THE PALEOGEOGRAPHIC DISTRIBUTION OF COLD WATER CARBONATES IN LATE ORDOVICIAN SEDIMENTARY ROCKS OF THE SOUTHWESTERN UNITED STATES

The Hirnantian (latest Ordovician) glaciation represents the first major Phanerozoic climatic perturbation, and also resulted the second largest mass extinction in Earth history. While the cause of the end-Ordovician crisis is well established to be the result of climatic disturbances related to the advance and retreat of Gondwanan glaciers, the climatic and oceanographic effects of the glaciation far afield from the ice sheet are poorly understood, as is the overall duration of the climatic event. The Upper Ordovician – Lower Silurian (Katian – Rhuddanian) Ely Springs Dolomite of eastern California was examined in order to determine the paleogeographic and temporal distribution of cold water and warm water carbonates and, as a result, gain a clearer picture of how southern hemisphere ice sheets affected a northwestern-facing carbonate ramp on the continental margin of Laurentia. Results indicate a distinct, but shifting boundary between warm, shallow waters dominated by rugose and tabulate corals, as well as Girvanella oncoids, and cherty cold water carbonates dominated by sponges and crinoids. Cold water carbonates are limited in geographic extent to outer shelf settings (Inyo Mountains, CA localities including Badger Flat, Tinemaha Reservoir and Willow Springs Canyon), while warm water carbonates are found in middle ramp and shallower environments (Nopah Range, CA), with intermediate localities exhibiting alternations between warm water and cold water carbonates (Talc City Hills, CA). The temporal distribution of cold water carbonates within outer shelf localities in the Inyo Mountains, CA stretches across much of the Katian Lower Member of the Ely Springs Dolomite, suggesting that cold, deep ocean waters were present from at least the mid-Katian, and support long-term global cooling across the Late Ordovician. Trace elemental data from the Willow Springs, CA, indicates periodic vigorous upwelling and enhanced organic matter burial associated with well-oxygenated bottom waters during deposition of the Hirnantian-aged Middle Member that may be the result of cycles of glacial advance and retreat. Chert is less common in the Rhuddanian (lowermost Silurian) Upper Member, but its presence in outer shelf localities may indicate a slow return to global warmth following the Hirnantian glaciation.