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James R. Wood, Michigan Technological University
In the Early to Middle Silurian strata of Michigan, large volumes of salts, most halite and anhydrite, dominate the rock column in the basin center where potassium salts are also present in large quantities. Like K-bentonites, potassium salts produce large gamma-ray excursions on well logs. And while it is difficult to distinguish potash salts from bentonites in well logs, it does not matter for purposes of stratigraphic correlation. The main objective of this study was to study the detailed Lower Paleozoic (Ordovician and Silurian) stratigraphy of the continental interior basins of central N. America using the well-documented Ordovician bentonites to constrain the time lines. What we have discovered in the Michigan Basin is that the Silurian potash salts serve the same purpose where they are present. Individual gamma-ray peaks from potash salts can be correlated over most (approximately 85%) of the Michigan Basin at a higher time resolution than bentonites. Accordingly the scope of the study has been expanded to include the Silurian Salina Formation and to use the potash salts to constrain the well-log tomography. As reported previously, we have collected and digitized approximately 500 digitized well logs and have identified the principal bentonites and salts in the Michigan Basin. The Salina Formation contains 5 main salt bodies (A, B, C, D and F) and the “A” salt contains by far the most potash as indicated by the gamma-ray and visual examination. We have discovered that this unit can be divided into ~12 subunits based on strong gamma-ray excursions and are in the process of (a) mapping them and assigning time durations. The time span for the “A”-salt appears to be 10 My +/- 1 My. Accordingly each subunit would appear to be on average less than 1 My. We will examine the available salt cores at the Michigan Core Repository this winter to see if any recognizable or datable bentonites beds are present in the unit. If so, samples will be taken and submitted for age dating so that the time spans can be further constrained. Finally, the gamma-ray tomography has be supplemented with the density (RHOB) logs and maps of paleogeographic environments determined.
In the Early to Middle Silurian strata of Michigan, large volumes of salts, most halite and anhydrite, dominate the rock column in the basin center where potassium salts are also present in large quantities. Like K-bentonites, potassium salts produce large gamma-ray excursions on well logs. And while it is difficult to distinguish potash salts from bentonites in well logs, it does not matter for purposes of stratigraphic correlation. The main objective of this study was to study the detailed Lower Paleozoic (Ordovician and Silurian) stratigraphy of the continental interior basins of central N. America using the well-documented Ordovician bentonites to constrain the time lines. What we have discovered in the Michigan Basin is that the Silurian potash salts serve the same purpose where they are present. Individual gamma-ray peaks from potash salts can be correlated over most (approximately 85%) of the Michigan Basin at a higher time resolution than bentonites. Accordingly the scope of the study has been expanded to include the Silurian Salina Formation and to use the potash salts to constrain the well-log tomography.
As reported previously, we have collected and digitized approximately 500 digitized well logs and have identified the principal bentonites and salts in the Michigan Basin. The Salina Formation contains 5 main salt bodies (A, B, C, D and F) and the “A” salt contains by far the most potash as indicated by the gamma-ray and visual examination. We have discovered that this unit can be divided into ~12 subunits based on strong gamma-ray excursions and are in the process of (a) mapping them and assigning time durations. The time span for the “A”-salt appears to be 10 My +/- 1 My. Accordingly each subunit would appear to be on average less than 1 My. We will examine the available salt cores at the Michigan Core Repository this winter to see if any recognizable or datable bentonites beds are present in the unit. If so, samples will be taken and submitted for age dating so that the time spans can be further constrained. Finally, the gamma-ray tomography has be supplemented with the density (RHOB) logs and maps of paleogeographic environments determined.
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