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The AAPG/Datapages Combined Publications Database

Wyoming Geological Association

Abstract


The Thrust Belt Revisited; 38th Annual Field Conference Guidebook, 1987
Pages 149-161

Lower Triassic Rocks Within and Adjacent to the Thrust Belt of Wyoming, Idaho, and Utah

Richard A. Paull, Rachel K. Paull

Abstract

Twenty-nine measured sections form the basis of a conodont biostratigraphic study of the Lower Triassic Dinwoody, Woodside, Red Peak, and basal Thaynes formation in the Wyoming-Idaho-Utah thrust belt region. Twenty-two sections are distributed throughout the five major plates of the thrust belt, five sections are in the Wyoming foreland, and two are in the hinterland of Idaho and Utah.

Greenish gray siltstone, shale, and limestone interbeds of the Dinwoody Formation were deposited during an Early Triassic transgression onto the Wyoming shelf. This unit disconformably overlies Upper Permian strata with little physical evidence of a hiatus. The Dinwoody is transitional with the overlying Thaynes in the western part of the study area, and grades laterally and vertically eastward into red beds of the Woodside Formation.

The thickness of the Dinwoody ranges from 664 m (2180 ft) within the western thrust belt to 38 m (115 ft) in the Wyoming foreland. Thickness variations across major thrust faults are generally less than changes within a given plate.

The Woodside Formation thins westward from a maximum thickness of 216 m (710 ft) in the central part of the thrust belt, to merge into the Dinwoody in the Meade plate. Like the Dinwoody, thickness variations in the Woodside are generally greater within plates than across major thrusts.

The Thaynes Formation generally overlies the Woodside with an abrupt, but conformable, contact. To the west, the basal Thaynes consists of the Meekoceras-bearing limestone member, but this unit and younger limestones are progressively replaced eastward by Woodside and younger red beds. Where the Thaynes loses lithologic identity in the Wyoming foreland, the Woodside is no longer recognized and the red beds are assigned to the Red Peak and younger formations of the Chugwater Group.

Conodont biostratigraphy of the Dinwoody establishes that the initial Triassic (Griesbachian) transgression was extensive and rapid. This advance was followed by a major regression during the Dienerian, when red beds of the Woodside Formation graded westward from the Red Peak Formation. A second major Early Triassic (Smithian) transgression resulted in deposition of Meekoceras-bearing limestone as the basal unit of the Thaynes Formation throughout much of the study area.

Although conodont biostratigraphic zonation of Lower Triassic rocks created a useful framework for evaluating changes in thickness and lithofaces within the Wyoming-Idaho-Utah thrust belt, this information provided few insights into displacements between major thrust plates. as previously described, variations within major plates are generally greater than changes across intervening faults.


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