INTEGRATION OF SEQUENCE AND EVENT STRATIGRAPHIC FRAMEWORKS OF MIDDLE SILURIAN CARBONATE PLATFORM DEPOSITS FROM GOTLAND (SWEDEN) AND THE CENTRAL UNITED STATES

The Late Wenlock Mulde Event caused major extinctions among biostratigraphically important taxa (e.g., graptolites, conodonts and chitinozoans). From an environmental point of view, associated perturbations in the global carbon cycle make this drop in bioproduction potentially the most important Silurian event. Coeval environmental changes are now evident also from the neritic carbonate system, both from the Baltic and Illinois Basins. In the Baltic Basin of southeast Scandinavia and the East Baltic, this includes a change from prolific reefal facies fringed by well-oxygenated sea-floors to deposition of dark mudstones, followed by regression, regional siliciclastic influx, karstification and complete reef termination. Reef recovery was delayed during the subsequent platform innovation when, instead, non-skeletal carbonate deposition prevailed. Integration of high-resolution sequence- and event stratigraphic frameworks shows that platform termination may have preceded eustatic regression and that extinctions took place independently of sea-level position.

Similar events may have taken place in the central U.S. during the same time interval, although biostratigraphic control is less precise. A significant disruption in carbonate shelf deposition has been observed within the upper Laurel Limestone in Indiana, followed by an abrupt change in sedimentation marked by siliciclastic deposition of the Waldron Shale. These features coincide with a major change in the composition of some macrofossil groups, such as trilobites. Reef development appears to have been terminated locally in areas where the Waldron Shale was deposited, but reefs returned when siliciclastic deposition ceased. In Illinois, a similar break in deposition between the Sugar Run Dolomite and the Racine Dolomite, which can be dated more precisely by graptolites and conodonts as occurring near the Shienwoodian/Homerian boundary. These occurrences suggest a widespread disruption in shelf carbonate deposition around the Illinois Basin during the late Wenlock, coinciding with the Mulde Event.