Deltaic Origin of Difunta Group (Late Cretaceous to Paleocene), Parras Basin, Coahuila and Nuevo Leon, Mexico: ABSTRACT

The Difunta Group is predominantly gray calcareous mudstone, siltstone, and sandstone with 3 wedge-shaped redbed units. The redbeds provide a basis for subdividing the group into 7 formations with a composite thickness of 12,000 ft. Simple structure, continuous exposures, and lithologic variability provide an excellent opportunity for paleoenvironmental interpretations. The Difunta Group is largely the product of deltaic sedimentation in the Parras basin, a shallow embayment off the ancestral Gulf of Mexico. Major delta progradation was from southwest to northeast as shown by facies changes, thickness trends, and paleocurrent data. The redbed units are interpreted as delta-plain deposits that accumulated as lake and bay muds and silts and fluvial and distributary-channel sands. Redbeds are bounded by resistant gray blanket sandstones 20-60 ft thick that have sharp bases, scour-and-fill bedding, Ophiomorpha, and local mudstone, wood, and oyster clasts. These sandstones are interpreted as delta-front and delta-destructional deposits formed during progradation and retrogradation of the deltaic complex. Prodelta facies are gray sequences of either (1) burrowed mudstone or muddy sandstone, many beds of which have ball-and-pillow structure and a sparse molluscan fauna, or (2) interbedded graded sandstone and burrowed mudstone. The graded sandstone beds of the later facies were deposited by turbidity currents that were generated at the delta front probably by hyperpycnal flow. Patchlike carbonate banks up to 1,000 ft thick developed in the north during episodes of low terrigenous influx in this area. Bank deposits are chiefly micritic bioclastic rocks; boundstone is rare. End_of_Article - Last_Page 352------------