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Alaska Geological Society

Abstract


Journal of the Alaska Geological Society, Fourth Volume, 1984
Pages 12-34

Structure and Stratigraphy of an Upper Triassic Unit, Healy–A Detailed Study of Part of the Pingston Terrane in the Central Alaska Range

Paul J. Umhoefer

Abstract

A 3,000-meter-thick Upper Triassic unit has been defined in the upper Wood River valley of the Alaska Range, just south of the Hines Creek fault. A suite of conodonts from 23 sample localities is dominated by Paragondolella polygnathiformis and Epigondolella forms. The unit has two lithofacies: (1) regularly interbedded very fine-grained calcarenites and calcareous shales with minor massive quartzites (calcarenite facies), and (2) argillites and phyllites with minor thick-bedded calcarenites (argillite facies). Trace fossils are locally common in slaty rocks of the argillite facies. The unit was deposited in a slope-to-basin environment below a carbonate shelf with a significant siliceous component. The calcarenites are distal turbidites and contourites; the quartzites are liquefied grain flows.

The unit was folded into tight recumbent folds in the Early Cretaceous, when the rocks south of the Hines Creek fault were juxtaposed to the Yukon-Tanana terrane to the north. A swarm of gabbro sills intruded the unit and adjacent rocks before a 95-m.y.-old granodiorite pluton was intruded across the Hines Creek fault. The area was regionally metamorphosed to the lower greenschist facies in the Late Cretaceous, followed by initiation of movement on the McKinley fault and deposition of the Paleocene nonmarine clastic rocks of the Cantwell basin. Strike-slip movement on the McKinley fault continued through Eocene time, producing large open folds in the tightly folded Triassic rocks. The late Tertiary Wood River fault-kink zone formed a new set of folds and complicated the orientations of older structures.

Regionally, the Upper Triassic rocks are correlative with rocks of the northern flank of the Alaska Range, which form the Pingston terrane, one of the many elongate terranes of south-central Alaska that are associated with large areas of Jurassic-Cretaceous flysch and lie between the Yukon-Tanana terrane to the north and Wrangellia to the southeast.


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