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Paleo-Canyons and Contemporaneous Oil Seeps Near the Paleocene/Eocene Boundary, Tampico-Misantla Basin, Eastern Mexico


At least five paleo-canyons have been identified on the western flank of the Tampico-Misantla Basin in eastern Mexico. These were formed at ~56 Ma (During Wilcox Formation deposition in the deepwater Gulf of Mexico) when fluvial systems eroded into the subaerially exposed, unconsolidated Paleocene bathyal sediments during an extremely large fall in sea-level. Previous workers had not identified the Lower Eocene sediments filling the canyons because of ubiquitous, reworked Upper Paleocene foraminifera and nannofossils. Our experience shows that detrital zircon analysis and palynology provide the most accurate depositional age control for the canyon-fill sequences. Following the rapid drop in relative sea-level, a rapid rise provided the accommodation space for the deposition of hundreds of meters of coarse-grained, conglomeratic, and mud-rich canyon-fill sediments. The northern (Acatepec) canyon was filled with more than 200 m of slumps, pebbly-mudstones, channel sandstones, and thin-bedded channel-levee sediments. The upper part of the sequence also contains several paleo-karsted intervals which may indicate additional large relative sea-level fluctuations. The San Lorenzo paleo-canyon contains a mass transport deposit at its base and overlies over 600 m of bathyal sediments. We have also identified two bitumen beds in outcrop, both of which overlie the ~56 ma unconformity when the canyons formed. One of the beds overlies and preserves evidence of a deep desiccation crack (80 cm deep) in the underlying bathyal sediments, on the once-exposed off-axis areas of the terrestrial canyon. Over 15 well penetrations of bitumen beds up to 11 meters thick have been identified in the subsurface of the basin, one of which is cored. These multiple occurrences of bitumen beds are tied to the large drop in the Gulf of Mexico water level near the Paleocene/Eocene boundary which would have decreased the lithostatic pressure gradient and resulted in a breach of deep hydrocarbon traps in the basin. The pre-erosion and canyon-fill sediments were all deposited in a bathyal environment, begging the question of what caused the rapid fall and rise of relative sea-level. The Paleocene Eocene Thermal Maximum (PETM) has not yet been identified in the outcrops, but Wheeler diagram reconstructions indicate that it should be present near the canyon bases, overlying the regional unconformity on which the bitumen beds were deposited.