Model for beach shoreline in Gallup Sandstone (Upper Cretaceous) of northwestern New Mexico

Abstract

A model stratigraphic cross section for beach deposits in the Gallup Sandstone (Upper Cretaceous) in the Ship Rock area of northwestern New Mexico was contructed for comparison with other ancient deposits as well as with modern beaches. The model shows a lower regressive beach sandstone overlain unconformably by an upper transgressive offshore-bar sandstone. Landward, the beach sandstone interfingers with coal-swamp deposits or is truncated and overlain by dune sandstone. Seaward, the beach sandstone grades through shoreface sandstone into a transition zone that gives way to offshore siltstone and mudstone. Offshore siltstone and mudstone overlie and, in part, grade laterally into offshore-bar sandstone, which has no contiguous landward equivalents and transgresses all beach and related facies. Beach and offshore-bar sandstones, as well as their related facies, are identified by sedimentary structures within a particular stratigraphic framework; lithologic and paleontologic data support the interpretation. Both the beaches and the offshore bars consist of imbricate patterns of parallel beds. In the backshore of the beach, cross laminae dip either in diverse directions or landward; in the foreshore, cross laminae dip uniformly seaward at low angles. Shoreface sandstone is either churned by burrowing organisms or composed of large-scale, truncated wave-ripple laminae. Offshore-bar sandstone is generally churned but may show high-angle cross laminae that dip parallel with the length of the bar. This model provides a basis for predic - ting the location of petroleum reservoirs, evaluating source and seal relationships, determining volumes of both the reservoir and the aquifer providing the water drive, and predicting flow patterns within the reservoir.