ABSTRACT: Persistent Myth About the Lower Triassic Little Medicine Member of the Goose Egg Formation and Eastward Termination of the Lower Triassic Dinwoody Formation, Central Wyoming

Richard A. Paull, Rachel K. Paull

Uncertainties about Upper Permian-Lower Triassic stratigraphic relations across a west to east transition from shelfal marine to marginal marine-continental conditions in central Wyoming have persisted for more than 80 years. The presence of the poorly defined Paleozoic-Mesozoic unconformity of more than a million years duration within this interval adds another complexity.

The facies change, the unconformity, poor exposures, and the unquestioned acceptance of early interpretations account for the present situation. Although recent work has clarified some aspects of the problem, a fundamental misunderstanding about correlation of the Lower Triassic Dinwoody Formation of west-central Wyoming to the Little Medicine Member of the Upper Permian-Lower Triassic Goose Egg Formation in east-central Wyoming still exists.

Detailed examination of Permian-Triassic strata in numerous measured sections along the north flank of the Owl Creek Mountains and in the southern Wind River basin indicates the Dinwoody was deposited during a geologically instantaneous eastward transgression during the earliest Triassic (Griesbachian) that did not persist into eastcentral Wyoming. Correlation establishes that the Little Medicine was not deposited during this transgression as generally believed. There is also no evidence that this unit is a shelfal marine deposit.

Although the Little Medicine Member is not physically continuous with the Dinwoody to the west, it is correctly assigned as partly age-correlative. Our conclusions are not intended to detract from the stratigraphic validity or usefulness of this member, but merely to end a persistent myth about its relationship to the Dinwoody.

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