Abstract
A 2,500 m thick section of Lower Cretaceous sedimentary rocks from the Lampazos region, east-central Sonora, is described. It was divided, for descriptive purposes, into seven informal formations, which, from base to top, are El Aliso, Agua Salada, Lampazos, Espinazo del Diablo, Los Picachos and its equivalent Nogal and La Mesa formations. Thin to medium bedded, with minor amount of shale and fine-grained sandstone, are characteristics of the El Aliso, Lampazos, Los Picachos and the member 1 of the Nogal formations, and their depositional environment is interpreted as restricted shallow marine (according to Rose, 1978). The main lithology from the lower half of the Agua Salada formation is well bedded, wavy black radiolarian chert containing ammonites (Parahoplites sp.), while black shale with nodular and thin bedded chert predominates in its upper half. Due to the presence of Dufrenoyia justinae and Hypacanthoplites sp., a late Aptian (Gargasian) age is considered for this unit. Download full text.
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