Stacked paleosol profiles comprise a substantial part of the variegated mudstone facies of Wolfcampian cyclothems in the mid-continent.  Flooding surfaces within these mudstone intervals are marked by lags of intraclasts, fish bone, and ostracodes, and are overlain by thin beds of micrite or laminated calcareous mudstone.  These surfaces divide the variegated mudstone intervals into laterally traceable meter-scale cycles.  The paleosol-bearing cycles thus defined can be correlated for tens of kilometers or more (Miller & West, 1998).  This has enabled time-equivalent packages of paleosol profiles to be compared from locality to locality.  Lateral variations in paleosol development within correlated cycles have been found to be small relative to the differences among vertically stacked paleosol profiles at a single locality.  Furthermore, the vertical succession of paleosol types observed within individual cyclothems are consistent from locality to locality.

    For most of the thicker variegated mudstone units within the Council Grove and lower Chase Groups (ie. Roca Shale, Eskridge Shale, Blue Rapids Shale, Speiser Shale, Matfield Shale) a very similar vertical succession of paleosol profiles has been observed (Miller et al, 1996).  Paleosols from the lower part of these units have calcic clay-rich profiles with carbonate nodules and rhizocretions.  By contrast, the uppermost paleosols within these units are characterized by pseudoanticlines and other features of vertic paleosols.

    Paleosols from the lowest cycles of the variegated mudstones have angular blocky to sub-angular blocky ped structure, root traces, and carbonate nodules, stacked nodules, and rhizocretions similar to those observed by Blodgett (1988).  The lower paleosols are also characterized by distinct color horizonation, and have prominent grayish-red to reddish-brown horizons in the middle of the profiles.  Clay cutans are well-developed around the blocky peds, indicating clay illuviation.  These paleosols can be classified as calcic Argillisols according to Mack and others (1993), or as Alfisols in the USDA classification (Soil Survey Staff, 1992). 

    Certain enigmatic features of the lower calcic paleosols indicate that they are polygenetic profiles.  The carbonate nodules and rhizocretions are concentrated in the upper part of the profiles, and overlie argillic horizons with well-developed clay cutans.  This pattern is the reverse of what would be expected in a typical calcic soil, where carbonate precipitation occurs below the zone of seasonal leaching and clay illuviation (Birkeland, 1984; Marriott & Wright, 1993; Smith, 1994).  Climates wet enough to translocate clay downward through the profile would also leach soluble salts from the upper soil horizons.  It is therefore inferred that these Wolfcampian paleosols developed first under humid to subhumid conditions.  The precipitation and preservation of the carbonate then occurred under semi-arid conditions later in pedogenesis, when rates of evapotranspiration greatly exceeded mean annual precipitation (Mack, 1992).  A diagram illustrating the proposed climatic changes associated with the formation of these complex paleosol profiles is shown in Figure 8.

Figure 8. Diagram showing interaction of sealevel change and climate change on the formation of paleosols as interpreted by the model described herein. Arid to semi-arid conditions prevailed during sealevel rise and highstand. Seasonally wet climates were associated with falling sealevel and deposition of siliciclastic muds. Pedogenesis followed first under humid conditions resulting in illuviated clays, and subsequently under sub-humid to semi-arid conditions producing carbonate nodules and rhizocretions.

    Within some cyclothems, rooted reddish-brown siltstones are preserved at the tops of silty paleosol profiles. Some siltstone beds are also distinct lithologic units. They weather spheroidally and bear a striking resemblance to loessites described from the Ancestral Rockies and elsewhere. The silts of the midcontinent may have been derived from the extensive Permian eolian dunefields to the west. Wind directions determined from both eolian dune crossbeds and paleoclimate models indicate winds from the northwest during the Wolfcampian (Johnson, 1989; Soreghan, 1992). If these silts were windblown, their occurrence at the top of paleosol horizons is consistent with sediment trapping by a vegetated surface. Relatively high rates of sediment aggradation would also account for only the early stages of pedogenic development being present in the upper silty horizons. Siltstones also increase in abundance upsection coincident with other lithologic indicators of increased aridity during the course of the Permian (West et al, 1997).

    Another paleosol type is commonly found within cyclothems of the Council Grove Group. These paleosols are characterized by columnar-shaped ped structures with rounded tops, and typically occur in underlying or overlying carbonate units with evaporitic features. In modern soil-forming environments, these columnar peds occur only under the influence of sodium domination, and are referred to as natric horizons (Birkeland, 1984). The columnar peds in the Wolfcampian paleosols of northcentral Kansas all display the typical morphology found in extant natric horizon soils (McCahon & Miller, 1997). Columnar peds are distinguishable from argillic prismatic peds, also present in the paleosols, by the presence of domed rather than flat surfaces at the top of the peds in the B horizons.

    The paleosols occurring at the top of the variegated mudstone intervals are dominated by vertic features. These paleosol profiles lack clearly developed horizonation and have a uniform greenish gray to yellowish gray color. Their most prominent features are pseudoanticlines (mukkara structure) with pedogenic slickensides. The hummocky surface relief (gilgai) associated with these structures are especailly well-displayed if horizontal exposures are available (see Miller et al., 1996). In some profiles the curved fractures of the pseudoanticlines are enhanced by carbonate precipitation along the fractures. The carbonate is generally micritic with a botryoidal surface texture, although it may also be sparry calcite. Carbonate nodules, isolated or less commonly stacked, and small (<5mm) sesquioxide nodules are also present in the matrix. Root traces are abundant and sometimes take on a "concertina" appearance. These paleosols can be classified as Vertisols (Soil Survey Staff, 1992) or calcic Vertisols (Mack et al., 1993).

    The pseudoanticlines and pedogenic slickensides of the vertic paleosols were produced by the shrinking and expansion of clay-rich soils in response to wetting and drying in a markedly seasonal or monsoonal climate.  The uniform green to yellowish color of these profiles suggests saturation of the soil during part of the year, and the lack of horizonation is consistent with the extensive turbation characteristic of Vertisols.  The relatively high organic carbon contents (~1-2%) and presence of disseminated charcoal also reflect high rates of soil turbation, and an extended dry season.  As for the calcic soils, the abundant carbonate nodules found in the upper part of some vertic soil profiles, such as in the Roca Shale, suggest a trend toward drier conditions during later phases of soil development.


    As stated above, the early Permian was a time of continental galciation in the southern Hemisphere continents of Gondwana.  Glacial advance and retreat would be associated with both global cyclical sealevel fluctuation and climate change.  The consistent carbonate to siliciclastic pattern of meter-scale cycles, within both the open marine facies and paleosol-bearing intervals of cyclothems, indicates that glacio-eustatic seaelvel change alone is not adequate to explain the observed cyclicity.  A model for climatic control over facies development proposed by Cecil (1990) provides a basis for developing a more complete model of cyclothem formation.  In this model, clastic sediment transport is predicted to be highest in seasonal wet-dry climates and lower in both arid and tropical wet climates  (see also Wilson, 1973; Perlmutter & Matthews, 1989).  Carbonates and evaporites accumulate during arid and semiarid conditions, and mappable coal beds form during relatively wet climates when clastic influx is low.  Because carbonates typically overlie the flooding surfaces of meter-scale cycles, acceptance of Cecil's model would indicate that drier conditions were associated with sealevel rise (interglacials) and wetter climates associated with sealevel fall (glacials).

    Combining the climate model of Cecil (1990) with the paleosol evidence gives the following scenario for cycle formation.  Arid to semi-arid conditions prevailed during sealevel rise and highstand resulting in carbonate precipitation.  Falling sealevel was associated with a transition to seasonally wet climates that initiated the influx of siliciclastic muds.  These siliciclastic muds were subaerially exposed with continued sealevel fall.  Pedogenesis followed first under humid conditions resulting in clay illuviation, and subsequently under semi-arid conditions producing carbonate nodules and rhizocretions.  Flooding of the land suface by rising sealevel truncated the soils and formed the thin intraclastic and skeletal lags.  At the cyclothem-scale, a general trend toward wetter conditions is recorded by the change from calcic to vertic paleosols.  The proposed relationship between cycles formation and sealevel climate change is illustrated in Figure 9.

Figure 9. Stratigraphic section for Tuttle Creek spillway with interpreted sealvel and climate surves. Note the short-term climatic fluctuations superimposed on the cyclothem-scale pattern.

    Because both eustacy and climate are intimately connected to the dynamics of glacial advance and retreat, some consistent relationship should be expected. A model incorporating both climate change and glacio-eustatic sealevel fluctuation has been proposed for the Wolfcampian of the mid-continent (Miller et al., 1996). Global circulation models (Parrish & Peterson, 1988; Kutzbach & Gallimore, 1989; Patzkowsky et al., 1991; Parrish, 1993) indicate a change from the dominance of zonal circulation in the Late Pennsylvanian, to the increasing influence of monsoonal circulation in the Permian, diverting the moisture-laden equatorial easterlies flowing from the Tethys, and resulting in a drying of equatorial Pangea. Furthermore, both climate models (Kutzbach & Guetter, 1984) and paleoclimate data (Fairbridge, 1986; Crowley & North, 1991) indicate that monsoons are strengthened during interglacial periods and significantly weakened during glacial periods.

    The proposed climatic/eustatic model assumes that the climate of the mid-continent was strongly affected by a Pangean monsoon, and that fluctuations in the intensity of the monsoon produced oscillations between wetter and drier conditions. During interglacial periods when the monsoon was strong, the wet equatorial air would have been diverted to the north or south, resulting in a dry midcontinent. However, the weakening of the monsoon during glacial periods would have permitted the equatorial easterlies to penetrate into the continental interior. This model predicts that strong monsoons during sealevel highstands would have been associated with more arid conditions resulting in the precipitation of pure carbonates and evaporites, and weakened monsoons during sealevel lowstands would have been associated with wetter conditions resulting in argillaceous and silty carbonates and mudstones (Miller et al., 1996). Figure 10 shows the contrasting atmospheric circulation patterns associated with glacial maximums and interglacials according to our model.

Figure 10. Changes in equatorial circulation patterns across Pangea between glacial and interglacial periods during the Lower Permian. According to the model presented here, low pressure cells over the supercontinent intensified during interglacials deflecting moisture-laden equatorial easterlies. This monsoonal circulation weakened during glacials allowing more moisture to reach the continental interior.

        Ice volume changes during the Pleistocene have been attributed to variations in solar insolation resulting from periodic changes in the Earth's orbit and axial tilt. the five primary "Milankovitch periodicities" are 413,000, 100,000, 41,000, 23,000, and 19,000 years (Crowley & North, 1991). Time estimates for the duration of Pennsylvanian and Permian cyclothems are not well constrained, varying between 250,000 and 400,000 years. Although probably falling with the range of 40,000 to 150,000 years (Heckel, 1986; Busch & West, 1987), the absolute time duration of meter-scale cycles is at present impossible to obtain. Most time within a cycle is represented by paleosols, exposure surfaces and flooding surfaces. Some individual paleosol profiles could easily have developed over time periods of several tens of thousands of years. Cycle periodicities thus will tend to be overestimated by an order of magnitude or more by a simple division of stage duration by number of cycles (Algeo & Wilkinson, 1988). Thus, while estimated cycle periods fall within the Milankovitch band, there is no way at present to identify specific Milankovitch orbital periodicities.


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