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The “Squaw Bay Formation,” a middle Devonian mixed carbonate-clastic unit located in the Michigan basin, was named almost 100 years ago by Warthin and Cooper (1935) for a single, poorly exposed outcropping of condensed strata. Since then the stratigraphic term has been used inconsistently in the subsurface where the unit is stratigraphically expanded, but should also be revised because it is a derogatory term for Native American Women. The goal of this study is to aid in the renaming process for this formation by determining its variability and defining boundaries. To accomplish this, the current research combines lithostratigraphy, paleoecology, and geochemical analyses of the Krocker 1-17, State Chester Welch 18, and Paxton Quarry cores to provide the background information needed to revise the formation. The “Squaw Bay Formation” in core is composed of predominantly argillaceous limestone and calcareous shale and is characterized by a crinoid and brachiopod fauna. Further lithologic analysis found that there are three main facies common in the studied cores; black shale with frequent pyrite nodules and silt laminae, calcareous fossiliferous shale, and a highly bioturbated calcareous shale with few fossils and pyrite nodules. The lower “Squaw Bay Formation” also contains zones of concentrated fossil debris, especially near the contact with the Traverse Limestone which is primarily a pyritized hardground. The studied formation also has an increasing black shale content up-section and a transitional contact with the overlying Antrim Shale. Future analysis of magnetic susceptibility, pXRF, and total organic carbon data will further illuminate the variability within the formation and characteristics of its boundaries with other stratigraphic units. Ultimately, the combination of lithostratigraphic, paleoecologic, and geochemical analysis of the Krocker 1-17, State Chester Welch 18, and Paxton Quarry cores will not only facilitate a better understanding of the “Squaw Bay Formation,” but also contribute to the process of revising and renaming the unit.
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Comparing lithologies and geochemical signals of the Middle Devonian Thunder Bay Formation (Michigan Basin) in core and at the type section
Lithologic correlation of the Middle Devonian Traverse Group in the Michigan Basin is challenging due to a paucity of outcrops and the difficulty in correlating shallow water marine facies across the basin.This study correlates strata in the State Chester Welch #18 core (SCW-18) to outcrop exposures at the type sections of the Thunder Bay Formation and neighboring units using litho- and chemostratigraphy. Lithologically, the texture, grain sizes, and fossil assemblages found in the SCW-18 core match the outcrop samples and type section description of the Thunder Bay Formation. Both successions are gray calcareous shale and limestone with abundant fossils and stylolites. δ13C isotopes reveal a shift to more negative isotopic values through time for both bulk-rock and drilled micrite powders from the SCW-18 core, but a trend toward more positive values with relative stratigraphic position for the outcrop sample. This suggests that Thunder Bay strata in the core and outcrop are not chronostratigraphically equivalent even though they appear similar lithologically. The core data also revealed two groupings based on relationships between δ18O vs δ13C values that correspond to differences in lithology. The lower samples in the core that are situated in shaley limestone illustrate a linear trend than the upper portion which was more crystalline and isotopic values noisier. This variation might suggest differences in diagenetic processes affecting the subsurface Thunder Bay Formation as δ18O values in carbonates are often altered to some extent. This study improves our lithologic and isotope geochemistry understanding of the Thunder Bay Formation in the subsurface. Future work will incorporate elemental data from the core and outcrop to better understand diagenetic processes.
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- Award ID(s):
- 2050697
- PAR ID:
- 10508884
- Publisher / Repository:
- Geological Society of America Abstracts with Programs
- Date Published:
- Volume:
- 56
- Issue:
- 3
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Petrographic analysis can reveal information about original depositional environments and subsequent diagenetic conditions. Here we examine a well-developed pyritized interval at the contact between the Traverse Limestone (a fossiliferous dolomitic packstone) and the “Squaw Bay Formation” in the Michigan Basin. Overlying the pyritized discontinuity, the “Squaw Bay Formation” is a calcareous, highly bioturbated shale, indicating Devonian sea level rise and/or basin subsidence. Little prior petrographic analysis has been done on these formations, and, given the stratigraphic consistency of the pyritized interval across geographically diverse cores, detailed petrographic study will elucidate the diagenetic history of each formation and the contact itself. Ten thin sections were cut from three cores with samples taken at the contact and in both the underlying Traverse Limestone and overlying “Squaw Bay Formation”. Thin section petrography, SEM, and µXRF technologies are used to delineate diagenetic overprints and stratigraphic relationships. Analysis revealed extensive dolomitization of the Traverse Limestone and a variable history of fossil recrystallization in the “Squaw Bay Formation”. The presence of chalcedony, glauconite, various forms of calcite, and several crystal morphologies of pyrite at and near the contact imply a complicated diagenetic history. The nature of the calcite and pyrite generations in these samples contextualize the diagenetic history of each formation and of the contact. Calcite crystals, overgrowths, and deformations distinguish the Traverse from the “Squaw Bay,” and suggest multiple and distinct generation events. The pyrite at the contact exhibits both cubic and needle-like forms. Future SEM analyses may reveal whether the needle-like form is a result of a second generation of sulfide mineralization or diagenetic alteration of the original pyrite. Continued research will result in the development of a paragenetic sequence for this mineralized stratigraphic contact.more » « less
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