Microbial Mats on Siliciclastic Bedding Surfaces - in the Del Rio Kinneyia Formation (Cenomanian), West Texas

The Del Rio Formation outcrops in Val Verde County, Texas, consist of calcareous shales with thin limestones and sandstones. The fauna is varied but is dominated by exogyrid oysters and arenaceous forams. The sandstone beds display hummocky cross stratification, grading (reworked oysters concentrated at the base) and oriented sole marks, all indicating deposition as tempestites. Locally, the top surfaces of sandstone beds are covered with microbial wrinkle mats of the type known as Kinneyia. This type of mat is common in the Proterozoic but rare in younger strata. The youngest occurrence previously reported is from the Jurassic. The Kinneyia mats are also noteworthy in that microbial mats are much more common on carbonate sediments than on siliciclastics.

A section through the formation near Terlingua, 240 km to the west, has only discontinuous lenses of sandstone, with gutter casts. Another example of Kinneyia mat was found on one of these lenses, indicating the widespread nature of the appropriate hostile environmental conditions.

Microbial mat development and survival to burial is conditional on protection from grazing invertebrates, suggesting hostile environmental conditions at the time of sandstone emplacement. While other oyster-rich strata, such as the Valanginian of the Jura area of France, have been attributed to eutrophism following disturbance of sea floor sediments, a major influx of low salinity water following major storms is considered a possible explanation in the Del Rio example.

Microbial mats are commonly considered indications of shallow water environments, on the assumption that they are produced by photosynthetic cyanobacteria. While this assumption is not necessarily valid, other reports of Kinneyia are from apparantly shallow environments, and the Del Rio sediments are generally consistent with modest water depths.