Valley anticline associated with the Warrensburg Sandstone in Missouri.
Key Words: valley anticline, Warrensburg Sandstone, Pennsylvanian, Desmoinesian Series, Cherokee Group, slumping, channel sandstone, valley incision.
In the Warrensburg, MO area, U.S. Highway 50 exposes a two mile wide cross-section of the northsouth trending Pennsylvanian Warrensburg Sandstone as well as dipping layers of the older Marmaton limestones and shales on both the east and west margins of the sandstone body. These Marmaton Group strata along both edges of the Warrensburg Sandstone dip about 25-30 degrees away from the sandstone body on both sides, giving the impression that the sandstone was deposited along the axis of an eroded anticline (Emerson, 1975; Beall, 1975; Emerson and Nold, 1981; Nold and Emerson, 1989). This relationship extends for at least 10 km to the north and 20 km to the south of Warrensburg (Figure 1). More importantly, where Cenozoic erosion along the Blackwater River and its tributaries has cut through the bottom of the Warrensburg Sandstone, the same anticlinal dips are observed in the Desmoinesian rocks beneath the channel (Figures 2 and 3). Furthermore, where the Warrensburg Sandstone bifurcates south of Warrensburg, ther e is a valley anticline present along both branches (Figure 1). The coincidence between the branches of the Warrensburg Sandstone and the valley anticlines implies some relationship between the two.
In contrast, the regional structure in Johnson County is dominated by northwest and northeast trending faults and folds (McCracken, 1971). Structure contour maps of Ordovician and Mississippian rocks in the subsurface of Johnson County, based on dozens of well logs, show the northwest-southeast strike. The valley anticline in the Desmoinesian rocks beneath the Warrensburg Sandstone bears no relationship to regional structures.
The Warrensburg Sandstone is a north-south trending Pennsylvanian alluvial valley-fill which crops out for more than 80 kilometers in Lafayette, Johnson, and Henry Counties, Missouri (Figure 1 and Emerson, 1979). The present maximum thickness of the Warrensburg Sandstone is about 100 feet based on missile site cores. The original thickness is unknown as the Warrensburg is the youngest formation exposed in the outcrop area. The Warrensburg Sandstone has been assigned to the Pleasanton Group, Missourian Series, by the Missouri Geological Survey (Howe and Koenig, 1961; Thompson, 1995). The Warrensburg Sandstone Lies with angular unconformity upon cyclothemic rock units belonging to the Desmoinesian Series (Figures 3 and 4). The lower portion of the Desmoinesian, the Cherokee Group, Consists mainly of shales, claystones, coals, and thin sandstones, the total thickness of the Group ranging from 200 to 300 feet thick within the region. The overlying Marmaton Group has a maximum thickness of 100 feet and is composed mainly of shales and thin limestones.
The above thickness of the Desmoinesian rocks is taken from water well logs and missile site cores. Both Groups are poorly exposed due to soil development and vegetative cover. Outcrops in Lafayette County and northern Johnson County are further obscured by a mantle of glacial deposits.
The Post Oak Creek Cross-section at Old Highway 13 North
Figure 3 is a structural cross-section along Post Oak Creek showing the angular unconformity between the horizontal Warrensburg Sandstone and an excellent section of underlying dipping Desmoinesian Series sedimentary rocks. On Figure 1 the section is exposed about two miles north of Warrensburg where Post Oak Creek has downcut through the Warrensburg Sandstone. The section is 400 feet in length and, except for rocks covered by 44 feet of bridge abutment, has nearly continuous exposure. The section is shown with west on the right and east on the left because it is exposed on the south side of Post Oak Creek and is observed while facing toward the south. The dip angle of the tilted rocks averages about 30 degrees on the west end of the section and gradually changes to approximately 60 degrees on the east end of the section. The tilted Demoinesian rocks are a series of shales, claystones, sandstones, coals, and limestones. No attempt has been made to divide these rocks into individual Formations because when the Pennsylvanian cyclothemic sedimentary rocks are structurally deformed and not in their normal stratigraphic order, identification is nearly impossible. We are certain that the tilted rocks are mostly Marmaton Group, with perhaps some underlying Cherokee Group rocks present as well.
Two normal faults are present within the tilted Demoinesian Series rocks, the west fault being at 45 feet and the east fault at 300 feet on the 400 foot section (Figure 3). The west fault strikes north and dips about 60 degrees west and drag of beds on both sides definitely show it to be west side down, east side up. The east fault strikes a little west of north and has a vertical dip; no dragged beds are observed and the fault is inferred to have the same type of displacement as the one on the west, that is west side down, east side up. Adjacent to the east fault is a small fault that displaces coal and shale (Figure 3). The two normal faults are inferred to divide this section into three slump blocks which are present in this portion of the east side of the valley anticline.
Inferences about the amount of displacement of the two normal faults are of considerable interest. First, the west fault. If the sequence coal, shale, sandstone that is exposed between zero and 30 feet is the same sequence, repeated by the west fault, as that exposed from 153-183 feet (Figure 3), then the stratigraphic separation and the slip of the fault would be approximately 80 feet. Estimating the amount of movement of the east fault is more tenuous. If we are correct in our inference that the fault is west side down, then a minimum of about 140 feet of stratigraphic separation would be required in order for the three limestones in the east fault block not to be exposed in the central fault block.
The three limestone units in the east block are in themselves a stratigraphic problem. We suspect them to be Marmaton Group limestones but three limestones should not be present so close together within the Group. Perhaps there is faulting which is not exposed that is present between some of the limestones.
Similar Midwestern Structures
Hinds (1912), and Hinds and Greene (1915) mentioned several localities in east-central Missouri where the Moberly channel sandstone (Missourian Series) overlies dipping Cherokee Group and Marmaton Group rocks. Both Jackimovicz (1970) and Gentile (1976) found channel sandstones overlying tilted Marmaton limestones in Bates County, western Missouri. Unklesbay (1952) commented on a channel sandstone unconformable on a tilted Marmaton limestone in Boone County. The present authors did field work on the Moberly Sandstone and found that outcrops in that area, in comparison to the Warrensburg area, were extremely poor. In addition, several specific localities were visited that were described by Hinds (1912) and Hinds and Greene (1915), and it was found that the outcrops no longer existed.
In nearby eastern Kansas, dipping strata have been noted along the margin of the valley-fill Ireland Sandstone (Pennsylvanian System, Virgilian Series). In Franklin County, steeply dipping Robbins Shale is found below the Ireland Sandstone (Ball et al., 1963). O'Connor (1960) found undisturbed beds of Ireland Sandstone overlying steeply dipping Weston Shale and Stranger Formation in Douglas County.
Simmons (1966) describes valley anticlines formed during the Recent within Ordovician limestones and shales in central Kentucky.
Utah Valley Anticlines
Huntoon (1982) discussed anticlinal river valleys in Utah. The Meander anticline in Utah, has its axial trace along the Colorado River for 41 km and on the southeast side of the river, eight tributary canyons also contain valley anticlines. Dips on the limbs of these anticlines range from a few degrees to more than 30 degrees. The rims of the canyons are tilted away from the river as much as a mile from the axis. These canyons are eroded to a depth between 122 and 548 m into Permo-Pennsylvanian sedimentary rocks.
Harrison (1927), Shoemaker (1973), and Huntoon (1982) considered the Meander anticline an unloading feature and both Harrison (1927) and Huntoon (1982) believed that the Meander anticline is still growing. The mechanisms proposed for the origin of these features include salt flowage, salt solution, and brittle plate gliding.
Deformation Due to Valley Incision - Examples from Civil Engineering
The terms stress release, rebound, unloading, shale flow, and bulging have all been used by civil engineers and engineering geologists to describe the formation of upraised valley floors and tilted valley margin rocks developed after stream incision or after excavation. One of the earlier references to the formation of valley margin anticlines and valley axis bulges is by Hollingworth et al. (1944). This study of the Middle Jurassic age Northampton Ironstone Field in England found that upward movement of valley floors, composed of Upper Lias clay, caused high dips on rocks of the valley margin. Steep dips on valley side limestones are shown in cross sections (Figure 5), with dips from 10 degrees up to 80 degrees found in one valley for more than a kilometer. Contorted valley floor clays and marginal limestones which dip away from the axis are confined to valleys. Cross sections of Lias clay exposed in dam trench excavations for large reservoirs show highly contorted and thrust faulted layers.
Lydekker (1883) stated that broken, contorted, and steeply dipping strata adjacent to the valley bottom have been recognized in England.
Valley floor rebound due to stream incision in Tertiary and Cretaceous siltstones, clay shales, and sandstones of the Great Plains of Canada and the western United States is well documented in engineering literature concerning damsite investigations. These valley anticlines have been studied by several investigators (Crandell, 1958; Peterson, 1958; Matheson, 1972; Matheson and Thompson, 1973). Similar structures in Romania were described by Zaruba (1956). All described raised, tilted valley rims and contorted valley floor rocks. Ferguson (1967) found that in the Allegheny Plateau, that the valley bottoms were deformed by arching and thrust faulting, apparently caused by stress release during valley incision.
Nichols (1980) reviewed the literature concerning valley-floor expansion in terrains composed of clay, clay shale, and shales interbedded with limestones and dolomites. These studies from the United States, Canada, and western Europe indicated that unloading response begins at the time of incision and is continuous over long periods of time. Long-term valley floor uplift may be up to ten percent of valley depth. Rebound of .01 m (.04 ft) per year has been measured at Fort Peck Dam, Montana, since 1937 (Matheson and Thompson, 1973).
Unloading response can be rapid in both competent and incompetent rocks. Legett (1973) described a newly excavated limestone quarry floor in St. Louis County, Missouri, which overnight developed a ridge 60 cm high and 90 m long. Hollingworth et al. (1944) noted that Sir Malcolm Watson visited the Panama Canal in 1913 and observed bulging in the floor of the Culebra Cut. At one place a steam shovel working the Cut had been raised and tilted so that it fell over.
Incision of the Warrensburg Valley and Formation of the Valley Anticline
The Warrensburg Sandstone has a maximum present day thickness of 100 feet as shown by missile site cores and by water well logs. The maximum thickness before surface erosion is unknown. The outcrop is 2-3 miles wide in the Warrensburg area (Figures 1 and 2). For comparison, a nearby reach of the Missouri River, with a channel of similar width, has incised its bedrock channel about 300 feet below the bluffs and presently contains more than 100 feet of Quaternary alluvium.
The deep incision of the Warrensburg valley was probably due to the rapid Carboniferous drops in sea level documented by Vail et al. (1977) and similar to valley incision caused by Early Cretaceous rapid lowering of sea level (Weimer, 1982). Heckel (1986) presented a Late Pennsylvanian glacial-eustatic sea level curve for the midcontinent. A major sea level drop at the Desmoinesian-Missourian boundary correlates well with the time of incision of the Warrensburg valley. The only known fossil age assignment was obtained from a thin coal in the basal conglomerate of the Warrensburg Sandstone collected by Emerson (1988). An age assignment of latest Desmoinesian by Palinex International for the basal Warrensburg fits well with the observed stratigraphy.
The very coarse boulder conglomerate derived from erosion of the Marmaton Group limestone beds and contained in the basal Warrensburg Sandstone in Johnson and Henry Counties is evidence of steep valley sides and of mass movement to introduce this material as channel lag deposits (Emerson, 1975, 1977).
The incision of the valley in which the Warrensburg Sandstone was deposited apparently allowed the development of the valley anticlinal structure. Two possible mechanisms for development of this structure are suggested. The first method is slumping of large blocks of steep valley sides. Bristol and Howard (1974) studied the sub-Pennsylvanian unconformity in the Illinois Basin. A Late Mississippian (Upper Chesterian Series) sea level drop (Vail et al., 1977) recognized in the Illinois Basin and southern Appalachians, caused entrenchment of a system of valleys in the Chesterian marine limestones and shales followed by alluvial valley-fill of Pennsylvanian Caseyville Formation clastics. Electric logs and cross-sections from drill holes show great slump blocks of Chesterian strata arranged en echelon along the sides of the valley bottoms (Figure 6). Each block has rotated along the curved plane of a listric normal fault, so that the dip of the displaced strata is away from the valley axis. The slump blocks range from 10 to 125 feet in thickness and the maximum vertical displacement is 200 feet. Individual blocks are several hundred to 3000 feet long and up to several hundred feet wide. The northward advance of the Pennsylvanian sea across the area caused the streams that had been actively downcutting to begin aggrading and filling their valleys with the Caseyville alluvial sediments.
We believe that the rapid sea level drop (Heckel, 1986) at the end of Pennsylvanian Desmoinesian time caused the incision of the Warrensburg valley system and slumping of blocks of Marmaton (Upper Desmoinesian) limestones and shales along the valley sides. Subsequent sea level rise and alluviation during Missourian time deposited the Warrensburg Sandstone in this valley. Post-Paleozoic erosion of the land surface in this area has exposed the lower part of the Warrensburg valley fill sandstone underlain and flanked by the tilted slump blocks. The slump blocks tilted backwards as they moved downward into the valley giving the appearance of an anticline caused by structural deformation.
Figure 7 is an interpretive cross-section showing the Warrensburg river valley and the Warrensburg Sandstone during Pennsylvanian time. Also shown are the rotated slump blocks which have caused the tilting of the bedding away from the channel axis resulting in the valley anticlinal structure. This mechanism also explains the apparently excessive stratigraphic thickness for the tilted Desmoinesian strata beneath and adjacent to the Warrensburg Sandstone by repetition in individual slump blocks. The Marmaton Group is the only part of the Desmoinesian Series containing distinctive limestones more than 4 feet thick. These limestones are, in ascending order, the Blackjack Creek, the Higginsville, the Myrick Station, and the Coal City. Well logs and missile site cores from the area show that these four limestones occur in no more than 70 feet of stratigraphic section (Thompson, 1995, p.104). The north-south striking belts of tilted Marmaton strata (and perhaps some underlying Cherokee Group strata) beneath and adja cent to the sandstone body make up at least several hundred feet of stratigraphic thickness. Repetition of the 70-100 foot thick Marmaton strata is the most logical explanation for this anomalous thickness. The Post Oak Creek cross-section at old Highway 13 north (Figure 3) shows two of the faults on the east side of the valley anticline which repeat the Marmaton strata within individual slump blocks. In general, relatively poor exposures within the area have allowed the inference of other faults between the slump blocks in only a few localities.
The alternative mechanism for the development of the valley anticline is that of tilting of the strata on the valley sides from bulging due to flow of the Cherokee Group shale and claystone toward the axis of the developing valley. This flow would be due to a pressure gradient caused by valley incision. This mechanism would be similar to that suggested by Hollingsworth et al. (1944) for Jurassic strata in England (Figure 5). Though exposures are poor in the area, exposures of shale beneath the Sandstone do not show the types of internal deformation that would be expected to be present if the structure was caused principally by shale-flow. Perhaps if this mechanism was operative, it was minor in importance compared to slumping.
Summary and Conclusions
Detailed field mapping of the structures under and adjacent to the Warrensburg Sandstone shows that the Desmoinesian strata have been deformed into a valley anticlinal structure. Analysis of our field maps and structural cross-sections indicates that the tilted Desmoinesian strata adjacent to and beneath the Sandstone have an excessive thickness that is probably due to repetition. The valley anticlinal structure and the amount of repetition can best be explained by the breaking up of the valley walls into numerous slump blocks which moved toward the axis of the valley, the resulting tilt of the bedding being caused by rotation of the blocks. In addition, a pressure gradient within the underlying Cherokee Group shale and claystone may have caused flow toward the developing valley which resulted in thickening of the shales and tilting of the valley sides away from the center.
Lastly, examination of the literature leads us to believe that valley anticlines are a widespread phenomenon but, with a few notable exceptions, they are not well known to a majority of the geologic profession other than those involved in dam construction.
We acknowledge D. Barber and J. Beall for assistance in mapping. In addition, three anonymous reviewers made suggestions which were helpful in improving the manuscript.
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|Author:||Nold, John L.|
|Publication:||Transactions of the Missouri Academy of Science|
|Date:||Jan 1, 2001|
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