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43539-AC8
Establishing Chronostratigraphic Frameworks in Carbonate Rocks Using Dispersed Siliciclastic Sediment Pulses and K-Bentonites
Dispersed siliciclastics are silt- to clay-size particles that settle out of the water column and are entrained into carbonate sediments as a minor component. These particles produce a strong gamma-ray (GR) signal in many chemical sediments such as carbonates and salts (anhydrite, halite, sylvanite) in amounts that are easily detected by wireline logs. These particles may be added in the form of distal muds transported large distance by fluvial systems or as wind borne volcanic ash, which remain in the record as bentonite beds. Salts containing potash salts also produce strong GR signals (due to the decay of Potassium 40 to Argon-40). These GR signals tend to occur as spikes in the logs and since they resemble high-GR bentonites , these signals can be used in a similar manner to subdivide salt sequences. The primary purposes of this research are: (1) to use Well Log Tomography (WLT) to identify and map the distribution of the dispersed siliciclastics in the Ordovician carbonates of the North-Central United States and (2) to use the WLT technique to map the internal structure of the massive Silurian halite deposits in the Michigan Basin. Relevant data for both of these Periods can be obtained from the large number of well logs available for this area. Numerous Ordovician ash falls (K-bentonites) that have been documented in the subsurface and from outcrops in the region can be used to subdivide the Ordovician into smaller time stratigraphic units. Similarly the potash beds in the overlying Silurian salts can be used to subdivide the salt. Both can be used to demonstrate that WLT can provide a much improved, semi-quantitative picture of the wider Ordovician-Silurian depositional environments that will include details regarding the fluvial systems involved and their interbasinal relationships.
We have collected and digitized approximately 500 digitized well logs over a four state area and have identified the principal bentonites and salts in the Paleozoic of Michigan. We are also compiling a regional set of digital well log curves and digital cross sections together with a regional set of stratigraphic correlations that now include the entire geologic column for Michigan, akin to the earlier Lillenthal sections. The first set of log curve amplitude visualizations (avi animations) that will be used to display the distribution of dispersed siliciclastics and potash salts has been completed for the basal (“A”) salt. The results show the distribution of the potash in the basin through A-salt time. The potash deposition began at several widely separated locations (depo-centers) but at times they coalesced so that potash production extended over nearly the entire Michigan basin and seem to have spilled over the bounding arches into Ohio and Indiana. Work is in progress to extend the study to the upper salts (“B”, “C”, “D” and “F”). The carbonates and shales that separate these major salt beds will also be included. The study will conclude with the Bass Island Formation, the nominal top of the Silurian in Michigan.
