A DETERMINATION OF THE TIMING AND CONSEQUENCES OF THE LATE ORDOVICIAN GLACIATION USING THE ELY SPRINGS DOLOMITE OF EAST CENTRAL CALIFORNIA

The Late Ordovician is characterized by an anomalous glacial episode that caused profound changes in global oceanic circulation. Although the Late Ordovician glaciation is well recognized from sedimentological observations, the timing/extent of glaciation is poorly understood, with two competing hypotheses: 1) Glaciation during the Late-Ordovician was short-lived (<1 my) and occurred during the Hirnantian (e.g., Brenchly et. al., 1994); and, 2) Glaciation during the Late Ordovician was long-lived (lasting ~10-35 my) and began during the Sandbian (e.g. Saltzman and Young, 2005). The Late Ordovician-Early Silurian Ely Springs Dolomite, deposited as part of a homoclinal carbonate ramp along the northwestern margin of Laurentia, may help determine the nature and timing of observed glaciation. Two inner to outer carbonate ramp settings were examined in south-central California, including the Nopah Range and Hard Scramble Nose (Talc City Hills). Evidence of prolonged glaciation during the Late Ordovician is expected to show pronounced sequence boundaries and cool-water carbonate grain associations related to the upwelling of cool waters (James, 1997) during enhanced thermohaline circulation (Wilde, 1991). Evidence of a short lived glaciation is expected to show smaller scale sea level fluctuations prior to the Hirnantian, along with cool-water carbonate assemblages limited to the uppermost Ordovician. Analysis of the Ely Springs Dolomite will shed light on the nature of the Late Ordovician glaciation and its effects on a carbonate ramp far from the locus of glaciation. Current findings from Talc City Hills and Nopah Range indicate high frequency, low magnitude carbonate parasequences, most likely as a result of a short glacial period during the Late Ordovician.