ABSTRACT

Sandstone units of the Upper Cretaceous San Miguel Formation in south Texas are wave-dominated deltaic sequences deposited during an overall marine transgression. San Miguel sediments were deposited in the Maverick basin within the Rio Grande embayment. Cross sections and sandstone maps reveal that during deposition of the San Miguel Formation, the Maverick basin consisted of two subbasins. A western subbasin received sediments from the northwest; an eastern subbasin received sediments from the north.

The most common San Miguel vertical sequences, produced during delta progradation, coarsen upward from silt and clay to find sand. Burrows dominate, and the few primary structures in cores are of small scale. Large-scale cross beds 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 Formation. In most places, only the lower shoreface is preserved. The upper parts of the sequences, which normally bear large-scale primary structures, were lost by marine reworking during subsequent transgression. Intense burrowing destroyed any primary structures at the tops of the truncated sequences.

Net-sandstone patterns show that the thickest parts of the sandstone bodies are generally strike oriented. Where preserved, sand-feeder (fluvial) systems are indicated by dip-aligned net-sandstone trends on the updip sides of the sandstone units. Variability of these net-sandstone patterns indicate that a wide spectrum of wave-dominated delta types is represented. The spectrum includes wave-influenced lobate, arcuate, cuspate, and strike-elongate deltas, in order of increasing importance of wave processes.