PALEOCEANOGRAPHY OF THE MICHIGAN BASIN IN SILURIAN TIME AS REVEALED FROM STABLE ISOTOPIC ANALYSIS OF BRACHIOPODS

Stable C and O isotopic analyses of microsampled brachiopod shell material from the middle Silurian Burnt Bluff Group has established fundamental information on the paleoceanographic conditions in the Michigan Basin during the Silurian. These data indicate that, despite clear lithological evidence for highly restricted conditions at times during the Silurian, the Michigan Basin was also characterized by periods of normal marine circulation.

The Michigan Basin is an intracratonic basin that was an active depocenter throughout much of the Paleozoic. Lithofacies patterns and distribution in the basin indicate that circulation within the basin varied widely throughout the Paleozoic. Fluctuations between normal marine conditions indicated by periods of luxuriant reef growth with deposition of restricted marine facies and evaporites are especially prevalent during the Silurian. Little geochemical evidence exists, however, to establish the timing and magnitude of variability in paleoceanographic conditions within the basin and how minor fluctuations in salinity and circulation patterns may have controlled the distribution of reef growth during this time.

Samples of pentamerid brachiopod shells were isolated from limestone intervals of cored material and examined with both cathodoluminescence and petrographic methods to establish that the shells had not been diagenetically altered. The samples have average isotopic values of -4.50 ±1.0‰ (δ¹⁸O) and +1.21‰ ± 0.99 (δ¹³C). These values are consistent with other established normal marine data for the Silurian at other sites around the world indicating that deposition of the Burnt Bluff Group took place during a period of open marine circulation within the Michigan Basin.

Establishing isotopic conditions of marine waters in the Michigan Basin provides additional data points for the composition of normal Silurian seawater for comparison with other basins, while also shedding new insight into the circulation patterns within the Michigan Basin. Fluctuations between highly restricted and normal marine conditions in the Michigan Basin may be a result of Milankovitch band orbital variability in response to resulting high amplitude sea level change, and/or to periods of increased structural sag.