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Outcrop versus core and geophysical log interpretation of mid-Cretaceous paleosolsfrom the Dakota Formation of Kansas

Gregory J. Retallack a,⁎, David L. Dilcher b,c

a Department of Geological Sciences, University of Oregon, Eugene, OR 97403, U.S.A.b Department of Biology, Indiana University, Bloomington, IN 47401, U.S.A.c Department of Geology, Indiana University, Bloomington, IN 47401, U.S.A.

a b s t r a c ta r t i c l e i n f o

Article history:Received 18 February 2011Received in revised form 9 February 2012Accepted 14 February 2012Available online 23 February 2012

Keywords:CretaceousPaleosolKansasPaleoclimateDiagenesisWeathering

Paleosols of the mid-Cretaceous (late Albian to early Cenomanian) Dakota Formation in Kansas and Nebraskaare well exposed in road, river and clay pit exposures, but show systematic overprinting by modern weath-ering. Examination of deep drill cores reveals that modern weathering effects include (1) oxidation of pyriteto jarosite, (2) reaction of jarosite and calcite to clear selenite, and (3) oxidation of sphaerosiderite and sid-erite nodules to hematite. Surface oxidation in this region extends no deeper than 2 m from the surface. He-matite nodules and coatings of leaves are found in core only at one stratigraphic horizon, which was not asappears in outcrop, a laterite. Transformed matrix density and photoelectric absorption logs of cores revealthat kaolinite clays are most abundant within deeply weathered paleosols (Sayela pedotype) at this strati-graphic level, near the Albian–Cenomanian boundary, when angiosperm forests were especially diverseand lived in a subtropical humid climate. Other paleosols of the early Cenomanian were less deeply weath-ered, and fossil plants of this age included temperate humid angiosperm mangal and swamp woodland,but conifer-dominated floodplain forests. This short-lived mid-Cretaceous warm–wet spike has also beenrecognized in other paleosols of Minnesota, Utah, Nevada, and New Mexico, and coincides with marineblack shale of the Breitstroffer event, midcontinental marine transgression, a sequence-stratigraphic bound-ary, and a marked increase in early angiosperm diversity.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Weathering of paleosols in outcrop can compromise interpreta-tion of their past environments. The best recourse when surfaceweathering is suspected is to examine the paleosols in deep drillcore. Confusion of modern and ancient weathering was controversialfor interpretation of redox and base status of Cambrian paleosols(Retallack, 2008) in South Australia and of Archean paleosols(Ohmoto et al., 2007) and banded iron formations (Hoashi et al.,2009) in Western Australia, but solved by examination of core andgeophysical core logs. Furthermore, core logs come with useful geo-physical logs such as gamma ray profiles (MacFarlane et al., 1989),and this paper attempts to convert such data to information criticalfor interpretation of paleosols (Retallack, 1997). This comparison ofpaleosols of the mid-Cretaceous Dakota Formation in Kansas withincores and outcrop (Fig. 1) thus itemizes common alterations of paleo-sols on exposure, and explores paleosol interpretation using wirelinelogs.

This study of cores and geophysical logs has ramifications for in-terpretation of paleoclimates and paleoenvironments. For example,paleosols of the Dakota Formation identified as laterites in outcropby Thorp and Reed (1949) and Reed (1954) are shown here to havebeen subtropical Ultisols, but not tropical laterites (Retallack, 2010).Furthermore, not all red paleosols in outcrops of the Dakota Forma-tion were originally red or deeply weathered. One horizon of Ultisolpaleosols records global warmth and humidity from a CO2 green-house spike, recorded elsewhere as the Breitstroffer marine anoxiaevent (Retallack, 2009), but other paleosols of the upper Dakota For-mation now red in outcrop, were originally gray, sideritic paleosols ofwaterlogged coastal plains.

Paleosols were first recognized in the mid-Cretaceous Dakota For-mation by soil scientists (Thorp and Reed, 1949; Reed, 1954), but alsohave attracted geological attention (Retallack and Dilcher, 1981a,b;Joeckel, 1987, 1991; Porter et al., 1991). The Dakota Formation haslong been known for its sandstone fossil floras, collected initially byGeorge Miller Sternberg (Rogers, 1991) for Leo Lesquereux (1892),and more recently collected for shale floras with preserved cuticles(Dilcher et al., 1976; Dilcher, 1979; Basinger and Dilcher, 1984;Dilcher and Crane, 1984; Dilcher and Kovach, 1986; Skog andDilcher, 1992, 1994; Wang, 2002; Wang and Dilcher, 2006; Dilcherand Wang, 2009; Wang et al., 2011). The Dakota Formation is also

Palaeogeography, Palaeoclimatology, Palaeoecology 329–330 (2012) 47–63

⁎ Corresponding author.E-mail address: [email protected] (G.J. Retallack).

0031-0182/$ – see front matter © 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.palaeo.2012.02.017

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rich in fossil pollen (Farley and Dilcher, 1986; Kovach, 1988; Kovachand Dilcher, 1988; Ravn and Witzke, 1994, 1995; Hu et al., 2008a,b),but poor in fossils other than plants. Moths, weevils, leaf beetles,and midges are inferred mainly from damage to fossil leaves(Labandiera, 1998), and a few insect fossils (Retallack and Dilcher,1981a). Fossil molluscs are rare molds and casts of freshwater tobrackish forms (White, 1894; Siemers, 1976). A locality near Wilson(Fig. 1) has yielded 6 species of shark teeth, 6 different rays and 7kinds of bony fish (Everhart et al., 2005), localities near Carneiroand Salina produced the giant crocodile Dakotasuchus kingi (Mehl,1941; Vaughn, 1956), and another crocodile was found in Big Creeknear Russell (D.E Hattin pers comm., 1978). A species of nodosaur(Silvisaurus condrayi Eaton, 1960) from near Minneapolis and alsonear Wilson (both in Kansas), and a large ornithopod from nearDecatur, Nebraska (Barbour, 1931; Galton and Jensen, 1978), are theonly dinosaur bones from the Dakota Formation, though dinosaurgastroliths and tridactyl footprints are also recorded (Joeckel et al.,2001; Liggett, 2005). This unbalanced pattern of preservation alsomay be related to paleosols (Retallack, 1998), and this paper also

attempts to detail taphonomic biases in reconstruction of past ecosys-tems from paleosol-rich sequences.

2. Geological background

The age of the Dakota Formation in Kansas is bracketed by marinefossils in underlying and overlying rocks (Fig. 1). Fossil foraminiferaare evidence for an Albian age of the underlying Kiowa Formation(Loeblich and Tappan, 1961) and an age of Cenomanian from forami-nifera in the overlying Graneros Shale (Eicher, 1965) is supported byassociated ammonites and clams (Hattin, 1967; Hattin et al., 1978).Palynofloras are evidence that the Lower Dakota Formation is lateAlbian, but the middle and upper Dakota Formation are lower to mid-dle Cenomanian (Pierce, 1961; Farley and Dilcher, 1986; Ravn andWitzke, 1994, 1995; Hu et al., 2008a).

The Albian–Cenomanian boundary has been recognized using pal-ynology and carbon isotope chemostratigraphy within the Rose Creekclay pit south of Fairbury, Nebraska (Gröcke et al., 2006). This bound-ary is marked by exceptionally developed paleosols near the Rose

Fig. 1. Map of locations examined within the Dakota Formation in central Kansas and Nebraska after Retallack and Dilcher, 1981a,b) and cross-section from borehole data afterHamilton (1994).

48 G.J. Retallack, D.L. Dilcher / Palaeogeography, Palaeoclimatology, Palaeoecology 329–330 (2012) 47–63


Creek pit and at other localities such as Carneiro and Delphos (Sayelapedotype of Fig. 2) at the marine regression forming the junction of Jand D sequence stratigraphic units recognized in regional correlations

(Franks, 1975; Hamilton, 1994). This key sequence boundary is with-in the Terracotta Clay Member of the Dakota Formation, as defined byPlummer and Romary (1947) and Plummer et al. (1960), and not at

Fig. 2. Measured sections of paleosols in the Dakota Formation of Kansas (B–N) and Nebraska (A). Paleosols are classified into pedotypes using Lakota Sioux descriptors (Table 1),and the thickness is shown by boxes whose width corresponds to degree of development. Scales of development and calcareousness from field application of dilute acid followRetallack (1997), and colors are from a Munsell chart.

49G.J. Retallack, D.L. Dilcher / Palaeogeography, Palaeoclimatology, Palaeoecology 329–330 (2012) 47–63


the boundary of the Terracotta and overlying Janssen Clay Member(Hamilton, 1994). The deeply weathered Sayela paleosols can beregionally correlated through Nebraska (Porter et al., 1991) andIowa (Witzke and Ludvigson, 1994), where they separate a lowerNishnabotna and upper Woodbury Member of the Dakota Formation(Karl, 1976). The deeply weathered paleosols developed in granite(Goldich, 1938), quartzite (Austin, 1970) and paleokarst (Sloan,1964) beneath the base of the Dakota Formation in Minnesota areof comparable geological age and exceptional degree of weathering(Setterholm, 1994). Sandstones of the lower Dakota Formation(upper J) include large (up to 6 m diameter) spherical to ellipsoidalnodules of cross bedded sandstone cemented by calcite, notably atMushroom Rock State Park (N38.726005° W98.030718°) south ofCarneiro, at Rock City Park (N39.089852° W97.739759°) southwest ofMinneapolis, and Quartzite Quarry Park (N39.028497° W97.162289°)southwest of Lincoln (McBride and Milliken, 2006). Sandstones ofthe upper Dakota Formation (unit D) lack such features, although theyweather into hoodoos and show meandering paleochannels suchas the Rocktown channel, near Russell (N38.966597° W98.857589°;Siemers, 1976; Retallack and Dilcher, 1981a).

The Dakota Formation in Kansas is flat-lying and unmetamor-phosed. Smectite and kaolinite are the principal clay minerals in theDakota Formation of Kansas (Plummer and Romary, 1947; Plummeret al., 1960), Nebraska (Joeckel, 1987, 1991), Illinois (Frye et al.,1964) and Minnesota (Parham and Hogberg, 1964; Parham, 1970).These clays are unillitized by burial, which would not have exceeded0.8 km in Kansas through Minnesota (Dyman et al., 1994; McBrideand Milliken, 2006). Ptygmatic folding of clastic dikes to 38% oftheir former thickness has been observed in carbonaceous shales ofthe Dakota Formation in northwestern North Dakota (Shelton,1962), where there was 1.6 km of overlying Cretaceous and 0.2 kmof Cenozoic rocks (Dyman et al., 1994). Standard compaction formu-lae for paleosols (Sheldon and Retallack, 2001) give original thick-nesses of Ultisols (Su in cm) and Histosols (Sh in cm) from thicknessof comparable paleosol (Bp in cm) for a known burial depth (K inkm), as follows.

Su ¼ Sp.

−0:58. 0:42

e0:2K−1

� �� ð1Þ

Sh ¼ Sp.

−0:06. 0:94

e2:09K−1

� �� ð2Þ

Using these equations for kaolinitic paleosols (Ultisols) of theDakota Formation gives reduction to 90% of former thickness at0.8 km burial, and 82% at 1.8 km, and coals (Histosols) give reductionto 7% at 0.8 km and 6% at 1.8 km. Early diagenetic sphaerosideritefrom methanogenic pedogenesis was in some locations followed bypyritization due to sulfate reduction (Brenner et al., 1981). Large(6 m) spherical nodules in sandstone paleochannels of the lowerDakota Formation in Kansas have oxygen isotope compositions inter-preted as evidence of warm precipitation temperatures (30–50 °C)

and shallow (ca. 0.4 km) depths of burial (McBride and Milliken,2006). Although called “concretions” by McBride and Milliken(2006), they lack the growth banding required of concretions in soilscience terminology (Brewer, 1976), and are thus nodules. Most he-matite in Dakota Formation paleosols is paleomagnetically reversedand so compatible with a Cretaceous age, but one sample is normaland oxidized well after burial (Faulds et al., 1996).

3. Materials and methods

Stratigraphic sections were measured using tape and level on aBrunton compass, with degree of development and of dilute acid reac-tion estimated in the field using the scale of Retallack (1997) and colorfrom a Munsell chart with tropical and gley pages (Fig. 2). Localities ex-amined for this study in stratigraphic succession were: 1, Carneiro,Ellsworth Co. (Acme Brick Company Ramsay pit N38.708031°W98.090387°); 2, Fairbury, Jefferson Co. (Endicott Clay Company, RoseCreek pit N40.050094° W97.169206°); 3, Delphos, Cloud Co. (Braun'sfarm N39.336428° W97.604351°); 4, Kanapolis, Ellsworth Co. (AcmeBrick Company, Vondra pit N38.714624° W98.115197°); 5,Hoisington, Barton Co. (3 localities in Kansas Brick and Tile companyquarry, “north” N38.474784° W98.791444°, “southwest” N38.471302°W98.781502°, and “south” N38.469133° W98.780099°); 6, Ellsworth,Ellsworth Co. (gully at N38.816884° W98.137002°); 7, Bunker Hill,Russell Co. (2 sites on Linnenberger brothers ranch; “west”N38.908973° W98.663604° and “east” N38.910887° W98.656723°); 8,Concordia, Cloud Co. (Cloud Ceramic Company pit N39.533929°W97.597685°); 9, Black Wolf, Ellsworth Co. (gully at N38.739122°W98.369881°); 10, Russell, Russell Co (bluffs north of Saline RiverN38.966597° W98.857589°); 11, Wilson, Ellsworth Co. (gully atN38.782809° W98.493196°). Boreholes examined include KGS Braun#1 (N38.986373° W99.354256°) in Ellis Co., Jones #1 (N39.219036°W98.176458°) in Lincoln Co., and Kenyon #1 (N39.683807°W97.710725°) in Republic Co. The Dakota Formation from 146.9 mdown to 239.3 m in Braun #1 (Macfarlane et al., 1990) is a useful refer-ence section for relative stratigraphic level of these localities, estimatedfrom local level relative to Sayela paleosols near D–J sandstone contactand Graneros Shale (Retallack and Dilcher, 1981a,b) as follows: locality1 at 204–210m, 2 at 197–204m, 3 at 184–192m, 4 at 169–176 m, 5at 151–163m, 6 at 157–175 m, 7 at 157–172 m, 8 at 147–164 m, 9 at147–161 m, 10 at 147–168 m, and 11 at 135–169 m.

Paleosols were classified into different field categories (pedotypesof Table 1) and named for non-genetic descriptions in the LakotaSioux Native American language (Buechel, 1970). Pedotype terminol-ogy developed here can also be applied to other described paleosols,for example, Sayela paleosols near Brookville, Kansas (Thorp andReed, 1949; Porter et al., 1991) and Endicott, Nebraska (Joeckel,1991), and Casmu, Sagi and two Sapa paleosols along Little BlueRiver near Fairbury, Nebraska (Joeckel, 1987). The “Morton profile”of Sloan (1964) developed on granite beneath the Dakota Formationin Minnesota (Goldich, 1938) is an additional mid-Cretaceous pedo-type. Comparable paleosols are found in the Cenomanian Dunvegan

Table 1Diagnosis and classification of pedotypes of the Dakota Formation in Kansas.

Pedotype Lakotameaning

Diagnosis Soil taxonomy FAO soil map Australianclassification

Cahli Coal Thick coal (O) over white clayey underclay with carbonaceous roottraces (A) and bedded siltstone (C)

Fibrist Dystric Histosol Fibric Organosol

Casmu Sandy Pyrite encrusted carbonaceous root traces (A) in bedded sandstone (C) Sulfipsamment Thionic Fluvisol Intertidal HydrosolKazela Shallow Carbonaceous root traces (A) in gray shale (C) Fluvent Dystric Fluvisol Stratic RudosolMakazi Sulfur Carbonaceous claystone with pyrite encrusted root traces (A) over bedded shale (C) Sulfaquept Thionic Fluvisol Intertidal HydrosolSagi Auburn colored Ferruginized sandstone and root traces (A) in bedded sandstone and shale (C) Psamment Ferralic Arenosol Arenic RudosolSapa Dirty Carbonaceous claystone with root traces (A) over sphaerosideritic claystones (By) Kandihumult Humic Gleysol Oxyaquic HydrosolSayela Reddest Light gray clayey siltstone (A) over red mottled claystone (Bt) Kandiudult Ferric Acrisol Red Chromosol



Formation of Alberta and British Columbia, Canada (McCarthy andPlint, 1999, 2003; McCarthy et al., 1999; McCarthy, 2002), especiallytheir “type 1” (like Casmu here),” type 2” (like Sapa), “type 3” (likeCahli), “type 4” (Cahli–Sapa–Casmu pedocomplex) and “type 5”(Sapa–Cahli pedocomplex). Type profiles of the pedotypes (Fig. 3)were sampled for field laboratory study. These and other data werethe basis for pedotype-specific paleoenvironmental interpretations(Table 2), and taphonomic biases and outcrop overprints discussedin this paper.

Samples of paleosols were collected in the field and petrographicthin sections prepared using kerosene as a non-polar coolant andepoxy resin as a specimen stabilizer of these friable specimens (Tateand Retallack, 1995). Samples were analyzed for major oxides usingatomic absorption by Christine McBirney (University of Oregon),who also determined ferrous iron using the Pratt titration. Weightpercent organic carbon was analyzed by the soil analytical laboratoryat Oregon State University using the Walkley-Black titration. Bulk

density was determined weighing paraffin-coated clods some 2 cm2

in and out of water, in order to calculate mass transfer and strain ofelemental composition.

Thin sections were point-counted using a Swift Automated stageand collator and an eyepiece micrometer to determine proportionsof sand–silt–clay and mineral components of the paleosols (Fig. 3).Molar ratios were also calculated from bulk chemical analyses togive products over reactants of common soil forming chemical pro-cesses (Retallack, 1997), such as salinization, calcification, clayeyness,base loss and gleization (Fig. 2). A more detailed accounting of geo-chemical change following Brimhall et al. (1991) is mass transfer ofelements in a soil at a given horizon (τj,w in mol) calculated fromthe bulk density of the soil (ρw in g cm−3) and parent material (ρpin g cm−3) and from the chemical concentration of the element insoils (Cj,w in wt.%) and parent material (Cp,w in wt.%). Changes in vol-ume of soil during weathering are called strain by Brimhall et al.(1991), and estimated from an immobile element in soil (such as Ti

Fig. 3. Grain size and mineral (from point-counting) and chemical composition (from Walkley-Black titration and Atomic Absorption) of paleosols in the Dakota Formation ofKansas. Molar weathering ratios are designed to reveal degree of common soil forming reactions, such as base loss (Retallack, 1997).



used here) compared with parent material (εi,w as a fraction). The rel-evant Eqs. (1) and (2) (below) are the basis for calculating divergencefrom parent material composition (origin in various panels of Fig. 4).

τj;w ¼ ρw ⋅Cj;w

ρp ⋅Cj;p

" #εi;w þ 1h i

−1 ð3Þ

εi;w ¼ ρp ⋅Cj;p

ρw ⋅Cj;w

" #−1 ð4Þ

Clear weathering trends were found in Sayela paleosols (Fig. 4),but other paleosols show variations reflecting sedimentary parentheterogeneity.

These data also enable interpretation of the pedotypes in terms ofmodern soil classifications, such as those of the Soil Survey Staff(2000) of the United States Natural Resources Conservation Service,the Food and Agriculture Organization (1974) of the United NationsEducational, Scientific and Cultural Organization based in Paris, andthe Australian soil classification (Isbell, 1996). Within the US system,for example, prominent relict bedding (Casmu, Sagi andMakizi) mark

some of the pedotypes as weakly developed (Entisols), and coalmarks the Cahli pedotype as peaty (Histosols). Kaolinitic clays, lowbase status and thick clay-enriched subsurface horizons markothers as Ultisols, with some showing hematite mottles as evidenceof good drainage (Sayela pedotype) and others carbonaceous withsphaerosiderite as evidence of poor drainage (Sapa pedotype).

4. Features of the paleosols

Large woody root traces are a conspicuous feature of many paleo-sols in the Dakota Formation, and include carbonaceous remains ofroots heavily encrusted with pyrite (Fig. 5D). Other root traces arefilled with clay and sphaerosiderite, but retain the downward taper-ing, bracing and irregular distribution of plant roots. Most paleosoltops can be recognized in the field from the level at which root tracesare truncated by overlying beds.

The Dakota Formation has well preserved sedimentary structures,such as paleochannels and flaser and linsel bedding (Siemers, 1976),but also thick horizons in which primary bedding has been destroyed.These unbedded rock units have two particular characteristics of soilhorizons: sharply truncated tops and diffuse alteration downward

Table 2Interpretation of pedotypes of the Dakota Formation in Kansas.

Pedotype Paleoclimate Ecosystems Paleotopography Parentmaterial

Time forformation

Cahli Not diagnostic for climate Angiosperm (Sapindopsis bagleyae,Liriophyllum kansense) swamp woodland

Swampy lakes and oxbows Smectite clay 3–13 ka

Casmu Not diagnostic for climate Angiosperm (Araliopsoides cretacea)colonizing woodland

Supratidal estuarine flats Smectite clay andquartz silt

0.01 ka

Kazela Not diagnostic for climate Angiosperm (Aquatifolia fluitans) aquaticvegetation

Streamside clayey swales Smectite clay 0.01 ka

Makizi Not diagnostic for climate Angiosperm (Pabiania variloba) mangalwith mussels (Brachidontes filiscuptus)

Intertidal estuarine flats Smectite clay andquartz silt

2–5 ka

Sagi Not diagnostic for climate Angiosperm (Araliopsoides cretacea)colonizing woodland

Stream levee tops Quartz sand 0.01 ka

Sapa Warm temperate (14.2±4.4 °C meanannual temperature) humid(1482±182 mm mean annual precipitation),short dry season

Conifer (Sequoia condita) swamp woodland Seasonally waterloggedfloodplain

Smectite clay andquartz silt

112–640 ka

Sayela Subtropical (17±4.4 °C mean annualtemperature) humid (1527±182 mmmean annual precipitation)

Angiosperm (Rogersia parlatoriii) wet forest Well drained floodplain Kaolinite clay andquartz silt

25–107 ka

Fig. 4.Mass balance geochemistry of paleosols in the Dakota Formation of Kansas, including estimates of strain from changes in an element assumed stable (Ti) and elemental masstransfer with respect to an element assumed stable (Ti, following Brimhall et al., 1991). Zero strain and mass transfer is the parent material lower in the profile: higher horizonsdeviate from that point due to pedogenesis.



(Fig. 5C,E). Carbonaceous surface horizons (A horizons) and coals (Ohorizons) are common in these paleosols, as are subsurface zones ofred mottling (Bw horizon), clay enrichment (Bt horizon) and abun-dant replacive sphaerosiderite (Bg horizon).

Paleosols of the Dakota Formation also have a variety of soil struc-tures including subangular clayey units (blocky peds) surrounded byslickensided clay skins (argillans), and vertical structures (columnarpeds) outlined by ferruginized planes (ferrans). Some of the intenselyferruginized units have been interpreted as laterite (Thorp and Reed,1949) or plinthite (Joeckel, 1991), but chemical (Fig. 4) and petro-graphic analysis (Fig. 3) of likely rocks analyzed in this study fallsshort of the degree of iron enrichment found in plinthite, murramor laterite (Retallack, 2010). In thin section also, these paleosolsshow sepic plasmic fabrics of highly oriented and birefringent claywithin fields of less oriented clay (Fig. 6C–D), which develop within

soils due to the complex and highly deviatoric stress fields imposedby desiccation, rooting and burrowing (Brewer, 1976). A distinctivemicrostructure revealed by examination under crossed nicols ofunderclays of Cahli pedotype paleosols is inundulic plasmic fabric(Fig. 6A,B, see also color figure of Retallack, 1997, color photos 55,56). This nearly isotropic fabric may result from clay flocculation inwetlands (Brewer, 1976).

5. Paleoenvironmental interpretations

Paleoenvironmental interpretations of fossil plants and soils aredetailed in the following paragraphs and illustrations (Figs. 6and 7), before considering the degree to which these interpretationsmay be compromised by surface weathering and paleoenvironmentalbiases in fossil preservation.

Fig. 5. Field appearance of Cretaceous paleosols of the Dakota Formation Kansas; A, Sayela pedotype (below siltstone bench near top) in Acme clay pit east of Kanapolis; B, Sagipaleosol (in sun immediately below hammer) in levee facies, Kansas Brick and Tile Company pit south of Hoisington; C, sequence of paleosols along Saline River north of Russell(see Fig. 2B for interpretation); D, pyritized root traces (shining and golden) in gray Makizi pedotype of Cloud Ceramic Company pit south of Concordia; E, Cahli pedotype paleosolfrom Linnenberger's Ranch west, near Bunker Hill; F, Gypsum crystals littering gray shales along Saline River, north of Russell; G, red mottles in from oxidized sphaerosiderite fromSapa pedotype near Ellsworth.



5.1. Paleoclimate

Paleosols of the Dakota Formation in Kansas, Nebraska, andMinnesota are non-calcareous (pedalfers), and such soils indicate ahumid climate in which precipitation exceeds evaporation (Retallackand Dilcher, 1981a, b; Joeckel, 1991). A more precise estimate ofpaleoprecipitation can be gained from the paleohyetometer ofSheldon et al. (2002), using chemical index of alteration without pot-ash (R=100mAl2O3/(mAl2O3+mCaO+mNa2O), in mol), which in-creases with mean annual precipitation (P in mm) in modern soils(R2=0.72; S.E.=±182 mm), as follows.

P ¼ 221e0:0197R ð5Þ

This formulation is based on the hydrolysis equation of weather-ing, which enriches alumna at the expense of lime, magnesia, potashand soda. Magnesia is ignored because it is not significant for mostsedimentary rocks, and potash is excluded because it can be enrichedduring deep burial alteration of sediments (Maynard, 1992). Unlikeprofile specific mass balance (Fig. 4), this method conflates weather-ing that contributed to the parent material as well as within thepaleosols. The estimated mean annual precipitation for a Sayelapaleosols of the Dakota Formation is 1527±182 mm, and two Sapapaleosol received 1482±182 and 1507±182 mm (Fig. 9D), muchmore humid than modern Concordia, Kansas (740 mm average from1964 to 1993: Wood, 1996).

Deep weathering and kaolinitic composition of Dakota Formationpaleosols are evidence of warm climates (Retallack and Dilcher,1981a,b; Joeckel, 1991), and of paleoclimatic cooling in the smectiticupper Dakota Formation and overlying formations (Austin, 1970). Auseful paleotemperature proxy for paleosols devised by Sheldonet al. (2002) uses alkali index (N=(K2O+Na2O)/Al2O3 as a molarratio), which is related to mean annual temperature (T in °C) in mod-ern soils by Eq. (6) (R2=0.37; S.E.=±4.4 °C).

T ¼ −18:5 N þ 17:3 ð6Þ

Estimated mean annual temperature using Eq. (6) for a Sayelapaleosol of the Dakota Formation is 17.0±4.4 °C, and for two Sapapaleosols 14.2±4.4 and 14.6±4.4 °C (Fig. 9C), thus quantifying thetemperature decline suspected on the basis of clay mineralogy byAustin (1970). All estimates of paleotemperature are warmer thanmodern Concordia, Kansas (11.8 °C average from 1964 to 1993:Wood, 1996). The basal Dakota Formation thus enjoyed a subtropicalhumid climate, and the upper Dakota Formation a warm temperatehumid climate in the Köppen system (Trewartha, 1982). There is noevidence from paleosols of frost heave or other periglacial structuresto support inferences of glaciation from paleochannel morphology inthe Dakota Formation (Koch et al., 2007).

Paleosols of the Dakota Formation contain abundant charcoal,even in wetland Makizi and Cahli pedotypes (Fig. 2), as an indicationof a dry season of wildfires. Wetland wildfires of the Everglades ofFlorida are generally in the driest months of December to February(Smith et al., 2001; Winkler et al., 2001). Dakota Formation paleosolsdo have some pedogenic slickensides and some cracks, but they arenot organized into lentil peds or mukkara structure like those of theVertisols in the modern Gulf coastal plain of Texas, where evaporationexceeds precipitation for 5 months of the year (Nordt et al., 2004).

These paleoclimatic estimates from paleosols are in broad agree-ment with those from isotopic composition of pedogenic sphaerosi-derite. Two analyzed sets of sphaerosiderite have δ18O of −4.7‰±0.2‰ (SCB4B-119.6) and −2.9‰±0.5‰ (SCB4A-139) from the mid-dle Dakota Formation in cores at Sergeant Bluff, Iowa (Ludvigson etal., 1998) and of −5.1‰±0.3‰ (8.6 m) and −4.1‰±0.1‰ (7.6 m)at the Rose Creek locality near Fairbury Nebraska, and modeled to in-dicate mean annual precipitation of 1800±1000 mm (Ufnar et al.,2008a).

Fossil plants of the Dakota Formation are also evidence of warm,seasonallywet paleoclimate. Angiospermdominance of swamp paleo-sols (Cahli pedotype) and intertidal paleosols (Makizi: Retallack andDilcher, 1981a; Upchurch and Dilcher, 1990) is now only found incentral America south of the United States, where intertidal vegeta-tion is salt marsh rather than mangal (Avicennia germinans), and

Fig. 6. Thin section photomicrographs of Cretaceous paleosols of Kansas: A–B, inundulic plasmic fabric from A horizon of Cahli pedotype on Linnenberger's Ranch east of Bunker Hill,under crossed nicols and plane light, respectively; C, climobimasepic plasmic fabric from Bw horizon of Makizi pedotype from Fairbury under crossed nicols; D, mosepic plasmicfabric from A horizon of Sapa paleosol along the Saline River, north of Russell. Specimens in collections of University of Oregon are R108 (A–B), R110 (C) and R101 (D).



swamp woodlands are dominated by cypress (Taxodium distichum)rather than angiosperms (Hartshorn, 1983; Barbour and Billings,1988). This indication of warm paleoclimate using modern analogsof swamp andmangal angiospermsmust be viewedwith caution con-sidering the early stage of angiosperm evolution and globalmigrationsrepresented by the Dakota flora (Retallack and Dilcher, 1986; Fieldet al., 2011).

The unusually diverse Fort Harker fossil leaf assemblage of themiddle Dakota Formation has been taken as physiognomic evidenceof mean annual temperature of 24 °C, because of its common large,entire margined leaves (Upchurch and Wolfe, 1987). Mean annualtemperature estimated from leaf margins and mean annualprecipitation from leaf size for successive localities (after Sloan,1964; Setterholm, 1994; Gröcke et al., 2006) in the upper DakotaFormation have been estimated as 21.1±3 °C and 990 mm for Court-land in southeastern Minnesota, 18.6±3 °C and 840 mm for Fairburyin Nebraska, 14.9±3 °C and 790 mm for Delphos, and 18.3 °C and1270 mm for Hoisington (Dilcher et al., 2005; errors from Peppeet al., 2010).

These lines of evidence demonstrate an unusual spike of warm–

wet conditions in the lower Dakota Sandstone, which is also evidentfrom the percentage of base-poor clay inferred by Macfarlane et al.(1989) from photoelectric absorption and density logs of Braun #1core (Fig. 9B). Base-poor clay (kaolinite rather than smectite) is in-ferred from a cross plot of apparent matrix density (RHOMAA ing cm−3) and apparent matrix photoelectric absorption (UMAA inbarns cm−3), because other minerals of comparable physical charac-ter such as muscovite and biotite are rare in point counts (Fig. 3). Athick interval of dominantly kaolinitic clay (Fig. 9B) is at the samelevel as Sayela paleosols in the field (Fig. 2). Thick kaolinitic paleosolsare widespread in the “Dakota Formation” (see discussion of Witzkeand Ludvigson, 1994) in Utah, where a recent paleoclimatic study ofcalcareous paleosols (Retallack, 2009) had to go west as far as theBaseline Sandstone and Willow Tank Formation of Nevada to obtaina pedocal record through that time interval (Fig. 9A). Comparablelatest Albian–Cenomanian anomalously deep weathering also hasbeen recorded in paleosols of New Mexico (Leopold, 1943; Mack,1991), and Minnesota (Goldich, 1938; Sloan, 1964; Austin, 1970).

Fig. 7. Reconstructed sedimentary setting, marine communities and pedotypes of the upper Dakota Formation in Kansas.



Triplehorn's (1971) objection that the change from deep weatheringinferred by Austin (1970) was due to comparison of paleosol withshale is unjust, because paleosols like the Sapa pedotype are commonin the upper Dakota Formation of Minnesota as well. This Albian–Cenomanian boundary (99 Ma) warm–wet paleoclimatic spike corre-sponds with the Breitstroffer oceanic anoxic event (Gradstein et al.,2004) and with a marked perturbation in isotopic balance of the car-bon cycle (Gröcke et al., 2006). The basal Turonian Bonarelli oceanicanoxic event (93 Ma), was a similar warm–wet climatic spike, associ-ated with a spike in atmospheric CO2 spike estimated from ginkgo(Retallack, 2009) and laurel leaf stomatal index (Barclay et al.,2010), but at a stratigraphic level above the Dakota Formation andGraneros Shale in Kansas (Austin, 1970; Hattin et al., 1978).

Large (6 m) nodules in paleochannel sandstones (Fig. 10) at thesame stratigraphic level as deeply weathered Sayela paleosols and in-ferred warm–wet paleoclimatic spike is additional support for theconclusion of Swineford (1947) based on facies distribution thatthese exceptionally large nodules formed shortly after deposition.Unusually warm–wet climate could explain both the mobilization oflarge amounts of Ca and its precipitation within a deep weatheringzone of a soil, as an alternative to the deep diagenesis model ofMcBride and Milliken (2006). Their chemical data is strongly sugges-tive of a role for meteoric water and soil formation: displacivecarbonate (requiring low confining pressure), calcite replacement offeldspar (open system alteration), calcite 87/86Sr values of 0.707739to 0.708700 (meteoric), calcite δ13C of −36‰ to −4‰ (methano-genic to meteoric), calcite δ18 O of −13‰ to −6‰ (meteoric), andpyrite δ34S of +8.5‰ and 12‰ (microbial sulfate reduction). Further-more a cross plot of 60 analyses of calcite δ13C and δ18O (by McBrideand Milliken, 2006) shows a significant linear trend (R2=0.61, t testpb0.001) with a high slope (m=0.4), as found in pedogenic carbon-ate in a highly evaporative warm climate, as opposed to marine anddeep diagenetic carbonate (Ufnar et al., 2008b). This unusual giantnodule horizonmay be another consequence of unusual paleoclimaticperturbation.

5.2. Past vegetation

Paleosols of the Dakota Formation are themselves evidence ofplant formations. Pyritic carbonaceous claystones with abundantpyrite nodules (Makizi pedotype) and brackish–marine bivalves ingrowth position (Fig. 11A) are distinctive paleosols of mangal

Fig. 8. Reconstructed biota of stream margins (Casmu pedotype) of the Dakota Formation in Kansas.

Fig. 9. Paleoclimatic time series from calcareous paleosols of Utah and Nevada (A, fromRetallack, 2009), compared with proportion of base poor clay inferred a cross plot ofapparent matrix density (RHOMAA in g cm−3) and apparent matrix photoelectric ab-sorption (UMAA in barns cm−3) values (B: after Macfarlane et al., 1989); and paleo-temperature and paleoprecipitation (C–D) inferred from paleosols (solid squares,herein) and fossil plants (open diamonds, Upchurch and Wolfe, 1987; Dilcher et al.,2005).

Fig. 10. Giant subspherical nodules in lower Dakota Formation sandstones of Kansas, atMushroom State Park, near Carneiro (A) and Rock City Park, near Minneapolis (B).



communities, the pyrite derived from microbial reduction of marinesulfate (Altschuler et al., 1983). Sparsely pyritic coals with largewoody roots in their underclay (Cahli pedotype) represent freshwa-ter swamp woodland communities, as are well known in coal mea-sures (Greb et al., 2006). Thick paleosols with clay enrichedsubsurface horizons (Ultisols) form over long periods of time underold-growth forest communities (Markewich et al., 1990). These welldeveloped paleosols would have been well drained in the case ofdeeply rooted and oxidized Sayela pedotype, which are not so highlyoxidized or ferruginized to qualify as laterites (Retallack, 2010) withwhich they have been compared (Thorp and Reed, 1949). Pedogenicsphaerosiderite (Ludvigson et al., 1998; Ufnar et al., 2008a) of carbo-naceous Sapa pedotype paleosols is known to form in waterloggedsoils of coastal marshes (Pye, 1981). Weakly developed paleosols inwhich rooting and burrowing have not proceeded far enough to de-stroy primary bedding features are comparable with soils supportingvegetation early in ecological succession after disturbance by flooding(McKenzie et al., 2004), and also come in a variety of redox conditionslike comparable wetland soils today (Vepraskas and Sprecher, 1997).Sandy paleosols ferruginized in outcrop and in drill cores (Sagi pedo-type) are comparable with modern freely drained soils, as indicatedalso by their deep patterns of rooting. Carbonaceous sandy and clayeyCasmu pedotypes have siderite nodules of seasonally waterloggedsoils (Pye, 1981), whereas gray shaley Kazela pedotype has lost littleorganic matter to aerobic oxidation. These three pedotypes can thusbe interpreted to represent well drained levee top riparian woodland(Sagi), swale woodland (Casmu) and pond vegetation (Kazela).

These different kinds of paleosols preserve floristically distinctfossil plant assemblages, and thus provide a guide to autochthonousplant communities. Some fossil plant localities (Hoisington, Courtlandand Delphos: Wang, 2002; Wang and Dilcher, 2006; Wang et al.,2011) are laminated lake deposits in which plant remains from a

variety of distinct communities have been mixed. Other localitiesinclude subautochthonous plant remains close to growth position,such as mangal vegetation dominated by Pabiania variloba andPandemophyllum kvacekii in Makizi pedotype paleosols at Fairbury(Upchurch and Dilcher, 1990), and swamp woodland vegetation ofLiriophyllum kansense and Sapindopsis bagleyae in underclay of theCahli pedotype at Bunker Hill east (Retallack and Dilcher, 1981a,b).These two localities also had a prominent understory component offerns (Skog and Dilcher, 1994). Sandstone floras of the Dakota Forma-tion come in two varieties: those in ferruginized siderite nodules(Casmu pedotype at Sternberg localities of Fort Harker illustrated byRogers, 1991, and Hoisington specimens F111088–11091 in CondonCollection, University of Oregon) and those in ferruginized sand-stone (Sagi pedotype: F111083–111087 from Hoisington in CondonCollection, and other historic museum specimens of Fig. 11). Boththese streamside early successional vegetation types were dominatedby Araliopsoides cretacea. A Kazela pedotype paleosol at Carneiro alsoyielded a small collection including redwood (Sequoia condita), epi-phytic lycopsid (Onychiopsis goepperti), and sycamore (Eoplatanusserrata: F111092–F111098 Condon Collection). Conifers such asredwood are found in lake deposits of the Dakota Formation as well,and dominate its pollen (Pierce, 1961; Ravn and Witzke, 1994,1995). This may be in part because the conifers were wind pollinated,whereas most angiosperm pollen types were insect pollinated (Huet al., 2008b), but it is also likely that pedotypes for which fossil florasare unknown (Sayela and Sapa pedoptypes) supported coniferousforests (Retallack and Dilcher, 1981a,b).

These inferences from paleosols and their distinct plant communi-ties are compatible with what has already been inferred from fossilplants of the Dakota Formation. Large leaves, drip tips and vine likeleaf shapes of the Fort Harker leaf assemblage have been interpretedas closed canopy multistratal rain forest (Upchurch andWolfe, 1987),

Fig. 11. Fossil preservation in paleosols, including (A) mussel (Brachidontes filiscuptus) in life position within a clayey mangal paleosol (Makizi pedotype) from Fairbury, and (B–E)fine preservation of fossil plants within ferruginized sandstones (Sagi pedotype) near Ellsworth, including inflorescences (B) of Eoplatanus serrata, seed capsule in side (D) and su-perior (C) view of Nordenskioldia borealis, and leaves of Araliopsoides cretacea and Brasenites kansense (E). Depositories and specimen numbers are Florida Museum of Natural His-tory 15713–3313 (A), Smithsonian Institution Museum of Natural History 5005A (B), University of Kansas Natural History Museum 11043 (C–D) and 2951 (E).



and these authors were prescient in noting that this sandstone flora(of Casmu pedotype at stratigraphic level of Sayela pedotype) mayhave been of short duration, anticipating discovery here of the basalCenomanian warm–wet paleoclimatic spike (Fig. 9). Upper DakotaFormation floras are slightly less diverse (87 species, rather than 93for the Fort Harker sandstone floras), and have variety of features oftemperate deciduous woodlands: moderate size leaves with limitedinsect and fungal damage, and stipules or other protection for leafbuds (Wang, 2002). These floras were a marked advance in diversityand modernization of early angiosperms associated with extensiveEarly Cretaceous coastal migrations and paleoclimatic changes(Retallack and Dilcher, 1981a, 1986; Wang et al., 2009; Field et al.,2011; Friis et al., 2011).

5.3. Past animal life

The crocodile (Dakotasuchus kingi) and nodosaur (Silvisaurus con-drayi) were both found in large ellipsoidal siderite nodules (Mehl,1941; Vaughn, 1956; Eaton, 1960), characteristic of Casmu pedotypepaleosols, so were a part of the early successional streamside commu-nity of plants noted above from nodules within that pedotype (Fig. 8).The alkaline geochemical environment required by siderite nodulesmay have been important for preservation of bone, otherwise dis-solved by acidic deep weathering that has stripped most of the Dako-ta Formation of calcite, including the shells of bivalves (Fig. 11A). Avariety of marine trace fossils have been described from the upper-most parts of the Dakota Formation (Siemers, 1976; Hattin et al.,1978), but few trace fossils were found in the intermittently water-logged paleosols of the lower parts of the formation.

5.4. Paleotopography

Mangal to freshwater swamp paleosols of the Dakota Formationnoted above are additional evidence for the fluvial to linear clasticshoreline sedimentary setting inferred for the formation from evi-dence of sedimentary facies and structures (Siemers, 1976; Hattin etal., 1978). Even the best drained pedotypes have oxidation (Fig. 5A),sepic plasmic fabrics (Fig. 6C–D), clay skins and root traces reachingdown no more than 2 m, to levels of pyrite, sphaerosiderite, coal orother indications of a stagnant water table. Subdued paleotopographyof alluvial valley landscapes is preserved at the base of both D and Junits of the Dakota Formation preserved in boreholes in Kansas(Fig. 1: Hamilton, 1994) and Iowa (Witzke and Ludvigson, 1994). InIllinois and Minnesota the low hills of Precambrian granite andquartzite and Paleozoic limestone emerge from beneath cover ofDakota Formation (Frye et al., 1964; Sloan, 1964), although softenedby thick paleosols from intense chemical weathering (Goldich,1938; Austin, 1970; Triplehorn, 1971).

5.5. Parent material

Source terranes for the Dakota Formation were low hills of Pre-cambrian quartzites, schists and granite, with a cover of Paleozoicquartz sandstone and limestone. Very few rock fragments or lime-stone clasts, and little feldspar were delivered to the quartz-richsands (Witzke and Ludvigson, 1994) that formed parent materialsto Dakota Formation sandy paleosols (Fig. 3). This itself is evidenceof profound weathering of the source terrane, which is also apparentin the dominantly kaolinitic composition of shales and claystones ofthe lower and middle Dakota Formation ((Plummer and Romary,1947; Plummer et al., 1960; Frye et al., 1964; Parham and Hogberg,1964; Parham, 1970; Joeckel, 1987, 1991).

Smectite is abundant near the top of the Dakota Formation at thetransition to the marine Graneros Shale (Fig. 12), but the clay is notentirely of marine origin. A bentonite seen at Black Wolf (Fig. 2)and others within overlying marine rocks (Hattin et al., 1978) are

evidence that smectite clays were derived from alteration of volcanicash derived from the western Cordillera (Dyman et al., 1994). Withinthick (Sapa and Sayela) profiles, smectite and illite were weathered tokaolinite, as reflected by profound base depletion (Figs. 3 and 4).

5.6. Time for formation

The best developed paleosols in the Dakota Formation are thickkaolinitic Ultisols comparable with those of the modern coastalplain of the southeastern United States (Markewich et al., 1990),where strongly developed soils of increasing age (A in ka) show thefollowing increases in depth of solum (Ds in cm) and depth of argillichorizon (Dt in cm) with R2=0.79 and 0.80, and standard errors onage of 140 and 137 ka, respectively.

A ¼ 4915:2Ds−343466 ð7Þ

A ¼ 4844:4Dt−196090 ð8Þ

For application to paleosols, observed solum and argillic depthsof Sayela (60 and 40 cm) and Sapa pedotypes (110–160 cm and90–100 cm respectively) need to be corrected for compaction usingEq. (1), above. These calculations reveal that these Cretaceous paleo-sols compare in age with mid-Pleistocene soils of the Georgia coastalplain (Markewich et al., 1990): 25±140 ka from solum and 107±137 ka from argillic horizons of a Sayela paleosol, and 332–640±140 for solum and 112–138±137 ka from argillic of several Sapapaleosols. The shorter time of development of Sayela paleosols, yetgreater intensity of chemical weathering (Fig. 4) is another indicationof the basal Cenonamian paleoclimatic spike. Coaly paleosols (Histo-sols like Cahli) are also indicators of durations because growth ofwoody plants in precursor peats is curtailed at subsidence rates of

Fig. 12. Log of Braun #1 well through the Dakota, Kiowa and Cheyenne Formations,showing variation in lithological composition (A), and proxies for paleosol develop-ment (B), calcareousness (C) and hue (D). Data is from Macfarlane et al. (1989, 1990).



more than 1 mm per year, and peat cannot survive aerobic decay atsubsidence rates of less than 0.5 mm per year (Retallack and Jahren,2008). Using known compaction of peat to coal (Eq. (2)), givestimes of formation of 2.8 to 12.8 ka for Cahli paleosols with organichorizons now 20–45 cm thick. In coastal plain and alluvial settingspaleosols with the degree of development seen in Casmu, Sagi andKazela pedotypes have historical artifacts indicating durations of for-mation from decades to centuries (McKenzie et al., 2004), but thosewith large (5–10 cm) siderite or pyrite nodules (Casmu and Makizi)may have taken millennia to form, like modern coastal soils (Pye,1981). While these weakly developed paleosols are common in theformation, the abundance of moderately to strongly developed paleo-sols (Fig. 2) is an indication of relatively low sediment accumulationrates on a tectonically stable low relief coast that was well vegetated.

6. Core-outcrop comparisons of Dakota Formation paleosols

Comparison of outcrops with cores of paleosols can be useful forunderstanding unusual modes of fossil preservation, overprints ofmodern upon ancient weathering, and additional sources of geophys-ical data on paleosols.

6.1. Leaves preserved in red sandstone?

A paradox presented by the 19th century fossil leaf floras of theDakota Formation collected by the Sternberg family is their superbpreservation in coarse ferruginized sandstone or red siltstone nodules(Rogers, 1991). Many leaves are wrinkled and partly decayed as ifdried and curled on a leaf litter (Fig. 11E). Thin leaves of aquaticplants show less relief, but superbly detailed venation (Fig. 11Eupper). Delicate three dimensional structures such as fruit capsules(Fig. 11C–D) and seed heads (Fig. 11B) appear uncompressed by buri-al, whereas formerly calcareous shells of bivalves preserved in growthposition within clayey mangal paleosols (Makizi pedotype) arecrushed flat in life position (Fig. 11A). Observations of such fossilsfrom a Casmu and Sagi paleosol in Hoisington brick pit, and fromJones #1 core, show that the red color of the nodules is a surficial ox-idation product of dark gray siderite nodules seen in core. In contrast,the red color of leaf-bearing sandstones is found also in core, and wasred and oxidized before burial in the Cretaceous. In the fossil preser-vation terminology of Schopf (1975), the nodules are a form of chem-ically reducing authigenic cementation and the red sandstone fossilsare oxidized impressions.

Authigenic cementation is best known in the Francis Creek Shale(Middle Pennsylvanian) in that region of Illinois strip mines bestknown as Mazon Creek (Shabica and Hay, 1997). The mechanism ofpreservation of plants, fish, crabs and other fossils is decay, especiallydeamination, of the fossil itself, creating in surrounding mud an alka-line and reducing geochemical halo that promoted early diageneticprecipitation of siderite. Unlike hard durable siderite nodules withleaves found at Hoisington, the Dakota Formation leaves collectedfrom Fort Harker by the Sternbergs (Rogers, 1991) are preserved inporous red silty nodules, which have been oxidized in outcrop, by ox-idative dissolution of siderite to hematite.

Ferruginized siderite nodules do not explain the high fidelity, red,leaf impressions in Sagi paleosols, which may be “microbial deathmasks” in the sense of Gehling (1999), but not of pyrite. In the DakotaFormation, pyrite forms massive nodules and root encrustations inmangal paleosols (Fig. 4D), but does not form surficial “deathmasks”. Pyrite framboids are ubiquitous within pyritized fossils, butnot on their surfaces which retain original ribbing and striae(Grimes et al., 2002). Pyrite framboids leave characteristic clusteredcavities when oxidized (Altschuler et al., 1983), not seen on close ex-amination of Dakota Formation red leaves (Spicer, 1977). A predepo-sitional death mask of goethite (ferric hydroxide) was proposed forsuch leaves by Spicer (1977), based on observations of rusty coatings

of modern leaves created by Sphaerotilus natans. These filamentousproteobacteria settle on surfaces by entanglement and by an adhesivebase (Pellegrin et al., 1999). Sphaerotilus, and the similar Leptothrixare heterotrophic sludge thickeners, and thus fueled by partial leafdecay. Although widely considered as iron oxidizing bacteria, the ne-cessity of iron for metabolism of Sphaerotilus and Leptothrix is contro-versial (van Veen et al., 1978). Ferric hydroxide encrustations onmodern leaves observed by Spicer (1977) were only 2–3 μm thick,so were not responsible for observed differences in relief of aquaticleaves (Fig. 11E upper), densely veined leaves (Fig. 11E lower), andof woody reproductive structures (Fig. 11B-D). These observationsare evidence that resistance to burial compaction was conferred byremnant refractory lignin rather than iron-hydroxide death maskswhich preserve surficial detail. Other fine-grained impressions inred sandstones for which this preservational mechanism is appropri-ate include Evolsonia from the Permian of Texas (Mamay, 1989),Sanmiguelia from the Triassic of Colorado (Tidwell et al., 1977), andDickinsonia from the Ediacaran of South Australia (Retallack, 2007).

6.2. Jarosite and gypsum in coaly paleosols?

Many Dakota Formation coals and carbonaceous shales in outcrop(especially Makizi pedotype) are coated with powdery masses of theyellow, rotten-egg-smelling mineral jarosite [KFe3+3(OH)6(SO4)2] andlittered with clear crystals of gypsum (CaSO4), but neither mineralwas seen in cores. Both are common soil minerals, jarosite in “catclay” supratidal soils and mine tailings (Doner and Lynn, 1977) andgypsum in desert soils, where it forms replacive nodules and sandcrystals including much pre-existing soil matrix (Retallack andHuang, 2010). These crystal forms are quite distinct from the clearselenite crystals found in Kansas (Fig. 5F), where paleosols of theDakota Formation are evidence of much more humid climate thanfor modern gypsic soils (Table 2). Jarosite is found in close associationwith pyrite and carbonaceous root traces and claystone (Fig. 5D), butboth jarosite and gypsum crystals were found littering marine shalesof the Graneros Formation as well. The acidophilic and thermophilicarchaebacterium Acidianus brierleyi and proteobacterium Thiobacillusferrooxidans are known to oxidize pyrite to sulphuric acid or jarosite(Larsson et al., 1990; Olsson et al., 1995). Neutralization of this acidby calcite from limestone or mollusc shells (Fig. 11A) produces gyp-sum in modern coastal soils (Doner and Lynn, 1977). Both jarositeand gypsum are assumed to be modifications of these paleosols inoutcrop until they can be demonstrated in core.

6.3. Hematite in carbonaceous paleosols?

A surprising discovery from examination of Dakota Formationcores was that the Sapa paleosols with scattered red buckshot color-ation in outcrop (Fig. 5C,G) were entirely gray in core. Sayela, likeSagi paleosols, were red in both core and outcrop, as noted for compa-rable stratigraphic levels in cores from Iowa (Witzke and Ludvigson,1994). In thin section the red coloration of Sapa paleosols can beseen to be thin weathering rinds on individual sand-size spheroidalaggregates of sphaerosiderite, formed by oxidative dissolution of sid-erite to hematite in outcrop. The formation of sphaerosiderite duringmicrobial respiration of these paleosols was thus remote from oxidiz-ing surface conditions, as in modern coastal swamp soils (Pye, 1981).This result is compatible with methanogenic carbon isotopic compo-sitions found in some of the Dakota Formation sphaerosiderites(Ludvigson et al., 1998).

6.4. Geophysical signals of paleosols in core?

Projects to characterize the Dakota Formation aquifer in Kansas havegenerated a large amount of geophysical borehole data (Macfarlaneet al., 1990), some of which is helpful in interpreting these mid-



Cretaceous paleosols. For example, degree of pedogenic development,calcareousness and hue are key variables in paleosol interpretation(Fig. 2), and can potentially be inferred from geophysical logs (Fig. 12).

A general lithological log of the formation can be obtained frommatrix density (RHOMAA in g cm−3) and matrix z-number inferredfrom photoelectric absorption (UMAA in barns cm−3), which incross-plot arrays base-poor clays (kaolinite), base-rich clays (illite,smectite), and light minerals (quartz, feldspar) in different fields(Macfarlane et al., 1989). These have been converted to a proportion-al log of the three components using a matrix algebra computer algo-rithm by Doveton (1986). These data (Fig. 12A) show proportions ofwell developed clayey paleosols and of sandy sediment with weaklydeveloped paleosols through the Braun #1 core. Spikes in base-poorclay are evidence of deeply-weathered kaolinitic paleosols, and ele-vated abundance of quartz and feldspar reveals local paleochanneland levee facies.

A suitable proxy for paleosol development in the Dakota Forma-tion is photoelectric index (Pe): a proxy for atomic number (z) of el-ements in the matrix, which in shales is effectively a proxy for ironabundance (Ellis, 1987). This proxy thus quantifies density of sphaer-osiderite in Sapa paleosols and occasional thin hematitic Sagi paleo-sols in the upper Dakota Formation, and density of hematiticferruginization in Sayela paleosols of the middle Dakota Formation.There is less evidence of well developed paleosols in the sandylower Dakota Formation (Fig. 12B).

A suitable proxy for paleosol pH or calcareousness, for which thefield test is acid reaction (Fig. 2), is apparent matrix photoelectricabsorption (UMAA), calculated from photoelectric index (Pe) andneutron density. The values of known local calcareous (GreenhornLimestone) and sandy rock units (Longford Member of lowerKiowa Formation) divided limestones from clastics at a value of9.6 barns cm−3 (Macfarlane et al., 1990). These data reveal that theBraun #1 core (Fig. 11B) like the outcrop of the Dakota Formation iscalcite poor, with exception of nodules in paleochannel sandstones(McBride and Milliken, 2006) and rare molluscan fossils (White,1894; Siemers, 1976).

A suitable proxy for redox status of paleosols, for which the fieldindex is Munsell hue (Fig. 2), is Th/U ratio, derived from measure-ments of natural gamma ray radiation filtered to obtain ppm valuesfor these different radioactive elements (Macfarlane et al., 1989).Values of this ratio of less than 2 reflect leaching of U under chemical-ly reducing conditions, but values of 7 or more result from fixation ofU under chemically oxidizing conditions (Adams and Weaver, 1958).These values for the Braun #1 core (Fig. 11D) show the expectedhighly oxidized level of Sayela paleosols atop sandstones of the mid-dle Dakota Formation. Three other zones of oxidation in the upperDakota Formation are non-calcareous, sandy, and iron poor, so reflectredeposited soil oxides in paleochannels.

Critical contributions of these geophysical logs include demon-stration that most of the clayey upper Dakota Formation is chemicallyreduced (Fig. 12D), as physical inspection of the cores confirms. Theoxidation in outcrop of the upper Dakota Formation is thus a mislead-ing indication of paleodrainage of these paleosols. In addition, thelogs confirm that base-poor clay is most abundant at one stratigraphiclevel of Sayela paleosols in the middle Dakota Formation (Figs. 9Band 12A).

7. Conclusions

The Dakota Formation of Kansas includes common paleosols withwell preserved root traces, soil horizons and soil structures (Retallack,1997). The variety of paleosol types can be classified into pedotypeswhich reveal both spatial and temporal variations in paleoenviron-ments. Evidence from paleosols confirms that the Albian–Cenomanianboundary spike of paleoclimate was warmer and wetter than later inthe Dakota Formation, as also inferred from regional paleosol studies

(Retallack, 2009) and fossil plants of the Dakota Formation (Upchurchand Wolfe, 1987; Dilcher et al., 2005). A variety of mangal, coastalswamp, and riparian communities dominated by angiosperms and up-land communities of conifers are confirmed by different kinds of paleo-sols in the upper Dakota Formation (Retallack and Dilcher, 1981a,b).Strong development ofmanypaleosols in theDakota Formationmay re-flect formation on a tectonically stable and well vegetated coastal plain,comparable with modern lowland Georgia, U.S.A. (Markewich et al.,1990).

Examination of paleosols of the Dakota Formation in drill core re-veals that some features of the paleosols that might otherwise be con-sidered significant to paleoenviromental interpretation have beencompromised by alteration in outcrop. Oxidative weathering ofpaleosols in outcrop has created red color and ferruginous weather-ing rinds of siderite nodules and spherulites (sphaerosiderite) notseen in the cores. Oxidation of pyrite to jarosite and formation of gyp-sum by reaction of sulfuric acid and calcite also are restricted to out-crops, and not found in the cores. Nevertheless, siderite and pyrite areoriginal features of the paleosols, as in comparable modern freshwa-ter swamp and mangal (Pye, 1981; Ludvigson et al., 1998).

Fossil leaves in red nodules and sandstones from the Dakota For-mation widely distributed in fossil collections around the worldfrom the 19th century activities of the Sternbergs (Rogers, 1991)are of two kinds. Some were oxidized in modern outcrop from sider-ite nodules seen in cores and deep clay pits: a style of preservationcomparable with that better known from Pennsylvanian fossils ofMazon Creek, Illinois (Shabica and Hay, 1997). Other fine impressionsof leaves and fruits in red sandstones were preserved by microbialdeath masks of ferric hydroxide, not pyrite (Spicer, 1977).

Drill cores are not only valuable because they provide convenientvertical sections of paleosols, but proxies for many pedogenically sig-nificant variables can be obtained from geophysical logs of boreholes.Iron content as an index of paleosol development for example can bequantified by photoelectric index. Carbonate content as an index ofpaleosol pH can be quantified by apparent matrix photoelectric ab-sorption (UMAA), calculated from photoelectric index (Pe) and neu-tron density. Finally, redox status of paleosols can be quantified byTh/U ratio, derived from measurements of natural gamma ray radia-tion (Macfarlane et al., 1989). Such remote sensing of paleosols canconfirm field observations, but does not supplant the need for groundtruth provided by drill core and outcrops.

Core and geophysical logs thus demonstrate that not all paleosolsof the Dakota Formation were oxidized or deeply weathered. A short-lived deep-weathering spike is represented by Sayela paleosols in themiddle Dakota Formation of Kansas, and was continent-wide judgingfrom comparable kaolinitic paleosols in Utah (Witzke and Ludvigson,1994), Nevada (Retallack, 2009), New Mexico (Leopold, 1943; Mack,1991), and Minnesota (Goldich, 1938; Sloan, 1964; Austin, 1970).This Albian–Cenomanian boundary (99 Ma) warm–wet paleoclimaticspike corresponds with the Breitstroffer oceanic anoxic event(Gradstein et al., 2004), with a marked perturbation in isotopic bal-ance of the carbon cycle (Gröcke et al., 2006), and a significant adap-tive radiation and modernization of angiosperms (Wang et al., 2009;Field et al., 2011).

Acknowledgments

This project was initiated and partly funded by David Dilcherwith funds provided by NSF (BMS75-02268, DEB77-04846, DEB79-10720, BSR83-00476, BSR 84-01148, BSR86-16657) and Kansas Geo-logical Survey grant 41-246-02. Roger Pabian, Bill Crepet, HowardReynolds, Stephen Manchester, Mike Zavada, Carolyn Bagley, TomHalter, Terry Lott, Judith Skog, Peter Dilcher, Charles Beeker, KarlLongstreth, Bill Macklin, Bob Schwartzwalder, Chuck Huang, WarrenKovach, Hongshan Wang, Xin Wang, Shusheng Hu, Ben Sanders andFrank Potter aided with fieldwork. John Doveton helped interpret



geophysical logs. Alan Wade, P. Allen Macfarlane, and Janet Colemanallowed access to cores of the Kansas Geological Survey.

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