Shoreface Sandstones Research Articles

Depositional Model--upper Cretaceous Gallup Beach Shoreline, Ship Rock Area, Northwestern New Mexico

To provide an example of ancient deposits across a sand-beach shoreline for comparison with both Recent and other ancient deposits, a cross-sectional model was constructed from surface exposures across the Upper Cretaceous Gallup shoreline in the Ship Rock area of northwestern New Mexico. Tile model shows lower Gallup regressive-beach sandstone overlain unconformably by upper Gallup transgressive offshore-bar sandstones. Landward, the beach sandstone interfingers with coal-swamp deposits or is truncated and overlain by dune standstone. Seaward, the beach sandstone grades through shoreface sandstone into a transition 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. Within their particular stratigraphic frameworks, sedimentary structures, supported by lithologic and paleontologic data, identify both the beach and the offshore-bar sandstones as well as their related facies. Both the beaches and the offshore bars consist of imbricate patterns of parallel beds. Beds in the backshore beach consist of cross laminae that dip either in diverse directions or landward and that compose laminasets, which are swale fillings; cross laminae in the foreshore beach dip uniformly seaward at low angles. Beds of shoreface sandstone are either churned by burrowing organisms or composed of large-scale truncated wave-ripple laminae. in the transition between shoreface and offshore siltstone and mudstone, beds of mudstone interfinger with bells of sandstone and siltstone, which show small-scale truncated wave-ripple laminae and churned structure. Siltstone and mudstone beds alternate in offshore rocks, and the siltstones contain both current-ripple and truncated wave-ripple laminae, except where burrowing organisms have destroyed the laminae. Most beds of offshore-bar sandstone are churned, but some show high-angle cross laminae that dip parallel with the length of the bar and approximately parallel with the distant shoreline. Burrowing organisms have completely churned the muddy sandstone beds of the offshore-bar transition.