HIGH-RESOLUTION CARBON AND OXYGEN STABLE ISOTOPE SAMPLING OF THE LOWER ORDOVICIAN POGONIP GROUP: IMPLICATIONS FOR CHEMO-STRATIGRAPHIC CORRELATION

High-resolution sampling of carbon and oxygen stable isotopes was performed across two known sequence boundaries in the Lower Ordovician House Limestone of the Pogonip Group, in the Ibex basin of west central Utah. Sampling was performed to test the utility of carbon and oxygen stable isotope chemo-stratigraphy in high resolution correlation. Sampling also enabled testing of whether shifts in δ¹³C and δ¹⁸O values could be used to identify subaerial exposure surfaces and/or freshwater influences across sequence boundaries, as observed in previous studies of Upper Ordovician rock units in the Nashville Dome in Tennessee. The carbon and oxygen stable isotopic record of the Ordovician Pogonip Group has been examined by multiple workers. These studies have shown that δ¹³C­_PDB and δ¹⁸O_PDB values range between -5.21 and 1.48‰ and -15.69 and -3.09‰, respectively. In addition, large magnitude positive and negative carbon stable isotope excursions found within the Pogonip Group isotopic record have been shown to correlate with other contemporaneous rock units, on both local and global levels. In the present study, four to five transects per sequence boundary were sampled vertically at 10 cm intervals and at 1 to 5 m intervals horizontally along stratigraphic horizons. Preliminary data suggest that shifts in carbon stable isotope ratios relating to subaerial exposure surfaces are not clearly evident within the House Limestone. However, the data show a surprisingly high amount of lateral variability in the δ¹³C and δ¹⁸O values across stratigraphic horizons at or near the sequence boundaries. Data collected along a single stratigraphic horizon demonstrate that the variability in δ¹³C and δ¹⁸O values can be as high as 2.83‰ and 3.34‰, respectively. Also, a previously-unidentified negative carbon stable isotope excursion of approximately 3.0‰ was revealed by high-resolution sampling just above a sequence boundary, this excursion did not appear laterally in all transects sampled. These findings suggest that carbon and oxygen isotope stratigraphies based on single, widely-spaced samples may be insufficient to properly characterize the carbon and oxygen isotopic signatures of rock units.