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Upper Cretaceous (Maastrichtian) Paleosols in Trans-Pecos Texas

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Upper Cretaceous (Maastrichtian) Paleosols in Trans-Pecos Texas Upper Cretaceous (Maastrichtian) paleosols in Trans-Pecos Texas 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 Aguja Formation 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 olive, gray, compares them with similar modern soils. Al- tion during Late Cretaceous (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. Campanian to lower Tertiary section, only those sols are characterized by pale gray leached Calcareous nodules are abundant in the mud- developed within Maastrichtian sediments 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 "series" (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 dinosaurs—the Alamosaurus calcite nodules, coalesced masses of nodules, Tertiary section (for example, Udden, 1907; fauna—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 dinosaur 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 climate in this re- gion may have fluctuated between regimes of humid and semiarid character, each at least several thousand years in duration. This in- ferred long-term climatic cyclicity may cor- respond to climatically induced sedimentation cycles observed in the marine realm that are ascribed to Milankovitch cycles. Fossil wood suggests that the flood-plain paleosols sup- ported coniferous forests, whereas adjacent stream 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 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 UPPER CRETACEOUS PALEOSOLS, TRANS-PECOS, TEXAS 189 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 faunas. part of the Aguja Formation ("upper shale 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 North America (the Triceratops 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 Paleocene 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 Eocene basalt flows. A thick ert Mountain Overlook; 3, Rattle- tions (Wolleben, 1966). In Brewster County, the paleosol of probable Eocene age is developed snake 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. 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 UPPER CRETACEOUS PALEOSOLS, TRANS-PECOS, TEXAS 191 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 limestone (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).
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