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Deltaic and Associated Deposits of Difunta Group (Late Cretaceous to Paleocene), Parras and La Popa Basins, Northeastern Mexico
Approximately 6,000 m of terrigenous sediment were deposited marginal to the ancestral Gulf of Mexico in the Parras and La Popa basins. Detritus, predominately volcanic, was derived from the west and fed by separate high-gradient rivers to each basin. Lobate deltas were constructed by the interplay between distributaries that supplied sand-and silt-rich sediment to a marine basin characterized by moderate waves and low tides. Four major cycles of delta progradation and marine transgression are recorded in each basin. Lateral progradation of at least 200 km is recorded in the Parras basin. Most deltaic facies are hundreds of meters thick because of the high rate of basin subsidence.
Delta-plain deposits are lenticular varicolored siltstone and claystone beds that include repetitious upward-coarsening and upward-fining lake deposits, locally including soil zones, that are interrupted by distributary-channel sandstone beds from 200 to 600 m wide and 3 to 10 m thick. Larger, multistory fluvial-distributary sandstones are present only locally.
Delta-front sheet sandstone units, 3 to 15 m thick and up to 20 km wide, are characterized by parallel laminae and lesser trough cross-beds, Ophiomorpha, and transported oysters.
Delta-platform deposits are characterized by bioturbated clayey siltstone beds with marine molluscs that are intercalated with laminated sandstone beds introduced by flood discharge, many of which foundered to form ball-and-pillow structures. Some sand flowed down the prodelta slope as turbidity currents to produce flysch-like interbeds of turbidites and bioturbated background mudstone beds that contain forminifers, Exogyra, and ammonites.
Delta-flank deposits include mudstone, oyster reefs, marine mud- and sand-flat deposits, and mixed terrigenous sand-oolite deposits.
Shelf deposits include rare, thin glauconite beds, phosphatic limestone beds, and 9 lenticular biostromes. The latter are up to 300 m thick and 19 km long, and are red-algal packstones that contain sparse rudists and other frame builders. Several biostromes are marginal to evaporite diapirs that may have been active during Late Cretaceous time.
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