Abstract

A unit composed of interbedded sandstone and siltstone with bedding oriented at an angle to underlying and overlying strata is present within the Black Dragon Member of the Lower Triassic Moenkopi Formation. The rocks are exposed in southeastern Utah in the Island in the Sky District of Canyonlands National Park and in the Orange Cliffs of Glen Canyon National Recreation Area. This study of the unit establishes a depositional rather than structural origin for the dipping beds, and clarifies the unit’s relation to underlying and overlying strata.

Based on field studies, the dipping beds are interpreted as foresets of a prograding delta, as indicated by sedimentary structures, lithology, and both lateral and vertical facies relations. The dipping foresets have a sigmoidal form, with the beds gradually becoming horizontal both downsection and upsection. The unit overlies the Hoskinnini Member of the Moenkopi Formation and/or the Permian White Rim Sandstone. It is overlain by horizontal beds of similar lithology that locally truncate the angular sets. Paleocurrent indicators demonstrate east to north-east flow, parallel to the dip of the angular sets.

In the study area, the Black Dragon Member reflects deposition within the Hoskinnini basin during a period of local sea-level rise. The basin formed initially because of erosion during a relative sea-level fall represented by the Tr-1 unconformity. As the basin was transgressed from the north, fluvial systems transported sediment into the basin from the west. Progradation of these fluvial systems into the water-filled basin led to its eventual filling with sediment. Fluvial systems then flowed across the former location of the Hoskinnini basin, transporting sediment north toward the miogeocline. The uppermost beds of the Black Dragon Member indicate that the area was transgressed from the northwest during their deposition