LOCATING THE CARNIAN–NORIAN BOUNDARY IN THE MARTIN BRIDGE LIMESTONE, BLUE MOUNTAINS PROVINCE, OREGON: INITIAL RESULTS OF CONODONT BIOCHRONOLOGY AND STABLE ISOTOPE GEOCHEMISTRY

The Late Triassic period is one of the most poorly constrained time intervals in the Phanerozoic, lacking both a robust numerical chronology, and a well-defined stable isotope stratigraphy. Work is underway now to define a Global Stratotype Section and Point (GSSP) in British Columbia (Black Bear Ridge) or Italy (Pizzo Mondello) for the Carnian-Norian Boundary (CNB) using both halobiid clam and conodont biostratigraphy. We present new data from an age correlative section of the Martin Bridge Formation in the southern Wallowa Mountains of northeastern Oregon. The type locality of the Martin Bridge Fm. is an excellent location to further define the biostratigraphy and stable isotope stratigraphy of this important time interval, with implication for debate over a “long” or “short” Norian Stage. Lithologies consist predominantly of thin-bedded, laminated limestone interpreted to represent deep-marine calcilutite turbidity flow deposits interbedded with calcareous to carbonaceous Halobia-rich shale. Preliminary analysis of conodont elements, halobiid bivalves, and ammonoids collected near the base of the type section suggest an Upper Carnian age. We preliminarily identify Quadralella carpathicus, Q. nodosus, and Parapetella. n. sp. Q on the basis of comparison to conodont elements from the possible GSSP section at Black Bear Ridge. ­­­Ammonoids found in the lower half of the section include Tropites cf. T. nevadanus and Tropiceltites? sp.. Halobiid clams from the basal half of the type locality represent Halobia oregonensis. Age assignments for samples near the top of the section are underway. Whole-rock samples for C and O stable isotope analysis have been collected at the decimeter-scale across 30-m of strata. Initial results indicate a robust carbon-isotope signature using 350–500 µg of powdered material. Existing global stable isotope data appear to show a consistent trend in δ ¹³C starting at ~1‰ at the base of the Carnian, towards more positive values over most of Carnian time. By early Norian time values stabilize around ~4‰, a trend that has been attributed to a long-term increase in global organic carbon burial rates. Results of this study may provide a high-resolution record of stable isotope stratigraphy across the CNB and may help refine an important global correlation tool for Late Triassic time.