Sedimentary Features and Depositional Setting of the Ordovician Maquoketa Group in Indiana
The late Ordovician Maquoketa Groups is a shale-rich succession that was deposited in a shallow epicontinental sea, shed westward from the eastern Taconic highlands. The shales vary in carbon content and constitute an important source rock in the Illinois Basin and adjacent areas. Three drill cores were examined in Indiana along a SW-NE transect to document sedimentary features, internal stratigraphy, and lateral variability. The Maquoketa Group consists of interbedded silty mudstones and limestones, with thin interbeds of coarser-grained material (silt, sand, pebbles up to 5 mm). Shale-dominant intervals are typically bioturbated to the point that primary sedimentary structures have been largely destroyed. Intervals with alternating gray and black shale beds show escape burrows and traces of sediment swimmers (mantle and swirl "burrows"), suggesting water rich sediments and event deposition of cm- to dm-thick shale beds. Intermittent erosion is indicated by coarse clastic lags, shell hash beds, and knife-sharp boundaries between shale beds. At least some of these probably represent sequence boundaries, whereas intervals with abundant phosphate nodules can probably be interpreted as maximum flooding horizons. The carbonaceous shales of the Maquoketa Group were probably deposited in largely dysaerobic to intermittently aerobic setting. Notwithstanding the pervasiveness of bioturbation, the dark gray to black color of the majority of shale intervals relates to substantial quantities of buried organic matter (up to 8%). Whereas for various other Paleozoic source rock intervals anoxic bottom water conditions were suggested as a cause of organic matter preservation, such a scenario is not applicable to the Maquoketa Group.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009