A PALEOCEANOGRAPHIC ANALYSIS OF THE ELY SPRINGS DOLOMITE, EAST-CENTRAL CALIFORNIA: CONSTRAINING THE DURATION OF THE HIRNANTIAN GLACIATION

In order to better understand the mechanics behind modern climatic change, other periods in Earth history when rapid temperature shifts had a substantial effect on Earth’s biota need to be examined. The End-Ordovician mass extinction likely resulted from climatic cooling associated with Hirnantian (latest Ordovician) glaciation, which invigorated ocean circulation and altered ocean chemistry far from the ice sheet. Examination of the Ely Springs Dolomite (east-central California) provides a means to study the effects of shifts in ocean circulation on Earth’s tropical biota during an interglacial period and constrain the duration of the Hirnantian glaciation by determining how long changes in ocean circulation persisted. The Ely Springs Dolomite was examined at two localities; Willow Spring Canyon (Mazourka Canyon), CA and Talc City Hills, CA. At Talc City Hills, the Ely Springs Dolomite contains abundant crinoid columnals scattered throughout the unit, rugose corals near the base, and storm deposits consisting of crinoid hash beds near the Ordovician-Silurian boundary. The change in fauna is indicative of a shift from warm, tropical waters to cool waters that were introduced to the shelf via upwelling. At Willow Spring Canyon, the formation is divided into three informal members: an Upper Ordovician member consisting of chert-rich dolostone; a middle, lowermost Silurian member consisting of massive dolostone; and an upper, Lower Silurian member consisting of interbedded cherty dolostone and thick chert beds. This sequence records a drop in sea level followed by a relative rise in sea level caused by the glaciation and deglaciation of Gondwana, while the chert-rich nature of the unit is indicative of vigorous ocean circulation, upwelling, and high primary productivity. Initial trace element geochemistry and lithologic analysis of the Ely Springs Dolomite suggest that upwelling and high primary productivity were present during deposition of much of the formation, demonstrating that vigorous ocean circulation was persistent and driven by a long-lived Gondwanan glaciation.