LUTZ, SUSAN JUCH, University of Utah Research Institute, Salt Lake City, UT

ABSTRACT: Sequence Stratigraphic Interpretation of a Mixed Carbonate-Siliciclastic Shelf Environment: The Triassic Moenkopi Formation in Central Utah

The Moenkopi Formation in central Utah contains four major depositional packages: the siliciclastic Black Dragon Member, the carbonate Sinbad Limestone Member, the evaporitic Torrey Member, and the siliciclastic Moody Canyon Member. Log, outcrop, and core analyses from the northern San Rafael swell area define regional depositional patterns for the shallow-marine and tidal deposits of the Moenkopi shelf. Detailed facies analysis of cores and well logs from the Grassy Trail Creek field has enabled workers to make a sequence stratigraphic interpretation of the Moenkopi.

A prograding tidal delta system produced channelized sandstones in the middle Black Dragon Member. Herringbone cross-bedding and the presence of Thalassinoides in calcareous siltstones overlying the channel sandstones indicate marine reworking and flooding of the estuarine channels with a sea level rise. A thin shale bed on well logs may identify the actual transgressive surface and parasequence boundary.

Continued transgression lead to the deposition of the Sinbad Limestone Member. The Black Dragon-Sinbad Limestone contact is gradational. Fossiliferous wackestones and packstones of the lower Sinbad record subtidal deposition on a shallow carbonate shelf. The mudstones of the upper Black Dragon Member (above the transgressive surface) and the shallow-marine limestones of Sinbad Member are interpreted as a transgressive system tract.

Overlying the skeletal limestone facies in the Sinbad are cross-bedded oolitic grainstones that represent barrier shoals along a retrograded shoreline. Oolitic tidal channels incise into the oolitic shoal deposits and indicate a progradation during late highstand. The upward succession of facies in the overlying Torrey Member, from basal tidal channel sandstones to intertidal mudstones to supratidal anhydritic mudstones, records the steady progradation of tidal deposits over the carbonate shoals in the parasequence.

AAPG Search and Discovery Article #90993©1993 AAPG Rocky Mountain Section Meeting, Salt Lake City, Utah, September 12-15, 1993.