MIDDLE-UPPER DEVONIAN (LATE GIVETIAN-EARLY FAMENNIAN) CONODONT SEQUENCE IN THE SUBSURFACE OF SOUTHEASTERN IOWA: STRATIGRAPHIC CONDENSATION, LEAKS AND SUBMARINE EROSIONAL REWORKING ALONG THE WESTERN MARGIN OF THE ILLINOIS BASIN

In this study the conodont sequence is outlined as documented from continuous sampling at intervals ranging from 3 to 30 cm from the lower 80 samples taken through the upper part of the Cedar Valley Group (Coralville and Lithograph City fms.), “Sweetland Creek Shale” (=new un-named member of the Lime Creek Fm.), and Grassy Creek Shale in IGS Sullivan Slough Core from southeastern Iowa. In other well documented core (IGS IPSCO #3) and quarry sections in eastern Iowa, the Coralville Formation features conodonts of the Late Givetian Upper Icriodus subterminus Fauna. Conodonts from the Coralville in the Sullivan core include Pandorinellina insita indicating a stratigraphic leak of species of the P. insita Fauna into burrows or paleokarst cavities in the Coralville from the overlying Lithograph City Formation. Most of the overlying Andalusia Member of the Lithograph City yields shallow water conodonts of the latest Givetian-Early Frasnian P. insita Fauna, with Ancyrodella alata of Montagne Noire (M.N.) Zones 3 to 4 in the upper Andalusia, succeeded by faunas of M.N. Zone 4 in the Buffalo Heights Member. One upper Andalusia sample features mixed faunas of Early Frasnian M.N. Zones 4 and 10-11, evidence of a significant stratigraphic leak of Late Frasnian conodonts into paleokarst cavities in Early Frasnian carbonates (analogous to Independence Shale Leaks in eastern and central Iowa). Five feet of overlying hemipelagic carbonate mudstones and calcareous mudstone (“Sweetland Creek Shale") yields a highly condensed Late Frasnian sequence spanning M.N. zones 11 to 12, with Zone 13A faunas in the lower 1.8 feet of the Grassy Creek Shale. A cryptic disconformity in the Grassy Creek is indicated by faunas of the Middle triangularis Zone immediately above strata with M.N. Zone 13A faunas, evidence of submarine erosion of an interval spanning M.N. Zones 13B-13C and the Lower triangularis Zone. The overlying five feet (1.5 meters) of the Grassy Creek yield insitu Middle triangularis Zone faunas. Mixed faunas of M.N. Zone 13B are mixed with Middle triangularis Zone taxa in a sample associated with a pyritic lag in the lower Grassy Creek, and is evidence of a second submarine erosional horizon where very Late Frasnian conodonts were eroded from some other location, transported and admixed with insitu Early Famennian taxa.