Reconnaissance geologic map of the Petersburg A-3 quadrangle, southeastern Alaska

Abstract

The geologic information presented here has been released previously in generalized form at 1:250,000 scale (Brew and others, 1984); that map is based on reconnaissance field mapping and therefore the individual maps in this series of 1:63,360-scale maps (Brew, 1997a-m;Brew and Koch, 1997) do not have the density of field-station control, samples, or field observations that are expected in published U.S.Geological Survey 1:63,360-scale geologic maps. There are both a combined description and a combined correlation of the map units for this whole series of maps (Brew and Grybeck, 1997).The available information on known mineral deposits in the whole Petersburg/Wrangell area was released previously (Grybeck and others, 1984) and Brew and others (1989, 1991). Bedrock, stream-sediment, and other geochemical data were released and interpreted by Karl and others (1985), Karl and Koch (1990), Cathrall and others (1983a-w), and Tripp and Cathrall (1984. Aeromagnetic and aeroradioactivity surveys information was released by the U.S. Geological Survey (1978Survey ( , 1979 and Bouguer gravity information by Barnes and others (1989). Remotely-sensed features were described by LeCompte (1981). Burrell and others (1982) released a preliminary bibliography of Petersburg and Port Alexander quadrangles-related items.Assessments of the undiscovered mineral resources for the whole Petersburg/Wrangell area are also available (Brew and others, 1989(Brew and others, , 1991(Brew and others, ,1996Brew and Drinkwater, 1991). Some of the mineralresource-assessment tract information in neighboring areas was revised by Brew and others (1996). Brew (1993) presented a generalized view of metallogenic belts that includes this area.Detailed information on the Late Cretaceous plutonic rocks in the Petersburg 1:250,000-scale quadrangle is found in Burrell (1984abc); major-element chemical and other data for the area were reported by Douglass and others (1989), and relatively young volcanic features were described by Brew and others (1984) and by Brew (1990). McClelland and Gehrels (1990) reinterpreted some of the geology in and around the Duncan Canal area, which lies to the northwest of this quadrangle. The index map on the over-size sheet shows the major geological elements of the Petersburg/Wrangell area. They are, from west to east, (1) the Alexander belt, consisting of generally unmetamorphosed Lower Paleozoic through Upper Triassic rocks intruded by scattered mid-Cretaceous plutons, (2) the Gravina belt, consisting of unmetamorphosed to highly metamorphosed, variably deformed Upper Jurassic(?) through mid-Cretaceous flysch and volcanic rocks intruded by both mid-and Upper Cretaceous plutons, and (3) the Mainland belt, consisting of metamorphic rocks intruded by Upper Cretaceous, lower Tertiary, and mid-Tertiary plutons. The lower to middle Tertiary Kuiu-Etolin belt is younger than almost Massive Limestone-Limestone and minor shale where exposed in the Petersburg A-2 quadrangle to the east;weathers light to medium gray; light gray on...