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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 63 (1979)

Issue: 3. (March)

First Page: 547

Last Page: 548

Title: Lower Cretaceous Carbonate Shelf in Southeastern Arizona and Northeastern Sonora, Mexico: ABSTRACT

Author(s): E. Robert Warzeski

Article Type: Meeting abstract


The Mural Limestone (Bisbee Group, Lower Cretaceous) in southeast Arizona and northeastern Sonora exposes a broad carbonate shelf and an irregular, migrating shelf margin at the northwest end of the Chihuahua trough. The upper Mural represents the culmination and initial regressive phases of an Aptian-Albian transgression. It is underlain by nearshore clastic strata and limestones of the lower Mural and the Morita Formations, and overlain by clastic beds of the Cintura Formation. Study of the narrow outcrop belt from the Mule Mountains in Arizona to where the Mural disappears beneath Quaternary volcanics, 70 km to the south in Sonora, reveals an overall pattern of southward-deepening water. The upper Mural thickens from 50 to nearly 300 m over this distance. Facies presen in Arizona are, from north to south; (a) shallow lagoonal packstones and wackestones; (b) a broad oolite and pelletoid-sand shoal; and (c) a muddy, open shelf with small, isolated reefs in waters at least 10 m deep. In Mexico,

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larger isolated reefs give way southward to reef banks 5 to 8 km across, surrounded by mudstones and wackestones containing pelagic microfossils. Shallow-water deposits of the southernmost bank "step" progressively southeastward over deeper water limestones.

Reef cores in most of this belt consist of platy, branching megacolonies of the coral Microsolena, encrusted by thick, laminated stromatolites. In the north, rudists are abundant only on reef flanks and caps, and in shallow interreef and backreef areas. Rudists increase in abundance on reef banks to the south, as do various encrusting and head-forming algae. Thoroughly bound rudist-coral-algal frames dominate the southernmost reefs.

Lagoonal limestones of the early regressive phase overlie the reef interval, and are punctuated in the south by the thick sandstone and shale wedge. Nearshore and continental clastic units of the Cintura Formation ended carbonate deposition in this area.

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