TRACKING AUTHIGENIC MINERAL CEMENTS IN RELATION TO LITHOLOGY AND PALEOENVIRONMENT: A CASE STUDY IN THE TRIASSIC ISCHIGUALASTO FORMATION OF NORTHWESTERN ARGENTINA

Numerous recent studies explore vertebrate fossil preservation in relation to the links between authigenic minerals and host facies. The Upper Triassic Ischigualasto Formation of northwestern Argentina yields abundant vertebrate fossils in a variety of lithofacies, and thus affords an opportunity to test for associations. The formation is subdivided into four members, each characterized by different alluvial architecture, different paleosols, and distinct taphonomy. In this study, we explore authigenic infill patterns in Ischigualasto fossil vertebrates from three of the four members using SEM-EDS and petrographic microscopy. The common authigenic minerals in Ischigualasto bones are calcite and hematite, similar to other Triassic assemblages in the region. However, these cements vary in relative abundance and are associated with other authigenic minerals in relation to specific paleoenvironments. For example, the La Peña Member is interpreted as deposits of a high sinuosity fluvial system with a poorly-drained floodplain. Fossil bones from this unit are characterized by authigenic apatite, abundant Fe-rich minerals and barite, and a second phase of calcite filling remnant voids. The fluvial Cancha de Bochas Member preserves evidence of increased aridity and seasonality (exemplified by Calcisols) and temporal mixing of fossils. Authigenic cements in this unit vary, although again secondary apatite is abundant, and is associated with calcite, along with thin linings of Fe-rich minerals and clay. Fluvial and floodplain facies of the overlying Valle de la Luna Member show evidence of increased rainfall, and preserve abundant volcaniclastics. Fossils from this unit exhibit evidence of advanced weathering and include deposits of Fe-rich minerals in voids. Moreover, many fossils from this unit are permineralized with silica, which presumably reflects volcaniclastic input. The suite of Ischigualasto fossils suggests that the diagenetic emplacement of authigenic minerals tracks regional patterns in climate. Fossil permineralization also appears to track local paleoenvironment, specifically in relation to volcaniclastic input and redox conditions.