PALEOGEOGRAPHY OF THE SALT ANTICLINE REGION OF THE PARADOX BASIN DURING DEPOSITION OF THE ALI BABA MEMBER OF THE LOWER TRIASSIC MOENKOPI FORMATION, WESTERN COLORADO AND EASTERN UTAH

The Ali Baba Member of the Lower Triassic Moenkopi Formation is confined to the salt anticline province of the Paradox basin in eastern Utah and western Colorado. This coarse-grained fluvial deposit records the rise of the salt anticlines and concurrent subsidence in intervening salt withdrawal basins.

Paleocurrent data from 11 localities show a consistent N-NW paleoflow direction for Ali Baba rivers, parallel to the rising anticlines and the western margin of the Uncompahgre highlands. This indicates a system of axial N-NW flowing fluvial systems that ran between the salt anticlines, and a local sediment source within the anticlines. Sandstone and pebble conglomerate that make up the Ali Baba consist dominantly of quartz, feldspar, granite rock fragments and carbonate rock fragments, indicating a sediment source of reworked upper Paleozoic strata exposed on the rising flanks of the salt anticlines. Regionally, this pattern suggests a drainage outlet somewhere to the northwest, out of the region of salt uplift, where the rivers were able to escape the confines of the anticlines and flow unimpeded to the shallow sea that lay to the west.

A recently identified locality on the northeast flank of the Castle Valley salt anticline well illustrates the relations between sedimentation and salt anticline uplift. Here the lower part of the section, which is dominated by planar tabular cross-stratification, shows northwest-directed paleoflow (315º). In contrast, the upper part of the section which is dominated by trough cross-stratified granule/pebble conglomerate shows an abrupt switch with a northeast paleoflow direction (57º). This shift is concurrent with the appearance of up to 30% fossilifereous carbonate clasts. Fossils in these clasts include rugose corals and crinoids columnals. This section records an initial northwest-flowing axial fluvial system confined between the Fisher Valley and Castle Valley salt anticlines. The upper part of the section records the progradation of a short tributary system that drained off the limb of the Castle Valley anticline. This tributary drainage tapped into sediment of the Permian Cutler Formation and lower Cutler beds as it cut headward into the rising east flank of the anticline.