Dakota Aquifer Program--Geologic Framework
The total thickness of sedimentary rocks above the Precambrian varies considerably across the study area ranging from less than 2,000 ft. in southeastern Colorado on the Sierra Grande and Apishapa uplifts to almost 7,000 ft. in the western Kansas basin. The strata range in age from Cambrian to the Holocene and consist of both consolidated and unconsolidated deposits that were deposited in marine and nonmarine environments. Periodic episodes of deposition, uplift, and erosion over geologic time and local variations in the rates of these processes are largely responsible for the observed variation in thickness of sedimentary strata in the study area. Table 1 shows the subdivision of the strata above the Permian Sumner Group into geologic units.
The Dakota aquifer is composed of the Cheyenne Sandstone, and the Dakota and Kiowa Formations, and is overlain by the Graneros Shale (Table 1). The Cheyenne Sandstone consists predominantly of cross-bedded, fine to medium sandstone with lenses of shale and conglomerate. It was deposited in fluvial to deltaic environments and rests unconformably on Permian and Jurassic rocks (Latta, 1946; Hamilton, 1994). The Cheyenne is variable in thickness, ranging up to 260 ft. (Merriam, 1957), but is typically less than 100 ft. thick, and tends to thicken into paleotopographic lows (Latta, 1946; Merriam, 1957; Hamilton, 1994). This basal Cretaceous unit has been termed the Plainview Member of the Purgatoire Formation on the basis of correlation to the type area in Colorado (Hamilton, 1994; Scott et al., in press).
The Kiowa Formation overlies the Cheyenne Sandstone, is an open-marine shale typically 100-150 ft. thick (Scott, 1970). The Kiowa also contains interbedded sandstone, siltstone, and shale deposited in open-marine, barrier-bar, nearshore, and deltaic environments (Franks, 1966, 1975; Scott, 1970). Where the Cheyenne is absent, the basal Kiowa lies directly on Permian and Jurassic rocks. In south-central Kansas outcrops of the contact between the Cheyenne and Kiowa have been described as both abrupt and unconformable (Franks, 1975). In other outcrop areas the contact is apparently conformable (Latta, 1946). In the subsurface, the transition from Cheyenne to Kiowa is commonly gradational with an upward increase in the proportion of shale, a decrease in sandstone bed thickness, and an increase in bed continuity. The Kiowa sandstones, as observed on subsurface data, are thickest near the base of the Kiowa (typically 30 ft. thick) and show coarsening-upward characteristics. These sand-rich deposits are interpreted to represent transgressive deltaic deposits. Locally, with an extensive log data base, some basal Kiowa deltaic sections can be laterally correlated to basal Cheyenne-type sandstones. Isolated sandstone bodies in the middle and upper Kiowa tend to be thin (less than 10 ft.) and laterally continuous for at least 10-20 mi. and are interpreted as shelf storm deposits. The Longford Member of the Kiowa Formation originally was restricted to regressive siltstones, sandstones, and mudstones in the lower Kiowa in north-central Kansas (Franks, 1979). These facies were interpreted to have been deposited in lagoonal, estuarine, and fluvial environments (Franks, 1980). The term has since been extended to include a range of nearshore facies transitional between the Kiowa Formation and Cheyenne Sandstone throughout Kansas (Hamilton, 1994).
The Dakota Formation in Kansas lies between the Kiowa Formation below and Graneros Shale above (Figure 1). The lower contact is erosional and separates the underlying marine to deltaic deposits from Dakota Formation terrestrial facies (Hamilton, 1994). Thick, cross-bedded unfossiliferous sandstone bodies in central Kansas have been described as marine barrier bars in the Kiowa Formation (Franks, 1979; Bayne et al., 1971). However, evidence in favor of interpreting them as fluvial deposits within incised valleys in the basal Dakota Formation includes concave-up erosional bases, abundance of clay pebbles, and the lack of marine fossils or bioturbation (Hamilton, 1989, 1994). The Dakota is generally 200-300 ft. thick in Kansas (Macfarlane et al., 1990) and consists of fluvial and deltaic/estuarine sandstone bodies encased in a matrix of alluvial plain to shallow marine claystone and siltstone deposits. Plant fossils and thick paleosols are common. In cores and on outcrop the sandstones are fine to medium, well sorted, and contain large-scale and small-scale cross-beds. These sandstone bodies are interpreted as fluvial channel deposits. Paleocurrent indicators throughout the Dakota Formation generally trend west to southwest (e.g., Siemers, 1976; Franks et al., 1959). Basal Dakota sandstone bodies thicken appreciably into paleotopographic lows on the unconformity reaching a maximum thickness of approximately 120 ft. In the lower half of the formation, sandstone beds are abruptly capped by fine-grained deposits or have a fining-upward succession. The Rocktown channel sandstone in the upper Dakota is a long-recognized outcrop expression of a channel sandstone in Russell County, Kansas. This channel contains cross-bedded, fine to coarse, fluvial sandstone in a discontinuous, narrow (1-2 mi. wide or less), sinuous belt that has been traced along a straight-line distance of 27 mi. (Rubey and Bass, 1925; Siemers, 1971, 1976). The upper Dakota fluvial complexes were transgressed and evolved into deltaic and estuarine environments (Siemers, 1976).
The lower Graneros Shale is composed of dark-gray silty shale with starved ripples, irregularly laminated siltstone and fine sandstone, thin-bedded sandstones, and bone and shell lags (Hattin and Siemers, 1987). The Graneros/Dakota contact appears transitional on outcrop and in shallow subsurface cores (Feldman, 1994; Gellasch and Hattin, 1994).
Rocks that constitute the Dakota aquifer are best understood within the framework of sequence stratigraphy (Hamilton, 1989, 1994; Coleman and Turbek, 1992). Sequence stratigraphic models for early Cretaceous rocks of the Western Interior seaway were initially developed along the Front Range in Colorado, where intertonguing of marine and continental rocks and deeply incised sequence boundaries provided key stratigraphic markers (Weimer and Land, 1972). The correlations of sequences on the western edge of the basin were subsequently extended throughout the Western Interior and into Kansas (Weimer, 1984) and then to the Texas Gulf Coast with the addition of biostratigraphic control (Scott et al., in press).
Following the models of Weimer (1984), Hamilton (1989, 1994), Coleman and Turbek (1992), and Scott et al. (in press), the units that make up the Dakota aquifer are divided into three sequences, the Cheyenne-Kiowa sequence, the Dakota J sequence, and the Dakota D sequence (Figures 2 and 3). A major Cretaceous unconformity lies at the base of the Dakota aquifer; Permian and Jurassic rocks were exposed and eroded prior to Cretaceous deposition. The oldest recognized Cretaceous sequence in Kansas is the Cheyenne/Kiowa sequence, which extends from the base of the Cheyenne Sandstone to the top of the Kiowa Formation. The Cheyenne Sandstone was deposited over the unconformable surface in fluvial and coastal-plain environments prior to transgression of the Kiowa sea into Kansas. The sharp contact between the Cheyenne Sandstone and Kiowa Formation is a transgressive surface of erosion that formed by shoreface erosion during the eastward migration of the shore line (Hamilton, 1994; Scott et al., in press). The overlying Kiowa Formation is a transgressive deposit that records widespread marine flooding of the Western Interior basin. The maximum extent of the Kiowa sea is not known because of post-Kiowa erosion, but Franks (1979) suggested that the distribution of nearshore facies in the type Longford Member indicates that the present eastward limit of the Kiowa Formation approximates the maximum extent of the transgression.
The Dakota J sequence extends from base of the Dakota Formation to the D unconformity near the middle of the Dakota Formation (Figures 2 and 3). A major drop in sea level resulted in subaerial exposure and erosion of Kiowa deposits. Thick, cross-bedded fluvial sands and alluvial-plain sediments were deposited within paleovalleys and then across the interfluves. Thick paleosols are common within the J section, and evidence of marine influence is lacking except in extreme western Kansas. The basal Dakota unconformity is correlated in central Colorado with the base of the J sandstone (Weimer, 1984). In the Denver basin this J sequence culminates upwards with the marine Huntsman Shale, which is present only locally in northwestern Kansas (Hamilton 1994). Weimer (1984), Hamilton (1994), Coleman and Turbek (1992), and Scott et al. (in press) all have recognized a mid-Dakota sequence boundary in Kansas that correlates with the unconformity between the D and J sandstones along the Front Range in central Colorado. In northwestern Kansas the boundary between the D and J sequences is placed at the unconformity between the Huntsman Shale below and fluvial facies above. The mid-Dakota sequence boundary is difficult to correlate across Kansas because of the lack of marine facies, but Hamilton (1989, 1994) identified the boundary in central Kansas at an upward abrupt change from meandering-stream deposits to braided-stream deposits. Associated with the braided river deposits are granules of chert and quartz that may indicate a lowering of base level (Hamilton, 1989). This sequence boundary has been identified in the Amoco Bounds #1 core from western Kansas as falling within a 30 ft. section of paleosols and overbank deposits (Hamilton, 1994; Scott et al. in press). The D sequence extends from the mid-Dakota sequence boundary to the unconformity at the base of the Carlile Shale (Hamilton, 1989) above the Graneros Shale.
Thick, clean, discontinuous fluvial channel sandstones were deposited above the D unconformity. In the upper Dakota, sandstone bodies are thinner and more laterally continuous, reflecting a transgressive deltaic/estuarine setting. In outcrop uppermost Dakota deposits show evidence of marine influence, including bioturbation and rare body fossils (Seimers, 1971; Gellasch and Hattin, 1994). The contact between the Dakota and overlying marine Graneros Shale has been interpreted as a transgressive surface of erosion (Hamilton, 1994).
Geophysical logs are the continuous records of physical properties of rocks in boreholes drilled for hydrocarbons, minerals, or water. They are made by electrical, acoustic, and nuclear tools suspended on a wireline and winched upwards through the formations penetrated by the borehole. Most logging tools are run by the oil industry in both exploration holes and producing wells. Consequently, log analysis is a methodology geared mainly to the location of oil and gas. Fortunately, many of the rock properties that are used to characterize hydrocarbon reservoirs are also key parameters for aquifers. However, there are also some significant differences, so that adaptations of traditional methods of data analysis must be made with caution. The long history of oil exploration in Kansas has resulted in the recording of hundreds of thousands of logs across the state. The Dakota Aquifer Program has developed a large regional database of digital gamma-ray logs drawn from this library, which are being used for detailed mapping of inferred aquifer and aquitard units. This data base can be found on Volume 2 of this CD set.
Gamma-ray logs record natural gamma radiation, which can be attributed mainly to sources of potassium-40 and radioactive isotopes of the uranium and thorium families. The logs are conventionally recorded in API (American Petroleum Institute) units, which although arbitrary, do allow consistent comparisons between boreholes (Doveton, 1986). The sandstones of the Dakota aquifer have fairly low radioactivity since they consist mainly of quartz and water, with only minor amounts of clays, feldspars, and other accessory minerals (Franks, 1966). By contrast, the interbedded shales have moderate radioactivity caused by thorium adsorbed on the clay platelets, potassium in the composition of some clays, and variable amounts of uranium generally associated with organic matter and fixed under reducing conditions. Further details on the use of logs for determining the lithologic character of the subsurface can be found in the Petrophysics directory on this volume of the CD set.
Digital gamma-ray logs from wells along a lengthy east-west traverse along T. 16 S. in central and western Kansas were transformed to gray-level image strips, where the darkness intensity is a function of the natural gamma radioactivity of the logged formations. Under this system, sandstones and limestones appear as white or pale gray, while shales register as dark gray or black. When hung together on a common stratigraphic horizon (the top of the Graneros Shale) and arranged in correct geographic order, the result is a regional gray-level image of the subsurface geology of the Permian to Cretaceous sequence shown schematically in Table 1. In order to accentuate differences between the various lithologies, the gray levels were further converted to a range of false colors (Figure 4). Lighter colors were selected to represent low gamma-ray values and darker colors for high gamma-ray values. The major advantage of this approach is that the information is coded in a visual form that is close to a simulation of how the subsurface geology would actually appear, when making allowance for the vertical exaggeration. The immediacy of the image frees the observer to focus easily on structural and stratigraphic features of all kinds. In addition, gamma-ray-log cutoff criteria, discussed later in this paper, for aquifers and aquitards can be applied immediately in visual assessments of aquifer geometries.
The original version of the regional image required remedial processing to normalize the gamma-ray logs on which the image is based. Two stratigraphic units were selected to function as calibration standards: the Graneros Shale (Upper Cretaceous) as the "hot" standard (high gamma-ray values and darker colors) and the Stone Corral Formation (Lower Permian) as the "cold" standard (low gamma-ray values and lighter colors). The use of two different standards allows correction of random errors that involve both shift and stretch of the log with respect to its true scale. The choice of each was based on a geological perception that major local changes (at a scale less than the distance between section wells) would be fairly minor. Instead, the gamma-ray response of these units would be dominated by a regional trend but confounded by random measurement error.
On the cross section in Figure 4, the Stone Corral Formation forms a distinct and continuous light-colored band at the base of the section. The Cedar Hills Sandstone (Table 1) is the most obvious feature in the overlying Permian as a thick wedge that subcrops at the base of the Cretaceous to the east. In the interval that includes the Dakota Formation, the Kiowa Formation, and the Cheyenne Sandstone, most of the strata register as blue, purple, and red bands on the cross section. This results from the shaly character of the mudstone that makes up most of the Dakota aquifer. In contrast, the sandstones register as light yellow to orange in the vertical section. Sandstones in the Cheyenne-Longford sequence appear to be traceable for considerable distances, and contrast with the rather patchy sandstone developments within the Dakota Formation. The patchiness is the expected outcome of the intersection of the east-west cross-section plane with the sinuous deposits of alluvial channel networks and the complex facies mosaics of the coastal and alluvial plains. The more shaly part of the marine transgressive sequence in the Upper Cretaceous above the Dakota Formation (Graneros Shale, Greenhorn Limestone, Carlile Shale in Table 1) shows as darker-colored blue, green, and black regional bands near the top of the section. The lighter orange to reddish-purple bands are chalky shales and limestones in the Greenhorn Limestone. At the very top, the chalky limestone of the Fort Hays Member, Niobrara Chalk, shows as a yellow to orange band in the western part of the cross-section.
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Weimer, R. J., 1984, Relation of unconformities, tectonics, and sea-level changes, Cretaceous of Western Interior, U.S.A., in J. S. Schlee, ed., Interregional Unconformities and Hydrocarbon Accumulation: American Association of Petroleum Geologists, Memoir 36, p.7-36.
Dakota Home || Geologic Framework Index
Kansas Geological Survey, Dakota Aquifer Program
Updated April 8, 1996.
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