Cerro del Pueblo Fm (Difunta Group, Upper Cretaceous), Parras Basin, southern Coahuila, Mexico:: reference sections, age, and correlation

Principal and supplementary reference sections provide data that clarify the stratigraphic relationships and depositional history of the Cerro del Pueblo Formation (CdP) - an important unit in Late Cretaceous paleobiogeographic studies in northeastern Mexico and the Western Interior of North America. At Saltillo, the CdP is 162 m thick, much thinner than previously reported. To the west, however, the CdP thickens to 449 m at Rincon Colorado (35 kin west of Saltillo) and 540 m at Porvenir de Jalpa (70 km west of Saltillo). A substantial stratigraphic interval of interbedded grey-green and red beds is present above the CdP throughout the field area; for consistency, it is assigned to the overlying Cerro Huerta Formation (CH). Westward-thickening of the CdP indicates an increased rate of subsidence and accommodation in that direction, and further suggests that sediment was supplied from the west along a narrow. east-west oriented trough that paralleled the modern Sierra Madre Oriental south of the field area. This trough was likely the location of an embayment in which CdP sediments first aggraded and then slowly prograded to the east and northeast. Overall, the CdP records deposition in very low gradient lower coastal plain frequency changes in relative sea-level and shallow marine (ramp) settings that were influenced by high-frequency changes in relative sea-level and coastal storm events. Magnetostratigraphic data from 66 samples collected in the Saltillo area indicate that the CdP was deposited it? magnetochronozones 32n.3r-32n.2n. This magnetostratigrophic interval falls within the combined Western Interior ammonite biozones of B. reesidei and B.jenseni, and suggests a maximum absolute age of 72.5 Ma for the CdP in the Saltillo area. The Campanian-Maastrichtian boundary (here accepted as 32n-31r) lies 90 m below the top of the overlying CH. A minimum sediment accumulation rate of 5 5 cm/1,000 yrs is proposed for the CdP.