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Thunder Bay Limestone is a geologic formation in
The name for the formation originates from the description of strata near the "south cape of Thunder Bay" by C. C. Douglas in 1841, this area also represents the type locality of the formation. Due to negligence by multiple later workers, the area was later referred to as the Partridge Point Formation by Warthin & Cooper in 1935. It wasn't until 1943 that Warthin & Cooper would rename the formation back to its original name.
The type locality (Locality 30-8-11 SE) is the largest outcrop of the formation, exposing six units that have a total depth of about 4.8 meters. These units are largely made from light-colored
Michigan
Michigan ( ) is a peninsular U.S. state, state in the Great Lakes region, Great Lakes region of the Upper Midwest, Upper Midwestern United States. It shares water and land boundaries with Minnesota to the northwest, Wisconsin to the west, ...
that preserves fossils dating back to the Middle Devonian
In the geological timescale, the Middle Devonian epoch (from 393.3 ± 1.2 million years ago to 382.7 ± 1.6 million years ago) occurred during the Devonian period, after the end of the Emsian age.
The Middle Devonian epoch is subdivided into two ...
and is the uppermost formation of the Traverse Group
The Traverse Group is a geologic group in Michigan, Indiana and Ohio comprising middle Devonian limestones with calcareous shale components. Its marine fossils notably include Michigan's state stone, the Petoskey stone (the extinct coral ''Hexagon ...
.
Description
The name for the formation originates from the description of strata near the "south cape of Thunder Bay" by C. C. Douglas in 1841, this area also represents the type locality of the formation. Due to negligence by multiple later workers, the area was later referred to as the Partridge Point Formation by Warthin & Cooper in 1935. It wasn't until 1943 that Warthin & Cooper would rename the formation back to its original name.
The type locality (Locality 30-8-11 SE) is the largest outcrop of the formation, exposing six units that have a total depth of about 4.8 meters. These units are largely made from light-colored limestones
Limestone is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Limestone forms when these ...
though the bottom-most unit has bluish limestone. The most unique unit would be unit 3 which mostly is made up of grey shales
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g., kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
that grades into granular limestone at the surface.
The environment that the formation represents is a shallow carbonate platform with large amounts of coral and shelly fauna. Later formations show an increase in depth as water levels raised in the Eastern Interior seaway. Over time, the seaway would develop a stratified water column. Eventually, younger formations, like the Squaw Bay limestone, would show a deep basin environment.
Paleobiota
Anthozoa
Brachiopoda
Bryozoa
Conodonta
Echinodermata
Mollusca
Placodermi
Stromatoporoidea
Tentaculita
Trilobita
See also
* List of fossiliferous stratigraphic units in MichiganReferences
{{Stratigraphic column of Michigan Devonian Michigan Middle Devonian Series Limestone formations of the United States Shale formations of the United States