The Dakota Formation lies unconformably above the Kiowa Formation and is transitional with Graneros Shale above. The Dakota Formation is Albian to Cenomanian in age, comprises predominately non-marine variegated mudstone and channel sandstone, and crops out extensively in the central and north-central parts of Kansas (Franks, 1975; Hattin, 1965). Average thickness of the Dakota Formation is 76 m in Ellsworth and Russell counties (Siemers, 1971), with a thickness range here of 60-91 m (Frye and Brazil, 1943). Sedimentary deposits within the upper 30-40 feet of the Dakota section in Russell and Ellsworth counties represent a transition from the non-marine deposits of the Dakota to the shallow-water marine deposits of the overlying Graneros Shale (Siemers, 1971).
In central Kansas, the Dakota Formation is subdivided into the basal Terra cotta Clay and the overlying Janssen Clay members (Plummer and Romary, 1942; Figure 1). The Terra Cotta Clay Member comprises grey and red-mottled mudstone, with thin-bedded siltstone and cross-bedding sandstone lenses interspersed (Plummer and Romary, 1942). The Terra Cotta Clay Member is 45-75 m thick, and comprises about the lower two-thirds of the Dakota section (Franks, 1966). Franks (1975) interpreted sandstone of the Terra Cotta to represent channel deposits of southwest-flowing low-sinuosity streams. He further attributed fine-grained facies of this member to deposition in floodplains adjacent to these rivers.
Rubey and Bass (1925) identified sandstone-rich strata in the uppermost part of the Terra Cotta Clay Member of Russell County as the Rocktown Channel Sandstone (Figure 1). The Rocktown Channel Sandstone is well exposed in central Kansas, especially in the Saline valley and the Wilson Lake areas of Russell County. It contains grey cross-bedded sandstone and white, grey, red, or brown mudstone. Sandstone strata is characterized by small- to medium-scale (0.04-1.5 m) planar cross-bedding. Much of the mudstone is silty, and contains approximately 30% silt and 70% mud. Rubey and Bass (1925) interpreted Rocktown Channel strata to represent amalgamated channel deposits derived from mostly meandering streams.
Figure 1--Idealized lithologic units and contacts shown as defined by Siemers (1971) (adapted from Hamilton, 1989).
The Janssen Clay Member is the upper member of Dakota Formation, and is characterized by grey and dark-grey siltstone and clay interspersed with seams of lignite and sandstone beds (Plummer and Romary, 1942; Karl, 1976). Sandstone generally is less abundant in Janssen strata than it is in the Terra Cotta Clay Member. The Janssen Clay Member is 15-30 m thick, and comprises about the upper one-third of the Dakota section (Franks, 1966). The Janssen Clay Member is interpreted to represent transitional nonmarine to marine strata (Karl, 1976).
Siemers (1971) identified the sandstone bodies directly above the Rocktown Channel Sandstone as the upper flat-bedded unit. This upper flat-bedded unit was interpreted by Hamilton (1989) as part of the basal Janssen Clay Member (Figure 1). The upper flat-bedded unit is well exposed in central Kansas, and is dominated by horizontally stratified, ripple-laminated brown sandstone with locally abundant burrow structures. Siemers (1971) identified the upper flat-bedded unit as a marginal marine deltaic complex.
Hamilton (1989) interpreted a sequence boundary to exist in central Kansas within the Terra Cotta member. He interpreted the upper Terra Cotta and all of the Janssen member as correlative with the late Early Cretaceous part of the D sequence in the Denver Basin of eastern Colorado. He further interpreted the contact between the Terra Cotta and Janssen members to represent a regional marine flooding surface, and he correlated the lower Terra Cotta with the J sequence of the Denver Basin.
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