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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 66 (1982)

Issue: 9. (September)

First Page: 1429

Last Page: 1429

Title: Upper Jurassic-Lower Cretaceous Fan-Delta Complex--La Casita Formation of Saltillo Area, Coahuila, Mexico: ABSTRACT

Author(s): K. S. Fortunato, W. C. Ward

Article Type: Meeting abstract

Abstract:

The La Casita Formation represents a major influx of terrigenous sediment on the epicontinental shelf of northeastern Mexico during the Late Jurassic and earliest Cretaceous. Near Saltillo, the La Casita can be divided into three major units: (1) a basal unit of carbonaceous siltstone and mudstone, (2) a middle unit which is predominantly sandstone and pebble conglomerate, and (3) an upper unit of siltstone and fine-grained sandstone with thin limestone and dolomite layers becoming more abundant upward, and grading into the overlying Taraises Formation. The La Casita-Taraises terrigenous sequence is underlain and overlain by shallow-marine limestones.

In the Saltillo area, the La Casita crops out in narrow canyons in breached anticlines of the Sierra Madre Oriental. Consequently, interpretations concerning the depositional framework of the La Casita must be made from vertical sequences exposed in widely spaced canyons. A first-order, single-dependence Markov analysis aids in identifying preferred vertical transitions in lithologically variable parts of the section.

Important aspects of the La Casita in the Saltillo area include: (1) the large-scale depositional sequence of basal shallow-marine mudstones overlain by shallow-marine and alluvial sandstone, conglomerate, and mudstone, which are in turn, overlain by shallow-marine, fine-grained sandstone, mudstone, and carbonate rocks; (2) conglomeratic shallow-marine sediment; (3) textural and mineralogical immaturity; (4) the nonrandom nature of upward lithofacies transitions; and (5) a predominance of coarsening-upward sequences in the middle unit. These characteristics suggest that the La Casita in the Saltillo area records the progradation of a complex of fan deltas.

The lower, fine-grained unit represents Late Jurassic "prodelta" deposits that accumulated on the submarine shelf prior to the influx of coarse sediment. The middle, coarse-grained unit was deposited during the period of maximum seaward advance of the fan-delta complex during latest Jurassic time. This unit contains predominantly coastal and shallow-shelf conglomerate, sandstone, and mudstone, with some distal alluvial-fan conglomerate and sandstone. The upper La Casita records the waning input of coarse sediment as the fan-delta system retreated in earliest Cretaceous time.

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