FLUVIAL LINKAGES FROM CALIFORNIA TO COLORADO IN THE MID-CRETACEOUS? PROVENANCE OF THE DAKOTA GROUP FROM DETRITAL ZIRCON U-PB GEOCHRONOLOGY AND CHERT PEBBLE GEOCHEMISTRY (Invited Presentation)

The existence of Early to mid-Cretaceous west-directed fluvial systems linking the Sierran arc to the Sevier foreland in Utah and Colorado is a prediction of recent tectonic models that bear on the evolution of the Basin and Range, Colorado Plateau, and central Rocky Mountains. The mid- Cretaceous Dakota Group contains intervals of sandstone and pebble conglomerate that would have been deposited contemporaneously with the Sevier Orogeny and which are exposed throughout south-central Colorado and northern New Mexico on the eastern edge of the Rocky Mountains. These sandstones, such as the Lytle Sandstone of the Purgatoire Formation, are typically inferred to have a fluvial origin, thus presenting an opportunity to assess sediment dispersal pathways during the Sevier Orogeny. Detrital zircon U-Pb geochronology of the Purgatoire Fm reveals the presence of ca. 100 Ma zircons (12% of 96 grains) that have slightly abraded, non-euhedral shapes, confirming an Albian-Cenomanian depositional age and Sierran source link. An additional feature of the Purgatoire Fm is the dominance of chert as pebbles in the conglomerates; these clearly are robust during sedimentary transport and, if individual chert sources can be differentiated, offer the potential to record the erosional history of the predominantly carbonate Cordilleran miogeocline succession. Chert pebbles from multiple sampling localities were analyzed in cut and polished slabs by handheld XRF and compared to both local chert sources (e.g., Mississippian in CO) as well as distal (Paleozoic) chert sources from Nevada and California. We used methods common in archeology, and previously adapted to geological studies (Reifenstuhl et al., 2009). The Dakota Group contains chert pebbles derived from relatively local sources, but also a component of Paleozoic chert pebbles, presumably sourced from the present-day Great Basin. These results highlight a potentially promising new provenance tool, complementary to zircon U-Pb geochronology, that appear to link the Dakota Group via important fluvial systems to source terranes in the Great Basin and Sierran arc in mid-Cretaceous time.