BIO-, CHEMO- AND SEQUENCE STRATIGRAPHY OF LOWER ORDOVICIAN UNITS FROM DIFFERING DEPOSITIONAL SETTINGS ALONG THE WESTERN MARGIN OF NORTHERN LAURENTIA

The Ordovician Garden City Formation, located within the Northern Utah Basin of northeastern Utah, and the Pogonip Group, located within the Ibex Basin of west central Utah, are coeval units deposited under differing depositional settings. The Pogonip Group is characterized by higher subsidence rates and greater clastic input. A common age is indicated by similar conodont faunas at the base and the occurrence of clastic sands at the top of the units. Detailed correlation between these units is difficult, however, due to differences in thicknesses and lithologies. The Garden City Fm, which consists of two informal members (a lower intraformational conglomerate and upper cherty members), has a thickness which ranges from approximately 366 to 549 meters. The Garden City Fm is a generally continuous succession of fine-grained carbonates with interspersed thrombolite bioherm horizons and storm horizons which are evidenced by large intraclasts and hummocky cross-strata. The Pogonip Group, which consists of six units (the House Limestone, Fillmore Fm, Wah Wah Fm, Juab Limestone, Kanosh Shale and Lehman Fm) has an approximate thickness of 1,014 meters. The Pogonip Group contains alternating thick shales and carbonate units, the latter contain abundant hardgrounds with encrusting fauna. Eight third-order sequences are observed within the Pogonip Group whereas sequence expression within the Garden City Fm is greatly subdued. To enable detailed correlations between these units, trilobite and conodont biostratigraphy, sequence stratigraphy and stable isotope stratigraphy were employed. Presented for the first time are carbon and oxygen stable isotope ratio data for the Garden City Fm. Preliminary data show a δ¹³C_PDB range between -1.83 and 0.16‰ and a δ¹⁸O_PDB range between -12.58 and -8.46‰. These data suggest a correlation between a positive carbon stable isotope excursion and trilobite extinctions occurring at or near the Skullrockian/Stairsian Stage boundary during the early Tremadocian.