Storm Events in Geological Record - a Lecture from 92 MYA

Nebrigic, Dragana D.¹, Boyan K. Vakarelov¹ (1) The University of Texas at Dallas, Richardson, TX

The ongoing debate on greenhouse effect and modern global warming strengthen interest for abrupt warming events in the geological past. Cenomanian-Turonian greenhouse interval is related to a broad- scale global warming which peaked in the basal Turonian and led to major changes in oceanic environments.

Siliciclastic-carbonate-bentonitic unit, deposited during Cenomanian-Turonian transgression, the Eagle Ford Formation of Texas, has been a focus of study over several decades, due at least in part to the hydrocarbon resources generated from it. However, its upper lower Turonian Kamp Ranch unit (“member”) (Collignoceras woolgariZone) has never been adequately described probably due to the insignificant thickness of the unit, laterally discontinues, and poorly exposed. It represents a locally prominent sequence of alternating dark gray to grayish black, noncalcareous , laminated shales and silty shales interbedded with a distinctive, ledge-forming well-indurated light brown to grayish orange coquinoid, shell hash limestone, calcarenite, which makes prominent marker beds within the outcrops and occur approximately 30 m below the Eagle Ford-Austin Chalk contact. Average thickness of the entire unit varies from 0.03-1.5 m. A recent investigation of four Kamp Ranch exposures in North Texas which included detailed analyses of trends in grain size, lithology, biogenic and sedimentary structures, faunal assemblages of coquinoid limestone beds, has revealed evidence of storm deposition. Regional correlation of these beds is based on event lithostratigraphy, as well ammonite and foraminiferal biostratigraphy. Moreover, detailed sequence stratigraphic analysis allowed a reinterpretation of sea-level history for Cenomanian-Turonian time of this part of Gulf Coast.