PALEOENVIRONMENTAL RECONSTRUCTION OF AN UPPER TRIASSIC FOSSIL SITE BASED ON EVIDENCE FROM CALCIUM CARBONATE NODULES

Analysis of calcium carbonate nodules can provide important clues to the early diagenesis of a rare archosaur fossil bed from the Upper Triassic Chinle Formation of Southern Utah. Nodules are associated with a nearly complete articulated Poposaurus gracilis and an unidentified crocodylomorph in a siltstone overlying the basal Shinarump Conglomerate. In thin section, radiating calcite crystals exhibit extinction under cross-polarized light. Quartz crystals from the surrounding matrix are incorporated into the nodules. Where nodules have not been weathered out of the matrix, minerals near the edge of the nodule align concentrically. Nodules are generally 85% calcite and 10% quartz, whereas the surrounding matrix is less than 50% calcite and over 25% quartz according to QEMSCAN and XRD analyses. Quartz crystals incorporated into the nodules are reasonable evidence for displacement by calcite of softer clay minerals in the surrounding sediment. Nodules formed in place and could be considered concretions due to periodic growth visible in thin section. These rings could be linked to seasonal monsoonal cycles that dominated deposition of the basal Chinle. Because crystal morphology is controlled largely by ion concentration of the precipitating solution, further analysis of nodules and detailed stratigraphy will help determine the paleoenvironment responsible for the preservation of these unique fossils.