A detailed ~(87)SR/~(86)SR isotopic curve for the mid-Cincinnatian (Upper Katian-Lower Hirnantian, Upper Ordovician), NE North American Shelf (Ontario, Canada) transition to the Hirnantian glaciation

摘要

We examined the Sr-isotopic composition of one species of well-preserved brachiopod species from the mid-Cincinnatian (Upper Katian-Lower Hirnantian, Upper Ordovician) in closely spaced samples from one borehole on the western shelf of the Upper Ordovician Appalachian low latitude retroarc basin in southwestern Ontario. The section spans the transition from passive margin carbonate ramp through anoxic shales into foredeep orogenic clastics and encompasses the transition into the late Ordovician glaciation. The primary nature of the Sr-isotopic composition of the shells shows several stable Sr-isotopic excursions that refine the Upper Ordovician Sr-isotopic curve, and indicate several marked changes that may be related to the competing influences of island arc and ophiolite nappe emplacement and erosion, increasing continental crust erosion during the Taconic orogeny and the glacio-eustatic drop in sea-level related to the onset of the late Ordovician Hirnantian glaciation. These changes begin with low Sr isotope ratios (~. 0.7090) consistent with the overall trends but the ratios then change to much more positive, reaching high Sr isotope ratios (~. 0.7084) not elsewhere attained until the end Silurian. These and other minor fluctuations are attributed to periodic increases in Sr isotope ratios in the semi-isolated Taconic retroarc basin caused by rivers draining unroofing continental thrust sheets and earlier Ordovician carbonate cover rocks, with their higher Sr isotope ratios, together with increased continental erosion during regression associated with the onset of the main phase of the late Ordovician glaciation and possible glacial/interglacial phases.