Preliminary reconnaissance geologic map of the Petersburg and parts of the Port Alexander and Sumdum 1:250,000 quadrangles, southeastern Alaska

Abstract

Pleistocene)~Includes alluvium, colluvium, tidal mudflat deposits, and some glaciofluvial deposits. The distribution of most large areas of surficial deposits was mapped in the field, but the deposits have not been studied in detail; many small areas are not shown. KUIU-ETOLIN BELT Belt informally named by Brew and others (1979) and redefined by Brew and Morrell (1983). EXTRUSIVE AND INTRUSIVE VOLCANIC ROCKS OF KUIU-ETOLIN VOLCANIC-PLUTONIC BELT (Quaternary and Tertiary)-Diverse volcanic rocks exposed in a broad area extending from northeastern Kuiu southeastward through Kupreanof and Zarembo Islands; as mapped, divided into: Extrusive Basaltic Rocks and Underlying Sediments (Holocene and(or) Pleistocene)-Fresh, locally polygonally jointed, dark greenish gray, dense, very fine-grained to aphanitic magnetite-bearing olivine basalt and minor pyroxene basalt; individual flows are as much as 10 m thick and are columnar jointed; most flows are less than 1 m thick. Underlain locally by aa flows and mafic volcanic breccia in layers up to 0.5 m thick and by locally derived, poorly sorted, well-bedded brown-to gray-weathering conglomerate, pebbly sandstone, sandstone and minor siltstone deposited in fluvial or beach environment. Quarry on peninsula in Kah Sheets Bay exposes polymitic glacial till that is mapped with this unit in small lens under dense aphanitic basalt; whole unit is interpreted to be Pleistocene or younger. Three whole-rock K-Ar ages on basalts from a few miles south of Kah Sheets Bay gave ages of 0.272+0.085, 0.262+087, and 4.04+6.95 Ma (M. A. Lanphere, written commun., 1972). ExpoTed along south shore of Kupreanof Island from Kah Sheets Bay to Douglas Bay and from west of Totem Bay to beyond Point Barrie and at Indian Point and on High Castle Island in Duncan Canal. Equivalent rocks may be included with Basalt and Other Mafic Extrusive rocks unit (QTb), particularly along Rocky Pass and near the mouth of Irish Creek. occurring repeatedly throughout the section. Some suggestion that Altered Dellenite, etc. unit (QTf), and Gabbro and Microgabbro unit (Tmgb) occurs only low in the section. Siliceous Volcaniclastic Rock unit (QTc) occurs in and around Rhyolite, Rhyodacite, etc. unit (QTr); see also Muffler (1967); as mapped, divided into: Vent Breccia Angular to subangular blocks of fine-grained light gray silicic volcanic rock ranging from 5 mm to 15 cm with either no matrix or little (less than 15 percent) matrix of very fine grained dark gray volcanic rock or chalcedony; crops out on southeast shore of Zarembo Island southwest of Round Point, and in Kadake Creek drainage, west of Kadake Bay and north of Washington Bay, all on Kuiu Island. Rhyolite, Rhyodacite, and Related Siliceous Extrusive and Intrusive Rocks Aphanitic to finely crystalline, generally quartz and feldspar porphyritic; C.I. less than 1. Locally layered, spherulitic, and(or) miarolitic; light gray fresh; buff, white, green lavendar, maroon, or pink where altered; generally rusty weathering. Pyrite and zeolites common. Many exposures are t...

Metamorphic history of the Coast Mountains orogen, western British Columbia and southeastern Alaska

“…Post-tectonic plutons and stocks of Oligocene and Miocene age intruded into the western metamorphic belt and the central pluton-gneiss belt from Zarembo Island (Douglass and Brew, 1985;Brew et al, 1984) to just north of Prince Rupert (Fig. 4).…”

“…1) comprise chlorite + white mica + zoisite + albite + quartz + opaque minerals ± actinolite, chlorite or biotite + plagioclase + quartz + epidote + calcite, white mica + chlorite + epidote + plagioclase, and chlorite + white mica + plagioclase, or chlorite + epidote (McClelland et al, 1991b;Brew et al, 1984). On Douglas Island and northeastern Admiralty Island (Figs.…”

Cretaceous-Tertiary evolution of the western Coast Mountains, central Southeastern Alaska

“…2; Brew et al, 1984). These rocks are generally interpreted as a backarc to intra-arc basinal assemblage that unconformably overlies the eastern margin of the Alexander terrane (Berg et al, 1972;Brew and Karl, 1988;McClelland and Gehrels, 1990;Rubin and Saleeby, 1991b;Cohen and Lundberg, 1993).…”

“…Kinematic relations within the shear zone that dominantly record east-side-up displacement Stowell and Hooper, 1990;Ingram and Hutton, 1994) Figure 2. Geologic map of central southeastern Alaska and adjacent British Columbia modified from Elliott and Koch (1981), Brew et al (1984), Wheeler and McFeely (1991), and McClelland et al (1992c).…”

“…An uncertainty of 0.2% is assigned to the 206 Pb/ 238 U ratio based Figure 3. Geologic map of Bradfield Canal region modified from Elliott and Koch (1981) and Brew et al (1984). With exception of traverse in southeast corner of figure, modifications are based on geologic mapping along shoreline traverses and data presented herein.…”