The Lower Cretaceous marine strata in east-central Sonora are as much as 2,500 m thick and consist of six mappable lithostratigraphic units. This sequence is important not only for delineating the paleogeography of western Mexico, but also for correlating the sections in eastern Mexico with those in western Mexico and Baja California. The Lampazos sequence is near the boundary between the North American and Yaqui blocks, and the stratigraphy is clearly similar to that of the North American block.

Calcareous algae, benthic foraminifers, corals, and rudists described here complement the molluscan assemblage previously reported. Three species of algae are widespread...