CHRONOSTRATIGRAPHICALLY CALIBRATED SEQUENCE STRATIGRAPHY CLARIFIES RESOURCE ASSESSMENT OF THE ILLINOIS BASIN MAQUOKETA GROUP

The Upper Ordovician Maquoketa Group (450–440 Ma) in the Illinois Basin contains untapped phosphate rare earth elements (REE) resources and locally shows potential as an unconventional oil and gas play. Further, regional interest in the Maquoketa Group is in its properties as an aquitard, both for groundwater studies of the overlying Silurian bedrock aquifer and as a secondary seal in carbon sequestration within the underlying Knox Group. However, quantifying reserves and targeting development areas is impeded by spatial heterogeneity of unknown origin. Sequence stratigraphic analysis of the Maquoketa Group, integrating a strong component of new bio- and chemostratigraphic study, is resolving this uncertainty.

Sampling targeted key sections across the Illinois Basin region from Wisconsin to Iowa, Arkansas, Illinois, and Indiana. New graptolite and chitinozoan results suggest regional diachroneity within this shale-dominated succession. Details of this diachroneity are provided by stable carbon isotope stratigraphy displaying positive and negative excursions and offsets at unconformity surfaces. Built upon this temporal foundation, facies analysis, including pXRF elemental analysis, provides assessment of shifting composition in relation to basin geometry, eustasy, and ocean chemistry. Based on these results, the Maquoketa Group is divisible into multiple 3^rd-order (1–3 m.y.) depositional sequences, each containing nested 4^th-order (100–1000 k.y.) sequences that laterally replace one another from the margins to the center of the basin.

The resulting chronostratigraphic framework for the Maquoketa Group in the Illinois Basin demonstrates that REE and total organic carbon concentrations are specific to time-rock units reflecting environmental collapse at the end of the Ordovician. Seal characterization for both groundwater and carbon sequestration studies requires calibration of petrofacies to the chronostratigraphic framework as the young, shale-dominated facies of the basin center differ markedly from the older, mixed carbonate-siliciclastics of the basin margin areas.