THE WINNESHIEK SHALE AND THE DECORAH IMPACT STRUCTURE: WINDOWS INTO THE SAUK – TIPPECANOE MEGASEQUENCE BOUNDARY

The mid-Ordovician (Darriwilian) Winneshiek Lagerstätte in northeast Iowa has opened a spectacular window documenting a non-shelly fauna of a special marine environment. The shale dominated Winneshiek Lagerstätte is characterized by its highly unusual fossil composition, extraordinary fossil preservation including tissue and soft bodies, and its unusual geologic setting of being preserved within the 5.6 km diameter Decorah meteorite impact structure. Investigations of the Winneshiek fauna and the Decorah impact structure have not only extended our knowledge of the mid-Ordovician biota, but also revealed the potential to reconstruct a portion of the missing geologic record of the late Tremadocian through middle Darriwilian in the inboard region of the cratonic interior, northern midwest U.S.A.

The Sauk – Tippecanoe megasequence boundary represents a major stratigraphic gap or hiatus of approximately 20 m.y. of record in the Midwest including Iowa, Minnesota, and Wisconsin (Sloss, 1963, 1988; Taylor et al., 2012). In Iowa, the missing geologic record can be reconstructed in part from the studies of the Winneshiek fauna and the deposits preserved in the Decorah impact structure. In addition to the Winneshiek, similar shaly deposits and coeval fossils, including conodonts, eurypterids, and phyllocarids, have also been reported from the Ames impact structure in Oklahoma (Repetski, 1997) and the Rock Elm impact structure in Wisconsin (Peters et al., 2002). These data indicate that a similar marine environment with special faunas was likely widespread in this region during this time interval. Therefore, it becomes feasible to reconstruct a portion of the geologic history omitted by the Sauk – Tippecanoe megasequence boundary in the Midwest by studies of the strata and fossils preserved in these craters. Among the mentioned three structures, the Decorah crater preserves the most complete and accessible crater-fill deposits and abundant well-preserved fossils although a crater modeling (French et al., 2004, 2008) suggests that the Decorah crater also experienced approximately 300 m of erosion. Combined with information from other impact structures, these crater-fill materials can provide direct geologic and paleontologic evidence concerning regional earth history reconstruction.