Diagenesis, as used here, corresponds to Sujkowski's (1958, p. 2692) definition as "changes occurring in a freshly deposited sediment, commonly while still in the sedimentary basin" and including the "processes which turn a fresh sediment into a stable rock of some hardness, under conditions of pressure and temperature not widely removed from those existing on the earth's surface."
The Johnson Shale affords little direct evidence of such diagenesis. Although the lamination and fissility of the shale is mostly ascribed to primary sedimentary processes, some of the shaly parting may have developed by compaction of the Johnson clay mud during diagenesis. Shepard and Moore (1955, p. 1587) observed that 8,500-year-old mud buried 69 feet beneath similar currently accumulating mud in the central Texas Gulf Coast shows shalelike parting planes. The younger deposits have not yet acquired these structures. Hence, it is suspected that some of the shaly partings in Bennett and Roca clay shale may be of similar diagenetic origin. In the same way, diagenesis probably accentuated the bedded or laminated structures produced by other means (Sujkowski, 1958, p. 2716).
Diagenesis in Glenrock, Bennett, and Howe limestone is demonstrated by fossil interiors and matrix interstices filled with secondary calcite and, rarely, pyrite. The most conspicuous fossil fillings occur in Glenrock fusulinids and in the Howe Limestone nautiloid cephalopod (Pl. 7). Calcite in the matrices of the limestone is commonly microcrystalline-aphanitic. The calcite is thought to have been derived mostly by solution of calcareous material within the limestone masses, and by nearly simultaneous reprecipitation in open spaces. Judging from the known composition of living animal shells, many of the shells found in the limestone originally may have been aragonitic. It is assumed that recrystallization of aragonitic materials gave rise to much of the secondary calcite recognizable in the Red Eagle Limestone. Traces of short, impressed or indented contacts between pellets in the Howe Limestone osagite may reflect solution and reprecipitation of calcite, probably aided by compactive pressure.
The few chert nodules in the Bennett limestone at the Allen sections are further evidence of diagenesis. The secondary origin of these siliceous materials is affirmed by their content of silicified fossils. Individually silicified, milky, chalcedonic fossil brachiopod fragments and beekite are associated with the chert nodules. No explanation is offered for the preferential replacement of these fossils. Sujkowski (1958, p. 2695) postulated a diagenetic mechanism to explain the concentration of silica necessary for such chert structures. Initially, his mechanism requires saline water of a PH near 8, trapped with the sediments when they accumulate. [Note: The fauna of Glenrock and Bennett limestone reflects normal marine conditions. Normal sea water has a pH about 8.1 or 8.3. If Sujkowski's explanation is correct for silica in the Bennett limestone, his required pH lends support to the implication that Glenrock and Bennett pH values were near 8.0. Conversely, the implicit normal marine pH and presence of chert give support to the tenebility of Sujkowski's explanation.] The amount of this water is reduced by squeezing during compaction. To provide the required source of silica for the nodules, Sujkowski's explanation calls for organic and inorganic silica in the sediments. If silica was dispersed through Glenrock and Bennett sediments, the chert nodules constitute the only record.
Clay accumulations commonly undergo volume reduction of between 50 and 78 percent (Weller, 1960, p. 298) during diagenesis. Although it is certain that Johnson, Bennett, and Roca shale was compacted, the only direct evidence of compaction is visible in the black Orbiculoidea-bearing shale at the base of the Bennett Shale. Here, many specimens of cone-shaped Orbiculoidea have been crushed flat by compression under the weight of overlying sediments.
Traces of "muscovite" have been found with clay and fine quartz silt in many of the insoluble residues studied. It is believed that the "muscovite" is an alteration product of clay minerals such as illite, common in shale of the Red Eagle cyclothem. The small amounts of chlorite commonly present in many of these shale units could have been altered from the illite or from montmorillonite if appropriate ions were present during diagenesis. Some of the illite could have been altered from montmorillonite.
Evidence of oxidation of iron salts in shale is present where thin zones adjacent to fractures in greenish-gray shale are changed to a faint maroon-gray color. Some of the green coloration in Johnson and Roca shale is due to the presence of finely divided ferrous iron silicates.
The general light color of the Bennett limestone and the random orientation of its shell fragments may have resulted from the activities of mud-eating and scavenging organisms during early diagenesis. "The importance of living creatures as agents of early diagenesis in natural sedimentary environments can not be overstressed" (Shepard and Moore, 1955, p. 1586).
Much of the gray shale and some of the black shale of the Bennett Shale Member weather to light brownish gray or buff. This weathering is so deep in some places that the shale could be mistaken for unaltered buff shale. The light color is believed to be the result of oxidation, bleaching, or removal of dark carbonaceous and other rock-coloring materials during weathering.
Necessarily, some episodes in this history rest on inference within a framework of conclusions that can be supported by observation. These conclusions are listed after the following summary of environmental history.
The Red Eagle cyclothem was deposited in Wolfcampian time in marine waters spread thinly over a wide, very flat, shelflike basin in the Midcontinent region of Kansas, Oklahoma, and Nebraska. The basin floor subsided slowly and permitted the accumulation of some 5O feet of sediments that now make up the Red Eagle cyclothem. Lowlands bordered this arm of the Wolfcampian sea on the north, east, and south. From time to time uplifts in southern Oklahoma supplied large quantities of clastics which were washed into the southern side of the basin and thence were spread northward by gentle currents. These sediments mingled with those from the lowlands to the north and east. The generally moist climate of the region varied from warm to cool, with dry, warm conditions occurring periodically.
During the first half of Johnson time, the sea shallowed until shorelines were comparatively near to the area of study. At the same time the climate became warm, with alternating wet and dry seasons, so that red soil developed on the land. When the waters were shallowest (high intertidal?), red soil detritus washed into the northern and southern margins of the basin. The Red Eagle cyclothem sequence began with accumulation of these red sediments in warm and possibly hyperhaline waters. While red detritus was carried toward the center of the basin, its ferric oxides were chemically reduced so that the sediment was deposited as a greenish-gray aggregate. After a few inches of red mud accumulated, the sea deepened very slightly, circulation with the ocean became freer, and the water became normally saline. Greenish-gray mud was deposited throughout the Kansas part of the basin. Little light could reach the sea bottom because waters were turbid from suspended mud. Hence, few benthonic plants and few animals inhabited the water, although planktonic algae were active near the surface. The algae precipitated calcium carbonate particles which settled with the suspended sediments to form moderately calcareous greenish-gray mud.
The sea cleared occasionally during the last of Johnson time because of: (1) short; warm, dry periods (which lessened runoff and inhibited subaerial erosion); (2) low relief and gradients in source areas; or possibly (3) development of submarine barriers to clay influx. During these times relatively more algal calcium carbonate was produced so that local layers of argillaceous lime mud accumulated within the preponderant calcareous clay mud. The water was less than 10 feet deep, so storms were able to stir up some of the bottom sediments. Tidal and wind-induced currents exerted daily smoothing and laminating action on bottom sediments and were an important agent of transportation of suspended clay detritus.
Near the end of Johnson time, rapid influxes of water from land during pluvial periods temporarily lowered the salinity. Ostracodes were the only shelled animals that flourished. Fragments of gymnosperm wood washed in from lands to the north and east. In southern Kansas (Cherokee Basin) the late Johnson waters were only slightly deeper than in the north. The land to the south was low. During relatively dry periods the supply of mineral and rock detritus from the south periodically diminished, and so allowed the waters to clear. During these periods a mixed shelly fauna developed in southern Kansas, while only ostracodes lived in the northern part of the basin. Small amounts of detritus regularly entered the Midcontinent basin, mainly from the north and east.
Johnson deposition terminited when the water deepened from less than 10, to 20 or 30 feet. Whether or not subsidence aided the rising water, the rise in sea level forced the shorelines to move considerable distances over the adjacent lowlands. Hence, in Glenrock time little clastic sediment from the land reached the area of study, so waters were very clear. The sea floor was extremely flat. Benthonic life thrived. Particulate algal calcium carbonate settled from near-surface waters amid a profusion of benthonic calcareous shelled invertebrates, of which many were fusulinids. The result was a richly bioclastic, clean, calcareous bottom ooze. During Glenrock time the basin floor subsided slightly less in southern Kansas than in the north, so that a lesser thickness of Glenrock lime mud accumulated there than in the north. However, in northern Kansas and Nebraska, Glenrock sediments were deposited at a remarkably uniform rate. Minor local uplifts occurred along the Nemaha Anticline, causing shoals near the Alma, Saffordville, and Elmdale localities just before or during Glenrock time, so that Glenrock sediments were scarcely deposited in these areas. Fragments of Johnson sediment were eroded from the Alma shoal and deposited in the Glenrock sediments at Manhattan and Paxico. At the end of Glenrock time, the depositional interface was a strikingly level, uniform Surface over much of northeastern Kansas and southeastern Nebraska and parts of southeastern Kansas.
The sedimentary events that closed Glenrock time happened even more suddenly than those which began it. The sea shallowed quickly from 30 or 40 feet to less than 10 feet. The accumulation of clay clastics resumed in the area of study, and free circulation of water with the open ocean was restricted by an unknown barrier somewhere to the southwest. This change accounted for the euxinic sea floor at the beginning of Bennett time in Kansas and Nebraska. The top of the Glenrock Limestone was so flat that these changes occurred almost simultaneously throughout the area of study.
As the available oxygen was depleted, odorous Bennett mud began accumulating on the clean bioclastic Glenrock Limestone sediments. Black mud killed the last Glenrock fusulinids and washed into the open tubes of animals that burrowed into the upper Glenrock sediments. Organic matter and calcium carbonate settled from the surface waters to the toxic bottom (low in oxygen), where the organic matter was partly preserved and partly decayed by bacteria. This increased the toxicity. A few Lingula and Orbiculoidea were able to live in and on the black bottom, but they did not thrive.
During early Bennett-time the water deepened slightly. The ensuing freer circulation improved oxygenation, reduced the toxicity, and encouraged the establishment of a mixed shelly benthonic fauna. Gray mud was deposited from moderately turbid water 40 or 50 feet deep. The southern supply of clastics to the area of study was reduced while waters shallowed in the middle of Bennett deposition. Faint upwarp of the Bourbon Arch further interfered with circulation and prevented northern clastics from reaching southern Kansas. Hence, lime mud was laid down in shallow clear water during the rest of Bennett time in southern Kansas, while calcareous clay mud was deposited from more turbid water in Nebraska and northern Kansas. Up to 5 feet of lime mud or ooze accumulated in central Kansas during medial Bennett time, some in calcareous banks stabilized by ramose crustose algae, but calcareous clay mud with a mixed shelly fauna followed when the water deepened in the latter part of Bennett time. In a small shoal area south of Eskridge, Kansas, double the normal thickness of lime mud accumulated in algal banks amid calcareous clay mud during medial and late Bennett time.
After the euxinic conditions which began Bennett time, all Bennett waters were mildly warm (about 70° F) and normally marine in salinity and pH. The bottom was swept by moderate to gentle currents. In medial and late Bennett time, calcareous crustose algae grew on the lime ooze in central and southern Kansas and helped to stabilize and accumulate fine particulate carbonate detritus.
Sedimentary events that concluded characteristic Bennett deposition were less abrupt and extreme than events at the beginning. Surrounding land masses were worn low by the end of Bennett time. The sea became very shallow again and the deposition of Howe calcareous sediments began in clear, warm, nearly hyperhaline water. A profuse benthonic fauna, rich in foraminifers, ostracodes, tiny gastropods, and small clams, was established south of the Manhattan area. Shortly after the beginning of Howe time the water was warm, shallow, and clear enough for algae to thrive on the bottom. The algae (Osagia) secreted calcareous coatings around shell fragments, thus forming the pseudo-oolites observable in the pelletoid Howe Limestone (osagite) of central and southern Kansas. Commonly gentle, but sometimes strong, currents swept the floor of the Howe sea. In Nebraska and northern Kansas, algal particles accumulated to form aphanitic calcareous ooze containing few shelly fossils.
After the water had shallowed to less than 10 feet by the end of Howe time, there followed a sudden influx of clastics and the establishment of water conditions similar to those of late Johnson time. Greenish-gray mud began to accumulate. Regional climatic changes and uplifts in the southern source area caused a flood of red clastics to move northward across the shelf-basin in early Roca time. Smaller quantities of red clastics washed in from sources to the north and east. Most of these were chemically reduced to a green color before they reached central Kansas. Greenish-gray calcareous clay mud similar to that deposited late in Johnson time settled from the turbid Roca waters. The water shallowed to nearly intertidal depths and the shorelines drew nearer to the area of study. Occasionally, as in late Johnson time, thin layers of lime mud accumulated during temporary incursions of deeper water. When the climate changed and the intertidal waters became slightly hyperhaline near the middle of Roca time, red clastics spread across the area. This return to conditions similar to those which caused deposition of the Johnson redbeds marked the end of the cycle of depositional events recorded within the Red Eagle cyclothem.
Kansas Geological Survey, Geology
Placed on web Jan. 4, 2007; originally published Dec. 1963.
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