Depositional history of the Upper Ordovician Trenton Limestone, Lexington Limestone, Maquoketa shale and equivalent lithologic units in the Illinois Basin: An application of carbonate and mixed carbonate-siliciclastic sequence stratigraphy

The Upper Ordovician Trenton Limestone, Lexington Limestone, Maquoketa Group, and equivalent lithologic units in the Illinois Basin present a distinctive stratigraphic section notable for its complex assemblage of carbonates and siliciclastics. Ambiguous stratigraphic relationships between the carbonate-dominated part of the section and the siliciclastic-dominated part of the section have greatly hindered the understanding of the section's depositional history. A regional sequence stratigraphic framework of unconformities and marine-flooding surfaces was constructed to clarify these stratigraphic relationships and to develop a better understanding of Upper Ordovician depositional history.; Sequence stratigraphic relationships indicate the Trenton-Lexington-Maquoketa section can be subdivided into four shallow-water carbonate platforms (Decorah, Trenton, Lower Lexington, and Upper Lexington Platforms) and a basin that was filled initially with a mixture of deep-water siliciclastics and carbonates (Lower Maquoketa Basin) which were followed by only siliciclastics (Upper Maquoketa Basin). The depositional history of these deposits was a complex interplay of carbonate and siliciclastic deposition with time that recorded a six-stage collapse of an extensive carbonate shelf into a foredeep basin.; The carbonate shelf that characterized deposition prior to collapse was the Black River Platform, which is typified by the widespread pure carbonate mudstone that underlies the Trenton-Lexington-Maquoketa section. The first stage of collapse was subsidence and the resulting accumulation of Decorah Platform shaly carbonates upon the Black River Platform mudstone. The second stage of collapse was the tilting and subsidence of both the Black River and Decorah Platforms. Transgression over the ramp created by the tilting deposited the pure carbonates of the Trenton Platform. The third stage of collapse was continued subsidence and the resulting deposition of shaly carbonates of the Lower Lexington Platform along with the introduction of siliciclastics and carbonates representing Lower Maquoketa Basin foredeep deposits. The fourth stage of collapse was subsidence that resulted in the deposition of Upper Lexington Platform shaly carbonates along with widespread, additional Lower Maquoketa Basin foredeep siliciclastics and carbonates. The fifth stage was continued subsidence and deposition that resulted in additional Lower Maquoketa Basin foredeep siliciclastics and carbonates. The sixth stage was subsidence that resulted in the deposition of shale and silty sandstone that characterized Upper Maquoketa Basin foredeep deposition.