Geologic Setting

Structure

Kansas is characterized by broad basins and domes covering thousands of square kilometers. Prominent uplift and subsidence occurred episodically throughout the Phanerozoic, separated by periods of gradual deformation (Newell et al., 1989). Although the location of the structural features varied somewhat throughout the Paleozoic, there appear to be consistent patterns of repeated localized uplift and subsidence (Baars and Watney, 1991). Two prominent structural uplifts that affected the Paleozoic rocks in Kansas are the Nemaha uplift and CKU (Figure 7A). Structural maps of Precambrian basement and isopach maps of the Arbuckle indicate the presence of elements of the CKU and Nemaha Ridge prior to Arbuckle deposition. In addition, extensive differential erosion of Precambrian rocks (presumably in late Precambrian time) resulted in an irregular topography onto which Cambro-Ordovician strata were deposited (Walters, 1946).

The Midcontinent rift system and related Humboldt fault zone constitute a north-northeast by south-southwest-trending fault swarm that was reactivated several times during the Paleozoic. Local uplift and interstratal truncations suggest rejuvenation of structural blocks during the Late Ordovician (Baars and Watney, 1991). The CKU and Nemaha Ridge became prominent positive structural elements during later significant Early Pennsylvanian deformation likely associated with similarly aged plate convergence along the Ouachita Mountains orogenic belt in Arkansas (Newell et al., 1989). This uplift and erosion locally affected Arbuckle strata, especially on the Central Kansas Uplift where Pennsylvanian strata directly overlie Arbuckle strata or basement rocks where Arbuckle strata are absent (Figure 7B).

Arbuckle Distribution in Kansas

The Arbuckle is present in most of Kansas and is absent only in areas of northeastern and northwestern Kansas, and over ancient uplifts and buried Precambrian highs (Walters, 1946; Keroher and Kirby, 1948; Cole, 1962, 1975; Denison, 1966; Figure 7B). The Arbuckle is thin to absent in parts of northeastern Kansas due to pre-Simpson uplift and erosion (Newell et al., 1987). The Arbuckle is locally absent on the Nemaha uplift, Cambridge arch, and Central Kansas uplift due to pre-Pennsylvanian erosion (Walters, 1946; Jewett, 1951, 1954; Merriam, 1963). Arbuckle rocks occur at depths ranging from about 500 feet in southeastern Kansas to more than 7,000 feet in southwestern Kansas (Keroher and Kirby, 1948; Cole, 1975). Arbuckle strata thicken as a whole from north to south and are thickest, up to 1390 feet, in the southeastern corner of Kansas (Cole, 1975).

Pre-Pennsylvanian Unconformity

Near the end of the Early Ordovician, falling sea level drained the continents and left behind a landscape underlain by predominantly carbonate Cambro-Ordovician strata and Precambrian basement rocks. Deep weathering and erosion produced a regional unconformity and a regionally extensive karst system over most of the North American craton. This regional system has been called the Arbuckle-Ellenburger-Knox-Prairie du Chien-Beekmantown-St. George karst plain (Kerans, 1988). A vast system of caves, sinkholes, joint-controlled solution features, and collapse breccias developed. These features are found throughout the Midcontinent in the Arbuckle, Knox, Beekmantown, Prairie du Chien, St. George, and Ellenburger groups (Gore, 1954; Merriam and Atkinson, 1956; Fisher and Barrett, 1985; Kerans, 1990; Hammes, 1997). However, the nature of the post-Sauk unconformity surface is quite variable (Sloss, 1988). In some areas (such as the CKU) there has been extensive erosion during the post-Sauk regression and numerous additional prolonged periods of subaerial exposure up through the Early Pennsylvanian.

Studies over the years have focused on the erosional and karsted nature of the upper Arbuckle surface in Kansas (e.g. Walters, 1946; 1958; 1991). This attention was driven largely by petroleum production in the Arbuckle Group of Kansas, which is associated with the unconformity surface. A recent study by Cansler and Carr (2001) documented details of karst geomorphology of the upper Arbuckle surface at various scales over the southern part of the CKU (Barton, Ellsworth, Rice, and Stafford counties). They identified major karst landform geometries (dolines, cockpits) and karst erosional features related to ground water sapping processes (discussed further in the Discussion section).

Arbuckle Stratal Characteristics

Arbuckle Group rocks are part of the craton-wide Sauk Sequence, which is bounded at its base and top by major interregional unconformities (Sloss, 1963). These interregional unconformities represent major regressions of the sea and erosion and subaerial exposure of vast areas of the craton. Arbuckle shallow shelf carbonate strata in Kansas and age-equivalent strata elsewhere are part of the Cambro-Ordovician “Great American Bank” that stretched along the present southern and eastern flanks of the North American craton (Wilson et al., 1991). The bank consists of hundreds of meters of largely dolomitized intertidal to shallow subtidal cyclic carbonates overlain by a regional unconformity (Wilson et al., 1991). The shallow subtidal to intertidal environment persisted throughout Arbuckle depositional time and is reflected in a suite of rocks that are highly heterogeneous, but form a consistent suite found throughout North America (Bliefnick, 1992).

Figure 7: A) Map of Kansas showing major structural elements. Red dots show locations of Arbuckle cores studied to date. B) Diagrammatic cross section and maps showing locations of cross section in Kansas. Note Arbuckle relationships (blue) to basement rocks (brown) and overlying strata, especially across the CKU (modified from Adler et al., 1971).