Polygenesis of a Pleistocene paleosol in southern Iowa

K. WOIDA Department of Geology, University of Iowa, Iowa City, Iowa 52242 M. L. THOMPSON Department of Agronomy, Iowa State University, Ames, Iowa 50011

ABSTRACT tallack, 1981; Wright, 1986; Reinhardt and than most modern soils (Ruhe, 1969; Hall- Sigleo, 1988). The stratigraphie and paleoen- berg and others, 1980). The paleosol appears The Yarmouth-Sangamon Paleosol (YSP) is vironmental significance of Pleistocene pa- remarkably uniform with depth, lacking any a major soil-stratigraphic unit occurring leosols within the glacial record of the North clear horizon differentiation. It has also been across a large portion of the U.S. Midwest. American midcontinent has long been recog- depleted by weathering of many of the chem- Study of a YSP exposed in a toposequence near nized (Valentine and Dalrymple, 1976). Pleis- ical constituents, such as carbon and phos- Earlham, Iowa, demonstrates the paleosol's tocene buried soils are a key record of cli- phorus, commonly used in soil-development usefulness for deciphering paleoenvironments matic, biotic, and geomorphic conditions studies. The paleosol's uniformly fine- in the U.S. midcontinent before the last glacial during interglacial stages, long intervals of grained texture and lack of sedimentary maximum, environments for which very little time for which little depositional record ex- structures have hindered consistent identifi- other evidence exists. Field description and mi- ists. Buried soils that lay just beyond the limit cation of its parent material as well. As a re- cromorphologjcal, textural, and mineralogical of glaciation during some or all of the ice ad- sult of these constraints, very little has been analyses were used to identify the mloess; and Pearce, 1920; Leighton and MacClintock, main processes at work during development (iii) an upper unit of loess and reworked loess, 1930; Simonson, 1941, 1954; Thorp and oth- of the YSP through field description, soil mi- possibly Illinoian in age. A stone zone at the top ers, 1951; Frye and others, 1960; Ruhe, 1956, cromorphology, and textural and mineralog- of the till indicates a period of erosion before 1967; Ruhe and Cady, 1967; Hallberg, 1980; ical analyses of five soil profiles along a burial by the fine-textured sediments. Filled Hallberg and others, 1980). Because of toposequence (catena) in Madison County, animal burrows and granular and spongy mi- the polygenetic nature of these soils and Iowa (Figs. 1 and 2). The paleosol had never crostructure at the top of the middle unit sug- postburial alteration, however, little of been studied as a suite of related soils in a gest a former surface horizon and imply that their paleoenvironmental history has been catena setting, and documenting its lateral the YSP is a composite, welded profile consist- deciphered. variability across a buried landscape was crit- ing of at least two sola. Micromorphological The thickest Pleistocene buried soil in the ical to properly evaluating its parent materials and mineralogical features suggest soil forma- midcontinent is the Yarmouth-Sangamon Pa- and pedogenetic history. The results indicate tion under paleoenvironmental conditions that leosol (YSP), which occurs throughout much that the paleosol is a complex polygenetic ranged from periglacial to intensely seasonal of the Midwest, including southern Iowa, soil, consisting of more than one solum, climates. Profile differentiation during periods eastern Nebraska, northeastern Kansas, which has been influenced by several climatic of landscape stability was offset by the homog- northern Missouri, and portions of other sur- and biotic regimes during its lengthy tenure at enizing effects of shrink/swell processes and rounding states. The YSP developed over the the landsurface. The study illustrates the use- postburial diagenesis, resulting in an isotropic longest time span of any Quaternary soil fulness of buried soils for deciphering Mid- profile that belies a genetically complex his- known in the U.S. midcontinent and poten- western environments before the last glacial tory. As a result of diagenesis, toposequence tially records significant information about maximum, environments for which very little members at the site exhibit little chemical and the paleoenvironments of at least one glacial other evidence exists. mineralogical variability. and two interglacial stages (Simonson, 1941; Ruhe, 1969; Hallberg, 1986). Past attempts to REGIONAL GEOLOGY AND SITE INTRODUCTION understand the genesis of the YSP through STRATIGRAPHY traditional field description and chemical The importance of paleosols as tools for analysis have been ineffective, however, In Iowa, the YSP is preserved on the high- paleoenvironmental reconstruction through- largely because the paleosol is so unlike mod- est geomorphic surfaces of the loess-mantled out the geologic record has become increas- ern soils in the midcontinent. Reported thick- Southern Iowa Drift Plain landform region ingly apparent in the past few decades (Re- nesses of 6.5 m are three to six times thicker (Prior, 1991). The study site, located at Earl-

Data Repository item 9338 contains additional material related to this article.

Geological Society of America Bulletin, v. 105, p. 1445-1461, 9 figs., 5 tables, November 1993.

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Wisconsinan loess), but studies of the past two decades have demonstrated the inade- quacy of the classic four-stage division of the Pleistocene in the U.S. Midwest (for exam- ple, Boellstorff, 1978a; Hallberg, 1980), and ™ Earlham only an undifferentiated Pre-Illinoian stage is currently recognized in Iowa (Hallberg, 1986). The general stratigraphic sequence at Earl- Madison ham Quarry (Fig. 2) consists of Upper Penn- County sylvanian (Missourian) limestone overlain in turn by Pre-Illinoian till, undifferentiated Yarmouth-Sangamon materials, a basal Wis- consinan loess, and the Wisconsinan Peoria Loess. The basal Wisconsinan loess is equiv- alent to the Roxana Silt in Illinois and is called the Pisgah Formation in Iowa (Bettis, 1990; Forman and others, 1992). A shallow drain- ham Quarry near the town of Earlham, Iowa 50 km north of the Aftonian type area of Kay ageway, or swale, on the paleo-landsurface is (EK2NW4 sec. 8, T. 77 N„ R. 28 W.; Fig. 1), and Apfel (1929) and 30 km northeast of exposed along the west wall of the quany. is situated on a broad, tabular upland divide Ruhe's (1956, 1967) Greenfield quadrangle Insofar as local relief on this paleo-landsur- that separates drainage basins of the Racoon stucfy area. The stratigraphy of this area was face is only about 2.5 m and well-drained or River and the North River, tributaries to the formerly interpreted as a classic Nebraskan, moderately well drained pedons are absent, Des Moines River. Earlham Quarry is about Aftonian, Kansan sequence (mantled by the toposequence is incomplete.

described profile rO involutions 200 m stone line I L 5 m top of unleached zone Figure 2. Cross section of the Earlham Quarry site, showing the general stratigraphy of materials exposed along west and north walls. Major pedologic features and locations of Profiles 1-5 are also indicated.

1446 Geological Society of America Bulletin, November 1993

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METHODS content consists of clasts >2 mm in diameter solum, Ruhe (1967) concluded that such a (Soil Survey Staff, 1951), extracted from 2-kg unit could not be recognized because of the Sections were described and sampled at samples. Lithologies of 2- to 4-mm clasts intensive weathering the paleosol had under- five locations (Profiles 1-5, Fig. 2) along the were identified using procedures of Boell- gone. Nevertheless, Guccione (1983) later two quarry walls. Although data presented in storff (1978b). Soil pH was measured using a proposed a loessial origin for a Yarmouth- this paper are taken primarily from Profiles 3 1:1 soil-to-water ratio, and organic carbon Sangamon paleosol in north-central Missouri and 5, complete field and micromorphologi- was analyzed by the Walkley-Black proce- on the basis of textural and mineralogical cal descriptions and analytical results for the dure (Nelson and Sommers, 1982). Total data. To understand the genesis of the paleo- other profiles are available through the GSA phosphorus content was determined by sol at the Earlham study site, it was neces- Data Repository.1 bromine oxidation (Dick and Tabatabai, sary to determine which, if any, of these in- Field descriptions followed standard pro- 1977), and pedogenetic oxides were meas- terpretations was applicable. cedures (Soil Survey Staff, 1951; Hallberg ured by citrate-bicarbonate-dithionite ex- Although the considerable thickness of the and others, 1978), although horizon nomen- traction (Jackson, 1975) and atomic ab- YSP suggests a composite profile developed clature was assigned mainly on the basis of sorption spectrophotometry. in multiple parent materials, the paleosol ex- micromorphological data. For micromorpho- Calcite/dolomite contents were deter- posed at Earlham Quarry exhibits no visible logical analysis, undisturbed blocks were ex- mined with a Chittick apparatus (Boellstorff, discontinuities throughout its 4.5-m profile, cavated from representative horizons of each 1978b), and barite was identified by X-ray dif- with the exception of a weakly expressed profile, air dried, and impregnated with a fraction and scanning electron microscopy. stone zone at —7- to 8-m depth at the north polyester resin. Thin sections of each block Qualitative analysis of clays was performed end of the exposure. For the most part, par- were prepared by the procedures of Murphy on the total clay (<2 |xm) fraction according ticle-size distribution plots (Fig. 3) also fail to (1986), analyzed with a petrographic micro- to methods of Whittig and Allardice (1986). show any abrupt textural changes within the scope, and described with terminology from Semiquantitative estimates of clay mineral YSP. Other parameters, however, including Bullock and others (1985). "Related distribu- percentages were made according to the au- medium:coarse silt ratios, gravel content, tion pattern" (RDP) refers to the distribution thors' own method; illite content was esti- clay mineralogy, and certain micromorpho- of small units in relation to larger units within mated on the basis of total potassium, deter- logical features indicate the presence of at the soil matrix: for example, "porphyric" mined by HF digestion. least two discontinuities within the YSP par- RDP, in which the larger (>20 (xm) units oc- The University of Texas Radiocarbon ent materials. As a result, three depositional cur in a dense groundmass of finer units. "Bi- Laboratory and the University of Arizona- units (DU-3 through DU-5) have been as- refringence fabric" (b-fabric) refers to the National Science Foundation Accelerator signed to the YSP (Fig. 4) and are described pattern of interference colors, produced by Facility for Radioisotope Analysis provided below. During the Wisconsin Glacial Stage, the parallel orientation of fine particles, the 14C dates. these units were buried by two loesses, the mainly clays, within the soil groundmass. It is mid-Wisconsinan Pisgah Formation (DU-2) approximately equivalent to Brewer's (1964) PARENT MATERIALS and the Woodfordian-aged Peoria Loess "plasmic fabric.'' Striated b-fabrics represent (DU-1). Because the focus of this study was a greater degree of orientation of clay parti- Ruhe considered the Yarmouth-Sangamon soil stratigraphy rather than lithostratigraphy, cles by wetting and drying stresses than do surface to be "essentially a weathered relict depositional units were numbered from the speckled b-fabrics. Reported porosity per- of the Kansan drift plain that has not been top down to correspond to the soil horizon centages are for total area of pores greater changed by erosion since Kansan time" nomenclature (Soil Survey Staff, 1951). than —10 (jim. Estimates of porosity and (Ruhe, 1956, p. 441). The view that Pleis- abundances of textural and amorphous ped- tocene paleosols in the Midwest were prod- Depositional Unit 5 ologjcal features (pedofeatures) were made ucts of in situ weathering of till ("gumbotil") by comparing microscopic fields to charts in had been held by many workers since the DU-5, exposed to a depth of 14 m, was Bullock and others (1985). turn of the century (McGee, 1891; Kay, 1916; identified as glacial till by its clay loam tex- Particle-size distribution was determined Thorp and others, 1951), although Simonson ture, poor sorting, over-consolidation, and by the pipette method (Walter and others, (1954) noted that the upper portion of many jointing. In addition, medium (16-31 jim) to 1978), using the particle-size classification of gumbotil profiles was lacking in quartz peb- coarse (31-62 |j.m) silt ratios (Fig. 3) are sim- clay (<2 |jim), fine silt (2-20 p.m), coarse silt bles and sand grains and seemed to consist of ilar to values for tills in Illinois described by (20-62 |xm), and sand (62 urn to 2 mm). Later loess that had merged with the underlying till. Follmer (1982). Hallberg (1986) designated calculation of particle-size ratios using the Others later interpreted "gumbotil" materi- the uppermost "Kansan" till in southern Wentworth intervals of medium silt (16-31 als in Iowa, Illinois, and Kansas to be pri- Iowa as an "Al" till, equivalent to the Hick- jim) and coarse silt (31-62 |xm), to allow marily swale-fill sediments (Frye and others, ory Hills Till Member of the Wolf Creek For- comparison with other Midwestern data 1960). Ruhe (1956, 1967) believed that the mation in eastern Iowa. However, because (Follmer, 1982; Hall and others, 1991), ac- YSP occurred in weathered till on upper the till at Earlham Quarry contains consider- counts for the overlap between "fine," "me- slope positions—based on an orderly de- ably more kaolinite (Tables 1 and 2) than dium," and "coarse" silt in the study. Gravel crease in heavy-mineral weathering ratios Wolf Creek tills in eastern Iowa (Hallberg, with depth—and in till-derived colluvium in 1980; Hallberg and others, 1980), its strati- swale positions. Although he did not rule out graphic status is uncertain. the possibility of a loess parent material 'GSA Data Repository item 9338, descriptive The transition from clay loam (DU-5) to data, is available on request from Documents Sec- (namely, the Illinoian-aged Loveland Loess) overlying silty clay (DU-4) is gradual (Fig. 3). retary, GSA, P.O. Box 9140, Boulder, CO 80301. for the fine-textured upper part of the YSP There is also a decrease in the abundance and

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size of clasts moving upward in the profile. A PROFILE 3 weakly expressed stone zone—a "layer more than one stone thick" (Johnson, 1988)—is 16-31 /Lsoil structure and increased channel porosity compared with the overly- 0 50 100 1 2 3 20 40 60 80 100 ing materials (Fig. 6A). As discussed later in this paper, granular structure is typical of soil cumulative % cumulative % A horizons, and an abundance of channel Figure 3. Textural data and clay mineral composition for Profile 3 and Profile 5 (£s = fine silt; pores also suggests bioturbation fairly close cs = coarse silt; Exp = expandable clay minerals; K + C = kaolinite plus chlorite). to the landsurface. In addition, abundant krotovinas (filled animal burrows) at a depth of 6-7 m in many of the profiles suggest a sedimentologic mixing with the underlying at two railroad cuts, separated by an "intra- former landsurface near this level. For these till. We propose that the sand is colluvial in Illinoian" soil. It is possible that DU-4 reasons, two sola are designated in Figure 4, origin, added during beveling of the Pre-Dli- corresponds to their lower, thinner loess which together form a composite Yarmouth- noian landscape. On the other hand, the unit and DU-3 to the upper unit. A Pre-Illi- Sangamon profile. abrupt change in clay mineralogy from DU-4 noian age for DU-4, however, is just as Thé presence of an erosional stone zone at to DU-5 suggests the input of some externally plausible. the top of DU-5 precludes the possibility of derived material such as loess. A loess rich in an in situ till parent material for DU-4, be- medium silt, similar to the overlying Wiscon- Depositional Unit 3 cause the process of till deposition, had it oc- sinan loesses in the section, could also ac- curred, would surely have destroyed the un- count for the fining-upward trend of DU-4. DU-3 is nearly uniform in texture across derlying stone zone. For reasons that follow, Slow aeolian deposition would allow new silt the toposequence, and no stratification is ev- the most likely parent material for DU-4 is a to become incorporated into the surface ma- ident at any level of the unit in the field or in mixture of loess and till-derived colluvium. terials or even translocated to lower levels by thin sections. The similarity in medium: DU-4 is sandier than overlying materials way of cracks, channels, and burrows. Ad- coarse silt ratios between DU-3 and DU-1 (Fig. 3). Sand-rich loesses commonly occur ditional loess deposits would cause the sur- (Fig. 3) suggests a loessial origin for DU-3, in areas close to the loess source, but because face to migrate upward with progressively with perhaps a source similar to that of the all of the overlying loess materials in the sec- less mixing through time, until the surface Peoria Loess. In the depression (Profile 3), tion contain very little sand, such a proximal was composed completely of loess. DU-3 is overthickened and may also contain source is questionable. Furthermore, the Ruhe and Cady (1967, p. 29) described some reworked loess derived from the same stone zone at the base of DU-4 suggests little two increments of Illinoian Loveland Loess unit on surrounding slopes.

1448 Geological Society of America Bulletin, November 1993

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PROFILE 5 others (1988). Radiocarbon dates from the site can be used as a rough guide to the net rate of accumulation of DU-2 sediments in depo, 16-31 pm the area (Table 4). The initially slow average unit horiz particle size 31 -62 ¡j m clay minerals rates of 1 cm per 100 yr would have allowed pedogenetic mixing to incorporate new silt A | additions into the top of the YSP, contribut- Bt ing to the blurred boundary between DU-3 1 - and DU-2. BC | 1 EVIDENCE FOR MULTIPLE PEDOGENETIC AND DIAGENETIC PROCESSES C Differentiation of three YSP parent mate- rials, with a pedological discontinuity at the top of DU-4, suggests a complex pedogenetic 2Ab 2 history for the paleosol. Certain macroscopic E 3Ab features of the paleosol (for example, strong 3BEb Q. soil structure and abundant clay coatings) are a) ' 3 also suggestive of advanced pedologic devel- 3Btb opment. However, the paleosol's uniform

- 3Bqb texture and almost complete absence of ho- rizon differentiation are inconsistent with the 4Btgb type of morphologies usually associated with 4 4BCtgb advanced weathering and soil development, particularly in composite, or welded, profiles (Ruhe and Olson, 1980). These characteris- tics, along with the absence of organic mate- rial and the uniformity of chemical indicators, 5C 5 have frustrated past attempts at pedogenetic reconstruction for the YSP. The absence of chemical discontinuities in the YSP at the study site is illustrated in Ta- 0 50 20 40 60 80 100 bles 1 and 2. In general, the data show only cumulative % cumulative % that the paleosol is highly weathered com- Figure 3. (Continued). pared with most modern soils of the midcon- tinent. The only data of significance are (i) uniformly neutral pH values, which suggest A case can be made for an Illinoian age for pedologic mixing processes such as freezing resaturation of the paleosol by alkaline earth DU-3, making it equivalent to Loveland and thawing. A colluvial origin for DU-3 is cations (primarily Ca+2) derived from the Loess deposits farther west in Iowa. If also unlikely in that DU-3 is laterally contin- Wisconsinan loess units that buried the YSP, Ruhe's (1969) equation is used to calculate uous across the toposequence both texturally and (ii) the general decrease in free Fe oxides the expected thickness of Loveland Loess and mineralogically. The study site is situated in the very poorly drained position on the based on distance from the source (assumed at the highest level of the modern local land- paleo-landsurface. to be the Missouri River), the study area, lo- scape, and there is no evidence for the exis- The limits of chemical investigation were cated 160 km from the Missouri Valley, tence of a former topographic high that could emphasized by Ruhe (1956) and Ruhe and should have received about 2 m of loess, have supplied 2 m of colluvium to the area. Cady (1967), who performed mineralogical which is close to the actual thickness of The transition from DU-3 (the top of the analysis of the sand and clay fractions to DU-3. More proximal sources for the loess, YSP) to DU-2 in the lower slope positions is characterize the differences among facies of however, cannot be ruled out. texturally abrupt (Fig. 3, Profile 3) and is the paleosol at various isolated landscape po- Coarse sand grains and a few clasts 2-3 marked by numerous krotovinas. In the up- sitions (that is, not part of a catena). This mm in diameter occur at the bottom of the per slope positions, the upper boundary of study also considered the YSP's clay mineral unit (Table 3) and gradually decrease in abun- the YSP is indiscernible. Color, structure, composition, but study of its sand mineralogy dance upward. Such particles were viewed in and texture all change gradually over a ver- was impractical due to the scarcity of heavy the past as partial evidence for a parent ma- tical distance of >1 m, possibly as the result mineral grains in the major part of the paleo- terial of weathered till (for example, Ruhe, of sedimentological or pedological mixing sol—typically <0.5% by weight, of which 1969), but it seems much more plausible that processes during the initial deposition of more than half are unidentifiable opaque a few coarse particles from DU-4 gradually DU-2, similar to processes ascribed to basal grains. This study focused primarily on the worked their way upward in the profile by mixing zones in loess by Schumacher and morphological properties of the paleosol,

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Time Pedo- (Fig. 6B). Diameters of the granules range Lithostratigraphy Major Morphological Stratig. stratigraphy Features from 200-2,500 (Am, but most are 500-1,000 (xm. Granular structure commonly results from bioturbation by soil fauna and is char- Peoria acteristic of the epipedons of many modern DU-1 Loess soils, such as Mollisols and Alfisols (Babel,

Stag e massive 1975; Pawluk and Bal, 1985; Bullock and

/isconsina n Thompson, 1985). Modern soil fauna that s- 4 Pisgah Fm. DU-2 Farmdale Soil form fecal pellets in the 200 to 2,500-^m size range include millipedes, pill bugs, earth- d) c granular micVoyructure'.-.. . _ worms, and fly and beetle larvae (Rusek, abundant kro'tovinas 1985). Most of these pellets contain large c Solum loess 0 E DU-3 E pieces of organic litter fragments. Such pel- (undiff.)

Paleós e abundant channels & krotovinas colluvium DU-4 Solum compounds and clay (Shipitalo and Protz, c slickensides 1989). Pellets of some earthworm groups ac- (undiff.) 2 blocky microstructures .55 in armouth-Sa r O (D >- -8 tive just below the litter layer, however, are c o> •o«0 o«o«OoO composed mainly of mineral matter with = to O a glacial 0 „0 O some very small pieces of organic matter and d> 3) till DU-5 ° O are usually quite stable (Rusek, 1985). Such Q- O ° (undiff.) massive earthworm pellets can be very abundant in 0 0 - 0 ° modern uncultivated soils, making up the en- 0 0 tire humus horizon and upper parts of the mineral horizon (Kubiena, 1953; Edwards, involutions unconformity 1985; Pawluk and Bal, 1985; Thompson and Figure 4. Late Pleistocene stratigraphy and prominent macro- and micromorphological fea- others, 1990). tures of buried soils at the Earlham Quarry site. Note division of the Yarmouth-Sangamon Granular structure in surface horizons can Paleosol into an upper and a lower solum (see text). also form in solifluction lobes on steep slopes (Van Vliet-Lanoe, 1982,1985). Even though prominent involutions higher in the section both macroscopic and microscopic. Morpho- boundaries and welded the two sola together are evidence of periglacial conditions at the logical features have greater preservability into one composite profile or (ii) diagenetic Earlham site, the low-relief landscape (slopes and are generally more reliable genetic indi- processes that further masked evidence of <5%) was probably too gentle for granules to cators than geochemical properties in pa- the original sola. Accordingly, the Yarmouth- form by solifluction. Solifluction granules are leopedological studies (Finkl and Gilkes, Sangamon pedologic unit(s) is (are) referred also less rounded and larger (5 mm) than 1976; Retallack, 1981). In an earlier study in to in this paper by these terms: "Solum 1" or those in Solum 1. Smaller granules (300 p,m north-central Iowa, Thompson (1986) used "Solum 2" when discussing properties and in diameter) produced experimentally by morphology and clay mineralogy to infer a processes believed to be associated with the freeze-thaw by Pawluk (1988) were restricted polygenetic origin for a Pre-Wisconsinan pa- pedogenesis of the upper or lower solum, re- to clay loam textures. Although the interac- leosol buried by Wisconsinan till. spectively, or the "Yarmouth-Sangamon Pa- tions between structure and texture (Sanborn Macromorphological and micromorpho- leosol" (YSP) or "paleosol" when discussing and Pawluk, 1989) complicate interpreta- logical features of profiles from the highest the composite, welded profile or processes tions, the texture; excellent sorting of the par- (Profile 5) and lowest (Profile 3) positions of believed to have contributed to the welding of ent materials; and size, roundness, and abun- the toposequence are summarized by hori- the profile. dance of granules in Solum 1 strongly suggest zon in Tables 3 and 5, respectively. Morpho- formation by earthworms. At the very least, logical and mineralogical properties of the pa- Morphological Features and Their the granules imply formation at or veiy near leosol are discussed in the following section. Implications the paleo-landsurface and are thus evidence Certain of these features, particularly ped- for at least partial preservation of the Solum ologic structures and evidence of bioturba- Pedologic Structure. In the field, the YSP 1A horizon. Formation by present-day earth- tion, indicate that the YSP is a compound soil exhibits predominantly subangular or angular worms may be ruled out because of the depth consisting of at least two sola, designated blocky soil structure (Table 3). Weak blocky of the zone—3-4 m below the modern land- Solum 1 and Solum 2 (Fig. 4). These sola rep- structure extends V2 m into the underlying till surface—and the absence of pellets in the in- resent two distinct periods of pedogenesis. at the north end of the quarry. tervening 3 m of sediment. Other properties, such as certain textural and In thin section, the upper zone of Solum 1 Microstructures in the 3B horizons are amorphous pedofeatures and clay mineral (3A horizons) exhibits granular and spongy subangular or angular blocky (Table 5; composition, appear to have resulted from (i) (coalesced) microstructure and porosity that Fig. 6C). Microstructure in Solum 2 above processes that obscured original horizon consists mainly of compound packing pores the stone zone is very similar to that in the 3B

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TABLE 1. CHEMICAL PROPERTIES AND CLAY MINERALOGY, PROFILE 3

Unit/horizon Depth pH Org. C Total P CBD—extractable' Exp.s Illite Kaolinite 1.7 nm (%) (or zone') (cm) (g/kg) (g/kg) (g/kg) (%) (%) (cps) Fe Mn Al

Peoria Loess MDL 191-218 0.6 67 12 21 1,204 MDL 218-246 1.1 MDL 246-275 1.5 0.51 6.19 0.07 0.22 71 8 21 1,289 Farmdale Soil 2Ab 275-287 7.0 2.9 0.78 0.00 0.39 69" 9 22 1,156 Y-S Solum 1 3Ab 287-300 7.0 2.6 0.18 6.06 0.09 0.51 76 6 18 581 3Ab 300-315 1.6 4.09 0.10 0.51 77 6 17 581 3Atb 315-330 7.0 1.4 0.12 3Atb 330-348 0.8 1.32 0.19 0.48 73 6 21 361 3ABtb 348-366 0.8 0.10 73 7 20 331 3ABtb 366-384 7.0 0.7 0.53 0.15 0.53 72 7 21 778 3Btbl 391-404 0.8 0.11 74 7 19 718 3Btbl 419-434 7.1 0.6 0.18 2.76 0.16 0.62 74 10 16 876 3Btgb2 452-467 0.2 75 6 19 718 3Btgb2 483-498 7.0 0.3 0.12 3Btgb2 498-513 0.2 0.45 0.11 0.42 75 9 16 801 3Btgb2 513-531 0.1 0.14 3Btgb2 531-549 0.2 74 8 18 615 3Btgb3 549-561 0.7 0.12 3Btgb3 561-574 0.7 73 10 17 773 3BCgb 574-587 7.0 0.1 0.20 3BCgb 587-599 0.2 1.95 0.06 0.46 72 8 20 980 3BCgb 599-615 0.2 0.15 71 7 22 650 Y-S Solum 2 4BCgb 615-630 0.1 4BCgb 630-645 0.2 0.18 1.45 0.02 0.46 71 8 21 566 4BCgb 645-660 0.1 68 6 26 515 4BCgb 660-676 0.1 0.16 8.04 0.13 0.94 4BCgb 676-691 0.1 67 7 26 655 Pre-Illinoian till 5Cg 691-716 0.1 0.24 3.21 0.17 0.41 52 8 40 615 MOL 747-777 0.42 4.99 0.14 0.69 46 11 43 1,258 OJL 826-853 0.1 6.36 0.09 0.49 37 12 51 942 OJU 853-884 3.95 0.31 0.49 40 12 48 795 OJU 914-945 0.1 4.05 0.28 0.13 40 12 48 961

"Weathering zones: D = deoxidized; J = jointed; L ~ leached of carbonates; M = mottled; O = oxidized; U = unleached. ^Citrate-bicarbonate-dithionite extracts. 5Smectite plus interlayercd smectite. * 'Trace vermiculite.

horizons and probably resulted from second- Krotovinas indicate biological activity rel- color to the soil matrix beneath them. Their ary pedogenetic effects associated with atively close to the landsurface. A krotovina resemblance to slickenside surfaces that oc- Solum 1. At the top of Solum 2, the blocky from Solum 1 examined in thin section con- cur in the same zone suggests formation by microstructure overprints coalesced granules tained a few fragments of microlaminated stress orientation of clay rather than by illu- and associated spongy microstructure. The clay coatings, but lacked clay coatings on viation of clay translocated from higher levels granules, and increased channel porosity void walls like those in the surrounding ma- in the profile. Thick clay coatings clearly of over that of overlying horizons, were inter- trix. This and the absence of soil structure illuvial origin were noted only in the depres- preted to be relict features of the former A suggest that burrows in Solum 1 were created sion profile (in Solum 1) and in krotovinas in horizon of Solum 2. after formation of soil structure and clay coat- the till. The till is structureless except in Profile 1, ings in the surrounding matrix, that is, rela- Micromorphology shows that many of the where it exhibits subangular blocky micro- tively late in the paleosol's history. "coatings" on ped surfaces do in fact consist structure to a depth of 9 m. This may be re- Krotovinas in the till exhibit a texture and of stress-oriented clay that makes up the po- lated to processes originating in the overlying related distribution pattern similar to that of rostriated b-fabric common in much of the sola, but it is also possible that it is a relict overlying 4B horizons, but strong, very fine paleosol (described in the next section). Mi- feature from soil formation that occurred be- subangular blocky microstructure like that of crolaminated clay coatings of illuvial origin fore burial by DU-4. the surrounding till, so it appears that the are very thin and occur mainly on the walls of Krotovinas. Oval or cylindrical pods of ma- krotovinas were created by backfilling of channels. One exception is at the top of trix, 1-5 cm in diameter, with a different tex- burrows during deposition of DU-4, before Solum 2, where numerous clay coatings oc- ture, consistency, and color than the sur- significant pedogenetic alteration. cur on the walls of packing voids around rounding soil matrix, are prominently Clay Coatings and Other Textural Pedofea- granular peds (Fig. 6A). These coatings most exposed in the YSP. These were interpreted tures. One of the most characteristic features likely formed from clay translocation associ- to be krotovinas, possibly from moles, of the YSP noted in the literature is the prev- ated with Solum 1, a process that contributed ground squirrels, gophers, or crayfish. The alence of clay coatings on ped faces. At field to the welding of the two sola. Clay coatings krotovinas occur at or near the tops of Solum scale, continuous, gray clay coatings are on ped surfaces are also abundant in 3Bt ho- 1 and Solum 2, and at the top of the till in abundant in the lower half of the paleosol at rizons of the depression profile (Fig. 6D). Profile 1. Krotovinas in the till are as much as Earlham Quarry as well. Many of these are This distribution is the reverse of that in mod- 40 cm long and are subvertically oriented. quite thin, however, and nearly identical in ern soils at the site, in which illuviation is

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TABLE 2. CHEMICAL PROPERTIES AND CLAY MINERALOGY, PROFILE 5

Unit/horizon Depth PH Cal/Dol Org. C Total P CBD-extractable* Exp.® IUite Kaolinite 1.7 nm (or zone*) (cm) (sfcg) (g/kg) (g/kg) (g*g) (%) (%) (%) (cps) Fe Mn Al

Peoria Loess MDU 254-269 8/63 0.6 MDU 269-285 0.8 5.68 1.11 0.32 68 11 21 1,576 MDL 305-325 1.2 68 12 20 1,584 MDL 325-343 0.5 6.70 0.41 0.32 MDL 343-361 0.8 72 11 17 1,927 MDL 361-376 1.1 0.65 3.66 0.07 0.30 Farmdale Soil 2Abl 376-389 7.1 1.4 3.04 0.01 0.30 66 9 25 1,289 2Ab2 389-404 1.4 0.26 58 9 33 986 2Bwb 404-422 7.1 0/0 1.2 0.25 7.14 0.05 0.65 60 12 28 596 Y-S Solum I 3Ab 422-442 0.9 0.21 5.06 0.21 0.74 3Ab 442-455 0.8 64 9 27 692 3Ab 455-467 7.0 0.6 0.16 3BEb 467-480 0.8 7.39 0.18 0.70 69 8 650 3BEb 480-493 0/0 1.6 0.17 3Btbl 493-505 7.0 1.0 0.15 6.90 0.46 0.78 75 8 17 335 3Btbl 518-531 0.8 0.15 3Btbl 531-544 6.9 0.3 74 7 19 853 3Btb2 544-556 0.5 0.12 3Btb2 556-569 0.3 3.43 0.20 0.53 76 7 17 586 3Btb2 569-582 0.3 0.13 3Bgb 582-594 7.0 0.2 73 7 20 581 3Bgb 594-610 0/0 0.2 0.14 75 5 20 778 Y-S Solum 2 4Btgb 610-625 0.1 74 8 18 681 4Btgb 625-640 0.3 0.14 4.04 0.28 0.62 73 9 18 493 4Btgb 655-671 0.1 0.15 72 8 20 335 4BCtgb 671-686 7.0 0.2 4BCtgb 686-701 0.3 0.15 7.18 0.42 0.77 71 5 24 418 4BCtgb 701-716 0/0 0.3 4BCtgb 716-737 0.1 0.16 69 5 26 543 Pre-Iflinioan till 5C-MOL 737-762 0.1 8.11 0.13 0.76 61 6 33 930 MOL 762-792 0.2 0.11 55 7 38 1,232 MOL 792-823 50 9 41 1,318 MOL 823-853 7.27 0.21 0.32 MJOL 853-884 44** 11 45 751 MJOU 884-914 33/26 MJOU 945-975 0.0 44 11 45 924 MJOU 975-1,006 47/50 6.66 0.16 0.17 MJOU 1,097-1,128 46/64 MJOU 1,158-1,209 43 14 43 524 MJOU 1,209-1,240 0.2 6.98 0.28 0.19

•Weathering zones: D = deoxidized; J = jointed; L = leached of carbonates; M = mottled; O = oxidized J = unleached. fCitrate-bicaibonate-dithionite extracts. 5Smectite plus interiayered smectite. * "Trace veimiculite.

most pronounced in the better-drained, up- shrinking and swelling of the soil (Bullock slowly accreting at the surface, and they are per slope positions of the toposequence. The and Thompson, 1985). This can cause voids thus compatible with a loessial origin for abundance of clay coatings in the depression to permanently swell shut, enclosing the DU-3. may be the result of large amounts of water coatings. The embedded clay coatings seen in Silt coatings and fillings occurring at the moving vertically through the profile at the both sola of the YSP most likely represent top of Solum 1 (Fig. 7A) exhibit excellent point where overland flow and downslope clay illuviation during (an) early cycle(s) of sorting. This suggests that the coatings throughflow from rainfall converged. The pedogenesis. formed by deposition of silt derived from Edina soil (Typic Argialboll) in Iowa is an Thin sections also reveal dark brown DU-2 and/or DU-1, rather than by depletion example of a poorly drained modern soil clayey patches embedded in the soil matrix of or winnowing of finer particles from the ma- that displays a better-developed argillic hori- Solum 1 (Table 5). These small (200-700 [Am) trix, and are thus diagenetic features. zon than adjacent, better-drained Mollisols rough-edged patches, or elongated and Stress-Oriented Clay. Reorganization of (Thompson, 1987). weakly laminated stringers (Fig. 6F), appear the expandable-clay fraction of a soil by In addition to clay coatings on channel and to consist of birefringent clay and dark gray- shearing stresses associated with wetting and ped surfaces, abundant microlaminated clay ish brown colloidal material. The patches' drying often results in striated birefringence fragments are embedded in the soil matrix of ragged edges and occurrence within the ma- fabrics, which show a preferred orientation of the peds (Fig. 6E). These embedded clay trix suggest that they are fragments of larger clay particles (Brewer, 1964; McCormack bodies are clearly visible in plane-polarized disrupted pedofeatures. One hypothesis is and Wilding, 1974; Nettleton and others, light, suggesting an illuvial origin. Most retain that they are fragments of surface crusts, 1983; Bullock and Thompson, 1985). In this their elongated shape and were interpreted to which can be relatively stable, resisting trans- study, clay concentrations were identified as be former clay coatings. The processes re- formation into groundmass material (G. stress-oriented if (i) they had very diffuse sponsible for incorporating clay coatings into Stoops, personal commun., 1990). Crusts are boundaries with the soil matrix in cross-po- the soil matrix are not well understood, but most likely to form and become incorporated larized light and (ii) they were indiscernible in the most likely agent is physical stress due to into the soil profile where materials are plane-polarized light. Such striated b-fabrics

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depo, glossiness of pedogenetic slickensides in- creases with depth, from about 95% of all fer- unit crease with depth to a zone of optimum ex- ruginous nodules in the 3A horizon to about pression; the optimum depth and maximum 40% in the 3Bt horizon. A similar trend was depth of occurrence depend upon rainfall noted in both the modern soil and the Farm- amounts and seasonal distribution (Yaalon dale Soil at the site. The nodules are similar in and Kalmar, 1978). At Earlham Quarry, morphology to subrounded nodules attrib- slickensides occur in a zone 0.9 to 2.9 m be- uted to authigenic formation by Pawluk and low the top of the YSP and are largest and Dumanski (1973) and may have resulted from most abundant in the zone between 1.1 and pedoturbation (shrink/swell processes), bio- 2.1 m. Knight (1980) found that the ratio turbation, or cryoturbation of the upper ho- "depth of maximum number:maximum rizons of the solum. depth of occurrence" of slickensides typi- Iron oxide hypocoatings, which impreg- cally has a value of about 0.5. The corre- nate the soil matrix or clay coatings adjacent sponding value (0.55) for the slickensides in to pores, are common in Solum 1 in the bet- the YSP is thus compatible with a pedoge- ter-drained profiles. Because these hypocoat- netic origin. ings are not overlain by clay, they postdate Short, isolated clay separations, which clay illuviation and may be the result of post- make up the random striated b-fabric domi- burial translocation of Fe oxides from the de- nant in Solum 1, probably also formed by oxidized Peoria Loess. Lower horizons of stress orientation of clays within the matrix Solum 1 contain common Mn oxide coatings, and are similar to what Nettleton and others a few of which also overlie clay coatings. Al- (1983) called "microslickensides." As inter- though the origin of the Mn in these latter % gravel preted above, the embedded clay coatings coatings was not determined, it is question- present in much of the paleosol became em- able whether Solum 1 would have retained Figure 5. Gravel content (percentage by bedded as a result of wetting and drying sufficient amounts of unoxidized Mn at this weight of >2-mm clasts) for Profile 2, showing stresses as well, although they owe their orig- relatively late stage in its formation (that is, an abrupt increase in gravel at the contact be- inal formation to clay illuviation. Thus, we after multiple cycles of illuviation). Thus, the tween DU-4 and DU-5. interpret slickensides, striated b-fabrics, and coatings may be another diagenetic feature, embedded clay coatings all to represent composed of Mn ions from DU-2 and DU-1. churning of the soil matrix by shrink/swell Sharply bounded Fe oxide nodules again are dominant throughout both sola of the processes, probably during alternating wet increase at the top of Solum 2 and decrease YSP (Table 5). Porostriated b-fabric is asso- and intensely dry seasons. Such disturbance with depth. Thin Fe oxide coatings which ciated mainly with planar voids and probably of the matrix probably occurred after burial overlie, but do not impregnate, clay coatings constitutes the stress cutans mistaken for of Solum 2 but may have occurred before are common in Solum 2. Such coatings are clay coatings in the field (Fig. 6C). Circular- burial of Solum 2 as well. not present in Solum 1, which suggests that striated b-fabric in the 3A horizons (Fig. 7b) Amorphous Pedofeatures (Gleying, Mot- they formed from iron derived from Solum 1 may have formed as clay particles were tling, Nodules, and Coatings). Matrix colors rather than from iron originating in the Peoria smeared during passage of the soil materials of Solum 1 are generally grayish brown to Loess. A diagenetic origin for the iron oxides through the digestive tracts of earthworms, dark gray or olive gray. Solum 2 is gleyed would imply that migrating ions bypassed similar to "stress neo-cutans" described by (chroma <1.5, hue 5Y in Munsell notation) Solum 1 to precipitate out in the now-gleyed Bal (1973) in modern soils. throughout the toposequence (Table 3). Nod- Solum 2. Mn oxide coatings overlain by illu- Stress cutans are particularly well ex- ules and coatings of secondary oxides are vial clay represent formation during an early pressed in slickensides occurring along common in the better-drained profiles but un- part of the pedogenetic cycle, whereas those straight, smooth planar voids (Fig. 7C). In the common in Profile 3, indicating saturated, re- that overlie clay coatings (Fig. 7E) represent field, the slickensides occur in both YSP sola ducing conditions for extended periods of a later, undetermined part of the cycle. (Table 3; Fig. 8) throughout the topose- time in the depression. Upper Boundary of the Yarmouth-Sanga- quence, spaced 50-100 cm apart. Pedoge- Thin sections show that some of the Fe mon Paleosol. The boundary between the netic slickensides result from swelling pres- oxide nodules observed in the field are actu- YSP and the overlying Farmdale Soil is gra- sures induced by expanding clay (Soil Survey ally pseudomorphic nodules, composed of dational; changes in color, structure, and tex- Staff, 1975) and occur primarily in Vertisols charcoal flecks in which plant tissue has been ture occur over a vertical distance of at least and vertic intergrades (Wilding and Puentes, almost completely replaced by goethite. yi> m (Table 3). The boundary is further ob- 1988; Dasog and others, 1987; Mermut and Krotovina fillings at the top of Solum 1 are scured by the presence of involutions. Invo- Acton, 1985). similarly impregnated with Fe oxides. Sec- lutions at this and other southern Iowa sites Judging by their depth, the YSP slicken- ondary iron-oxide impregnation of charcoal are probable evidence of active cryoturbation sides appear to be pedogenetic in origin. In and krotovinas was probably a diagenetic of the landsurface under periglacial condi- Vertisols, slickensides occur in the subsoil, process, contemporaneous with weathering tions during the last glacial maximum below the maximum depth of seasonal crack- of primary iron-bearing minerals in the over- (Woida, 1992). ing and above the maximum depth of sea- lying Wisconsinan loesses. Thin sections provide a closer look at the sonal wetting (Wilding and Tessier, 1988). In In Solum 1, the percentage of sharply nature of the Yarmouth-Sangamon/Farmdale many Vertisols, the abundance, size, and bounded, rounded nodules (Fig. 7D) de- boundary. Although the A horizon of the

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Figure 6. A. Partially coalesced granules and associated spongy microstructure at the top of Solum 2, 4Atb horizon of Profile 1 (circularly polarized light). B. Granular microstructure with compound packing voids at the top of Solum 1,3Ab horizon of Profile 5 (plane-polarized light). C. Subangular blocky microstructure in the 3Btb horizon of Profile 1. Bright bands along the edges of peds make up the porostriated b-fabric common in both Y-S sola (cross-polarized light). D. Microlaminated clay coatings of depositional origin nearly filling pores in the argillic (3Btb) horizon of the depression, Profile 3 (circularly polarized light). E. Clay bodies, interpreted to be embedded clay coatings, in the 3Btb horizon of Profile 3 (circularly polarized light). F. Dark brown clayey patches or stringers, possibly remnants of surface crusts, embedded in the soil matrix of 3Btb horizon of Profile 1 (plane-polarized light).

1454 Geological Society of America Bulletin, November 1993

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TABLE 3. MACROMORPHOLOGICAL FEATURES OF BURIED SOILS AND WEATHERING ZONES, PROFILES 3 AND 5

Unit/horizon Depth Texturet Matrix Mottles/ Structure** Coatings/ Slickensides® Other features Lower 5 Kn 55 (or zone*) (cm) colors accummulations fillings boundary

PROFILE 3 Peoria Loess MDL 191-275 Sil 2.5Y 5/2 m3p 7.5YR 4/6 0 f 10YR 4/2-cl a wavy cip 5YR 2.5/2 Farmdale Soil 2Ab 275-287 sie 10YR 2/1 mlf 10YR 5/4 lmbk f 10YR 4/1 Matrix colors occur as c 10YR 3/2 (nonpedogenic) f 10YR 3/1 bands. Few charcoal 10YR 5/2 Hecks Y-S Solum 1 3Ab 287-310 sie 10YR4/1 m3p 10YR 5/8 lvf&f sbk f 2.5Y 4/4-cl Common krotovinas g 10YR 4/3 m SYR 4/6-fe 3Atb 310-348 sie 10YR 4/1 flp 7.5YR 2/0 2vf sbk m 5YR 4/6-fe c 10YR4/3 3ABtb 348-384 sie 10YR 5/1 fid 7.5YR 3/2 2-3vf sbk f-c 5Y 5/1-cl Few krotovinas g f5YR 4/6-fe 3Btbl 384-452 sie 10YR 5/1 flf 2.5Y 5/4 3 f&m abk c-m 2.5Y 4/0-cl c a f2.5Y 4/4-cl c 5YR 5/8-fe c 10YR 4/6-fe 3Btgb2 452-549 sie 5Y5/1 cl-2f 2.5Y 5/4 2 f-m pr c 2.5Y 4/0-cl c Few krotovinas g cl-2p 7.5YR 4/6 2 f abk f 5Y 4/1-cl f 5YR 2.5/2-fe/mn 3Btgb3 549-574 sie 5Y5/1 vf2-3p 7.5YR 4/6 l-2vf&f sbk f 5Y 4/1-cl c Few coarse sand grains c 5Y6/2 f5YR 2.5/2-fe/mn 3BCgb 574-617 sie 5Y5/1 c3p 7.5YR 4/6 2m pr c 5Y 4/1-cl a Common coarse sand c 5Y6/2 lm sbk c 2.5YR 3/2-fe/mn grains Y-S Solum 2 4BCgb 617-693 sie 5Y5/1 c3p 7.5YR 4/6 lm sbk 7.5YR 2/0-mn c Abundant coarse sand c 5Y6/1 cl f 5Y 5/4 to c 2.5YR 3/2-fe/mn grains. Few krotovinas 10YR 5/6 flp 2.5YR 4/8 0 f 10YR 5/8-fe Pre-Illinoian till 5Cg 693-716 cl 5Y6/1 flp 2.5YR 4/8 0 m 2.5YR 3/2-fe/mn Common pebbles g 10YR 5/6 MOL 716-820 1 7.5YR 5/8 c2p 5Y 7/2 0 c 10YR 3/1 Common pebbles a f 5Y 7/2 PROFILES Peoria Loess MDL 361-376 siel 5Y5/2 m2d 10YR 5/3 0 c5YR 5/8-fe a cip 10YR 5/6 Farmdale Soil 2Abl 376-389 siel 10YR 4/1 elf 10YR 3/2 0 c 7.5YR 2.5/0 Few charcoal flecks a 10YR 4/4 mid 10YR 5/6 2Ab2 389-404 siel 10YR 4/1 c2 f 10YR 5/4 0 f7.5YR 4/6-fe Few charcoal flecks a flp 10YR 5/6 2Bwb 404-422 siel 10YR 5/2 mlf 7.5YR 5/8 lvfsbk c 7.5YR 5/8-fe g 7.5YR 5/6 f 10YR 7/2-si Y-S Solum 1 3AB 422-465 siel 10YR 5/2 ml-3 f 7.5YR 5/8 lvf sbk- f 5YR 5/8-fe Common charcoal g 7.5YR 5/6 2-3f sbk c 10YR 7/2-si flecks 3BEb 465-490 sie 10YR 5/2 2-3f sbk f 10YR 4/2-cl g c 10YR 7/2-si 3Btbl 490-544 c 2.5Y 5/2 2-3f sbk f 10YR 4/2-cl g c 5YR 2.5/1-mn 3Btb2 544-580 sie 2.5Y 5/2 c2-3p 7.5YR 4/6 2m sbk- f 2.5YR 4/6-fe f g 2f sbk c 10YR 5.5/1-cl c 5YR 2.5/1-mn 3Bgb 580-610 sie 2.5Y 5/2 c2-3p 7.5YR 4/6 2m sbk f2.5YR 4/6-fe f Few small pebbles g 5Y5/1 c 10YR 5.5/1-cl c 5YR 2.5/1-mn Y-S Solum 2 4Btgb 610-670 sie 5Y5A 2m sbk c 7.5YR 5/2-cl c Common pebbles. Few g 7.5YR 5/6 c 7.5YR 2/0-mn barite coatings and nodules. Abundant krotovinas 4BCtgb 670-737 sie 5Y5/1 lm sbk c Common pebbles. Few g 7.5YR 5/6 barite coatings and nodules Pre-Hlinoian till 5C 737-864 cl 10YR 5/6 0 c 7.5YR 2/0-mn c Common large pebbles a 10YR 5.5/2

•Weathering zones: D - deoxidized; L = leached; M = mottled; O - oxidized. fc = clay; 1 = loam; si = silt. = few; c = common; m = many; a = abundant; 1 - fine; 2 = medium; 3 - coarse; f = faint; d = distinct; p = prominent. • *0 - structureless; 1 = weak; 2 = moderate; 3 = strong; vf = veiy fine; f = fine; m = medium; c = coarse; pr = prismatic; gr = granular; abk = angular blocky; sbk = subangular blocky. ,fcl = clay coating; fe = Fe-oxide coating; mn - Mn-oxide coating; fe/mn - Fe/Mn-oxide coating; si = silt coating. 56a = abrupt; c - clear; g = gradual; d = diffuse.

Farmdale Soil shows evidence of minimal Solum 1 is composed almost entirely of sim- ogenesis to keep pace, resulting in upbuild- pedogenetic alteration, its B horizon exhibits ilar granules, the granules in the Farmdale ing of the 3A horizon and the formation of a features typical of a more advanced stage of Soil are probably inherited features, in the cumulic soil profile (Birkeland, 1984; John- pedogenesis, such as spongy microstructure sense that the same zone is both the A hori- son, 1985). Accelerated loess deposition may and greater porosity relative to the A horizon zon of the lower soil and the B (or C) horizon have resulted in infilling of pores between (Table 5). Granular peds are strongly coa- of the upper soil. Initial deposition of DU-2 granular peds and the coalesced, spongy mi- lesced or surrounded by apedal silty matrix silts onto the Yarmouth-Sangamon surface crostructure of the lower part of the Farm- (Fig. 7F). Because the upper zone of the Y-S appears to have been slow enough for ped- dale Soil.

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TABLE 4. RADIOCARBON AGES OF SELECTED SAMPLES, PROFILE 1 Mineralogical Components

Depo, unit Depth Lab No. Age Accumulation rate (cm) (RCYBP)5 (cm/100 yr) Clay Mineralogy. The predominant clay minerals in both sola are smectite and kao- DU-2 394-404 TX-6619* 21,380 ± 740 linite, with minor amounts of illite and inter- 4.0 DU-2 404-414 TX-6620 21,630 ± 890 layered smectite (Tables 1 and 2). The rela- 3.0 DU-2 425-436 TX-6621 22,440 ± 850 tively uniform content of expandable clay 0.9 f minerals, both with depth and laterally across DU-3 488-499 AA-4830 29,610 ± 550 1.0 the toposequence, suggests that the bulk of 509-519 DU-3 TX-6750 31,500 ± 950 the smectite was inherited from the parent •University of Texas Radiocarbon Laboratory. material and/or was subsequently added to University of Arizona-National Science Foundation Accelerator Facility for Radioisotope Analysis. 'Radiocarbon years before present. the surface and gradually incorporated into the soil profile. If much of the smectite had been derived from alteration of clay mica or from neoformation, it is likely that the abun-

TABLE 5. MICROMORPHOLOGICAL FEATURES OF BURIED SOILS AND WEATHERING ZONES, PROFILES 3 AND 5

Unit/horizon Depth Micro- Porosity Void type5 (%) Sorting RDP" B-fabric" Textural pedofeatures55 Amorphous pedofeatures55 Fecal (or zone*) (cm) (%) pellets59 pi ch pk vg clay emb. dark silt Fe Fe Fe Mn Mn coat. clay brown coat. nod. mottle coat. nod. coat.

PROFILE 3 Peoria Loess MDL 223-229 massive 5-10 50 50 well ssp stipple sp 0 c c Farmdale Soil 2Ab 275-280 crack 10-15 50 50 well dsp stipple sp. o o c r Y-S Solum 1 3Ab 289-296 spongy 20-25 25 25 50 mod dsp circular str. o c o o r r va 3Atb 317-323 sbk (w) 15-25 35 35 30 poor dsp random str. c c c r m m spongy circular str. 3ABtb 357-363 sbk (m) 25-30 20 15 65 poor dsp random str. o m 0 r r c spongy circular str. 3Btbl 392-399 sbk (s) 15-20 65 10 25 poor dsp random str. a a c o o o r porostriated 3Btbl 423-430 sbk (s) 20-25 65 35 poor dsp random str. a m o r porostriated 3Btgb2 488-495 abk (m) 15-20 65 15 20 poor dsp random str. m m c r r r porostriated 3Btgb3 559-566 sbk (m) 15-20 70 25 5 poor dsp porostriated c o c a r r random str. 3BCgb 580-587 crack 15-20 65 20 15 poor dsp porostriated o 0 r c c r monostriated Y-S Solum 2 4BCgb 633-640 abk (m) 15-20 70 10 20 poor dsp mozaic sp. o r r a c o granostriated Pre-Illinoian till 5Cg 697-704 massive 8-10 10 20 70 unsort dsp mozaic sp. r va 0 o granostriated

PROFILE 5 Peoria Loess MDL 371-378 crack 5-10 60 40 well dsp stipple sp. r o c r r Farmdale Soil 2Abl 384-391 platy (wk) 5-10 50 50 mod dsp stipple sp. r o o c r r 2Ab2 391-396 platy (wk) 5 40 40 10 10 mod dsp stipple sp. 0 r o r r 2Bwb 401-406 channel 5 15 65 10 10 mod dsp stipple sp. o r o o o o 2Bwb 414-419 spongy 10-20 40 10 50 mod dsp circular str. r o o m r o va Y-S Solum 1 3Ab 432-437 spongy 20-30 10 90 mod dsp circular str. r r o o m r va 3Ab 462-467 granular 30-40 5 10 80 5 mod dsp circular str. o r o m r c r va sbk (s) 3Btbl 493-498 abk (m) 10-15 80 20 mod dsp random str. r a r m o r 3Btb2 559-566 abk (m) 15-20 70 10 20 poor dsp random str. c m r o c c m r porostriated 3Bgb 604-610 abk (m) 15-20 90 5 5 poor dsp porostriated o o c c m 0 c monostriated Y-S Solum 2 4Btgb 610-616 abk (m) 15-20 40 40 20 poor op porostriated m c 0 c c a c monostriated 4Btgb 645-650 abk (m) 15-20 85 15 poor ssp porostriated m c r a c granostriated 4BCtgb 670-676 sbk (w) 15-20 80 15 5 poor ssp porostriated m c r a o granostriated Pre-Illinoian till 5C 808-815 massive 5-10 70 5 10 15 unsort ssp mozaic sp. r o a c o granostriated

•Weathering zones: D = deoxidized; L - leached; M = mottled. fabk = angular blocky; sbk - subangular blocky; w - weak; m = moderate; s - strong. 5pl = planar void; ch - channel; pk = compound packing void; vg = vugh. ••Related distribution pattern (based on a coarse/fine limit of 20 jim): ssp = single-spaced porphyric; dsp = double-spaced porphyric; op = open porphyric. ^Birefringence fabric: stipple speckled, mozaic speckled; circular striated, random striated. ^Estimated abundances: r — rare (<1%); o = occasional (l%-3%); c = common (3%-5%); m - many (5%-10%); a = abundant (10%-20%); va - very abundant (>20%). (Textural pedofeatures: emb. clay - embedded clay coating; dark brown = dark brown clayey patch.)

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Figure 7. A. Silt coatings (indicated by arrows) on granular peds in the 3Ab horizon of Profile 5. Top of the photomicrograph is at left (circularly polarized light). B. Porostriated b-fabric in the 3Ab horizon of Profile 5. Diffuse boundaries between the rings of oriented clay (bright) and the soil matrix suggest a stress-related origin for the clay (circularly polarized light). C. Slickenside, or porostriated b-fabric, in the 3Bgb horizon of Profile 5, consisting of separate, parallel strands of stress-oriented clay parallel to straight planar voids (circularly polarized light). D. Iron-oxide nodules in the 3Ab horizon of Profile 5. Sharp, smooth boundaries with the surrounding matrix suggest rounding by pedoturbation (see text) (plane- polarized light). E. Compound coating on a channel wall in 4Btgb horizon of Profile 5. Coating consists of Fe oxides impregnating the soil matrix, microlaminated clay deposited later, and opaque Mn oxides that were deposited last (plane-polarized light). F. Granules in the 2Bwb horizon of the Farmdale Soil, separated by apedal silty matrix and interpreted to be inherited from the Solum 1 A horizon (plane-polarized light).

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ing discussion of barite). Similar dissolution of hydroxy-Al interlayers in vermiculite was interpreted by Lietzke and others (1975) to have occurred in some Michigan floodplain materials when the environment shifted from neutral to mildly alkaline.

Authigenic Barite. Barite (BaS04) occurs as soft spherical nodules (0.5-2 cm in diam- eter) and thin coatings in Solum 2, except in the depression where barite does not occur. Under scanning electron microscopy, barite crystals in the coatings do not appear to be covered by clay or other colloids, suggesting formation during a relatively late stage in the pedogenetic cycle. In situ formation is likely in that (i) barite is believed to be essentially immobile in soil profiles because of its ex- tremely low solubility (Lynn and others, 1971) and (ii) the barite crystals viewed under the scanning electron microscope (SEM) are euhedral (Fig. 8). Figure 8. Slickensides (light areas near center of photo), oriented 50° from horizontal, in the A few cases of secondaiy barite in modern 3Btb horizon of Profile 1 (knife-handle is 10 cm long). soils have been documented (Lynn and oth- ers, 1971; Stoops and Zavaleta, 1978; Crum and Franzmeier, 1980; Darmody and others, 1989), but its in situ formation in soils is dance of such products would differ between loess to the soil surface maintained a rela- poorly understood. Darmody and others landscape positions (Allen and Hajek, 1989). tively high pH, preventing extensive Al-in- (1989) demonstrated that concentration of There is also scant micromorphological evi- terlayering of smectitic clays and, at the same barium, rather than sulfate, is the limiting fac- dence for clay mica alteration in either solum. time, adding smectite to the profile. Alterna- tor in formation of barite in Illinois loess soils. High smectite content is compatible with tively, any hydroxy-Al interlayers that did Jones (1986) demonstrated that barium con- the loess or partial-loess origin proposed ear- form may have been subsequently destroyed centrations in surface horizons in Illinois lier in this paper; loesses rich in expandable by high-pH soil solutions percolating down were greater in loess-derived soils than in till- clay are common in the midcontinent (Bor- from the calcareous Peoria Loess (see follow- derived soils, and also greater in Wisconsinan chardt, 1989; Frye and others, 1968; Ruhe, 1967). Some of the smectite in the YSP parent materials may have come from pre-weath- ered materials such as surface soils that were overridden by glacial ice and eventually con- tributed to the loess fraction. Additional sources may have been Quaternary volcanic ash or dust derived from Cretaceous shales, from a westerly direction, but these hypoth- eses were not tested. There is limited evidence for weathering of clay minerals in the Yarmouth-Sangamon Figure 9. Scanning electron sola. Generally, lower values of the 1.6- to image of euhedral crystals in a 1.7-nm XRD (X-ray diffraction) peak for gly- barite nodule from the 4Btgb colated clay samples from the YSP (Tables 1 horizon of Profile 1 (frame and 2) suggest greater weathering in the YSP width is 180 p,m). materials compared with overlying materials, despite similar percentages of expandable clay (Frye and others, 1968). However, the complete collapse of expandable minerals (to d-spacings of 1.0 nm) upon potassium satu- ration and heat treatment (550 °C) indicates that Al-interiayering of smectites is only poorly expressed, or absent. There is virtu- ally no chlorite in the paleosol. It is possible that continual additions of calcareous dust or

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loesses originating in the Missouri River val- bases and Mn and Fe oxides, oxidation of the lack of absolute dates precludes age de- ley than in Mississippi River loesses. If DU-3 organic carbon, depletion of phosphorus and termination of this and other events. Micro- and DU-4 consist wholly or partially of loess other diagnostic elements, and possible de- laminated clay coatings, later disrupted and from the Missouri River valley, as suggested struction of smectite interlayers. Preburial embedded in the soil matrix, are relict fea- earlier, the barium may have originated in pedoturbation and bioturbation may also be tures from clay deposition onto void walls those units. partially responsible for the absence of stan- during this stage. Clay translocation and illu- Because nearly all the soil barite found to- dard depth curves for chemical constituents viation suggest landscape stability, subhumid day occurs in soils with a pH of about 5 or and the uniform clay mineral composition of climatic conditions, seasonal moisture defi- lower, the barite probably formed before the two Yarmouth-Sangamon sola. cits, and prior leaching of carbonates from burial and base resaturation of the paleosol Solum 1 (Bullock and Thompson, 1985). Il- by Wisconsinan loess, during a time of rela- Evolution of the Yarmouth-Sangamon luvial clay composing the embedded coatings tively acidic conditions. Poorly drained con- Paleosol was probably sufficient for this horizon to ditions were also likely, as most documented have qualified as an argillic horizon (Soil Sur- occurrences in modern soils are in hydromor- The YSP's textural, mineralogical, and vey Staff, 1975). Dining this time, welding of phic settings. Time of formation is further morphological characteristics point to multi- Solum 1 to Solum 2 began and included the constrained by the absence of illuvial clay on ple pedogenetic processes at work over a overprinting of blocky structure over the the barite coatings, which implies formation long period of time within the composite pro- granular structure of the buried A horizon. after the last episode of clay illuviation that file. The following reconstruction of the pa- Stress-related features that occur in both affected Solum 2. Uncertainty over the tim- leosol's polygenetic history, although some- sola strongly suggest the effects of argilli- ing of this illuviation event precludes a more what speculative, is offered as a starting point pedoturbation on the profile sometime after precise time determination. for further discussion and research. the first cycle of clay illuviation. Vertic shrink/ The Pre-Illinoian diamicton (DU-5) must swell processes and surface cracking may oc- DISCUSSION have undergone at least one period of surface cur in response to an intrinsic threshold weathering, probably during the first intergla- (Muhs, 1984) such as the buildup of expand- Lateral Variability in the Earlham cial stage (Yarmouthian?) following its dep- able clays in an argillic horizon. However, Toposequence osition, although the extent of soil develop- because slickensides in the YSP occur well ment on this surface is unknown. During below the paleosol's main clay accumulation Theoretically, paleocatenas might be ex- some later period of landscape instability, a zone (3Bt), extending through both sola, such pected to exhibit lateral variations among stone zone developed as the surface was a cause is questionable. Slickensides are their members analogous to the differing soil truncated by erosion. This truncated surface most commonly attributed to precipitation types found in catenary association on the was later buried by a sediment influx (DU-4) patterns, and longer, more intense moisture modem landsurface. Prior to this study, the consisting of colluvium from low-relief slopes deficits result in thicker slickenside zones degree to which toposequence members of (since beveled) and aeolian materials. Fining- (Yaalon and Kalmar, 1978). The common de- the YSP differ morphologically, mineralogi- upward medium:coarse silt ratios suggest the nominator in soils with slickensides is a large cally, and chemically had never been evalu- influence of sedimentologic mixing processes expandable clay content (at least 30% clay, ated. The results show less variability than during this time. When deposition slowed or mostly smectite) and a seasonal moisture def- expected, even considering the absence of ceased, DU-4 underwent soil development icit of four to eight months (Ahmad, 1983). well-drained members in the partial topose- and Solum 2 formed. Its A horizon, now bur- Slickensides in the YSP may point to a cli- quence. Most of the differences that do exist ied, is partially preserved, as evidenced by matic regime during this stage that included are morphological in nature. Most obvious is thin sections of granular or spongy micro- long, very dty seasons. the difference in degree of mottling and glei- structure. Krotovinas in Solum 2 are evi- Clay coatings on ped surfaces in Solum 1 zation between the YSP profile in the depres- dence of bioturbation, and embedded clay (Profile 3) indicate another episode of clay il- sion and the better-drained profiles, clearly coatings imply clay illuviation after carbon- luviation, which may or may not have af- visible in the field (Table 3) and confirmed by ates had been leached. fected Solum 2. The great thickness (>2 m) of micromorphology (Table 5) and extractable Renewed loess deposition, possibly during the clay accumulation zone (argillic horizon) iron oxide data (Tables 1 and 2). Less obvi- the Illinoian Glacial Stage, resulted in the ac- in the depression, where it actually extends ous is the difference in amount of illuvial clay cretion of DU-3 and burial of Solum 2, ter- up into the 3At horizon, suggests upbuilding in Solum 1, which increases in the depres- minating soil formation in the lower solum. of the solum as silty clay materials from sur- sion, in contrast to the modern soil at the site. Deposition of DU-3 appears to have been rounding slopes were redeposited in the de- The thickness of Solum 1, and in particular continual because the unit exhibits no micro- pression by colluviation or wind. As a result, the Solum 1 A horizon, is considerably morphological evidence for surface weather- the 3Bt horizon migrated upward with time. greater in the depression than elsewhere, sug- ing zones that might suggest significant hia- Evidence for an argillic horizon and the pres- gesting upward growth of the solum through tuses during this stage. Dark brown clayey ence of secondary barite, indicating acidic aeolian and possibly colluvial additions to the patches in Solum 1 suggest pedogenetic as- conditions, suggest forest vegetation during surface. similation of surface crusts during slow sed- this stage. Members of the toposequence exhibit rel- iment accretion. During early or middle Wisconsinan time, atively few differences in chemistry and clay As sediment deposition slowed or ceased, slow deposition of aeolian silts (DU-2) began. mineralogy. This may be due largely to the DU-3 underwent weathering and soil forma- The gradual boundary between Solum 1 and homogenizing effects of diagenesis, which in- tion. This stage may correspond to the early the Farmdale Soil indicates upward growth of cluded enrichment of the YSP profile with part of the Sangamon Interglacial Stage, but the solum as these silts were assimilated into

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the profile by pedologjc and biologic mixing ACKNOWLEDGMENTS ewan: Soil Science Society of America Journal, v. 51, p. 1243-1250. processes, including earthworm activity. The Dick, W. A., and Tabatabai, M. A., 1977, An alkaline oxidation method for determination of total phosphorus in soils: Soil greater thickness of the Solum 1 A horizon This study was supported by Geological Science Society of America Journal, v. 41, p. 511-514. (and the Farmdale Soil) in the depression Society of America Research Grant 4340-89; Edwards, C. A., 1985, Earthworms in soil formation, structure and fertility, in Faunal influences on soil structure: Proceedings of than elsewhere suggests assimilation in the a grant-in-aid of research from Sigma Xi, the a symposium held at the University of Alberta: Quaestiones Entomologicae, v. 21, p. 517-522. swale of some loess-derived colluvial mate- Scientific Research Society; and a University Finkl, C. W., and Gilkes, R. J., 1976, Relationships between mi- rials as well. Krotovinas and the worm-re- cromorphological soil features and known stratigraphic layers of Iowa Collegiate Associations Council re- in Western Australia: Geoderma, v. 15, p. 174-208. worked A horizon of Solum 1 are compatible search grant. The Iowa Geological Survey Follmer, L. R., 1982, The geomorphology of the Sangamon surface: Its spatial and temporal attributes, in Thorn, C. E., ed., Space with an interpretation of grassland vegetation Bureau and the Iowa Agriculture and Home and time in geomorphology: London, England, Allen & Un- during this time (—31,500 radiocarbon yr Economics Experiment Station provided fi- win, p. 117-146. Forman, S. L„ Bettis, E. A., Ill, Kemmis, T. J., and Miller, B. B., B.P.), which is more or less in agreement nancial support and field or laboratory equip- 1992, Chronologic evidence for multiple periods of loess dep- osition during the Late Pleistocene in the Missouri and Mis- with paleoenvironmental data from a nearby ment. We are very grateful to Art Bettis and sissippi River Valley, United States: Implications for the ac- tivity of the Laurentide ice sheet: Palaeogeography, site (Baker and others, 1991) that suggested Tim Kemmis of the Iowa Geological Survey Palaeoclimatology, Palaeoecology, v. 93, p. 71-83. open, prairie-like upland environments at Bureau for assisting in the field work and for Frye, J. C., Shaffer, P. R„ Willman, H. B„ and Ekblaw, G. E., 1960, Accretion-gley and the gumbotil dilemma: American -33,000 to 35,000 yr B.P. providing a valuable forum for discussion of Journal of Science, v. 258, p. 185-190. Frye, J. C„ Glass, H. D„ and Willman, H. B., 1968, Mineral zo- Radiocarbon ages and pedologic develop- ideas during the entire four years of this nation of Woodfordian loesses of Illinois: Illinois State Geo- ment suggest an increase in the rate of loess project. We also thank Walter Wulf, Jr., of logical Survey Circular 427, 44 p. Guccione, M. J., 1983, Quaternary sediments and their weather- accumulation after -22,000 yr B.P. The Monarch Cement Company for granting ac- ing history in northcentral Missouri: Boreas, v. 12, cess to Earlham Quarry. Tim Kemmis, Art p. 217-226. Yarmouth-Sangamon (more accurately, the Hall, R. D., Jensen, C. A., Stephens, N. G., and Zobbe, L. A., 1991, "Yarmouth-Sangamon-Wisconsin") surface Bettis, and Dick Baker edited early drafts of Complexity in the parent materials of the Sangamon Soil, central Indiana: Geological Society of America Abstracts was buried, and diagenetic alteration of the manuscript. with Programs, North-Central Section, v. 23, no. 3, p. 16. Hallberg, G. R., 1980, Pleistocene stratigraphy in east-central Iowa: the Yarmouth-Sangamon sola commenced. Iowa Geological Survey Technical Information Series No. 10, Postburial diagenesis continued through dep- 168 p. Hallberg, G. R., 1986, Pre-Wisconsin glacial stratigraphy of the Cen- osition of DU-1 and included cryogenic de- REFERENCES CITED tral Plains region in Iowa, Nebraska, Kansas, and Missouri: Quatemaiy Science Reviews, v. 5, p. 11-15. formation of the uppermost zone of Solum 1, Ahmad, N., 1983, Vertisols, in Wilding, L. P., Smeck, N. E., and Hallberg, G. R„ Fenton, T. E„ and Miller, G. A., 1978, Standard probably between 21,500 and 16,500 yr B.P. Hall, G. F., eds., Pedogenesis and soil taxonomy, Volume 2. weathering zone terminology for the description of Quater- The soil orders: Amsterdam, The Netherlands, Elsevier, nary sediments in Iowa, in Hallberg, G. 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1460 Geological Society of America Bulletin, November 1993

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G., 1967, The relation of Pleistocene Pliocene buried soils of central United States: Journal of Soil MANUSCRIPT ACCEPTED MARCH 23, 1993 geology and soils between Bentley and Adair in southwestern Science, v. 2, p. 1-19. IOWA QUATERNARY STUDIES GROUP CONTRIBUTION NO. 55

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