Lithologies and Diagenesis of the Lacustrine Sediments of the Lockatong Formation (Upper Triassic) in the Newark Rift Basin
Mohamed El Tabakh, B. Charlotte Schreiber, 1994. "Lithologies and Diagenesis of the Lacustrine Sediments of the Lockatong Formation (Upper Triassic) in the Newark Rift Basin", Lacustrine Reservoirs and Depositional Systems, Anthony J. Lomando, B. Charlotte Schreiber, Paul M. Harris
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The Lockatong Formation lacustrine sequences are the most extensive Upper Triassic deep-lake deposits exposed in eastern North America. Sedimentation took place within tropical lakes in a developing rift basin. These lacustrine deposits and their sedimentary structures exhibit a generally upward-shallowing sedimentary pattern and follow an internal repetitive pattern of cyclicity. The sequences suggest that lake depth responded to continuing changes in climatic conditions, although the basinal bounding faults were also active during sedimentation and may have had some general control. The identified lithologies formed in these lakes are black and gray shales, carbonate-rich mudstones, siltstones, and evaporite-bearing mudstones. The authigenic minerals present are analcime, dolomite, and calcite.
Petrographic and electron microprobe investigations suggest that (1) analcime formed both as a direct chemical precipitate from lake and groundwaters and as an early diagenetic mineral, due to the early alteration of original clay minerals by the increasingly concentrated alkaline waters; (2) the lacustrine calcites are primary and/or early diagenetic; and (3) dolomite also was formed penecontemporaneously during evaporative concentration of shallow groundwaters. These observations are supported by carbon and oxygen isotopic ratios of the carbonates taken from major sections of the Lockatong Formation, and show that variations in the stable isotope ratios closely match the lithological alternations of the Lockatong Formation. The sedimentary succession shows an overall upward trend of increased evaporation, upsection increase in evaporite development, and, with this, a concomitant enrichment in δ18O. The δ13C isotope ratio, on the other hand, remains quite constant throughout the section, having a value appropriate to lacustrine deposition.
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Lacustrine Reservoirs and Depositional Systems
Lacustrine depositional systems are intriguing from sedimentologic, stratigraphic, and paleoclimatic perspectives. In localizing hydrocarbon source and reservoir rocks and also valuable evaporate minerals, they are also extremely important from an economic viewpoint. Lakes are dynamic systems that are susceptible to fluctuations in climate. They are also highly variable in their tectonic and depositional settings. The papers included in this volume, although not a comprehensive collection, provide coverage of a broad spectrum of modern and ancient lacustrine examples.