GEOLOGIC SETTING OF THE WINNESHIEK LAGERSTATTE - DECORAH, IOWA

The Middle Ordovician (Whiterockian) Winneshiek Lagerstätte is found within the provisionally named “Winneshiek Shale” which lies disconformably beneath the Tonti Member of the St. Peter Sandstone at Decorah, Iowa. The 60 to 120 ft. thick Lagerstätte-bearing shale is the uppermost facies of a 600+ ft. sedimentary succession that fills a 3.6 mile diameter circular basin of suspected meteorite impact origin. This thick sedimentary sequence differs substantially from pre-St. Peter incised-valley and paleokarst fills as typified by subsurface sections assigned to the Readstown or Kress members of the St. Peter.

Water well cuttings and two short cores comprise the samples available for study. Basin facies beneath the “Winneshiek Shale” consist of sandstone, conglomerate and breccia containing abundant angular clasts of Lower Ordovician Prairie du Chien Group dolomite and chert, and lesser amounts of Upper Cambrian glauconitic sandstone. A deep well near the basin edge provides evidence for normal faulting with 350 ft. of offset of the Jordan Sandstone. A second deep well, with gamma log, provides data to suggest that the Cambrian Wonewoc Sandstone forms the floor of the basin near its center; it also documents the presence of a marine shale facies containing eurypterid fragments and chirognathid conodonts in the lower part of the sedimentary fill.

The possible origin of the structure by a meteorite impact event is suggested by the general circular shape, the anomalous stratigraphy, and the presence of quartz grains in well cuttings and core that show planar microdeformation features that resemble shock-produced planar fractures (PFs) or cleavage similar to those found at the Rock Elm impact structure, Wisconsin. However, unlike Rock Elm, no zone of central uplift or rebound is apparent. We are continuing studies of this unusual structure, and especially of the quartz deformation features, to explore the possibilities of a second impact structure contemporaneous with Rock Elm, and the possible role of multiple impact events in the Great Ordovician Biodiversification Event.