CALCRETES AND PALUSTRINE CARBONATES AS INDICATORS OF CLIMATIC SHIFT, LATE TRIASSIC AND EARLY CRETACEOUS OF THE SOUTHWESTERN US

Palustrine carbonates and calcretes have received much interest lately because of their inherent treasure troves of environmental information. Depositional facies of palustrine carbonates favor sub-humid conditions. Calcrete overprinting is common in palustrine carbonates and signifies periods of aridity interrupting palustrine deposition. Palustrine deposits of the Owl Rock Member (ORM), Chinle Formation document a change from sub-humid to more arid climate patterns during the late Triassic, whereas calcretes of the Cedar Mountain Formation (CMF) characterize a trend toward increasingly humid conditions during the Early Cretaceous. Calcrete overprinting increases upward in the ORM, as palustrine facies are lost up section to calcrete facies exhibiting strong evidence for subaerial exposure. These observations have led to the conclusion that sub-humid conditions persists through most of the ORM with an increase in aridity over the region leading to calcretization and desiccation of palustrine deposits. Three trace fossils in carbonates of the ORM indicate burrowing in soils, wetlands, or saturated palustrine deposits. Crayfish burrows extend approximately two meters below the surface of burrowing, suggesting crayfish burrow more deeply during periods of aridity. Adhesive meniscate burrows and Scoyenia isp. occur where soil moistures exceed ten percent, signifying that the trace makers burrowed during sub-humid climates. Occurrences of these trace fossils decreases up section in the ORM. In the lower CMF, groundwater calcretes dominate over pedogenic calcretes. Calcretes typically form in nodular, displacive forms in lower CMF mudstones, but grade upward to comprise coalesced nodule beds. This gradual change is seen in CMF sections throughout eastern Utah. Basal calcretes lack major desiccation features whereas up section calcretes display desiccation features at their top contacts. The lower calacretes lack root structures, pisoids or bioturbation. These data suggest major calcium mobilization in subsurface waters saturated with respect to bicarbonate. Phosphor imaging will determine concentrations of datable radiogenic materials in the calcretes. As the calcretes signify unconformities, the information used will constrain a terrestrial sequence stratigraphy for the CMF and ORM.