ABSTRACT: Evidence for an Unconformity at the Base of the Ericson Formation along the East Flank of the Rock Springs Uplift
Michael L. Hendricks
The existence of an unconformity at the base of the Ericson Formation along the east flank of the Rock Springs uplift has been widely debated. The change from fine-grained sandstones, shales, and coals of the upper Rocks Springs Formation to medium and coarse-grained fluvial sandstones at the base of the Ericson indicates a significant increase in depositional energy. But was a significant amount of Rock Springs section removed before or during deposition of the Ericson?
On outcrop, the contact between the Rock Springs Formation and the Ericson does not show extensive downcutting by Ericson channels. Instead, low-angle truncation and only minor scouring occur at the top of the Rock Springs Formation. Detailed section measurement and mapping of the upper regressive member of the Rock Springs Formation (the McCourt member) indicate that a significant unconformity is present at the base of the Ericson Formation.
Strata along the east flank of the uplift are offset by more than 25 northeast-trending normal faults. Most of these faults are Laramide in age. However, recurrent movement on certain faults along trend with the Wamsutter Arch produced a horst block before basal Ericson deposition. Erosion on this block removed over 250 ft of upper Rock Springs section on outcrop and up to 550 ft of section southeast of the uplift in the Amoco #1 Bitter Creek well (See. 22, T.16 N., R.99 W). Formation of the unconformity by channel incisement within a graben is inconsistent with observed thinning of lower Ericson beds (Trail member) across the block.
AAPG Search and Discovery Article #91002©1990 AAPG Rocky Mountain Section Meeting, Denver, Colorado, September 16-19, 1990