Paleoceanographic Conditions of the Michigan Basin in the Silurian as Revealed from Stable Isotopic Composition of Brachiopod Shell Material

Peter Voice
Western Michigan University, Department of Geosciences
Kalamazoo, MI
Peter.voice@wmich.edu

Carbon and Oxygen isotope analyses of microsampled brachiopod shell material from the middle Silurian Burnt Bluff Group has provided fundamental new information on the paleoceanographic conditions in the Michigan Basin during the Silurian. Although there is abundant lithologic evidence for highly restricted conditions at times during the Silurian, isotopic data have shown that during Burnt Bluff Group deposition, the Michigan Basin was characterized by normal marine circulation.

The Michigan Basin is an intracratonic basin that was an active depocenter throughout much of the Paleozoic. During the Silurian, paleoceanographic conditions fluctuate between normal marine conditions as indicated by luxuriant growth of pinnacle reefs, and deposition of restricted marine and evaporites facies. Geochemical evidence for the timing and magnitude of variability in paleoceanographic conditions within the basin is lacking.

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 normal marine values recorded from 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 compositions of marine waters in the Michigan Basin provides additional data points for the composition of normal Silurian seawater for comparison with other basins. Isotopic data sheds new insight into the circulation patterns within the Michigan Basin and potentially yields new models for facies distribution, reservoir development and the source of hydrocarbons in the Michigan Basin during Silurian time.