THE RISE OF PINNACLE REEFS DURING THE MID-TELYCHIAN MANITOWOC EVENT

The oldest pinnacle reef tract in the rock record is associated with the newly discovered mid-Telychian Manitowoc Event. This short-lived early Silurian event, now recognized globally, was first identified in the Michigan Basin. The first appearance of pinnacle reef tracts, protruding tens to hundreds of meters above the surrounding mid-Silurian seafloor, represents a step change in the evolution of the marine biosphere. This change in seafloor morphology opened a host of new ecological niches that served as “evolutionary cradles” for organism diversification.

Integration of δ¹³C_carb chemostratigraphy, facies analysis, and mapping of discontinuity surfaces led to the discovery of the Manitowoc event. The early Silurian strata of the Michigan Basin form a complex northward-thickening wedge, largely restricted to the deep subsurface that was mapped in eastern Wisconsin using a series of new and archived cores that penetrated this lower Silurian succession along a 400 km transect. A core from Manitowoc Wisconsin was designated the type-section for the event as it showed clear separation of the Valgu Event below and the Ireviken above. Recent biostratigraphic study extends the known spatial distribution of the Manitowoc Event and reinforces age interpretations across the US midcontinent.

While the Manitowoc Event shows many features in common with older Silurian events, it is most similar to the younger Ireviken, Mulde, and Lau events in also displaying a well-developed pinnacle reef tract. Measurement from paleokarst horizons underlying reefs up to their crests indicates sea level fluctuation on the order of 50 to 100 m that, according to integrated geochronology, grew in approximating 100 k.y. Regressive and transgressive episodes are linked to shifts in the δ¹³C_carb record. These patterns suggest that Paleozoic reef evolution was the product of environmental forcing by perturbations of the global carbon cycle.