The Upper Ordovician Maquoketa Group equivalent strata in Indiana and Ohio were part of a westward-thinning shale-dominant succession. Large amounts of fine-grained siliciclastics were shed from the eastern highlands during the Taconic Orogeny.
The detailed lithofacies analysis of the Upper Ordovician shales has yielded recognition of a series of genetically related sequences in a seemingly homogenous succession. The lower succession is pyritic laminated/banded organic-rich mudstone that accumulated after the onset of a major flooding event. Cryptobioturbation, bottom current ripples, graded silt/clay couplets and well preserved benthic fossils indicate an oxygen-depleted dysoxic condition. In addition, layers enriched in phosphatic fossils, phosphate and pyritic grains appear to mark flooding surfaces and sediment starvation. The maximum organic-matter enrichment mainly occurred within black homogenized mudstone in the middle succession. Upsection, more extensive bioturbation and carbonate production are observed. The intermittent yet frequent wave and current activity, suggested by cross-lamination, wavy-lenticular stratification and hummocky cross stratification, indicate a shallower and proximal settings with enhanced sediment influx.
The deposition of the Upper Ordovician shales in the Maquoketa Group reflects a complex interplay between storms, sediment supply, and eustatic sea-level changes. Nonetheless, with distinct characteristics of lithofacies, wireline logs, and organic carbon isotope data, a high-resolution sequence stratigraphic framework of the Upper Ordovician shales can be compiled for the study area. The whole studied interval comprises an entire 3rd order sequence, wherein the lower part appears to be a transgressive systems tract and the remaining overlying strata represent a highstand systems tract. This project is an example how integration of sedimentological observations, geophysical data, petrographical and geochemical data enable a better understanding of the accumulation of this mudstone succession in a regional sequence stratigraphic context.
|Commitee:||Basu, Abhijit, Rupp, John A.|
|School Location:||United States -- Indiana|
|Source:||MAI 55/06M(E), Masters Abstracts International|
|Keywords:||Depositional environment, Facies, Geochemistry, Maquoketa group, Ordovician, Shale|
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