EVIDENCE FOR EARLY ARAGONITE IN PALEO-LACUSTRINE SEDIMENTS

Distinct pseudomorphs of displacive pseudohexagonal aragonite crystals are found in organic-rich lake deposits of the rift basins of the eas t coast of North America (Triassic-Jurassic). These crystals developed while the sediments were relatively uncompacted and soft, and hence a re displacive, readily mistaken for molds and casts of preexisting hal ite and/or thinolite (ikaite). A second form of early aragonite is fou nd in more evaporative sediments of the Newark Basin (Passaic Formatio n), developed as pseudomorphic replacement of precursor, more soluble minerals such as glauberite and/or gypsum that formed during depositio n. In all instances the aragonite is now re placed by calcite and dolo mite, but still preserves much of the original and unusual crystal for m and associated internal structure, This aragonite is probably formed due to release of CO2 by bacterial breakdown of organic matter in bot tom sediments. Elevated Mg/Ca in the pore waters may also have promote d preferential formation of aragonite over calcite.