UPPER ORDOVICIAN – LOWER SILURIAN CARBONATES FROM THE SOUTHWESTERN UNITED STATES INDICATE LONG-TERM GLOBAL COOLING BUT A SHORT-TERM GONDWANAN GLACIATION

The Hirnantian (latest Ordovician) glaciation is the first significant climatic perturbation of the Phanerozoic, and resulted in the second largest mass extinction in Earth history. Climatic perturbations driven by a Gondwanan glaciation are generally agreed upon as the cause of the end-Ordovician extinction, however, much is unknown about the climatic and oceanographic effects of the glaciation far from the southern hemisphere ice sheet, as well as the overall timing and span of climatic perturbations. The lower (Katian), middle (Hirnantian), and upper (Rhuddanian) members of the Upper Ordovician – Lower Silurian Ely Springs Dolomite of eastern California were examined in order to determine paleoceanographic conditions and variation across the interval of climatic and biotic instability via the temporal and spatial distribution of cool water carbonates and, as a result, gain a clearer picture of how southern hemisphere ice sheets affected a northwest-facing carbonate ramp on the continental margin of Laurentia. Results indicate a distinct, but shifting boundary between warm, shallow waters dominated by corals and oncoids, and chert-rich cool water carbonates dominated by sponges and crinoids. Cool water carbonates are geographically limited to outer ramp settings, while warm water carbonates are found in middle ramp and shallower environments. The distribution of cool water carbonates extends across much of the Katian Lower Member of the Ely Springs Dolomite in deeper water settings, suggesting that cold, deep ocean waters were present from at least the mid-Katian, and indicate an extended period of global cooling across the Late Ordovician. However, significant sea level changes within the Ely Springs Dolomite are limited to the middle member, implying that the Late Ordovician glaciation and associated sea level drop was limited to the Hirnantian. Trace element data from Willow Springs indicates episodes of vigorous upwelling 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 ramp localities may indicate a slow return warm conditions in the Early Silurian.