CORRELATING ORDOVICIAN CRATONIC SEQUENCES; SHOREWARD SEDIMENT CONDENSATION AND TRANSGRESSIVE SANDY FACIES IN THE ANCELL, PLATTEVILLE, AND GALENA GROUPS, MIDCONTINENT NORTH AMERICA

Although many Paleozoic cratonic sequences display sediment condensation or starvation in an offshore direction (especially in TSTs), examples from the Ancell and Platteville groups (upper Darriwilian-Sandbian, mid Whiterockian-Blackriveran) in the Iowa area show the opposite patterns. Depositional shallowing towards the Transcontinental Arch is evident in these Ordovician sequences, and some are characterized by dramatic stratal thinning in a shoreward direction. The upper Ancell Group (Glenwood Shale) in northern Iowa is only 1-2 m thick, but this unit correlates with strata southward in Illinois that are 30-130 m thick. The Platteville Group comprises four carbonate sequences, three of which include basal transgressive reworked quartz sand units in central Iowa and northern Missouri. Platteville strata are 100-200 m thick in southern Illinois and Missouri, but correlative strata are less than 6 m thick across western Iowa. Locally in northern Iowa the entire Platteville succession is only 1.3 m thick, represented by sandstone, shale, and carbonate strata with multiple hardground surfaces. Greatly thinned shoreward sequences in the Ancell and Platteville groups imply significantly reduced sediment accumulation over vast areas of the cratonic seaway during part of the Sandbian. Standard sequence stratigraphic models are not directly applicable to these cratonic sequences. The overlying Galena Group carbonate and shale succession also displays reworked quartz sand (and phosphatic grains) at the transgressive bases of five sequences (within the Decorah and Dunleith formations), some of regional extent. Interbedded organic shales are recognized in all sequences of the Ancell, Platteville, and Galena groups, including the globally recognized GICE carbon isotope excursion (first identified in Iowa). Precise correlations of the Midcontinent succession to the international GSSP-defined stages (especially the graptolite-based Sandbian) have proven difficult. A more refined stratigraphic framework (litho-, bio-, chemo-, sequence) and detailed sea-level history are needed for improved inter-regional correlations of the Midcontinent Ordovician succession.