Differentiation of Delta-Front and Barrier Lithofacies of the Upper Cretaceous Pictured Cliffs Sandstone, Southwest San Juan Basin, New Mexico

Abstract

The Upper Cretaceous Pictured Cliffs Sandstone represents a regressive littoral marine unit deposited during the final retreat of the Cretaceous epeiric sea in the southwestern San Juan Basin, New Mexico. Differences in rock type, internal and penecontemporaneous deformation structures, textural sequences, mineral composition, and trace fossil content permit recognition of laterally contemporaneous delta-front and barrier lithofacies in the Pictured Cliffs Sandstone. The delta-front lithofacies consists of distal bar, distributary mouth bar, and distributary channel deposits. The barrier lithofacies consists of shoreface, beach, washover channel, tidal inlet, tidal channel, and ebb-tidal delta deposits; these lithofacies are coarsening-upward sequences of shale, siltstone, and sandstone, locally scoured in the upper part by fining-upward channel deposits. The delta-front lithofacies contain abundant penecontemporaneous deformation structures in the form of ball-and-pillow, diapiric, and slump structures as well as convolute bedding and microfaults. In contrast, the laterally equivalent shoreface deposits of the barrier lithofacies lack these deformation structures. The channel deposits of the delta-front lithofacies contain large-scale convolute lamination and a few burrows, in contrast to the channel deposits of the barrier lithofacies, which contain common herring­bone crossbeds separated by reactivation surfaces, bidirectional but ebb dominated festoon crossbeds, and more abundant burrows. Quartz content of the barrier lithofacies varies from 43 to 74 percent, whereas in the delta­ front lithofacies it varies from 29 to 60 percent. The difference in quartz content between these lithofacies suggests some cleaning-up process in the deposition of the barrier lithofacies, particularly in the preserved beach deposit. The delta-front and barrier lithofacies were probably deposited along mesotidal shorelines. The recognition of laterally contemporaneous delta-front and barrier lithofacies of the Pictured Cliffs Sandstone allows delineation of potentially economic coal-producing deposits in the delta-plain lithofacies flanked by back­ barrier lithofacies in the overlying prograded Fruitland Formation.