Missourian (early late Pennsylvanian) climate in Midcontinent North America

International Journal o f Coal Geology, 5 (1985) 111--140 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

MISSOURIAN (EARLY LATE PENNSYLVANIAN) MIDCONTINENT NORTH AMERICA

111

CLIMATE IN

STEPHEN R. S C H U T T E R ' and PHILIP H. HECKEL 2 1E xxo n Production Research Co., P.O. Box 2189, Houston, T X 77001 U.S.A. 2 Department o f Geology, University o f Iowa, Iowa City, IA 52242 U.S.A. (Received January 14, 1985)

ABSTRACT Schutter, S.R. and Heckel, P.H., 1985. Missourian (early Late Pensylvanian)climate in Midcontinent North America. In: T.L. Phillips and C.B. Cecil (Editors), Paleoclimatic Controls on Coal Resources of the Pennsylvanian System of North America. Int. J. Coal Geol., 5: 111--140. The abrupt decrease in mineable coals from Desmoinesian to Missourian rocks in Midcontinent North America has been related by several lines of evidence to the probability that Missourian climate became at least seasonally drier than Desmoinesian climate. This represents a transition from the equatorial Desmoinesian rainforest climate to the arid Permian climate, as North America moved northward from the equator. This change is reflected in the progression of evaporites from western Colorado in the Desmoinesian to Kansas in the Permian. Direct climatic evidence from soils in two Missourian shales deposited at low stands of sea-level includes caliche horizons, incompletely leached mixed-layer clays (in contrast to intensely leached Desmoinesian kaolinite), and soil profiles resembling Vertisols, which develop under semi-arid conditions. Less direct climatic evidence includes the greater proportion of marine limestone deposited at intermediate sea-level stands in the Missourian than in the Desmoinesian part of the sequence, in conjunction with the greater abundance and thickness of oolite and shoreline facies in Missourian limestones than in Desmoinesian counterparts. This probably reflects increasing dryness of the climate, which would have led to decreased detrital influx and increased salinity as rainfall and runoff diminished. Indirect climatic evidence in offshore black phosphatic shales deposited at highest sea-level stands involves possible seasonality in organic and phosphatic laminations, related to periodicity of upwelling in the tropical trade-wind belt under the strong monsoonal influence of Pangaea. Midcontinent Missourian climates probably ranged from tropical monsoon to tropical savanna or hot steppe, all having wet-dry seasonality. The coincident extinctions of conodont, brachiopod, and other marine genera as well as the loss of swamp lycopods at the end of the Desmoinesian suggest the possibility that a greater than usual drop in sea level affected both realms at this time, while the climate was becoming drier.

INTRODUCTION O n o f t h e m o s t p r o n o u n c e d f e a t u r e s o f c o a l d i s t r i b u t i o n in t h e M i d c o n t i n e n t r e g i o n o f N o r t h A m e r i c a is t h e a b r u p t d e c r e a s e o f c o a l in Mis-

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© 1985 Elsevier Science Publishers B.V.

112

sourian rocks compared to underlying Desmoinesian rocks. Long-term drying of the climate is one possible cause of this loss, and evidence for the nature of the change can be f o u n d t h r o u g h o u t the cyclic sedimentary sequence as well as in the decrease in abundance of coals. On a very broad scale, the Late Paleozoic in Midcontinent North America shows a drying trend, with Early Pennsylvanian rainforests eventually giving way to widespread Permian evaporite deposition. This may reflect the gradual northward movement of North America, which would have taken it into increasingly arid climatic zones in the Late Paleozoic (Rowley et al., 1985). Support for this is found in the pattern of evaporite occurrences. In the Desmoinesian, evaporite deposition was largely restricted to the Paradox and Eagle Basins, and the Lusk e m b a y m e n t of the Denver Basin. As the Pennsylvanian progressed, evaporite deposition spread into the Williston Basin and west Texas. In the Early Permian it became well established in the Midcontinent region centered around western Kansas (Fig. 1). Between the rainforest environment and the arid environment both geographically and temporally, a transition zone would be expected. Because

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Fig. 1. Eastward migration of Pennsylvanian and Lower Permian evaporite (sulfate and halite) deposition in Midcontinent U.S. Information from Peterson and Hite, 1969 (Paradox Basin); McKee, 1982 (Arizona); Lovering and Mallory, 1962 (Eagle Basin); Hoyt, 1962 (Denver Basin); Agatston, 1954 (Wyoming); Schoon, 1967 (Williston Basin); O'Connor et al., 1968 (Midcontinent); Harbour, 1972 (west Texas).

113

the seasonal shift in the maximum angle of insolation causes a sympathetic shift in the region of maximum rainfall associated with the equator, these transitional climates are characterized by seasonal rainfall rather than by continuously moderate rainfall. In Midcontinent North America during the Pennsylvanian, this seasonal effect likely would have been modified by the powerfull monsoonal effect of Pangaea, which was a large continent extending across temperate latitudes where winter and summer air pressures would vary considerably, and with them the wind direction and consequent rainfall (Rowley et al., 1985). CYCLIC SEDIMENTARY SEQUENCE The upper Desmoinesian and Missourian sedimentary sequence in Midcontinent North America is characterized by an alternating sequence of marine limestones with thin shale members and nearshore marine to terrestrial shales with sandstones and coals {Fig. 2). This general alternation is very persistent laterally, and many individual units are traceable for hundreds of miles along outcrop and into the subsurface. The marine limestone and shale portion of the sequence was deposited during times of widespread marine inundation, whereas the intervening detrital rocks and coals were deposited during times of general marine withdrawal when much of the sediment surface was emergent. Within this generally alternating sequence, cyclic successions of individual rock units have long been recognized as cyclothems (Wanless and Weller, 1932; Moore, 1950). Deposition of most Midcontinent cyclothems is considered to have been the result of Pennsylvanian glaciations in the Southern Hemisphere (Wanless and Shepard, 1936; Crowell, 1978). The calculated periodicity of major Midcontinent cyclothems is a b o u t 400,000 years, comparable to that of the major Pleistocene glacial cycles (Heckel, 1980). The effects of the eustatic cycles are especially pronounced in the northern Midcontinent, where the shelf lay inundated at high sea-level stand and emergent during low sealevel stand and where the complicating effects of Missourian deltas are rare. Whatever climatic changes t o o k place, long-term or short-term, should be reflected in depositional changes within homologous phases of the cyclothems. The basic "Kansas c y c l o t h e m " of Heckel (1977, 1980) characterizes with minor modification much of the upper Desmoinesian, Missourian, and lower Virgilian sequences north of the latitude of Tulsa, Oklahoma. These cyclothems cover essentially the broad northern shelf region extending along the north side of the Arkoma--Anadarko basin complex of central Oklahoma, and across Kansas and much of Missouri, Iowa, and Nebraska. The basic cyclothem consists of the following ascending sequence (Fig. 3): (1) generally thick, sandy, locally nonmarine and coal-bearing, shale formation (stratigraphically " o u t s i d e " the limestone formation); (2) thin "middle" limestone member, typically skeletal calcilutite, but with local calcarenite

114

at the base; (3) thin nonsandy " c o r e " shale, c o m m o n l y with fissile black facies; (4) thicker " u p p e r " limestone, typically skeletal calcilutite grading upward to calcarenite and locally to m u d d y shoreline facies; and thick, sandy nearshore shale (unit 1 ) above. These cyclothems are generally persistent across the Midcontinent. Related cyclothems, which generally lack the "middle" limestone (unit 2), South Bend ~ _ _ ~ 1 ~

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Fig. 2. Generalized Desmoinesian and Missourian stratigraphic sequence in northern Midcontinent region, including Forest City Basin (modified from Heckel, 1984). Arrows with names on right indicate shales studied in detail by Schutter (1983); C denotes named coals, with names in parentheses. Horizontal names in column are formations (capital) and members (lower case), most of them limestone, that represent marine inundative cycles. B-P marks cycles with black phosphatic shale facies on northern shelf. Intervening shale formations (with names omitted, except for those studied in detail and contained coals) represent low stands of sea level. Symbol # marks those limestones for which depositional facies are known through detailed petrographic studies: Breezy Hill, F o r t Scott (K.L. Knight, pets. commun., 1984); Pawnee (Price, 1981); A l t a m o n t (Schenk, 1967); Lenapah (Parkinson, 1982; H. Greenberg, pers. commun., 1984); "Lost Branch" (P.H. Heckel, unpublished data); Exline (M.A. Nielsen, pets. commun., 1984); Hertha (Ravn, 1981); Swope (Payton, 1966; Mossler, 1973); Dennis (Payton, 1966; Frost, 1975); Cherryvale (Siebels, 1981); Drum (Stone, 1979); Iola (Mitchell, 1981); Wyandotte (Crowley, 1969); Plattsburg (Harbaugh, 1959, 1960); Stanton (Heckel, 1975, 1978).

115 are f o u n d in t h e Illinois Basin ( H e c k e l et al., 1 9 8 0 ) . O f f s h o r e facies are r e m a r k a b l y persistent, and a l t h o u g h facies changes due to t o p o g r a p h i c relief are k n o w n , t h e y are generally n o t s u f f i c i e n t to radically alter t h e d e p o s i t i o n a l patterns. Facies c h a n g e s are m o r e c o m m o n and abrupt in t h e n e a r s h o r e t o terrestrial " o u t s i d e " shales and t o p s o f upper l i m e s t o n e s . T h e t h i c k , s a n d y " o u t s i d e " shales represent t h e t i m e s o f l o w e s t sealevel stand at m a x i m u m regression b e t w e e n t h e marine i n u n d a t i o n s . S o m e o f t h e s e shale units are thinner, grey-green to red b l o c k y m u d s t o n e s , w i t h local caliche and features o f clay m i n e r a l o g y that are characteristic o f soil profiles ( W a t n e y , 1 9 8 0 ; S c h u t t e r , 1 9 8 3 ) . S o m e o f these b l o c k y m u d s t o n e s are overlain by coals. Other " o u t s i d e " shale units are t h i c k e r and sandier, l o c a l l y w i t h plant fossils, and represent various detrital alluvial to deltaic e n v i r o n m e n t s . Marine p o r t i o n s o f these shales are t y p i c a l l y t h i c k e r Basic Cyclothem (simplified meggcyclothem) in K a n s a s - I o w a oufcrop belt

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Fig. 3. Basic vertical sequence of individual "Kansas cyclothem", the transgressiveregressive depositional unit that characterizes, with only minor modification, most of Marmaton and Missourian sequence on northern Midcontinent shelf. Positional terms derive from Moore (1931, 1936) for limestones and from Heckel and Baesemann (1975) for shales. Conodont faunas conspicuously differentiate the two shale members and illustrate that the two limestone members are essentially mirror images of one another• (Adapted from Heckel, 1977•)

116 and contain sparse c o n o d o n t and shelly invertebrate faunas of low diversity, reflecting fluctuating nearshore conditions of rapid sedimentation. Most of the thick shales and sandstones of the Illinois Basin are of these types. The relatively thin (0.1--1.0 m) middle limestones are more difficult to interpret because of their thinness and presence in most places of only one facies, typically a diversely skeletal calcilutite deposited below effective wave base. Taken in conjunction with the underlying nearshore to nonmarine shales, middle limestones apparently represent a cessation of detrital influx and an increasing stability of marine conditions, both of which would be expected during a transgression. Supporting this is the observation that when a wave-washed calcarenite facies is present in the middle limestone, it is always at the base below the skeletal calcilutite facies, where shallow-water agitation would have affected the sediment earlier during transgression. Thus, the middle limestones are referred to depositionally as transgressive limestones. Transgressive limestones are rare and discontinuous in the Illinois Basin. The nonsandy, thin (0.1--2.0 m) " c o r e " shales all contain a mediumto-dark grey, sparsely to abundantly, and usually diversely fossiliferous facies, representing stable marine salinity conditions below wave base. Most also contain a phosphatic fissile black facies within the grey facies. Core shales are remarkably continuous along outcrop considering their thinness; most are traceable from Iowa to Oklahoma, a distance of about 300 miles (500 km). Both facies also contain an extremely abundant, relatively diverse c o n o d o n t fauna. Their lack of coarse terrigenous detritus, their widespread distribution for such a thin unit, their great abundance of conodonts, and their position between the most offshore facies of carbonate deposition in both limestone members, all provide strong evidence that the core shales were deposited in deepest water under conditions of sedimentary near-starvation, at the times of highest sea-level stand during m a x i m u m marine inundation. The black facies, which typically lacks definitely benthic fossils, represents anoxic conditions on the sea b o t t o m developed when vertical water circulation was eliminated by development of a thermocline in the water column as deeper, cooler, denser water extended over large parts of the Midcontinent shelf during highest sea-level stand (Heckel, 1977). Supporting this is the observation that in several cyclothems (e.g., Iola and Stanton) where the black facies disappears laterally, it is typically over a paleotopograpic high, such as the Bourbon Arch. The " c o r e " shales of the Illinois Basin are remarkably similar (Heckel et al., 1980; Schutter, 1983), although since they were closer to the major coal swamps toward the east, they are usually higher in humic matter. The relatively thick upper limestones display a classic shoaling-upward sequence from below-wave-base diversely skeletal calcilutites at the base to more wave-washed calcarenites (including oolites) toward the top. In places, the tops display a variety of m u d d y shoreline facies, including locally mud-cracked, fenestral laminites characteristic of tidal-flat deposition.

117 A number of these sequences are diagrammed in Heckel et al. (1979) from the northern outcrop. Faunas of the upper m u d d y beds are patchy in distribution, low in diversity, and dominated by ostracodes, foraminifers, and snails, reflecting the fluctuating conditions of the shoreline environment. Thus, the upper limestones were deposited during a drop in sea level, and they are described depositionally as regressive limestones. Most of the limestones of the Illinois Basin are regressive limestones (Heckel et al., 1980), although all but a few were smothered by prograding clastics before shoal-water facies developed at the top. TERRESTRIAL EVIDENCE FOR CLIMATE -- PALEOSOLS

Blocky mudstones The subaerial portions of the " o u t s i d e " shales in Missourian cyclothems include a number of blocky mudstones, which provide substantial evidence of seasonal climates. Blocky mudstones are particularly evident in those areas where there was little or no fluvial influence, and the subaerial environm e n t is expressed as a weathering profile on the exposed marine to transitional sediments. These weathering profiles are decidedly Pennsylvanian, because all the features found on outcrop have also been identified in cores. Furthermore, systematic variation within a single cyclothem precludes the possibility that the features resulted from burial diagenesis. Several features identify these Missourian soils as products of seasonal semiarid climates. Most striking (and yet least conspicuous) is the pattern in clay mineralogy, which previously has been recognized (e.g., by Eberl, 1980; Rimmer and Eberl, 1982) but generally not attributed to pedogenic processes. Beneath the regional coals (those below marine transgressive deposits and not restricted to alluvial plains), the seatearths appear generally as blocky mudstones in which the clays are weathered and leached at the top adjacent to the coal, but become progressively less so downward from the coal horizon. This is expressed as a downward decrease in hydrous layers in the illite; that is, farther down, less K+ has been removed from the illite, and thus it is more regularly crystalline (i.e., less altered). Conversely, closer to the coal the clays have more expandable layers, and behave more like swelling clays (Fig. 4). These mixed-layer, swelling clays are characteristic of seasonal climates, falling in that area where the climate is sufficiently wet to produce leaching, but not intensely wet enough to leach clays to kaolinite. Kaolinite, an expected product of lateritic weathering in humid climates, is present in Missourian strata only in transported sediments, either those derived from more humid areas to the east or from older Desmoinesian soils (Schutter, 1983). Because the Missourian strata of the northern Midcontinent had almost no detrital input, kaolinite is rare in them (Siebels, 1981; Schutter, 1983). The swelling clays, under the influence of an alternately wet and dry

118

climate, produce the blocky structure characteristic of the blocky mudstones, both those that are seatearths and those that have no overlying coal but appear to be of similar origin. When the clays dry and shrink, deep cracks form, which may fill with clay that falls in from the surface. ILLITE CRYSTALLINITY INDEX I 2 I i

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SOIL DRYS, CRACKS

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Fig. 5. Illustration of wet-dry cycle in Vertisols (terminology from Soil Survey Staff, 1975). Surface mounding during re-expansion produces "gilgai" microtopography. Adapted from Buol et al., 1980, p. 235 (reptile courtesy of W.W. Thomson).

]19

When the clays becom e wet, t hey expand, and the added volume of the fallen clay produces stress, which is released by low-angle slickensiding, producing the oblique bl ocky fabric. This process, " a r g i l l o p e d o t u r b a t i o n " (Buol et al., 1980) produces a slow vertical circulation of the clays; thus, such soils are called Vertisols (Fig. 5). Because of this circulation, these soils have essentially no horizon development, and t hey are very clay-rich. Vertisols typically form in climates with rainfall of 25--100 cm, ranging f r o m p r e d o m i n a n t l y dry to having dry periods of only a few weeks (Buol et al., 1980). These soils also pr oduc e a m o u n d e d or "gilgai" surface, which is c o m m o n in m o d e r n soils in n o r t h e r n Australia (Buol et al., 1980) and was observed by Donaldson et al. (1983) in Late Pennsylvanian mudstones of the Appalachian Basin. Caliche The leaching that p r o d u c e d the clay profile is also expressed in other ways, of which the most apparent is the distribution of calcium carbonate. Many seatearths are noncalcareous, a striking feature in units sandwiched between marine deposits; the carbonate was apparently leached out, with some deposited at the base of the profile as caliche. Sometimes this caliche is a crust on the underlying limestone (Heckel, 1983, pp. 753--754), or it may be a horizon of carbonate nodules. Some, though not all, of the so-called "freshwater limestones" of Illinois are pedogenic carbonate nodules or caliche horizons (Fig. 6). The nodular caliche horizons are " m a t u r e " caliches in the terminology of Reeves (1970), with a profusion of small nodules, c o m m o n fractures and interior cracks, and with the bulk of the parent soil volume now carbonate (Schutter, 1983). In addition to the caliche horizons f ound by the authors in the Midcontinent, Upper Pennsylvanian caliches (or " c o r n s t o n e s " ) have been widely report ed in central and even eastern North America (Knight, 1929; Donahue and Rollins, 1979; Dubois, 1979; Watney, 1980; L oope and Schmitt, 1980). Caliche horizons are f ound in subarid to subhumid climates that are seasonally dry, Reeves {1976) reports that caliches form in rainfall regimes ranging from 10 to 130 cm annually, but that 50 cm seems to be about the o p t i m u m . In addition to rainfall, the factors of temperature, runoff, and relief all influence caliche formation, but are not easily categorized. Caliches are characteristic of Aridisols, soils that form in areas of low rainfall. Many of the features t hat Buol et al. {1980) attribute to Vertisols are listed by Steila (1976) as characteristic of Aridisols, so these soils appear to form a spectrum, with caliche mainly toward the Aridisol end. R e d color Several o th e r features support the interpretation of these blocky mudstones as a soil profile. Some thicker mudstones are oxidized to a red color,

120

Fig. 6. Thin sections of Illinois caliche soil nodules. A. Nodule from base of seatearth in Fithian cyclothem, near Fithian, Illinois, consisting mostly of brown and black micrite, apparently from organic staining. Note probable root holes (light), some bifurcating, lined with multiple generations of calcite growth (plane light). B. Nodule from base of seatearth below Shoal Creek (Carthage) Limestone in Charleston, Illinois, core (-740 foot level). Note dark areas, variably stained by organics, multiple layers of calcite growth in voids, and extensive (including circumgranular) fracturing (plane light).

a n o t h e r characteristic o f Aridisols, w h i c h f o r m in climates w h e r e it is difficult to preserve organic material and the iron goes into oxides. These red m u d s t o n e s have a green transition z o n e at the t o p b e n e a t h the overlying m a r i n e unit, and this shows w h y m o s t Missourian seatearths are n o w green or grey. With transgression o f the sea, a source o f r e d u c i n g organic material b e c o m e s available, as well as a w a y to preserve it. In their generally d r y clim a t e , m o s t Missourian soils m a y originally have been o x i d i z e d and t h e n d e h y d r a t e d to a red color. If the soil profile were thin e n o u g h , it was later all r e d u c e d t o a green c o l o r b y marine organic material d u r i n g the subs e q u e n t transgression (with or w i t h o u t f o r m a t i o n o f coal), a l t h o u g h red o x i d i z e d glaebules in the seatearth and m o t t l e s in the u n d e r l y i n g l i m e s t o n e m a y remain. A m o d e r n analog was described b y J o h n s o n {1982) in the G a s c o y n e delta o f Western Australia, w h e r e Pleistocene r e d b e d s were d r o w ned by the H o l o c e n e sea-level rise, and the t o p few inches were c o l o r e d

121

MISSOURIAN PALEOSOLS

A. VILAS TYPE

B. C O F F E Y V I L L E TYPE

blocky~ elickensided, no orgonlce or root trOCel COOl rare gloebulee of hematite a n d / o r colcite may have coliche near bose may be 5 m thick sl~pandoble cloys c o m ~ o n ; mica and quartz rare usually red; may hove top reduced tO green by Succeeding sea level r i l e Esomple: Yilas Shale of Stonzel~ Iowa 1VFRTISOL

few diagnOstic criteria; t h i n usually in fluviO~ Section CO011 and r o o t e d zones prolont may hove weokly developed~ immature coliche horizon little or rio a l t e r a t i o n of (::lays or detritcl mineroll kaolinite present b u f f or g r e y Eiample: Coffeyvine Formation below Cedar Bluff Cool at Cherryvole, KOnlOI FLUVENT

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blocky) Ilickensided t root traces rare variable carbonaceous material n o n - c a l c a r e o u s at t o p may be imprinted o v l r older soil U p t o 2 m thick= COOl Ot top illits increasingly leached u p w a r d ; * k a o l i n i t e r a r e l y mica or q u a r t z green, usually pyritic S U L F A Q U E N T

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Fig. 7. Missourian paleosols, showing characteristics of types recognized by Schutter (1983) and classified as Vertisol (A), Fluvent (B), and Sulfaquent (C, D), after Buol et al. (1980) as discussed in text.

122

greenish, p r o b a b l y b y d o w n w a r d p e r c o l a t i o n o f w a t e r r e d u c e d b y m a r i n e organic m a t e r i a l . This p a t t e r n can be seen in t h e Vilas Shale o f t h e F o r e s t City Basin (Fig. 7A) a n d is r e p o r t e d f r o m t h e G a l e s b u r g Shale a n d similar s u b s u r f a c e h o r i z o n s in w e s t e r n Kansas b y W a t n e y (1980).

Other considerations D e t r i t a l minerals, such as q u a r t z a n d m u s c o v i t e , are a b s e n t in Missourian b l o c k y m u d s t o n e s in m u c h o f the n o r t h e r n M i d c o n t i n e n t , w h e r e t h e r e w e r e f e w rivers c o m p e t e n t e n o u g h t o i n t r o d u c e t h e m . T h e clay p r e s e n t in the m u d s t o n e s either u n d e r w e n t long-distance aerial or littoral t r a n s p o r t , or was derived f r o m t h e u n d e r l y i n g r o c k (usually l i m e s t o n e ) . In Illinois, w h e r e detrital minerals are c o m m o n in u n d e r l y i n g rocks, s e a t e a r t h s also are usually g r e a t l y d e p l e t e d in detrital minerals, r e f l e c t i n g t h e intense w e a t h e r i n g in the soil t h e r e as well. T h e a b o v e f e a t u r e s are largely a b s e n t f r o m s a n d y alluvial soils, classified as F l u v e n t s , w h i c h are a s u b o r d e r o f Entisols, soils t h a t are t o o y o u n g to have significant profile d e v e l o p m e n t . Because o f t h e a g g r a d a t i o n a l n a t u r e o f a f l o o d plain, a soil profile r a r e l y has t i m e to d e v e l o p , a n d t h e clays rarely

Fig. 8. Exposure of paleosol near Davis City, Iowa, showing a complete cycle: from Bethany Falls (BF) limestone (top of regressive limestone of underlying Swope cyclothem), through Galesburg (G) seatearth, Davis City (DC) coal (new unit), Stark (S) core shale of Dennis cyclothem, showing lower grey, middle black fissile (overhanging ledge), and upper grey facies, to base of Winterset (W) regressive limestone. Staff at center is 91.5 cm (3 feet) long. June, 1979. See Appendix for exact location.

123 evolve in place. Even so, a few seatearths of this t y p e do show development o f a " y o u n g " caliche horizon. This t y p e of caliche preserves the original sediment t e x t u r e and has a b u n d a n t parent material (Reeves, 1970). It is typically f o und in an aggradational envi ronm ent where it is p r o t e c t e d f r o m exposure and does n o t form a laminated crust. An example can be f o u n d locally in the Galesburg-equivalent Coffeyville F o r m a t i o n of southeastern Kansas (Fig. 7B). As dynamic systems, soils usually best reflect the last thing to happen to them. Thus, Missourian seatearths, while basically seasonally wet-dry soils, are overprinted by a marine transgression. This is expressed not only by the iron r e duct i on causing the red-to-green color change m e n t i o n e d above, but also by the f r e q u e n t presence of pyrite in the seatearth and the overlying coal. These characteristics closely parallel those of modern soils f o u n d below tidal marshes, or Sulfaquents, which constitute a marine t y p e o f immature waterlogged soils or Aquents, anot her suborder of Entisols. (If their p o o r horizon de ve l opm ent were considered to be due to unfavorable conditions rather than lack of time, t hey would be more properly placed in the Inceptisol order as Sulfaquepts.) Because spores of the herbaceous l y c o p o d Polysporia (Chaloneria) dominate some thin Missourian coals, it is possible that some Missourian paralic systems may have been

Fig. 9. Upper paleosol-influenced surface of regressive Bethany Falls Limestone near Davis City, Iowa (same locality as Fig. 8), showing solution pitting and possible root holes. This rock is calcilutite containing only sparse ostracodes.

124

coastal marshes rather than tree-dominated swamps (DiMichele et al., 1979). Examples of drowned Vertisols partly converted to Sulfaquents would be the Fithian seatearth of the Illinois Basin (Fig. 7C) and the Galesburg Shale of the Forest City Basin (Figs. 7D, 8, 9).

Summary To summarize, many Missourian seatearths show features expected to develop in seasonally wet-dry climates, producing Vertisols with some Aridisol characteristics. These were usually overprinted with Sulfaquent features developed during the following sea-level rise, although relict features, such as blocky fabric, leached clay profiles, caliche horizons, and sometimes red color remain to show that the precursor soil was formed under drier conditions than the final Sulfaquent. Aquents and Fluvents are c o m m o n immature soils in areas of inundation or more rapid deposition, respectively, in climatic regions where Vertisols might otherwise be expected (Buol et al., 1980). The Missourian soils of the Midcontinent contrast markedly with the Desmoinesian and earlier soils of the same region. The fire clays of the Cheltenham Formation of Missouri contain diaspore as well as abundant kaolinite (Keller et al., 1954). These represent intense Desmoinesian (and perhaps earlier) weathering of the pre-Pennsylvanian land surface in a very humid climate. Brown et al. (1977) noted that upper Desmoinesian terrestrial sediments of south-central Iowa and adjacent Missouri are kaolinitic, and they attributed the kaolinite to acid leaching of flood-plain sediments. The a m o u n t of kaolinite in northern Midcontinent sediments decreases fairly abruptly upward into the Missourian. Brown et al. (1977) report it up to the Exline horizon (lowest Missourian), but it is not present in the higher Galesburg Shale (Schutter, 1983) or the Cherryvale Formation (Siebels, 1981). MARINE EVIDENCE FOR CLIMATE

Nature of limestones Marine rocks in Midcontinent Missourian cyclothems consist almost entirely of limestone and shale. The limestones are dominantly calcilutitic, ranging from skeletal-rich to skeletal-poor, with locally thick horizons of oolite in the shoal-water facies. The dominance of the carbonate mud indicates a warm tropical to subtropical climate where fine-grained carbonate of any type of marine origin is readily preserved, rather than dissolved as small particles tend to be in cold water (Lees, 1975). The mere presence of abundant marine ooids at local horizons implies a warm climate that is also relatively dry. Modern marine ooids seem to be forming only at the slightly more saline end (> 35.8 ppt) of normal open marine salinity

125 (Lees, 1975), which t o d a y is established only in the dry tropical to subtropical zones. Thus the gross nature of Missourian marine limestones is strongly compatible with the warm, at least periodically dry climates indicated by the paleosols.

Offshore shales Of the two different major types of marine shales in these cyclothems, the offshore (or core) shale was deposited essentially below the combination of the photic limit for algal production of carbonate mud and the thermal limit for preservation of carbonate mud (Heckel, 1984). These shales generally display two basic facies: (1) gray with calcareous benthic fossils and recording an oxygenated, moderately deep, marine shelf bottom, which displays little climatic imprint; and (2) black with phosphatic laminae and only pelagic fossils, deposited below a thermocline and affected by upwelling, which is strongly influenced by climate, particularly prevailing wind direction. The fine interlamination of black organic-rich layers with lighter layers dominated by peloidal apatite in these shales (Fig. 10) suggests an episodically fluctuating environment, and it may be that the apatite nodules in these shales also require a fluctuating environment to aggrade from the

!~ ~ii!i~~

Fig. 10. Laminae of apatite peloids (light gray) in black Eudora Shale near Lewis, Iowa. Irregular dark line toward bottom is artifact of acrylic coating.

126 peloids as Burnett (1977) discussed for m o d e r n apatite nodules. The rare benthic faunas f ound locally in the black shales display criteria f o u n d in opportunistic faunas (Pianka, 1970), such as low diversity and high abundance. Also, th e y are dominated by small simple forms, which are f o u n d in low numbers t h r o u g h o u t the c y c l o t h e m but becom e abundant only where periodic stress inhibits more stenotopic, less opportunistic forms. The last observable fauna before the onset of intense black shale conditions is dominated by the brachiopod Crurithyris, a small form occurring in great numbers, supplemented by lingulid brachiopods, which have a high tolerance for low-oxygen conditions (Manwell, 1960). The best developed black shales contain mostly pelagic fossils, with some opportunistic faunas of agglutinated foraminifers and pectinids. All these sedimentologic and faunal features are compatible with the periodic tropical upwelling postulated by Heckel (1977, 1980) for these black shales. The climate on the adjacent coast in areas of m o d e r n upwelling is typically at least seasonally dry (Brongersma-Sanders, 1971). These upwellings tend to wax and wane episodically, often partly in response to monsoonal changes in wind direction and intensity brought about by seasonal fluctuation in air masses over the temperate part of the neighboring c o n t i n e n t (Parrish, 1982). {In the Pennsylvanian case, this would be Pangaea.) Thus, opportunistic faunas periodically colonized a temporarily o x y g e n a t e d b o t t o m only to be killed off when upwelling returned. It is worth noting that the fauna from an area o f modern upwelling (Sen Gupta et al., 1981) resembles in gross detail that f o u n d in portions of the Pennsylvanian black shales (Fig. 11). PENNSYLVANIAN

BENTHIC FORAMS HIPPOCREP~NA AMMODISCUS PECTEN DUNBARELLA

RECENT

BENTHIC FORAMS BOLIVINA CABSIDULINA PECTEN ARGOPECTEN

Fig. 11. Comparison of Pennsylvanian black shale fauna (from Schutter, 1983) with modern upwelling fauna (from Sen Gupta et al., 1981). Both have opportunistic patterns of low diversity and high populations. Both are also supplemented by pelagic forms such as fish, and, in the Pennsylvanian, conodonts.

Nearshore shales The o th er major t y p e of marine shale in the cycl ot hem is the nearshore facies of the outside shale member, which was deposited in shallow water where normal carbonate p r o d u c t i o n was diluted or suppressed by tero

127 rigenous detrital influx in shoreline or prodeltaic environments. The abundance o f this facies implies enough rainfall and r u n o f f to carry large am ount s o f terrigenous argillaceous and siliceous m ud o n t o the marine shelf, and this implies a climate with at least periodically significant rainfall. Because the offshore shales are uniformly thin, bot h laterally across the shelf and vertically f r o m c y c l o t h e m to c y c l o t h e m , most of the thickness variation in marine shales from one part to a not h er of the cyclic sequence is due to variation o f overwhelming detrital influx in the nearshore shales. Thickness o f these nearshore shales reflects n o t only climate (i.e., rainfall as developed above) but also nearness to detrital source (both shoreline and provenance) and t o p o g r a p h y of the hinterland. There is enough comparative information from upper Desmoinesian and Missourian rocks (from cores through most or all of those strata in the Forest City Basin of southwestern Iowa and adjacent Missouri and Nebraska) t h at some preliminary data can be applied to the nature of the change f r o m Desmoinesian to Missourian sedimentation in this area. First of all, it is readily apparent f r om Table I that the p r o p o r t i o n of limestone increases dramatically f r om a range of 13 to 33% of the Desmoinesian sequence to a range of 42 to 60% of the Missourian sequence. F u r t h e r m o r e , the proportion of limestone in the Missourian ranges from 1.7 to m ore than 4 times the p r o p o r t i o n in the Desmoinesian in the four cores that penet rat ed both sequences. The remainder of the sequence is p r e d o m i n a n t l y shale and mudstone in both ages, with significant sandstone only in the Desmoinesian. As developed above, thickness of offshore shales is uniform from c y c l o t h e m to c y c l o t h e m in both ages. Thus all of the decrease in the shale-sandstone p r o p o r t i o n from Desmoinesian to Missourian has taken place in the nearshore to terrestrial " o u t s i d e " shales. Although the detailed study of the cores has not y e t progressed to the p oin t that nearshore marine, nonmarine alluvial deltaic, and paleosol facies o f these shale units have been differentiated in all cores (and thus we cannot r e p o r t exact changes in p r o p o r t i o n s of these facies from Desmoinesian to Missourian), we can offer the following considerations relating the gross change to climate: {l_) As developed above, the a m o u n t of nearshore marine shale should vary directly with the a m o u n t of rainfall in the climate. (2) The a m o u n t of nonmarine alluvial to deltaic detritus also should vary directly with the a m o u n t of rainfall. Because sandstone is most likely to accumulate in this environment, the conspicuous reduction of sandstone from Desmoinesian to Missourian rocks implies a similar reduct i on in alluvial-deltaic facies. (3) The thickness of d e v e l o p m e n t of m u d s t o n e paleosols from any parent rock also should vary directly with the a m o u n t of rainfall in the climate (other factors remaining equal) because, as developed previously, it is the leaching effects of fresh gr oundw at er flow that dest roy parent carbonate and detrital minerals. Thus, whatever the d o m i n a n t n o n c a r b o n a t e facies

4.5

84 (25) 13

Desmoinesian Thickness, f t ( m ) Limestone, %

R a t i o Msrn.Ls.% Desm.Ls.%

212 (64) 58

NAC Cass 11-11-12

Core: County : Location:

Missourian Thickness, f t ( m ) Limestone, %

Nebraska east

Area •

2.9

131 ( 3 9 ) 21

149+(45) 60

NOC Sarpy 11-13-13

Nemaha Uplift ~- closer

2.4

149 (45) 21

--

ILC Harrison 19-79-42

Iowa west

167+(50) 51

ISC Madison 5-75-29

central

1.9

147 ( 4 4 ) 24

--

CP37 Clarke 2-72-26

farther

--

2 7 0 (81) 46

IBC Taylor 4-67-34

south

1.8

160 (48) 33

305 ( 9 2 ) 58

MNC Nodaway 10-65-36

Missouri northwest

1.7

141 (42) 25

341 (102) 42

MRC Andrew 4-59-34

C o m p a r i s o n o f l i m e s t o n e p r o p o r t i o n s of lithic s e q u e n c e in eight cores f r o m F o r e s t City Basin. Six cores c u t c o m p l e t e u p p e r Desm o i n e s i a n ( M a r m a t o n G r o u p ) s e q u e n c e , six cores c u t c o m p l e t e or n e a r l y c o m p l e t e M i s s o u r i a n s e q u e n c e , a n d f o u r cores i n c l u d e b o t h sequences. F o r p r e s e n t purposes, M a r m a t o n G r o u p i n c l u d e s all s t r a t a f r o m base o f E x c e l l o shale to t o p o f C o o p e r Creek L i m e s t o n e M e m b e r o f " L o s t B r a n c h F o r m a t i o n " o f Heckel ( 1 9 8 4 ) , a n d Missourian s e q u e n c e i n c l u d e s all s t r a t a f r o m t o p o f M a r m a t o n t o t o p o f S t o n e r M e m b e r o f S t a n t o n F o r m a t i o n . B o t h i n c o m p l e t e Missourian cores ( d e n o t e d +) s t a r t in u p p e r m e m b e r s o f W y a n d o t t e Form a t i o n . All cores are h e l d b y respective s t a t e geological surveys.

TABLE 1

129

in either age, the abrupt drop in proportion of shale in the sequence from Desmoinesian to Missourian strongly suggests a significant overall drying of the climate. Because both change to a more distant detrital source and reduction of hinterland topography also potentially could decrease the proportion of shale in the sequence, the change from Desmoinesian to Missourian paleogeography around the Forest City Basin must be considered. The Nemaha uplift, which abruptly bounds the west side of the Forest City Basin, formed from Late Mississippian to Early Pennsylvanian (Kluth and Coney, 1981). The adjacent basin then filled rapidly with detritus, which eventually lapped up over the top of the uplift. Most of the basin filling was accomplished before deposition of the late Desmoinesian Marmaton Group. Wanless and Wright (1978, fig. 6) show that a progressively decreasing area of the Nemaha uplift, centered on the Kansas--Nebraska border area, remained emergent during Marmaton and Missourian deposition. A trend in the significance of the Nemaha uplift as a detrital sediment source with respect to the proportion of limestone in the sequence is shown in Table 1. The lowest percentages of Desmoinesian limestone (13--21) occur in the three cores closest to the uplift in eastern Nebraska (NAC, NOC) and westernmost Iowa (ILC). This contrasts with two of the highest Missourian percentages (58 and 60) in two of these cores, established after more of the pre-Pennsylvanian provenance was covered. Nevertheless, the remaining cores farther from the uplift, in central Iowa and northwestern Missouri still show much greater percentages of limestone in the Missourian (42--58) than in the Desmoinesian (24--33), even though the percentage of Desmoinesian limestone does increase away from the uplift, as would be expected if the uplift were more of a detrital source at that time than in the Missourian. Shoal-water and shoreline carbonates

This complicating factor prompts us to look for further lithic evidence in the marine sequence relating to climate. Oolites, which imply a drier climate as developed previously, are reported from nearly every Missourian cyclothem on outcrop and dominate the shoal-water facies of several (e.g. Swope, Dennis). In contrast, oolites are known (but rare) in only two Desmoinesian cyclothems on outcrop (Altamont, Breezy Hill), even though these cyclothems have received as much detailed study as Missourian cyclothems (Fig. 2). Shoal-water and shoreline carbonate facies (e.g., tidal fiats) also are well developed at the tops of nearly all regressive limestones in the Missourian on the northern shelf. In contrast, few shoreline and shoal-water carbonates are reported from Desmoinesian cycles on outcrop, and very little of either was noted in the cores studied in detail. The significance of carbonate shoal-water and shoreline facies is that they develop only

130 in very shallow water at the time of shoreline retreat in places where carbonate p r o d u c t i o n was not overwhelmed by detrital influx, and thus t h e y are more c o m m o n l y developed in relatively drier climates. The converse, however, th at scarcity or absence of carbonate shoreline facies at the t op o f regressive limestones signifies w e t t e r climates, must be t e m p e r e d by the possibility t ha t a nearer detrital source in the Desmoinesian could have been more overwhelming, even in a drier climate. Interestingly enough, the few thin shoreline facies n o t e d in the Desmoinesian cores were f o u n d in two of those (NOC, ILC) closest to the Nemaha uplift. F u r t h e r m o r e , detailed correlation of the cores in this area indicates convergence and thinning of the entire Marmaton Group (Table 1) into an area where Wanless and Wright (1978, fig. 6) considered most of it to be absent. These considerations suggest that the Nemaha uplift was not a major detrital sedim e n t source (at least to those places cored) during Marmaton deposition. This, in turn, suggests that perhaps the much greater p r o p o r t i o n of limestone in the Missourian than in the upper Desmoinesian of the Forest City Basin resulted more f r om a generally drier climate than from a more distant detrital source.

Summary During the deposition of Desmoinesian regressive limestones, prograding detrital clastics rapidly dr ow ned out carbonate product i on, so t hat the later regressive p o r t i o n of the c y c l o t h e m is represented largely by prodeltaic and deltaic shales. In contrast, Missourian carbonate p r o d u c t i o n across the Forest City Basin was rarely interrupted by overwhelming clastic influx, and shoal-water to shoreline lithologies, such as cross-bedded calcarenites, oolites, and fenestral laminated calcilutites, c o m m o n l y cap the upper limestones. The thin outside shales above t hem rarely show signs o f deltaic development, as t he y lack detrital minerals, and show little sign o f wedging over large distances. These features can be explained in terms o f a simple residual deposit on top of the exposed carbonate surface. This contrast is most readily explained by a loss of detrital influx from the Desmoinesian, when streams carried large volumes of sediment into the region, to the Missourian, when the drying climate cut detrital input to the p o in t where it rarely prograded across the carbonate shelf, even when the water was very shallow. OTHER EVIDENCE FOR CLIMATE Other lines of can be derived than diagnostic argument. (1) Kaolinite,

evidence for the drier, seasonal climate of the Missourian f r om the literature. Most are merely compatible rather by themselves, but taken together strongly support the which is p r o d u c e d under conditions of intense leaching,

131

disappeared from the sediments as the climate dried, and (excluding diagenetic kaolinite in sandstones) disappeared from Pennsylvanian sediments in Arizona (Wanless and Patterson, 1952), Utah (Tooker, 1962), Colorado (Raup, 1966), and the Midcontinent before it did in the east (Potter and Glass, 1958). This reflects the migration of drier conditions from west to east. Eventually, with the onset of truly arid conditions, magnesium-rich evaporitic clays such as corrensite appear in the sediments. They also appear earlier in the west, in the Pennsylvanian of Arizona (Hauff and McKee, 1982) and the Paradox Basin (Schutter, unpublished data), than eastward in the Permian of Texas (Bassett and Palmer, 1981) and Kansas (Kopp and Fallis, 1974), again reflecting the northwestward migration of North America during this time. (2) The expansion of areas of arkosic alluvial fans from the Middle Pennsylvanian Fountain and Madera formations of Colorado into the Upper Pennsylvanian "granite wash" of Oklahoma and Texas may reflect both tectonism and the changing nature of the sources. Dutton (1979), however, described these granite-wash fan-deltas as products of flashy runoff, which is more characteristic of arid areas. This type of immature detrital deposit contrasts with the clay-rich subgreywackes of the east, which were more likely deposited from meandering streams in areas of heavy constant rainfall and intense leaching. (3) The sand-shale distribution in the Midcontinent may also reflect the seasonal climate. Such a wet-dry climate would tend to separate the sand from the mud, as the sand is transported only during the stormy rainy season, with only mud deposition prevailing at other times. Such a regime tends to produce a chenier plain (Reineck and Singh~ 1980). Modern analogs can be found in similar climates in northern Australia {Rhodes, 1982) and northern South America {Wells and Coleman, 1981). A mudrich system (such as a large cratonic river) might enhance the separation, if it had a circulation similar to that of the modern Amazon {Wells and Coleman, 1981), which produces a mud-rich coast down-current from the river m o u t h , with very little sand; the sand is deposited offshore, but bottom currents carry the mud downwind and back toward shore. This nearshore mud suppresses wave activity (Wells and Coleman, 1981), which might explain the paucity of beach deposits and other evidence of paralic winnowing that is observed generally in the Midcontinent Pennsylvanian and was analyzed and discussed by Keulegan and Krumbein (1949). (4) Several examples of Missourian wood with growth rings that might reflect wet-dry seasonality have been reported, by Baxter and Hartmann {1954) from Dichophyllum wood to Kansas, and by Jensen (1982) from cordaitan wood in Oklahoma. This stands in contrast to the apparent absence of growth rings in Desmoinesian wood in the Midcontinent {Phillips et al., 1985). (5) The rise of reptiles, which became diverse and abundant during the Missourian, would be encouraged by a drying climate (Colbert, 1980).

132 Missourian reptiles are known from Kansas (Reisz et al., 1982), Nebraska (Martin, 1971), Illinois (Cope, 1875), and the Appalachian Basin (Lund et al., 1979). (6) The rise and spread of conifers would be encouraged by a drying climate (Moore et al., 1936; Winston, 1983). These apparently evolved in the west and spread east (Read, 1947). By the end of the Missourian a well-developed drier-climate flora grew in eastern Kansas (Moore et al., 1936) during deposition of the Rock Lake Shale, which represents a glacial low-stand of sea level between the Stanton and South Bend marine cycles. The overall flora also shows this change in the loss of the aquatic arborescent lycopods discussed at greater length elsewhere in this volume. (7) Both Hall (1961) and Hamblin (1969) found that current directions in regressive Missourian calcarenites of the Midcontinent were bimodal to the northeast and southwest. Although this could be due to tidal currents as they suggested, it could just as well be due to seasonally reversing wind-driven circulation. CONCLUSIONS AND IMPLICATIONS The gross temporal and geographic setting of the Midcontinent in the Missourian suggests that it was a time of general drying, expressed as increasingly longer dry seasons and shorter wet seasons. This interpretation is supported by mineralogic, petrologic, sedimentologic, stratigraphic, and paleontologic evidence. A close analogy can be drawn between the postulated Missourian shoreline depositional system (which reflects the climate) and similar transitional climates today. In the Kbppen system of climate classification, these are the tropical monsoon (Am), tropical savanna (Aw), and hot steppe (BSh) climates. These climates are found t o d a y across much of the tropical world, including India, northern Australia and northwestern South America (Fig. 12). There the soils are Vertisols and Aridisols when the topography and parent materials are favorable (Fig. 13), and Entisols and related immature soils (Inceptisols) on alluvial plains and coastal areas. The vegetation patterns include coastal swamps and marshes, corridor forests on the floodplains, and savanna in the hinterland. This would closely parallel the Missourian distribution of paralic swamps and marshes, local floodplain forests of calamites and pteridosperms, and upland (savanna?) floras of conifers (Fig. 14). In areas where river-borne clastics were dominant, a chenier plain might form, while in areas away from clastic influx, carbonate deposition with a wide range of shallow-water facies would extend up to the shoreline. These climates are also those typically adjacent to areas of coastal upwelling~ if the coastal geometry is favorable. In fact, the northwestern coast of South America was one of the examples used by Brongersma-Sanders (1971) to illustrate upwelling, and the west coast of Australia also displays upwelling. This model encompasses a broad variety of transitional climates, and no at-

133

tempt is made at this point to tie the features observed in the cyclothems to a more specific climate. It. should be emphasized that the onset of this climatic drying was not a sudden event. Many of the features found in the sedimentology and clay mineralogy of the Missourian can be found in upper Desmoinesian strata as well. For example, Swade (1982) found that the proportion of carbonate in the transgressive deposits, which is high in most Missourian cycles, progressively increases upward in the six uppermost Desmoinesian major marine inundations in two Iowa cores. Thus, climatic drying in itself was probably not the sole factor responsible for the abrupt extinction

~

_e ~ ~ ' - " ~ ~ ~s

~ - - ]

TROPICAL MONSOON Am

(B)

:~ Sh

:~

~'~:=TROPICAL MONSOON(Am) ~TROPICAL SAVANNA(Aw) ~ H O T STEPPE(BSh)

~

HOT STEPPE BSh

TROPICAL SAVANNA AW

R(cm)

T°C g 20-

II

°il

I

-2C-I

IIII

I

IJ

ZO

I0

,I

M

d

llllllJ,,,

o

Fig. 12. Modern climate analogs. (Modified from Espenshade, 1960, pp. 8--11.) A. Global distribution of tropical monsoon (Am), tropical savanna (Aw), and hot steppe (BSh) climates. (Climate nomenclature from KSppen system.) B. Climate diagrams showing mean annual temperature (heavy curve) and average monthly precipitation (bars), as typical examples of conditions in these climates.

134

Fig. 13. Global distribution of Vertisols and Aridisols. Comparing with Fig. 12,A, Vertisols tend to develop in areas of savanna and steppe climate, particularly in Australia and India. Most monsoon and savanna areas in Africa and South America have unfavorable parent material or mountainous terrain; most Aridisols occur in climates drier than those modelled for Midcontinent Upper Pennsylvanian. (After map from U.S. Soil Conservation Service, published in Buol et al., 1980, p. 328. )

~

R

BONA~ /

Fig. 14. Sketch of environments developed during lower sea-level stands in Midcontinent during Missourian. Note paralic swamp (or marsh) and corridor forest on flood plains, which would be Missourian coal environments. Streams would tend to be flashy, braided, and carrying arkosic sediments when draining upland areas (such as Amarillo and Wichita Mountains). Terrestrial environments developed characteristic soil types, with savanna having Vertisols, floodplain having Fluvents and Aquepts, and paralic swamps having Sulfaquents. Away from areas of clastic influx, carbonate deposition would extend into shallow water. Cyclic deposition resulted from rise and fall of sea level superimposing the various environments.

135 of the aquatic arborescent lycopods noted by many others at the Desmoinesian--Missourian boundary. In fact, the nearly coincident extinction in the Midcontinent marine realm of the brachiopod genus Mesolobus, the cono d o n t genus Neognathodus, and the problematical chaetetids, along with changes in fusulinid and goniatite faunas, strongly suggest an ecologic crisis that transcended simple climatic drying. The abruptness of this change in both terrestrial and marine realms above the highest Desmoinesian marine horizon (provisionally termed Lost Branch Formation by Heckel, 1984) suggests that perhaps a marine withdrawal of greater than usual magnitude, both geographically and temporally, closed that particular marine inundative cycle. This greater drawdown of marine water into the steeper-sided basins of Oklahoma and Texas not only crowded and eliminated habitats and niches in the marine environment, but also eliminated, on the steeper coastline, the widespread coastal and floodplain swamp environments of the water-loving arborescent lycopods, for a sufficiently long time that they too became extinct. If China, considered a small independent continent at this time (Rowley et al., 1985), were sitting lower on the crust than Pangaea, such a large drawdown may not have uncovered the entire shelf there, thus explaining the persistence of the swamp-dwelling lycopods there into the Upper Pennsylvanian (Phillips et al., 1985). More data are needed to support this hypothesis, but it accords well with current information. The apparent Virgilian wet period, which produced such widespread coals as the Pittsburgh of the east and the Nodaway of the Midcontinent, may not contradict the prevailing drier pattern. A wetter period at that time could reflect local changes in orographic effects resulting from the known increase in tectonism that gave rise to the thick sequence of Douglas Group clastics at the base of the Virgilian, or from other changes in landsea distribution. The change in coals may also be an artifact of slower marine transgressions in the Virgilian, which would have given coals more o p p o r t u n i t y to form, and may not be related to general climate at all.

APPENDIX

Davis City Coal (new unit) The Davis City Coal is newly recognized as a named bed in the Galesburg Shale. It is a thin, banded coal resting upon the underclay that dominates the Galesburg Shale in the northern Midcontinent, and is overlain by the fossiliferous grey facies of the Stark Shale Member of the Dennis Formation. The Davis City coal is exposed in south-central Iowa and adjacent Missouri. Type locality: Quarry in SE 1/4 NE 1/4 Sec. 4, T67N, R26W, 2 miles west of Davis City, Decatur Co., Iowa.

136 ACKNOWLEDGEMENTS P a r t o f t h i s p a p e r is a m o d i f i e d e x c e r p t f r o m a d i s s e r t a t i o n u n d e r t a k e n at the University of Iowa by Schutter (1983). We would like to thank T . L . P h i l l i p s f o r t h e i n v i t a t i o n a n d e n c o u r a g e m e n t t o p a r t i c i p a t e in t h i s symposium, and W.L. Watney, H.H. Damberger, and L.R. Follmer for r e v i e w i n g t h e m a n u s c r i p t . We a l s o e x p r e s s o u r a p p r e c i a t i o n t o t h e Missouri, Nebraska, Iowa, and Illinois Geological Surveys for access to the long cores that they hold, and to the Kansas Geological Survey and Petroleum Research Fund administered by the American Chemical Society for financial support of the research program supervised by P.H. Heckel, of w h i c h t h i s w o r k is a p a r t .

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