Abstract

Stratigraphic and petrologic studies, supported by geologic mapping north of the San Marcos fault (central Coahuila), indicate that as much as 3,000 m of coarse Upper Jurassic and Lower Cretaceous detritus was derived from south of the fault. Two coarse Jurassic submarine conglomerate units (Las Palomas and Sierra El Granizo beds of the La Casita Formation) reflect separate episodes of displacement. Younger Jurassic eolianite and marine sandstone (Colorado and Tanque Cuatro Palmas beds of the La Casita Formation) record subsequent (Tithonian) fault inactivity. Neocomian rejuvenation of the fault produced alluvial deposits (San Marcos Formation); finer-grained, paralic or marine strata that are distal and suprajacent, and manifest the same faulting, are the La Mula Formation. Interruption of Neocomian displacement is indicated by a tongue of carbonate (Padilla and Barril Viejo Formations) that divides the San Marcos into upper and lower members.

The principal source of this detritus was the wildflysch of the late Paleozoic, arcproximal Las Delicias basin and the plutonic rocks that cut it. Rb-Sr ages on clasts and from outcrops of granitic rocks are Triassic and probably reflect late-stage magmatism within the Las Delicias arc. Although this arc supplied most of the detritus to the Las Delicias basin, Devonian ages on some clasts reveal that an unknown source area lay somewhere nearby during the late part of the Paleozoic Era.

The San Marcos fault may have served as part of the system of transform faults that connected sea-floor-spreading ridges of the Atlantic with those of the Pacific. If so, then the Jurassic units record periods of Jurassic activity and inactivity of that system. Neocomian displacment may have been caused by subsequent isostatic adjustment.

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