Stratigraphy and sedimentology of the upper Aptian–upper Albian Mural Limestone (Bisbee Group) in northern Sonora, Mexic

Abstract

The Cretaceous Mural Limestone in north-central Sonora, Mexico, is similar in stratigraphic succession and lithofacies to correlative strata of the Bisbee Group in the southwestern USA. It consists of 800–900 m of shallow-marine limestone and shale that range in age from late Aptian to mid-late Albian. Carbonate lithologies generally consist of oyster-bearing wackestone–packstone and rudistid boundstone, the latter forming discontinuous, mound-like bioherms. Siliciclastic intervals are dominated by dark gray to black shale with ammonoids and trigoniids. The Mural Limestone in north-central Sonora consists of six formal members, defined here. In ascending order, these are the Cerro La Ceja, Tuape Shale, Los Coyotes, Cerro La Puerta, Cerro La Espina and Mesa Quemada members. Based on ammonoids, trigoniids, and benthic foraminifers, the lower two members are late Aptian and the upper four members range from latest Aptian–mid-late Albian. The marine strata of the Mural Limestone separate two continental red bed successions that are difficult to differentiate on the basis of lithology and so provide an important marker in the Lower Cretaceous section of Sonora. Consistent stratigraphic successions throughout northern Sonora correlate with similar successions in the southwestern USA and northeastern Mexico, well beyond the limits of the Bisbee Basin, suggesting that global eustasy played an important role in deposition of these strata. Unusually thick Aptian shales of the Mural Limestone probably accumulated during post-rift tectonic subsidence, which enhanced the thickness of these marine-shelfal deposits. Consistent, predictable stratigraphy in the Mural Limestone across northern Sonora and the USA–Mexico border region likewise indicates that depositional models involving deep-marine gravitational emplacement of shallow-water facies proposed by some workers are untenable.