Theresa A. Taylor | Geology Reports

USGS2012

Early Tertiary exhumation of the flank of a forearc basin, southwest Talkeetna Mountains, Alaska

New geochronologic and thermochronologic data from rocks near Hatcher Pass, southwest Talkeetna Mountains, Alaska, record earliest Paleocene erosional and structural exhumation on the flank of the active Cook Inlet forearc basin. Cretaceous plutons shed sediments to the south, forming the Paleocene Arkose Ridge Formation. A Paleocene(?)-Eocene detachment fault juxtaposed ~60 Ma metamorphic rocks with the base of the Arkose Ridge Formation. U-Pb (analyzed by Sensitive High Resolution Ion Micro Probe Reverse Geometry (SHRIMP-RG)) zircon ages of the Cretaceous plutons, more diverse than previously documented, are 90.3±0.3 (previously considered a Jurassic unit), 79.1±1.0, 76.1±0.9, 75.8±0.7, 72.5±0.4, 71.9±0.3, 70.5±0.2, and 67.3±0.2 Ma. The cooling of these plutons occurred between 72 and 66 Ma (zircon fission track (FT) closure ~225°C). 40 Ar/ 39 Ar analyses of hornblende, white mica, and biotite fall into this range (Harlan and others, 2003). New apatite FT data collected on a west-to-east transect reveal sequential exhumation of fault blocks at 62.8±2.9, 54±2.5, 52.6±2.8, and 44.4±2.2 Ma. Plutonic clasts accumulated in the Paleocene Arkose Ridge Formation to the south. Detrital zircon (DZ) ages from the formation reflect this provenance: a new sample yielded one grain at 61 Ma, a dominant peak at 76 Ma, and minor peaks at 70, 80, 88, and 92 Ma. The oldest zircon is 181 Ma. Our apatite FT ages range from 35.1 to 50.9 Ma. Greenschist facies rocks now sit structurally between the plutonic rocks and the Arkose Ridge Formation. They are separated from plutonic rocks by the vertical Hatcher Pass fault and from the sedimentary rocks by a detachment fault. Ar cooling ages (Harlan and others, 2003) and new zircon FT ages for these rocks are concordant at 61-57 Ma, synchronous with deposition of the Arkose Ridge Formation. A cooling age of ~46 Ma came from one apatite FT sample. The metamorphic protolith (previously considered Jurassic) was deposited at or after 75 Ma based on new DZ data. The probability curve has a major peak from 76 to 102 Ma, minor peaks at 186, 197, 213, 303, 346, and 1,828, and two discordant grains at ~2,700 Ma. This is similar to DZ populations in the Valdez Group. The short period of time between deposition, metamorphism, and exhumation are consistent with metamorphism in a subduction-zone setting. Ductile and brittle structures in the metamorphic rocks are consistent with exhumation in a transtensional setting.

Alaska↗

USGS2011

Geologic map of Saint Lawrence Island, Alaska

Saint Lawrence Island is located in the northern Bering Sea, 190 km southwest of the tip of the Seward Peninsula, Alaska, and 75 km southeast of the Chukotsk Peninsula, Russia (see index map, map sheet). It lies on a broad, shallow-water continental shelf that extends from western Alaska to northeastern Russia. The island is situated on a northwest-trending structural uplift exposing rocks as old as Paleozoic above sea level. The submerged shelf between the Seward Peninsula and Saint Lawrence Island is covered mainly with Cenozoic deposits (Dundo and Egiazarov, 1982). Northeast of the island, the shelf is underlain by a large structural depression, the Norton Basin, which contains as much as 6.5 km of Cenozoic strata (Grim and McManus, 1970; Fisher and others, 1982). Sparse test-well data indicate that the Cenozoic strata are underlain by Paleozoic and Proterozoic rocks, similar to those exposed on the Seward Peninsula (Turner and others, 1983). Saint Lawrence Island is 160 km long in an east-west direction and from 15 km to 55 km wide in a north-south direction. The east end of the island consists largely of a wave-cut platform, which has been elevated as much as 30 m above sea level. Isolated upland areas composed largely of granitic plutons rise as much as 550 m above the wave-cut platform. The central part of the island is dominated by the Kookooligit Mountains, a large Quaternary shield volcano that extends over an area of 850 km 2 and rises to an elevation of 630 m. The west end of the island is composed of the Poovoot Range, a group of barren, rubble-covered hills as high as 450 m that extend from Boxer Bay on the southwest coast to Taphook Mountain on the north coast. The Poovoot Range is flanked on the southeast by the Putgut Plateau, a nearly flat, lake-dotted plain that stands 30?60 m above sea level. The west end of the island is marked by uplands underlain by the Sevuokuk pluton (unit Kg), a long narrow granite body that extends from Gambell on the north to near Boxer Bay on the south. Headlands having rugged cliffs or narrow, boulder-strewn beaches characterize the southwest coastline. The geologic map of Saint Lawrence Island was prepared from published and unpublished field investigations carried out between 1966 and 1971 by W.W. Patton, Jr., Bela Csejtey, Jr., T.P. Miller, J.T. Dutro, Jr., J.M. Hoare, and W.H. Condon (Patton and Csejtey, 1971, 1980) and data from Ormiston and Fehlmann (1969). Fossils collected during these investigations are reported in the Alaska Paleontological Database (www.alaskafossil.org), and mineral resource information is summarized in the online Alaska Resource Data File (Hudson, 1998).

Alaska↗