|Original published in D.F. Merriam, ed., 1964, Symposium on cyclic sedimentation: Kansas Geological Survey, Bulletin 169, pp. 199-203|
Stanford University, Stanford, California
Marine banks, consisting of locally thickened masses of limestone, are known to occur in southeastern Kansas in seven stratigraphic units in the Lansing and Kansas City Groups of Pennsylvanian age. The banks are interpreted to have formed as moundlike masses of loose carbonate material heaped above the level of the surrounding sea floor. In some respects, the banks may be considered offshore bars partly controlled by waves and currents and partly by biological processes. Leaflike calcareous algae contributed much sedimentary material to the banks, and also served to trap and bind fine carbonate mud. Other organisms that contributed carbonate material include crinoids, bryozoans, brachiopods, sponges, and fusulinids.
Some of the banks in different stratigraphic units tend to occur stacked one upon another in the same general localities. This, plus other considerations, suggests that the open sea, the banks, and a broad depositional apron spreading from a distant southerly source area, remained in about the same general geographic locations during Lansing-Kansas City time. Thus, large-scale transgressions and regressions of the sea may not be as essential for the origin of cyclic Pennsylvanian and Lower Permian sediments in eastern Kansas as previously supposed.
This paper deals with the significance of marine banks in the origin of cyclic Pennsylvanian sedimentary rocks in southeastern Kansas. Marine banks are defined as deposits forming above the general level of the surrounding sea floor. Marine-bank deposits are known to occur in seven different stratigraphic units (Fig. 1) and at various geographic locations (Fig. 2) in southeastern Kansas.
Figure 1--Schematic graphic column showing Pennsylvanian rock units in southeastern Kansas in which marine banks are known to occur. Units not involved in bank development are omitted in lower part of column.
Marine banks in southeastern Kansas are characterized by two to tenfold increases in thickness over the ordinary thickness of the limestone units in which the banks occur. In general, an increase in thickness of a bank is compensated by a corresponding decrease in thickness of the overlying shale unit. Thicknesses of banks range from about 20 to 90 feet, and their areal extent is also variable. For example, a large bank in the Plattsburg Limestone (Harbaugh, 1959, Pl. 1) extends over an area of more than 100 square miles, and a bank in the Iola Limestone (Fig. 2) is known to extend along the outcrop belt for about 25 miles, although its east-west dimension is not known. On the other hand, some of the banks probably extended over less than a square mile.
Figure 2--Map showing location of known Pennsylvanian marine banks in southeastern Kansas.
The banks are composed mainly of calcitic limestone, with minor proportions of dolomite and clay. The carbonate material is formed by (1) whole or fragmented skeletons of lime-secreting organisms, (2) lithified carbonate mud, and (3) coarsely crystalline calcite formed principally by inorganic precipitation in water-filled voids (Harbaugh, 1960, 1961). Many kinds of organisms are represented, but leaflike algae are the most important in volume; others of importance are crinoids, bryozoans, and brachiopods. The algal leaves are generally fragmented and have not been observed in growth position. Locally, fusulinids and sponges are important. With the possible exception of a bank in the South Bend Limestone Member (Fig. 1), leaflike algae greatly influenced bank development by contributing, trapping, and binding sediment. In addition, leaflike algae commonly exerted an "umbrella effect" (Harbaugh, 1960, 1961, 1962), in which individual algal leaves caught lime mud on their upper surfaces, creating sheltered, water-filled voids beneath, which were later filled with coarse calcite. The thickest parts of the banks generally tend to be richest in leaflike algae.
Toward the south the banks interfinger with shales and sandstones containing thin, lensing limestones rich in oolites and well-sorted, well-rounded skeletal particles. Toward the north the banks pass into limestones partly composed of poorly sorted skeletal material containing smaller proportions of leaflike algae.
Acknowledgments--1 wish to thank Daniel F. Merriam for long-continued encouragement of my work in southeastern Kansas, John C. Davis and Richard L. Schuman for intermittent accompaniment in the field, and Mrs. Sally Liggett Brown for typing the manuscript.
The tendency for the marine banks to grade into shales and sandstones toward the south suggests that the banks formed near the edge of a large depositional apron over which terrestrially derived clay, silt, and sand were deposited (Fig. 3). It is proposed that the upper surface of this apron alternated between slightly above and slightly below sea level, and that meandering streams carried sediment from a distant southerly source across the apron. The configuration of land and sea is a matter of speculation, but it would seem likely that the margin of the apron and the open sea was a complex of shallow bays, brackish and salt ponds, and mud flats. These features were subject to frequent changes as the irregularly embayed shoreline shifted back and forth over moderate distances in response to the balance between subsidence and addition of new sediment. The apron may have been similar in some respects to large sedimentary deltas, such as that formed by the Mississippi River where it enters the Gulf of Mexico.
Figure 3--Map showing hypothetical relations of land and sea in southeastern Kansas and northeastern Oklahoma during part of Lansing time. A broad alluvial apron is interpreted to have existed in southern part of area. Meandering streams are interpreted to have carried terrestrially derived sediment from a distant southerly source.
It is suggested that the limestone banks formed in the open sea offshore from the edge of the land, and that they are somewhat similar to offshore bars in origin. There is little evidence that the banks were hard, reeflike masses when formed; instead they probably consisted of loose skeletal material and soft lime mud that was consolidated later. Initially, the banks may have been localized by waves and currents that caused both argillaceous and calcareous material to be heaped up into submerged bars. Once initiated, the bars or banks tended to perpetuate themselves due to a feedback effect in which shallower depths over the banks stimulated production of lime-secreting organisms, which in turn contributed more sedimentary material to the banks.The upper part of the large bank in the Plattsburg Limestone appears to grade laterally into shale. I have suggested (Harbaugh, 1959, p. 322) that this may be due to deposition of carbonate material over the banks, while clay, mud, and silt were deposited simultaneously in deeper water. The silt and clay may have been borne as turbid clouds of water that tended to hug the bottom because of greater density. Thus, much of the time, the banks may have remained in clear water above the level at which clay and silt was being deposited. Algae flourished where the water was clear, whereas the sea floor adjacent to the banks may have been too poorly illuminated for vigorous algal growth and may have been inhospitable to other organisms. The banks may also have served as barriers, inhibiting northward movement of sand.
It has been supposed that the cyclic aspects of Pennsylvanian (Moore, 1936, 1962) and Lower Permian (Elias, 1937, 1962) strata in eastern Kansas could be best explained by rhythmic changes of sea level with respect to land level. According to this view, differences in organisms and in sediment type are partly related to depth of water and distance from shore. As sea level rose and fell, the shoreline migrated back and forth over substantial distances. This general interpretation has the merit of simplicity and will probably be proven correct in many respects. However, I suggest that it may place too much emphasis on widespread transgressions and regressions of the sea. Based on observations of Upper Pennsylvanian rocks in southeastern Kansas, I propose alternately, that transgressions and regressions were moderate and that the general configuration of land and sea in this area remained more or less the same during part of Late Pennsylvanian time. Evidence to back this contention is provided by the tendency for banks in different stratigraphic units within the Lansing Group to be stacked one upon another (Harbaugh, 1960, p. 230-231). The locations of the banks were seemingly governed by gross geographic features, and it would be unlikely that the banks would form in the same places several times in succession if large-scale migration of the shoreline took place during or between deposition of banks.
The question arises how alternating marine and nonmarine deposits (marine limestone and shale, coal, nonmarine sandstone) can occur in the same sequence of strata if widespread transgressions and regressions of the sea have not occurred. I suggest that some of the transitions from marine to nonmarine sediments are not primarily due to changes of sea level with respect to land, but instead are due to (1) ecologic succession of organism communities, and (2) differences in types and volumes of sedimentary materials supplied to the sites of deposition. The occurrence of a coal bed may record the lateral expansion of a coal swamp due to progressive colonization of shallow-water marine areas by coal-forming plants capable of living in salt water. For example, isolated patches of mangroves are presently colonizing shallow-marine areas in parts of Florida Bay in southern Florida. Peat-forming mangrove swamps are probably modern analogues of late Paleozoic coal-forming swamps. Thus, the presence of a coal bed does not necessarily record cessation of marine deposition due to change in sea level in respect to land. Of course, it is necessary to postulate that land level continued to sink with respect to sea level, allowing thousands of feet of shallow-water sediments to accumulate.
In summary, it may be stated that the origin of cyclic sediments in eastern Kansas and adjacent areas is far from well understood. Future study of the relationships between fossil organism communities and petrology of sedimentary rocks should throw new light on the origin of cyclic sediments.
Elias, M. K., 1937, Depth of deposition of the Big Blue (late Paleozoic) sediments in Kansas: Geol. Soc. America, Bull., v. 48, p. 403-432.
Elias, M. K., 1962, Comments on recent paleoecological studies of late Paleozoic rocks in Kansas: Kansas Geol. Soc., 27th Field Conf. Guidebook, p. 106-115.
Harbaugh, J. W., 1959, Marine bank development in Plattsburg Limestone (Pennsylvanian), Neodesha-Fredonia area, Kansas: Kansas Geol. Survey, Bull 134, pt. 8, p. 289-331. [available online]
Harbaugh, J. W., 1960, Petrology of marine bank limestones of Lansing Group (Pennsylvanian), southeast Kansas: Kansas Geol. Survey, Bull. 142, pt. 5, p. 189-234. [available online]
Harbaugh, J. W., 1961, Relative ages of visibly crystalline calcite in late Paleozoic limestones: Kansas Geol. Survey, Bull. 152, pt. 4, p. 91-126. [available online]
Harbaugh, J. W., 1962, Geologic guide to Pennsylvanian marine banks, southeast Kansas: Kansas Geol. Soc., 27th Field Conf. Guidebook, p. 13-67.
Moore, R. C., 1936, Stratigraphic classification of the Pennsylvanian rocks in Kansas: Kansas Geol. Survey, Bull. 22, p. 1-256.
Moore, R. C., 1962, Geological understanding of Cyclic sedimentation represented by Pennsylvanian and Permian rocks of northern Midcontinent region: Kansas Geol. Soc., 27th Field Conf. Guidebook, p. 91-100.