ABSTRACT: Allochthonous Rocks from the Western Part of the Early Triassic Miogeocline: Hawley Creek Area, East-Central Idaho

Rachel K. Paull, Richard A. Paull

Allochthonous, marine Lower Triassic rocks were first described from the Hawley Creek area in the Beaverhead Mountains of east-central Idaho in 1963, but the age (pre-Smithian), thickness (305 m), lower contact (conformable?), and formational assignment (Dinwoody) were incorrect. Because the Hawley Creek rocks are the northwesternmost record of deposition in the U.S. part of the Early Triassic miogeocline, it is important that stratigraphic relations be clarified.

Three sections of Lower Triassic rocks were measured, sampled, and described during this study. Twenty-two carbonate samples were processed for conodonts, and 7 yielded age-diagnostic elements. In addition, Meekoceras zone ammonites, polished slabs, and thin sections were studied.

Our field and laboratory study establishes that: (1) Triassic rocks disconformably overlie Upper Permian Tosi Chert with little physical evidence of this major hiatus, although significant local relief may have existed, (2) the thickness of Triassic strata exceeds 645 m, (3) the basal 260 m is Smithian and the remainder is Spathian, and all are assignable to the Thaynes Formation, (4) at least the lower 330 m of black shale was deposited under basinal conditions, while the upper part records shallower water carbonate deposition and an influx of coarser terrigenous clastics, and (5) these rocks are similar in age and lithology to Triassic rocks to the south in Idaho and northeast Nevada.

AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990