Paper No. 4
Presentation Time: 2:15 PM
THE MIDDLE ORDOVICIAN WINNESHIEK SHALE OF NORTHEAST IOWA -- CORRELATION AND PALEOGEOGRAPHIC IMPLICATIONS
A recently identified circular basin (5.5 km diameter) of probable impact origin at Decorah, Iowa, disrupts Cambrian and Lower Ordovician strata. The upper part of this basin is infilled by “post-impact” deposits of the Winneshiek Shale (18-27 m thick), a dark laminated shale unit containing a marginal-marine or estuarine Lagerstätte of arthropods, conodonts, fish, and other taxa. The Winneshiek Shale is overlain disconformably by the Tonti Member of the St. Peter Sandstone, and forms a pre-Tonti depositional sequence included within the Ancell Group. The lower Tonti in northern Iowa locally contains a diverse conodont fauna that is correlated with the Cahabagnathus friendsvillensis Zone of late Whiterockian/Darriwilian (Chazyan) age, which constrains the upper age limit of the underlying shale. The Winneshiek Shale shares some conodont taxa with the Tonti, but it lacks several key species. However, it includes additional taxa not seen in the Tonti, notably Multioistodus subdentatus and several undescribed prioniodinid and coniform assemblages. The range of M. subdentatus spans the early to middle Whiterockian (Rangerian-earliest Chazyan) on the Midcontinent Composite Standard Section of Sweet et al. (2005), ranging as high as the basal friendsvillensis Zone. Multioistodus has been recognized in the Ancell Group of southwest Indiana, southeast Missouri, Arkansas, and Michigan, as well as the lower Simpson Group of Oklahoma. All these occurrences are more than 550 to 700 km from northeast Iowa. Although previous paleogeographic interpretations showed the mid Whiterockian seas penetrating the continental interior no further than southern Illinois, it is now apparent that marine transgression extended at least as far as northern Iowa. Because the Winneshiek Shale contains a restricted-marine fauna deposited in an oxygen-deficient basin, the transgression into Iowa likely migrated up embayments or estuaries marginal to the main Whiterockian seaway.