THE JURASSIC HUASTECA SERIES IN THE SUBSURFACE OF POZA RICA, EASTERN MEXICO

The Author has studied 120 cores containing ammonites and lamellibranchs of Middle and Late Jurassic ages selected from the 230 wells drilled in the Poze Rica district, eastern Mexico. The wells are spread over an area of 8,000 sq. km. The Huasteca Series is a marine sedimentary sequence that ranges in age from Bathonian to late Tithonian, and is about 1,000‐m thick. This unit is subdivided into ammonite zones and substages which have previously been established by the Author. These zones are: Wagnericeras, Kepplerites, Reineckeia, Discosphinctes, Ataxioceras, Idoceras, Virgatosphinctes, Mazapilites, Suarites, and Parodontoceras‐Salinites. The restricted vertical distribution of these fossils and the general availability of gamma‐ray logs from the wells has made it possible for the first time to delimit the lateral facies changes that took place, especially in the late Bathonian—middle Callovian (calcarenite‐shale‐silstone) and in the early Tithonian (calcarenite‐argillaceous limestone). Structural and isopach maps for each interval show the presence in the northern half of the study area of the NW‐SE striking La Mesa syncline, and, in the southern half of the area, the east‐dipping Sultepec homocline. The transgressive phase of Huasteca Series sedimentation began during Bathonian time in the NW part of this region. Evaporites and calcarenites were deposited above rocks of continental origin. This transgression advanced progressively during Callovian to Kimmeridgian time towards the central and eastern parts of the area, where the initial marine deposits overlie igneous, metamorphic, and continental sedimentary rocks. The last phase of this transgression took place during early Tithonian time in the SE part of the Poza Rica district. It is represented here by the San Andrés was deposited on the same types of rocks—igneous, metamorphic, and continental‐sedimentary. Remnants of this late transgressive phase, characterized by calcarenites and deposited on continental sediments, are preserved in very limited areas NE of the Poza Rica district. Beginning in Kimmeridgian time, two well‐defined depocentres developed. The Huizotate Basin, with argillaceous limestones and shales of the Tamán Formation, formed in the central part of the district, while the Papantla Platform with calcarenite units of the San Andrés Member (Tamán Formation) formed around the northern, eastern, and SE margins. These depocentres are named here for the first time An important result of this study is that it explains the presence in structural traps of hydrocarbons of Middle and Late Jurassic ages