Regional Controls on Low-Latitude, Shallow-Water Heterozoan and Photozoan Carbonate Facies Distribution, Lower Mississippian, Continental U.S.
This study includes detailed examination of core, well log, seismic, and outcrop data, and extensive literature study to document distribution patterns of shallow-water heterozoan and photozoan facies across the continental US. The results will provide an understanding of the controls on deposition, stratigraphic packaging, and reservoir character.
Lower Mississippian carbonates form important petroleum reservoirs (e.g., western Kansas and the Williston Basin), are potential target for CO2 storage, and represent one of the best-known examples of a tropical heterozoan carbonate system in the rock record. During that time, most of the continental US was in low latitudes and carbonates were widely deposited in a shallow tropical sea that covered most of the area (Lowe, 1975; Gutschick and Sandberg, 1983). Initial results from us (Franseen, 2006; Franseen, 2012; Ortega-Ariza and Franseen, 2021) and by others, show shallow-water settings are dominated by heterozoans (± siliceous sponge facies), reflecting adverse photic zone conditions near basin margins, whereas photozoans are abundant in areas away from the basins, reflecting more normal photic zone conditions. The adverse conditions resulting in nutrients (and silica), and likely cooler water, affecting shallow-water environments are thought to be created by documented upwelling in the basins.
Plans for Study:
Kansas provides an excellent location for sedimentology, stratigraphy and facies relationships study. Because the Lower Mississippian is an important petroleum reservoir in Kansas, there are abundant well logs and cores. Core, well-log, and seismic (if available) data will be used to:
1) Test hypotheses on expected facies distributions near, and away from, proposed zones of upwelling.
2) Develop a detailed stratigraphic model to better understand the controls of sea level on facies character and distribution.
3) Develop detailed paleogeographic maps to evaluate the effect on facies characteristics and geometries.
The study will also include detailed examination of the literature to collect data that can provide a regional picture of facies distributions for the continental US during the Lower Mississippian, and aid in adding more detail to paleogeography. This effort will allow:
1) Documentation of facies characteristics and testing of initial results that show increase in photozoan components away from proposed zones of upwelling and heterozoans (± biosiliceous) components dominant in areas adjacent to proposed zones of basinal upwelling.
2) Paleogeographic details include further testing of preliminary results that indicate photozoans are present near exposed land areas, suggesting that land runoff was not a major contributor of nutrients and silica, which would be consistent with the arid climate interpreted for this time interval.
