Geologic map of the West Susitna area, parts of the Tyonek C-3, C-4, D-4, and D-5 quadrangles, Alaska

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Frequently anticipated questions:

What does this data set describe?

Title:
Geologic map of the West Susitna area, parts of the Tyonek C-3, C-4, D-4, and D-5 quadrangles, Alaska
Abstract:
Geologic map of the West Susitna area, parts of the Tyonek C-3, C-4, D-4, and D-5 quadrangles, Alaska, Preliminary Interpretive Report 2026-1, prevents surficial and bedrock geology for approximately 500 mi2 of the Tyonek C-3, C-4, D-4, and D-5 quadrangles in western Cook Inlet. These products summarize findings of the West Susitna STATEMAP project, a 1:50,000-scale geologic mapping campaign across an area north of Cook Inlet that has attracted interest for resource development, alternative energy, and recreation. The map area includes approximately 50 miles of the West Susitna Access Corridor, which is proposed to connect Anchorage to the Happy River valley on the western margin of the Susitna Basin. Previous surficial and bedrock geologic mapping in the region has primarily been reconnaissance-level and limited to 1:250,000-scale detail. Field data were collected during a helicopter-supported program from July 16-28, 2024. Surficial geology, initially interpreted through a desktop study in ArcGIS, was verified in the field during the 2024 campaign. The desktop study incorporated new interpretations of remotely sensed imagery combined with previously published geologic mapping and relevant literature. Together, these datasets form the basis for updated geologic unit definitions, geomorphic interpretations, and structural observations presented in this map series. This mapping offers stakeholders information about geologic hazards, including those associated with seismic activity, near the proposed corridor alignment. The resulting surficial and bedrock framework further supports refined interpretations of Quaternary and bedrock geologic history in the western Susitna Basin region. The map is published on two sheets to accommodate its scale and geographic extent. The dataset provides geologic, structural, stratigraphic, and geochronologic information organized according to the GeMS and AK GeMS mapping schemas. The complete report, geodatabase, and ESRI fonts and style files are available from the Alaska Division of Geological & Geophysical Surveys website: http://doi.org/10.14509/32075.
Supplemental_Information:
This geodatabase is organized according to the AK GeMS mapping schema Hendricks, M.D., Macpherson, A.E., Athey, J.E., Wyatt, W.C., Montayne, Simone, Buchanan, Wes, Rivera, P.G., and Steinleitner, A.M., 2024, AK GeMS data dictionary: A description of the Alaska geologic mapping schema: Alaska Division of Geological & Geophysical Surveys Miscellaneous Publication 170 v. 2, 13 p. https://doi.org/10.14509/31172. The distribution package includes ESRI fonts and style files, along with usage instructions. Further documentation of the symbology is provided by Ekberg, P.G., Hendricks, M.D., and Athey, J.E., 2021, AK GeMS symbology: A description of the AK GeMS style file: Alaska Division of Geological & Geophysical Surveys Miscellaneous Publication 169, 7 p. https://doi.org/10.14509/30584
  1. How might this data set be cited?
    Walser, S.L., Gillis, R.J., Bernt, J.D., Salisbury, J.B., Diaz, A.V., and Hubbard, T.D., 2026, Geologic map of the West Susitna area, parts of the Tyonek C-3, C-4, D-4, and D-5 quadrangles, Alaska: Preliminary Interpretive Report PIR 2026-1, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 10 p., 2 sheets, scale 1:63,360
  2. What geographic area does the data set cover?
    West_Bounding_Coordinate: -157.494883
    East_Bounding_Coordinate: -157.338946
    North_Bounding_Coordinate: 60.967378
    South_Bounding_Coordinate: 60.902996
  3. What does it look like?
  4. Does the data set describe conditions during a particular time period?
    Beginning_Date: 2005
    Ending_Date: 2025
    Currentness_Reference:
    ground condition
  5. What is the general form of this data set?
    Geospatial_Data_Presentation_Form: geodatabase
  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?
      The horizontal datum used is NAD83.
      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.
  7. How does the data set describe geographic features?
    west_susitna_ak_gems_db
    Geodatabase, layer files, style files, and fonts (Source: DGGS)
    west_susitna_gems_db
    Geodatabase, layer files, style files, and fonts (Source: DGGS)
    west_susitna_gems_geopackage
    SQLite Database file (Source: DGGS)
    west_susitna_gems_shapefile
    Shapefiles and open data files (Source: DGGS)

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?
    This geologic map (or report or conversion or product, etc.) was funded by the USGS National Cooperative Geologic Mapping Program under STATEMAP award number G23AC00584, 2023, and the State of Alaska General Fund. The views and conclusions contained in this document are those of the authors and should not be interpreted as necessarily representing the official policies, either expressed or implied, of the U.S. Government.
  3. To whom should users address questions about the data?
    Alaska Division of Geological & Geophysical Surveys
    GIS Manager
    3354 College Rd
    Fairbanks, AK
    USA

    907-451-5020 (voice)
    dggsgis@alaska.gov
    Hours_of_Service: 8 am to 4:30 pm, Monday through Friday, except State holidays

Why was the data set created?

The purpose of this report is to provide new geologic mapping for the Tyonek quadrangles to inform resource planning, hazard assessment, and understanding of the western Susitna Basin, supporting responsible development and infrastructure decisions across Alaska. This updated framework offers statewide benefits by improving evaluations of natural resources, geologic hazards, and land-use opportunities in a region of growing economic interest.

How was the data set created?

  1. From what previous works were the data drawn?
    Ekberg, P.G. and others, 2021 (source 1 of 3)
    Ekberg, P.G., Hendricks, M.D., and Athey, J.E., 2021, AK GeMS symbology: A description of the AK GeMS style file: Miscellaneous Publication MP 169, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 7 p
    Type_of_Source_Media: paper
    Source_Contribution: GIS analysis and digital cartography
    Hendricks, M.D. and others, 2021 (source 2 of 3)
    Hendricks, M.D., Ekberg, P.G., Athey, J.E., Wyatt, W.C., Willingham, A.L., and Naibert, T.J., 2021, AK GeMS data dictionary: A description of the Alaska geologic mapping schema: Miscellaneous Publication MP 170, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 10 p
    Type_of_Source_Media: paper
    Source_Contribution: GIS analysis and digital cartography
    U.S. Geological Survey National Cooperative Geologic Mapping Program, 2020 (source 3 of 3)
    U.S. Geological Survey National Cooperative Geologic Mapping Program, 2020, GeMS (Geologic Map Schema) - A standard format for the digital publication of geologic maps: Techniques and Methods TM 11-B10, U.S. Geological Survey, United States.

    Online Links:

    Other_Citation_Details: 1 p
    Type_of_Source_Media: paper
    Source_Contribution: GIS analysis and digital cartography
  2. How were the data generated, processed, and modified?
    Date: 2023 (process 1 of 2)
    Geologic mapping - DGGS geologists mapped surficial and bedrock geology in the Tyonek C-3, C-4, D-4, and D-5 quadrangles using a combination of desktop analysis and field verification. They developed initial geologic interpretations in ArcGIS from remotely sensed imagery, previously published maps, and relevant literature. From July 16-28, 2024, DGGS geologists conducted helicopter-supported fieldwork to verify geologic units, geomorphic features, and structural observations. They integrated field observations with desktop interpretations to refine geologic unit definitions and produce the final geologic maps.
    Date: 2026 (process 2 of 2)
    Data packaging - DGGS organized and attributed the geologic and geographic features according to the AK GeMS schema, then programmatically transformed the data into four distribution formats: (1) an AK GeMS data package containing the full database and supporting files, (2) a GeMS data package converted to the USGS base standard, (3) a SQLite database file, and (4) a simplified shapefile package with shapefile representations of the data.
  3. What similar or related data should the user be aware of?
    Gillis, R.J., Walser, S.L., Bernt, J.D., and O'Sullivan, P.B., 2026, Zircon U-Pb geochronology for bedrock samples from the West Susitna STATEMAP project, southcentral, Alaska: Raw Data File RDF 2026-8, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 9 p
    Walser, S.L., Gillis, R.J., Bernt, J.D., and Montayne, Simone, 2025, Major-oxide and trace-element analyses of rock samples from the West Susitna area STATEMAP project, southcentral, Alaska: Raw Data File RDF 2025-21, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 2 p
    Zechmann, J.M., and Salisbury, J.B., 2025, Lidar-derived surface elevation data for Dickason Highlands, southcentral Alaska, collected August 14, 2024: Raw Data File RDF 2025-12, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 10 p
    Zippi, P.A., Walser, S.L., Gillis, R.J., and Montayne, Simone, 2025, Palynological and thermal maturity analysis of outcrop samples from the West Susitna area STATEMAP project, southcentral, Alaska: Raw Data File RDF 2025-27, Alaska Division of Geological & Geophysical Surveys, Fairbanks, Alaska, United States.

    Online Links:

    Other_Citation_Details: 3 p

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

  1. How well have the observations been checked?
    Attribute fields within each feature class are populated using the map symbology and corresponding applicable values from the GeMS and AK GeMS specifications. When known, certainty in attribute interpretation is indicated in the relevant fields. Attribute accuracy varies throughout the map and is scale-dependent.
  2. How accurate are the geographic locations?
    Certainty in feature boundaries or location varies throughout the map. Data fields within each feature class and the accompanying orientation and location confidence lookup tables record the authors' 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 publication is a DGGS Preliminary Interpretive Report (PIR). A PIR provides timely release of maps and reports with a preliminary interpretation of geology or analytical data. Several scientists familiar with the geology of map area reviewed this product for general readability and logical consistency, but it has not undergone technical peer review.
  5. How consistent are the relationships among the observations, including topology?
    All polygon features were topologically validated according to the AK GeMS geologic mapping schema specifications.

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)
    State of Alaska, Department of Natural Resources, Division of Geological & Geophysical Surveys
    3354 College Road
    Fairbanks, AK
    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
    Contact_Instructions:
    Please view our website (http://www.dggs.alaska.gov) for the latest information on available data. Please contact us using the e-mail address provided above when possible.
  2. What's the catalog number I need to order this data set? PIR 2026-1
  3. What legal disclaimers am I supposed to read?
    The State of Alaska makes no express or implied warranties (including warranties of merchantability and fitness) with respect to the character, function, or capabilities of the electronic services 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, any failure thereof, or otherwise, and 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: 10-Mar-2026
Metadata author:
Simone Montayne
3354 College Road
Fairbanks, AK
USA

907-451-5020 (voice)
Metadata standard:
FGDC Content Standard for Digital Geospatial Metadata (FGDC-STD-001-1998)
Metadata extensions used:
Generated by mp version 2.9.50 on Tue Mar 10 15:40:19 2026