UNRAVELING THE ORDOVICIAN-SILURIAN BOUNDARY INTERVAL IN THE UPPER MIDWEST—INTEGRATED FACIES ANALYSIS AND HIGH-RESOLUTION CARBON ISOTOPE STRATIGRAPHY IN A SEQUENCE STRATIGRAPHIC FRAMEWORK

Many sequence stratigraphic studies are hampered by biostratigraphic zonations with low temporal resolution or intervals lacking zonal index fossils. Integration with high-resolution (cm-scale) δ¹³C_carb stratigraphy provides a means to greatly refine these chronostratigraphic hypotheses through comparison with global trends and recognition of temporal gaps manifest as flat-line offsets and negative "spikes".

Late Katian- (Richmondian) through Hirnantian-age strata in the upper Midwest contain a complex succession of facies and macrofaunas that record multiple environmental perturbations in the run-up to the end Ordovician extinction. The succession begins with a regional karstic unconformity developed on Galena Group (Edenian) carbonates and abrupt flat-line offset in δ¹³C values. It is overlain by dark gray to black shales, which record at least three depositional sequence (Scales and Fort Atkinson formations) and fluctuating δ¹³C values. In the middle part of the Maquoketa Group dark gray shale facies transition rapidly upward into green, purple and red calcareous shales (Brainard Formation) as δ¹³C values fall to a more negative baseline. A sharp, Thalassinoides-burrowed sequence boundary marks the contact with a thick oolitic ironstone (Neda Formation) associated with a large negative 4‰ excursion. This unit is cut by a sequence boundary underlying evaporite-bearing greenish-gray carbonates, which display a return to flat-line δ¹³C trend. This Richmondian-age succession is variably overlain and laterally cross-cut by a late Hirnantian-age high-relief sequence boundary and incised valley fill composed of dark gray to black argillaceous carbonates (Wilhelmi Formation) showing a +2‰ flat-line offset in δ¹³C values.

The integration of facies analysis and δ¹³C_carb stratigraphy in this study reveals several late Katian-age (Richmondian) depositional sequences previously unrecognized in the Midcontinent and elsewhere. This stratigraphic architecture is predicted by the carbon isotope record—highly complex where δ¹³C _{values are} relatively positive and erratic (dark gray rocks), simple in those intervals with relatively negative and stable values (green and red rocks)—suggesting a direct link to carbon burial and associated climate change.