Origin of a Coarse-Grained Shallow Marine Sandstone Complex: The Coniacian Tocito Sandstone, Northwestern New Mexico. (Volumes 1 and 2).

Abstract

Study of the Coniacian Tocito Sandstone, one of the shale-encased, shallow marine sandstone complexes in the Cretaceous Western Interior Basin, indicates that transport of coarse sediment to distal marine locations was accomplished by an increase in the efficiency of the marine dispersal system (tidal currents). Enhanced tidal currents were a consequence of bathymetric variations formed by uplifts along the pre-existing Four Corners Lineament system. The Tocito Sandstone forms an entirely marine, progradational-transgressive sequence and overlies an unconformity localized along uplifted submarine highs. The lower progradational strata represent the marine portion of a tide-dominated delta that prograded into a tectonically-created "embayment", and include prodelta facies at the base and the deposits of subtidal dunes and tidal channels at the top. The upper transgressive facies overlie a tectonically-enhanced transgressive erosion surface and contain the deposits of shelf sand ridges and dune fields. Comparison to a progradational-transgressive sequence driven by "regional" sea level fluctuations (Gallup Sandstone) reveals differences between progradations resulting from regional sea level fall and those resulting from local tectonic movements. The Gallup Sandstone shows an early aggradational phase (highstand) followed by a strongly progradational phase associated with a seaward shift of the subaerial shoreline and fluvial incision (sequence boundary and falling sea level systems tract) and finally a late aggradational phase (late lowstand). Progradations resulting from regional sea level fall are associated with a seaward shift in the shoreline, similar lithologies above and below the sequence boundary, and an unconformable sequence boundary in updip locations changing to a conformity in prodelta facies. Progradations resulting from intrabasinal tectonism (Tocito Sandstone) need not affect the position of the subaerial shoreline, display unconformities localized on tectonic highs, and may contain a higher proportion of feldspars and lithic fragments. Intrabasinal tectonics can enhance the marine dispersal system and allow for progradation of high-energy, coarse-grained deposits far into the marine environment. This mechanism provides an alternative to that of regional sea level fluctuations as a method for emplacing coarse sediments in distal marine environments.