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Bureau Seminar, September 7, 2012

An Answer to a Controversy: Depositional Features and Associated Processes in the Upper Cretaceous, Deepwater Eagle Ford (Boquillas) Carbonate dominated A-Member

Dr. Robert Loucks, Greg Fr├ębourg, and Steve Ruppel
Senior Research Scientist, BEG

Click here for streaming video: September 7, 2012 8:55am

The Upper Cretaceous Boquillas-A member (Equivalent to the Eagle Ford) is well exposed in a series of roadcuts along Highway 90 west of Del Rio, Texas. These roadcuts display an interesting array of depositional features that have sparked a controversy regarding the depositional environment of these sediments. This controversy hinges on whether the sediments were deposited on a storm-dominated shelf above storm-wave base or were they deposited on a deeper water slope below storm-wave base. To address this controversy all data must be analyzed and weighed and not let just one piece of evidence force the conclusion. The Boquillas-A unit was deposited on the drowned landward of the paleoStuart City shelf margin. The sediments are composed of deeper water biota including: Globigerinids, calcispheres, coccoliths, pelagic crinoid plates, thin-shelled bivalves, and small ammonites. No evidence of bioturbation is present. Sedimentary features include debris and mud flows (debrites), low-density turbidites, hummocky-like cross stratification, bottom current ripples and sand waves, load features, and slumps and slides. Hummocky cross stratification (HCS) is common on storm-dominated shelves, and some researchers take this feature as indisputable evidence of deposition above storm-wave base. Historically, once HCS is observed, the default interpretation is deposition above storm wave base, and contradictory evidence often cannot override this dogma. However, all the deeper water biota, the lack of any bioturbation, and the predominance of sediments displaying slope processes strongest suggest a slope setting below storm-wave base. Recent literature has documented hummocky-lie cross stratification in deepwater carbonates in the Upper Cretaceous in France and attribute the process of formation not to storms, but to antidunes related to Kelvin-Helmholtz instability occurring between a relatively dense layer at the base of a thick turbidity current and a less dense upper layer made up of fine-grained particles. Therefore, when all the evidence is weighed, a deeper water slope setting is the logical interpretation for the depositional setting of the Boquillas-A member.


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