Paleocanyons in the subsurface of eastern Mexico: Facts and uncertainties

The misuse of stratigraphic information from oil wells in eastern Mexico generated the concept of the Chicontepec paleocanyon (Busch and Govela, 1975, 1978). Its original definition, a paleogeographic entity parallel to the Gulf coastline in the subsurface of the Poza Rica district, Veracruz, was based on the use of petrophysical logs for correlation of stratigraphic sections. The methods used to define the Chicontepec paleocanyon are questioned here, and its existence at the original proposed location is rejected on the basis of the following criteria: (1) the unconformity surface does not reveal the absence of lithologic formations; (2) the correlated stratigraphic sections lack accurate chronostratigraphic control; (3) Paleocene-Eocene lateral facies changes are erroneously considered unconformities; (4) the so-called reference horizon, Horizon C (top lower Eocene), for the stratigraphic correlation is not documented with lithostratigraphic and biostratigraphic data; and (5) the marks on check-shot points used to determine the contact between the Paleocene-lower Eocene formations are not accurate. Two other areas, however, do contain identifiable paleocanyons: (1) The San Andrés paleocanyon is a paleogeographic province that lies perpendicular to the Gulf coastline, southeast Poza Rica district, Veracruz, eastern Mexico. This paleocanyon is characterized by the absence of upper Tithonian to Maestrichtian strata, and lower Tithonian rocks are unconformably overlain by Paleocene strata. (2) The Bejuco paleocanyon is another such feature that lies perpendicular to the Gulf coastline, south of Tampico, eastern Mexico. This paleocanyon is characterized by the absence of Cretaceous to Paleocene rocks, and Lower Cretaceous rocks are unconformably overlain by Eocene strata. The San Andrés and Bejuco paleocanyons are contemporaneous, with similar morphology and depth. In these two paleocanyons, hydrocarbon reservoirs are confined to the Upper Jurassic and Lower Cretaceous rocks. The structural nature of the crystalline basement influenced the configuration of the paleocanyons. In fact, successive crustal movements of igneous and metamorphic blocks seemed to have impeded the consolidation of the Jurassic and Cretaceous sedimentary columns in the San Andrés paleocanyon, southeast Poza Rica, and in the Bejuco paleocanyon, south of Tampico, eastern Mexico.