NEW INSIGHTS on DEFORMATION STRUCTURES OF THE CHICONTEPEC FORMATION AND THEIR POSSIBLE RELATION WITH TERTIARY OIL ACCUMULATION IN NE HIDALGO, MEXICO

The Chicontepec Formation, in East Central Mexico, is an Early Paleocene to Early Eocene linear turbiditic belt with oil and gas reservoirs, currently exploited in at least 29 oil fields. The Chicontepec turbidites represent deep water sedimentary rocks, initially deposited in a NNW-SSE trending basin adjacent to the Sierra Madre Oriental fold and thrust belt. The turbiditic deposits are characterized by thick- to thin-bedded sandstones and calcareous sandstones, marls, and shales, with abundant sedimentary structures and bioturbation. The Chicontepec Formation unconformably overlies the lowermost Paleocene Velasco Formation and Upper Cretaceous sediments, and it is locally overlaid by Miocene to Pliocene volcanic rocks.

Our structural investigations reveal that thrust faults and folds generate a predominant northwest to southeast trending structural grain in the Chicontepec Formation, in the NE area of the state of Hidalgo. Fault-bend folds, kinks, and imbricated faults affecting the turbiditic deposits are consistent with a southwest-to-northeast direction of tectonic transport (N48E trend). Slickenfibers occur on bedding surfaces of apparently undeformed sequences and are also consistent with a predominatly southwest to northeast subhorizontal displacement over shallow dipping planes; additionally, discrete vertical up displacements (60 - 80 cm) occur over short ramps.

Preliminary results show that migration and subsequent accumulation of organic matter occur in zones subparallel to bedding, close to the surfaces with slickens; this suggests that thrusting is, at least in part, responsible for the formation and accumulation of hydrocarbon deposits of Tertiary age in these rocks.

Although characterized by a different structural style, thrust-related structures in the Chicontepec Formation are coaxial with Laramide chevron folds of the Upper Cretacic Mendez Formation. Based on our observations, we interpret the thrusting event as a result of a mainly SW-NE shortening of the turbiditic sequence that likely occurred during a thin-skinned latest event of the Laramide Orogeny in NE Mexico.