Paleosols Spanning the Cretaceous-Paleogene Transition, Eastern Montana and Western North Dakota

Paleosols spanning the Cretaceous-Paleogene transition, eastern Montana and western North Dakota

DAVID E. FASTOVSKY* Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706 KEVIN McSWEENEY Department of Soil Science, University of Wisconsin, Madison, Wisconsin 53706

ABSTRACT lock (Ludlow) exposures, extensive ponded With the retreat of the Western Interior Sea- deposits are preserved. Vegetation accumu- way at the close of the Cretaceous, a large, Paleosols occur in exposures of the latest lated at a rate sufficient for coal formation. southeastward-prograding alluviual plain and Cretaceous Hell Creek and Paleocene Tul- The amount of fluctuation apparently was re- delta system developed in eastern Montana, lock (= Ludlow) Formations in Montana and duced, and pedogenesis was further inhibited, parts of Wyoming, and the Dakotas. The source western North Dakota. These units are com- as indicated by the virtual absence of illu- for the sediment has been attributed by Gill and posed mainly of interbedded siltstones and viated clays in the sediments. Cobban (1973) to the present Elkhora Moun- sandstones of meandering fluvial origin. The tains area, a Cordilleran orogenic belt about 300 paleosols indicate that changes in ancient soil INTRODUCTION km to the west (Smedes, 1966). The paleosols development occurred concomitantly with examined in this study formed proximally on the the better-known faunal transition. This paper is an overview of paleosol devel- delta plain, which itself occurred geographically In order of decreasing abundance, silty opment during the Cretaceous-Paleogene transi- and temporally midway in the continuum of del- soils, sandy soils, organic soils, and volcanic tion in eastern Montana and western North taic progradations bracketed by the Campanian- soils are preserved. Pedogenic features pres- Dakota. The paleosols reported here are from Maastrichtian Sheridan (Gill and Cobban, ent in these soils include roots, microscopic uppermost exposures of the latest Cretaceous 1973) and Danian Marmarth (Cherven and segregations of amorphous material, bire- Hell Creek Formation of eastern Montana and Jacob, 1985) deltas. The broad upper deltaic fringence fabric, and soil structures. The western North Dakota, and from lowermost ex- plain was drained by low-energy meandering features suggest that throughout the Creta- posures of the Paleocene Tullock and Ludlow river systems (Fastovsky, 1986), which pro- ceous-Paleogene transition pedogenic proc- Formations of eastern Montana and western duced flood plains in which Hell Creek paleo- esses in the region produced immature North Dakota, respectively.1 The Hell Creek sols developed. The tendency for channel profiles, an observation consistent with the and Tullock consist of complexly interbedded migration, in combination with periodic sheet unstable, fluvial setting in which the ancient siltstone, sandstone, and lignitic facies that have floods that must have occasionally covered de- soils formed. Gleization was a dominant been interpreted as representing a meandering veloping soils with clay-, silt-, and sand-sized process in this setting, and podzolization fluvial environment (Butler, 1980; Belt and oth- material, contributed to landscape instability. modified some sandy soils. The association of ers, 1984; Fastovsky and Dott, 1986; Fastovsky, The sediments consist of intensely weathered quartz, feldspars, lithic fragments, and smectites the features enables recognition of O, A/E, 1986). In this study, we provide evidence that ultimately from the volcanically active orogenic Btg, Bhs, Bg, BC, Cg, and C horizons. concomitant with the changes in biota during belt to the west, but penultimately reworked During middle Hell Creek time, soils the Cretaceous-Paleogene transition, changes in from local sources. Parent materials in Hell formed in a poorly drained setting that was paleosol development occurred. Creek and Tullock paleosols are of a broadly only stable enough to permit incipient pedo- Six localities, from which were studied fifty- uniform mineralogy, deposited in an area of low genesis. The bull; of the pedogenesis occurred two columnar sections, constitute the database topographic relief (Fastovsky, 1986). in levee and flood-plain deposits; soils also of this report. The localities range across an area occurred on point-bar and crevasse-splay of several hundred square miles, and include In this study, we infer a uniformity of soil deposits. In topographically depressed re- some of the most fossiliferous K-P exposures in processes, rather than a uniformity of soil type. gions, organic ¡accumulations formed with the Western Interior (Fig. 1). Two-hundred and Typological designations based upon modern minimal soil development. Matted plant de- six specimens were examined in hand sample, soil classifications are avoided because of (a) the bris is the product of this environment. Gley 150 were studied in thin section, and, in a com- incompleteness of the database, (b) the interpre- features and segregations of iron oxides plementary study (McSweeney and Fastovsky, tive limitations of such an approach, and (c) the around voids suggest fluctuation of the water 1987), 20 were studied by SEM. A composite design and intent of modern soil classifications. table. section from one of the localities (Brownie Butte Soil taxa cannot be accurately diagnosed with a By latest Hell Creek time, the mean level of in eastern Montana) is presented here in detail to limited database that does not provide all of the the water table rose, and in the lowest Tul- exemplify Hell Creek paleosol sequences and information used in the ordering of the classifi- stratigraphic context. cation. Furthermore, interpretations rooted in taxonomies devised for modern soils do not *Present address: Department of Geology, Univer- sity of Rhode Island, Kingston, Rhode Island 'Here, the term "Tullock" is equated with and sub- permit the recognition of ancient soil types that 02881-0807. sumes the term "Ludlow." are not classifiable within modern schemes

Geological Society of America Bulletin, v. 99, p. 66-77, 13 figs., 5 tables, July 1987.

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Figure 1. Location of six localities used in study. Locality 2, Brownie Butte, is in Trumbo Ranch, Mon- tana, 7.5 minute quadrangle, T. 21 N., R. 37 E. Sections in Figure 10 are indicated by letters. All samples figured below are from the Brownie Butte region; specimens figured in this study are housed in the Repository, De- partment of Geology & Geophysics, University of Wisconsin-Madison.

(Jungerius and Mucher, 1972; Valentine and PALEOSOL FEATURES Roots occur either as (a) flat-lying (or nearly Dalrymple, 1976). Retallack (1981, p. 73) has so) material associated with other plant debris in noted: "Fossil soils should not be strained to fit The soil features recognized in this study are sedimentary accumulations or (b) vertically ori- into classifications of modern soils, as some (a) the presence of roots, (b) clay and metallic ented structures, orthogonal to relict bedding kinds of soils once formed on the earth are now oxide accumulations, (c) birefringence fabric, plane surfaces, presumably in life position. Iso- extinct." Finally, soil classifications have been (d) soil structures, and (e) color. The interrela- lated root rami range from just a few microns to developed for multiple purposes (for example, tionships of some of these features in soils are as much as 2 cm in diameter. Evidence of plant see Soil Survey Staff, 1975). In accomplishing shown diagrammatically in Figure 2. Pedofea- activity and growth is abundant. Delicate fea- divergent tasks, taxonomies impose arbitrary ty- tures are described using the standardized no- tures such as rootlets, roothairs, or internal cellu- pological systems upon natural continua of over- menclature of Bullock and others (1985). lar structures of roots are commonly preserved lapping processes. Information is not adduced (Fig. 3). by the imposition of a classification upon soils in Roots Modes of preservation vary. Much of the the way that, for example, our knowledge of material is partially lignitic, although structure is evolution is furthered by the development of Features. Roots dating from Hell Creek and still preserved. Some material has undergone phylogenetic classifications from biological taxa. Tullock time can be observed in a wide range of partial cementation or complete replacement by In the section that follows, pedological fea- grain sizes within both formations, from clay- ferric oxides. Microscopic pyritic nodular tures preserved in Hell Creek sediments are de- rich sediments to fine sandstones. With the ex- growths (Eswaran, 1967) are commonly asso- scribed, and their significance in processes of soil ception of sandstone facies interpreted as ciated with root material (Fig. 4). In some cases, formation are briefly considered. channel fills, roots are abundant in all sediments. original root material has disappeared and been

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Figure 2. Diagram of soil features described in this study: (1) soil void, (2) hypocoating (or halo) around void, (3) coat- ing lining a void, (4) birefringence fabric, (S) segregation with hypocoating, (6) blocky ped structures, and (7) platy ped structures. Figure 3. Root microstructure, Hell Creek Figure 4. Pyritic nodular growths infilling Formation, showing preservation of relict eel- a root cast (*200). lular material (x400). translocated; clay-sized material has filled voids left by the decaying roots. The infillings range (cements), (c) as horizontal bands, and (d) asso- such banding appears as crudely developed from dense, complete to loose, discontinuous ciated with soil voids. planar stratification; however, in thin section, (Bullock and others, 1985). Because of a lack of Replacement of framework grains in sand- bands are seen to be discontinuous clay accumu- crystal orientation or pattern in the infilling, the stones by amorphous clays (phyllosilicates) is lations, commonly not much greater than 1 mm clays are interpreted to be the result of physical, fabric selective and is preferential for volcanic in length and about 0.05 mm in width. as opposed to chemical, processes. Orange- and sedimentary lithic fragment loci. Fastovsky In some sandstones, amorphous organic mat- colored and drab-colored halos (rhizospheres) (1986) has reported syndepositional cements on ter is assiociated with metallic oxides in the inter- are observable around some roots, root traces, single and aggregate clasts. Oriented clay skins stices between sand grains (Fig. 6). This appears and root infillings. Roots in the Hell Creek and around individual framework grains are pre- in outcrop in conjunction with thin (5-20 cm), Tullock Formations show none of the ac- served; however, such coatings are commonly laterally continuous, indurated ledges. cordian-like compaction features described by obscured by diagenetic overprints. The presence Microscopic voids of pedogenic origin in the Retallack (1985), and therefore we infer that the or absence of pedogenic clay bridges between sandstones are commonly lined with clay min- sediments have not been greatly compacted. In grains is hard to verify because of postdeposi- erals and associated iron oxides. Infilling may be all cases, ancient roots can be distinguished from tional alteration and replacement fabrics. observed in several stages, from partial infilling modern roots by preservation and location. Horizontally banded accumulations of clay (Fig. 7) to complete clogging of voids. Not un- Process. The presence in sediments of roots and iron oxides occur interstitially in some fine- commonly, some of the clays have filtered into in situ is acknowledged to be an excellent indica- grained sandstones (Fig. 5). In hand sample, the sediments surrounding the void (Fig. 7), tor of soil processes (Jenny, 1980; Bown and Kraus, 1981; Re:allack, 1985), and for some authors, a soil is by definition a substrate that can support plant life (Joffe, 1949; Soil Survey Staff, 1975). Transport of soil materials can occur through living root tissue, by root infilling, and by the physical reorganization of soils during root invasion, enlargement, and decay. The presence of Figure 5. Banding of rhizospheres in pileosols is an indicator of ion clays and metallic oxides transport around roots. Rhizosphere color re- formed by illuviation in flects local redox (Eh) conditions at the time of fine sandstone (x50). burial.

Segregations of Clay, Organic Material, and Metallic Oxides Features. In m edium-textured sediments (silt sized to fine sand sized), clay segregations occur (a) as replacement products, (b) as coatings

most fragmental along exposed surfaces. Deeper, carbonates, periodic chemical precipitation of where Pleistocene and Recent weathering have iron with subsequent flocculation of the colloids, been less destructive than at the surface, the évapotranspiration at the wetting front, or siev- cracks become fewer. Within cracks, metallic ing by a layer of fine pores (Dijikerman and oxides have been precipitated in thin isopachous others, 1967; Bond, 1986). Ideally, the mor- sheets. Hell Creek siltstones commonly also phology of illuvial clay can be directly related to show precipitated manganese oxides in such depositional mode. cracks. In some rocks, ferric and manganese ox- In thin sections of medium- to fine-textured ides are co-precipitated. Halos suggest some in- material, illuvial morphologies include (1) iso- filtration of the ion-saturated fluids into the pachous coatings of void walls, (2) microlami- matrix. Some specimens show metallic oxide nations exhibiting strong orientation of clay in zonations, suggesting that precipitation of the consistent patterns, (3) clay coatings clearly dif- oxides occurred along an advancing front. ferentiable from supporting matrix, and (4) dis- Microscopic voids commonly do not exceed tinctive extinction patterns between crossed 0.25 mm and exhibit clay and metallic oxide polars that are the result of the strong orientation accumulations as internal void coatings and as patterns (above) and probably of small differ- external matrix hypo-coatings. Void infillings ences in the fabric not directly evident from range from dense, complete to loose, discontinu- optical examination (FitzPatrick, 1984). Excep- ous (Bullock and others, 1985). Some of the tions to these features have been noted (Gile and voids, now blocked, are filled with microscopi- Grossman, 1968; Nettleton and others, 1969; cal pyritic concretions (Fig. 4). Bullock, 1983), but in combination with other Figure 6. Podzolization: segregations of Clay accumulations occur in weakly impreg- soil features discussed here, illuvial morpholo- clay, amorphous organic material, and metal- nated amorphous associations. This feature is gies are good indicators of soil development. lic oxides (x200). (a) Clay, (b) organic mate- present in much of the finer-textured material In medium-textured material, clay illuviation rial, and (c) metallic oxides. and commonly occurs in nongeometric interca- is expressed as microscopic clay bridges between lations. It is not visible in polished slabs but sand grains or as coatings around sand grains. appears as a translucent feature in thin sections, Where well developed, clay illuviation can give revealing megascopic dimensions. D. Norton rise to the distinctive banding, termed "illuvia- producing halos (hypo-coatings of Bullock and (1986, personal commun.) has observed similar tion banding" (Dijikerman, 1967; Bond, 1986; others, 1985). features in modern soils developing in coastal see Fig. 5). Clay segregations within fine-textured (clay- plains. Nonpedogenic (diagenetic) clay transloca- and fine-silt-sized) sediments occur (a) around Process. Discrimination between pedogenic tions are common in sandstones. Mineralogi- voids; (b) in discrete nodules; and (c) in weakly clay translocations (illuviation) and nonpedo- cally immature or submature sandstones contain impregnated amorphous associations. genic diagenetic clay translocations (alteration, sources for the release of requisite authigenic Two major void types are present: (1) large replacement, and/or authigenic precipitation) is clay-forming ions, and sufficient flushing by (megascopic) cracks, probably formed as a re- essential in soil reconstructions. Clay illuviation pore fluids saturated with these materials is sult of Recent weathering and (2) small (micro- requires dispersion of the particles in an overly- enough to initiate and maintain cementation and scopic) voids of pedogenic origin. Both are sites ing horizon, so that they can be transported by replacement processes (Blatt, 1979; Berner, for the accumulation of iron oxides, but only the water through the soil voids. Precise mode and 1980). For those reasons, pedogenic processes in microscopic voids show evidence of significant timing of clay deposition in the subsoils is sandstones are difficult to isolate. accumulation of clay-sized material. caused by one or more of the following events: Sandstones that show translocated clays (clay Cracks are most numerous and the rocks flocculation by free iron oxides, flocculation by coatings, hypo-coatings, and segregations) in combination with root development and associated features are good candidates for having undergone pedogenesis. Likewise, illuvial banding commonly represents pedological development (Bond, 1986). The translocation of organic matter and metallic oxides (podzolization) also indicates pedogenesis. This latter interpretation is reinforced by cemented layers observed in Figure 7. Illuvial fabric outcrop, which are zones of accumulation of in thin section (xlOO). organic and metallic oxides. Such indurated (a) Coatings, (b) hypo- zones are typical products of podzolization coatings, (c) void, and (d) (Buol and others, 1981). birefringence fabric. Fine-textured sediments from the Hell Creek and Tullock preserve clay segregations and mi- crolaminated, oriented coatings and hypo-coatings, which are characteristic of illuviated horizons (Fig. 8). Ferric oxide segregations provide further refinements of soil-horizon designation (see below).

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Figure 8. Illuvial mixed il lite-smectite clay species showing diagnostic planar alignment (xlOO).

study. A close relationship between ped formation and pre-existing structures in the substrate fabric is evident. Platy ped development commonly occurs along sedimentary laminae. Very fine, weak to moderate platy peds are common, although inconsistently developed in Hell Creek and Tullock rocks. Less common are very fine to fine subangular blocky peds, which show weak to moderate development. Weak, very fine to fine granular (or crumb) structures are Birefringence Fabric slickensides with associated movement along preserved. fracturing planes (Smart and Tovey, 1981; Bul- Process. Those processes relevant in ped Features. Birefringence fabric (b-fabric; Bul- lock and others, 1985; McSweeney and Fas- formation include close juxtaposition of soil par- lock and others, 1.985) is used to characterize tovsky, 1987). Retallack (1985) and Thomasson ticles by root invasion, removal of moisture by sample matrices in thin-section microscopy via and Bullock (1975) propose that slickensides roots and associated shrinkage and cracking, the relationships of birefringent, aligned clay can be an expression of pedogenesis. animal activities (physical disturbances), wetting minerals to each other and to features of the B-fabrics, by their distribution and develop- and drying cycles, and freezing and thawing matrix, such as voids and grains. B-fabrics in ment, provide an indication of the degree of soil (White, 1966; Thomasson and Bullock, 1975; fine-grained Hell Creek and Tullock sediments development. Although parallel and cross- Foth, 1984). All of these events initially involve vary from absent (o dense, packed (Bullock and striated patterns due to suspension settling have mechanical particle segregations. Platy struc- others, 1985). Cross-striated fabrics are most been reported from sedimentary rocks (Brewer, tures, such as are found in Hell Creek and Tul- abundant, although parallel and subparallel 1976), complex patterns of b-fabrics in the ma- lock sediments, are linked with inherited sedi- striated fabrics are common. The b-fabrics stud- trix, as well as rings around soil voids, aggregate mentary fabrics (FitzPatrick, 1980; Birkeland, ied here show first-order yellow birefringence, boundaries, and intraclasts, suggest pedogenesis. 1984), as well as compaction (FitzPatrick, which is indicative of smectites and illites in 30 B-fabrics in the Hell Creek and especially the 1980), freezing and thawing cycles, and cemen- micron thin sections (FitzPatrick, 1980). Bire- lower Tullock are not particularly well devel- tation (Birkeland, 1984). Subangular blocky fringent clay rings around voids and around oped and probably include superimposed orien- structures originate as a result of soil-moisture grains are uncommon, but present in some sam- tations due to multiple pedogenic and nonpedo- changes (Birkeland, 1984; FitzPatrick, 1980); ples (Fig. 7). All b-fabrics are discontinuous and genic causes: wetting and drying, suspension crumb or granular structures are the products of tend to be thin (~ 20 microns). settling, and pressure along slickensides, as well wetting and drying, as well as mineral and or- Process. B-fabrics reflect the planar align- as from Cenozoic overburden. ganic colloids loosely binding particles (Fitz- ments of clay minerals. In the case of mixed Patrick, 1980). layer illite-smectite species (the dominant clay Soil Structure The fine-textured sediments studied here minerals in the Hell Creek; identified in this commonly preserve very little megascopic struc- study by X-ray diffraction), the b-fabric is Features. Peds are best expressed in silt- and ture. This is in agreement with the ubiquitous formed by the diffraction of light perpendicular very fine sand-sized material in the Hell Creek presence of relict sedimentary structures. to the alpha ray (IDeer and others, 1966). and Tullock and are intimately tied with sedi- The mechanisms of b-fabric alignment are mentary history. Siltstones in both formations Color only partially understood. Brewer (1976) and were deposited as flood-plain materials asso- others (White, 1966; Brewer and Sleeman, ciated with meandering river channeling systems Features. Numerous workers (Frye, 1969; 1970; Thomassoi and Bullock, 1975; Fitz- during the waning of flood stages (Leopold and Butler, 1980; Archibald, 1982) have remarked Patrick, 1980, 1984; Retallack, 1985) have all others, 1960; Retallack, 1986). The bulk of such upon the somber Hell Creek colors: grays, implicated swelling and shrinking resulting from deposition was by suspension settling, the major greens, browns, and blacks. In this study we wetting and drying cycles, a condition enhanced mechanism of flood-plain aggradation during have attempted to quantify the colors. This work in Hell Creek sediments by the dominantly Hell Creek time. Evidence of suspension settling is preliminary, and we present the method for smectitic mineralogy. Shear stress and associated is visible on most Hell Creek siltstones as very more detailed applications in further studies. plastic deformation of clay minerals along lines fine, planar laminations of size-segregated clays, Colors are associated with thick outcrops of of pressure anc tension generate b-fabric silts, and macerated organic matter. gray lithic arenites, bands of siltstone, and lig- (Greene-Kelly arid Mackney, 1970; Brewer, Superimposed upon the sedimentary features nites. Using a modified Markov statistical proc- 1976). Alignments of clay minerals also occur in are the pedogenic features described in this ess based upon log-linear modeling (Carr,

TABLE 1. CONDENSATION OF MUNSELL COLORS TO guished within this study. The diagonal of the Process. Soil color is an important character- GENERALIZED COLOR CATEGORIES transitions matrix thus contains only structural istic of soil identification and recognition. No

Munsell color Color category Munsell color Category zeros (Carr, 1982) and the model fitted was consistent quantifiable relationship between soil a log-linear model of quasi-independence color and soil type exists, however, because soil 10YR 3/1 Brown 5Y4/4 Green color is largely controlled by the amount and 10YR 2/2 Brown 5Y 5/3 Green (Goodman, 1968, Carr, 1982; Feinberg, 1983). I0YR 2/1 Black 5GY 5/1 Green With this modeling procedure, we tested the hy- valence of iron present in the soil, as well as by 10YR 3/2 Brown 5Y6/1 Gray 5Y 5/1 Gray 5Y4/2 Green pothesis that the distribution and number of the amount of organic matter (FitzPatrick, 1980; 5Y4/I Gray 2.5Y 2/0 Black 5Y 2.5/2 Gray 5G 6/1 Gray transitions observed in the Hell Creek is the Potter and others, 1980). Low chromas are 5YR 6/4 Pink 2.5Y 6/2 Green product of random processes. A goodness-of-fit generally considered to be indicative of reducing 2.5Y 4/2 Brown 5Y 2.5/1 Black 2 5Y 3/1 Green 2.5Y 5/2 Gray statistic (G of Feinberg, 1983; this statistic ap- conditions, which in soils can easily be induced 2.5Y 3/2 Green 2.5Y 4/4 Brown 2.5Y4/1 Gray proaches a chi-squared statistic) was used to test by extended saturation with water (Thomasson the model. We were able to reject quasi- and Bullock, 1975; FitzPatrick, 1980; Buol and independence (G2 = 11.6 on 5 degrees of free- others, 1981; Pickering and Veneman, 1984). It dom), suggesting that the transitions observed in has been suggested that soil chromas of consist- 1982; Feinberg, 1983), we tested for the the matrix are nonrandom. This was the result of ently less than or equal to 2 are correlative with presence of sequential patterns of colored bands. cell 14, which showed a significantly lower poorly drained soils, especially if the parent In selected columnar sections, each colored number of transitions from black up into gray materials do not have strong colors (Moore, band observed in the field was sampled. Because than would be expected by random processes. 1974). moisture alters soil color, samples were mois- Rejection of the model of quasi-independence Dark soil colors are commonly equated with tened and then described by hue, value, and was not possible when the effects of this cell high organic content (FitzPatrick, 1980; Potter 2 chroma (after Munsell, 1975). Twenty-three dif- were removed from the matrix (G = 4.377 on 4 and others, 1980). Exceptions have been noted ferent colors in 95 samples were recognized in degrees of freedom). (Soil Survey Staff, 1981), but within Hell Creek, Hell Creek and Tullock rocks (Table 1). From Two interpretations are possible from these large amounts of preserved organic material the stratigraphie relations of the color-keyed data: (a) color changes within the Hell Creek occur, and their presence is interpreted as a samples, a matrix of 72 color transitions was and Tullock are random and are unrelated to further indication of reducing conditions. established. The difference between the number soil development; or (b) color changes in the Somber colors of low chroma, mottles, the of samples and the number of transitions in the Hell Creek and Tullock may not be random, but presence of dark, organic-rich horizons, and the matrix results from the fact that transitions were mitigating factors of sampling and erosion have poor development of ped structures are typical not counted across recognizable erosion surfaces produced a dominant random component with- of gley conditions. Gleyed soils are formed as a and from the fact that some apparently different- in the pattern. Given the statistically significant result of water saturating the soil for extended colored weathered surfaces yielded similarly associations of color and other paleosol features parts of the year (Rudeforth, 1970; Thomasson colored wet samples. (below), we reject interpretation a. Interpreta- and Bullock, 1975). During those times when A 23 x 23 matrix constructed of only 72 tion b is reasonable in light of the sampling diffi- the soil is not saturated, channels of permeability color transitions cannot be meaningfully mod- culties described above (the lack of one-to-one allow oxidation of iron locally at void margins. eled because of the large number of empty cells correspondence between weathered, dry, out- Flooding of the soil reduces available oxygen to (cells with no transitions observed). Therefore, crop color patterns and moistened colors used only that which is dissolved in the flood waters. we condensed the matrix by subsuming several for comparison with standardized soil color This small amount of oxygen is metabolized by hue/value/chroma combinations within a single chips) and in light of the fact that in unstable microorganisms. Under such conditions of in- category. For example, 5YR 7/1,2.5Y 4/0, and fluvial settings, horizons are missing. We can tense depletion, ferric iron is reduced to ferrous 5BG 5/1 would all, in the field, be preceived as show the superposition of two identical horizons iron, leaving in the soil matrix the pale colors varieties of gray. Original Munsell colors and separated by an unrecognized hiatus (see be- associated with gleying. Ferrous iron is mobil- condensed generalized color categories are low). Superposed, identical horizons are indis- ized to zones where it can again be oxidized, shown in Table 1. A matrix of generalized color tinguishable in outcrop and in hand sample, and accounting for local concentrations of ferric iron categories was established (Table 2), including thus the transitions from such settings may not associated with voids and mottles. Lepidocrocite green, gray, black, and brown. Condensation by be correctly represented. Furthermore, the un- and "green rusts" (Taylor and Mackenzie, other criteria lost too much information. doubted presence of reworked pedogenic clasts 1980), two iron-oxide species characteristic of The fact that there is not a one-to-one corre- incorporated within a horizon (Bown and spondence between weathered outcrop bands Kraus, 1981; see below) is a factor in the ran- TABLE 3. PEDOFEATURES USED IN CLUSTER ANALYSIS and moistened sample colors suggests that if two domization of the transitions matrix. Our matrix-collapsing procedure may also have con- colors, similar when dry, were contained within Features a single band, they would not have been distin- tributed to the randomness of these data, by subsuming significant nonrandom variation in the 1. Pores categories. Our work suggests only that we may 2. Birefringence fabric 3. Roots TABLE 2. MATRIX OF TRANSITIONS OF not expect to observe a predictable pattern of 4. Clay and metallic oxide segregations GENERALIZED COLOR CATEGORIES 5. Dominant particle size: clay-sized particles dominant color transitions by crude observations in the sill-sized particles dominant field. sand.sized particles dominant Green Gray Brown Black 6. Dominant structure type: ped structures dominant sedimentary structures dominant Along with color banding, Hell Creek soils indeterminant Green 0 4 1 3 7. Black color Gray 4 0 7 5 are mottled. The mottles are variably sized, rang- 8. Brown color Brown 0 7 0 1 ing from 0.5 mm to as much as 1 cm. They are 9. Gray color Black 3 2 6 0 10. Green color predominantly light colored.

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reducing settings, are present in selected thin sec- TABLE 4. ASSOCIATIONS MEASURED BY DICE CLUSTER COEFFICIENT OF PEDOFEATURES USED IN CLUSTER ANALYSIS tions. FitzPatrick (1980) noted that gleyed soil Comparing Coefficient values development is less a reflection of climate than pedofeatures of topography, ar d that it occurs where water can accumulate rather than in specified climatic Matrix 1 2 3 4 5 6 7 8 9 regimes. Drainage can be impeded by cemented 1 2 0.72 0.72 0.72 0.72 0.72 0.72 0.72 0.72 0.72 1 3 0.73 0.73 0.73 0.73 0.73 0.73 0.73 0.73 0.73 horizons. Gleization can occur after soil burial 1 4 0.89 0.89 0.89 0.89 0.89 0.89 0.89 0.89 0.39 1 5 0.17 0.17 0.17 0.68 0.68 0.68 0.33 0.33 0.33 and is thus not necessarily dependent upon surfi- 1 6 0.61 0.23 0.48 0.61 0.23 0.48 0.61 0.23 0.48 cial factors. In the Hell Creek, however, local- 1 7 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 0.15 1 8 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 ized zones of oxidation (for example, iron-oxide 1 9 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 1 10 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 coatings on voids:) suggest that gleization oc- 2 3 0.63 0.63 0.63 0.63 0.63 0.63 0.63 0.63 0.63 curred during pedogenesis. 2 4 0.76 0.76 0.76 0.76 0.76 0.76 0.76 0.76 0.76 2 5 0.26 0.26 0.26 0.78 0.78 0.78 0.10 0.10 0.10 2 6 0.63 0.23 0.43 0.63 0.23 0.43 0.63 0.23 0.43 2 7 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 0.18 CLUSTER ANALYSIS 2 8 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 2 9 0.46 0.46 0.46 0.46 0.46 0.46 0.46 0.46 0.46 2 10 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 0.40 3 4 0.78 0.78 0.78 0.78 0.78 0.78 0.78 0.78 0.78 Soil development can be quantitatively 3 5 0.13 0.13 0.13 0.62 0.62 0.62 0.42 0.42 0.42 assessed by measurement of preserved soil 3 6 0.56 0.26 0.46 0.56 0.26 0.46 0.56 0.26 0.46 3 7 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 indicators. In principle, ordered (preferred) 3 8 0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32 0.32 3 9 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 0.49 associations of soil indicators suggest greater soil 3 10 0.46 0.46 0.46 0.46 0.46 0.46 0.46 0.46 0.46 development than random (nonpreferred) asso- 4 5 0.16 0.16 0.16 0.70 0.70 0.70 0.37 0.37 0.37 4 6 0.64 0.27 0.44 0.64 0.27 0.44 0.64 0.27 0.44 ciation of the indicators. Here, cluster analysis of 4 7 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 0.14 4 8 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 0.37 categorical data (see Davis, 1973; Gauch, 1983) 4 9 0.51 0.51 0.51 0.51 0.51 0.51 0.51 0.51 0.51 is used to recognize ordered associations. 4 10 0.42 0.42 0.42 0.42 0.42 0.42 0.42 0.42 0.42 5 6 0.22 0.37 0.05 0.53 0.26 0.50 0.20 0.40 0.20 5 7 0.58 0.58 0.58 0.13 0.13 0.13 0.05 0.05 0.05 5 8 0.16 0.16 0.16 0.39 0.39 0.39 0.09 0.09 0.09 Methods 5 9 0.11 0.11 0.11 0.46 0.46 0.46 0.43 0.43 0.43 5 10 0.05 0.05 0.05 0.40 0.40 0.40 0.26 0.26 0.26 6 7 0.19 0.38 0.05 0.19 0.38 0.05 0.19 0.38 0.05 The categories established for the cluster 6 8 0.13 0.33 0.36 0.13 0.33 0.36 0.13 0.33 0.36 6 9 0.42 0.33 0.33 0.42 0.33 0.33 0.42 0.33 0.33 analysis are based upon the soil indicators listed 6 10 0.42 0.14 0.27 0.42 0.14 0.27 0.42 0.14 0.27 in Table 3. The form of other categories was 7 8 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 7 9 0.00 0.00 0.00 0.00 O.OO 0.00 0.00 0.00 0.00 governed by two statistical imperatives: (a) pres- 7 10 0.00 0.00 0.00 0.00 O.OO 0.00 0.00 0.00 O.OO 8 9 0.00 0.00 0.00 O.OO 0.00 O.OO 0.00 0.00 O.OO ervation of independence of the categories and 8 10 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 (b) presentation of the data in binary (presence 9 10 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 or absence) form. Independence is violated by categories 5 and 6, which involve states exclu- sive of each other. To preserve independence, these data were calculated in the manner of Steel with the observation that mottling within Hell nine different cluster matrices were studied, and Torrie (1960). Creek rocks is not abundant. When colors are using each of the three states of particle size and observed to be coincident, black is associated structure as single binary categories (Table 3). Results with brown, in contradistinction to gray and Colors are independent because two colors can green. Blacks and browns are negatively corre- appear in a single sample. A database of 98 Table 4 shows the Dice associations for lated with pores, b-fabric, roots, and silt domi- samples was used in the construction of the 9 the nine matrices. In this study, only those nance. Gray and green colors, on the other hand, data matrices. associations which fall outside the 95% confi- are negatively correlated with the dominance of The Dice coefficient of association was used dence interval are considered significant. Thus, clay-sized material. to measure association. This is because it does only associations of greater than 0.61 (showing 4. Dominance of ped structures shows affini- not measure data that consist of mutual ab- positive correlation) and less than 0.39 (show- ties with pores and b-fabric. Peds are negatively sences, and because with 100 random samples ing negative correlation) are considered non- correlated with black and brown coloration. (approximately the number of samples used in random. Dominance of sedimentary structures, converse- this study), it closely approximates a binomial The following relationships are shown in ly, is negatively correlated with pores, b-fabric, distribution (Steel and Torrie, 1960; Archer and Table 4. roots, and clay segregations. Dominance of sed- Maples, 1986). We elected not to measure mu- 1. Pores, b-fabric, roots, and clay and metal- imentary structures is negatively correlated with tual absences because of the fallibility of absence lic segregations are very strongly associated, and the color green, although not positively corre- data in the geological record and because of a each may be considered an excellent predictor of lated with any color. The category in which tendency for apparent dependence among the the others. structures are indeterminate is negatively corre- colors that, in the presence of one color, leads to 2. The dominance of silt-sized material is as- lated with black and brown. high associations of absences in the others. Link- sociated with these four features, whereas a age was by the unweighted pair-group method. dominance of sand- or clay-sized material is HORIZON DEVELOPMENT The clustering program used was the AZtel negatively correlated with them. CLUpack, and the; data were run on a Kaypro 2 3. Colors are largely unassociated with each Horizon development is resolved by cluster personal computer. Confidence intervals for other, a statistical inference which is concordant analysis when soil materials differ between ho-

Figure 9. Organic soil; plant microstructure little decomposition (xlOO).

rizons. A variety of organic soils are preserved in the Hell Creek Formation. The most organic- rich examples are the coals, which contain as much as 44% organic carbon (Fastovsky, 1986). Hell Creek coals are of lignitic grade, commonly weakly developed, and rarely exceed 100 cm in thickness. They appear only in the uppermost parts of the formation and are commonly inter- calated with fissile brown (10YR 2/1) shales and stem and root debris. Organic accumulations in the Hell Creek can also occur as dark brown sediments showing abundant, matted, planar-aligned, organic material composed of well-preserved stem and root material inter- in the micropore network. The inferred eluvial ticularly fine-grained, thin (<1 m), lenticular bedded with varying amounts of clay. Such horizons are more porous, and silt and sand par- sediments, commonly contain roots and illuvia- matted woody accumulations commonly reach ticles are uncoated. A horizons (organic-rich sur- tion bands. Roots are also common on large, 100 cm in thickness. Tullock coals reach thick- face units in the soil) are not easily identified, fine-grained sand bodies that preserve lateral ac- nesses of 1.5 m and occur throughout the forma- although in some cases organic enrichment in cretion surfaces. Illuviation in such sediments is tion. Thick, matted accumulations of plant siltstones suggests A-horizon development. The difficult to distinguish from non-pedogenic dia- matter are not common within the lower Tul- paucity of A horizons identified in this study is genesis; however, the ledge-forming podzolic lock, and Tullock coals do not exhibit well- consistent with their apical position which horizons are clear indicators of pedogenesis. In developed shaly partings such as are found in the makes them most subject to erosion, as well as modern soils, podzolization is associated with Hell Creek Formation. with the fact that even if successfully buried, the specific vegetational suites tolerant of low soil Organic soils are evaluated by their state of distinctive ped structures are subject to decom- pH and low nutrient resources (Jenny, 1980). decomposition (Buol and others, 1980), an es- position and compaction (Mucher and Moro- Vegetational relationships to sandy Hell Creek timation difficult to make in ancient sediments. zova, 1983; Fenwick, 1985). It is likely that and Tullock soils are poorly understood. It is clear, however, from detailed morphology some of the apparently eluviated horizons in this Rooted volcanic ash (Tan, 1984) and tonstein that is preserved in some of the matted plant study could encompass A horizons with organic layers (Williamson, 1970) are preserved within fragments that much fibrous material remains constituents removed by oxidation. For these, the coals but are a very minor part of Hell Creek intact in these sediments, and that many of them we have used the designation "A/E." Refine- deposits. The occurrence, mineralogy, and sig- are relatively undecomposed (Fig. 9; see Thies- ments of the broad horizon categories can be nificance of these unusual layers have been dis- sen, 1925). Pedogenesis in Hell Creek and Tul- recognized in Hell Creek sediments and are cussed elsewhere (Bohor and others, 1984; Izett lock O horizons typically is not advanced. shown in Figure 10. and Bohor, 1986). It is clear, however, that Pedogenic processes are evident in samples Sandy soil horizons are numerically less where present, these are commonly sites of ped- that contain roots, pores, b-fabric, peels, and clay abundant than silty ones. Most medium-grained ogenesis. Layers range from <1 cm to 4 cm segregations. The cluster analysis here, however, sand bodies within the Hell Creek and Tullock thick. Commonly the upper 0.5 cm is a dark is based upon binary data and thus does not are channel-fill deposits and do not show evi- brown color (10YR 2/1), separated from a provide an indication of the degree to which the dence of pedogenesis. Other sandy deposits, par- lower, pink (5YR 7/3) portion by an abrupt, soils and component horizons are developed.

For example, an eluviated horizon (E horizon) TABLE 5. POINT COUNT DATA FOR SELECTED ILLUVIATED AND ELUVIATED with residual clay accumulations is not distin- HORIZONS (IN PERCENTAGES OF TOTAL SAMPLE) guished by cluster analysis from an illuviated Sample no. horizon (B horizon) with well-developed clay Feature accumulations. Degree of accumulation can be recognized by point counting. Voids 2.4 0.6 2.0 0.3 0.4 0 0 0 0 0 Coatings 1.2 6.2 6.3 2.4 0.7 21.5 24.0 25.8 30 24.6 Table 5 compares point-count data from se- "Clean" grains 17.6 25.2 20.0 9.0 8.6 0 0.2 2.8 8.2 2.8 Matrix and b-fabric 70.1 67.5 65.5 87.0 76.5 50.8 66.2 61.4 60.0 57.3 lected horizons within Hell Creek paleosols. The Clay segregations 0.8 0.6 6.3 1.2 13.8 27.7 4.2 9.2 21.8 9.3 Roots, organics inferred illuviated horizons show higher percen- with hypo-coatings 8.2 0 0 0 0 0 5.4 0.8 0 6.0 tages of clay hypocoatings, void coatings, and Interpretation Eluviated horizon Illuviated horÌ7on clay segregations, as well as slightly reduced Nlne: 500 - 560 point counts per slide. porosity. This is reflective of clay-sized material

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Silts tone -I-E:

Mudstone & Laminated Plant Debris Lignite • Roots 'A

Figure 10. Composite stratigraphic section from Brownie Butte locality (Fig. 1). Interpre- tations of soil horizons indicated to right of stratigraphie column: O, O horizon (organic accumu- j-p—.. weathered lation); A, A horizon; A/E, inde- HJj Bhs terminate A or E horizon; B, B Bg horizon; Bg, gleyed B horizon; Bhs, B horizon with podzoliza- 362 Cg tion; Btg, B horizon with translocated amorphous material and gley features; BC, horizon with Bhs to B and C features; C, C horizon; 137 covered interval Cg, C horizon with gley features. 95 A/E 65 50

562

67 covered interval 18-3= C Btg 171

83 Cg A/E 150 11

wavy boundary. Figure 11 shows an ashy soil presence of illuvial banding as well as indurated horizon from the Hell Creek. ledge development and the microscopic features associated with podzolization indicate pedo- Stratigraphy genesis. Medium-textured channel fills may have been too unstable for the development of soils. Figure 10 encompasses the Cretaceous-Paleo- Woody material accumulated in depressed gene transition and exemplifies the interrelation- areas. Highly localized depressions may repre- ships of channels and soils in upper Hell Creek sent scouring of the flood plain by erosive flood and lower Tullock strata. It is a composite strati- waters or may be the late stages of channel graphic columnar section from the Brownie abandonment. More extensive organic accumu- Butte locality, Garfield County, Montana. Exact lations probably arose as a result of abandoned correlations between the three columnar sections meander cut-offs. The deposits of woody mate- are impossible because of complex, lenticular rial show little evidence of soil development. lithofacies and isolated outcrops. Nevertheless, Figure 13 presents a generalized reconstruc- topographic correlations are possible because tion of lateral variations in soil development Hell Creek and Tullock sediments are not along a topographic profile (catena) in the Hell deformed at that scale. Creek. The topographically highest horizons are There is a tendency for weak soil develop- recognized by zones of oxidation, from which ment high in the Hell Creek Formation and for are inferred fluctuations of the water table. In- marginal soil development in the lowermost tense gleying is reflective of a diminution in oxi- Tullock. Megascopic and microscopic indicators Figure 11. Pedogenic alteration of a ton- dation, interpreted here to reflect progressive of soil development, including ped structures stein. (a) Illuviation in void (xlOO). depressional settings within the landscape. Fi- and clay illuviation features are less common in nally, topographic lows were swampland and the uppermost part than they are lower in the sites for organic soil development. stratigraphic column. This is correlated with the presence of pedorelicts (Brewer, 1976), ripped By latest Hell Creek and earliest Tullock time, greater preservation of sedimentary structures, up and redeposited soil intraclasts (Fig. 12). The the mean level of the water table rose from its such as planar laminations, that exhibit reduced bulk of the pedogenic processes occurred on the position in mid-Hell Creek time, concomitant or no pedogenic influences. The increased levees and flood-plain sediments, where pre- with a reduction in the amount of its fluctuation. presence of sedimentary planar laminations is served features and gley morphologies indicate This is reflected by the onset of peat-swamp coincident with the increased presence of coal- the presence of a high water table. In some formation and by the presence in the region of swamp deposition and has been correlated by horizons, mottling and localized accumulations broad expanses of ponded-water deposits by the Fastovsky (1986) with increased ponding in the of oxidized zones around pores are interpreted earliest Paleogene (Fastovsky, 1986). The peat region. The absence of abundant, well-devel- as evidence for the fluctuation of the water table swamps developed slowly enough for soils to oped soils in the uppermost Hell Creek Forma- (Rudeforth, 1970; Thomasson and Bullock, form in them, as well as in ash layers that were tion is attributable to a more unstable landscape, 1975). Extensive rooting in these soils indicates deposited on them. in which poor drainage retarded soil develop- that there was abundant vegetation. Evidence for paleoenvironmental change is ment. Deposits high in the formation are laced Vegetation on the sandy point bars and ubiquitous across the K-P boundary in the study with thin lenticular sandstones and silty units crevasse-splay deposits is reflected by roots. The area, and the geographic extent of the change far with laminae attributable to suspension settling, all of which reflect an unstable, rapidly changing landscape (see Dingus, 1984).

INTERPRETATIONS OF HELL CREEK PALEOSOLS

A coherent picture of pedogenesis during the Cretaceous-Paleogene transition in eastern Mon- Figure 12. Pedorelict. tana and western North Dakota emerges. By Older soil clast with middle Hell Creek time, soils formed in a poorly cross-striated b-fabric drained fluvial system that was only stable contained within younger enough to allow incipient pedogenesis. All non- soil that has granostriated channelized facies underwent soil development. b-fabric (xlOO). They present minimally developed pedogenic features, including modest illuviation and b-fabric development, and weak ped structures showing strong sedimentary influences. Strong sedimentary influences are also indicated by the

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physics, University of Wisconsin-Madison. Field assistance for this project was provided by J. A. Hogler and P. M. Hartshorne. Special thanks are owed to ranchers H. Braddack, W. Collins, L. Engdahl, R. Engdahl, A. Mac- Donald. A. Sonsella, M. Sonsella, and their families. Our work has benefitted greatly from careful readings by R. H. Dott, Jr., M. Habecker, Figure 13. Generalized catenary profile of horizon relationships in fine-grained soils dating G. S. LeMasters, F. W. Madison, and C. G. from middle Hell Creek time. Topography constrains reductomorphic settings, with water- Maples. Discussions with D. R. Kerr, E. H. Ko- logged organic accumulations in the lowest regions (O horizon), intensification of gley features nopka, F. W. Madison, and C. G. Maples pro- below the water table (Bg horizon), and localized oxidation and illuviation in horizons within vided useful input. A. Foster contributed the the zone of water-table fluctuation (Btg). plates. Finally, this manuscript was improved by A surficial horizon of organic accumulation and eluviation (A/E) covers the setting. the constructive criticisms of its two reviewers, G. Kocurek and D. Johnson.

exceeds the scale of the fluvial deposits in the transition in the Western Interior. This report is

region (Fastovsky, 1986), as well as that of re- but an overview of the paleosols, and there is REFERENCES CITED constructed delta lobes (Cherven and Jacob, much detail yet to be unraveled. Exposures of Archer, A. \V„ and Maples. C. G., 1986, Monte Carlo simulations of binomial 1985). For this reason, larger-scale effects may the Hell Creek are suitable for refining the clustering coefficients: Effects of numbers of variables and "zero" data: North American Paleontology Convention, 4th, Abstract, p. Al. have caused the change in paleosols. These generalized catenary sequence presented here, Archibald, .1. D., 1982, A study of Mammalia and geology across the changes may have been induced tectonically which will not only elucidate lateral relation- Crctiiceous-Tertiary boundary in Garfield County, Montana: Berkeley, California, University of California Publications in Geological Sciences, (through a decrei.se in subsidence), eustatically ships of the paleosols but will reflect topographic 286 p. Belt, E. S., Flores, R. M„ Warwick, P. D„ Conway, K. M., Johnson, K. R„ and (by a mild transgression during a predominantly subtleties in the ancient fluvial setting. Close in- Was'iowilz, R. S., 1984, Relationship of lluviodeltaic fades to coal regressive phase of the end of the Cretaceous vestigation of the cemented horizons could be deposition in the Lower Fort Union Formation (Palaeocene), south- western North Dakota, in Rahmani, R. A., and Flores, R. M., eds., eustatic sea-level cycles), or they may have been the key for establishing soil-development rates, Sediinentology of coal and coal-bearing sequences: International Asso- ciation of Sedimentologists, Special Publication, Volume 7, p. 177-195. induced climatically (by, for example, increased as well as testing the longevity of the modern- Berner, R. A., 1980, Early diagenesis, a theoretical approach: Princeton, New rainfall). vegetation-podzolic-horizon relationship. We Jersey, Princeton University Press, 241 p. Birkeland, P. W., 1984, Soils and geomorphology: New York, Oxford Univer- In combination with the absence of correla- are convinced that a major feature in the further sity l"ress, 372 p. Blatt, H., 1979, Diagenetic processes in sandstones, in Scholle, P. A., and tive tectonic evidence of a rise in sea level, meg- understanding of Cretaceous-Paleogene transi- Sehl iger, P. R., eds.. 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On the basis of leaf morphol- elucidation of the iron chemistry may provide a wooJ Formation, northwest Wyoming, U.S.A.) and their significance for paleoecology, paleoclimatology, and basin analysis: Palaeogeog- ogy, they reconstructed for the uppermost Hell definitive explanation for the marked color dif- raphy, Palaeoclimatology, and Palaeoecology, v. 34, p. 1-30. ference between the Hell Creek and Tullock, a Brewer, R., 1976, Fabric and mineral analysis of soils: New York, Roben E. Creek a sub-humid climatic regime with a mean Krieger Publishing Co., 482 p. annual temperatu re of about 18 °C with incip- distinctive feature that is probably due to syn- Brewer, R., and Sleeman, J. R., 1970, Some trends in pedology: Earth Science Reviews, v. 6, p. 297-335. ient seasonality (estimated paleolatitude of 56° depositional as well as diagenetic causes. 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D., 1980, Soil genesis and tables 20-30 cm (Thomasson and Bullock, the ash soils may have implications for our classification: Ames, Iowa, Iowa State University Press, 406 p. Butler, R. D., 1980, Stratigraphy, sedimentology, and depositiona! environ- 1975) and could account for the changes ob- knowledge of the stability of their mineralogy, of ments of the Hell Creek Formation (Late Cretaceous) and adjacent their utility as stratigraphic indices, and of the strata, Glendive area, Montana [Ph.D. thesis]: Grand Forks, North Da- served in the stratigraphically highest exposures kota, University of North Dakota, 398 p. of the Hell Creek. nature of their origins. Carr, T. R., 1982, Log-linear models, Markov chains, and cyclic sedimentation: Journal of Paleontology, v. 52, p. 905-912. Because there is no evidence in Hell Creek Cherven, V. B., and Jacob, A. 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