Global chronostratigraphic correlation of the llandovery series (Silurian system) in Iowa, USA, using high-resolution carbon isotope (δ13ccarb) chemostratigraphy and brachiopod and conodont biostratigraphy

Abstract

Chronostratigraphic correlation of Silurian units in Iowa is complicated by complex carbonate depositional environments and poor biostratigraphic control. Carbon isotope (δ13Ccarb) chemostratigraphy, when integrated with the relatively sparse conodont data, provides an extremely useful chronostratigraphic tool. Here, we integrate conodont biostratigraphic data with carbon isotope chemostratigraphic data (δ13Ccarb) for the Llandovery of Iowa. Three carbon isotope excursions were recognized within the Hopkinton and Scotch Grove formations. The late Aeronian (sedgwickii graptolite zone) and likely a heretofore unrecorded Aeronian carbon isotope excursion were recorded from the Hopkinton Formation in the SS-10 Core (Jones County). The Valgu Excursion was recorded from the uppermost Hopkinton Formation through the overlying Buck Creek Quarry Member of the Scotch Grove Formation in the Garrison Core (Benton County). The integration of conodont biostratigraphic and carbon isotope chemostratigraphic data from the Silurian of Iowa allows for the first regional chronostratigraphic correlation of these strata at a resolution finer than stage level. The oxygen and carbon isotope values from the Garrison Core and the evidence for post-diagenetic karstification and fluid movement may provide further evidence that the dolomitization process of the LaPorte City Formation was halted by the influx of meteoric phreatic water.