The shale preserves a unique ecosystem, the Winneshiek biota, which is among the most remarkable Ordovician lagerstätten in the United States.

The shale's depositional environment is reconstructed as a calm marine basin or estuary with an anoxic, low-pH seabed.

[2][1] The Winneshiek Shale has no radiometric dating and little overlapping fossil content with nearby formations, making precise age estimates difficult.

This may be correlated with the lower half of the mid-Darriwilian isotope carbon excursion (MDICE), a chemostratigraphic event observed worldwide.

The MDICE is preceded by the lower-Darriwilian negative isotope carbon excursion (LDNICE), which likely occurred at the same time as the Decorah impact.

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.

[7] 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.

[9] Other fossils include chelicerates,[3][10] bivalved crustaceans,[11][12] algae,[13] linguloid brachiopods,[1] a single gastropod specimen,[11] and head shields from armored agnathans (jawless fish).