Abstract

The Parras and La Popa basins of Coahuila and Nuevo Leon, Mexico, have a composite thickness of about 5.4 km of paralic sedimentary rock (Parras Formation and Difunta Group) deposited during Late Cretaceous to Paleocene time. Detritus of sedimentary, volcanic, and hypabyssal rocks eroded from a highland to the west and southwest of the Parras basin and northwest of the La Popa basin was deposited in deltaic, delta-flank, strand-plain, and marine-shelf environments. Progradation and retrogradation of deltas in each basin in response to basin subsidence and variable sediment influx resulted in a complicated intertonguing of rock units. Using wedge-shaped red-bed bodies and sandstone marker-beds, the Difunta Group is divided into nine formations in the Parras basin and five formations in the La Popa basin. The correlation of stratigraphic units between the basins is uncertain, because only one formation (Muerto Formation) can be mapped across the basin margins, and diagnostic fossils are absent. Fence diagrams and isopach maps document the geometry of the major stratigraphic units.

The clastic basin-fill was deformed during a post-Paleocene (Laramide) event that formed the main folds of the Sierra Madre Oriental. Both tight and broad folds, thrust faults of small displacement, and minor tear faults characterize the deformation within the study area.

The relations of the stratigraphic units, the geometry of the units, and the internal anatomy of the units suggest that the Parras and La Popa basins acted as subsiding elements on the craton and were filled by deltaic and shelf sediments. The basins were not sites of linear geosynclinal deposition as suggested by some workers, and characterization of the basins as foredeeps is undesirable because of the ambiguous meaning of the term.

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