Abstract

Buda Limestone in northern Coahuila ranges in thickness from 15 to 35 m and consists mostly of lime wackestone and mudstone. Two major facies developed in the area in response to contrasting environmental conditions: a northern mudstone-wackestone facies composed mostly of benthic fossils deposited in water depths less than 100 m and a southern wackestone facies with abundant pelagic fossils deposited in water as deep as 500 m. A fossiliferous, intraclast packstone facies developed on shoal areas in adjacent trans-Pecos Texas. These shoals may have been related to areas of tectonic adjustment.

Subfacies developed in the north differ in intensity of bioturbation, fossil content, and amount of terrigenous clastic content. The lower and middle Buda contain a diverse benthic fauna, abundant burrows, and up to several percent quartz silt and clay in places. The upper Buda contains a benthic fauna low in diversity and number of specimens, rare burrows, and a lesser amount of terrigenous elastics.

Distribution of the two principal facies of the Buda in northern Coahuila conforms to late Albian paleogeographic features—the broad flat carbonate platform north of the Stuart City reef, and the deeper basinal area on the south. In the north, a broad low-relief shelf over the preexisting platform was characterized by open marine conditions although there is some evidence of restricted water circulation in the northeast part. The water was deep enough so that the sea floor was rarely disturbed by wave action. Normally clear waters were periodically muddied by an influx of terrigenous clay and silt. South of the preexisting Stuart City platform margin, waters were clear and deep.

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