Rocks Of Continental Origin Research Articles

Geochemical studies of shales from the Asu River Group, Lower Benue Trough: Implications for provenance and paleo-environment reconstruction

Combined use of trace elements, rare earth elements (REEs) with hydrogen (δH) and oxygen (δ18O) stable isotope composition to elucidate origin and paleo-environment-conditions and reconstruction is a contemporary trend in the field of geochemistry. Geochemical investigations were carried out on the shale deposit from five wells within the Asu River Group Formation, exposed at Ikwo, Lower Benue Trough (LBT). Results of trace elements for the shale deposits examined showed averages of Co (19.10 ppm), Th (16.50 ppm), Zn (103.73 ppm), Sr (203.71 ppm) and Zr (292.80 ppm) compared to PAAS and UCC. Observed negative Eu anomalies, enriched LREEs and depleted HREEs patterns have shown that the shales are from rocks of continental origin. This is provided by Al/(Al + Fe + Mn) with a value > 0.2. Plots of Zr/Sc vs Th/Sc, La/Th vs. Hf and Cr/V vs. Y/Ni indicated felsic igneous rock precursors for the shale samples. C-parameter, ratios of Rb/Sr and Sr/Cu signify paleo-climatic conditions of semi-humid to arid, Ba/Al showed low paleo-productivity of the basin during shale deposition, largely within the freshwater setting according to the ratio of Sr/Ba. V/(V + Ni), U/Th, Ni/Co ratios and Ce/Ce∗ anomalies have revealed an oxic depositional environment with Fe/Ti > 20 suggesting hydrothermal activity. The temperature of formation (120 °C–∼195 °C) coupled with δ18O (+17 to + 23‰) and δH (−46.90 to −38.80‰) is consistent with materials of sedimentary origin from chemically weathered felsic precursors under humid climatic conditions with an influence of hydrothermal activity.

Decompression and high-temperature–low-pressure metamorphism in the exhumed floor of an extensional basin, Alboran Sea, western Mediterranean

Leg 161 of the Ocean Drilling Program (ODP) has made a major contribution to our understanding of the origin of the Alboran Basin by demonstrating that it is underlain by rocks of continental origin that have undergone high-temperature metamorphism and melting at exceptionally low pressure after exhumation and decompression. Basement rocks recovered from Site 976 consist of high-grade schist and gneiss derived from aluminous sediments, and minor amounts of marble, granitic dikes, and migmatitic segregations of granitic material. Mineral assemblages and textural relations show that an early assemblage including biotite, garnet, staurolite, plagioclase, and rutile is overprinted by a second assemblage of biotite, sillimanite, plagioclase, potassium feldspar, and ilmenite. Both assemblages are overprinted by andalusite, potassium feldspar, and minor garnet. Migmatitic gneiss contains relict andalusite, overprinted by sillimanite and cordierite coexisting with granitic leucosome. Preliminary pressure-temperature estimates suggest that the metamorphic evolution followed an approximately isothermal decompression path from 7 to 3 kbar at temperatures in the range 580 to 630 °C. After decompression, granitic melts formed at 670 °C, after andalusite breakdown and within the sillimanite stability field. The cored rocks closely resemble high-grade metamorphic rocks in the adjacent Betic Cordillera of southern Spain, which yield early Miocene radiometric dates. At ODP Site 976 they are overlain by middle Miocene marine sediments. The combination of exhumation in an extensional tectonic environment and the evidence for high and increasing temperature during exhumation provide support for and new constraints on current models for the basin that involve the removal of lithospheric mantle below a zone of continental collision, accompanied or followed by extension.

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 timeAn important result of this study is that it explains the presence in structural traps of hydrocarbons of Middle and Late Jurassic ages