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Evidence of Below Storm-Wave Base, Deeper Water Depositional Setting for the Upper Cretaceous, Boquillas (Eagle Ford) Carbonate Dominated A-Member


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 sparked a controversy regarding the depositional environment of these sediments. This controversy hinges on whether the sediments (now wackestones and mud-dominated lime packstones and terrigenous-rich mudstones) were deposited on a storm-dominated shelf above storm-wave base or were they deposited on a deeper water, inclined drowned shelf 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 shelf landward of the paleoStuart City shelf margin. The statra are composed of only deeper water biota including: globigerinids, calcispheres, coccoliths, pelagic crinoid plates, Inoceramus fragments, and small ammonites. No evidence of bioturbation is present. The mudstones contain elevated TOC. Sedimentary features include debris and mud flows (debrites with boulder-sized blocks), low-density turbidites, hummocky-like cross stratification, bottom-current ripples and sand waves, load features, and regional-wide slump and slide units. 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, the deeper water biota, the lack of any bioturbation, a dysareobic to aerobic environment, and the predominance of sediments displaying slope-like processes strongly suggest deposition on an inclined, drowned shelf below storm-wave base. Recent literature has documented hummocky-like cross stratification in deepwater carbonates in the Upper Cretaceous in France and attributes 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, below storm-wave base setting is the logical interpretation for the depositional setting of the Boquillas-A member.