ABSTRACT: Cretaceous Stratigraphy Along the Moxa Arch, Green River Basin, Southwestern Wyoming

Robert E. Mueller, Steve R. Clawson

Years of hydrocarbon exploration along the Moxa arch established a large subsurface database for Cretaceous studies. Subsurface reference sections illustrate complex stratigraphy affected by depositional thickening, shoreline fluctuations along the margin of the Western Interior Seaway, and erosional unconformities. The entire Cretaceous section can be subdivided into four megasequences bounded by regional unconformities formed during Early Cretaceous, Turonian, Campanian, and Paleocene time.

Sequence one of Aptian-Albian age unconformity overlies the Jurassic Morrison Formation. Formations include nonmarine Cedar Mountain Formation and mixed marine and nonmarine lower Dakota Group that thicken from 250 (south) to over 500 ft (north). Sequence two is bounded by the tentatively identified "Muddy unconformity" of Albian age and a mid-Turonian unconformity within the Frontier Formation. This sequence contains marine and nonmarine upper Dakota Group, marine Shell Creek and Mowry shales, and predominantly marine lower Frontier Formation. Significant thickening from approximately 250 to over 1000 ft occurs in a west-northwest direction. Sequence three of Turonian-Campanian age thickens west-northwest and includes upper Frontier, Hilliard, Adaville, and Rock Springs formations.

Major structural uplift in late (?) Campanian time, most pronounced along the southern Moxa, resulted in erosional truncation of 3500+ ft of Rock Springs and Hilliard formations. The pre-Ericson unconformity that underlies Sequence four is overlain by nonmarine, Campanian Ericson Formation. The Ericson Formation thins northward, principally by erosional truncation below the Fort Union unconformity. These Cretaceous megasequences were formed during the complex interplay between variable sediment supply, tectonics, and absolute sea-level fluctuations.

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