Geology of the Sleetmute C-7, C-8, D-7, and D-8 quadrangles, Horn Mountains, southwestern Alaska

Geology of the Sleetmute C-7, C-8, D-7, and D-8 quadrangles, Horn Mountains, southwestern Alaska, Report of Investigation 98-12, presents a geologic map of the Horn Mountains area in southwestern Alaska. Between the 1990, 1991, and 1992 field seasons, DGGS geologists mapped four 1:63,360-scale quadrangles in the lower Kuskokwim River basin in southwest Alaska. The mapped formations span from the Late Cretaceous to the Holocene. Central to the stratigraphy is the Kuskokwim Group, a ~2,300-meter-thick sequence of marine turbidites and fluvial deposits formed in a post-accretionary basin, containing Turonian to Santonian fossil assemblages. This group is intruded and overlain by meta-aluminous to peraluminous igneous rocks, including the Iditarod Volcanics, the Horn Mountains volcanic-plutonic complex, and granite porphyry dikes and sills--features likely associated with a north-verging subduction zone and related extensional tectonics. Unconsolidated deposits from the Late Tertiary to Holocene cover approximately 65% of the area, with four Pleistocene glaciations producing a radial pattern of outwash deposits surrounding the glacially sculpted Horn Mountains. Thermoluminescence and radiocarbon dating indicate glaciations during the Illinoian (~140,000 years BP) and Late Wisconsin (~13,600 years BP) periods. Bedrock units exhibit multiple folding and faulting events, which may have facilitated hydrothermal mineralization. The region hosts notable mineral resources, including mesothermal, boron-enriched base and precious metals in hornfels and cupolas; mercury-antimony-gold lodes in granite porphyry and altered mafic dikes; and heavy mineral gold placers. The complete report, geodatabase, and ESRI fonts and style files are available from the Alaska Division of Geological & Geophysical Surveys website: http://doi.org/10.14509/2588.

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Organization: State of Alaska