Wave-Dominated Delta Systems of the Upper Cretaceous San Miguel Formation, Maverick Basin, South Texas

Upward-coarsening sandstone units of the Upper Cretaceous San Miguel Formation in South Texas were deposited in wave-dominated deltas during minor regressive phases, periodically interrupting a major marine transgression. Sediments accumulated in the Maverick Basin within the Rio Grande Embayment. Cross sections and sandstone maps reveal that during deposition of the San Miguel, the Maverick Basin consisted of two subbasins that received sediments from the northwest and the north. Net-sandstone patterns show that the thickest parts of San Miguel sandstone bodies are generally strike oriented. Where preserved, updip fluvial systems are indicated by dip-aligned sandstone trends. San Miguel deltas vary considerably in morphology and compose a spectrum of wave-dominated delta types. Modern analogs of San Miguel deltas include the Rhone, Nile, Sao Francisco, Brazos, Danube, Kelantan, Grijalva, and Senegal deltas. Sandstone geometry ultimately depended on three primary factors: (1) rate of sediment input, (2) wave energy, and (3) rate of relative sea-level change. Original delta morphology was determined by all three factors, but the degree of reworking of deltaic sediments after delta abandonment was determined by wave energy and rate of transgression. The most common vertical sequences exhibited by the subsurface San Miguel coarsen upward from mudstone and siltstone to fine sandstone. Burrows are the dominant structures. Rare primary structures are small scale; large-scale crossbeds are observed only in outcrop. Strandplain or barrier-island facies sequences, which prevail in most wave-dominated delta deposits, are incomplete in the San Miguel. Typically, only the lower shoreface is preserved. Upper parts of the sequences, which normally containlarge-scale primary structures, were destroyed by marine reworking during subsequent transgression. Intense burrowing obliterated primary structures in the upper parts of the truncated shoreface sequences. Most of the San Miguel sandstones are arkoses, but the mineralogical composition of the sandstones changes vertically. Generally within each sandstone, quartz content increases upward with increasing mean grain size. Cements include sparry and poikilotopic calcite, quartz overgrowths, feldspar overgrowths, illite rims, and kaolinite. Porosity was eliminated principally by two types of calcite cement, which tend to cement completely the coarsest, best sorted, and originally most porous zones of the San Miguel sandstones. Zones of secondary porosity resulted from leaching of shell material, calcite cement, and feldspars. Laterally, the distribution of high secondary porosity and calcite-cemented zones is unpredictable.