Kansas Geological Survey, Open-file Report 99-50
by
Evan K. Franseen
KGS Open File Report 99-50
Osagean-Meramecian (Mississippian) strata in several provinces in the United States, including the southern midcontinent, are composed predominantly of dolomite with local major components of chert (cherty dolomite). Some areas have been almost completely altered to chert (known as "chat"). Much attention has been given to the depositional and diagenetic history of these strata in regional studies and from surrounding areas (e.g. Indiana, Tennessee, Alabama, Illinois, Iowa, Kentucky). In contrast, partly because most of the Osagean-Meramecian strata occur in the subsurface in Kansas, relatively few sedimentologic and diagenetic studies have been done on these rocks. Also, because these strata form important petroleum reservoirs in Kansas, much of the focus in previous study of these strata in the subsurface has centered on reservoir characteristics and the post-Mississippian subaerial exposure that truncate these strata as the major control on reservoir character. Although focusing primarily on reservoir characteristics, some previous studies have documented depositional features and diagenetic complexities of these strata and noted, in part, the control of depositional facies and early diagenesis on reservoir character, including evidence for early silicification. (e.g. Ebanks et al., 1977; McCoy, 1978; Ebanks, 1991; Johnson and Budd, 1994; Rogers et al., 1995). Montgomery et al. (1998) provides a recent summary on current knowledge of these strata in Kansas, pointing out the poorly understood complexities.
In addition to understanding controls on reservoir character, the Osagean and Meramecian strata in central Kansas, consisting of a mixture of dolomite and chert, also provide an opportunity to add new insight into controls on depositional environments, including those related to paleogeography and paleo-oceanographic conditions. The results presented in this paper are part of a larger project addressing producibility problems in the numerous Kansas fields such as the Schaben field in Ness County that produce from Meramecian and Osagean dolomites beneath the sub-Pennsylvanian unconformity (e.g. Adkins-Heljeson et al., 1999; Carr et al., 1996). Reservoir characteristics of cores from the Schaben Field have been reported elsewhere (Franseen et al., 1998; Byrnes, and Franseen, 2000; Montgomery et al., in press).
This study is the first known that relates the importance of depositional setting in Kansas, including shallow-water environments dominated by brynoderm and sponge spicule facies, and early diagenetic processes to paleogeography, paleo-oceanography and climate. The results of this study suggest that ramp deposition and early diagenetic processes were variously controlled by: 1) cool, nutrient-rich water, most likely from upwelling under a tropical to subtropical climate; 2) a change to more and conditions near the close of the Osagean; 3) early tectonic movements; and 3) relative sea-level changes. These results have broad implications for better understanding controlling factors on equivalent strata throughout the region.
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Kansas Geological Survey, Energy Research
Placed online Aug. 18, 2011
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