Bedrock Geologic Map of the Northern Alaska Peninsula Area, Southwestern Alaska Compiled by Frederic H. Wilson, Robert B. Blodgett, Charles D. Blome, Solmaz Mohadjer, Cindi C. Preller, Edward P. Klimasauskas, Bruce M. Gamble, and Warren L. Coonrad Pamphlet to accomopany Scientific Investigations Map 2942 2017 U.S. Department of the Interior U.S. Geological Survey Contents Abstract ...........................................................................................................................................................1 Introduction and Previous Work .................................................................................................................1 Geographic, Geologic, and Physiographic Framework ...........................................................................2 Geologic Discussion ......................................................................................................................................3 Ahklun Mountains Province ................................................................................................................4 Lime Hills Province ...............................................................................................................................4 Alaska-Aleutian Range Province .......................................................................................................4 Map Units Not Assigned to a Province .............................................................................................4 Digital Data......................................................................................................................................................5 Acknowledgments .........................................................................................................................................5 Description of Map Units ..............................................................................................................................5 Unconsolidated Deposits and Rocks Not Assigned to a Province ...............................................5 Ahklun Mountains Province ................................................................................................................6 Sedimentary Rocks ......................................................................................................................6 Structural Assemblage ...............................................................................................................7 Lime Hills Province ...............................................................................................................................9 Sedimentary Rocks ......................................................................................................................9 Alaska-Aleutian Range Province .....................................................................................................11 Sedimentary Rocks ....................................................................................................................11 Structural Assemblages ...........................................................................................................17 Igneous Rocks ............................................................................................................................17 Metamorphic Rocks ..................................................................................................................19 Igneous Rocks, All Provinces .......................................................................................................... 20 References Cited......................................................................................................................................... 23 Table 1. Radiometric ages from the northern Alaska Peninsula map area ...................................28 iii iv Bedrock Geologic Map of the Northern Alaska Peninsula Area, Southwestern Alaska Compiled by Frederic H. Wilson, Robert B. Blodgett, Charles D. Blome, Solmaz Mohadjer, Cindi C. Preller, Edward P. Klimasauskas, Bruce M. Gamble, and Warren L. Coonrad Abstract (Detterman and Hartsock, 1966; Detterman and Reed, 1973, 1980; Magoon and others, 1976) and unpublished mapping of The northern Alaska Peninsula is a region of transition J.N. Platt and E.H. Muller in the Taylor Mountains quadrangle from the classic magmatic arc geology of the Alaska Peninsula (J.N. Platt, unpub. data, 1957) and the Dillingham quadrangle to a Proterozoic and early Paleozoic carbonate platform and (J.N. Platt and E.H. Muller, unpub. data, 1958). Field notes then to the poorly understood, tectonically complex sedimen- of J.M. Hoare and W.H. Condon (1969–1970) were espe- tary basins of southwestern Alaska. Physiographically, the cially useful in compiling the western Taylor Mountains and region ranges from the high glaciated mountains of the Alaska- Dillingham quadrangles map area. A preliminary map of the Aleutian Range to the coastal lowlands of Cook Inlet on the east Paleozoic rocks of the northeastern Taylor Mountains quadran- and Bristol Bay on the southwest. The lower Ahklun Mountains gle was published by LePain and others (2000). Early mapping and finger lakes on the west side of the map area show strong by J.B. Mertie and P.A. Davison in the 1930s (Mertie, 1938) effects from glaciation. Structurally, a number of major faults laid the general framework for the region; subsequent mapping cut the map area. Most important of these are the Bruin Bay by Cady and others (1955) filled in parts of that framework. Fault that parallels the coast of Cook Inlet, the Lake Clark Fault Re-examination of the field notes of Mertie, Davison, Hoare, that cuts diagonally northeast to southwest across the eastern Condon, and W.L. Coonrad helped define a number of the map part of the map area, and the presently active Holitna Fault to units in the western part of the map area. Limited new field the northwest that cuts surficial deposits. mapping was conducted as part of this compilation effort (for Distinctive rock packages assigned to three provinces example, Blodgett and Wilson, 2001; Wilson and others, 2003), are overlain by younger sedimentary rocks and intruded by and extensive photo interpretative work has also contributed to widely dispersed latest Cretaceous and (or) early Tertiary this product. W.K. Wallace graciously provided field maps from granitic rocks. Much of the east half of the map area lies in the his work in the region during the 1980s. Papers on the plutonic Alaska-Aleutian Range province; the Jurassic to Tertiary Alaska- rocks by B.L. Reed and his coworkers provided the basis for Aleutian Range batholith and derivative Jurassic sedimentary much of the map in the southern Alaska Range and north- rocks form the core of this province, which is intruded and ern Aleutian Range (Reed and Lanphere, 1969, 1972, 1973). overlain by the Aleutian magmatic arc. The Lime Hills prov- Studies around Redoubt and Iliamna Volcanoes (Till and others, ince, the carbonate platform, occurs in the north-central part of 1993; Waythomas and Miller, 1999) also contributed. However, the map area. The Paleozoic and Mesozoic Ahklun Mountains new mapping underway at the time of this map compilation is province in the western part of the map area includes abundant not included here. chert, argillite, and graywacke and lesser limestone, basalt, and During the course of this project, a surficial geologic map tectonic mélange. The Kuskokwim Group, an Upper Cretaceous of the region, especially the Lake Clark National Park and turbidite sequence, is extensively exposed and bounds all three Preserve region, was produced based on photo interpretation provinces in the west-central part of the map area. (Wilson and Preller, 2003; F.H. Wilson and T.K. Bundtzen, unpub. data, 2006). As a result, areas of apparent bedrock exposure were noted that were not shown on available bedrock maps. In general, our assignment of a bedrock rock unit to these Introduction and Previous work previously unmapped areas in the Lake Clark quadrangle is based on extrapolation of the outcrop patterns on the maps by This map, located on the west side of Cook Inlet and north Nelson and others (1983) and Eakins and others (1978), data of Bristol Bay, shows an area we nominally call the northern from W.K. Wallace (Wallace and others, 1989; Wallace, written Alaska Peninsula. It is a region of transition from the classic commun., 2003), and additional interpretation by the authors magmatic arc geology of the Alaska Peninsula to the accretion- of this map. In the Taylor Mountains and Dillingham quadran- ary geology of south-central and southwest Alaska and to the gles, previously unmapped apparent bedrock exposures, where poorly understood, tectonically complex sedimentary basins appropriate, are assigned to map units based on outcrop patterns of southwestern Alaska. A wide range of sources were used to on the maps of Hoare and Coonrad (1978) and from the earlier compile this map, including the published mapping of Robert unpublished maps of J.N. Platt and E.H. Muller of the U.S. Detterman and others in the Iliamna and Kenai quadrangles Geological Survey, dating from the 1950s (J.N. Platt, unpub. 1 data, 1957; J.N. Platt and E.H. Muller, unpub. data, 1958). modern Aleutian arc but do not form prominent features. In fact, In 1998, Wilson and others (1998) published a compi- Black is yet to be located following Coats’ (1950) report. lation of the geology of central Alaska that brought together The