THE DEL RIO FORMATION (LOWER CENOMANIAN) OF WEST TEXAS: STORMS AND MICROBES - A CONNECTION?

The Del Rio Formation has been studied in outcrops in Val Verde and Terrell counties, and in Brewster County, all in West Texas. The section varies from zero thickness through more than 30 feet (0 to 10 meters) and consists primarily of calcareous mudstones and argillaceous limestones. Dominant paleontology (arenaceous forams, oysters) suggests a near shore, brackish to marine setting. Of particular interest are thin sandstone beds with graded bedding, hummocky cross stratification, tool marks, dewatering structures, and gutter casts. Rip-up coral fragments more than 4 in (10 cm) in diameter are found at the base of some beds in the Black Gap Wildlife Management Area east of the Big Bend National Park. These beds are interpreted to be tempestites, with sand from a close by shoreline swept out to sea by storm surge relaxation currents. Of note is the presence of a microbial wrinkle mat (Kenneyia) on the top surfaces of many of the tempestites. The tempestite-Kenneyia relationship, observed elsewhere, implies the persistence of conditions inimicable to browsing organisms following major storms. Kenneyia is common in Precambrian strata, rarer through the Paleozoic and previously last described from the Jurassic. The Del Rio examples extend its range by about one hundred million years.