SEQUENCE STRATIGRAPHY, CHEMOSTRATIGRAPHY, AND BIOSTRATIGRAPHY OF LOWER ORDOVICIAN UNITS IN NORTHEASTERN AND WESTERN CENTRAL UTAH: REGIONAL IMPLICATIONS

The Lower Ordovician Garden City Formation and Pogonip Group, located within western and northeastern Utah, respectively, provide a unique opportunity to examine the effects of eustasy, sedimentation, and subsidence rates in two adjacent shelf basins on the western margin of Laurentia. The Garden City Formation primarily consists of argillaceous limestones while the Pogonip Group contains a diverse suite of lithologies (carbonates to siliciclastics). These rock units have been interpreted as roughly coeval and have been provisionally correlated using biostratigraphy. Detailed correlation of these very different rock units is critical in order to understand how these packages have responded to the same external forcing function (eustasy). Correlation is being attempted through chemostratigraphy (carbon/oxygen isotopes and total organic carbon), biostratigraphy (trilobite and conodont), and sequence stratigraphy. Preliminary chemostratigraphic and biostratigraphic data indicates a correlation between two positive carbon isotope excursions and trilobite extinctions near the Skullrockian/Stairsian Stage boundary (Early Tremadocian). Matching time equivalent significant surfaces generated by eustatic oscillations has been more challenging. For example, eight third-order sequences are observed within the Pogonip Group, whereas sequence expression within the Garden City Formation is greatly subdued. By combining detailed chemostratigraphic and biostratigraphic correlation with the strongly expressed sequence stratigraphic framework of the Pogonip Group, the more subtle sequence stratigraphic architecture of the Garden City Formation may be revealed.