WIDESPREAD LATE ORDOVICIAN SILICIFICATION EVENT ON LAURENTIA: A PROXY FOR THE DURATION OF GONDWANA GLACIATION?

Late Middle to Late Ordovician bedded carbonate, chert and phosphate deposits of the U.S. Cordillera, southern Laurentia and the North American Midcontinent are unique in the Paleozoic because they represent the influx of cool oceanic waters, oftentimes hundreds of kilometers onto the interior of this continent. Late Middle to Late Ordovician subtidal ramp carbonates of New Mexico, Texas and Oklahoma contain abundant organic-rich chert, biogenic chert, phosphate and glauconite indicating these rocks formed in an extensive upwelling zone. Upwelling began in the Late Middle Ordovician (~454 Ma) and persisted until the end of the Ordovician. Late Ordovician cherty carbonates also occur along the U.S. Cordilleran margin, lying inboard of organic-rich graptolitic shale and chert. The widespread occurrence of Late Ordovician cherty and phosphatic carbonates on Laurentia, in addition to phosphate-rich, cool water carbonates over much of the North American Midcontinent suggests vigorous thermohaline circulation. Abundant Late Ordovician bedded chert deposits and the dearth of these units in the Middle Ordovician and during the Early Silurian suggests a global climatic or oceanographic origin for these deposits. The abundant evidence of widespread upwelling on Laurentia during the Late Middle to Late Ordovician fits well with recent oceanographic computer modelling indicating enhanced equatorial transfer of oceanic heat during the Late Ordovician glaciation. The initiation of upwelling in the Late Middle Ordovician also corresponds with cool (13-19° C) surface waters in the Appalachian Basin, a northward expansion of cool water trilobite faunas in North America, a shift to cool water benthic faunas across eastern North America, and the initiation of glaciogenic deposits in Africa. Thus, widespread upwelling around Laurentia over an ~14 Ma period suggests vigorous thermohaline circulation, that was likely driven by a prolonged glaciation on Gondwana.