border: A polygon that encompasses the maximum geographic extent of the data observations SubCropDepthContours: Lines that record depth to the top Mesozoic (base Tertiary) unconformity surface in the Cook Inlet Basin. Depth is measured to top of Mesozoic unconformity in feet below sea level. Contour interval is 1000 ft. The structural surface onto which the subcrop map is projected is taken directly from the top Mesozoic unconformity depth map published by Shellenbaum and others (2010). SubCropFaults: Lines that record faults that bound map units and dangling faults, modified NCGMP09 format. SubCropMapUnitContacts: Lines that record map unit contacts and degree of certainty, modified NCGMP09 format. SubCropMapUnitPolys: Polygons that record the distribution of interpreted geologic units, modified NCGMP09 format.
Gregersen, L.S., and Shellenbaum, D.P., 2016, Top Mesozoic unconformity subcrop map, Cook Inlet basin, Alaska: Report of Investigation RI 2016-4, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.
This is a vector data set.
Horizontal positions are specified in geographic coordinates, that is, latitude and longitude. Latitudes are given to the nearest 0.00001. Longitudes are given to the nearest 0.00001. Latitude and longitude values are specified in decimal degrees.
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 1/298.257222101.
The contour lines are classified as: depth contour, index depth contour, depth contour depression, index depth contour depression, depth contour rise, and index depth contour rise.
|Range of values|
The location confidence values are: approximate, out of seismic control, certain, in seismic control, and inferred or doubtful.
Contour lines were labeled at the following intervals: 0, -10000, -15000, -20000, -5000, and -25000.
The faults were classified as: thrust fault, strike-slip fault, generic fault, and normal fault.
The faults were classified as: approximate, certain, and highly uncertain.
The faults were derived or interpreted from: Magoon and others (1976), seismic, and Wilson and others (2012).
The labeled features are: Bruin Bay Fault, Castle Mountain Fault, Border Ranges Fault, Lake Clark Fault, and Eagle River Fault
The contacts were classified as: certain and approximate.
|JTrtp-Jbrum||TALKEETNA FORMATION-BORDER RANGES ULTRAMAFIC AND MAFIC COMPLEX, UNDIVIDED -- Outcrops of the Talkeetna Formation on the east side of the basin are found to the northeast in the Matanuska Valley. These outcrops are highly faulted and are in fault contact with ultramafic and mafic plutonic rocks referred to as the Border Ranges ultramafic and mafic complex (BRUMC). Interpretations of aeromagnetic data suggest that both the BRUMC and the Talkeetna Formation are present on the east side of Cook Inlet basin along the Border Ranges fault (Mankhemthong and others, 2013; Burns and Winkler, 1994; Saltus, written commun.; Burns, 1985; Burns and others, 1991). Outcrops of volcaniclastic rocks, limestone, coal, and tuffaceous argillite (interpreted to be correlative with the Talkeetna Formation) are also present on the east side of the basin at Pogibshi Point near Seldovia (Magoon and others, 1976; Bradley and others, 1999; LePain and others, 2013). Kelley (1980) named these outcrops the Pogibshi Formation. Limestone, contorted chert, and greenstone interpreted as the Port Graham Formation of Kelley (1980) are also found in outcrop in this area. Because no other well data or seismic data are available on the east side of the basin to clearly determine which of the Talkeetna Formation, BRUMC, Pogibshi Formation, or Port Graham Formation subcrops the top Mesozoic unconformity, we include the BRUMC, Pogibshi Formation, and Port Graham Formation with the Talkeetna Formation on the east side of the basin and designate this as a separate map unit, Talkeetna-BRUMC undivided.|
|Jc-Jt||CHINITNA FORMATION-TUXEDNI GROUP, UNDIVIDED -- In outcrop, the Chinitna Formation and Tuxedni Group are described as light to dark gray and green, marine, fossiliferous, siltstone, sandstone, and shale with the Chinitna Formation having many limy concretions (Detterman and Hartsock, 1966; Detterman and Reed, 1980). Both the Chinitna Formation and the Tuxedni Group contain a mixture of plutonic and volcanic detritus and clasts presumed to be derived from erosion of the Talkeetna Formation and related plutonic rocks of the Early Jurassic magmatic arc (Detterman and Hartsock, 1966; Trop and others, 2005). Two wells, Iniskin Unit Beal (no. 110 on map) and Iniskin Unit Zappa (no. 111 on map), are drilled in surface outcrops of Tuxedni Group. Green-gray and brown-gray siltstone, sandstone, conglomerate, and shale are documented in cuttings in these wells. A Middle Jurassic age was assigned to the rocks at the top Mesozoic unconformity in the State 364651 1 well (no. 105 on map) (Zippi, 2006). The cuttings in those wells are described as light gray to greenâ€“gray and brown, claystone, silty claystone, indurated, argillaceous, weathered, and slightly micaceous at the top Mesozoic unconformity. The remaining wells were inferred to penetrate subcrop of Chinitna Formationâ€“Tuxedni Group because of similar cuttings, wireline log signature, and/or proximity to the age-dated wells.|
|Jn||NAKNEK FORMATION -- The Naknek Formation is described in outcrop as light to dark gray, fossiliferous, marine arkosic sandstone, conglomerate, siltstone, and shale having a primarily plutonic provenance (Detterman and Hartsock, 1966; Detterman and others, 1996; Trop and others, 2005; Wartes and others, 2013b). Multiple wells in the west-central portion of the basin include strata that are assigned an age of Late Jurassic at the top Mesozoic unconformity: Bell Island 1, State SRS 1, MGS State SRS 1, Kalgin Island State 1, and Oldmans Bay State 1 (nos. 102, 59, 65, 54, and 53 on map) (Zippi, 2006; Boss and others, 1976). Cuttings in these wells at the top Mesozoic unconformity are described mostly as gray to grayâ€“green claystone, siltstone with some sandstone, arkose, tuff, and argillite. In the Swanson River field area, the cuttings interpreted to be Naknek Formation include gray to grayâ€“green siltstone, sandstone, claystone, mica, tuff, and glauconite.|
|Jtk||TALKEETNA FORMATION -- In outcrop, the Talkeetna Formation is described as dark green to black, red, and multicolored lava, agglomerate, breccia, and tuff with interbedded sandstone and shale (Detterman and Hartsock, 1966). The Talkeetna Formation is interpreted to be a product of extrusion and erosion of an Early Jurassic arc (Detterman and Reed, 1980; Trop and Plawman, 2006). The cuttings and core chips interpreted as Talkeetna Formation are described as grayâ€“greenâ€“brownâ€“black volcanics, basalt, and tuffs.|
|Kkg-Km||KAGUYAK-MATANUSKA FORMATIONS, UNDIVIDED -- The Matanuska Formation is described in outcrop as dark-gray, marine shale, siltstone, sandstone, and conglomerate with Inoceramus fragments and ammonites (Grantz, 1964; Trop, 2008). The Kaguyak Formation is described in outcrop as dark gray to pale brown, marine, fossiliferous, siltstone, sandstone, and shale (Detterman and Miller, 1985; Wartes and others, 2013a). Nonmarine sedimentary rocks correlative with the Kaguyak Formation have been documented in the COST well (no. 94 on map) and in outcrop (Magoon, 1986; Magoon, and others, 1980; LePain and others, 2012). The cuttings interpreted as Kaguyakâ€“Matanuska Formations, undivided, are described as dark gray shale, grayâ€“green to grayâ€“brown siltstone, and sandstones with Inoceramus prisms. Palynology data from these wells suggest a Late Cretaceous age (Zippi, 2006), which supports the Kaguyakâ€“Matanuska Formation, undivided, interpretation.|
|MzPzmb||METAMORPHIC ROCKS NEAR THE BORDER RANGES FAULT, UNDIVIDED -- Metamorphic rocks, primarily metasedimentary, are observed at the top Mesozoic surface along the southeastern edge of the subcrop map. The Metamorphic rocks near the Border Ranges fault, undivided, map unit are described as metasedimentary, metaplutonic, and metavolcanic rocks, metachert, slate, metasandstone, and marble (Magoon and others, 1976). Cuttings descriptions in these wells include sandstone, chert, argillite, quartzite, muscovite, schist, and loose grains of quartz. Haeussler and Saltus's (2011) interpretation of aeromagnetic data suggests that these metasedimentary rocks may be correlative with the McHugh Complex.|
|TMzpu||PLUTONIC ROCKS, UNDIVIDED -- Plutonic rocks are observed at the top Mesozoic unconformity in wells at three different locations. Two of the areas, which correspond to the Bachatna Creek Unit 7 (no. 31 on map) and the Cottonwood State 1 (no. 49 on map) wells, are in the north-western corner of the subcrop map near outcrops of Jurassic quartz diorite, diorite, and granodiorite (Magoon and others, 1976). The third area, which corresponds to the Fishhook 1 well (no. 7 on map), is in the northeastern corner of the subcrop map near outcrops of Jurassic quartz diorite, diorite, and granodiorite and outcrops of Tertiary-Cretaceous granodiorite and quartz diorite (Magoon and others, 1976). The cuttings in these wells are described as granite, granodiorite, and/or diorite.|
|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.|
|2020||CMYK value - C:20%, M:0%, Y:20%, K:0%|
|0770||CMYK value - C:0%, M:70%, Y:70%, K:0%|
|AA00||CMYK value - C:8%, M:8%, Y:0%, K:0%|
|6040||CMYK value - C:60%, M:0%, Y:40%, K:0%|
|X270||CMYK value - C:100%, M:20%, Y:70%, K:0%|
|2070||CMYK value - C:20%, M:0%, Y:70%, K:0%|
Pattern fill used to delineate a sub-unit.
The authors gratefully acknowledge Spectrum Geo and Seitel, Inc., for authorizing the use of CI88 and CI89 Cook Inlet marine speculative 2-D seismic survey in the construction of this map. The authors also gratefully acknowledge WesternGeco and the U.S. Geological Survey for providing the seismic reflection data for the purpose of this research. Significant improvements were made to the map thanks to feedback and discussions with David LePain, Rick Stanley, Paul Decker, Dan Seamount, Art Saltmarsh, Rick Saltus, Dwight Bradley, Laurel Burns, Robert Blodgett, Al Hunter, Bob Gillis, Marwan Wartes, Trystan Herriott, Robert Swenson, and many other geoscientists. We are especially grateful to our reviewers: David LePain and Rick Stanley. The map benefited greatly from the cartographic expertise provided by Joseph Rolfzen and Patricia Gallagher.
Oil in Cook Inlet basin is primarily sourced out of the Middle Jurassic Tuxedni Group, thus the subcrop trends of the Middle Jurassic oil source rock and other potential Mesozoic-age reservoirs are an important consideration in oil exploration in the inlet. This map was prepared as part of a multi-year effort by the Alaska Department of Natural Resources to provide the public with the most accurate information possible on the geologic framework of this economically important area.
Gregersen, L.S., and Shellenbaum, D.P., 2015, Preliminary top Mesozoic unconformity subcrop map, Cook Inlet basin, Alaska (poster): Alaska Geological Society Technical Conference, May 15, 2014: Poster and Presentation Gregersen, L.S. and Shellenbaum, D.P., 2015, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.
Gregersen, Laura, and Shellenbaum, D.P., 2010, Progress on Developing a Cook Inlet Mesozoic Subcrop Map (presentation): U.S. Geological Survey Cook Inlet Geology Review Meeting, Anchorage Alaska, September 21-22, 2010: Poster and Presentation Gregersen, Laura and Shellenbaum, D.P., 2010, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.
Shellenbaum, D.P., Gregersen, L.S., and Delaney, P.R., 2010, Top Mesozoic unconformity depth map of the Cook Inlet basin, Alaska: Report of Investigation RI 2010-2, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.
Shellenbaum, D.P., and Silliphant, L.J., 2008, Cook Inlet Mesozoic subcrop and base Tertiary depth structure mapping (presentation): DNR Spring Technical Review Meeting, Anchorage, March 26-27, 2008: Poster and Presentation Shellenbaum, D.P. and Silliphant, L.J., 2008, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.
The supporting data for the subcrop map vary regionally, and within each region the uncertainty varies. Lower Cook Inlet has the greatest control of all regions: formation picks in the OCS wells, age data, 2-D seismic control, and outcrop. In upper and middle Cook Inlet, well formation picks, age data, and outcrop extents primarily define the subcrop limits, with seismic character and fabric substantiating the interpretation. Along the eastern edge of the subcrop map, where little to no seismic data or well data exist, interpreted aeromagnetic data, outcrop extents, and unpublished zircon age dates (R. Gillis, written commun.) are the main controls on the subcrop map unit boundaries. Map unit boundaries are dashed and are designated with question marks where little to no control is present. The structural surface onto which the subcrop map is projected is taken directly from the top Mesozoic unconformity depth map published by Shellenbaum and others (2010). Contours outside of the CI88 and CI89 two-dimensional seismic coverage are dashed to indicate uncertainty. Contours outside seismic coverage with little well control have even higher uncertainties, and are designated with question marks. There is also a certain amount of spatial uncertainty regarding the intersection of the top Mesozoic unconformity horizon with the Bruin Bay and Castle Mountain-Lake Clark fault systems that bound the basin to the west. The fault traces (Magoon and others, 1976) indicate surface expression, whereas the top Mesozoic surface lies at varying depths. The Border Ranges and Eagle River faults are modified from Wilson and others (2012). The modifications conform to aeromagnetic boundaries shown by data provided by R. Saltus (written commun.) and Haeussler and Saltus, in press. The uncertainty when picking formation tops is qualified by assigning a confidence rating (A = high confidence, B = moderate confidence, and C = low confidence). Lithologic data were unavailable for seven wells. In those cases, wireline log correlations and proximity to known formation picks were used to assign the age. Without core or age date data available at the top Mesozoic unconformity, it is difficult to clearly identify the Upper Jurassic Naknek Formation and the Middle Jurassic Chinitna Formation-Tuxedni Group, undivided, from only cuttings and wireline log signature. Seismic character, proximity to well control with known picks, and proximity to mapped bedrock helped differentiate between the Upper and Middle Jurassic rock units. A complete reference list of supporting data sources that were used to define and delineate geologic units can be found on the associated mapsheet.
The supporting data for the subcrop map vary regionally, and within each region the uncertainty varies. Map unit boundaries are dashed and are designated with question marks where little to no control is present.
This data release is complete.
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