EVOLUTION OF LATE TRIASSIC RIFT BASIN EVAPORITES (PASSAIC FORMATION) - NEWARK BASIN, EASTERN NORTH-AMERICA

The Passaic Formation of the late Triassic Newark Supergroup is 2700 m thick and was deposited in series of wide, deep to shallow lacustrine environments in the Newark rift basin (eastern North America). The Pa ssaic Formation can be divided into lower, middle, and upper sections based on depositional structures, composition and the distribution and morphology of its evaporites. Evaporites formed as a result of syndia genetic cementation and/or displacive processes. Evaporitive minerals now include gypsum and anhydrite, although other mineral species, such as glauberite, may have originally existed. Most of the evaporites of the Passaic Formation occur within massive red mudstone and siltstone lithologies in the form of diffuse cements, void-fillings, euhedral c rystals, crystal clusters and nodules. These evaporites grew displaciv ely within the fine siliciclastic matrix as a result of changes in the hydrochemical regimes of the rift basin. A well-developed upward incr ease in the amount of evaporite material is present in the Passaic For mation. This resulted from: (1) long-term, progressive increase in ari dity, and (2) significant increase in evaporation surface area of the basin during its tectonic evolution. A nonmarine source for the evapor ites is evident from the isotopic data. Sulphate delta(34)S ranges fro m 11 parts per thousand to 3.3 parts per thousand CDT, while delta(18) O ranges from +15.1 parts per thousand to +20.9 parts per thousand SMO W, indicating derivation from early diagenetic oxidation of organic su lphur and pyrite within the organic-rich, lacustrine deposits. The Sr- 87/Sr-86 ratios in sulphate are radiogenic (average 0.71211), showing the interaction of basin waters with detrital components and that the Newark Basin was isolated from the world ocean. Most of the original e vaporites show evidence of diagenetic change to polycrystalline and po lymineralic pseudomorphs now filled with recrystallized coarse-grained anhydrite (1-3 mm size) and low-temperature albite. Homogenization te mperatures of fluid inclusions within the coarse-grained anhydrite ind icate crystallization temperatures for anhydrite in the range of 150 d egrees to 280 degrees C. Such elevated temperatures resulted from circ ulation of hot water in the basin. Later exhumation of these rocks cau sed partial to total replacement of anhydrite by gypsum in the upper p art of the section. The resulting increase in volume due to hydration of anhydrite at shallow depths also emplaced non-evaporative satin-spa r veins (fibrous gypsum) along bedding planes and in fractures. While the local geology of the Newark rift basin controlled the distribution of facies, the sedimentological development of the Passaic Formation evaporites resulted from the world-wide climatic aridity that prevaile d during the late Triassic.