The Chicontepec Formation, Onshore Eastern Mexico: Compelling Evidence for the Eocene Gulf of Mexico Drawdown Theory

摘要

A detailed biostratigraphic analysis and stratigraphic framework of the Paleocene and Eocene Chicontepec Formation in the Tampico-Misantla Basin, onshore eastern Mexico was conducted in 2012 using 33 wells. This study enabled a better understanding of the geological history of the this basin and how it fits into the history of the entire Gulf of Mexico. The regional stratigraphic framework is defined by five sequence boundaries at 65.5 Ma, 60.4 Ma, 54 Ma, 46 Ma, and 38.1 Ma. A major depositional change occurs at the 54 Ma sequence boundary (SB), prior to which a single canyon feeder system (Tanlajas Canyon) in the northwest fed turbidites southeastward into the elongate basin. Prior to SB 54, the Tampico-Misantla Basin was much more elongate and the classic "Chicontepec Canyon" did not exist because the Paleocene paleocurrents continue to show a southeast trend, even in the southernmost outcrops of the basin. Using the composite standard developed from the 2012 biostratigraphic study, a unique 6-inch thick coal bed in the Chicontepec is dated as occurring between 54.9 Ma and 53.8 Ma. This coal is immediately underlain and overlain by upper bathyal turbidites, and can only be explained by a major, rapid lowering of sea level on the order of 2000 meters as described by others. This is postulated to have occurred when the Gulf of Mexico was isolated from the world's oceans as Cuba docked against the Florida/Bahama platform. Once isolation took place, evaporation caused sea level to lower and at about 54.9 Ma at least 5 fluvial systems started draining directly into the basin from the southwest, cutting several canyons into the lithified Paleocene turbidites. At about 53.8 Ma, a breach of the Cuban barrier flooded the Gulf of Mexico and the Chicontepec Basin and drowned the fluvially-cut valleys, preserving what is now called the "Chicontepec Canyon." This relatively rapid sea-level rise created accommodation space and allowed five lobate submarine fans to develop and prograde into the basin for a period of about 8 Ma. At about 46.3 Ma, the authors believe a second isolation of the Gulf of Mexico occurred allowing evaporation to lower sea level again, this time for a shorter period of about 500,000 years. The fluvial systems again fed directly into the Gulf of Mexico and deepened the already existing fluvial valley. The timing of this event correlates well with an observed unconformity in Cuba (based on communication with James Pindell). At about 45.7 Ma, the Cuban barrier was again breached, causing sea level to rise rapidly and allowing the lobate submarine fan systems in the Chicontepec Basin to re-establish themselves in exactly the same location as before.