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Deltaic and Associated Facies of Difunta Group (Late Cretaceous to Paleocene), Parras and La Popa Basins, Coahuila and Nuevo Leon, Mexico (1)
E. F. McBride (2), A. E. Weidie (3), J. A. Wolleben (3)
The Difunta Group is gray calcareous mudstone, siltstone, and sandstone that interfingers with wedge-shaped redbed units. These complexly interfingered units are divided into 13 formations and numerous members, tongues, and lentils; the group has a thickness of 10,000 feet.
Low sinuosity, high-bedload streams transported volcanic and sedimentary rock debris from the Sierra Madre Oriental eastward to the ancestral Gulf of Mexico. Separate rivers fed the Parras and La Popa basins. Sediment accumulated in arcuate, wave-dominated deltas that prograded slowly into water several hundred feet deep. Some sediment was transported by marine processes to shallow delta-flank environments.
Delta-plain lake and interfluve deposits are bioturbated mudstone containing sparse leaves, charophyte oogonia, dinosaur bones, and oyster debris. Many beds were reddened by postdepositional oxidation of iron-bearing minerals. Delta-front sheet sands are 20 to 60 feet thick, well sorted, flat bedded, and have sparse Ophiomorpha. Delta-platform deposits are characterized by fantastic sequences of ball-and-pillow structure and a sparse molluscan fauna. Prodelta deposits are interbedded graded sandstone and bioturbated mudstone; the sandstone beds were deposited by turbidity currents generated at the delta front by hyperpycnal inflow.
Carbonate banks up to 1000 feet thick developed in distal prodelta (shelf) environments on submarine highs that were sea-floor expressions of rising evaporite diapirs.
At least four major regressive-transgressive cycles developed in response to episodic basin subsidence. In contrast to the Rocky Mountain Cretaceous transgressions, the Difunta accumulated sheet sandstone units 20 to 60 feet thick. Parts of the transgressive units are delta-destructional deposits, but many are coalesced progradational sheet sands.
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