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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.
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An analysis of stratigraphic and paleoecologic variability in the “Squaw Bay Formation,” Michigan Basin
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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- Award ID(s):
- 2050697
- PAR ID:
- 10508883
- 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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