 m ice-contact lava flows along the east side of Indecision Creek valley that were emplaced against east margin of a valley-filling glacier. Ice-contact flows contain large masses of breccias, quenched glassy lava, discontinuous pods of intensely jointed lava, high cliffs displaying fans of radiating columnar joints, and numerous faults, indicative of interaction with water and ice. Also includes lava flows on south and north-northwest flanks that overlie deposits of Pleistocene block-and-ash flows (unit Pnba. Forms a lava bench on the northwest flank consisting of greater than 1-m-thick stacked lava flows. Lava flows have glacially eroded tops and are covered discontinuously by deposits of till (unit Pgt too small to map. 40Ar/39Ar ages of 15 +/- 11, 38 +/- 9, 44 +/- 18, 54 +/- 13, 55 +/- 8, 64 +/- 8, 72 +/- 9, and 75 +/- 3 ka and chemical diversity suggest eruptions occurred during several episodes in late Pleistocene time.
    PlaHydrothermally altered lava flows - Weakly to intensely hydrothermally altered porphyritic low silica dacite (63.4 and 63.8 weight percent SiO2; two analyses; most silicic analyses of any sampled Chiginagak rocks lava flows and intervening breccias of the upper south flank; some thick, massive outcrops may represent lava domes or dome breccia; gray, red, orange, and yellow in color; locally altered to clay and contains minor gypsum. Fresher lava contains abundant phenocrysts of plagioclase, hypersthene, and augite as well as common mafic enclaves. This unit is prominently exposed along the canyon walls flanking the south glacier under a carapace of less altered lava flows of the younger unit Pl; also underlies less-altered lava flows and lava breccias of unit HPlb in walls of summit crater. 40Ar/39Ar age of 89 +/- 6 ka in the east wall above south flank glacier. Rockfalls and rock avalanches from this unit and lava breccias of unit HPlb create thick, orange-brown, superglacial debris mantle (unit dcg).
    PloBasaltic andesite lava flows of north flank - At least two basaltic andesite lava flows (54.9 and 56.2 weight percent SiO2; two analyses that lie within block-and-ash-flow deposits of north flank (unit Pnba; exposed in cliffs above Indecision Creek, approximately 100 m from eroded top of block-and-ash-flow deposits. Abundant 1 mm plagioclase, orthopyroxene, and clinopyroxene in a dark gray sugary groundmass. An unsampled lava flow that forms an inaccessible bench below main flow is included in unit, but its stratigraphic position is not well constrained. A diamict of radially fractured blocks in a sandy matrix, which crops out about 400 m north of exposure of upper lava flows and appears to be interbedded in block-and-ash flows of north flank (unit Pnba, has a basaltic andesite composition (55.9 weight percent SiO2; one analysis similar to that of the lava flows in unit and is likely a fragmental deposit related to it. Block in diamict yielded a 40Ar/39Ar age of 251 +/- 6 ka.
    PlseAndesite lava flows of southeast flank - Porphyritic andesite (52.7 to 59.5 weight percent SiO2; five analyses lava flows and intervening breccias that form broad, glacially eroded ridges on lower southeast flank; contain abundant phenocrysts and microphenocrysts of plagioclase, hypersthene, augite, and oxides; also abundant mafic enclaves (one analysis, 52.7 weight percent SiO2. Lava characterized by high Ce (40 to 53 ppm; some samples also have high Sr (greater than 500 ppm; 40Ar/39Ar age of 24 +/- 8 ka. Overlain by lava flows of flank vent 5567 (unit Hlf).
    PlvcAndesite lava flows of upper Volcano Creek - Porphyritic andesite (57.3 to 57.6 weight percent SiO2; four analyses containing phenocrysts of plagioclase to 1.5 mm; common mafic enclaves. Form glacially eroded ridge west of upper Volcano Creek; lava characterized by high Ce (44-50 ppm and high Sr (647-703 ppm and relatively high K2O (1.43 to 1.49 weight percent for silica content. Overlies deposits of block-and-ash flows and lahars of north flank (unit Pnba. 40Ar/39Ar ages of 47 +/- 10 and 63 +/- 6 ka.
    PnbBanded lavas of northwest flank - Porphyritic, vesicular to dense, and locally banded, andesite (59.6 to 60.6 weight percent SiO2; four analyses lava flows and breccias. Phenocrysts chiefly 1 mm plagioclase with glomerocrysts of intergrown plagioclase, clinopyroxene, and orthopyroxene; abundant oxides throughout. Overlain by a lava flow of south and west flank (unit Pl that, in turn, is littered with abundant glacially transported and strikingly banded blocks similar to lava flow. Analysis of darker-colored band is 58.4 weight percent SiO2; two lighter-colored bands are 61.2 weight percent SiO2. Lighter-colored bands have glassy groundmass while darker-colored band has microlite-rich groundmass. Source of transported lava lies upslope largely under glacier ice, but perhaps exposed below summit ridge. Blocks and in-situ lava are compositionally similar to clasts in north flank block-and-ash-flow deposits (unit Pnba and suggest that block-and-ash flows and banded lavas may date from same eruptive period. A probably unreliable 40Ar/39Ar age of 15 +/- 10 ka was obtained from light-gray, slightly vesicular lava; overlying lava flow of unit Pl yielded more reliable age of 38 +/- 9 ka.
    PnbaBlock-and-ash-flow and lahar deposits of north flank - Bouldery and cobbly, very poorly sorted diamicts of dense to vesicular andesite clasts in a sandy matrix; clasts commonly prismatically jointed. Clasts range from 57.6 to 62.8 weight percent SiO2 (nine analyses. Lava contains abundant 1 mm subhedral plagioclase and orthopyroxene with smaller and less abundant clinopyroxene; glassy groundmass contains less than 0.2 mm plagioclase laths; abundant oxide growth in pyroxenes. The upper approximately 150 m of deposit is accessible for study; individual beds are one to several meters thick; locally indurated, probably by glacial compaction and silica cementation. Chemical diversity of upper part of unit suggests emplacement over several eruptive periods. Lower part of the unit is largely inaccessible in cliffs on east side of Indecision Creek valley, but appears to be formed of multicolored sequences of sandy and finer-grained, well-stratified deposits as well as diamicts. We do not know how similar upper and lower parts are in age and origin. Great thickness of sequence (more than 300 m and height above floor of Indecision Creek valley (600 m suggests that unit may represent sediment that accumulated against thick glacier in valley. Chemical similarity of analyzed clasts from upper part of unit with banded lava flows of north flank (unit Pnb suggests they may date from same general eruptive period during which active lava flows or lava domes on the upper northwest flank were generating large amounts of debris. 40Ar/39Ar age of 189 +/- 14 ka was obtained from a prismatically jointed lava block within a ~5-m-thick flow unit near the eroded top of the unit, just below ~30 m of till.
    PnpPumice-rich fall and flow deposits of north flank - More than 10-m-thick (base not exposed deposit of light-gray to yellow, high-silica andesite (60.8 to 62.1 weight percent SiO2; seven analyses pumice fall and flow deposits. Deposit composed chiefly of pumiceous lapilli and ash, with less abundant component of small bombs, dense gray clasts, and accidental fragments. Known only from single exposure along tributary of Volcano Creek where it underlies block-and-ash-flow and lahar deposits of north flank (unit Pnba, whose base is rich in pumiceous clasts and breadcrust bombs. Their contact is disconformable but, owing to geochemical similarity to samples from units Pnba and Pnp, may not signify a long time break.
    dcgDebris-covered glacier - Poorly sorted, superglacial till covering much of the lower reach of the south flank glacier and a few areas near termini of glaciers on other flanks. The mantle on the south flank glacier is chiefly composed of hydrothermally altered rock and orange-brown clay eroded from the upper south flank and forms a series of convex-downstream concentric ridges.
    gGlaciers and perennial snow fields

    Name
    Full map unit name, identifies the unit within its hierarchical context. (Source: Alaska Division of Geological & Geophysical Surveys)

    When applicable, usage is consistent with U.S. Geologic Names Lexicon

    Age
    Stratigraphic ranges or age of the map unit. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    NULLnot applicable

    Formal codeset
    Codeset Name:Geologic Time Scale v. 4.0
    Codeset Source:Walker, J.D., Geissman, J.W., Bowring, S.A, and Babcock, L.E., comp., 2012, Geologic Time Scale v. 4.0: Geological Society of America

    Description
    Free-format text description of the map unit. (Source: Alaska Division of Geological & Geophysical Surveys)

    Commonly structured according to one or more accepted traditions (for example, lithology, thickness, color, weathering and outcrop characteristics, distinguishing features, genesis, age constraints).

    GenericPoints
    A point file that provides locations and labels for key geologic features or observations. (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    Type
    Type values specify the kind of features represented by a line, point, or polygon (Source: Alaska Division of Geological & Geophysical Surveys)

    Feature type(s): fumarole; thermal spring; vent

    MapUnit
    Short ASCII string that indicates the map unit. Note: map units in parentheses, such as (Qcf), indicate combined map units consisting of bedrock overlain by thin or discontinuous deposits of the map unit. Map units with a question mark, such as (Qus?), indicate an uncertain identification. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    HlfLava flows from vent 5567 on southeast flank - Rubbly-topped andesite (55.3-60.5 weight percent SiO2; eight analyses lava flows that form a high point about 1.4 km southeast of summit crater (x5567 ft on USGS Ugashik A-3 topographic map and a series of flow lobes that surround portions of Pleistocene lava flows (units Plse and Pl on the southeast and south flanks. Lava is porphyritic and contains variable proportions of 0.5-1.5 mm plagioclase, hypersthene, augite, and minor olivine and oxides. Northernmost lava flow underlies the southern margin of summit lava flows (unit Hls. Detterman and others (1987b map vent 5567 as a lava dome surrounded by lava flows, but we find that the vent lavas form flows that reach the base of east the flank. Two prismatically jointed blocks (57.4 and 58.1 weight percent SiO2 that are chemically similar to lava of this unit lie on deposits of block-and ash flows of Bear valley (unit Hba. We infer that they are blocks emplaced by late pyroclastic flows or rockfalls from active lava flows of vent 5567. Geomorphic relations at the head of north Bear valley drainage show that emplacement of lava flows of unit Hlf followed erosion of a valley through deposits of unit Hba and passage of pyroclastic flows and lahars of unit Hpd.
    HlsLava flows from summit vent - Rubbly-topped andesite (56.4-59.0 weight percent SiO2; seventeen analyses lava flows that form a nearly continuous apron on the north and east flanks and one flow on upper west flank. Lava is porphyritic and contains variable proportions of 0.5-1.5 mm plagioclase, hypersthene, clinopyroxene, and oxides. Flow surfaces are modified by glacial erosion and deposition within the limit of neoglacial advances, but are typically rubbly and blocky beyond. Southernmost flow of unit on east flank overlies lava flow from a vent on the southeast flank (unit Hlf. Analyses of summit flows define a geochemical trend that is slightly (about 0.1 weight percent higher in K2O than lava flows from southeast flank vent (unit Hlf.
    PguGlacial deposits, undifferentiated - Undifferentiated deposits of till and glaciofluvial and glaciolacustrine deposits that bury Pleistocene block-and-ash-flow deposits (unit Pnba, Pleistocene lava flows, and bedrock (unit JTu on the north flank. Also includes discontinuous mantle of chiefly fine-grained loessial sediments with interbedded tephra and buried soils.

    Symbol
    References the symbol value that was assigned to the point. (Source: Alaska Division of Geological & Geophysical Surveys)

    Formal codeset
    Codeset Name:FGDC Digital Cartographic Standard for Geologic Map Symbolization; Appendix A. Geologic map symbols, colors, and patterns, "REF NO" field
    Codeset Source:Federal Geographic Data Committee [prepared for the Federal Geographic Data Committee by the U.S. Geological Survey], 2006, FGDC Digital Cartographic Standard for Geologic Map Symbolization: Reston, Va., Federal Geographic Data Committee Document Number FGDC-STD-013-2006, 290 p., 2 plates.

    LocationConfidenceMeters
    Half-width in meters of positional uncertainty envelope; position is relative to other features in the data. A value of -9 indicates that this measure is not available. (Source: Alaska Division of Geological & Geophysical Surveys)

    Range of values
    Minimum:25
    Maximum:25
    Units:meters

    DataSourceID
    Provides information about the source of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    Foreign key to the DataSources_ID field in the DataSources table.

    Notes
    Free text for additional information about a feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    The notes indicate the source of the feature interpretation or provide more information about the processing or data acquisition procedure.

    GeochronPoints, GeochronPointsAnno, GeochronPointsAnnoMask
    Geochronology sample location points. (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    MapUnit
    Short ASCII string that indicates the map unit. Note: map units in parentheses, such as (Qcf), indicate combined map units consisting of bedrock overlain by thin or discontinuous deposits of the map unit. Map units with a question mark, such as (Qus?), indicate an uncertain identification. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    HlsLava flows from summit vent - Rubbly-topped andesite (56.4-59.0 weight percent SiO2; seventeen analyses lava flows that form a nearly continuous apron on the north and east flanks and one flow on upper west flank. Lava is porphyritic and contains variable proportions of 0.5-1.5 mm plagioclase, hypersthene, clinopyroxene, and oxides. Flow surfaces are modified by glacial erosion and deposition within the limit of neoglacial advances, but are typically rubbly and blocky beyond. Southernmost flow of unit on east flank overlies lava flow from a vent on the southeast flank (unit Hlf. Analyses of summit flows define a geochemical trend that is slightly (about 0.1 weight percent higher in K2O than lava flows from southeast flank vent (unit Hlf.
    PlAndesite lava flows of south, west, and northwest flanks - Porphyritic andesite (57.2-61.8 weight percent SiO2; eighteen analyses lava flows containing abundant 1-1.5 mm plagioclase; smaller and less abundant hypersthene, and augite to 0.5 mm; abundant oxide growth in the pyroxenes; common fine-grained mafic enclaves. Includes some thick (up to 350 m ice-contact lava flows along the east side of Indecision Creek valley that were emplaced against east margin of a valley-filling glacier. Ice-contact flows contain large masses of breccias, quenched glassy lava, discontinuous pods of intensely jointed lava, high cliffs displaying fans of radiating columnar joints, and numerous faults, indicative of interaction with water and ice. Also includes lava flows on south and north-northwest flanks that overlie deposits of Pleistocene block-and-ash flows (unit Pnba. Forms a lava bench on the northwest flank consisting of greater than 1-m-thick stacked lava flows. Lava flows have glacially eroded tops and are covered discontinuously by deposits of till (unit Pgt too small to map. 40Ar/39Ar ages of 15 +/- 11, 38 +/- 9, 44 +/- 18, 54 +/- 13, 55 +/- 8, 64 +/- 8, 72 +/- 9, and 75 +/- 3 ka and chemical diversity suggest eruptions occurred during several episodes in late Pleistocene time.
    PlaHydrothermally altered lava flows - Weakly to intensely hydrothermally altered porphyritic low silica dacite (63.4 and 63.8 weight percent SiO2; two analyses; most silicic analyses of any sampled Chiginagak rocks lava flows and intervening breccias of the upper south flank; some thick, massive outcrops may represent lava domes or dome breccia; gray, red, orange, and yellow in color; locally altered to clay and contains minor gypsum. Fresher lava contains abundant phenocrysts of plagioclase, hypersthene, and augite as well as common mafic enclaves. This unit is prominently exposed along the canyon walls flanking the south glacier under a carapace of less altered lava flows of the younger unit Pl; also underlies less-altered lava flows and lava breccias of unit HPlb in walls of summit crater. 40Ar/39Ar age of 89 +/- 6 ka in the east wall above south flank glacier. Rockfalls and rock avalanches from this unit and lava breccias of unit HPlb create thick, orange-brown, superglacial debris mantle (unit dcg).
    PloBasaltic andesite lava flows of north flank - At least two basaltic andesite lava flows (54.9 and 56.2 weight percent SiO2; two analyses that lie within block-and-ash-flow deposits of north flank (unit Pnba; exposed in cliffs above Indecision Creek, approximately 100 m from eroded top of block-and-ash-flow deposits. Abundant 1 mm plagioclase, orthopyroxene, and clinopyroxene in a dark gray sugary groundmass. An unsampled lava flow that forms an inaccessible bench below main flow is included in unit, but its stratigraphic position is not well constrained. A diamict of radially fractured blocks in a sandy matrix, which crops out about 400 m north of exposure of upper lava flows and appears to be interbedded in block-and-ash flows of north flank (unit Pnba, has a basaltic andesite composition (55.9 weight percent SiO2; one analysis similar to that of the lava flows in unit and is likely a fragmental deposit related to it. Block in diamict yielded a 40Ar/39Ar age of 251 +/- 6 ka.
    PlseAndesite lava flows of southeast flank - Porphyritic andesite (52.7 to 59.5 weight percent SiO2; five analyses lava flows and intervening breccias that form broad, glacially eroded ridges on lower southeast flank; contain abundant phenocrysts and microphenocrysts of plagioclase, hypersthene, augite, and oxides; also abundant mafic enclaves (one analysis, 52.7 weight percent SiO2. Lava characterized by high Ce (40 to 53 ppm; some samples also have high Sr (greater than 500 ppm; 40Ar/39Ar age of 24 +/- 8 ka. Overlain by lava flows of flank vent 5567 (unit Hlf).
    PlvcAndesite lava flows of upper Volcano Creek - Porphyritic andesite (57.3 to 57.6 weight percent SiO2; four analyses containing phenocrysts of plagioclase to 1.5 mm; common mafic enclaves. Form glacially eroded ridge west of upper Volcano Creek; lava characterized by high Ce (44-50 ppm and high Sr (647-703 ppm and relatively high K2O (1.43 to 1.49 weight percent for silica content. Overlies deposits of block-and-ash flows and lahars of north flank (unit Pnba. 40Ar/39Ar ages of 47 +/- 10 and 63 +/- 6 ka.
    PnbBanded lavas of northwest flank - Porphyritic, vesicular to dense, and locally banded, andesite (59.6 to 60.6 weight percent SiO2; four analyses lava flows and breccias. Phenocrysts chiefly 1 mm plagioclase with glomerocrysts of intergrown plagioclase, clinopyroxene, and orthopyroxene; abundant oxides throughout. Overlain by a lava flow of south and west flank (unit Pl that, in turn, is littered with abundant glacially transported and strikingly banded blocks similar to lava flow. Analysis of darker-colored band is 58.4 weight percent SiO2; two lighter-colored bands are 61.2 weight percent SiO2. Lighter-colored bands have glassy groundmass while darker-colored band has microlite-rich groundmass. Source of transported lava lies upslope largely under glacier ice, but perhaps exposed below summit ridge. Blocks and in-situ lava are compositionally similar to clasts in north flank block-and-ash-flow deposits (unit Pnba and suggest that block-and-ash flows and banded lavas may date from same eruptive period. A probably unreliable 40Ar/39Ar age of 15 +/- 10 ka was obtained from light-gray, slightly vesicular lava; overlying lava flow of unit Pl yielded more reliable age of 38 +/- 9 ka.
    PnbaBlock-and-ash-flow and lahar deposits of north flank - Bouldery and cobbly, very poorly sorted diamicts of dense to vesicular andesite clasts in a sandy matrix; clasts commonly prismatically jointed. Clasts range from 57.6 to 62.8 weight percent SiO2 (nine analyses. Lava contains abundant 1 mm subhedral plagioclase and orthopyroxene with smaller and less abundant clinopyroxene; glassy groundmass contains less than 0.2 mm plagioclase laths; abundant oxide growth in pyroxenes. The upper approximately 150 m of deposit is accessible for study; individual beds are one to several meters thick; locally indurated, probably by glacial compaction and silica cementation. Chemical diversity of upper part of unit suggests emplacement over several eruptive periods. Lower part of the unit is largely inaccessible in cliffs on east side of Indecision Creek valley, but appears to be formed of multicolored sequences of sandy and finer-grained, well-stratified deposits as well as diamicts. We do not know how similar upper and lower parts are in age and origin. Great thickness of sequence (more than 300 m and height above floor of Indecision Creek valley (600 m suggests that unit may represent sediment that accumulated against thick glacier in valley. Chemical similarity of analyzed clasts from upper part of unit with banded lava flows of north flank (unit Pnb suggests they may date from same general eruptive period during which active lava flows or lava domes on the upper northwest flank were generating large amounts of debris. 40Ar/39Ar age of 189 +/- 14 ka was obtained from a prismatically jointed lava block within a ~5-m-thick flow unit near the eroded top of the unit, just below ~30 m of till.

    FieldSampleID
    Identifier assigned by person who originally located the sample (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    NULLA sample number was not recorded or was not assigned.

    Generic example of a DGGS sample identifier: YYYYAAA9999XXXX; YY or YYYY= year, AAA=geologist's initials (one to three characters), 9999=unique station number, XXXX= optional alpha characters which indicate that multiple samples were collected at a given location or that multiple observations with possibly identical prefixes were recorded in the project database. Field stations observed by non-DGGS personnel use identifiers from the originating reference.

    LocationConfidenceMeters
    Half-width in meters of positional uncertainty envelope; position is relative to other features in the data. A value of -9 indicates that this measure is not available. (Source: Alaska Division of Geological & Geophysical Surveys)

    Range of values
    Minimum:25
    Maximum:25
    Units:meters

    Age
    The interpreted (preferred) age calculated from geochronological analysis. Note: units provided in AgeUnits field. (Source: Volcano Science Center geochronology lab, U.S. Geological Survey, Menlo Park, California)

    Range of values
    Minimum:-5
    Maximum:251

    AgePlusError
    The upper age plus error calculated from geochronological analysis. Note: units provided in AgeUnits field. (Source: Volcano Science Center geochronology lab, U.S. Geological Survey, Menlo Park, California)

    Range of values
    Minimum:3
    Maximum:63

    AgeMinusError
    The lower age plus error calculated from geochronological analysis. Note: units provided in AgeUnits field. (Source: Volcano Science Center geochronology lab, U.S. Geological Survey, Menlo Park, California)

    Range of values
    Minimum:3
    Maximum:63

    AgeUnits
    Units of the geochronology analysis. (Source: Alaska Division of Geological & Geophysical Surveys or originating reference)

    Units(s): ka

    LocationSourceID
    Provides information about the source of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    Foreign key to the DataSources_ID field in the DataSources table.

    Notes
    Free text for additional information about a feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    The notes indicate the source of the feature interpretation or provide more information about the processing or data acquisition procedure.

    GeologicLines
    Dikes, coal seams, ash beds, glacial features, anticline and syncline hinge-surface traces, and other kinds of key beds that do not participate in map-unit topology (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    Type
    Type values specify the kind of features represented by a line, point, or polygon (Source: Alaska Division of Geological & Geophysical Surveys)

    Feature type(s): form line on lava flow; flow lobe, lava-flow front; within unit lava flow boundary; crest line of lateral levee on lava flow; crater rim; ice contact flow lobe, lava-flow front

    IsConcealed
    This is a flag to identify contacts and faults covered by overlying map units. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    NThe feature IS NOT covered by an overlying map unit.

    ExistenceConfidence
    Indicates the degree of confidence and/or certainty in the interpretation of a a feature's existence. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    certainIndicates that the author has sufficient observational and/or other supporting data to be reasonably confident in the scientific credibility of the interpretation of the feature. The level of certainty is specific to the map scale.

    IdentityConfidence
    Indicates degree of confidence in the identification and classification of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    certainIndicates that the author has sufficient observational and/or other supporting data to be reasonably confident in the scientific credibility of the interpretation of the feature. The level of certainty is specific to the map scale.

    LocationConfidenceMeters
    Half-width in meters of positional uncertainty envelope; position is relative to other features in the data. A value of -9 indicates that this measure is not available. (Source: Alaska Division of Geological & Geophysical Surveys)

    Range of values
    Minimum:25
    Maximum:25
    Units:meters

    Symbol
    References the symbol value that was assigned to the point. (Source: Alaska Division of Geological & Geophysical Surveys)

    Formal codeset
    Codeset Name:FGDC Digital Cartographic Standard for Geologic Map Symbolization; Appendix A. Geologic map symbols, colors, and patterns, "REF NO" field
    Codeset Source:Federal Geographic Data Committee [prepared for the Federal Geographic Data Committee by the U.S. Geological Survey], 2006, FGDC Digital Cartographic Standard for Geologic Map Symbolization: Reston, Va., Federal Geographic Data Committee Document Number FGDC-STD-013-2006, 290 p., 2 plates.

    DataSourceID
    Provides information about the source of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    Foreign key to the DataSources_ID field in the DataSources table.

    Notes
    Free text for additional information about a feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    The notes indicate the source of the feature interpretation or provide more information about the processing or data acquisition procedure.

    MapUnitPolys, MapUnitPolysAnno
    Polygons that record the distribution of mapped or interpreted geologic units (Source: Alaska Division of Geological & Geophysical Surveys (DGGS)This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    MapUnit
    Short ASCII string that indicates the map unit. Note: map units in parentheses, such as (Qcf), indicate combined map units consisting of bedrock overlain by thin or discontinuous deposits of the map unit. Map units with a question mark, such as (Qus?), indicate an uncertain identification. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    HPbvAndesite lava flow of Bear valley - Andesite (57.1 and 57.3 weight percent SiO2; two analyses lava flow that underlies block-and-ash-flow deposits (unit Hba along south margin of Bear valley. Plagioclase is the dominant mineral phase; 1-1.5 mm plagioclase phenocrysts with less abundant 0.5 mm clinopyroxene and orthopyroxene. Chemical composition of relatively high TiO2 and FeO and low K2O, Ba, and Sr poorly matches other units. Riehle (1990 interpreted this flow as Holocene, but, owing to poor exposure that limits assessment of degree of glacial erosion prior to burial by deposits of block-and-ash flows, we leave open the possibility of late Pleistocene age. Undated.
    HPlbHydrothermally altered lava flows and breccias - Slightly to intensely hydrothermally altered lava flows and lava breccias that form the uppermost edifice on the south flank and the highest point on the crater rim. This unit is younger and has more steeply dipping beds than the more severely altered lava flows and intervening breccias of unit Pla, also on the south flank. Some may represent near-vent equivalent of summit lava flows of unit Hls. Unsampled.
    HaAlluvium of active channels and floodplains - Chiefly cobble and pebble gravel and sand in upstream areas grading to sandy pebble gravel, sand, and sand and silt in downstream areas near Mother Goose Lake, Chiginagak Bay, and the upper Dog Salmon River.
    HbaDeposits of block-and-ash flows and lahars of Bear valley - Bouldery andesitic (58.1 and 59.8 weight percent SiO2; two analyses diamicts with sandy matrix; poorly sorted, non-stratified; commonly with prismatically jointed clasts; clasts dense to vesicular; minor amount of hydrothermally altered clasts. Limited exposures suggest diamicts comprise numerous flow units on order of 1-3 m thick. They partly fill the unnamed stream valley on the east flank (which we informally call Bear valley up to 8 km distant from the presumed source at vent 5567. Likely related to collapse of growing lava dome or active lava flows. If lava-dome extrusion at vent 5567 fed the block-and-ash flows, that dome is buried by later lava flows of unit Hlf. Stratigraphic relationship of block-and-ash-flow deposits with unit Hlf is obscured by erosion and deposition from neoglacial advance, but we infer that both originated from eruptions near flank vent 5567. Geomorphic relations show that fill of unit Hba was eroded by stream along north side of valley prior to emplacement of lobe of unit Hlf at site 05JRSCH051. Riehle (1990 believed that much of the fill in Bear valley was a lava flow (our unit Plbv, except along the north side of the valley, and was only thinly mantled by block-and-ash-flow deposits. However, we find substantial cover of deposits and only limited outcrop of the lava flow. Above about 1,000 ft elevation, surface is largely barren of vegetation and reworked by solifluction and other surface processes. At lower elevations, the unit is covered by a discontinuous mantle of sand, silt, and organic material that contains a few thin tephra layers. The amount of soil development (tens of centimeters of oxidation suggests unit is probably of early Holocene age. Overlies lava flow of Holocene or Pleistocene age (unit HPbv exposed locally along south margin of Bear valley. Undated.
    HcColluvium and alluvium - Colluvium and alluvium of valley-wall fans and landslide deposits; gravel and sand; very poorly sorted to poorly sorted; non-bedded to crudely bedded. Forms sheets and cones of talus and fans of alluvium and debris-flow deposits. Along brush-covered east side of Indecision Creek valley includes small areas of pre-Quaternary bedrock (unit JTu, block-and-ash-flow and related deposits of north flank (unit Pnba, and Pleistocene glacial deposits (units Pgt and Pgu too small to show at map scale.
    HdfPyroclastic-flow and lahar deposits along Chiginagak creek - Gray bouldery and cobbly diamicts with blocks of juvenile lava and clay-rich lahar deposits rich in hydrothermally altered debris that form small terraces along informally named Chiginagak creek southeast of the volcano. Along upper Chiginagak creek, gray, sandy-matrix diamicts overlie yellow to orange-brown, clayey, massive lahar deposits. We infer that the clayey lahar deposits (individual units on the order of 1 to 2 m thick were generated by debris avalanches that contained sufficient water to transform into a lahar and the overlying gray diamict originated in collapses of growing lava domes or flows. Geochemical analysis of a block from the gray diamict resembles that of lava of unit Hls. Upper gray diamicts up to a few meters thick; clay-rich lahar deposits more than 10 m thick.
    HdiDebris-flow deposits of upper Indecision Creek - Poorly sorted gravel, sand, and orange-yellow clay; orange-brown sandy to silty-clay matrix; larger clasts are faceted andesite boulders, and gravel- to sand-sized angular clasts of hydrothermally altered andesite and gray dense andesite; forms terraces several meters above active channel along upper Indecision Creek. Probably originated from debris-covered glacier upstream (unit dcg. Varied terrace top elevations represent at least two ages of debris-flow deposits.
    HgoGlacial outwash of late Neoglacial age - Cobble and pebble gravel and sand, locally bouldery; crudely to moderately bedded; poorly sorted. Forms fans and channels fronting modern glaciers and moraines of late Neoglacial age. Includes small areas of late Neoglacial till and underlying lava flows too small to show at map scale. Merges downstream to alluvium of active channels (unit Ha and floodplains.
    HgtGlacial till of late Neoglacial age - Bouldery gravel and sand; very poorly sorted; in some locations the till forms sharp-crested moraines up to 1 km beyond margins of existing glacial termini. These likely date from the past several centuries. Includes patches of underlying lava flows too small to show at map scale.
    HlfLava flows from vent 5567 on southeast flank - Rubbly-topped andesite (55.3-60.5 weight percent SiO2; eight analyses lava flows that form a high point about 1.4 km southeast of summit crater (x5567 ft on USGS Ugashik A-3 topographic map and a series of flow lobes that surround portions of Pleistocene lava flows (units Plse and Pl on the southeast and south flanks. Lava is porphyritic and contains variable proportions of 0.5-1.5 mm plagioclase, hypersthene, augite, and minor olivine and oxides. Northernmost lava flow underlies the southern margin of summit lava flows (unit Hls. Detterman and others (1987b map vent 5567 as a lava dome surrounded by lava flows, but we find that the vent lavas form flows that reach the base of east the flank. Two prismatically jointed blocks (57.4 and 58.1 weight percent SiO2 that are chemically similar to lava of this unit lie on deposits of block-and ash flows of Bear valley (unit Hba. We infer that they are blocks emplaced by late pyroclastic flows or rockfalls from active lava flows of vent 5567. Geomorphic relations at the head of north Bear valley drainage show that emplacement of lava flows of unit Hlf followed erosion of a valley through deposits of unit Hba and passage of pyroclastic flows and lahars of unit Hpd.
    HlsLava flows from summit vent - Rubbly-topped andesite (56.4-59.0 weight percent SiO2; seventeen analyses lava flows that form a nearly continuous apron on the north and east flanks and one flow on upper west flank. Lava is porphyritic and contains variable proportions of 0.5-1.5 mm plagioclase, hypersthene, clinopyroxene, and oxides. Flow surfaces are modified by glacial erosion and deposition within the limit of neoglacial advances, but are typically rubbly and blocky beyond. Southernmost flow of unit on east flank overlies lava flow from a vent on the southeast flank (unit Hlf. Analyses of summit flows define a geochemical trend that is slightly (about 0.1 weight percent higher in K2O than lava flows from southeast flank vent (unit Hlf.
    HpdDeposits of pyroclastic flows and lahars - Lahar and poorly sorted pyroclastic-flow deposits consisting of andesite blocks (25percent are vesicular in a loose, pink to gray, sandy matrix. Most vesicular blocks are less than 10 cm, and the dense, juvenile, prismatically jointed blocks (60.2 to 60.8 weight percent SiO2 are larger, up to 1 m. Terraced surfaces of Hpd lie along the northeast side of Bear valley, where they are inset below the upper surface of block-and-ash-flow deposits of Bear valley (unit Hba, as well as in lower Bear valley, approximately 1,600 m farther down valley than the toe of unit Hba. The deposit is discontinuously mantled by unmapped, fine-grained loessial deposits with interbedded tephras and buried soils. A small, isolated outcrop of a similar diamict of possibly equivalent age is found along a tributary of Volcano Creek on the lower north flank surrounded by unit Pgu, where approximately 2 m of soil and tephra overlie a 1-m-thick gray, sandy, polylithic, matrix-supported lahar with a base deposit consisting of a pink monolithic pyroclastic-flow deposit with a variably cemented matrix. An organic layer in this upper mantle has a radiocarbon age of 3,110 +/- 80 yBP.
    JTuBedrock, undifferentiated - Chiefly Upper Jurassic Naknek Formation sandstone and siltstone (units Jns and Jnst of Detterman and others, 1987a, as well as intrusive, hypabyssal, and extrusive igneous rocks of Miocene to Pliocene age and nonmarine sedimentary rocks of Eocene age. Includes discontinuous and unmapped mantle of alluvial, colluvial, and glacial deposits.
    PgtGlacial till - Bouldery gravel and sand; massive; very poorly sorted. Weathering profile consisting of a few tens of centimeters of oxidized till and mantling fine-grained sediment. Only mapped in one area on the northwest flank of volcano where it lies on Pleistocene block-and-ash-flow deposits (unit Pnba.
    PguGlacial deposits, undifferentiated - Undifferentiated deposits of till and glaciofluvial and glaciolacustrine deposits that bury Pleistocene block-and-ash-flow deposits (unit Pnba, Pleistocene lava flows, and bedrock (unit JTu on the north flank. Also includes discontinuous mantle of chiefly fine-grained loessial sediments with interbedded tephra and buried soils.
    PlAndesite lava flows of south, west, and northwest flanks - Porphyritic andesite (57.2-61.8 weight percent SiO2; eighteen analyses lava flows containing abundant 1-1.5 mm plagioclase; smaller and less abundant hypersthene, and augite to 0.5 mm; abundant oxide growth in the pyroxenes; common fine-grained mafic enclaves. Includes some thick (up to 350 m ice-contact lava flows along the east side of Indecision Creek valley that were emplaced against east margin of a valley-filling glacier. Ice-contact flows contain large masses of breccias, quenched glassy lava, discontinuous pods of intensely jointed lava, high cliffs displaying fans of radiating columnar joints, and numerous faults, indicative of interaction with water and ice. Also includes lava flows on south and north-northwest flanks that overlie deposits of Pleistocene block-and-ash flows (unit Pnba. Forms a lava bench on the northwest flank consisting of greater than 1-m-thick stacked lava flows. Lava flows have glacially eroded tops and are covered discontinuously by deposits of till (unit Pgt too small to map. 40Ar/39Ar ages of 15 +/- 11, 38 +/- 9, 44 +/- 18, 54 +/- 13, 55 +/- 8, 64 +/- 8, 72 +/- 9, and 75 +/- 3 ka and chemical diversity suggest eruptions occurred during several episodes in late Pleistocene time.
    PlaHydrothermally altered lava flows - Weakly to intensely hydrothermally altered porphyritic low silica dacite (63.4 and 63.8 weight percent SiO2; two analyses; most silicic analyses of any sampled Chiginagak rocks lava flows and intervening breccias of the upper south flank; some thick, massive outcrops may represent lava domes or dome breccia; gray, red, orange, and yellow in color; locally altered to clay and contains minor gypsum. Fresher lava contains abundant phenocrysts of plagioclase, hypersthene, and augite as well as common mafic enclaves. This unit is prominently exposed along the canyon walls flanking the south glacier under a carapace of less altered lava flows of the younger unit Pl; also underlies less-altered lava flows and lava breccias of unit HPlb in walls of summit crater. 40Ar/39Ar age of 89 +/- 6 ka in the east wall above south flank glacier. Rockfalls and rock avalanches from this unit and lava breccias of unit HPlb create thick, orange-brown, superglacial debris mantle (unit dcg).
    PloBasaltic andesite lava flows of north flank - At least two basaltic andesite lava flows (54.9 and 56.2 weight percent SiO2; two analyses that lie within block-and-ash-flow deposits of north flank (unit Pnba; exposed in cliffs above Indecision Creek, approximately 100 m from eroded top of block-and-ash-flow deposits. Abundant 1 mm plagioclase, orthopyroxene, and clinopyroxene in a dark gray sugary groundmass. An unsampled lava flow that forms an inaccessible bench below main flow is included in unit, but its stratigraphic position is not well constrained. A diamict of radially fractured blocks in a sandy matrix, which crops out about 400 m north of exposure of upper lava flows and appears to be interbedded in block-and-ash flows of north flank (unit Pnba, has a basaltic andesite composition (55.9 weight percent SiO2; one analysis similar to that of the lava flows in unit and is likely a fragmental deposit related to it. Block in diamict yielded a 40Ar/39Ar age of 251 +/- 6 ka.
    PlseAndesite lava flows of southeast flank - Porphyritic andesite (52.7 to 59.5 weight percent SiO2; five analyses lava flows and intervening breccias that form broad, glacially eroded ridges on lower southeast flank; contain abundant phenocrysts and microphenocrysts of plagioclase, hypersthene, augite, and oxides; also abundant mafic enclaves (one analysis, 52.7 weight percent SiO2. Lava characterized by high Ce (40 to 53 ppm; some samples also have high Sr (greater than 500 ppm; 40Ar/39Ar age of 24 +/- 8 ka. Overlain by lava flows of flank vent 5567 (unit Hlf).
    PlvcAndesite lava flows of upper Volcano Creek - Porphyritic andesite (57.3 to 57.6 weight percent SiO2; four analyses containing phenocrysts of plagioclase to 1.5 mm; common mafic enclaves. Form glacially eroded ridge west of upper Volcano Creek; lava characterized by high Ce (44-50 ppm and high Sr (647-703 ppm and relatively high K2O (1.43 to 1.49 weight percent for silica content. Overlies deposits of block-and-ash flows and lahars of north flank (unit Pnba. 40Ar/39Ar ages of 47 +/- 10 and 63 +/- 6 ka.
    PnbBanded lavas of northwest flank - Porphyritic, vesicular to dense, and locally banded, andesite (59.6 to 60.6 weight percent SiO2; four analyses lava flows and breccias. Phenocrysts chiefly 1 mm plagioclase with glomerocrysts of intergrown plagioclase, clinopyroxene, and orthopyroxene; abundant oxides throughout. Overlain by a lava flow of south and west flank (unit Pl that, in turn, is littered with abundant glacially transported and strikingly banded blocks similar to lava flow. Analysis of darker-colored band is 58.4 weight percent SiO2; two lighter-colored bands are 61.2 weight percent SiO2. Lighter-colored bands have glassy groundmass while darker-colored band has microlite-rich groundmass. Source of transported lava lies upslope largely under glacier ice, but perhaps exposed below summit ridge. Blocks and in-situ lava are compositionally similar to clasts in north flank block-and-ash-flow deposits (unit Pnba and suggest that block-and-ash flows and banded lavas may date from same eruptive period. A probably unreliable 40Ar/39Ar age of 15 +/- 10 ka was obtained from light-gray, slightly vesicular lava; overlying lava flow of unit Pl yielded more reliable age of 38 +/- 9 ka.
    PnbaBlock-and-ash-flow and lahar deposits of north flank - Bouldery and cobbly, very poorly sorted diamicts of dense to vesicular andesite clasts in a sandy matrix; clasts commonly prismatically jointed. Clasts range from 57.6 to 62.8 weight percent SiO2 (nine analyses. Lava contains abundant 1 mm subhedral plagioclase and orthopyroxene with smaller and less abundant clinopyroxene; glassy groundmass contains less than 0.2 mm plagioclase laths; abundant oxide growth in pyroxenes. The upper approximately 150 m of deposit is accessible for study; individual beds are one to several meters thick; locally indurated, probably by glacial compaction and silica cementation. Chemical diversity of upper part of unit suggests emplacement over several eruptive periods. Lower part of the unit is largely inaccessible in cliffs on east side of Indecision Creek valley, but appears to be formed of multicolored sequences of sandy and finer-grained, well-stratified deposits as well as diamicts. We do not know how similar upper and lower parts are in age and origin. Great thickness of sequence (more than 300 m and height above floor of Indecision Creek valley (600 m suggests that unit may represent sediment that accumulated against thick glacier in valley. Chemical similarity of analyzed clasts from upper part of unit with banded lava flows of north flank (unit Pnb suggests they may date from same general eruptive period during which active lava flows or lava domes on the upper northwest flank were generating large amounts of debris. 40Ar/39Ar age of 189 +/- 14 ka was obtained from a prismatically jointed lava block within a ~5-m-thick flow unit near the eroded top of the unit, just below ~30 m of till.
    PnpPumice-rich fall and flow deposits of north flank - More than 10-m-thick (base not exposed deposit of light-gray to yellow, high-silica andesite (60.8 to 62.1 weight percent SiO2; seven analyses pumice fall and flow deposits. Deposit composed chiefly of pumiceous lapilli and ash, with less abundant component of small bombs, dense gray clasts, and accidental fragments. Known only from single exposure along tributary of Volcano Creek where it underlies block-and-ash-flow and lahar deposits of north flank (unit Pnba, whose base is rich in pumiceous clasts and breadcrust bombs. Their contact is disconformable but, owing to geochemical similarity to samples from units Pnba and Pnp, may not signify a long time break.
    dcgDebris-covered glacier - Poorly sorted, superglacial till covering much of the lower reach of the south flank glacier and a few areas near termini of glaciers on other flanks. The mantle on the south flank glacier is chiefly composed of hydrothermally altered rock and orange-brown clay eroded from the upper south flank and forms a series of convex-downstream concentric ridges.
    gGlaciers and perennial snow fields

    IdentityConfidence
    Indicates degree of confidence in the identification and classification of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    certainIndicates that the author has sufficient observational and/or other supporting data to be reasonably confident in the scientific credibility of the interpretation of the feature. The level of certainty is specific to the map scale.

    Label
    The label values provide text strings to display along with their associated feature. Depending on the context of the feature class, label values will contain applicable FGDCGeoAge font characters, feature labels, dip and plunge values for measured orientations, or other special characters. (Source: Alaska Division of Geological & Geophysical Surveys)

    Formal codeset
    Codeset Name:FGDCGeoAge font
    Codeset Source:Federal Geographic Data Committee [prepared for the Federal Geographic Data Committee by the U.S. Geological Survey], 2006, FGDC Digital Cartographic Standard for Geologic Map Symbolization: Reston, Va., Federal Geographic Data Committee Document Number FGDC-STD-013-2006, 290 p., 2 plates.

    Symbol
    References the CMYK color value that was assigned to the map unit. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    57A0CMYK value - C:50%, M:70%, Y:8%, K:0%
    4400CMYK value - C:40%, M:40%, Y:0%, K:0%
    50A0CMYK value - C:50%, M:0%, Y:8%, K:0%
    34A0CMYK value - C:30%, M:40%, Y:8%, K:0%
    00X0CMYK value - C:0%, M:0%, Y:100%, K:0%
    0020CMYK value - C:0%, M:0%, Y:20%, K:0%
    2630CMYK value - C:20%, M:60%, Y:30%, K:0%
    5600CMYK value - C:50%, M:60%, Y:0%, K:0%
    0000 Pattern 135-BCMYK value - C:0%, M:0%, Y:0%, K:0%
    0000CMYK value - C:0%, M:0%, Y:0%, K:0%
    0003CMYK value - C:0%, M:0%, Y:0%, K:30%
    0A50CMYK value - C:0%, M:8%, Y:50%, K:0%
    2770CMYK value - C:20%, M:70%, Y:70%, K:0%
    3620CMYK value - C:30%, M:60%, Y:20%, K:0%
    2000CMYK value - C:20%, M:0%, Y:0%, K:0%
    0430CMYK value - C:0%, M:40%, Y:30%, K:0%
    2200CMYK value - C:20%, M:20%, Y:0%, K:0%
    6350CMYK value - C:60%, M:30%, Y:50%, K:0%
    AX00CMYK value - C:8%, M:100%, Y:0%, K:0%
    73A0CMYK value - C:70%, M:30%, Y:8%, K:0%
    2130CMYK value - C:20%, M:13%, Y:30%, K:0%
    35X0CMYK value - C:30%, M:50%, Y:100%, K:0%
    A650CMYK value - C:8%, M:60%, Y:50%, K:0%
    3030CMYK value - C:30%, M:0%, Y:30%, K:0%
    41 14 05 00CMYK value - C:40%, M:13%, Y:%, K:13%
    5750CMYK value - C:50%, M:70%, Y:50%, K:0%

    Notes
    Free text for additional information about a feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    The notes indicate the source of the feature interpretation or provide more information about the processing or data acquisition procedure.

    DataSourceID
    Provides information about the source of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    Foreign key to the DataSources_ID field in the DataSources table.

    OtherPolys
    Polygons that record the location and extent of geologic or geographic features other than the geologic units (Source: Alaska Division of Geological & Geophysical Surveys (DGGS)This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    Type
    Type values specify the kind of features represented by a line, point, or polygon (Source: Alaska Division of Geological & Geophysical Surveys)

    Feature type(s): Fumarole Field; Map Neatline

    IdentityConfidence
    Indicates degree of confidence in the identification and classification of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    certainIndicates that the author has sufficient observational and/or other supporting data to be reasonably confident in the scientific credibility of the interpretation of the feature. The level of certainty is specific to the map scale.

    Symbol
    References the symbol value that was assigned to the point. (Source: Alaska Division of Geological & Geophysical Surveys)

    Formal codeset
    Codeset Name:FGDC Digital Cartographic Standard for Geologic Map Symbolization; Appendix A. Geologic map symbols, colors, and patterns, "REF NO" field
    Codeset Source:Federal Geographic Data Committee [prepared for the Federal Geographic Data Committee by the U.S. Geological Survey], 2006, FGDC Digital Cartographic Standard for Geologic Map Symbolization: Reston, Va., Federal Geographic Data Committee Document Number FGDC-STD-013-2006, 290 p., 2 plates.

    Notes
    Free text for additional information about a feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    The notes indicate the source of the feature interpretation or provide more information about the processing or data acquisition procedure.

    DataSourceID
    Provides information about the source of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    Foreign key to the DataSources_ID field in the DataSources table.

    RepurposedSymbols
    Non-spatial reference table that documents cases in which FGDC Standard symbols are supplemented with custom symbols or with FGDC symbols repurposed for the map. (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    FgdcIdentifier
    Zero-padded identifier string from FGDC standard. (Source: Alaska Division of Geological & Geophysical Surveys)

    Example: 01.01.03

    OldExplanation
    Explanatory text from FGDC standard for meaning of symbol. (Source: Alaska Division of Geological & Geophysical Surveys)

    Example: Contact identity and existence certain, location approximate

    NewExplanation
    Explanation of usage of symbol in this map portrayal (Source: Alaska Division of Geological & Geophysical Surveys)

    Example: Limit of tephra deposits from Holocene eruptions of Glacier Peak.

    Stations, StationsAnno
    Field observation locations and descriptions (Source: Alaska Division of Geological & Geophysical Surveys (DGGS).This table is organized according to the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1 by the USGS National Cooperative Geologic Mapping Program (NCGMP).)

    FieldID
    Identifier assigned by person who originally located station (Source: Alaska Division of Geological & Geophysical Surveys)

    Generic example of a sample identifier: YYYYAAA9999X; YY or YYYY= year, AAA=geologist's initials (one to three characters), 9999=unique station number, X= optional alpha character which indicates that multiple samples were collected at a given location or that multiple observations were recorded in the project database.

    MapUnit
    Short ASCII string that indicates the map unit. Note: map units in parentheses, such as (Qcf), indicate combined map units consisting of bedrock overlain by thin or discontinuous deposits of the map unit. Map units with a question mark, such as (Qus?), indicate an uncertain identification. (Source: Alaska Division of Geological & Geophysical Surveys)

    ValueDefinition
    HaAlluvium of active channels and floodplains - Chiefly cobble and pebble gravel and sand in upstream areas grading to sandy pebble gravel, sand, and sand and silt in downstream areas near Mother Goose Lake, Chiginagak Bay, and the upper Dog Salmon River.
    HbaDeposits of block-and-ash flows and lahars of Bear valley - Bouldery andesitic (58.1 and 59.8 weight percent SiO2; two analyses diamicts with sandy matrix; poorly sorted, non-stratified; commonly with prismatically jointed clasts; clasts dense to vesicular; minor amount of hydrothermally altered clasts. Limited exposures suggest diamicts comprise numerous flow units on order of 1-3 m thick. They partly fill the unnamed stream valley on the east flank (which we informally call Bear valley up to 8 km distant from the presumed source at vent 5567. Likely related to collapse of growing lava dome or active lava flows. If lava-dome extrusion at vent 5567 fed the block-and-ash flows, that dome is buried by later lava flows of unit Hlf. Stratigraphic relationship of block-and-ash-flow deposits with unit Hlf is obscured by erosion and deposition from neoglacial advance, but we infer that both originated from eruptions near flank vent 5567. Geomorphic relations show that fill of unit Hba was eroded by stream along north side of valley prior to emplacement of lobe of unit Hlf at site 05JRSCH051. Riehle (1990 believed that much of the fill in Bear valley was a lava flow (our unit Plbv, except along the north side of the valley, and was only thinly mantled by block-and-ash-flow deposits. However, we find substantial cover of deposits and only limited outcrop of the lava flow. Above about 1,000 ft elevation, surface is largely barren of vegetation and reworked by solifluction and other surface processes. At lower elevations, the unit is covered by a discontinuous mantle of sand, silt, and organic material that contains a few thin tephra layers. The amount of soil development (tens of centimeters of oxidation suggests unit is probably of early Holocene age. Overlies lava flow of Holocene or Pleistocene age (unit HPbv exposed locally along south margin of Bear valley. Undated.
    HdfPyroclastic-flow and lahar deposits along Chiginagak creek - Gray bouldery and cobbly diamicts with blocks of juvenile lava and clay-rich lahar deposits rich in hydrothermally altered debris that form small terraces along informally named Chiginagak creek southeast of the volcano. Along upper Chiginagak creek, gray, sandy-matrix diamicts overlie yellow to orange-brown, clayey, massive lahar deposits. We infer that the clayey lahar deposits (individual units on the order of 1 to 2 m thick were generated by debris avalanches that contained sufficient water to transform into a lahar and the overlying gray diamict originated in collapses of growing lava domes or flows. Geochemical analysis of a block from the gray diamict resembles that of lava of unit Hls. Upper gray diamicts up to a few meters thick; clay-rich lahar deposits more than 10 m thick.
    HdiDebris-flow deposits of upper Indecision Creek - Poorly sorted gravel, sand, and orange-yellow clay; orange-brown sandy to silty-clay matrix; larger clasts are faceted andesite boulders, and gravel- to sand-sized angular clasts of hydrothermally altered andesite and gray dense andesite; forms terraces several meters above active channel along upper Indecision Creek. Probably originated from debris-covered glacier upstream (unit dcg. Varied terrace top elevations represent at least two ages of debris-flow deposits.
    HgoGlacial outwash of late Neoglacial age - Cobble and pebble gravel and sand, locally bouldery; crudely to moderately bedded; poorly sorted. Forms fans and channels fronting modern glaciers and moraines of late Neoglacial age. Includes small areas of late Neoglacial till and underlying lava flows too small to show at map scale. Merges downstream to alluvium of active channels (unit Ha and floodplains.
    HgtGlacial till of late Neoglacial age - Bouldery gravel and sand; very poorly sorted; in some locations the till forms sharp-crested moraines up to 1 km beyond margins of existing glacial termini. These likely date from the past several centuries. Includes patches of underlying lava flows too small to show at map scale.
    HlfLava flows from vent 5567 on southeast flank - Rubbly-topped andesite (55.3-60.5 weight percent SiO2; eight analyses lava flows that form a high point about 1.4 km southeast of summit crater (x5567 ft on USGS Ugashik A-3 topographic map and a series of flow lobes that surround portions of Pleistocene lava flows (units Plse and Pl on the southeast and south flanks. Lava is porphyritic and contains variable proportions of 0.5-1.5 mm plagioclase, hypersthene, augite, and minor olivine and oxides. Northernmost lava flow underlies the southern margin of summit lava flows (unit Hls. Detterman and others (1987b map vent 5567 as a lava dome surrounded by lava flows, but we find that the vent lavas form flows that reach the base of east the flank. Two prismatically jointed blocks (57.4 and 58.1 weight percent SiO2 that are chemically similar to lava of this unit lie on deposits of block-and ash flows of Bear valley (unit Hba. We infer that they are blocks emplaced by late pyroclastic flows or rockfalls from active lava flows of vent 5567. Geomorphic relations at the head of north Bear valley drainage show that emplacement of lava flows of unit Hlf followed erosion of a valley through deposits of unit Hba and passage of pyroclastic flows and lahars of unit Hpd.
    HlsLava flows from summit vent - Rubbly-topped andesite (56.4-59.0 weight percent SiO2; seventeen analyses lava flows that form a nearly continuous apron on the north and east flanks and one flow on upper west flank. Lava is porphyritic and contains variable proportions of 0.5-1.5 mm plagioclase, hypersthene, clinopyroxene, and oxides. Flow surfaces are modified by glacial erosion and deposition within the limit of neoglacial advances, but are typically rubbly and blocky beyond. Southernmost flow of unit on east flank overlies lava flow from a vent on the southeast flank (unit Hlf. Analyses of summit flows define a geochemical trend that is slightly (about 0.1 weight percent higher in K2O than lava flows from southeast flank vent (unit Hlf.
    HpdDeposits of pyroclastic flows and lahars - Lahar and poorly sorted pyroclastic-flow deposits consisting of andesite blocks (25percent are vesicular in a loose, pink to gray, sandy matrix. Most vesicular blocks are less than 10 cm, and the dense, juvenile, prismatically jointed blocks (60.2 to 60.8 weight percent SiO2 are larger, up to 1 m. Terraced surfaces of Hpd lie along the northeast side of Bear valley, where they are inset below the upper surface of block-and-ash-flow deposits of Bear valley (unit Hba, as well as in lower Bear valley, approximately 1,600 m farther down valley than the toe of unit Hba. The deposit is discontinuously mantled by unmapped, fine-grained loessial deposits with interbedded tephras and buried soils. A small, isolated outcrop of a similar diamict of possibly equivalent age is found along a tributary of Volcano Creek on the lower north flank surrounded by unit Pgu, where approximately 2 m of soil and tephra overlie a 1-m-thick gray, sandy, polylithic, matrix-supported lahar with a base deposit consisting of a pink monolithic pyroclastic-flow deposit with a variably cemented matrix. An organic layer in this upper mantle has a radiocarbon age of 3,110 +/- 80 yBP.
    JTuBedrock, undifferentiated - Chiefly Upper Jurassic Naknek Formation sandstone and siltstone (units Jns and Jnst of Detterman and others, 1987a, as well as intrusive, hypabyssal, and extrusive igneous rocks of Miocene to Pliocene age and nonmarine sedimentary rocks of Eocene age. Includes discontinuous and unmapped mantle of alluvial, colluvial, and glacial deposits.
    PguGlacial deposits, undifferentiated - Undifferentiated deposits of till and glaciofluvial and glaciolacustrine deposits that bury Pleistocene block-and-ash-flow deposits (unit Pnba, Pleistocene lava flows, and bedrock (unit JTu on the north flank. Also includes discontinuous mantle of chiefly fine-grained loessial sediments with interbedded tephra and buried soils.
    PlAndesite lava flows of south, west, and northwest flanks - Porphyritic andesite (57.2-61.8 weight percent SiO2; eighteen analyses lava flows containing abundant 1-1.5 mm plagioclase; smaller and less abundant hypersthene, and augite to 0.5 mm; abundant oxide growth in the pyroxenes; common fine-grained mafic enclaves. Includes some thick (up to 350 m ice-contact lava flows along the east side of Indecision Creek valley that were emplaced against east margin of a valley-filling glacier. Ice-contact flows contain large masses of breccias, quenched glassy lava, discontinuous pods of intensely jointed lava, high cliffs displaying fans of radiating columnar joints, and numerous faults, indicative of interaction with water and ice. Also includes lava flows on south and north-northwest flanks that overlie deposits of Pleistocene block-and-ash flows (unit Pnba. Forms a lava bench on the northwest flank consisting of greater than 1-m-thick stacked lava flows. Lava flows have glacially eroded tops and are covered discontinuously by deposits of till (unit Pgt too small to map. 40Ar/39Ar ages of 15 +/- 11, 38 +/- 9, 44 +/- 18, 54 +/- 13, 55 +/- 8, 64 +/- 8, 72 +/- 9, and 75 +/- 3 ka and chemical diversity suggest eruptions occurred during several episodes in late Pleistocene time.
    PlaHydrothermally altered lava flows - Weakly to intensely hydrothermally altered porphyritic low silica dacite (63.4 and 63.8 weight percent SiO2; two analyses; most silicic analyses of any sampled Chiginagak rocks lava flows and intervening breccias of the upper south flank; some thick, massive outcrops may represent lava domes or dome breccia; gray, red, orange, and yellow in color; locally altered to clay and contains minor gypsum. Fresher lava contains abundant phenocrysts of plagioclase, hypersthene, and augite as well as common mafic enclaves. This unit is prominently exposed along the canyon walls flanking the south glacier under a carapace of less altered lava flows of the younger unit Pl; also underlies less-altered lava flows and lava breccias of unit HPlb in walls of summit crater. 40Ar/39Ar age of 89 +/- 6 ka in the east wall above south flank glacier. Rockfalls and rock avalanches from this unit and lava breccias of unit HPlb create thick, orange-brown, superglacial debris mantle (unit dcg).
    PloBasaltic andesite lava flows of north flank - At least two basaltic andesite lava flows (54.9 and 56.2 weight percent SiO2; two analyses that lie within block-and-ash-flow deposits of north flank (unit Pnba; exposed in cliffs above Indecision Creek, approximately 100 m from eroded top of block-and-ash-flow deposits. Abundant 1 mm plagioclase, orthopyroxene, and clinopyroxene in a dark gray sugary groundmass. An unsampled lava flow that forms an inaccessible bench below main flow is included in unit, but its stratigraphic position is not well constrained. A diamict of radially fractured blocks in a sandy matrix, which crops out about 400 m north of exposure of upper lava flows and appears to be interbedded in block-and-ash flows of north flank (unit Pnba, has a basaltic andesite composition (55.9 weight percent SiO2; one analysis similar to that of the lava flows in unit and is likely a fragmental deposit related to it. Block in diamict yielded a 40Ar/39Ar age of 251 +/- 6 ka.
    PlseAndesite lava flows of southeast flank - Porphyritic andesite (52.7 to 59.5 weight percent SiO2; five analyses lava flows and intervening breccias that form broad, glacially eroded ridges on lower southeast flank; contain abundant phenocrysts and microphenocrysts of plagioclase, hypersthene, augite, and oxides; also abundant mafic enclaves (one analysis, 52.7 weight percent SiO2. Lava characterized by high Ce (40 to 53 ppm; some samples also have high Sr (greater than 500 ppm; 40Ar/39Ar age of 24 +/- 8 ka. Overlain by lava flows of flank vent 5567 (unit Hlf).
    PlvcAndesite lava flows of upper Volcano Creek - Porphyritic andesite (57.3 to 57.6 weight percent SiO2; four analyses containing phenocrysts of plagioclase to 1.5 mm; common mafic enclaves. Form glacially eroded ridge west of upper Volcano Creek; lava characterized by high Ce (44-50 ppm and high Sr (647-703 ppm and relatively high K2O (1.43 to 1.49 weight percent for silica content. Overlies deposits of block-and-ash flows and lahars of north flank (unit Pnba. 40Ar/39Ar ages of 47 +/- 10 and 63 +/- 6 ka.
    PnbBanded lavas of northwest flank - Porphyritic, vesicular to dense, and locally banded, andesite (59.6 to 60.6 weight percent SiO2; four analyses lava flows and breccias. Phenocrysts chiefly 1 mm plagioclase with glomerocrysts of intergrown plagioclase, clinopyroxene, and orthopyroxene; abundant oxides throughout. Overlain by a lava flow of south and west flank (unit Pl that, in turn, is littered with abundant glacially transported and strikingly banded blocks similar to lava flow. Analysis of darker-colored band is 58.4 weight percent SiO2; two lighter-colored bands are 61.2 weight percent SiO2. Lighter-colored bands have glassy groundmass while darker-colored band has microlite-rich groundmass. Source of transported lava lies upslope largely under glacier ice, but perhaps exposed below summit ridge. Blocks and in-situ lava are compositionally similar to clasts in north flank block-and-ash-flow deposits (unit Pnba and suggest that block-and-ash flows and banded lavas may date from same eruptive period. A probably unreliable 40Ar/39Ar age of 15 +/- 10 ka was obtained from light-gray, slightly vesicular lava; overlying lava flow of unit Pl yielded more reliable age of 38 +/- 9 ka.
    PnbaBlock-and-ash-flow and lahar deposits of north flank - Bouldery and cobbly, very poorly sorted diamicts of dense to vesicular andesite clasts in a sandy matrix; clasts commonly prismatically jointed. Clasts range from 57.6 to 62.8 weight percent SiO2 (nine analyses. Lava contains abundant 1 mm subhedral plagioclase and orthopyroxene with smaller and less abundant clinopyroxene; glassy groundmass contains less than 0.2 mm plagioclase laths; abundant oxide growth in pyroxenes. The upper approximately 150 m of deposit is accessible for study; individual beds are one to several meters thick; locally indurated, probably by glacial compaction and silica cementation. Chemical diversity of upper part of unit suggests emplacement over several eruptive periods. Lower part of the unit is largely inaccessible in cliffs on east side of Indecision Creek valley, but appears to be formed of multicolored sequences of sandy and finer-grained, well-stratified deposits as well as diamicts. We do not know how similar upper and lower parts are in age and origin. Great thickness of sequence (more than 300 m and height above floor of Indecision Creek valley (600 m suggests that unit may represent sediment that accumulated against thick glacier in valley. Chemical similarity of analyzed clasts from upper part of unit with banded lava flows of north flank (unit Pnb suggests they may date from same general eruptive period during which active lava flows or lava domes on the upper northwest flank were generating large amounts of debris. 40Ar/39Ar age of 189 +/- 14 ka was obtained from a prismatically jointed lava block within a ~5-m-thick flow unit near the eroded top of the unit, just below ~30 m of till.
    gGlaciers and perennial snow fields

    Label
    The label values provide text strings to display along with their associated feature. Depending on the context of the feature class, label values will contain applicable FGDCGeoAge font characters, feature labels, dip and plunge values for measured orientations, or other special characters. (Source: Alaska Division of Geological & Geophysical Surveys)

    Formal codeset
    Codeset Name:FGDCGeoAge font
    Codeset Source:Federal Geographic Data Committee [prepared for the Federal Geographic Data Committee by the U.S. Geological Survey], 2006, FGDC Digital Cartographic Standard for Geologic Map Symbolization: Reston, Va., Federal Geographic Data Committee Document Number FGDC-STD-013-2006, 290 p., 2 plates.

    LocationConfidenceMeters
    Half-width in meters of positional uncertainty envelope; position is relative to other features in the data. A value of -9 indicates that this measure is not available. (Source: Alaska Division of Geological & Geophysical Surveys)

    Range of values
    Minimum:25
    Maximum:25
    Units:meters

    LocationMethod
    Term that categorizes technique used to determine station location (Source: Alaska Division of Geological & Geophysical Surveys)

    Location method(s): GPS

    TimeDate
    Time and date of observation at station. (Source: Alaska Division of Geological & Geophysical Surveys)

    Range of values
    Minimum:2004-08-21
    Maximum:2011-08-24

    Observer
    Name of the person who located station (Source: Alaska Division of Geological & Geophysical Surveys)

    Observer(s): Schaefer, J. R. G.; McGimsey, R. G.; Scott, W. E.

    DataSourceID
    Provides information about the source of the feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    Foreign key to the DataSources_ID field in the DataSources table.

    Notes
    Free text for additional information about a feature. (Source: Alaska Division of Geological & Geophysical Surveys)

    The notes indicate the source of the feature interpretation or provide more information about the processing or data acquisition procedure.

    GPSX
    Measured GPS coordinate (Source: Alaska Division of Geological & Geophysical Surveys)

    Range of values
    Minimum:-157.0908508
    Maximum:-156.868866
    Units:NAD83, degrees

    GPSY
    Measured GPS coordinate (Source: Alaska Division of Geological & Geophysical Surveys)

    Range of values
    Minimum:57.07942963
    Maximum:57.18186951
    Units:NAD83, degrees


Who produced the data set?

  1. Who are the originators of the data set? (may include formal authors, digital compilers, and editors)

  2. Who also contributed to the data set?

    Game McGimsey provided valuable field expertise during our excursion to Mount Chiginagak in 2005. His thoughtful review greatly improved the quality and content of the map and report. We thank Jessica Larsen for her helpful review and for providing expertise in geochemical data display. This work was made possible by cooperative agreements between the U.S. Geological Survey Volcano Hazards Program and the State of Alaska Division of Geological & Geophysical Surveys, including grants G16AC00054 and G16AC00165.

  3. To whom should users address questions about the data?

    Alaska Division of Geological & Geophysical Surveys
    Metadata Manager
    3354 College Road
    Fairbanks, AK 99709-3707
    USA

    (907)451-5020 (voice)
    (907)451-5050 (FAX)
    dggspubs@alaska.gov

    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays


Why was the data set created?

Mount Chiginagak is a hydrothermally active volcano on the Alaska Peninsula, approximately 170 km south-southwest of King Salmon, Alaska. This 1:25,000 scale geologic map and accompanying report document the age, geochemical, and spaital distribution of eruptive products of the Mount Chiginagak volcano.


How was the data set created?

  1. From what previous works were the data drawn?

  2. How were the data generated, processed, and modified?

    Date: 2011 (process 1 of 4)
    Field data collection - Geologic field mapping, rock sampling, and tephra stratigraphy were conducted at Chiginagak volcano during short field excursions (2-6 days each) in 2004, 2005, 2006, 2008, and 2011. Orthorectified black-and-white airphotos from 1953, and satellite imagery such as Quickbird, Worldview, and Landsat7 aided geologic contact mapping. In particular, the approximately 1-m-pixel Worldview image acquired August 8, 2008, served as the most useful basemap for post-fieldwork geographical information system (GIS) mapping.

    Date: 2011 (process 2 of 4)
    Rock sample analysis - 40Ar/39Ar age dating of lavas was conducted at the University of Alaska Fairbanks geochronology lab. Rock samples were analyzed for major and trace-element composition by the Washington State University (WSU) GeoAnalytical Laboratory

    Date: 2015 (process 3 of 4)
    Technical review - This is a final report of scientific research. It has received technical review and editing by several scientists familiar with the subject matter. We incorporated the reviewers' suggestions into the final draft when deemed appropriate.

    Date: 2017 (process 4 of 4)
    GIS analysis and digital cartography - The geologic and geographic features portrayed in this data distribution package were attributed according to the NCGMP09 geodatabase standard.

  3. What similar or related data should the user be aware of?

    Detterman, R.L., Case, J.E., Wilson, F.H., and Yount, M.E., 1987, Geologic map of the Ugashik, Bristol Bay, and western part of Karluk quadrangles, Alaska: Miscellaneous Investigations Series Map I 1685, U.S. Geological Survey, United States.

    Online Links:

    Other_Citation_Details: 1 sheet, scale 1:250,000
    Detterman, R.L., Wilson, F.H., Yount, M.E., and Miller, T.P., 1987, Quaternary geologic map of the Ugashik, Bristol Bay, and western part of Karluk quadrangles, Alaska: Miscellaneous Investigations Series Map I 1801, U.S. Geological Survey, United States.

    Online Links:

    Other_Citation_Details: 1 sheet, scale 1:250,000
    Schaefer, J.R., Scott, W.E., Evans, W.C., Wang, Bronwen, and McGimsey, R.G., 2013, Summit crater lake observations, and the location, chemistry, and pH of water samples near Mount Chiginagak volcano, Alaska: 2004-2012: Report of Investigation RI 2011-6 v. 2, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 25 p


How reliable are the data; what problems remain in the data set?

  1. How well have the observations been checked?

    Certainty in attribute interpretation varies throughout the map. Attribute fields within each table record the author's degree of confidence in the existence or identity of applicable features. This publication is a final report of research and has received two technical reviews as well as technical editing.

  2. How accurate are the geographic locations?

    Certainty in feature boundaries or location varies throughout the map. Data fields within each table record the author's degree of confidence in the location of applicable features.

  3. How accurate are the heights or depths?

  4. Where are the gaps in the data? What is missing?

    This DGGS Report of Investigations is a final report of scientific research. It has received technical review and may be cited as an agency publication. This publication provides information about the mapped and interpreted geologic deposits distinguishable at a scale of about 1:25,000. The reader is cautioned that unevaluated hazards or uncertainties may be present in areas that were inecessable to field geologists.

  5. How consistent are the relationships among the observations, including topology?

    All polygon features were topologically validated using ArcGIS prior to export to shapefile format. Polygon topology is consistent with the specifications of the NCGMP09 - Draft Standard Format for Digital Publication of Geologic Maps, Version 1.1.


How can someone get a copy of the data set?

Are there legal restrictions on access or use of the data?

Access_Constraints:
This report, map, and/or dataset is available directly from the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys (see contact information below).
Use_Constraints:
Any hard copies or published datasets utilizing these datasets shall clearly indicate their source. If the user has modified the data in any way, the user is obligated to describe the types of modifications the user has made. The user specifically agrees not to misrepresent these datasets, nor to imply that changes made by the user were approved by the State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys. The State of Alaska makes no express or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.

  1. Who distributes the data set? (Distributor 1 of 1)

    Alaska Division of Geological & Geophysical Surveys
    Metadata Manager
    3354 College Road
    Fairbanks, AK 99709-3707
    USA

    (907)451-5020 (voice)
    (907)451-5050 (FAX)
    dggspubs@alaska.gov

    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
  2. What's the catalog number I need to order this data set?

    RI 2017-10

  3. What legal disclaimers am I supposed to read?

    The State of Alaska makes no expressed or implied warranties (including warranties for merchantability and fitness) with respect to the character, functions, or capabilities of the electronic data or products or their appropriateness for any user's purposes. In no event will the State of Alaska be liable for any incidental, indirect, special, consequential, or other damages suffered by the user or any other person or entity whether from the use of the electronic services or products or any failure thereof or otherwise. In no event will the State of Alaska's liability to the Requestor or anyone else exceed the fee paid for the electronic service or product.

  4. How can I download or order the data?


Who wrote the metadata?

Dates:
Last modified: 03-Jan-2017
Metadata author:
Alaska Division of Geological & Geophysical Surveys
Attn: Simone Montayne
Metadata Manager
3354 College Road
Fairbanks, AK 99709-3707
USA

(907)451-5020 (voice)
(907)451-5050 (FAX)
dggspubs@alaska.gov

Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays
Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)
Metadata extensions used:


Generated by mp version 2.9.21 on Wed Jan 3 13:34:32 2017