GRAIN SIZE FRACTIONATION OF DETRITAL ZIRCON IN AN ANCIENT DELTAIC ENVIRONMENT (LATE CRETACEOUS TRINIDAD SANDSTONE AND PIERRE SHALE, RATON BASIN, COLORADO): IMPLICATIONS FOR U-PB ZIRCON PROVENANCE STUDIES

The relative proportions of detrital zircon U-Pb age populations from sandstones are commonly used to distinguish clastic units with different provenances. A single, restricted grain size fraction is often used for such studies, typically very fine sand (63-125 microns), but the extent to which hydraulic processes fractionate detrital zircon as a function of grain size has not been well-defined. In the Raton Basin at Trinidad, Colorado, Late Cretaceous Trinidad Sandstone and Pierre Shale formed as delta front and prodelta deposits stacked together during a progradation process. Detrital zircon was separated with other heavy minerals (>2.9 g/cc) using bromoform from 20-63 micron-size coarse silt sieve fractions from the Trinidad Sandstone (TSF) and Pierre Shale (PSM), and the 63-125 micron, very fine sand from the Trinidad Sandstone (TSC). The zircon and other heavy minerals were mounted randomly in epoxy round mounts, to avoid grain selection bias, and the mineralogy was identified and mapped using an automated TESCAN Integrated Mineralogical Analyzer (TIMA) at Colorado School of Mines. U-Pb ages for the mounted zircon was measured at the Mineral Isotope Laser Laboratory (MILL) at Texas Tech University using a Nu Instruments AttoM magnetic sector ICP-MS and NWR193 excimer laser ablation system. The major detrital zircon U-Pb age populations in all 3 samples are <200 Ma (mostly ~70 and 160 Ma), 1300-1500 Ma and 1600-1800 Ma, which is consistent with a common source but the proportions of zircon ages are different: 21:15:37% in TSC, 26:7:36% in TSF and 28:9:29% in PSM. This suggests that there is significant hydraulic fractionation of zircon as a function of grain size in sandstone-shale deltaic environments. Thus, in order to make quantitative estimates of the entire zircon source contributions to deltaic clastic sequences, both sand and silt sized zircon grain fractions should be analysed from constituent sandstone and shale units.