MAQUOKETA GROUP HOLOSTRATIGRAPHY: INTEGRATION OF BIO-, CHEMO- AND LITHOSTRATIGRAPHY (UPPER ORDOVICIAN, MIDCONTINENT, USA)

The Upper Ordovician shale-dominated Maquoketa Group in the Illinois Basin lacks detailed chronostratigraphic division due to a dearth of biostratigraphic studies and a general perception that this interval is dominated by long-ranging and relatively rare zonal taxa. However, recent advances in biostratigraphy and chemostratigraphy and access to hundreds of archived subsurface drill cores across the US midcontinent has significantly advanced our chronostratigraphic understanding of this interval. Integration of this new chronostratigraphy with facies analyses suggests compositional changes within the Maquoketa Group that reflect regional diachroneity.

This collaborative research has analyzed the Maquoketa Group and equivalent units in a series of continuous cores and outcrops across and surrounding the Illinois Basin. Hundreds of chitinozoan samples containing tens of thousands of identified chitinozoans, belonging to more than fifty species, have significantly refined the biostratigraphy of this interval. Importantly, this effort documents significant changes in chitinozoan relative abundance and extends the known geographic range of key zonal taxa. Conodont and graptolite samples are likewise producing exciting new collections. Chemostratigraphic results from samples collected for stable carbon isotope and elemental analysis (via portable X-ray fluorescence) number in the tens of thousands and have identified several correlatable features. Facies analysis within this new chronostratigraphic framework defines a range of paleoenvironments, from peritidal to deep marine and documents the stratigraphic and spatial relationships between units.

While refining Maquoketa Group depositional history remains a work in progress, several important conclusions can now be made: 1) the Maquoketa Group distribution within the Illinois Basin is demonstrably diachronous, 2) this complex stratigraphic architecture likely reflects major sea level oscillation associated with the Late Ordovician glaciation and 3) the Maquoketa Group contains previously unrecognized latest Katian and Hirnantian-age units. Excitingly, chronostratigraphically correlated, these sections contain one of the most temporally continuous and well-preserved Late-Ordovician records on the planet.