PETROGRAPHIC AND BIOSTRATIGRAPHIC ANALYSIS OF A MCKAY BAY MEMBER (BUSH BAY FORMATION) MID-SILURIAN KNOLL REEF IN MICHIGAN’S UPPER PENINSULA

Knoll reefs are described as massive elongated mud-skeletal mounds, with size ranging from a few square meters to hundreds of kilometers in size. Knoll reefs form in extremely shallow waters which stall their ability to grow vertically. This study focused on a mid-Silurian knoll reef found in the Hiawatha National Forest at the eastern end of Michigan’s Upper Peninsula. These dolomitized reefs are stratigraphically located in the McKay Bay Member, in the Bush Bay Formation, of the Engadine Group.

Previous efforts in the 1960’s and 1970’s conducted visual investigations of the knoll reef blocks to help identify reef distributions throughout the Michigan Basin. However, for the last 60 years the features have remained only partially described based on hand sample analysis. This current study was designed to further investigate the knoll reefs of the same location by conducting petrographic analysis, biostratigraphic analysis, and analysis by photogrammetric high-resolution panoramic images, collected using a Gigapan Epic Pro. The knoll reef mound selected for this study measured 24 meters, by 35 meters, and 5.5 meters in height. Four transverse sampling surveys were completed over the knoll reef and were collected every 5 meters.

The dolomitization of the knoll reef caused many of the megafossils to be lost, leaving behind the moldic voids where some fossil remnants dissolved. However, the stromatolites and stromatoporoids were well preserved and are known in other locations to incorporate underlying marine organisms as substrates. Horizontal and vertical thin sections of samples collected of the well-preserved stromatolites and stromatoporoids were made to determine if remnants of other marine organisms are identifiable.

This research represents the first phase of a project to collect more detailed information about the knoll reefs of the area and determine if they are true knoll reefs or another type of organic feature.