BRACHIOPOD SPECIES-LEVEL EXTINCTION IN A SEQUENCE STRATIGRAPHIC FRAMEWORK, MIDDLE ORDOVICIAN OF THE NORTHERN MISSISSIPPI VALLEY

Emerson et al. (2000) described the approximately synchronous disappearance of 26 brachiopod species, Mohawkian age, in the northern Mississippi Valley. This bioevent, which occurs at the top of the Decorah Formation, appears to correlate with an extinction recognized by Patzkowsky and Holland (1996) in the Appalachian Basin-Cincinnati Arch region (their M4-M5 contact). This bioevent is the subject of an ongoing investigation to determine the degree of synchronicity in species disappearances and to compare the brachiopod ranges with the sequence stratigraphy. The original recognition of the Decorah extinction was based on existing collections, for which range data were available only at the formation-level. New centimeter-scale collections have provided refined ranges. It is now clear that the disappearance of species at the top of the Decorah is not completely synchronous; some species disappear within the formation.

In addition, a sequence stratigraphic study of the Decorah has led to a new interpretation of its depositional history. Stratal geometries display a reciprocal facies pattern along a NW-SE cross-section. The shale-dominated lower Decorah (Spechts Ferry Member) thins southeastward (9 m in Rochester,MN to 2 m in Dickeyville,WI). The carbonate-dominated upper Decorah (Guttenberg and Ion Members) thickens southeastward (4 m in Rochester,MN to 10 m in Dickeyville,WI). We interpret this pattern to result from a water depth gradient, deepening westward toward the Twin Cities area from the Wisconsin Arch. During the Spechts Ferry cycle, fine clastics from the Transcontinental Arch were deposited in deep water, some in dysoxic conditions. These shales pinch out to the southeast against the Wisconsin Arch. Greater submergence in the next cycle (Guttenberg) provided for shallow carbonate deposition on the Wisconsin Arch (carbonates pinch out down ramp to the northwest).

By comparing the refined range chart with the new sequence interpretation, the timing and controls of the local extinction event can be assessed. The brachiopod species disappearances appear to be strongly facies dependant indicating a close link between environmental shifts and faunal changes.