The Winneshiek Shale is a thin and geologically homogeneous package of dark grey to greenish-brown sandy
shale. Drill core data has estimated a maximum thickness of 38 meters, This characteristic was not initially recognized, and the Winneshiek Shale was first believed to be a subunit of the St. Peter Sandstone.
Age and chemostratigraphy The Winneshiek Shale has no
radiometric dating and little overlapping fossil content with nearby formations, making precise age estimates difficult. The overlying St. Peter Sandstone is firmly late
Darriwilian in age based on its
conodont fauna. The Winneshiek Shale shares only a few taxa with other formations, namely
Multioistodus subdentatus and
Archeognathus primus. Both of these conodonts were originally known from the mid- to upper-Darriwilian
Dutchtown Formation of
Missouri. The mid-Darriwilian global stage corresponds to the late
Whiterockian regional stage in North American
biostratigraphy. Conodont material similar to
Iowagnathus grandis is also known from
Siberia. The correlation between the MDICE and the positive excursion found in the Winneshiek Shale has been questioned. Small organic carbon isotope excursions may be influenced by local environmental or ecological conditions, rather than worldwide events. Moreover, numerous excursions occur in every time period, so there is no reason to assume that the Winneshiek record is specifically correlated with the MDICE and LDNICE. The connections between the Decorah impact, other impacts worldwide, and the GOBE have also been criticized for their reliance on imprecise age estimates. Criticisms of the correlation between the MDICE and the Winneshiek excursion have been countered with the argument that alternative explanations have no direct evidence within the strata.
Multioistodus subdentatus was presented as a mid-Darriwilian fossil supporting the Winneshiek Shale's biostratigraphic connection to areas with similar chemostratigraphy, despite the lack of direct biostratigraphic overlap. == Paleobiota ==