2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 218-11
Presentation Time: 11:30 AM


WARD, Brittany M., Department of Geology, Grand Valley State University, 1 Campus Drive, Padnos Hall of Science, Allendale, MI 49401 and COLGAN, Patrick M., Department of Geology, Grand Valley State University, Padnos Hall of Science, 1 Campus Drive, Allendale, MI 49401

Previous studies of the Cordell Formation examine numerous localities throughout Upper Michigan and eastern Wisconsin; and provide detailed stratigraphic descriptions of unit lithology, and fossil faunal descriptions. This study is a sedimentological and paleoenvironmental analysis of the dolostones exposed at Seul Choix Point, Michigan.

Dolostones of the Cordell Formation were examined in hand sample and thin sections. The presence of more than 10% skeletal fragments and other allochems lead us to classify the rocks as wackestones and packstones. Corals and stromatolite-stromatoporid mounds are the most abundant fauna, with corals constituting ~70 to 75% of the total fauna, and stromatolite-stromatoporid mounds constituting ~25% of the fauna. A faunal map was constructed of a single bedding plane of the Cordell Formation to estimate spatial distribution of in situ organisms. On average three specimens of either corals or stromatolite-stromatoporid mounds were located in every square meter. Orientation data were collected of straight-shelled cephalopods and compared with probable Silurian wind directions. Mean orientations show three modes of direction: ~165/345 degrees, ~25/205 degrees, and ~65/245 degrees.

Packstones and wackestones indicate a moderate energy environment. The inferred energy level is consistent with those found at or above the effective wave base in an epicontinental sea. Additionally, the dominance of corals and stromatolite-stromatoporid mounds indicate a rich faunal community located at or above the effective wave base. Previous studies have suggested depths of ~10 to 30 m for this faunal community. During the Silurian the Michigan Basin was located south of the equator and was rotated ~45 degrees clockwise from its current orientation. Given the observed shell orientations it is possible that shells were orientated with the length of the shell perpendicular to ocean currents driven by Silurian winds.