LATE EOCENE DIATOMITES IN TEXAS REVEAL GULF OF MEXICO BIOSILICEOUS SEDIMENTATION

The discovery of Late Eocene diatomite deposits in central Texas records a previously unrecognized episode of marine biosiliceous sedimentation in the region. Previous work noted biosilica-rich Late Eocene sediment containing common sponge spicules, but the smaller diatoms were overlooked until a large diatomite was recognized.The discovery deposit is a 6-10 interval of diatomite (dominantly diatoms; 60-65 % porosity) containing a few thin storm sands and is overlain by 7 m sheet sands, a shoaling deposit capped by an exposure-generated erosion surface. Diatomite deposition occurred on a shallow shelf coastal marine setting, between storm wave base and fair weather wave base. Evidence of environment includes presence of diverse coastal marine diatoms, a few washed-in radiolaria, presence of wood clasts, and upward transition to sandstone with abundant Ophiomorpha traces. An occurrence of coastal zone diatomite is highly unusual and unexpected. Biosiliceous fossills have excellent preservation. The diatom assemblage is dominated by Paralia and diverse coastal tychoplanktic diatoms and contains transported freshwater diatoms. The diatomite occurs in strata containing beds of high-silica volcanic ash, providing evidence that the proliferation of biosiliceous organisms resulted from river input of dissolved silica and other nutrients. The ash is derived from silicic volcanism along western margin of North America. The abundance of Late Eocene silicic volcanic ash in the Texas coastal plain and the presence of sponge spicule-rich sediments laterally are reason to believe that Late Eocene biosiliceous sediments are extensive in the northwest Gulf of Mexico. The Texas deposits can be related to the “Ignimbrite Flareup” of the North American continent as it overrode part of the Pacific Ocean spreading axis.