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The AAPG/Datapages Combined Publications Database

GCAGS Transactions

Abstract


Gulf Coast Association of Geological Societies Transactions Vol. 58 (2008), Pages 635-644

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

Brian E. Lock

Department of Geology, University of Louisiana at Lafayette, Box 44530, Lafayette, Louisiana 70504-4530

ABSTRACT

The Del Rio Formation in Val Verde County, Texas, consists of calcareous shales with thin limestones and sandstones. The fauna is dominated by exogyrid oysters and arenaceous forams. The sandstone beds display hummocky cross stratification, graded bedding (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.

Microbial mat development and survival to burial is conditional on protection from grazing invertebrates, suggesting hostile environmental conditions at the time of sandstone emplacement. Anoxic or disaerobic conditions have been invoked to satisfy this requirement in other examples of preserved mats, and other oyster-rich strata, such as the Valanginian of the Jura area of France, have been attributed to eutrophism. A major influx of low salinity water following major storms provides another possible explanation for the Del Rio microbial mats and oyster-dominated fauna.

A section through the formation near Terlingua, 150 mi (240 km) to the west, has only discontinuous lenses of sandstone, with gutter casts. Another example of a Kinneyia mat was found on one of these lenses, indicating the widespread nature of hostile environmental conditions.

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 apparently shallow environments, and the Del Rio sediments are generally consistent with modest (but not intertidal) water depths.

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