INTEGRATION OF PETROGRAPHY AND GEOCHEMISTRY TO ESTABLISH PALEOCEANOGRAPHIC CONDITIONS IN THE SILURIAN MICHIGAN BASIN

A major question that arises in reconstruction of sedimentary systems in ancient basins is the paleoceanographic conditions present. The Silurian in Michigan is noted for its fluctuations between normal marine conditions indicated by luxuriant reef growth coupled with deposition of restricted marine facies and evaporites. A combination of petrographic and geochemical analyses was used on pentamerid brachiopod valves to elucidate information about the paleoceanographic conditions during the normal marine phases.

Pentamerid valve structures were analyzed with a combination of standard petrographic microscopy, SEM microscopy, and cathodoluminescence. Valves exhibited fibrous calcite crystals oriented perpendicular to growth lines under the petrographic microscope. Analysis under a SEM microscope, showed Pentamerids with detailed secondary layer preservation, and recrystallized fabrics of the tertiary layer. Samples used for isotopic analysis were examined with cathodoluminescence to distinguish diagenetic alteration in shells. Non-altered Pentamerid brachiopod shells have average isotopic values of -4.50 ±1.0‰ (ä18O) and +1.21±0.99‰ (ä13C). Other time-equivalent datasets from open marine basins in Eurasia and North America show similar ä18O values to this study, indicating that the Michigan Basin was also open to normal marine circulation.

A circulation model for the Michigan Basin during Silurian time incorporates a transgressive/ regressive carbonate ramp model with varying degrees of restriction of circulation.