The Cenomanian Del Rio Formation in West Texas
Brian Lock and Joseph Grimball
The Del Rio Formation sits with major unconformity on the Lower Cretaceous. The underlying surface is extensively reddened and discolored, suggesting sub-aerial exposure and fresh-water diagenesis during the hiatus is indicated by moldic dissolution of gastropods with basal Del Rio muds filling the molds. Borings and other typical hardground features are abundant. This is the unconformity which separates the Lower and Upper Cretaceous. The Del Formation has extremely variable thickness, attributed to basal onlap and erosional truncation at the top surface. A new localized basal member has been identified around Lake Amistad (Val Verde County), consisting of a white, chalky limestone with common Mariella brazoensis (Roemer), also known as Wintonia and previously as Plesioturrilites, and local concentrations of a terrebratulid brachiopod. The bulk of the formation is a mix of shales and limestones (storm concentrations of the abundant oyster, Ilymatogyra arietina) attributed to nearshore, brackish marine conditions, with a few sandstone beds interpreted as storm deposits (tempestites) derived from neighboring barrier islands or strandplains by retreating storm surge waters. Another unconformity overlies the Del Rio Formation, separating it from the Buda Formation. Clasts of the more resistant Del Rio lithologies are concentrated in the basal Buda, and are extensively bored, primarily by Gastrochoenites. Evidence for subaerial exposure is inconclusive, however, and the surface may represent submarine erosion.
AAPG Search and Discovery Article #90167©2013 GCAGS and GCSSEPM 63rd Annual Convention, New Orleans, Louisiana, October 6-8, 2013