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Upper Cretaceous (Maastrichtian) Paleosols in Trans-Pecos Texas

Upper Cretaceous (Maastrichtian) Paleosols in Trans-Pecos Texas

Upper Cretaceous () paleosols in Trans-Pecos

THOMAS M. LEHMAN Department of Geosciences, Texas Tech University, Lubbock, Texas 79409

ABSTRACT ico. Maastrichtian strata are wholly fluvial in cient river flood plains of the Tornillo Basin origin and include the El Picacho Formation in (Lawson, 1972; Schiebout, 1980; Lehman, Fluvial flood-plain deposits of the upper Presidio County and the upper part of the 1985a). The present study describes the proper- Aguja, Javelina, and El Picacho Formations and lower part of the Javelina ties of the Maastrichtian paleosols, interprets the in Brewster and Presidio Counties of West Formation in Brewster County (Fig. 1). These processes responsible for their formation, and Texas record multiple episodes of soil forma- strata consist of alternating beds of , gray, compares them with similar modern soils. Al- tion during (Maastrichtian) purple, and red mudstone interbedded with though paleosols occur throughout the upper time. These well-differentiated alluvial paleo- white, tan, and dark brown lenticular sandstone. to lower section, only those sols are characterized by pale gray leached nodules are abundant in the mud- developed within Maastrichtian will (albic) A horizons and purple or red, clay- stones and form a conspicuous lag pavement on be examined in this study, as these form a coher- and iron oxide-enriched (cambic, argillic) B weathered slopes. ent paleosol "" (sensu Retallack, 1983) or horizons. These characteristics indicate that The striking multi-colored appearance of "pedofacies" (sensu Bown and Kraus, 1987) podzolization and lessivage were important these sediments and their abundant nodule con- with generally similar characteristics (Fig. 1). pedogenic processes and suggest that these tent have long been noted in stratigraphie de- The development of this suite of paleosols paleosols are comparable to some modern Al- scriptions and are useful in the lithostratigrahic corresponds to the introduction of a distinctive fisols. Texturally diverse accumulations of subdivision of the Upper Cretaceous and lower assemblage of —the nodules, coalesced masses of nodules, Tertiary section (for example, Udden, 1907; —into the Trans-Pecos region (Fig. 1; rhizocretions, and thin discontinuous hard- Maxwell and others, 1967). Only recently, Lehman, 1987). This fauna is considered to be pans indicate that calichification also oc- however, has it been recognized that the color middle to late Maastrichtian in age (Lancian, curred. The extreme development of the banding and nodule development may reflect sensu Russell, 1975). The same as- petrocalcic horizons, partial silicification, as- repeated episodes of soil formation on the an- semblage occurs in contemporaneous Maas- sociation with sulfate minerals, and high posi- tion within the soil profiles indicate that calichification was in many cases not con- temporaneous with iron and clay illuviation. The compound paleosols thus formed indi- cate that the Maastrichtian in this re- gion may have fluctuated between regimes of humid and semiarid character, each at least several thousand in duration. This in- ferred long-term climatic cyclicity may cor- respond to climatically induced cycles observed in the marine realm that are ascribed to Milankovitch cycles. wood suggests that the flood-plain paleosols sup- ported coniferous forests, whereas adjacent courses were lined by angiosperm woodlands.

INTRODUCTION

Upper Cretaceous (Maastrichtian) and lower Tertiary sedimentary rocks accumulated in the broad southeast-trending Tornillo Basin in the Trans-Pecos region of Texas (Lehman, 1986). Sedimentation in this basin was in response to Laramide tectonism in Texas and adjacent Mex-

Geological Society of America Bulletin, v. 101, p. 188-203, 11 figs., 1 table, February 1989.

188

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trichtian strata elsewhere in the southwestern leosols are of value in interpreting the environ- lowest sandstone above which are prominent United States (Utah and New Mexico), where it mental and climatic implications of the Alamo- paleosols is mapped as the boundary between is associated with paleosols similar to those saurus fauna and in discerning the underlying the Aguja and Javelina Formations (Fig. 1; found in the Trans-Pecos deposits (Lehman, causes of the separation of this fauna from Maxwell and others, 1967). Hence, the upper 1981, 1985b). The link between these paleosols northern . part of the Aguja Formation ("upper and the Alamosaurus fauna suggests a strong member" of Lehman, 1985a), Javelina Forma- environmental control on the distribution of this STRATIGRAPHIC DISTRIBUTION tion, and El Picacho Formation are the focus of fauna. This assemblage of dinosaurs is distinct OF PALEOSOLS the present study. As many as 20 stratigraphi- from those found in the northern interior of cally successive paleosols can be recognized (the and Leptocer- Purple and gray banded paleosols, with ho- through the Maastrichtian section at some atops faunas of Lehman, 1987). The northern rizons strongly differentiated on the basis of localities. faunas occur in association with paleosols that color and texture, first appear in the Upper Cre- The Cretaceous-Tertiary boundary lies differ markedly from those described herein taceous stratigraphie sequence 50 to 90 m above within the Javelina Formation as it is presently (Fastovsky and McSweeney, 1987). Hence, pa- the top of the highest paralic sandstone in the mapped (Fig. 1; Schiebout and others, 1987). Aguja Formation (the "Terlingua Creek sand- Paleosols developed within the part stone member" of Lehman, 1985a; Fig. 1). In a of the Javelina Formation differ in color and few sections, the first such paleosol occurs as organic carbon content from those in the under- Figure IB. Correlation of measured low as 12 m above this sandstone; however, lying Maastrichtian part and are more like those sections of Upper Cretaceous and Pa- these examples are local and poorly developed. in the overlying Black Peaks Formation. In the leocene strata in Big Bend National In Presidio County, the lowest occurrence of southwestern part of the Tornillo Basin, Maas- Park (Brewster County, Texas). Sec- these paleosols is mapped as the boundary be- trichtian strata are eroded and unconformably tion locations: 1, Sierra Aguja; 2, Des- tween the San Carlos and El Picacho Forma- overlain by late basalt flows. A thick ert Mountain Overlook; 3, Rattle- tions (Wolleben, 1966). In Brewster County, the paleosol of probable Eocene age is developed Mountain; 4, west fork of Alamo 200 Creek; 5, Pena Mountain; 6, Maverick Mountain; 7, Dawson Creek; 8, Paint Gap Hills; 9, Croton Spring; 10, northwest Grapevine Hills; 11, south- west Grapevine Hills; 12, McKinney Springs; 13, Dagger Flat; 14, Tornillo Flat. (Features illustrated or discussed in more detail elsewhere in this paper: A, paleosol 1 of Table 1 and Figs. 2B, 5B, 6B; B, paleosol 2 of Table 1 and Fig. 5C; C, Fig. 6C; D, paleosol 3 of Table 1 and Fig. 6A; E, paleosol 4 of Table 1; F, Fig. 11; G, paleosol 6 of Table 1; H, Figs. 2A, 2C, 3).

Eocene erosional surface overlain by

Eocene paleosol

marine shale (tongue of PEN FORMATION

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/2/188/3380449/i0016-7606-101-2-188.pdf by guest on 24 September 2021 Figure 2. A. Sequence of flood-plain paleosols between channel sandstones exposed on northwest flank of McKinney Hills (a = top of section shown in Fig. 3). B. Channel swale paleosol on Rattlesnake Mountain. C. Detail of flood-plain paleosol sequence shown in A (detail in Fig. 3). D. Channel swale paleosol in El Picacho Formation. E. Flood-plain paleosol sequence northwest of McKinney Hills (section 12 of Fig. 1), spanning Javelina (KTj) and Black Peaks (Tbp) formational contact. F. Detail of D, showing channel swale paleosol with abundant krotovina.

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within the Cretaceous rocks beneath the basalt PALEOSOLS light gray (2.5 GY 8/1, 5 GY 8/1, 7.5 Y 7/1, (Fig. 1). This paleosol also differs from the older 10 Y 7/1) to olive gray (2.5 GY 6/1) or light Maastrichtian paleosols and is not considered in Paleosols occur in two broad environmental olive gray (2.5 GY 7/1, 5 GY 7/1). Such units the following discussion. settings within the sequence of fluvial deposits generally do not exhibit pronounced color mot- described above: (1) within channel deposits, tling, but they uncommonly have 5% to 20% DEPOSITIONAL SETTING particularly in swales developed on the upper coarse mottles of purplish gray (5 RP 6/1) to surfaces of point bars, and (2) in overbank dull reddish brown (7.5 R 5/3). Such mottles The Maastrichtian strata consist largely of flood-basin deposits laterally far removed from may be faint, with irregular diffuse boundaries, mudstone (75% of the section) with a lesser channels (Fig. 2). Although paleosols that de- or subcircular in cross section, with pronounced amount of lenticular sandstone (25%) and a few veloped in each of these settings are broadly boundaries. thin beds of (less than 1%; Figs. 1 and similar, there are differences in texture, glaebule Units described as "purple" in color range 2). The sandstone lenses are deposits formed by development, and pedoturbation that may cor- from light bluish gray (5 PB 7/1) to purplish the lateral migration and filling of highly sinuous respond to differences in environment. The most gray (5 P 6/1, 5 RP 6/1, 5 RP 5/2). Beds meandering stream channels (Lehman, 1985a). striking characteristic of the paleosols is the described as "red" in color include reddish gray There is a marked hierarchy in the size (thick- presence of alternating gray, purple, and olive (5 R 5/1 to 6/1, 7.5 R 6/1), dark reddish gray ness and lateral continuity) of these sandstone color bands and the presence of indurated cal- (5 R 4/1,7.5 R 4/1,10 R 4/1), grayish red (7.5 channel deposits. The largest channel sand bod- cium accumulations (Fig. 2). That the R 4/2, 7.5 R 4/2, 7.5 R 5/1 to 5/2, 10 R 4/2 ies exceed 1 km in width, are as thick as 30 m, color banding and nodule development are in- to 5/2), dull reddish brown (7.5 R 5/3), reddish and contain pebble and volcaniclastic con- deed pedogenic in origin is demonstrated by brown (10 R 4/3), and dark reddish brown (7.5 glomerates of extra-basinal origin. Smaller (1) the presence of both gray and purple mud R 3/2 to 3/3, 10 R 3/2 to 3/3). Red beds channel sand bodies seldom exceed several clasts, as well as carbonate nodules identical to commonly grade downward into a purple bed hundred meters in width and 5 m in thickness those preserved in situ, as a reworked basal lag and may also be overlain by a purple bed, al- and contain lag material of only local derivation in laterally equivalent channel deposits; (2) the though this is less common. In most examples, (calcite nodules, bone, and wood). The smallest association of floral and faunal pedoturbation purple beds exhibit distinct or prominent color channels are thought to represent tributary features with presumed soil horizons; (3) cal- mottles with irregular diffuse boundaries. Such drainages, developed during periods of flood- careous and ferruginous encrustation of asso- mottling varies from 5% to 20% in abundance, plain degradation, whereas the largest channels ciated weathered bone accumula- medium to coarse in size, and light gray (5 GY must represent major trunk that fed into tions; (4) the presence of reworked bone with 7/1 to 8/1) to light olive gray (2.5 GY 7/1) in the Tornillo Basin from the northwest (Lehman, comparable encrustation in adjacent channel color. Where peds are developed in purple beds, 1986). deposits; and (5) the presence of illuviation cu- the ped interiors exhibit drab colors compared tans ("clay skins") in some presumed paleosol Mudstone deposits are volumetrically more to those of the exteriors. Red beds may also horizons. Retallack (1981, 1983), Bown and important than the channel sandstones and exhibit mottling, but where present, it is gener- Kraus (1981, 1987), and others have discussed resulted from periodic overbank flooding. Levee ally much fainter in contrast, differing only in at length the criteria used to support the pedo- and crevasse accumulations are poorly devel- value or chroma by about two units from the genic origin of similar features in other strata; oped, and most of the overbank deposits reflect matrix color. Some red beds have very fine (1 to hence, such criteria will not be elaborated vertical aggradation in well-drained flood ba- 3 mm diameter) root traces delineated by drab herein. sins. Local lenses of unionid bivalve and haloes. Most red beds are, however, nearly uni- form in matrix color. Units described as "olive" gastropod shells indicate that some short-lived In the following discussion, emphasis is beds range in color from gray (7.5 Y 5/1) or flood-basin ponds probably existed. The rare placed on the general similarities between all of light gray (7.5 Y 7/2), yellowish gray (5 Y 8/1), limestone beds are generally between 30 and the paleosols in the Trans-Pecos Maastrichtian olive gray (5 Y 5/1) to grayish olive (7.5 Y 50 cm thick and range from nearly pure micritic section, rather than on differences between indi- 6/2), olive yellow (7.5 Y 6/3), or light yellow limestone to muddy limestone or calcareous silt- vidual paleosols. At present, it is possible to rec- (7.5 Y 7/3). stone. Such beds are commonly pervasively bur- ognize a spectrum of paleosol types, generally rowed; however, some retain varve-like lamina- similar to and gradational with one another, but Texture. Gray units range from muddy very tion. A wide variety of smooth, pellet-lined, it is premature in this case to establish "type" fine sandstone and sandy mudstone to mud- meniscate, straight, and branched burrows are examples of numerous paleosol series in the stone. Less commonly, such units will be clay- present that are comparable to those described manner accomplished by Retallack (1983). The stone. The fine-textured examples exhibit the by Bracken and Picard (1984) and Bown and definitions of soil properties, soil nomenclature, strongest color mottling. Flood-basin deposits Kraus (1983) from similar depositional settings and classification used in this study are based on yield gray beds of both coarse and fine textures; (as will be shown later). Unionid bivalve and current practices in the United States as given by however, only the coarser examples are devel- gastropod shells as well as calcified charophyte the Soil Survey Staff (1975) and in the work of oped in channel swales. Gray units are, almost are found in some of these beds. In most Brewer (1976) and Buol and others (1980). without exception, coarser than are underlying instances the overlie channel sand- Munsell color ranges are given for naturally purple- or red-colored units. stones and may be the late- filling of moist unweathered material. Purple and red beds in flood-basin deposits topographic depressions created by channel range from mudstone to claystone. Beds of sim- abandonment. The limestones are interpreted Paleosol Horizon Characteristics ilar color are, however, developed in muddy as deposits of shallow fresh-water calcareous Color. Units that appear in exposure as very fine sandstone in channel swales. Olive ponds. "gray" in color range from gray (7.5 Y 5/1) or units consist predominantly of mudstone.

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Structure. Gray units, particularly the fine- in the lower parts of some gray units. Vertebrate depositional in origin, as it surrounds and en- textured examples, exhibit moderate to strong bones in the paleosols are commonly coated gulfs patches of matrix. coarse subangular blocky structure and no with thin films of iron and manganese oxides, Chemistry. Gray beds contain significantly primary stratification. Some sandy gray units probably of similar origin. less total iron (range 1 to 5 wt% Fe) than do may, however, exhibit faint areas of primary Mineralogy. Gray units are composed pre- associated (directly subjacent) purple or red (depositional) stratification. This stratification dominantly of Ca-Mg smectite, quartz, and pla- beds (Table 1). Although most of the iron in consists of irregular discontinuous parallel lami- gioclase, with trace amounts of illite and both gray and purple/red beds resides in a ferric nae that in thin section, appear as segregations of (determined by X-ray diffractometry). state, gray beds have a higher percentage of fer- very fine sand and silt from clay. It is possible On the basis of their petrographic relationships, rous iron than do underlying purple/red beds that the sand-rich gray units are crevasse splay these minerals appear to be detrital and illuvial (as expressed in the ratio Fe2+/Fe2+ + Fe3+; see deposits subsequently modified by pedogenesis. in origin. Only kaolinite is observed as a pore- Table 1). Gray beds have less A1 than do pur- Visible in thin section or polished section, but filling, presumably authigenic phase. In addition, ple/ red beds. The higher A1 content of purple unusual in outcrop, are very fine (1 to 2 mm) gray and olive units commonly contain a signifi- and red units probably reflects their higher clay root casts filled peripherally by weakly oriented cant amount of calcite in the form of small glae- content compared to overlying gray beds. No argillans and centrally by carbonate. bules, coalesced masses of glaebules, void-filling clear trend is visible in Ca abundance, although Purple beds show very coarse granular struc- crystallaria, rhizocretions, and thin beds. Cal- some gray beds contain significantly more Ca ture in many examples. Red beds generally ex- cium carbonate nodules also occur in purple and than underlying purple/red beds. The total or- hibit a stronger coarse subangular blocky red units, but in fewer cases than in olive or gray ganic carbon content of some gray beds is structure, commonly with slickensided clay and beds (see section on petrocalcic horizons). Such slightly higher than that of underlying pur- manganese cutans delineating ped surfaces. At calcite accumulations are demonstrably post- ple/red beds; however, the greatest accumula- least two examples of red beds, however, show depositional in origin, as their growth has en- tion of organic carbon is generally found in red strong fine-granular structure. Olive beds exhibit gulfed and replaced detrital grains. beds (Table 1). abundant and readily visible primary stratifica- Purple and red beds contain more smectite Both purple and red beds owe their coloration tion. Apart from sparse burrows and calcium but otherwise do not differ substantially in bulk to the presence of iron oxides in the form of carbonate nodules, however, olive beds show composition from gray or olive beds. Both red amorphous clots or aggregates of minute hema- little pedogenic structure. and purple beds, however, contain abundant tite crystals distributed through the , as Weakly oriented clay films and coatings of iron oxides, either as diffuse amorphous clots described above. Purple and red beds are de- iron or manganese oxides, interpreted here as distributed in patches throughout the sediment scribed here together because the difference be- illuviation cutans, are found in purple or red or as fine 1- to 3-micron-size hematite crystal tween them is one of degree and not kind. The units and in the lower parts of gray units. These aggregates outlining granular soil peds. The total iron content of these units ranges from 4 to structures are found on presumed ped surfaces poorly crystalline character of this material 8 wt% Fe and generally exceeds that of overlying and in microscopic planar voids and root molds. makes its mineralogical identity obscure, al- gray units by several weight percent (Table 1). Strongly oriented cutans are rarely observed. though its color suggests hematite. Petrographic Purple units have slightly less total iron and tend Stress cutans are observed around ped surfaces relationships indicate that the hematite is post- to be slightly coarser in grain size than are red

TABLE 1. GEOCHEMICAL ANALYSES OF SELECTED PALEOSOLS

2 Sample Inferred Color Total Wt% FeO Wt% Fe203 Fe » Wt% Mn Wt% Al Wt% Ca Wt% T.O.C. paleosol wt% Fe horizon Fe2+ + Fe3t

Paleosol 1-upper Aguja Fm Sample 1 A2 2.5 GY 6/1 4.52 0.82 3.58 0.202 0.01 13.41 0.89 0.36 Sample 2 B 7.5 R 3/2-4/2, 7.61 0.68 6.78 0.101 0.01 13.75 1.57 0.35 5% mottles 7.5 R 6/1

Paleosol 2-upper Aguja Fm Sample 1 A2 2.5 GY 7/1, 1.23 0.10 1.11 0.089 0.15 2.78 39.80 1.23 5% mottles 5 RP 6/1 Sample 2 B 10 R 4/3 8.22 0.42 7.69 0.057 0.04 14.78 5.81 0.87

Paleosol 3-upper Aguja Fm Sample 1 A2 2.5 GY 8/1 3.08 0.78 2.20 0.283 0.03 12.38 2.60 0.47 Sample 2 B2 10 R 3/2-5/2, 4.93 0.66 4.16 0.149 0.01 14.09 0.54 2.16 15% mottles 7.5 R 6/1 Sample 3 B3 7.5 R 4/1-6/1 5.76 0.74 4.88 0.144 0.01 14.44 0.54 0.16

Paleosol 4-upper Aguja Fm Sample 1 A2 5 GY 8/1 3.70 1.41 2.12 0.425 0.01 13.07 0.77 1.77 Sample 2 B2 5 P 6/1 4.32 0.94 3.33 0.234 0.01 13.75 0.77 0.78 Sample 3 B3 5 PB 6/1-7/1 4.32 1.15 3.01 0.297 0.01 12.72 0.89 1.04

Paleosol 5-EI Picacho Fm Sample 1 A2 10 Y 7/1 3.29 0.43 2.78 0.147 0.02 12.38 2.37 0.01 Sample 2 B1 7.5 Y 7/1, 3.29 0.22 3.01 0.077 0.02 14.44 2.14 0.26 20% mottles 7.5 R 5/3 Sample 3 B2 5 R 5/1 to 7.5 R 5/2, 3.91 0.43 3.40 0.122 0.02 15.12 2.03 0.26 20% mottles 2.5 GY7/1

Paleosol 6-Javelina Fm Sample 1 A2 10 Y 7/1-7/2 4.93 2.14 2.53 0.485 0.04 11.70 11.30 2.70 Sample 2 Bl 7.5 Y 5/1 5.55 0.79 4.63 0.159 0.02 16.15 1.57 1.10 Sample 3 B2 7.5 R 3/2-3/3, 6.79 0.00 6.72 0.000 0.05 12.72 8.44 8.53 15% mottles 2.5 YR 5/2

Note: total iron, aluminum, manganese, and calcium were determined by atomic absorption analysis, using U.S. Geological Survey silicate rock standards in the linear range of absorbance. Precision is in the range of 2% to 3% relative. FeO was determined by titration, using the ammonium metavanadate method. Total organic carbon (T.O.C.) was determined by mass spectrometry.

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units. Both purple and red units have a lower development. It is difficult in many instances to Olive beds are interpreted as relatively un- percentage of reduced iron than do overlying establish the actual top of each particular paleo- modified flood-plain alluvium (C horizon where gray units (as expressed in the ratio Fe2+/Fe2+ + sol and hence to delineate the top of an A ho- associated with an overlying paleosol) and con- Fe3+ in Table 1). These findings concur with the rizon that had incremental cumulization follow- sist of silty smectitic claystone generally darker conclusions reached by Bown and Kraus (1981) ing its development. Gray (A2) horizons are and more olive, green, or yellow in color than is regarding the chemical differences between pur- distinguished from unmodified flood-plain allu- the A horizon of associated paleosols. Apart ple and red beds. vium (C horizons) primarily by color, texture, from color, slight texture and structure contrast, Thickness and Lateral Relationships. Gray and position (see below). and position, it is difficult to distinguish paleosol units average 1.5 m in thickness (n = 18, range = Both purple and red beds are interpreted as B A horizons from alluvium little affected by ped- 0.3 to 3.5 m) where intercalated with purple or horizons of paleosols. The predominant features ogenesis that accumulated between marked red beds. The top of a gray unit is in most cases of these horizons are their darker, redder color periods of soil formation. Unmodified alluvium well defined as a slightly wavy horizontal zone than in overlying and underlying horizons and contains approximately the same amount of or- where it is in contact with an overlying purple or the apparent illuvial concentration of iron and ganic carbon (1 to 2 wt% T.O.C.) as either gray red bed. On close inspection, such contacts are clay. In some cases, organic matter has also been or purple/red beds, but in some instances, it gradational over 10 to 20 cm. In contrast, the concentrated in these horizons (Table 1). Be- is visibly carbonaceous. This carbonaceous lower boundary of a gray bed is commonly cause illuvial iron and clay characterize the con- material is probably of allochthonous origin. higher irregular and intertonguing with either a dition of the B horizon, the subhorizon designa- Olive beds also exhibit more visible primary purple or red bed. Such intertonguing may occur tion "Birt" is appropriate. Where a purple or stratification. over a thickness of 0.2 to 0.5 m. In several ex- purple-mottled gray unit underlies a red unit, as amples, this tonguing has the form of vertical is the case in many instances, the purple unit is Petrocalcic Accumulations fissures as much as 20 to 30 cm deep and 2 to 5 appropriately designated B3 and the red unit B2. cm wide. Gray beds may seldom be traced later- Likewise, purple or purple-mottled gray units Indurated accumulations ally the full extent of most outcrops (less than 1 overlying a red unit are denoted as B1 (Fig. 3). occur in various positions within the paleosol km to several kilometers) and are observed to Some of the thicker examples may represent su- sequence, at a variety of different scales, and end by erosional truncation where adjacent to a perimposed or "compound" B horizons in exhibit a wide array of external forms and inter- channel sandstone. which an intervening A horizon has been re- nal structures (Figs. 4 and 5). These accumula- Individual purple and red units average 1.23 moved by erosion or obliterated by subsequent tions are pedogenic in origin and are readily pedogenesis. m in thickness (n = 71, range = 0.2 to 4.5 m). separated from lacustrine , found Purple or red beds are generally thinner than are associated overlying gray beds. In many cases, a II — MOTTLING red unit grades downward into a purple unit, that is in turn underlain by an olive or gray unit. The planar contacts between units are grada- tional over a thickness of 10 to 20 cm. Red and purple units are less continuous in outcrop than are olive units. Individual red or purple units may be traced laterally in good exposures sev- eral tens or hundreds of meters before pinching Figure 3. Sequence of out. In several instances where a purple bed is flood-plain paleosols ex- well exposed, it exhibits a lenticular geometry. posed on northwest flank There is, however, a distinct trend toward in- of McKinney Hills (H in creasing continuity of purple and red units with Fig. 1; Figs. 2A and 2C). increasing stratigraphic height above the Terlin- Color of horizons, mot- gua Creek sandstone member (Fig. 1). tled zones, petrocalcic ho- Interpretation. Gray beds, where underlain rizons, and grain size by a purple or red bed, are regarded as A2 (= trends (cl = claystone, si = eluvial, Ae or E) horizons of paleosols. This in- siltstone, ss = sandstone, terpretation is based on the lighter color and eg = conglomerate) are coarser texture of the gray beds, their lower clay indicated. Interpretation and total iron content compared to underlying of six paleosols and iden- purple or red beds (Birt horizons), and their po- tification of horizons is sition relative to purple or red beds. It is not shown on left. possible to establish the presence of an overlying A1 or O horizon in these paleosols, perhaps owing to the very small amount of organic car- bon originally present and still remaining in the sequence and the effects of cumulization during and following periods of pedogenesis. The rela- tively great thickness of some gray beds com- pared to underlying purple/red beds may be due, at least in part, to gradual cumulization of the flood-plain surface between periods of soil

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/2/188/3380449/i0016-7606-101-2-188.pdf by guest on 24 September 2021 Figure 4. A. Eocene Alamo Creek Basalt Member of Chisos Formation (Ta), resting unconformably on thinned Javelina Formation at Sierra Aguja (section 1 of Fig. 1). B. Well-developed petrocalcic horizon (type 6) in Birt horizon of paleosol in El Picacho Formation. Pick handle is 0.5 m long. C. Pervasively mottled upper Birt horizon of channel swale paleosol in El Picacho Formation, showing krotovina and rhizoliths. Lens cap is 5 cm in diameter. D. Variation in petrocalcic nodules: a, sectioned pisolitic nodule from type 5 horizon; b, segment of type 4 rhizolith; c, botryoidal type 3 nodule; d, type 2 nodules; e, sectioned segment of type 6 hardpan; f, tuberose type 2 nodule; g, broken barite nodule showing radial structure. Scale bar is 5 cm in length. E. Burrowing in lacustrine calcareous siltstone bed. F. Petrocalcic "stringers" in C horizon of paleosol. Width of hammer head is 17 cm.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/101/2/188/3380449/i0016-7606-101-2-188.pdf by guest on 24 September 2021 Figure 5. A. Rhizoeretions and krotovina in A2 horizon of channel swale paleosol. Lens cap is 5 cm in diameter. B. Channel swale paleosol showing A2, Birt, and C horizons and petrocalcic types 2 and 4 (ca). Hammer head is 17 cm across. C. Well-developed type 5 petrocalcic horizon with coalesced masses of nodules. Hammer head is 17 cm across. D. Sequence of paleosols with several type 6 petrocalcic horizons.

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elsewhere in the sequence, on the basis of their irregularity, nodular character, clearly interstitial mode of development, and the absence of bur- rowing or associated fauna. The carbonate deposits form relatively continuous horizons and in most cases are highly indurated and resistant. Hence, these paleosol-associated carbonate accumulations are appropriately termed "petrocalcic horizons." External Morphology. Six general types of petrocalcic horizons are recognized on the basis of their external appearance and relative amount of carbonate accumulation (Figs. 4D, 5, and 6). These groups compare well with the morpho- genetic classification scheme of soil carbonate accumulations described by Gile and others (1966), as indicated below. (1) Evenly dispersed very small (millimeter scale) nodules and root casts, in many cases hav- ing irregular or diffuse boundaries with matrix, not readily separated from matrix (stage I of Gile and others, 1966). Figure 6. Variation in morphology and position of petrocalcic horizons in flood-plain paleo- (2) Evenly dispersed large (centimeter scale) sols. A. Type 2 accumulation (E in section 4, Fig. 1). B. Type 4 accumulation (A in section 3, nodules, in many cases with well-defined Fig. 1). C. Type 5 accumulation (B in section 4, Fig. 1). D. Type 6 accumulation (lower part is boundaries with matrix (stage II of Gile and shown in Fig. 4B). Scale is in meters. others, 1966). (3) Large botryoidal aggregates of nodules, as much as 10 cm in diameter (perhaps stage II units or the boundary between gray and purple cumgranular cracks are present in the nodule of Gile and others, 1966, but see below). units. (Fig. 8A). Most of the cracks are filled with (4) Large, subvertically oriented root casts, Internal Structure and Mineralogy. Nod- radially oriented bladed and blocky equant rhizocretions, and burrow fillings, generally 1 ules, present in type 1 and type 2 petrocalcic calcite spar. Some cracks that penetrate from the to 3 cm in diameter, may be 50 cm or more in horizons, are generally subspherical in form, in exterior of the nodules, and the interiors of the length (stage II of Gile and others, 1966). some cases with lumpy or tuberose projections larger central cracks, show later filling by blocky (5) Coalesced masses of nodules and/or (Fig. 4D). The bulk of the nodule is composed ferroan calcite spar. Other, later-generation rhizocretions distributed along discrete horizons of microcrystalline calcite with a massive or ir- cracks are partially filled with weakly oriented and in vertical columns, enclosing patches of regular clotted texture and no visible lamination. argillans. The central area of the largest cracks matrix; horizons may be 10 to 50 cm in thick- Most have small amounts of detrital quartz, pla- may remain open. Some nodules have a central ness (stage III of Gile and others, 1966). gioclase, and smectite embedded in the micrite "core" or an outer "rind" with darker, reddish- (6) Sheet-like "hardpan," discontinuous lay- matrix (determined both petrographically and brown coloration due to impregnation with ers or beds, generally about 10 to 15 cm thick, in by X-ray diffractometry of HCl-insoluble resi- finely disseminated iron oxides. Type 2 nodules, most cases underlain by nodular type 5 accumu- dues). In most cases, both radial and cir- exhumed from the paleosols, are the most com- lation (incipient stage IV of Gile and others, 1966). Of 118 carbonate accumulations observed in unmodified inferred 15 measured sections, 52% occur in olive beds, alluvium O and/or AI 21% in purple or red beds, and 27% in gray beds or at the boundary between a gray bed and an underlying purple bed (Figs. 6,7). Types 1 and 2 A2 gray are the most common varieties and occur throughout the section, predominantly in gray Bl purple (A2) and olive (C) units of flood-basin paleo- sols. Type 3 is rare, exhibits a unique and dis- D> B2 red tinctive internal structure and crystal morphol- B3 purple ogy, and occurs only high in the section, particularly in purple (Birt) units. Type 4 is C olive common in channel swale paleosols and may transect both gray and purple units. Type 5 commonly occurs at the boundary between gray Figure 7. Variation in development of Birt horizons observed in the study section. Such and purple units. Type 6 is known only from variation may reflect duration of pedogenesis, topographic position, distance from an active three localities high in the section, from purple channel, local sedimentation rate, or some combination of these effects.

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mon constituent of the lag gravels present in cient caliches and also to the problematical do not differ petrographically from those de- adjacent channels. micro-organisms that are thought to produce it, scribed above under type 2 and type 4 Botryoidal nodules (type 3) differ notice- whether algae (Wray, 1977), bacteria (Esteban, accumulations. Their number, however, is such ably from other nodules in their external form, 1974; Chafetz and Butler, 1980), or fungi that in many cases, they exceed the volume of and particularly in their crystal morphology (Klappa, 1978). Chafetz and Butler (1980) have the matrix (Figs. 4B and 5C). Such accumula- (Figs. 8C, 8D, and 8E). The individual nodules also shown, however, that radial calcite fabric tions form discrete horizons, generally 20 to 50 are about 1 cm in diameter; have a very smooth, can originate neomorphically by recrystalliza- cm thick, that persist laterally the full extent of regular, spherical shape (in contrast to the lumpy tion of originally micritic calcite nodules. Apart most outcrops, although the density of nodules irregular surfaces of type 2 nodules); and gener- from soil environments, similar calcite textures may vary markedly along strike. Along some ally form "grape-bunch" aggregates, 10 to 15 occur in stromatolites produced by blue-green horizons, masses of rhizocretions and irregular cm wide, in which the individual nodules re- algae (for example, Calothrix, Entophysalis, patches of coalesced nodules form radiating main fairly discrete (Fig. 4D). The interiors of Schizothrix) in some fresh-water streams and patterns 0.5 to 2 m across, suggesting the former such nodules are characterized by radial and cir- lakes (Eggleston and Dean, 1976) and in some positions of individual (see Klappa, 1980; cumgranular cracks, similar to those observed in speleothems (Thrailkill, 1976). Consequently, his Fig. 3a). other nodules, that are likewise filled by coarse the origin of the botryoidal (type 3) nodules is Sheet-like layers or beds (hardpan) of calcium bladed and equant sparry calcite, ferroan calcite, uncertain; however, the inclusion of radially ar- carbonate (type 6 accumulations) are known and argillans. The crystal terminations of pore- ranged organic filaments suggests that an origi- from only a few examples (Fig. 5D). In all cases, filling spar in some nodules exhibit thin coatings nal (not neomorphic) radial fabric was present these beds are underlain and/or overlain by of lumpy hematite and relict growth banding. and indicates some organic participation in the nodule or rhizolith accumulations comparable The bulk of the nodule is, however, composed formation of the nodules. The association of the to type 5. Although the compact portion of such of fine (width 0.2 to 0.5 mm), radial bladed nodules with paleosols, three dimensionality of a bed is 10 to 20 cm thick, it may extend calcite markedly different from the clotted or the nodules, inclusion of abundant detrital grains downward as thin discontinuous platy "string- massive microcrystalline calcite seen in other within them, and absence of nearby lacustrine ers" to a total thickness of as much as 50 cm. nodules (Figs. 8C, 8D, and 8E). The radial facies suggest that they are pedogenic. In some Internally, hardpan layers consist of masses of bladed calcite occurs in discrete bundles of crys- places, petrocalcic horizons appear to pass later- nodules several centimeters in diameter, each tals, 5 to 10 mm in width, that radiate from ally into lacustrine carbonates; hence, a lacus- with an inner unstructured zone and an outer points about the center of the nodule. Relict trine origin for type 3 nodules cannot be ruled concentrically laminated zone (Fig. 8B). In some concentric growth bands are visible within the out. cases, the laminae engulf adjacent nodules. The radial fabric. An anastomosing network of or- A wide variety of calcite rhizoliths is present, nodules are composed of microcrystalline cal- ganic inclusions and fluid-filled vacuoles tran- ranging from simple millimeter-scale root casts cite; however, radial and circumgranular cracks sects the coarse radial crystal fabric and (type 1) to complex centimeter-scale (type 4, that transect the nodules and surrounding matrix delineates a much finer fibrous radial texture. It Figs. 5A and 5B) rhizocretions (terminology of are filled with an early generation of botryoidal appears that the coarse radial bladed calcite spar Klappa, 1980). In the simplest form of root chalcedonic microquartz (clear to dark red, 7.5 originated by recrystallization of a finer fibrous trace, probably associated with fine root hairs, R 3/6, in color due to iron oxide inclusions), a calcite . Calcite of identical crystal no calcite mineralization is present, and little or later generation of coarse radial bladed and morphology is observed as a thin coating on no trace of the original root hair remains; there equant calcite spar cement in most cases with many vertebrate bones preserved in these paleo- are only thin (1 to 5 mm diameter) drab-colored associated radial barite crystal rosettes, and a sols. Such bones commonly have a coating of vermicular mottles in a darker sediment matrix, final filling of coarse crystalline megaquartz. The iron and manganese oxides that underlies the the "drab haloes" of Retallack (1983). In some early botryoidal chalcedony is growth banded fibrous calcite. In at least one instance, similar examples, both drab and iron-manganese haloes and probably resulted from crystallization of fibrous calcite occurs as a coating on the upper are present. In larger roots, a void was left, fol- opal precursor. In some nodules, void space re- surface of a thin hematite-cemented sandstone lowing decay of the root, that filled marginally mains in the larger cracks. Surrounding the nod- within the lower part of a paleosol (Birt ho- with illuviated clay and centrally with micritic ules and forming the bulk of the hardpan is rizon). That the nodules with radial-fibrous tex- calcite. In still larger features, the drab halo and thickly laminated microcrystalline and radial fi- ture were formed contemporaneously with the adjoining matrix surrounding a root have been brous calcite with a pustular or botryoidal fab- more typical (type 2) nodules is indicated by the cemented with micritic calcite, and the root ric. The pustular fabric, concentric lamination, presence of both types in channel lag accumula- mold filled with coarse bladed and equant and microquartz cement are also shown in nod- tions. In nearly all observed occurrences of this sparry calcite. Some of the larger spar-filled root ules below the hardpan layer but are absent in peculiar petrocalcic texture, it is associated with casts are lined with coarse crystalline hematite nodules not associated with such a layer. a Birt horizon, in many cases with hematite en- (some pseudomorphic after pyrite) and contain crustation and/or bone substrates. As noted above, barite nodules and crystal abundant organic inclusions, , and spores. rosettes are commonly associated with the pet- Pedogenic calcium carbonate accumulations The most complicated, and largest, rhizoliths rocalcic horizons and can also form separate with similar radial-fibrous calcite textures and show multiple concentric layers of micritic cal- accumulations (Fig. 8F). Discrete barite nodules included anastomosing organic filaments are cite, alternating with layers and cracks filled or have a central core of equant inclusion-filled described as "spherulites" by Chafetz and Butler partially filled by radial bladed spar (similar to subhedral crystals (0.3 to 0.5 mm) and patches (1980), who noted the similarity of these fea- those depicted by Klappa, 1980; his Fig. 5d). The of clay matrix, surrounded by clear radial bladed tures to those identified by others as Microco- micritic layers show alveolar texture and crystals with relict growth bands. Discrete barite dium (Esteban, 1974; Klappa, 1978). The term "Microcodium-like" features (Figs. 9E and 9F). nodules may result from decomposition and "Microcodium" is applied to a peculiar radial The coalesced and densely packed nodules translocation of volcanic ash supplied to the soil calcite structure found in many modern and an- and rhizocretions of type 5 petrocalcic horizons surface (Lehman, 1985b).

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for many of the Birt horizons. Differences in iron tion and lessivage. The strong expression of albic Figure 8. Photomicrographs showing vari- and clay content (and hence color) within a sin- and argillic horizons suggests that these paleosols ation in calcite textures in petrocalcic nod- gle Birt horizon can be used to establish Bl, B2, are not Entisols, Inceptisols, or Vertisols. Devel- ules. All are in plane-polarized light except D and B3 subhorizons in some examples (Fig. 7). opment of these soils in clay-rich parent mate- (crossed polarizers). A. Interior of typical The common lateral variation in the develop- rial, and the absence of true spodic horizons, type 2 calcite nodule, with massive microcrys- ment of Birt horizons suggests the preservation indicates that these soils are not Spodosols. talline texture and radial spar-filled cracks. of catenas. In any event, these beds represent Likewise, the preponderance of 2:1 layer clays Scale bar is 0.5 mm in length. B. Interior of subsoil horizons and not surface horizons as im- (smectite) and abundance of relatively unaltered type 5 nodule, showing pisolitic texture. plied by McBride (1974) and Turner (1980) for plagioclase in the sediment indicate that these Voids are filled with: a, drusy calcite; b, bo- similar beds in the Difunta of Mexico. soils are not highly leached (Oxisols, Ultisols). tryoidal chalcedony; c, equant calcite spar. Strong color mottling in the upper parts of These paleosols compare most favorably with Scale bar is 0.5 mm in length. C. Part of type many of these paleosols is associated with root some modern Alfisols, particularly the Paleudalf 3 botryoidal calcite nodule with fine radial "haloes," burrows, and ped exteriors. This may and Paleustalf great groups (Soil Survey Staff, fibrous calcite texture and fine anastomosing reflect gleization within the realm of a fluctuat- 1975). These are thick red soils (formerly classi- organic-filled inclusions. Scale bar is 0.5 mm ing ground-water table or less likely at the sur- fied as Red Yellow Podzolic, Red Brown, Red in length. D. Same field of view as in C, but face of waterlogged soil. Iron and/or manganese Chestnut, and Red Prairie soils) found in sub- with crossed polarizers, showing coarser ra- , pans (placic horizons), or other fea- humid and semiarid regions. Vegetation on dial calcite texture. Scale bar is 0.5 mm in tures indicative of poor drainage are absent, modern Alfisols is typically deciduous forest or a length. E. Margins of several type 3 botryoi- however. The form and occurrence of much of mixture of grasses and woody . These soils dal nodules in "grape-bunch" aggregate. the mottling suggests that it results from filling exist under relatively warm soil-temperature re- Voids are filled by: a, calcite; b, clay; c, iron of burrows or root molds or from alteration gimes: mesic, isomesic, or warmer in the case of oxides. Scale bar is 0.5 mm in length. F. produced in the chemical microenvironment Udalfs, and thermic, isothermic, or warmer in Upper surface of type 6 petrocalcic horizon around original living roots (see Retallack, the case of Ustalfs. ("up" is to right), showing radial fibrous cal- 1983). Petrified logs and in situ stumps occur sporad- cite (a) and barite crystals (b) in matrix of The lighter color and apparent depletion of ically in the Trans-Pecos flood-plain deposits detrital silt and clay. Scale bar is 0.5 mm in clay and iron oxides in gray (A2) horizons sug- and are locally common in the upper parts of length. gests that these beds may be regarded as albic channel deposits and in olive beds (relatively (E or eluvial) subsurface horizons, varying in unmodified by pedogenesis). Of 39 well-pre- their degree of development. The O and A1 served logs observed, 61% were , 31% horizons that are inferred to have overlain gray were dicotyledonous angiosperms, and 8% were (A2) beds cannot be recognized, perhaps owing palms. Most (75%) of the wood exhibits INTERPRETATION AND to the following. distinct or indistinct growth rings, whereas in COMPARISON WITH MODERN (1) Loss of surficial organic-enriched hori- contrast, only 42% of the dicot wood exhibits SOILS zons by erosion prior to development of a growth rings. Abbott (1986) explained similar suprajacent paleosol or by oxidation following observations by suggesting that the angiosperm Although many ancient soils may not be burial. wood types represent a riparian flora that did directly comparable to modern ones, the proc- (2) Impingement of the B horizon of a su- not experience seasonal variations in water esses responsible for soil formation are still prajacent paleosol that engulfs the surficial O supply, whereas the conifer woods represent a discernible in ancient soils. By interpreting pa- and A1 horizon of underlying paleosol. flora growing on higher ground or away from leosols from a process standpoint, it is possible (3) Gradual cumulization during and follow- stream courses and so subject to more marked to compare them with modern soils wherein the ing soil development. fluctuation in water supply. Angiosperm wood same processes are active. The apparent illuvial (4) Originally low organic content of O and occurs mainly in channel deposits, whereas the concentration of iron oxides and clay, and in A1 horizons, making later recognition difficult. majority of logs and stumps encountered in some instances organic carbon, in purple or red (5) Inadequate sampling for organic matter flood-plain deposits are conifers. Such a recon- (Birt) beds suggests that these layers represent in the present study. struction is in accordance with studies of other Late Cretaceous floras that suggest angiosperrn spodic or argillic subsurface horizons. The high Future, more detailed, sampling may allow dominance of channel-margin environments and clay content of the parent alluvium, as well as in the discrimination of surficial organic horizons conifer dominance of interfluve areas (Doyle the resulting soils, indicates, however, that these in these paleosols; however, apical horizons and Hickey, 1976; Parker, 1975; Hickey, 1984). may not qualify as "true" spodic horizons as are commonly not preserved in ancient soils, Vegetation of this type is also consistent with defined by the Soil Survey Staff (1975). Many and the combination of effects given above interpretation of the paleosols as Alfisols. Over- of these units do meet the requirements for an necessitates that their former presence be all, 64% of all wood observed (excluding palms) argillic horizon. The processes of podzolization inferred here. displays growth rings, suggesting that seasonal and lessivage were important in the genesis of The marked differentiation of the paleosols variation in precipitation was the rule. these paleosols, although in most cases, neither into albic A2 horizons and cambic/argillic B process was effective in producing a true spodic horizons suggests that these are relatively well- The presence of well-developed petrocalcic or even argillic horizon. Therefore, cambic ho- developed soils comparable to some modern Al- horizons in the Trans-Pecos paleosols may ap- rizon may be the most appropriate designation fisols in reflecting the activity of both podzoliza- pear somewhat incompatible with the identifica-

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the same regime as did the argillic horizons but luviation of clays and free iron oxides, under a Figure 9. Photomicrographs in plane- by a process different from that believed to humid climate, produced differentiation of albic polarized light of paleosol textures. A. Ma- occur in modern soils. and cambic/argillic horizons resulting in the trix of purple bed (Birt horizon), showing (2) The petrocalcic horizons formed under formation of Alfisols. Later pedogenesis during a dispersed aggregates of iron oxides. Scale bar the same regime as did the argillic horizons and period of semiarid or subhumid climate em- is 0.05 mm in length. B. Iron oxides filling were emplaced in the A or upper B horizons of placed the petrocalcic horizon within the upper voids between granular soil peds in purple underlying paleosols. part of the previously formed soil (Fig. 10). This (Birt) horizon. Scale bar is 0.1 mm in (3) The petrocalcic horizons formed under hypothesis is supported by the superpositioning length. C. Matrix of Birt horizon, showing: a, an entirely different climatic regime from that of calcite over earlier-formed iron oxide coatings detrital quartz and clay; b, iron oxide coatings which resulted in the argillic horizons. on bone fragments in the paleosols and by evi- on ped surfaces; c, calcite nodules. Scale bar The first possibility seems unlikely, owing to dence for precipitation of calcite nodules follow- is 0.5 mm in length. D. Iron oxide (a) and the aforementioned similarity between these and ing the precipitation of iron oxides in Birt fibrous calcite (b) coating on weathered dino- modern petrocalcic horizons in their characteris- horizons (Figs. 9C and 9D). saur bone fragment in Birt horizon. Scale bar tics and developmental stages. The second alter- If this hypothesis is correct, the Maastrichtian is 0.5 mm in length. E. Alveolar and native is possible, but in those instances where climate in the Trans-Pecos region must have os- "Microcodium-like" textures in calcite rhizo- no overlying soil profile is present, the carbonate cillated between periods of humid and semiarid cretion. Scale bar is 0.1 mm in length. accumulation is demonstrably within the A ho- character. On the basis of degree of develop- F. "Microcodium-like" texture in calcite rizon. Carbonate rhizocretions are observed in ment of the argillic and petrocalcic horizons, rhizocretion. Scale bar is 0.1 mm in length. many cases extending through the A and into each of these periods of soil formation must the B horizon. In many areas, paleosols with have been of at least several thousand years du- well-differentiated albic and cambic/argillic ho- ration (for example, Soil Survey Staff, 1975). rizons are completely devoid of calcium carbon- Such long-term climatic cyclicity has been in- ate accumulation, and correspondingly, in other ferred for Late Cretaceous time, on the basis of tion of these soils with Alfisols. Although soils of areas petrocalcic horizons are developed where limestone/shale rhythms observed in pelagic the Petrocalcic Paleustalf subgroup exhibit no argillic horizon is apparent. This suggests that marine carbonate successions (for example, comparable petrocalcic horizons, in these soils the two soil regimes existed either in different Mount and Ward, 1986; Laferriere and others, such horizons occur within or below the argillic local subenvironments, and/or at different 1987). These sedimentary cycles are thought to horizon (hence, generally designated Cca ho- times, and that the products of these two regimes reflect alternating arid and humid climatic re- rizon; Soil Survey Staff, 1975). In the paleosols are spatially superimposed. I therefore advocate gimes induced by regular variations in the described herein, petrocalcic horizons occur not the third hypothesis above, and suggest that Earth's orbit. The so-called Milankovitch cycles only in the C horizon but commonly much many or most of these paleosols are superim- include cycles with periods of 21,000 yr (pre- higher in the profile, predominantly within the posed or "compound" paleosols (sensu Bown cession), 41,000 yr (obliquity), 100,000 and A2 horizon, at the contact between the A2 and and Kraus, 1981), having formed under differ- 400,000 yr (eccentricity). It is conceivable that underlying Birt horizon, or penetrating down- ent successive climatic/environmental condi- the long-term climatic cyclicity suggested by the ward into a Birt horizon from an overlying A2 tions. According to this hypothesis, weathering Trans-Pecos paleosols may be the terrestrial horizon. As discussed above, some of these pet- of the flood-plain alluvium accompanied by il- counterpart to corresponding sedimentary cycles rocalcic horizons exhibit fairly advanced stages of calichification, including partial silicification, as well as precipitation of sulfates generally as- gray paleo-A horizon sociated with Aridosols. It seems unlikely that Uno5rbank deposits / PurPle paleo-B horizon_ many of these petrocalcic horizons formed under the same regimen of soil-forming proc- §|I esses that resulted in the albic and cambic/argil- %0 lic horizons. In modern soils, removal of Is,/- carbonate (a powerful flocculant) from upper parts of the soil profile is seen as a necessary o precursor to lessivage and podzolization, even- MI H tually resulting in an argillic or spodic subsoil horizon (Soil Survey Staff, 1975). Calcium car- bonate plays an effective role in stopping clay and iron illuviation; hence, the occurrence of a PODZOLIZATION- -CALICHIFICATION • petrocalcic horizon above an argillic horizon v HUMID PHASE SEMI-ARID PHASE runs contrary to our understanding of the genesis of an argillic horizon. Owing to this incompati- Figure 10. Interpretation of observed relationships between paleosol horizons and hypothet- bility, several hypotheses are advanced in possi- ical climatic cycles. Podzolization and lessivage produce A2 and Birt horizons during a period ble explanation. of relatively humid climate. Calichification results in the emplacement of a petrocalcic horizon (1) The petrocalcic horizons formed under high in the existing profile during a later period of relatively arid climate.

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observed in the marine realm. The absence of creases higher in the section. No clear cyclicity argillic Birt horizons. O and/or A1 horizons are any clear correlation of the proposed cycles in soil development, as described by Kraus absent, or cannot be identified, possibly owing within the Trans-Pecos region, however, allows (1987), is apparent. In some areas, relatively to erosion or cumulization and subsequent su- no substantiation for this possibility at present. unmodified alluvium (olive beds) accumulated perimposed pedogenesis. Fossil logs and in situ to thicknesses of 20 or 30 m without notable stumps suggest that the soils supported conifer- RELATIONSHIP OF FLOOD- modification by soil formation (Fig. 1). Such ous forests and that areas around stream courses PLAIN PALEOSOLS TO areas occur primarily in the lower part of the supported angiosperm woodlands. Such a vege- ADJACENT CHANNELS sequence and in the vicinity of (lateral to) large tative cover is compatible with the identification channel sand bodies. Paleosols in such deposits of these soils as Alfisols. Growth rings in both In some smaller (tributary) channel deposits, show little or no evidence of clay or iron illuvia- conifer and angiosperm wood types indicate sea- where the channel margins can be directly ob- tion but have well-developed type 2 or type 4 sonality in precipitation. The presence of well- served in outcrop, it is possible to describe the petrocalcic horizons. Olive beds also intertongue developed petrocalcic horizons in the upper relationship between the flood-plain paleosols with marginal channel deposits. The presence of parts of many of the paleosols indicates that and adjacent channel. In such instances, the top petrocalcic horizons suggests that thick succes- many are "compound" paleosols, having formed of the channel coincides laterally with the top of sions of olive sediments were subject to periods under at least two different climatic regimes. It is a gray (A2) horizon (Fig. 11). The underlying of weak soil development; however, such epi- proposed herein that the Maastrichtian climate purple or red (Birt) horizon ends abruptly sodes left no apparent argillic horizon develop- of the Trans-Pecos region fluctuated between against the margin of the channel. An overlying ment. This suggests that depositional topography humid or subhumid periods, resulting in the purple (Birt) horizon rests on top of the channel or sedimentation rate may have influenced soil formation of Alfisols, and semiarid periods, re- fill, although it is irregularly developed directly maturation in the manner described by Bown sulting in the emplacement of petrocalcic hori- over the channel compared with the same ho- and Kraus (1987) or that the processes of calcite zons in the upper parts of the existing soils. The rizon traced laterally into the flood plain. This is precipitation and clay/iron illuviation were in developmental stages of the argillic and petrocal- perhaps due to greater permeability of the un- some other way decoupled. cic horizons suggest that each episode of soil derlying channel sands. These observations formation must have been at least several thou- It is very difficult to correlate individual pa- suggest the following sequence of events. sand years in duration. Such climatic cyclicity leosols beyond a given outcrop or group of (1) Channel incision occurred following, or less has previously been inferred for Late Cretaceous nearby outcrops (several kilometers). Groups of likely concurrently with, development of the first time on the basis of marine carbonate cycles. several closely spaced distinctive paleosols may, paleosol. (2) Channel and flood-plain aggrada- however, be correlated long distances, although Paleosols have also been described from tion occurred. (3) A second paleosol developed the number of soils within the group may vary. Maastrichtian fluvial flood-plain deposits in the on the newly aggraded flood-plain surface. The lack of continuity of individual paleosols of Montana and North Paleosols that developed in swales on the suggests that the flood-plain surfaces on which (Fastovsky and McSweeney, 1987). upper surfaces of large channel bars are, in they formed were topographically diverse and These paleosols developed contemporaneously general, coarser in texture than are flood-plain probably marked by low rounded hills. Periodic with those in the Trans-Pecos region. Paleosols paleosols. Such paleosols also exhibit more incision of the flood plain, as evidenced by the in the Hell Creek deposits have somber colors, abundant color mottling, both as drab root "ha- small channels described above, may have been gley morphologies, and abundant well-devel- loes" and as distinct burrows, and have more responsible for producing this topography. oped organic accumulations (O horizons), indi- prominent large discrete calcareous rhizocre- cating the presence of high water tables, poor tions (Fig. 2F, 4C). The Birt horizons of the CONCLUSIONS drainage, and local water-logged swampland in-channel paleosols do not attain colors as red conditions (Fastovsky and McSweeney, 1987). as flood-plain paleosols, perhaps owing to their Maastrichtian paleosols in the Trans-Pecos The marked differences in paleosols between the coarser texture and better drainage. region of Texas are comparable to some modern northern interior region and West Texas suggest In some areas, as many as six successive Alfisols (Udalfs, Ustalfs) and reflect the activities that pronounced climatic and environmental dif- flood-plain paleosols may be delineated between of both podzolization and lessivage. These pa- ferences existed between these regions in latest episodes of major channel sedimentation. The leosols are well differentiated into light gray Cretaceous time. lateral continuity of individual paleosols in- albic A2 horizons and purple or red cambic or ACKNOWLEDGMENTS

A2 I thank Scott Kelley, Francis Zimmer, Martin Sander, and Neal LaFon for their help with field work during this study. Richard Sanders, Melanie Barnes, James Browning, and Gary Strathearn helped perform and interpret the chemical analyses, and Mike Gower prepared excellent pétrographie thin sections from poorly indurated material. The officials of Big Bend Na- Figure 11. Field sketch illustrating the relationship between flood-plain paleosols and a small tional Park and the owners of the Mine (tributary) channel deposit, northwest of Paint Gap Hills (location F in section 8, Fig. 1). Ranch allowed access to exposures and supplied

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permission to collect lithologic samples. Prelim- Eggleston, J. R.t and Dean, W. E., 1976, Freshwater stromatolitic bioherms in imentary Petrology, v. 44, p. 760- 773. Green Lake, New York, in Walter, M. R., ed., Stromatolites: Develop- Mount, J. F., and Ward, P., 1986, Origin of limestone/ alternations in the inary work for this project was done as part ments in sedimentology 20: Amsterdam, the Netherlands, Elsevier, upper Maastrichtian of Zumaya, Spain: Journal of Sedimentary Petrol- p. 479-488. ogy, v. 56, p. 228-236. of a dissertation under the supervision of Gary Esteban, M., 1974, Caliche textures and Microcodium: Bollettino della Societa Parker, L. R., 1975, The paleoecology of the fluvial coal-forming swamps and Kocurek and Wann Langston, Jr., at the Uni- Geologica Italians, v. 92 (Suppl.), p. 105-125. associated floodplain environments in the Fastovsky, D. 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