New Mexico Geological Society Annual Spring Meeting — Abstracts

The sandstone member of the Lower Cretaceous U-Bar Formation in the East Potrillo Mountains, south-central New Mexico, ranges from 25 to 66 m thick, and consists of four intervals of coarse sandstone and conglomerate interbedded with very fine-grained sandstone and siltstone. Four Iithofacies have been identified: 1) pebble conglomerate, 2) crossbedded to hummocky stratified coarse sandstone, 3) horizontally laminated sandstone, and 4) bioturbated fine sandstone and siltstone. Each coarse interval consists of up to three stacked sequences of, in ascending order, pebble conglomerate, crossbedded to hummocky stratified sandstone, and horizontally laminated sandstone. The grain size of each coarse interval increases upward. The fine sandstone and siltstone lithofacies is predominantly bioturbated, although locally horizontal laminations and vertical burrows are found. Coarsening-upward sequences are present near the top of this lithofacies where it underlies the coarse intervals.

The sandstone member is interpreted as a stacked sequence of offshore sand ridges deposited in a shallow marine envoronment. The coarse intervals were deposited on the upcurrent side of the ridges; the fine-grained sandstone and siltstone on the down-current side. The presence of hummocky stratification indicates the importance of storms in th evoluion of the sand ridges, but bipolar paleocurrent data suggests an additional tidal influence.