Distinct pseudomorphs of displacive pseudohexagonal aragonite crystals are found in organic-rich lake deposits of the rift basins of the east coast of North America (Triassic-Jurassic). These crystals developed while the sediments were relatively uncompacted and soft, and hence are displacive, readily mistaken for molds and casts of pre-existing halite and/or thinolite (ikaite). A second form of early aragonite is found in more evaporative sediments of the Newark Basin (Passaic Formation), developed as pseudomorphic replacement of precursor, more soluble minerals such as glauberite and/or gypsum that formed during deposition. In all instances the aragonite is now replaced by calcite and dolomite, but still preserves much of the original and unusual crystal form and associated internal structure. This aragonite is probably formed due to release of CO 2 by bacterial breakdown of organic matter in bottom sediments. Elevated Mg/Ca in the pore waters may also have promoted preferential formation of aragonite over calcite.