PETROGRAPHIC ANALYSIS AND DISTRIBUTION OF SEDIMENT CAPTURED IN DOLOMITIZED MCKAY BAY MEMBER KNOLL REEF, UPPER PENINSULA, MICHIGAN

Research on the pinnacle reef systems of the Michigan Basin dates back to the 1930s, yet little attention has been paid to its outcropping knoll reefs. These carbonate mounds develop in shallow waters, limiting vertical growth and compelling horizontal expansion instead. The knoll reefs under investigation are situated within the McKay Bay Member of the Bush Bay Formation, part of the early-Silurian Engadine Group. They are located within the Hiawatha National Forest at the south-eastern tip of Michigan’s Upper Peninsula.

The first phase of the project in 2023 collected over 120 samples from the main knoll reef (Reef A) and two neighboring smaller reefs to its south/southeast, (Reef B and Reef C). The samples and cores were collected at 1-meter intervals in a transected grid pattern every 22.5 degrees. Fossil identification was also collected for biostratigraphic analysis.

This project investigates the origin and distribution of detrital sediment captured in the largest knoll reef in the study area. Each sample was prepared into a thin section. The thin sections were then analyzed and point-counted for detrital sediment that was originally captured in the reef. The counted detrital sediment per thin section was used to create rose diagrams and determine direction of sediment influx to the main reef. The petrographic analysis shows direction of sediment influx and also shows areas of intra-reef currents between the main reef and the two smaller reefs.