Sequence Stratigraphy

The placement of Middle Pennsylvanian strata into a sequence stratigraphic framework provides a better understanding of the variability in coal distribution, thickness, quality, and gas content. Determining the depositional history of an area is the first step to sequence stratigraphic analysis. Conditions that encourage thick peat accumulation include accommodation, rising base level, and sediment bypass. The period of maximum transgression is ideal for these conditions. Late transgressive systems tracts (TSTs) and early highstand systems tracts (HSTs) are therefore where the thickest coals will be found. Coals tend to thicken, have lower ash, and higher BTU values upwards through the TST to a maximum at the beginning of the HST. Coals tend to thin, have more ash content, and have lower BTU values from the maximum flooding surface (MFS) through the HST. Widespread coals tend not to develop during the lowstand given the lack of accommodation and have low preservation potential during the highstand due to high rates of sedimentation (Aitken, 1994).

The two figures below are twin strike-oriented cross sections: one showing a lithologic interpretation, and the other a sequence stratigraphic interpretation of Desmoinesian strata across the study area. Eight possible maximum flooding surfaces and nine sequence boundaries are recognized between the Mississippian-Pennsylvanian unconformity and Mulberry coal, and more of both are possible above this coal. Coals such as the Riverton, ‘Aw’, Tebo, Mineral, Croweburg, and Mulky--all close to MFS’s--are considered thick, extensive, “major” coals in this area.

Lithologic depositional strike-oriented cross section from Miami County to Wilson County, Kansas (Click on above image to show more detail)

Sequence stratigraphic interpretation of the same cross section (Click on above image to show more detail)