Kansas Geological Survey, Open-File Rept. 91-1a
Pre-Graneros Paleogeography--Page 5 of 16
Kiowa Formation and Cheyenne Sandstone
The additional subsurface data from this study extends the previously defined northeastern limit of Kiowa deposition to at least the Kansas-Nebraska state line. Franks (1979, 1980) could not distinguish Longford Member strata from the overlying Dakota Formation in outcrops north of T.3 S. in southern Washington County and presumed that these beds had been removed by prelocated in 1.3 mi (2.1 km) south of the Kansas-Nebraska border. Franks found a maximum Longford thickness of 100 ft (30 m) in central Kansas. The total thickness of Longford Member strata encountered in the test holes ranges from 43 ft (13 m) in the Gaydusek WII to 200 ft (60 m) in the KGS #1 Jones (Figure 8). Much of the thickness variation is attributable to pre-Dakota erosion and the resultant paleotopography on the Permian surface as evidenced by thickness variations in the overlying Dakota Formation and the facies succession of strata near the base of the Kiowa in the KGS #1 Jones.
Figure 8. Regional lithofacies variations and stratigraphy of the Dakota Formation, Kiowa Formation, and Cheyenne Sandstone in a cross section through the FY89-90 test holes.
Franks (1979, 1980) believed that the Longford Member is laterally equivalent to and overlapped by a Kiowa marine shale facies in central Kansas. A marine shale facies overlying deltaic Longford strata was not observed in any of the test holes because of either nondeposition or pre-Dakota erosion. In the KGS #1 Jones core, strata deposited in deltaic environments [(273-463 ft) (83.2-141 m)] similar to the Longford lie above a marine black shale (Figure 6). We interpret these strata to be deposits of a deltaic lobe of the Longford Member because the strata in the core are progradational deposits laid down at about the same time and along the same coastline as those in the nearby outcrop. Walters and Bayne (1959) and Mack (1962) reported a dark-gray clay shale at the base of the Cretaceous near the western edge of the Kiowa outcrop in central Kansas east of the KGS #1 Jones. An outcrop stratigraphic section reported by Bayne et al. (1971) in southeastern Ellsworth County also records the occurrence of black shale at the base of the Cretaceous. In the KGS #1 Jones test hole and in the areas reported on in the preceding text, absence of strata similar to the Longford above the Permian-Cretaceous unconformity indicates that Longford sediments did not accumulate before inundation by the Kiowa sea but were paralic deposits along the eastern margin of the Kiowa sea.
Several occurrences of red-mottled gray siltstone within the Longford have been documented. Red-mottled siltstones were only encountered in the KGS #1 Jones and the #1 Brungardt test holes, where they were associated with lignite. Franks (1979, 1980) described nonmarine red-mottled siltstones in the Longford and attributed the occurrence of red mottles to the initial stages of soil formation in alluvial sediments under the influence of a fluctuating water table. However, this occurrence of nonmarine red-mottled siltstones interbedded with fluvial sandstone in outcrop is a contrast to the interbedding of similar red-mottled siltstones with bioturbated sandstones in the KGS #1 Jones test hole to the west. We interpret the alternating sequences of red mottled gray siltstones and bioturbated fine-grained sandstones as stillstand units (Clifton, 1982) deposited by successive episodes of estuarine sedimentation near the boundary separating the supratidal and intertidal zones and associated with an overall rise in sea level.
The most important similarity between the Kiowa intervals penetrated by each test hole is the association that each has with tidally influenced deltaic environments, as the deposits prograde into the Kiowa sea (Galloway and Hobday, 1983; Clifton, 1982). In the Gaydusek WII, Kenyon #1, and KGS #1 Jones cores, evidence of tidal currents in a deltaic environment was observed in the tidal sand ridge, estuarine distributary, and tidal channel facies. Tidal influences are also alluded to by Franks (1979, 1980) in his description of Longford characteristics and his interpretation of depositional environment. He reported finding predominantly ripple laminated to wavy bedded to horizontally bedded strata and instances of flaser bedding interlaminated strata and low-angle strata and low-angle stratification in outcrops of his capping siltstone. He also reported finding abundant plant material and trace fossils preserved in the sediments. These are sedimentary and biogenic characteristics typical of estuarine, tidal flat, or channel depositional environments.
In the KGS #1 Jones test hole sand grains incorporated in the black shale may be the only remnant of a transgressive lag that was preserved during the transgression of the Kiowa sea. Immediately above this lag black shale and silty shales contain plant fragments and small fossilized mollusks. Walters and Bayne (1959) mention the absence of a transgressive lag deposit at the base of the black shale they found at the base of the Kiowa near the western edge of the outcrop belt. In the Gaydusek WII test hole, distal bar facies interbedded with debris flows are present just above the Cretaceous-Permian unconformity.
Nonmarine fluvial facies consisting of channel sandstones and associated overbank deposits are part of a transgressive sequence in the central Kansas outcrops of the lower Longford and occur as the Cheyenne Sandstone in the Haberer, Braun #1, and #1 Brungardt test holes. Sandstones make up the majority of the formation in the Braun #1 test hole and are probably locally derived from the underlying Permian deposits. However, in the Haberer and #1 Brungardt test holes the Cheyenne consists largely of nonmarine gray to greenish-gray and white sandy siltstone and silty very fine-grained sandstone that represent levee and channel deposits. Cheyenne Sandstone core samples recovered from the Haberer test hole display few definitive primary sedimentary structures and are nonfossiliferous. These strata are similar to siltstones found in the subsurface to the south and have been described from drill cuttings by McLaughlin (1949), Fent (1950), and Latta (1950).
The eastward thinning Cheyenne Sandstone pinches out by onlap between the KGS #1 Jones and the Haberer test holes in the cross section (Figure 8). Along the west side of the cross section the Cheyenne is present as only a thin, locally mappable unit. The underlying Permian Cedar Hills Sandstone subcrop is nearly coincident with the Cheyenne pinchout, suggesting that it may have been a positive topographic feature and sediment source are during Cheyenne deposition. After Cheyenne deposition in this area, a major transgression of the Kiowa sea shifted facies tracts to the east and drowned the Cedar Hills paleo-outcrop.
Lateral facies changes in Figure 8 are interpreted to occur within an overall transgressive-regressive phase of deposition from fluvially deposited sandstones and mudstones (Cheyenne Sandstone and basal Longford Member, Kiowa Formation) through open marine shales (marine shale facies, Kiowa Formation) to subaqueous and subaerial deltaic sediments (Longford Member, Kiowa Formation). This series of vertical changes suggests that the central Kansas area is near the maximum extent of transgression of the Kiowa sea. Furthermore, based on core evidence, such as that of the KGS #1 Jones, the initial transgression of the Kiowa sea must have occurred relatively swiftly over a land surface with some relief. Marine shales were deposited directly upon fluvial sediments in alluvial valleys, whereas, paralic deposits accumulated on the surrounding uplands to the east. Because the Dakota Formation unconformably overlies the Longford Member (Figure 8) and the original contact between the Kiowa-Longford and the Dakota has been removed by errosion, the maximum extent of the transgression of the Kiowa sea into this part of central Kansas is not known.
The Dakota Formation is part of an overall transgressive sequence that culminated in the deposition of the Greenhorn Limestone (Hamilton, 1989). During Dakota time in central Kansas sediments were deposited in response to landward-stepping progradational episodes during two periods of rising sea level (the J and D sequences) separated by a period of falling sea level (Hamilton, 1989). The base of the Dakota Formation is marked by an unconformity, the J unconformity, which is a result of ravinement by streams after the retreat of the Kiowa sea; this unconformity marks the base of the J sequence. The J unconformity has been dated in Colorado at 97 m.y. (Weimer, 1984). The maximim extent of the transgression associated with deposition of the J sequence in central Kansas is unknown because of truncation of the turnaround point by the D unconformity, located near the middle of the Dakota Formation. The D unconformity has been dated at 95 m.y. in Colorado (Weimer, 1984). Strata above the D unconformity in the Dakota Formation and the Graneros Shale are assigned to the D sequence (Hamilton, 1989).
Evidence of the J unconformity was recovered in the Kenyon #1 and Gaydusek WII cores where fluvial channel deposits directly overlie the marginal marine sediments of the Kiowa Formation at a sharp erosional contact. In the Gaydusek WII core, rotated blocks of lithifield marginal marine sediment of the Kiowa Formation up to 4 ft (1.2 m) thick are preserved within the basal Dakota channel sandstone, having slumped into the Dakota river channel, which was eroding the Kiowa Formation at this site. Franks (1966, p. 194), in describing an outcrop in T.7S., R.1E. (Clay County) notes slumping of Kiowa rocks induced by erosion and differential loading immediately below a basal Dakota sandstone. In the other test holes the erosional surface is inferred from the change in lithology seen from one side of the disconformity to the other in cores, drill cuttings, and probe responses recorded on the geophysical log traces.
The position of the D unconformity in the Haberer, KGS #1 Jones, and Kenyon cores in central Kansas is not known for certain. However, a fluvial channel sandstone containing an intraclastic conglomerate at the base and a fining-upward sandstone (from medium- to fine-grained) was recovered from the Gaydusek WII core at the expected position of the D unconformity. Deposition of coarser grained fluvial sediments above the D unconformity in the Gaydusek WII core and in the Braun #1 drill-cuttings and in outcrop suggests higher energy conditions in this part of the Dakota Formation than in the underlying upper part of the J sandstone sequence. Figure 8 shows that the D unconformity is a relatively flat surface over most of the cross section in contrast to the J unconformity and that it is parallel to the top of the Graneros Shale, which for all practical purposes is considered to be a time line. This suggests a low-relief depositional surface before deposition of D sandstone sediments.
Most of the sediments of the Dakota Formation in all the test holes were deposited above sea level in fluvial environments. The Dakota strata consist mostly of overbank variegated mudstone and very fine-grained levee and splay sandstones. The rest of the fluvially deposited strata consists of river channel fill, predominantly fine- to medium-grained crossbedded sandstone. However, in the abondoned channels [e.g. between 350-380 ft (107-116 m) in the Gaydusek WII test hole], the channel fill consists of poorly sorted, carbonaceous, sandy siltstone and mudstone. The maximum grain size in the river channel sandstones decreases upward reflecting the decrease in competence of the rivers as their base level rose with time because of sea level rise. The basal portion of the J sequence in all the test holes is nearly always marked by a fluvial channel sandstone. Above this section the dominant lithology is a variegated overbank mudstone, but in the Jones and Haberer test holes thick, vertically stacked sequences of channel sandstone predominate. This stacking of channel sandstones and dominance of overbank mudstone is a typical feature of fluvial deposition during landward stepping of the shoreline (Hamilton, 1989).
The upper 45-90 ft (14-27 m) of the D sandstone consists mainly of subaqueous, river-dominated deltaic sediments. There are coarsening-upward sequences of interdistributary bay fill (penetrated by large roots and capped by a lignite in the Kenyon #1 and KGS #1 Jones test holes) and delta-front distal bars as well as very poorly sorted, fining-upward distributary fill. The grain size within these deltaic sediments is rarely coarser than fine grained, reflecting the low competence of the upper Dakota rivers that supplied the sediment and the general lack of higher energy marine sediment supply processes. The cleanest sandstones were deposited under the relatively high-energy conditions of distributary mouth bars. They contain abundant mica and have a fairly high organic content. These bars constitute the uppermost shallow-water fill of interdistributary bays and the accumulations at the mouths of the main distributaries (Coleman and Prior, 1988). There is little evidence of marine processes within the subaqueous deltaic sediments. However, lenticular bedding, which may be due to tidal processes, is present in some of the distal bar sediments.
The most common sediments of the deltaic system at the top of the Dakota were deposited in shallow interdistributary bays and on the delta front. The sediments of these two subenvironments are similar coarsening-upward distal bar-distributary mouth bar sequences capped with lignites in completely filled bays. Active distributary channel-fill deposits are rare, indicating a lack of meandering and the consequent lack of preservation of sandy bedforms. The poorly sorted fill of abandoned channels is more common because distributary channels in a river-dominated delta are prone to abandonment (Coleman and Prior, 1988). In contrast, Siemers (1971), who worked in the outcrop areas of the upper Dakota Formation in central Kansas, observed planar tabular, high-angle cross-stratification and unidirectional paleocurrents in the Rocktown channel sandstone, an active distributary channel sandstone.
The contact of the Dakota Formation with the overlying Graneros Shale in the cores and in outcrop is commonly an abrupt transgressive disconformity where delta-front deposits were overstepped by the Graneros sea. Sudden reductions in sediment supply at a particular point because of deltaic lobe switching contributed to the sharp contact. Thus an abrupt disconformity does not necessarily reflect a sudden acceleration of sea-level rise. Although the coastline as a whole was moving landward, sections of it close to the main fluvial sediment supply, that is, in the vicinity of the mouths of major distributaries, were prograding. This progradation can be seen in the cores as coarsening-upward sequences. The combination of locally prograding delta lobes, an advancing Graneros sea, and a low-gradient depositional surface likely led to an irregular shoreline locally. The net result is that strata deposited in deltaic environments in the upper part of the Dakota Formation laterally interfinger with and are temporally equivalent to the lower section of the marine Graneros Shale.
Within this overall framework the test holes show that there is considerable variety in the character and thickness of the Dakota Formation in three ways. First, the total thickness of the Dakota Formation ranges from over 300 ft (91 m) in the northern counties of the state to 280 ft (85 m) in the Haberer core and 220 ft (67 m) in the KGS #1 Jones core (Figure 8). Assuming little tectonic activity during most of Dakota deposition, the large variation in thickness of the Dakota Formation [over 110 ft (34 m)] is due to the relief of the J unconformity surface. The base of the J sequence in the vicinity of the KGS #1 Jones test hole was probably a topographic high in this erosional surface cut into the Kiowa Formation. The unusually thick section of the Kiowa Formation preserved here has not been observed elsewhere in central Kansas because of the pre-Dakota erosion that resulted in the J unconformity.
Second, the deltaic sediments at the top of the Dakota Formation vary in character between different surface and subsurface sections. This is a function of the lobate nature of river-dominated deltaic deposition and is consistent with the large number of subenvironments typical of deltaic systems. Sand-dominated facies are concentrated in the relatively high-energy subenvironments, such as distributary mouth bars. That the distributary mouth bar sands were not redistributed as a sand sheet across the top of the Dakota Formation is evidence that river currents dominated over tidal currents and marine forces. The local nature of the river influence resulted in the patchy distribution of the relatively high-energy distributary mouth bar sediments.
Third, the subaqueous deltaic sediments in the Dakota Formation vary considerably in thickness (Figure 8). They are thickest at the Kenyon #1 and Gaydusek WII test holes [78 ft (21 m) and 68 ft (24 m), respectively] and much thinner in the KGS #1 Jones [53 ft (16 m)] and the Haberer [42 ft (13 m)] test holes. The expected trend in thickness of deltaic deposition was for a slight thinning toward the east because of the gradual lowering of the gradient of the depositional surface resulting in less accommodation space below sea level. This eastward thinning can be seen between the Kenyon #1 and the Gaydusek WII test holes. This difference in thickness may be tectonically related or a function of subsidence resulting from compaction of the great thickness of fluvial plain Dakota sediments at the Kenyon and Gaydusek sites, most of which is overbank mud. Subsidence at these sites occurred at a greater rate than at the Jones and Haberer sites possibly due to the extra thickness of unconsolidated muddy Dakota sediment. (The Kiowa Formation was already consolidated and more resistant to further compaction.) Thus the relief on the Dakota-Kiowa unconformity may be a major control on the thickness of the deltaic Dakota Formation section and a factor affecting total formational thickness. However, local thickness variations of the Graneros Shale and of the deltaic sediments in the D sandstone may also indicate that this part of the Central Kansas uplift and adjacent Salina basin were tectonically active during late Dakota and Graneros time. Uplift or subsidence as a result of slight vertical movement of Precambrian bault blocks would have significantly altered deposition locally by focusing sediment dispersal away from structurally higher areas (Sonnenburg and Weimer, 1981; Weimer, 1984). Thinning of the Graneros Shale in the Haberer and #1 Brungardt test holes over the Central Kansas uplift (Figure 8) suggests that this area may have been structurally higher than surrounding localities during late Dakota and Graneros deposition.
Previous page--Borehole Stratigraphy ||
Next page--Paleogeographic Implications
Start of this report || Table of Contents