Lock, Brian E.1, Ashley Walker Fife1
(1) Energy Institute, University of Louisiana at Lafayette, Lafayette, LA
ABSTRACT: Contourites and Related Outer Shelf/Upper Slope Sediments, Boquillas Formation, West Texas
The Upper Cretaceous Boquillas Formation consists of flaggy carbonate strata which crop out along the northern margin of the Maverick Basin, west of Del Rio, West Texas. Significant facies differences exist between the rocks exposed in superb road cuts of US 90 between Del Rio and Langtry, compared with extensive exposures in the vicinity of the Big Bend National Park about 120 miles to the west. The Big Bend strata are interpreted to have been deposited in mid-shelf waters while those near Del Rio, described here, accumulated in outermost shelf or, more likely, upper slope depths.
Previous studies have interpreted decimeter-scale lenticular bedding in the US 90 outcrops as storm-generated hummocky cross-bedding; we believe this interpretation to be wrong. Similar structures occur in carbonate contourites described from the Cambro-Ordovician of Nevada, Ordovician of China, Neogene of Japan and Oligocene of Cyprus.
The contourites in the Boquillas Formation occur in association with a typical deeper water assemblages of sediment types, including debris flows, turbidites and black, bituminous hemipelagic/pelagic shales. One distinctive turbidite can be traced for more than 25 miles, limited only by the extent of the highway exposures. Slump folds are common. Subtle channeling and intraformational scouring are seen. The fossils are predominantly small ammonites, saccocomid crinoids, planktonic forams and calcispheres. Bioturbation is rare to non-existant.
These characteristics, taken together, support an interpretation of upper slope deposition, in an oxygen-minimum depth zone.
AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004.