Linking rudist distribution to paleoenvironmental conditions: evidence from a Glen Rose Patch Reef (Albian, Cretaceous), Herff Falls, Boerne, Texas


Carbonate platforms, developed extensively during the Cretaceous, are considered as excellent archives for paleoenvironmental and paleoceanographic proxies. During this time period, large amounts of organic carbon were deposited and preserved as thick sequences of marine black shale, thin black beds in shelf chalks, and lenses in contemporaneous rudist reef limestones. These black shale deposits may be linked to the so-called "Ocean Anoxic Events." During the early Albian (Cretaceous), rudist patch reefs belonging to the Glen Rose Formation occupied a portion of the shallow marine shelf of the northwestern Gulf of Mexico, which is now preserved in the Texas Hill Country, north of San Antonio. The reef complex outcropping at Herff Falls in Boerne, Texas contains caprinid mounds; they are up to ten feet thick and are built up on the landward side of a large pile of coarse skeletal debris. The caprinid mound is capped by a thin layer of monopleurid and requinid rudists. This lower Albian rudist reef complex is overlain by thin cyclic sequences of wackestone, packstone, grainstone, and finely laminate mudstone and stromatolites. The demise of lower Albian rudist reefs is observed in other localities in Texas. Using chemostratigraphy (carbon stable isotopes) and paleoenvironmental proxies (e.g., phosphorus and barium content measured by X-ray fluorescence), this research highlights a link between this disappearance, the repartition of rudist type, and changes in ocean chemistry. In particular, a slight increase in biophile elements content is observed toward the top of the rudist reef complex. Its demise may thus be linked to an incipient installation of mesotrophic conditions in confined environments, as mirrored by the occurrence of stromatolites, followed by the subaerially exposure of this portion of the Comanche platform. This research also compares the carbon stable isotope curve and sequence stratigraphic interpretation of several outcrops located in Boerne, with established reference sections. This correlation provides a higher precision dating of the rudist reef and exposed section to ca 108 Ma-107 Ma.


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Caprinid, Comanche Platform, Cretaceous, Glen Rose Fm, Rudist, South Texas Stratigraphy