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The AAPG/Datapages Combined Publications Database
Sandstone units of the Upper Cretaceous San Miguel Formation in south Texas are wave-dominated delta sequences deposited during a major 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, the Maverick basin consisted of two subbasins. A western subbasin received sediments from the northwest; the eastern subbasin received sediments from the north.
Net-sandstone patterns show that the thickest parts of the sandstone bodies are generally strike oriented. Where not eroded, sand-feeder (fluvial) systems are indicated by dip-aligned components on the updip sides of the sandstone units. According to these net-sandstone patterns, the San Miguel deltas vary considerably and make up a wide spectrum of wave-dominated delta types.
The most common vertical sequences in the San Miguel coarsen upward from silt and clay to fine sand. Burrows dominate, and the few primary structures are of small scale. Large-scale cross-beds are observed only in outcrop. Strand-plain or barrier-island facies sequences, which prevail in most wave-dominated delta deposits, are incomplete in the San Miguel. 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.
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