Paleoenvironmental Record and Applications of Calcretes and Palustrine Carbonates (Special Paper) by...

Special Paper 416 THE GEOLOGICAL SOCIETY OF AMERICA

Paleoenvironmental Record and Applications of Calcretes and Palustrine Carbonates

edited by

Ana Mara Alonso-ZarzaDepartamento Petrologa y Geoqumica

Facultad de Ciencias GeolgicasUniversidad Complutense de Madrid

28040 MadridSpain

Lawrence H. TannerDepartment of Biological Sciences

Le Moyne CollegeSyracuse, New York 13214

USA

3300 Penrose Place, P.O. Box 9140 Boulder, Colorado 80301-9140, USA

2006

Special Paper 416

ii

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Library of Congress Cataloging-in-Publication Data

Paleoenvironmental record and applications of calcretes and palustrine carbonates / edited byAna Mara Alonso-Zarza, Lawrence H. Tanner. p. cm.--(Special paper; 416) Includes bibliographical references and index. ISBN-10 0813724163 (pbk.) ISBN-13 9780813724164 (pbk.) 1. Calcretes. 2. Rocks, Carbonate. 3. Paleopedology. I. Alonso-Zarza, Ana Mara, 1962-. II. Tanner, Lawrence H. III. Special papers (Geological Society of America) ; 416.

QE471.15.C27.P35 2007552/.58--dc22 2006041338

Cover: View of laterally continuous pedogenic calcretes in the Upper Triassic (Norian) Owl Rock Formation (Chinle Group), northern Arizona. Photo by L.H. Tanner. Back cover: Recent vertical calcrete formed by the penetration of tree roots on Miocene deposits of the Madrid Basin, Guadalajara, Spain. Photo by A.M. Alonso-Zarza.

10 9 8 7 6 5 4 3 2 1

Contents

iii

Preface .........................................................................................................................................................v

Ancient Landscapes, Climate and Sequence Boundaries

1. Calcic pedocomplexesRegional sequence boundary indicators in Tertiary deposits of the Great Plains and western United States ...................................................................................1D.L. Hanneman and C.J. Wideman

2. A Late Triassic soil catena: Landscape and climate controls on paleosol morphology and chemistry across the Carnian-age IschigualastoVilla Union basin, northwestern Argentina .....17N.J. Tabor, I.P. Montaez, K.A. Kelso, B. Currie, T. Shipman, and C. Colombi

3. Investigating paleosol completeness and preservation in mid-Paleozoic alluvial paleosols: A case study in paleosol taphonomy from the Lower Old Red Sandstone .......................................43S.B. Marriott and V.P. Wright

4. Calcareous paleosols of the Upper Triassic Chinle Group, Four Corners region, southwestern United States: Climatic implications .................................................................................................53L.H. Tanner and S.G. Lucas

5. Estimates of atmospheric CO2 levels during the mid-Turonian derived from stable isotope composition of paleosol calcite from Israel ......................................................................................75A. Sandler

6. Pedogenic carbonate distribution within glacial till in Taylor Valley, Southern Victoria Land, Antarctica ..................................................................................................................89K.K. Foley, W.B. Lyons, J.E. Barrett, and R.A. Virginia

Sedimentary Environments and Facies

7. Calcretes, oncolites, and lacustrine limestones in Upper Oligocene alluvial fans of the Montgat area (Catalan Coastal Ranges, Spain) .......................................................................105D. Parcerisa, D. Gmez-Gras, and J.D. Martn-Martn

8. The role of clastic sediment infl ux in the formation of calcrete and palustrine facies: A response to paleographic and climatic conditions in the southeastern Tertiary Duero basin (northern Spain) ..............................................................................................................................119I. Armenteros and P. Huerta

9. The Upper Triassic crenogenic limestones in Upper Silesia (southern Poland) and their paleoenvironmental context ............................................................................................................133J. Szulc, M. Gradzinski, A. Lewandowska, and C. Heunisch

iv Contents

10. A recent analogue for palustrine carbonate environments: The Quaternary deposits of Las Tablas de Daimiel wetlands, Ciudad Real, Spain ................................................................153A.M. Alonso-Zarza, M. Dorado-Valio, A. Valdeolmillos-Rodrguez, and M. Blanca Ruiz-Zapata

11. Depositional conditions of carbonate-dominated palustrine sedimentation around the K-T boundary (Facis Rognacien, northeastern Pyrenean foreland, southwestern France) ..............169D. Marty and C.A. Meyer

12. Reworked Microcodium calcarenites interbedded in pelagic sedimentary rocks (Paleocene, Subbetic, southern Spain): Paleoenvironmental reconstruction ...................................................189J.M. Molina, J.A. Vera, and R. Aguado

Dating of Calcretes: Applications

13. Calcite cement stratigraphy of a nonpedogenic calcrete in the Triassic New Haven Arkose (Newark Supergroup) ......................................................................................................................203E.T. Rasbury, E.H. Gierlowski-Kordesch, J.M. Cole, C. Sookdeo, G. Spataro, and J. Nienstedt

14. Calcrete features and age estimates from U/Th dating: Implications for the analysis of Quaternary erosion rates in the northern limb of the Sierra Nevada range (Betic Cordillera, southeast Spain) ...............................................................................................................................223J.M. Azan, P. Tuccimei, A. Azor, I.M. Snchez-Almazo, A.M. Alonso-Zarza, M. Soligo, and J.V. Prez-Pea

vPreface

The study of ancient soils continues at an accelerating pace as more geologists recognize the value of these ancient land surfaces as archives of important paleotopographic, paleoenvironmental, and paleoclimatic information. Indeed, a survey of one database yields over 600 citations containing the keyword paleosol for just the fi rst half of this decade, compared to only one-fourth this number from the fi rst half of the 1990s! Not all of these publications presented detailed descriptions and interpretations of paleosols, certainly, but many were broader studies that incorporated the description of ancient soil surfaces into examinations of tectonics, basin evolution, sedimentary processes, or climate change. Clearly, the variety of paleosols and their potential applications to geological problems is enormous. Given the breadth of this subject, we chose to focus this volume on the topic of calcretes and the closely related subject, palustrine carbonates.

Calcretes are perhaps the most commonly described of paleosols, owing to their ready preservation in the rock record and relative ease of recognition. The term calcrete, synonymous with caliche, is widely applied, although it is neither the name of a soil order nor of a soil horizon. In a broad sense, calcretes are, as proposed by Watts (1980, p. 663; after Goudie, 1973), terrestrial materials composed dominantly, but not exclusively, of CaCO3, which occurs in states ranging from nodular and powdery to highly indurated, and result mainly from the displacive and/or replacive introduction of vadose carbonate into greater or lesser quantities of soil, rock, or sediment within a soil profi le. This defi nition was restricted to calcretes of pedogenic origin, however Wright and Tucker (1991) later expanded the term calcrete to include, as initially recommended by Netterberg (1980), the effects of shallow groundwater. This broader sense sug-gests the importance of the interaction between sediments undergoing active pedogenesis and shallow groundwaters.

Palustrine carbonates exhibit many similarities with calcretes. As described by Freytet (1984, p. 231), a palustrine limestone must show the characteristics of the primary lacustrine deposit (organisms, sedi-mentary features) and characteristics due to later transformations (organisms, root traces, desiccation, pedo-genic remobilizations). Palustrine carbonates are common in alluvial sequences, often in association with calcretes, but their widespread recognition has been attained more slowly. Indeed, much of the research on alluvial carbonates has focused exclusively on either palustrine carbonates or calcretes, when in fact there is often a spatial transition from one to the other, revealing an interplay between pedogenic, sedimentary, and diagenetic processes.

Indisputably, these deposits contain information that is signifi cant to the interpretation of the sedimen-tary record and the evolution of the landscape in both recent and ancient settings (Alonso-Zarza, 2003). These terrestrial carbonates are widely distributed on fl oodplains and in the distal reaches of alluvial basins. Their presence and characteristics can be used as indicators of aggradation, subsidence or changing accom-modation rates, and therefore as indicators of different tectonic regimes. Although calcretes and palustrine carbonates are both commonly associated with semiarid climates, more detailed climatic information can be obtained from the depths of the carbonate-bearing horizons within paleosol profi les and from the oxygen isotope signature of the carbonate. The carbon-isotope composition, on the other hand, has been used quite successfully to track changes in atmospheric pCO2 through the Phanerozoic. Vegetation is important to the formation of many of these types of carbonates, and data on the prevailing vegetation may be obtained some-times from the analysis of the micro- and macrofabric of the carbonate.

This volume was inspired by a technical session on the topic of calcretes and palustrine carbonates (chaired by us) that was held at the 32nd International Geological Congress in Florence in August 2004. Six of the contributions presented here were fi rst delivered at this meeting, and the volume grew with additional

vi Preface

contributions that provided both a broad overview of calcretes and palustrine carbonates and the state of the art of their application. The papers presented here cover a wide array of environmental settings and ages of deposits where calcretes and palustrine occur. Moreover, the papers included in this monograph discuss a number of interesting applications, including: a possible modern analogue for palustrine carbonates, the interplay between palustrine, pedogenic and diagenetic processes, the utility of radio-isotopic dating of pedo-genic carbonates and its application to understanding the evolution of recent landscapes, the reconstruction of a diagenetic sequence, and the climatic and geomorphic controls on calcrete formation. The papers have been arranged in three groups.

Papers that apply calcretes and palustrine carbonates to the reconstruction of ancient landscapes, climate and sequence boundaries comprise the fi rst group. Hanneman and Wideman illustrate the utility of calcic pedocomplexes in delineating regional unconformities that are large-scale sequence boundaries. Their study, focused on the Tertiary of the Great Plains, shows that these pedocomplexes have distinct physical proper-ties that allow their identifi cation in seismic sections and well logs. Tabor and co-authors demonstrate that the distribution of the fl uvial channel sandstones and the characteristics of the paleosols are both controlled by geomorphic evolution during deposition of the Triassic Ischigualasto Formation in northwest Argentina. The preservational bias in paleosol formation is described in the contribution by Marriott and Wright. These authors analyzed mid-Paleozoic paleosols from the Lower Old Red Sandstone and show that reactivated, truncated cumulate horizons provide a means of assessing the dynamics of fl oodplains, including those from before the advent of rooted vascular plants in the mid-Paleozoic. The fourth paper, by Tanner and Lucas, relates the potential climatic control on the morphology of Upper Triassic paleosols in the Chinle Group of the southwestern United States. Temporal changes in the types of paleosols and the maturity of calcretes suggest a gradual aridifi cation across the Colorado Plateau during the Late Triassic. Sandler uses the isotopic composition of Mid-Turonian paleosol carbonate to estimate the atmospheric pCO2 level for this interval. His results, which indicate high mid-Turonian pCO2, correspond with the high temperatures that prevailed at that time. The last paper of this group, by Foley and co-authors, demonstrates that the relatively low carbon-ate concentrations in Antarctic polar desert soils can be attributed to the shallow active layer, low rates of weathering, and the extreme aridity of the landscape. Moreover, the differences in CaCO3 concentrations in these soils correlate with landscape position with respect to elevation and distance from the coast.

Six papers dealing with the sedimentary environments and facies of calcretes and palustrine carbonates comprise the second group. These papers provide an overview of the interrelationships between calcretes and palustrine carbonates in terrestrial environments, focusing on their similarities and on problems in their interpretations. Notably, some papers discuss the lack of a recent analogue for ancient palustrine carbon-ates. In the fi rst contribution to this group, Parcerisa and co-authors analyze the geochemistry of calcretes, oncolites and lacustrine limestones formed during the Upper Oligocene in two coalescent alluvial fans. They fi nd that the trace element and isotopic composition of the limestones were controlled mainly by the fl uvial regime and the lithology and altitude of the catchment areas in the sedimentary basin. Armenteros and Huerta studied calcretes and associated palustrine of the southeastern Tertiary Duero basin. The characteristics of both carbonate facies indicate their accumulation in semiarid climates with scarce clastic sediment supply, and that meter-scale cyclicity of the carbonate and siliciclastic sediments was controlled mostly by climate. The interrelationship between spring, fl uvial, palustrine, and pedogenic facies is discussed by Szulc and col-laborators in their study of the Upper Triassic freshwater carbonates from the Upper Silesian basin. These carbonates were deposited within a shallow swampy depression, fed by springs of deep-circulating ground-water. Alonso-Zarza and co-authors focused their study on a recent core in Las Tablas de Daimiel, Spain, one of the few freshwater wetlands preserved in southern Europe. Their studies of the core, including mineral-ogy, petrography, stables isotopes and pollen analyses indicates that these sediments are similar of those of ancient palustrine sequences, suggesting that Las Tablas is a suitable recent analogue for freshwater palus-trine sequences. Marty and Meyer analyze in detail a palustrine sequence (Facis Rognacien) encompassing the K-T boundary in southwestern France. The facies association indicates a seasonal, palustrine wetland system, with ephemeral ponds surrounded by vegetated areas of freshwater marshes under subarid to inter-mediate climates. The last paper of this group, by Molina and co-authors, describes an unusual occurrence of various types of calcarenites containing reworked Microcodium prisms. Their study of Paleocene marine deposits from southern Spain indicates that the Microcodium was reworked from exposed inland areas, thus providing evidence of emersion and clarifying the palebathymetry of the adjacent pelagic deposits.

Preface vii

The fi nal section contains two papers on different types of calcretes to which radio-isotopic dating techniques have been applied. Rasbury and co-authors describe the importance of cement stratigraphy to the application of U-Pb dating of calcite in Triassic calcretes from the New Haven Arkose, and demonstrate further that this analysis is a useful tool for distinguishing between pedogenic versus nonpedogenic calcrete. U-Th dating of calcretes is used to constrain the evolution of the Quaternary Ranges in the Betic Cordillera by Azan and co-authors. These authors describe how rapid incision by the rivers, and subsequent capture of the Guadix by the Guadalquivir River is constrained by initial dissection of a calcrete layer dated at 42 ka. This age is used to calculate the incision and erosion rates, demonstrating once again that calcretes play a major role in the evolution of landscape in many arid and semiarid regions.

This collection of papers in its fi nal form would not have been possible without the work of the review-ers who dedicated their time to careful reviews and revisions. We were truly lucky to have the help of the fol-lowing colleagues: J. Andrews, C. Arenas, B. Barclay, J. Bockheim, G. Bowen, Ll. Cabrera, J. Casanova, E. Cheney, C. De Wet, S. Dunagan, M.A. Garca del Cura, P. Ghosh, R. Goldstein, A.D. Harvey, M. Joeckel, A. Kosir, J. Lpez, G. Marion, A. Martn-Algarra, P. McCarthy, D. Nash, R. Palma, T. Peryt, N. Platt, G. Retal-lack, D. Royer, Y. Snchez-Moya, P.G. Silva, A.R. Soria, R. Swennen, M. Talbot, S.K. Tandon, A. Trav, D. Valero-Garcs, D. Varrone, and J. Wilkinson. Our sincere thanks also go to our departments: Departamento de Petrologa y Geoqumica de la Universidad Complutense de Madrid and the Department of Biological Sciences of Le Moyne College. We also have a special remembrance for F. Calvet, one of the pioneers in the studies of calcretes in Spain who passed away a few years ago. His ideas are tangibly present throughout this volume.

We hope the reader fi nds this collection of papers both stimulating and informative. This collection will, ideally, constitute a base for understanding how calcretes and palustrine carbonates form an integral part of ancient and recent landscapes and contribute to the broader knowledge of continental basins and their geo-morphic features.

REFERENCES CITED

Alonso-Zarza, A.M., 2003, Palaeoenvironmental signifi cance of palustrine carbonates and calcretes in the geological record: Earth-Science Reviews, v. 60, p. 261298, doi: 10.1016/S0012-8252(02)00106-X.

Freytet, P., 1984, Les sdiments lacustres carbonats et leurs transformations par mersion et pdognse: Importance de leur identi-fi cation pour les reconstitutions palogographiques: Bulletin Centres Rechercher Exploration-Production Elf-Aquitaine, v. 8, no. 1, p. 223246.

Goudie, A.S., 1973, Duricrusts in Tropical and Subtropical Landscapes: Clarendon, Oxford, 174 p.Netterberg, F., 1980, Geology of southern African calcretes: 1. Terminology, description, macrofeatures and classifi cation: Transac-

tions of the Geological Society of South Africa, v. 83, p. 255283.Watts, N.L., 1980, Quaternary pedogenic calcretes from the Kalahari (southern Africa): mineralogy, genesis and diagenesis: Sedi-

mentology, v. 27, p. 661686.Wright, V.P., and Tucker, M.E., 1991, Calcretes: an introduction, in Wright, V.P., and Tucker, M.E., eds., Calcretes: IAS Reprint

series 2, Oxford, Blackwell Scientifi c Publications, p. 122.

Ana M. Alonso-ZarzaLawrence H. Tanner

1Hanneman, D.L., and Wideman, C.J., 2006, Calcic pedocomplexesRegional sequence boundary indicators in Tertiary deposits of the Great Plains and western United States, in Alonso-Zarza, A.M., and Tanner, L.H., eds., Paleoenvironmental Record and Applications of Calcretes and Palustrine Carbonates: Geological Society of America Special Paper 416, p. 115, doi: 10.1130/2006.2416(01). For permission to copy, contact [email protected]. 2006 Geological Society of America. All rights reserved.

Geological Society of AmericaSpecial Paper 416

2006

Calcic pedocomplexesRegional sequence boundary indicators in Tertiary deposits of the Great Plains and western United States

Debra L. HannemanWhitehall Geogroup, Inc., Whitehall, Montana 59759, USA

Charles J. WidemanProfessor Emeritus, Montana Tech of the University of Montana, Butte, Montana 59701, USA

ABSTRACT

Calcic pedocomplexes are associated with regional unconformities in the Great Plains and western United States that have approximate ages of 30 Ma, 20 Ma, and 4 Ma. In southwestern Montana, the calcic pedocomplexes are readily identifi able on the surface, and a pedocomplex typically contains several partial soil profi les. In the most complete scenario, an individual profi le may contain an argillic or argil-lic/calcareous (Bt or Btk) horizon, a K horizon, and a C horizon. Often, however, the Bt(k) horizon is truncated or can be entirely absent from an individual profi le. The K horizon contains an upper laminated zone that is underlain by an indurated carbon-ate sheet. Carbonate nodules and chalky micritic matrix materials underlie the sheet carbonate. The calcic paleosols display carbonate morphology ranging from stage IV to stage VI.

The calcic pedocomplexes also possess distinct physical properties that aid in subsurface identifi cation. The combined density and velocity differences between paleosols and nonpedogenic strata result in bright refl ections on seismic sections and distinct well-log signatures.

Although the calcic pedocomplexes and regional unconformity associations were fi rst described within Tertiary strata of southwestern Montana, the same associa-tions exist in numerous localities in the Great Plains and in other parts of the western United States. The extensive occurrence of the calcic paleosols and regional uncon-formity associations throughout this large area underscores their utility as a regional correlation tool. Moreover, the delineation of regional unconformities that are large-scale sequence boundaries by pedocomplexes has broad implications for continental sequence stratigraphy.

Keywords: calcic, paleosol, sequence, Tertiary, pedocomplex.

RESUMEN

En las Great Plains y oeste de Estados Unidos, los complejos edfi cos clcicos estn asociados con las discontinuidades regionales cuyas edades aproximadas son: 30 Ma, 20 Ma, y 4 Ma. En el suroeste de Montana, estos edafocomplejos clcicos se observan fcilmente en afl oramientos de superfi cie y contienen varios perfi les edfi cos

2 Hanneman and Wideman

sols in the Cenomanian Dunvegan Formation of British Colum-bia to defi ne sequence boundaries. Weissmann et al. (2002) marked sequence boundaries in Quaternary Kings River alluvial fan strata near Fresno, California, by laterally extensive, mod-erately mature paleosols and incised valley bases. Demko et al. (2004) used laterally continuous, mature paleosols to delineate regional unconformities within the Jurassic Morrison Formation of the U.S. Western Interior.

Specifi cally for calcic paleosols, Gulbranson (2004) noted that calcretes within the Chinle Formation of the southwestern United States signify unconformities and delineate a terres-trial sequence stratigraphy for members of the Chinle Forma-tion. Tandon and Gibling (1997) observed pedogenic nodular and underlying groundwater calcretes at sequence boundaries in Upper Carboniferous cyclothems in the Sydney Basin of Atlantic Canada.

The purpose of this paper is to initially describe the calcic pedocomplexes in Tertiary basin fi ll of southwestern Montana. Because the calcic pedocomplexes do delineate regional uncon-formities, we will then detail their use as sequence boundary indi-cators in continental strata. The utility of using calcic pedocom-plexes as sequence boundary markers will be further enhanced by documenting their existence within Tertiary strata of the Great Plains and western United States.

CALCIC PEDOCOMPLEXES

Calcic paleosol pedocomplexes typically occur within the Tertiary basin fi ll of many valleys in southwestern Montana

incompletos. En los casos en los que estos complejos edfi cos estn ms completos, un perfi l individual puede contener un horizonte arglico (Bt) o arglico/calcreo (Bt[k]), un horizonte K, y un horizonte C. Sin embargo, a menudo el horizonte Bt(k) est truncado o puede estar ausente totalmente en un perfi l determinado. El horizonte K tiene una zona superior laminada que se sita por encima de una capa carbontica endurecida. Por debajo de la capa dura se reconocen ndulos carbonticos y mate-rial micrtico pulverulento. Los paleosuelos carbonticos tienen estadios morfolgicos que varan entre IV y VI.

Los complejos edfi cos clcicos tambin presentan propiedades fsicas que facili-tan su identifi cacin en el subsuelo. Las combinacin de las variaciones de densidad y velocidad en paleosuelos y estratos sin paleosuelos da lugar a refl exiones importantes en los perfi les ssmicos y a rasgos distintivos en sondeos.

Si bien la asociacin entre complejos pedoclcicos y las discontinuidades region-ales se describi por primera vez en estratos Terciarios del suroeste de Montana, estas mismas asociaciones se reconocen tambin en muchas otras zonas de las Great Plains y en otras partes del oeste de Estados Unidos Norteamericanos. La frecuente presen-cia de estas asociaciones en una zona tan amplia indica su utilidad como herramienta de correlacin regional. Adems, la delimitacin de las discontinuidades regionales que constituyen lmites de secuencias de gran escala, y que estn marcados por estos complejos edfi cos, tiene implicaciones importantes para aplicar en la estratigrafi a secuencial de cuencas continentales.

Palabras clave: paleosuelos clcicos, secuencias, Terciario, edafocomplejos.

INTRODUCTION

Tertiary continental strata of the Great Plains and western United States typically contain a multitude of various types of paleosols. In southwestern Montana, Tertiary paleosols com-monly contain cambic, argillic, and calcic horizons; oxic hori-zons occur only within the basal portions of the Tertiary section (Hanneman, 1989). Of particular interest within these Tertiary continental deposits are calcic paleosols. Because of a marked climatic change to drying and cooling conditions within much of this area from ca. 33 Ma to ca. 4 Ma (Prothero, 1994, 1998; Wing, 1998; Retallack, 1992, 1998; Retallack et al., 2000), calcic paleosols commonly occur throughout the age equivalent part of the Tertiary section.

Calcic paleosols with carbonate morphology stages IV and V occur within pedocomplexes at particular times within Ter-tiary basin fi ll of southwestern Montana. These times equate to regional unconformities in the northwestern United States that occurred at ca. 30 Ma, 20 Ma, and 4 Ma (Hanneman and Wide-man, 1991; Hanneman et al., 1994, 2003). Consequently, these pedocomplexes mark sequence boundaries within continental Tertiary strata in southwestern Montana (Hanneman and Wide-man, 1991; Hanneman et al., 1994); the sequence boundaries noted in southwestern Montana have recently been extended into central Washington (Hanneman et al., 2003) using criteria other than unconformity-bounding paleosols.

The concept of using paleosols to defi ne sequence boundar-ies in nonmarine strata has also recently been applied to other geologic settings. McCarthy et al. (1999) used interfl uve paleo-

Calcic pedocomplexes 3

(Fig. 1). The calcic pedocomplexes contain at least two calcic paleosols that are generally separated by small thicknesses of C horizon material. We defi ne calcic paleosols informally as paleo-sols that have a large amount of secondary carbonate present in the form of calcic horizons (Machette, 1985). Although calcic paleosols have been placed into classifi cations such as Aridosols (Retallack, 1993), Calcisols (Mack et al., 1993), or paleo-Ari-dosols (Nettleton et al., 2000), we have not yet identifi ed an A horizon within individual profi les of the southwestern Montana paleosol stacks, and there is typically, at best, only a truncated part of a B horizon within the profi les. Gardner et al. (1992) also noted the absence of the A and B horizons in Neogene calcic paleosol stacks of western Nebraska. These authors suggested that their absence may result from several factors such as: (1) the horizons generally not being well developed or very thick in some Aridosols, (2) the upward growth of the calcic horizon may

overprint the A and B horizon, and (3) the A and B horizons are more prone to erosion than petrocalcic horizons are. In any event, with the absence of a diagnostic surface horizon in the paleosol profi le, we fi nd that calcic paleosols is the most appropriate term for these paleosols.

In former publications, we referred to the vertical confi gura-tion of calcic paleosols that we observed in southwestern Mon-tana as calcic paleosol stacks (Hanneman and Wideman, 1991; Hanneman et al., 1994, 2003). However, instead of the term paleosol stack, we now prefer to use the term calcic pedo-complexes in accordance with the defi nition for pedocomplex as proposed to the Paleopedology Commission of International Union for Quaternary Research (INQUA). The proposed defi ni-tion states that a pedocomplex is composed of two or more paleo-sols that are separated over large areas by a thin deposit of C hori-zon material, and are overlain and underlain by greater amounts

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of strata that contain weak to no evidence of soil development (Catt, 1998). Additionally, individual paleosols within a pedo-complex often are discontinuous, being in places truncated or cut out by small disconformities and/or amalgamated with other paleosols (Morrison, 1998, p. 31). The term pedocomplex is synonymous with other terminology used in paleosol studies, such as compound and multistory paleosols.

Figure 2 depicts calcic pedocomplexes found in some val-leys of southwestern Montana. Each pedocomplex (Figs. 2A and 2C) contains at least two calcic paleosols and occurs between thick sections of nonpedogenically modifi ed strata. And, as pre-viously noted, individual paleosols (Figs. 2B, 2D, and 2F) may be discontinuous and/or amalgamated (amalgamated is syn-onymous with the terms welded and composite; see North American Commission on Stratigraphic Nomenclature, 1983; Morrison, 1998, p. 31) even when traced laterally over short dis-tances. Nonetheless, the pedocomplex itself may be traced over a considerable distance.

Surface Calcic Pedocomplex Paleosol Profi les

The pedocomplexes characteristically contain several partial soil profi les. An individual profi le may include in the most ideal-ized scenario, in descending order, (1) an argillic (Bt) horizon, (2) an argillic/calcareous (Btk) horizon, (3) a K horizon, and (4) a C horizon (Fig. 3A).

An argillic diagnostic subsurface B horizon (Bt) may be present in an individual soil profi le of a pedocomplex. Bt hori-zons contain blocky structure; illuviated clays form bridges between grains and coat ped faces. The majority of Bt horizons in southwestern Montana Tertiary deposits are developed within tuffaceous mudstone, and thus their color range is very similar to pedogenically unmodifi ed mudstone beds with very pale brown (10YR 5/4) to yellow gray (10Y 5/2) colors. In a few sections, Bt horizons are developed on sandy parent material, and the color range is more varied, from light reddish brown (5YR 6/3) to light brown (10YR 7/3). Root traces are common within the Bt hori-zon. Although the root casts and rhizoconcretions are typically calcareous, they may be also be composed of silica or sediments. Where these root structures are calcareous and are numerous, the horizon is better termed a Btk horizon. Root traces are from 0.1 cm to 2 cm in diameter and range up to 30 cm in length. The Bt(k) horizon is commonly truncated within the pedocomplex and can be entirely absent from a soil profi le within the pedocom-plex. However, where the horizon is preserved, it has a maximum observed thickness of 0.3 m.

The K horizon (Fig. 3B) is the locus of secondary carbon-ate accumulation within the profi le. As originally noted by Gile et al. (1965, p. 74) the carbonate is present as an essentially continuous medium. It coats or engulfs, and commonly sepa-rates and cements skeletal pebbles, sand, and silt grains. This type of carbonate is a K-fabric, and according to the defi nition originally set forth by Gile et al. (1965), a K horizon must have more than 90% K-fabric. Even though the K horizon has never

been formally accepted as a master horizon into Soil Taxonomy (Soil Survey Staff, 1975), we fi nd it extremely helpful for use in separating the more weakly developed calcic horizons (Bk) from those horizons with major authigenic carbonate accumulations.

The uppermost part of the K horizon contains laminations that range in thickness from 0.2 cm to 3 cm. The laminated part of the K horizon attains a maximum thickness of 0.3 m. A well-indurated sheet of carbonate occurs below the laminated zone. Floating skeletal grains, clasts, pisoliths, root casts, and some laminations are contained within the carbonate sheet (Figs. 3C and 3D). The hardpans are often fractured and brecciated. Maxi-mum thickness of the hardpan part of the K horizon is 1 m.

Powdery to indurated carbonate nodules are often present below the carbonate sheet (Fig. 3E). The nodular zone may also include micrite matrix material. More commonly, the chalky micritic matrix horizon underlies the nodular zone. This K hori-zon profi le is similar to the pedogenic calcrete idealized profi les detailed by Esteban and Klappa (1983), Goudie (1983), and sum-marized by Alonso-Zarza (2003).

Secondary silica, in the form of nodules, stringers, and silici-fi ed root traces commonly occurs in association with the K hori-zons (Fig. 3F). The silica nodules range from 5 to 20 cm along the long axis; the stringers vary from 1 to 5 cm in thickness. Both the nodules and stringers are usually located in the K-C horizon transition zone. The silicifi ed root traces occur throughout the K to upper C horizon. Contact of the K horizon with the underlying C horizon is gradational.

As stated already, the paleosol profi le described here and shown in Figure 3 is an idealized profi le. Not all features noted for the profi le are typically found in every southwestern Mon-tana calcic paleosol. The upper surface of the K horizon can be extremely irregular (Fig. 2B), and the entire paleosol can even be truncated when traced laterally. Individual paleosols become welded with other paleosols (Fig. 2D) within some pedocom-plexes. However, there are usually enough profi le characteristics present in fi eld exposures to identify calcic paleosols.

Subsurface Calcic Pedocomplexes

Hanneman et al. (1994) documented the identifi cation of cal-cic paleosol stacks, now termed calcic pedocomplexes herein, in the subsurface of the Deer Lodge Valley, southwestern Montana (Fig. 4A). Calcic pedocomplexes with accumulated thickness in excess of 10 m appeared in the subsurface as a collection of several relatively thin, high-velocityhigh-density zones within the basin fi ll. Zone thickness ranged from 1 to 1.5 m. Density varied within the zones by as much as 0.6 g/cm3, and differed by as much as 0.9 g/cm3 from material immediately above these zones. Velocity differed by as much as 10 ft/ms (3.3 m/ms) from the overlying material and caused bright refl ections on seismic sections. Synthetic seismograms were used to tie well-log and seismic data (Fig. 4B).

The high-velocityhigh-density zones in the Cenozoic basin fi ll were interpreted to be calcic paleosols based on data extracted


Btor

Btk

K

K - C

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NodulesSilica StringersMudstone

Horizo

ns

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A

Floating Grains

B C D

EF

K Horizon - Laminations

K Horizon - Floating Clasts in Carbonate Sheet

K Horizon - Root Casts

K - C Transition Zone - Silica Stringers K Horizon - Chalky/Nodular

ZoneFigure 3. (A) Idealized calcic paleosol profi le (detailed locations for features in profi le are given in Table 1). (B) Laminations in upper part of K horizon, lower Madison Valley. (C) Floating skeletal clasts in micrite of indurated sheet portion of K horizon, central Deer Lodge Valley. Lens cap is 67 mm in diameter. (D) Root casts in indurated sheet portion of K horizon, central Deer Lodge Valley. Film cap is 35 mm in diameter. (E) Chalky/nodular zone (indicated by arrow) present beneath indurated sheet of K horizon, lower Madison Valley. Quarter for scale. (F) Silica stringers (indicated by arrows) of the K horizonC horizon transition zone, Jefferson Valley. Hammer is ~0.45 m in length.

from a suite of well logs that included sonic, density, resistivity, neutron, and lithology logs, and from well-cutting analyses (Fig. 4C). The pedogenic origin of the zones was shown by (1) well-cutting chips from the high-velocityhigh-density zones that exhibited pedogenic features associated with calcic paleosols, (2) paleosol horizonation interpreted from well-log analysis, (3) the absence of minerals normally associated with lacustrine deposits, and (4) comparison with surface paleosols (Fig. 4D).

Morphology of Calcic Paleosols

The calcic paleosols within the calcic pedocomplexes of Tertiary basin fi ll in southwestern Montana have calcium carbon-ate morphologies consistent with the stage IV to stage V mor-

phologies as outlined by Machette (1985, p. 5; Table 1 therein). The stage IV morphology characteristics include laminae up to 1 cm in thickness in the upper part of the K horizon, with some laminae draped over fracture surfaces. Laminae of stage V are up to 3 cm in thickness. Fractures in the K horizon are typically coated with laminae, and pisolites are present. Thickness of the K horizon ranges from 0.5 to 1.5 m.

Lateral Variation within Paleosol Stacks

Although a calcic pedocomplex can be traced for several miles within a basin, lateral variation commonly occurs. The variance may be within individual paleosol profi les of the pedo-complex, in the vertical succession of horizons within a


complex, and in the overall thickness of the pedocomplexes (Fig. 5). Within individual profi les, soil descriptive features such as texture, color, root trace concentration, and horizon bound-ary distinctness often vary laterally, particularly within the Bt(k) horizons (Fig. 5A shows K horizon termination; Fig. 5B shows scoured K horizon top). These changes can be related to local soil-forming controls, such as topography, parent material tex-ture, and scour events (McCarthy and Plint, 1998; McCarthy et

al., 1999). Lateral changes that affect soil horizon succession and overall pedocomplex thickness may be correlated to calcic profi le initial development position and the variable deposition and/or erosion events associated with calcic profi le formation. Typically, soil profi le development begins on stabilized areas within a basin, such as interfl uves or distal portions of alluvial fans (Alonso-Zarza et al., 1998; McCarthy et al., 1999). How-ever, in order to generate a pedocomplex, episodic sedimentation

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Figure 4. Hanneman et al. (1994) used well-log data, seismic data, and well cuttings analyses to defi ne calcic paleosols and pedocomplexes in the sub-surface of the Deer Lodge Valley, southwestern Montana. (Figure was modifi ed from Hanneman et al., 2003.) (A) Geologic setting of the Deer Lodge Valley, southwestern Montana. Location of Montana State Prison (MSP) 1-25 well and seismic line of 1B are also shown. (B) Seismic-refl ection line from the Deer Lodge Valley. Synthetic seismogram generated from well-log data of MSP 1-25 is tied to bright refl ectors that occur on the seismic data at 1.01.1 s (~930980 m in depth). (C) Paleosol profi le delineated by resistivity and neutron log data. Argillic paleosol horizons are interpreted to have low resistivity; K horizons are interpreted where porosity is low on the neutron log. The overlay of these two logs depicts individual profi les within the mature pedocomplex. The K horizons also correspond to the intervals high calcium content on the lithology log. CNLcompensated neutron log; NPHIneu-tron porosity. (D) Matrix identifi cation depth plot correlated with a surface pedocomplex measured in the northern Deer Lodge Valley. The thickness and frequency of increased calcite-content zones compare reasonably well with the K horizons of the surface pedocomplex.


needs to occur. Periodic deposition of sediment over the initial calcic soil profi le provides more space for plant growth, and new cycles of calcic soil formation are begun. Given time, calcic soils will form over large parts of a basin, wherever surfaces become inactive. With episodic sedimentation, calcic pedocomplexes will eventually build over this larger area.

The result of this soil-forming activity on a basin-wide scale is that (depending on a variety of factors, such as differential sub-sidence, climate, hydrology, parent materials) over time, thicker sections of calcic soils, and soils with somewhat different pro-fi les, may exist in different areas. Alonso-Zarza et al. (1998) doc-umented this pedocomplex variability on Pleistocene alluvial fan surfaces of the Campo de CartagenaMar Menor Basin, Murcia, southeast Spain. These authors noted the formation of different calcrete profi les in proximal and distal fan areas. These differ-ences resulted from the interplay of erosion and deposition on the fan surfaces. In proximal fan areas, some soil horizons were stripped from a profi le, while on the more stable areas, rework-ing and brecciation occurred, which would presumably produce a brecciated horizon. Conversely, in the distal fan areas, episodic sedimentation initially disrupted calcic soil formation, leading to another cycle of calcic soil formation. Collectively, these dif-ferences in the individual soil horizons of the various fan areas resulted in complex composite profi les being thicker in the distal fan areas than in proximal areas.

CALCIC PEDOCOMPLEXES AS SEQUENCE BOUNDARY INDICATORS

The calcic pedocomplexes present in the Tertiary basin fi ll of southwestern Montana developed over extended periods of time as evidenced by their advanced carbonate morphology stages. Soil development ceased for brief intervals because of sediment

infl ux, but then resumed, adding yet another soil profi le to the pedocomplex. Collectively, the individual paleosol profi les con-tained within a pedocomplex represent signifi cant breaks within the Tertiary basin-fi ll record. Consequently, the calcic pedocom-plexes mark unconformities that occur between large-scale sedi-mentary packages.

The age of each unconformity is constrained by paying strict attention to well-documented fossil vertebrate and radioisotopic age data taken from units occurring on both sides of the uncon-formity. The regional unconformities marked by calcic pedocom-plexes occur at ca. 30 Ma, 20 Ma, and 4 Ma. The magnitude of each hiatus represented at these regional unconformities in southwestern Montana is estimated to be ~34 m.y. where all sequences are present. Because age data are derived not directly from the bounding surface itself but from strata that occur at some distance above and below pedocomplexes, there is yet a degree of uncertainty that exists for exact ages of the sequence bounding surfaces. Consequently, we are constantly looking for better age constraints on the regional unconformities.

Montana Unconformity-Bounded Sequences

Five unconformity-bounded sequences were initially delin-eated within continental Tertiary strata in southwestern Mon-tana (Hanneman and Wideman, 1991; Hanneman et al., 2003). The sequences have upper and lower bounding surfaces that are unconformities of regional extent. We refer to these unconfor-mity-bounded sequences as large-scale sequences, because they contain sizeable packages of basin-fi ll material. The unconfor-mity-bounded sequences can include several hundred meters of strata, many different lithologies, and represent several million years of the geologic record. Calcic pedocomplexes mark the unconformities that separate four of these unconformity-bounded

K horizontermination

Glove for scale

Scoured K horizon

A B

Figure 5. Examples of lateral variability that occurs within calcic pedocomplexes located in the lower Madison Valley of southwestern Montana (location of pedocomplex given in Table 1). (A) K horizon termination in a 20 Ma calcic pedocomplex. Terminated K horizon is ~0.3 m in thickness. (B) Scoured K horizon top (laminar zone and a part of the carbonate sheet). Glove is 24 cm in length.


typically contains a maximum of three paleosol profi les. The K horizon in these profi les has carbonate morphology equivalent to stage IV. However, in many locations, sequence 2 is overlain directly by sequence 4. In these areas, calcic paleosol stacks have several paleosol profi les and K horizons attain a carbonate mor-phology stage V.

Calcic pedocomplexes at the top of sequence 3 have sev-eral paleosol profi les, and the K horizons in each profi le reach a carbonate morphology stage V. It should be noted that in some past studies of southwestern Montana Tertiary basin fi ll, paleo-sols at this same stratigraphic level have been described as red, saprolitic, and kaolinite-rich (Thompson et al., 1982, p. 415;

sequences. The regional unconformity-bounded sequences delimited by calcic paleosol stacks are informally designated as: sequence 2middle/late Duchesnean to Whitneyan (ca. 3830 Ma), sequence 3Arikareean (ca. 2720 Ma), sequence 4Bar-stovian to Blancan (ca. 164 Ma), and sequence 5early Quater-nary (ca. 1.8 Ma) to the present (Fig. 6). Locations for examples of these calcic pedocomplexes and unconformity associations are given in Table 1.

There are some differences among the calcic pedocomplexes that occur on the upper bounding surfaces of sequences 2, 3, and 4 in southwestern Montana. Where sequence 3 directly overlies sequence 2, pedocomplex development at the top of sequence 2

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Wasatchian

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Duchesnean

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Arikareean

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North AmericanLand Mammal Ages

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PliocenePleistoceneHolocene

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LegendCalcic pedocomplexes

Approximatetime and duration of hiatusNewly definedhiatus

4 Ma

20 Ma30 Ma

Figure 6. Correlation of southwestern Montana sequences with central Washington (CW) sequences. The dashed lines within the CW Kittitas represent the newly recognized 30 Ma to 27 Ma hiatus. The gray area in between some of the wavy lines represents the estimated magnitude of the hiatus. Age estimates for the Cenozoic epochs are ones proposed by Berggren et al. (1995). Age estimates for Paleogene North American Land Mammal Ages (NALMA) are based on those given by Prothero (1995). Age estimates for Neogene NALMA are those delineated by Woodburne and Swisher (1995).


Fields et al., 1985). Strata at the locations sampled for the oxic horizon were originally thought to be ca. 2117 Ma. Later map-ping with more-detailed age control revealed that strata at these sample localities are much older than previously thought. Conse-quently, recent work has shown no evidence for an oxic horizon at this stratigraphic level and that the regional unconformity at ca. 2117 Ma is instead marked by calcic pedocomplexes (McLeod, 1987; Hanneman, 1989; Hanneman and Wideman, 1991; Portner and Hendrix, 2004).

Calcic pedocomplexes that mark the upper surface of sequence 4 are similar to those at the top of sequence 3. How-ever, these pedocomplexes are often absent in the southern areas of southwestern Montana, where there are no reported uppermost Tertiary strata and there are scant Quaternary age sediments. It may well be that much of this part of the section (including the calcic pedocomplexes) has been stripped from the basins due to recent uplift of the YellowstoneSnake River Plain area of Idaho, Wyoming, and Montana.

Washington Unconformity-Bounded Sequences

The Cenozoic unconformity-bounded sequences identi-fi ed in Montana extend into central Washington based upon work originally done by Cheney (1994, 2000). Hanneman et al. (2003) recognized that there are equivalent interregional uncon-formity-bounded sequences in this area: Lower Kittitasca.

3630 Ma, Upper Kittitasca. 2722 Ma, Walpapica. 204 Ma, and High Cascadeca. 4 Ma to present (Fig. 6). Although Cheney (1994, 2000) emphasized the importance of changes in lithology and provenance in initially delineating the Wash-ington unconformity-bounded sequences rather than using the identifi cation of pedocomplexes, the literature reports a caliche constraining the upper surface of the Walpapi Sequence at the Hanford Site (Pasco Basin) in south-central Washington. The caliche is developed on the Miocene-Pliocene Ringold Forma-tion, and middle to late Pleistocene sediments overly it. The thickness of the caliche ranges from 0 to 20 m, and the unit is bounded by irregular surfaces having as much as 25 m of relief. The number of carbonate layers differs with the thickness of the deposits. Carbonate morphology of the layers varies from stage I to stage V. The caliche is interpreted to be pedogenic, although some modifi cation to the paleosols by groundwater processes may have occurred (Slate, 1996).

EXTENSION OF SEQUENCE BOUNDARIES DELINEATED BY CALCIC PEDOCOMPLEXES INTO THE GREAT PLAINS AND OTHER WESTERN U.S. AREAS

The unconformity-bounded sequences cited above have been extended into the western United States and the northern Great Plains in previous studies by Hanneman and Wideman

TABLE 1. LOCATIONS FOR EXAMPLES OF TERTIARY CALCIC PEDOCOMPLEXES (PC) AND ASSOCIATED UNCONFORMITIES IN SOUTHWESTERN MONTANA

Approximate age of unconformity (Ma)

Valley location

Section, township range

NAD 1927, Zone 12, UTM easting/northing coordinates

U.S. Geological Survey 7.5quadrangle

Remarks

4 JeffersonBiltmore area

SE section 34, T 4 S, R 7 W

387150 m E; 5031420 m N Beaverhead Rock, Montana

Vertebrate fossils below PC are Hemphillian; mapped Quaternary gravels are above PC.

20 Upper Ruby Valley

NE section 3, T 9 S, R 5 W

406085 m E; 4992985 m N Belmont Park Ranch, Montana

Vertebrate fossils below PC are late Arikareean; vertebrate fossils above PC are Barstovian.

20 Central Deer Lodge

NW section 31, T 3 N, R 9 W

363500 m E; 5142162 m N Conleys Lake, Montana

Barstovian fossils are above PC; mapped Arikareean strata are below PC.

? 20 (Lower boundary age unconstrained)

LowerMadisonMadison

Bluffs area

SW section 34, T 1 N, R 2 E

463440 m E; 5070289 m N Manhattan SW, Montana

Barstovian vertebrate fossils are above PC; no age constraints were found below, so they could range from Chadronian to Arikareean in age (3719 Ma).

30 JeffersonGolden

Sunlight Mine

SE section 29, T 2 N, R 3 W

422640 m E; 5082290 m N Black Butte, Montana

Vertebrate fossils below PC are Chadronian; fragmentary oreodont fossils above PC most likely are Arikareean (W. Coppinger, July 2004, personal commun.).

30 JeffersonRenova area

NE section 28, T 1 N, R 4 W

413542 m E; 5073180 m N Whitehall,Montana

Vertebrate fossils below PC are Chadronian; thin veneer of Quaternary loess is above PC.


(1991), Cheney (1994, 2000), and Hanneman et al. (2003). Con-stenius et al. (2003, see their Fig. 19) expanded on these investi-gations and documented age-equivalent unconformity-bounded sequences throughout the Cordilleran orogenic belt that extends from southern Canada to Mexico. Based upon the interpretation of extensive structural data, Constenius et al. (2003) showed that the unconformity-bounded sequences record plate-tectonic inter-actions and continental deformation.

Because age-equivalent Cenozoic unconformity-bounded sequences can be extended throughout the Great Plains and west-ern United States, we expect that where equivalent soil-forming conditions prevailed, calcic pedocomplexes should delineate regional unconformities. A recent literature search revealed the likely identifi cation of the paleosolregional unconformity asso-ciations. Several occurrences of the calcic paleosolregional unconformity associations at ca. 30 Ma, 20 Ma, and 4 Ma from

these areas are listed next. It is possible that many other occur-rences of paleosolregional unconformity associations are pres-ent in these areas. The locations of the paleosolregional uncon-formity associations are shown in Figure 7.

Regional Unconformity at ca. 30 Ma

Pinnacles Lookout, Badlands National Park, Southwestern South Dakota

Pinnacle Series paleosols occur in the top of the Poleside Member (early Oligocene) of the Brule Formation. The Pinnacle Series contains calcic paleosols that have prominent horizons of hard calcareous nodules at shallow depths. Elongate calcare-ous concretions that are interpreted as rodent burrows are also abundant in the paleosols. The calcic horizons probably only correspond to a stage II or at maximum stage III carbonate

NORTHDAKOTA

SOUTHDAKOTA

NEBRASKA

KANSAS

OKLAHOMA

TEXAS

MONTANA

WYOMING

COLORADO

NEW MEXICOARIZONA

UTAH

IDAHO

WASHINGTON

OREGON

NEVADA

CALIFORNIA

LEGENDCalcic Pedocomplexes/ Regional Unconformity Associations

4 Ma

20 Ma

30 Ma

Southern HighPlains with4 Ma association

SouthwesternMontana - AllAssociations Are Present

0 500

Kilometers N

Figure 7. Locations of calcic pedocomplexes and regional unconformities in the Great Plains and western United States. Details of locations and age constraints for the calcic paleosol pedocomplexes and regional unconformities are given in text.


morphology. The Rockyford Ash (which has a radioisotopic age of close to 29 Ma) of the Sharps Formation unconform-ably overlies the Poleside Member of the Brule Formation (Retallack, 1983).

Banner County, Western NebraskaThe top of the upper Eocene to early Oligocene Brule Forma-

tion is locally cemented here with pedogenic nodular caliche. The Brule is overlain by gravel of the Neogene Ogallala Group (Gardner et al., 1992).

Medicine Lodge Creek Valley, South-Central IdahoNodular limestone armors the top of Medicine Lodge

beds present in the Medicine Lodge Creek Valley, in Clark County, Idaho, and in sparse locations to the southwest as far as the southern Lemhi Range (Hodges and Link, 2002). The nodular limestone is ~2 m thick at the head of the south fork of Deep Creek, where it lies stratigraphically above a tuffaceous mudstone unit that has yielded a 40Ar/39Ar age of 30.23 0.45 Ma. The nodular limestone is most likely pedogenic in origin (Hodges et al., 2004; M.K.V. Hodges, 2005, personal commun.).


South Killdeer Mountains, Southwestern North Dakota (Medicine Hole Plateau, Dunn County)

The Arikaree Formation contains a ledge-forming bed of carbonate/sandstone that is ~9 m in maximum thickness. This unit is known as the burrowed marker unit because it contains abundant fossil burrows (Forsman, 1986; Murphy et al., 1993). Delimata (1975) noted that this bed is an exceptional stratigraphic marker for the South Killdeer Mountains. He described the unit as containing tuffaceous limestone, nodular limestone, and banded limestone. Although Delimata interpreted the burrowed marker unit as a lacustrine deposit, its described features are more consistent with a pedogenic interpretation for the unit. The same marker bed may be present at White Butte, North Dakota, ~130 km southwest of the South Killdeer Mountains (Murphy et al., 1993). Presently, the burrowed marker unit is age con-strained by: (1) a fi ssion-track age of 25.1 2.2 Ma taken from the base of the burrowed marker unit, and (2) the occurrence of two genera of oreodonts, Merychyus and Merycochoerus, located ~27 m above the stratigraphic position of the fi ssion-track age. The range zones of these oreodonts overlap in the latest Arika-reean to earliest Hemingfordian (Hoganson et al., 1998).

Monroe Canyon, NebraskaThe terminal paleosol at the head of Monroe Canyon,

along the high rim, is ~4.6 m in thickness and is developed on the Harrison Formation. The paleosol appears to be a silcrete-calcrete intergrade (Nash and Shaw, 1998), and it contains con-centrations of rhizoliths and burrows, an upper laminar petrocal-cic horizon, and a surface cemented as silcrete. Remnants of this

paleosol, the terminal Harrison paleosurface, are on fl at-topped hills and buttes from Monroe Canyon west to the Nebraska-Wyoming state boundary (a distance of ~20 km). The Eagle Crag Ash, with a fi ssion-track age of 19.2 0.5 Ma, overlies the Harrison paleosurface by ~2 m; the Agate Ash, with a 40K/40Ar age of 21.9 Ma, occurs ~10 m below the Harrison paleosurface at Agate National Monument, in the Hoffman channel section (Hunt, 1990; MacFadden and Hunt, 1998).


Kimball and Banner Counties, Western NebraskaPedocomplexes of calcareous paleosols are present in the

uppermost Neogene Ogallala Group, at the top of the Ash Hol-low Formation, western Nebraska. The pedocomplexes are ~12 m thick, contain up to four paleosols, and each paleosol is ~1 m thick. The uppermost calcic paleosol in a pedocomplex has reached stage IV carbonate morphology, and the lower paleosols are between a stage III and IV carbonate morphology (Gardner et al., 1992).

Hagerman Fossil Beds National Monument, Southwest Idaho

A caliche is developed on Pleistocene-Pliocene gravels and forms a cap rock in most of the monument and the sur-rounding area. The caliche averages several meters in thick-ness, but thins to less than a meter locally. It is a very dense layer and contains vertical fractures that are often recemented (Farmer and Riedel, 2003).

Southern High Plains, Texas and New MexicoThe uppermost late Tertiary Ogallala Formation typi-

cally includes a stage V paleosol or up to two stage IV cap-rock calcic paleosols, and may have a stage VI calcic paleosol where the Quaternary Blackwater Draw Formation overlies it. Where the Blackwater Draw Formation is only a thin veneer or is entirely absent (as is the case in large portions of the western High Plains), the Ogallala calcic paleosol cap rock is 1.510 m thick, and has stage VI carbonate morphology. In these areas, it is probable that the pedogenic carbonate accumulations pres-ent within numerous buried calcic soils and the surface calcic soils of full sections of the Blackwater Draw have been welded onto the uppermost Ogallala calcrete (Gustavson, 1996, p. 37). It is also possible that in certain areas, the Ogallala cap rock may range in age from late Miocene to late Quaternary.

Roswell-Carlsbad, Southeastern New MexicoStage VI calcic paleosols are developed on the top of the

Ogallala Formation in this area. The age of the calcic paleosol is thought to be late Pliocene (Bachman, 1976; Machette, 1985).

Morman Mesa, Southeastern NevadaThe Morman Mesa calcic paleosol is ~2.5 m thick and has

stage VI carbonate morphology. It is developed on red quartz sand


of the Muddy Creek Formation. The age of the calcic paleosol is thought to be late Pliocene (Gardner, 1972; Machette, 1985). Vertebrate fossil remains of medial Hemphillian (late Miocene) age have been reported for the Muddy Creek Formation in the Morman Mesa area (Williams et al., 1997).

Vidal Junction, Southern CaliforniaA stage VI calcic paleosol is developed on the top of the

Mioceneearly Pliocene Muddy Creek Formation in this area. The age designated for this calcic paleosol is late Pliocene (Bull, 1974; Machette, 1985).

DISCUSSION

The signifi cant areas of discussion that follow from our work on calcic pedocomplexes and their association with regional unconformities center on the usefulness of calcic pedocomplexes, or in fact, any type of mature paleosol, as sequence stratigraphic tools. Even with lateral variation of pedocomplexes, sequence boundaries can be defi ned when one combines other techniques for mapping unconformities. Additionally, although the primary control on the sequences described herein is tectonic, higher-resolution work on the pedocomplexes, their adjacent strata, and better age constraints will help in understanding secondary con-trols of sequence and pedocomplex formation.

Calcic pedocomplexes and calcic paleosols with stage IV to stage VI carbonate morphology are associated with regional uncon-formities of ca. 30 Ma, 20 Ma, and 4 Ma from the Great Plains through a large part of the western United States. These paleo-solunconformity associations mark large-scale regional sequence boundaries and consequently aid in surface and subsurface mapping of regional sequences. The calcic paleosols are easily identifi able in surface sections and have distinct physical properties that can be recognized in various types of geophysical data. Where basins contain several thousand feet of fi ll, and only have basin margins sections exposed, the ability to identify calcic pedocomplexes and use them to separate the subsurface geology into at least large-scale unconformity-bounded sequences is extremely advantageous in basin research. Additionally, the widespread extent of the calcic paleosolsregional unconformities associations enhances their util-ity as a regional correlation tool.

It is important to note that the regional hiatuses recognized at ca. 30 Ma, 20 Ma, and 4 Ma are marked by many different sets of calcic pedocomplexes. Some of these pedocomplexes are laterally extensive over large areas, such as the Great Plains, but others formed within discrete depositional basins. Depositional basins began to form in the Cordilleran foreland fold-and-thrust belt by ca. 49 Ma (Hanneman, 1989; Hanneman and Wideman, 1991; Constenius, 1996; Constenius et al., 2003; ONeill et al., 2004). Thus, pedocomplexes that formed in discrete depositional basins may be physically traced only within a particular basin. The pedocomplexes that mark these regional hiatuses were probably developed at similar times in various locations due to regional tectonic and climatic controls.

Because the regional unconformities defi ned at ca. 30 Ma, 20 Ma, and 4 Ma can be recognized throughout the Great Plains and the western United States, other types of mature paleosols should mark these same unconformities where climatic condi-tions differed. For example, in the Painted Hills of central Ore-gon, the 30 Ma regional unconformity separates the upper Big Basin Member of the John Day Formation from the overlying Turtle Cove Member of the John Day Formation. Mature iron-rich paleosols are in the middle Big Basin Member and within the Big Basin Member, and the last one is located at the contact of the Big Basin Member and the overlying Turtle Cove Member of the John Day Formation (Bestland, 1997; Retallack et al., 2000).

Even though a pedocomplex can be traced for up to sev-eral miles within a basin, lateral variation commonly occurs. The variance may be within individual paleosol profi les of the pedocomplex, in the vertical succession of horizons within a pedocomplex, and in the overall thickness of the pedocomplexes. The lateral variation is most likely related to factors such as the location of initial pedocomplex development within a basin, or the complex interplay of erosion and deposition rates (Tandon and Gibling, 1997; Alonso-Zarza et al., 1998; McCarthy et al., 1999; Weissmann et al., 2002). Although the lateral variance is easily recognized on the surface, the resolution of subsurface data may mask these differences. Where calcic paleosols or ped-ocomplexes are not present, angular stratal relationships, abrupt changes in provenance or lithologies, and the bases of incised valleys can also defi ne sequence boundaries. These features can be mapped on the surface, and geometric patterns as indicators of unconformities can be recognized on seismic data. Collectively, these data types can be combined with paleosol information to complete the delineation of a sequence boundary.

The calcic paleosols observed in southwestern Montana at the 30 Ma boundary are not as well developed (in regard to car-bonate morphology and number of soil profi les within a pedo-complex) as those that mark the 20 Ma and 4 Ma regional uncon-formities. This appears to be a consistent feature of those bound-aries throughout the Great Plains and western United States. The cause for this may be related somehow to a broad range of cli-mate and/or tectonic controls, but presently, the actual reason for this difference in degree of paleosol development is not known. As stated previously, Constenius et al. (2003) have shown that the large-scale unconformity-bounded sequences defi ned in the northwestern United States by Hanneman and Wideman (1991), Cheney (1994, 2000), and Hanneman et al. (2003) are tectoni-cally controlled sequences. However, future high-resolution work on these sequences will probably lead to an understanding of other secondary controls on their formation.

The ages of the regional unconformities are given as approximate ages and are based upon currently available age constraints derived from radioisotopic age data and verte-brate faunal assemblages initially established in southwest-ern Montana. The ages appear to be fairly consistent across the Great Plains and western United States, but there is some range to these age designations. Historically, radioisotopic age


data have been acquired in these geographic areas in order to place constraints on defi ning North American Land Mammal boundaries, the Cenozoic time scale, and the Cenozoic magne-topolarity time scale. Sparse attention has been paid to refi ning age constraints on regional unconformities. With additional age constraints, it may become apparent that there are timing differences among the regional unconformities. If the timing differences exist, they may be correlated to the time sweep on unconformity-bounded sequences boundaries noted by Con-stenius et al. (2003) and linked to regional tectonic events, or they may be indicators of timing differences in regional cli-matic change.

In relation to the development of calcic pedocomplexes that mark regional unconformities, we fi nd it of interest to contem-plate the many Cenozoic relict calcic soils listed by Machette (1985, p. 11, Table 2 therein) for regions within the southwest-ern United States. These calcic soils may be young examples of the much older Tertiary calcic pedocomplexes. They may represent the different soils that could become pedocomplexes in a future geologic record.

SUMMARY

Calcic pedocomplexes with a maximum carbonate morphol-ogy of stage VI are associated with regional unconformities that have approximate ages of 30 Ma, 20 Ma, and 4 Ma. The cal-cic paleosols are easily identifi able in surface sections and have distinct physical properties that can be recognized in various types of geophysical data in the subsurface. The recognition of the calcic paleosolunconformity association enables the separa-tion of Cenozoic basin fi ll into at least large-scale unconformity-bounded sequences, which can greatly enhance both surface and subsurface basin research.

Although the 30 Ma, 20 Ma, and 4 Ma calcic pedocom-plexesregional unconformity associations were initially described in southwestern Montana, they can be traced through-out the Great Plains and western United States. The widespread extent of the calcic paleosolsregional unconformities associa-tions enhances their utility as a regional correlation tool. Because the pedocomplexes delineate regional unconformities that are also large-scale sequence boundaries, the identifi cation of the pedocomplexunconformity association has broad implications for continental sequence stratigraphy.

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