--> ABSTRACT: Paleotectonic Controls on Deposition of Upper Upper Jurassic La Casita Formation, East-Central Chihuahua, Mexico, by David C. Roberts; #91022 (1989)

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Paleotectonic Controls on Deposition of Upper Upper Jurassic La Casita Formation, East-Central Chihuahua, Mexico

David C. Roberts

Surface mapping of the basal Mesozoic La Casita Formation (upper Upper Jurassic) in east-central Chihuahua, Mexico, indicates initial Mesozoic sedimentation occurred in a segmented, interconnected subbasin of the Chihuahua trough.

La Casita Formation (1,200 m thick) is a tectonostratigraphic unit resting with angular unconformity on the Lower Permian Plomosas Formation. It consists primarily of siliciclastic material with sporadic interbedded limestones. The dominant lithofacies, approximately 1,000 m thick, consists of turbiditic sandstone units (10-20 m) alternating with thicker, monotonous shale sequences.

In the mapped area (approximately 30 km2), flute cast measurements indicate flows from both the northeast (N20°E) and southwest (S58°W). Turbiditic sandstone units appear to pinch out and/or interfinger as they extend from the north and south into the central portion of the area.

The initial opening of the Chihuahua trough is often associated with Late Jurassic block faulting, related to development of the ancestral Gulf of Mexico. Synrift depositional sequences of a similar age have been described in southern Coahuila, northern Zacatecas, and western Chiapas, Mexico.

The subbasin (graben?) examined here may be ascribed a paleoposition near the western edge of the early Chihuahua trough. The western boundary of the early trough may have comprised a series of these subbasins, forming a cuspate or serrated coastline. Late Jurassic ammonites recovered from this and other localities along the length of the Chihuahua trough suggest that the subbasins were interconnected by means of an eastern continuous seaway.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.