Early Permian Sedimentation and Paleogeography in East-Central Alaska Range, and Regional Implications: ABSTRACT

Abstract

A detailed study of the Early Permian strata in part of the east-central Alaska Range strongly supports the hypothesis that a volcanic island archipelago, possibly similar to modern island arcs of the circum-Pacific type, existed in the eastern Alaska Range during the late Paleozoic. The depositional history of Early Permian strata in the study area further suggests that the geologic history of the inferred island arc system was responsible for the nature of upper Paleozoic stratigraphic successions throughout the eastern Alaska Range. The study area of approximately 80 sq mi is near Gulkana Glacier in the east-central Alaska Range east of the Richardson Highway, 170 mi south of Fairbanks, Alaska. The Permian strata are divided into 2 formations, informally named the Phelan Creek formation, of early Wolfcampian age, and the Gulkana Glacier formation, of middle Wolfcampian age. The units are separated by an erosional unconformity of small magnitude. The Phelan Creek formation, approximately 4,000 ft thick, is composed of interstratified pyroclastics, graded volcaniclastics, pebbly mudstone, and lenticular, fossiliferous limestone. The formation was deposited in a neritic basin below wave base. Subaqueous gravity processes deposited the pebbly mudstone, volcaniclastics, and many of the limestone and pyroclastic units. In contrast, the Gulkana Glacier formation, approximately, 1,000 ft thick, is composed of interbedded fossiliferous limestone and black shale and was deposited, mostly, above wave base by tractive currents. Thickness and grain size changes in the pyroclastic units of the Phelan Creek formation indicate that during early Wolfcampian time a group of volcanoes was a few miles west of the study area. The volcanoes probably were the source of all pyroclastic and volcaniclastic deposits in the formation. By middle Wolfcampian time the volcanoes were no longer active and began to subside producing an extensive shallow-water platform. The carbonate rocks and black shales of the Gulkana Glacier formation were deposited on top of this subsiding platform. Upper Paleozoic strata exposed elsewhere in the eastern Alaska Range can be divided similarly into a lower sequence containing mostly volcanic and volcanic-derived deposits and an upper sequence containing limestone and black shale. The thickness of these 2 sequences varies, but usually is more than 1,000 ft. Preliminary study of these strata suggests that these deposits also record the growth, extinction, and burial of volcanic islands. The sequence of depositional and tectonic events suggested by the vertical change from volcanics to limestone and black shale in the upper Paleozoic deposits of the eastern Alaska Range is strikingly similar to the Tertiary and Holocene history of some modern island arc systems. For example, Guam and Saipan of the Marianas group and the Windward Islands in the Caribbean consist of a complex succession of mostly End_Page 2470------------------------------ volcanics and volcanic derived rocks that closely resemble the strata of the Phelan Creek formation. These modern volcanic islands are mostly inactive and have subsided. Thick limestone-black shale caps are now being formed on top of the subsiding volcanic platforms. The resulting stratigraphic succession is nearly identical with the stratigraphic sequence that characterizes upper Paleozoic deposits throughout the eastern Alaska Range. End_of_Article - Last_Page 2471------------