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

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.

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Organization: State of Alaska