DECORAH FORMATION OF THE UPPER MISSISSIPPI VALLEY: FACIES VERSUS KEY-BED CORRELATION

The Ordovician Decorah Formation is a mixed carbonate-shale package deposited in the midcontinent of North America during the sea level highstand belonging to Sloss’s Tippecanoe I Sequence. We examined the Decorah of the Upper Mississippi Valley region, specifically within the Hollandale Embayment, which lay between two topographic highs -- the Transcontinental Arch to the west and the Wisconsin Dome to the east. The formation consists of a basal shale-rich facies overlain by carbonate lithologies. Westward, toward the Transcontinental Arch, the shale facies is thicker and the carbonates are thinner. The traditional interpretation was that the carbonates fade out into coeval shales toward the clastic source. We constructed a NW-SE stratigraphic profile extending across the shale/carbonate facies change in order to place the Decorah within a sequence context and to re-interpret its depositional history. The profile is hung on a series of time-parallel event beds that are traceable regionally: four K-bentonites that can be identified via trace element fingerprinting, distinctive carbonate hardgrounds, gamma ray zones, a phosphate pebble horizon, an iron-ooid horizon, and epibole layers of the bryozoan Prasopora and the microfossil Gloeocapsomorpha prisca. These correlated event beds demonstrate that the Decorah is composed of two separate depositional packages. The lower package is shaly, thickening westward toward the source of clastic sediment (and freshwater influx, which produced a stratified, dysoxic water column). The upper package is dominated by carbonate sediments, which accumulated in a clear-water ramp setting on the flank of the Wisconsin Dome and thinned westward (downramp) into the Hollandale Embayment. These two packages thus form reciprocal wedges of sediment that are not time-equivalent. Evaluating the Decorah using a facies approach suggests lateral, interfingering facies changes between the carbonates and shales, whereas the high resolution key-bed correlation distinguishes separated ages for these facies with important biostratigraphic implications.