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Published online 29 October 2007
Published in Soil Sci Soc Am J 71:1878-1888 (2007)
DOI: 10.2136/sssaj2006.0351
© 2007 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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PEDOLOGY

Degradation of Pedogenic Calcretes in West Texas

D. R. Hirmasa,* and B. L. Allenb

a Soil and Water Sciences Program, Dep. of Environmental Sciences, Univ. of California, Riverside, CA 92521-0424
b Dep. of Plant and Soil Science, Texas Tech Univ., Lubbock, TX 79409-2122

* Corresponding author (daniel.hirmas{at}email.ucr.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Calcretes (petrocalcic horizons) are widespread indurated accumulations of carbonate in arid and semiarid regions of the world. Many studies have detailed the formation of these calcretes; however, little direct attention has been given to their degradation. The primary objective of this study was to evaluate the continuity of calcrete on the Llano Estacado as a step toward understanding processes of calcrete degradation across the region. The Llano Estacado is a tableland spanning eastern New Mexico and West Texas with a well-developed calcrete, known as the "caprock," exposed as an escarpment around most of its perimeter. We excavated several deep pits along a transect between the caprock on the eastern margin of the Llano Estacado and a similar outcrop on a playa slope. We documented degradation features in the calcrete and compared the extent of degradation with a calcrete in a more arid region (Diablo Plateau). Our findings show a lack of continuity of the calcrete on the Llano Estacado even in a relatively short transect (220 m) between the two outcrops. Individual calcrete fragments showed evidence of partial dissolution. These observations were confirmed in road cuts and pits excavated for road material across the area. Geomorphic position and surface slope control water movement across the landscape and apparently explain both the variation in the presence of degradational features and the distribution of calcrete fragments in the transect. Observations from the more arid Diablo Plateau suggest that calcrete degradation is widespread. The degradation of calcrete on the Llano Estacado makes the caprock unreliable in some areas as a stratigraphic marker in distinguishing the Ogallala and Blackwater Draw Formations.

Abbreviations: CCE, calcium carbonate equivalent • CMI, calcrete morphology index


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Calcretes (petrocalcic horizons) occur in many arid and semiarid regions of the world. One such calcrete that has been extensively documented is the "caprock" of the Llano Estacado, spanning West Texas and eastern New Mexico (Sidwell, 1943; Bretz and Horberg, 1949; Brown, 1956; Reeves, 1970; Walker, 1978; McGrath, 1984; Holliday, 1990, 2004). The Llano Estacado (Fig. 1 ) is an extensive (approximately 75,000 km2), essentially undissected tableland in the southwestern part of the Great Plains physiographic province (Fenneman, 1931). Playas, which are small (<5 km2), roughly circular, seasonally dry depressions with flat central floors, dot the landscape with estimates ranging from 30,000 to 37,000 in number across the entire region (Osterkamp and Wood, 1987; Sabin and Holliday, 1995). The Llano Estacado is bounded on its northern, eastern, and western margins by 50- to 200-m escarpments and gently grades into the Edwards Plateau toward the south (Sabin and Holliday, 1995). Annual precipitation varies from about 540 mm in the northeast to about 300 mm in the south (Garoogian, 2000).


Figure 1
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  		Fig. 1. Location of the sampling sites.

 
The caprock is a highly resistant calcrete, as much as 3 m thick, and is present around most of the perimeter of the Llano Estacado. Where exposed, it often forms ledges supporting overlying, usually thin soils (Fig. 2 ). The calcrete is mostly cemented by CaCO3, but in places is silicified to varying degrees (McGrath, 1984). The thickest, most indurated portions of the caprock (apparently formed as a result of pedogenesis) formed in the upper Ogallala Formation (Fig. 3 ), which is of Miocene to Pliocene age (Gustavson and Winkler, 1990). The Ogallala Formation is composed of gravelly, sandy, and loamy alluvial sediments derived from the southern Rocky Mountains (Reeves, 1970). Although deposited originally by fluvial processes, some of the sediments have been reworked by wind to various degrees (Reeves, 1984; Reeves and Reeves, 1996).


Figure 2
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  		Fig. 2. Photographs of the Crosby site showing (a) the virtually featureless tableland topography of the Llano Estacado overlooking Blanco Canyon and (b) the "caprock" calcrete-supported ledge. The caprock exposure is approximately 3 m thick.

 

Figure 3
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  		Fig. 3. Upper Cenozoic stratigraphic relationships between the Blackwater Draw and Ogallala Formations (modified from McGrath [1984] and Holliday [1990]).

 
The Blackwater Draw Formation (Fig. 3), a sandy to loamy, eolian deposit, overlies the Ogallala Formation across most of the area (Holliday, 1989). It generally ranges from barely identifiable to as much as 27 m thick (Reeves, 1976). These extensive Pleistocene deposits thicken from the southwest to the northeast, apparently the result of prevailing winds at the time of deposition (Seitlheko, 1975). Most of the land-surface soils on the Llano Estacado have formed in Blackwater Draw sediments (Holliday, 1990). Thick, well-developed calcic horizons are commonly observed in these sediments (Reeves, 1970; McGrath, 1984). Calcretes are rarely reported in the Blackwater Draw Formation. Where present, they are thin and often directly overlie the Ogallala caprock.

Calcretes are important because they significantly contribute (104 billion Mg of C) to the soil inorganic C pool (Lal and Kimble, 2000). Because they are indurated, they often form root-restrictive layers and impact the vegetation, geomorphology, and hydrology of landscapes (Goudie, 1973). Research on calcretes has, to a large extent, focused on their formation, morphology, and classification. Gile et al. (1966) proposed a widely used four-stage classification scheme based on the morphology of calcretes in gravelly and nongravelly sediments. The classification was later expanded by Bachman and Machette (1977) to include two additional stages identified by incipient or common brecciation and pisolith formation. Much less is known, however, about the degradation of calcretes, although degradation features were observed in previous studies.

Knox (1977), in a study on caliche formation in South Africa, outlined a dynamic environment of the cyclic formation of fractures, breccias, and solution hollows. Others have realized the need for a broader approach to carbonate accumulations and given increased attention to complex cyclic processes such as erosion, sediment deposition, reworking, carbonate dissolution, recrystallization, and brecciation (Watts, 1980; Klappa, 1983; Alonso-Zarza et al., 1998). Calcrete brecciation in the southwestern United States probably represents partial cyclic degradation, since it is thought to be a product of soil moisture fluctuations caused by alternating cool, moist pluvial and warm, dry interpluvial conditions (Machette, 1985). Disintegration of petrocalcic horizons was observed in southern New Mexico, with mechanical, hydrological, and biological mechanisms given as explanations (Gile and Grossman, 1979). Dissolution features, especially pipes, are very common on the Llano Estacado (Wood and Osterkamp, 1987) and the continuity of the calcrete has been severely compromised in some exposures. Brown (1956) noted that exposed caliche on the Llano Estacado showed dissolution features. In addition, present soil conditions on the Llano Estacado may be causing degradation of the clay minerals palygorskite and sepiolite, originally preserved within the caprock calcrete (Bigham et al., 1980). Despite these studies, little is known about the morphological sequence of calcrete degradation in West Texas and its relationship to landscape position, properties of overlying soil horizons, or paleoclimate interpretation.

We aimed to document morphological and micromorphological degradation features in a transect of the caprock as a step toward understanding calcrete degradation across the region. The primary objectives of our study were to: (i) assess the continuity of caprock calcrete in a transect on the Llano Estacado; and (ii) document degradation features in the calcrete, if present. A secondary objective was to consider the extent of calcrete degradation by making observations across the region and comparing our transect observations to an arid region.


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Two locations in West Texas were chosen for field investigations, the Crosby and Hudspeth sites (Fig. 1). The Crosby site, located in Crosby County, is approximately 16 km southeast of the town of Floydada. It is situated near the eastern margin of the Llano Estacado overlooking Blanco Canyon (Fig. 1). To assess the continuity of caprock calcrete, a 220-m transect was set up between two outcrops of the caprock. One outcrop was on a playa slope and the other on an escarpment on the western edge of Blanco Canyon. Six pits were excavated to depths ranging from 165 to 315 cm and spaced evenly along the transect. The pits are labeled in sequential order beginning with the one closest to the playa, C1, and ending near the outcrop of calcrete composing the escarpment, C6 (Fig. 1). The Hudspeth site (Pedon H), located in Hudspeth County, is a caliche quarry on the Diablo Plateau, approximately 110 km east of El Paso (Fig. 1). The slope gradient for the surface of Pedon H is <0.3% and annual precipitation is about one-half that of the Crosby site.

Pedons C1 to C6 were on gentle (<2%) slopes at an elevation of about 945 m in native rangeland covered dominantly by blue grama [Bouteloua gracilis (Kunth) Lag. ex Griffiths], buffalograss [Buchloe dactyloides (Nutt.) Engelm.], and threeawns (Aristida spp.). The Hudspeth site was nearly level, with an elevation of about 1280 m in native rangeland dominated by creosotebush [Larrea tridentata (DC.) Coville]. Parent materials for the soils at the Crosby site were sediments of the Blackwater Draw and Ogallala Formations, while the soil at the Hudspeth site formed in local eolian sediments with a possible contribution from underlying limestone residuum. Climatological data are not available for the two study sites; however, the present mean annual precipitation at Floydada, approximately 16 km northwest of the Crosby site, is about 545 mm. Annual precipitation at the Hudspeth site is about 220 mm based on precipitation records at Cornudas, which is slightly higher in elevation and 15 km to the northwest. Average mean annual atmospheric temperature for both of these areas is about 15.6°C (Garoogian, 2000).

To assess topographic relationships to calcrete degradation, surface slopes of the Crosby pedons were determined using a theodolite. All pedons were described according to the U.S. system of soil taxonomy (Soil Survey Staff, 2006) and sampled by morphologic horizon for analyses.

Particle size distribution was determined using a modified Bouyoucos hydrometer method (Bouyoucos, 1951). As recommended to assess soil field conditions, CaCO3 was not removed before particle size analysis (Nettleton et al., 1991). Sand was wet sieved, dried, and weighed. Calcium carbonate equivalent (CCE) was determined for all samples following U.S. Salinity Laboratory Staff (1954). Organic C content was determined following Prince (1955) to assess the upslope sediment contribution. Additional data including sand fractionation, pH, electrical conductivity, cation exchange capacity, and x-ray diffraction for the determination of clay and silt mineralogy for the Crosby and Hudspeth sites were given by Hirmas (2003). These data were used primarily to classify the pedons. Cation exchange capacity (CEC) values were lower than those typically reported for similar soils on the Llano Estacado. These values were determined using an automated syringe vacuum extractor (Hirmas, 2003), which may not have completely saturated exchange sites, resulting in an underestimate of CEC (Rhoades, 1982). Although these soils are reported here to have active CEC classes, they are more realistically superactive (Table 1 ).


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  		Table 1. Morphological properties, CaCO3 equivalent, and organic C content for the Crosby and Hudspeth sites.

 
Selected horizons were sampled from each pedon for micromorphology. Fabric characteristics and pedofeatures (Stoops, 2003) were identified and described according to the terminology of Brewer (1964). Micromorphological observations were made to supplement other analyses. They were made primarily to determine whether the deep reddish, carbonate-rich horizons qualified as Bt or Bkk horizons, or both. They were also made to elucidate the depositional sequence of mobile constituents (primarily silicate clay and carbonates). A quantitative micromorphological analysis was not an objective of the study.


   RESULTS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Soil and Calcrete Morphology
Family classifications and surface slopes of all the Crosby pedons are given in Table 1 and diagnostic subsurface horizons are illustrated in Fig. 4 . Evidence of pronounced pedogenesis, expressed by strong horizonation, including well-developed calcic horizons, was apparent to the depth exposed in all pedons, except Pedon C1, at the Crosby site. Pedon C1 was classified as an Aridic Calciustoll. All other pedons at the Crosby site were Aridic Paleustolls. Pedon C6 (the shallowest pit in the transect) was not excavated deeper because a strongly developed Ogallala Formation caprock was present. All pedons also had argillic horizons except for Pedon C1. While the Btkb horizon of Pedon C1 showed evidence of clay illuviation, it did not qualify as an argillic horizon due to an insufficient increase in clay content. Carbonate masses and calcrete fragments in the Bw1 and Bw2 horizons of Pedon C1 appeared to be inherited as parent material from a previous calcrete, as opposed to pedogenic formation in situ. These horizons were therefore designated cambic horizons. Pedon H was classified as a Typic Haplocalcid.


Figure 4
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  		Fig. 4. Profile morphology of the Crosby pedons illustrating epipedons and diagnostic subsurface horizons. Relative depths and distances between pedons are indicated. Note the large calcrete fragment in the Bkk horizon of Pedon C2.

 
Four morphological properties in Table 1 were used to distinguish calcrete expression or degradation: CaCO3 distribution, CCE, cementation class, and soil structure. These properties were combined into a calcrete morphological index (CMI) to semiquantify and relate calcrete degradation with geomorphic position. These indices were calculated for each horizon following a procedure similar to Harden and Taylor (1983) for profile development indices. Each property was scored according to Table 2 , normalized to the highest possible score for that property, and averaged across the four properties in each horizon.


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  		Table 2. Quantification of carbonate and horizon properties for calculating calcrete morphology index (CMI) values.

 
Figure 5 shows the profile distribution of CMIs for each of the six pedons at the Crosby site. Pedons C1 and C6, which are located on backslope positions, have profile distributions that terminate with strong expressions of calcrete, as represented by high CMI values. Soils beneath shoulder positions show a peak in the expression of calcrete in the middle of their profiles (119 and 169 cm for Pedons C2 and C5, respectively) but terminate with an increase in CMI with depth. Distributions for Pedons C3 and C4, which occur on level summit positions in the transect, show two peaks each, at 142 and 216 cm and 127 and 244 cm, respectively. Both of these pedons have CMI profile distributions that decrease below this second bulge.


Figure 5
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  		Fig. 5. Calcrete morphology index (CMI) depth profiles for the Crosby pedons C1 to C6. The CMI values are based on four horizon properties: CaCO3 distribution, CaCO3 equivalent, structure, and cementation class.

 
Slope gradients increase from summit to backslope positions along the Crosby site transect. This increasing slope is represented as a change in the CMI profile distributions of these pedons (Fig. 5). In addition, the importance of slope in calcrete degradation is evidenced by the strong correlation between slope gradient and the depth of free carbonates (r2 = 0.95), defined here as the depth to chemical reaction with 1 mol L–1 HCl (Fig. 6 ). This correlation is consistent with Honeycutt et al. (1990), who found maximum CaCO3 occurring at shallower depths on shoulder and backslope positions in two of the three transects used in their study.


Figure 6
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  		Fig. 6. Depth of free carbonates as a function of surface slope at the Crosby site.

 
The distribution of calcrete fragments in the Crosby transect are depicted in Fig. 4. Calcrete fragments were concentrated at the ends of the transect and occurred at shallower depths in pedons on backslope positions (Pedons C1 and C6) as opposed to shoulder positions (Pedons C2 and C5). Only very few fragments were present under the nearly level slopes of the summit (Pedons C3 and C4). A large angular slab of calcrete occurred at 270 cm in Pedon C2. Although calcrete fragments were present in all pedons, our excavations show that caprock calcrete is not continuous between the outcrops on the escarpment and playa slope. Assuming that the calcrete has been mostly degraded in Pedons C2 to C5, the lower part of the excavations should be in the Ogallala Formation. No appreciable differences could be detected, however, between the Ogallala and Blackwater Draw Formations.

Figure 7 shows a significant correlation (P < 0.001) between CCE and clay content for all horizons in the transect. This correlation exhibits a curvilinear trend such that CaCO3 is positively related to clay content until approximately 40% CCE, after which it is negatively related. Since CaCO3 was not removed before particle size analysis, Fig. 7 represents the relationship between CCE and the combined silicate and carbonate clay content of the soil.


Figure 7
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  		Fig. 7. Curvilinear regression between CaCO3 equivalent and clay content for all horizons at the Crosby site.

 
Relatively high amounts of CaCO3 were present throughout the Hudspeth (H) pedon (Table 1), as contrasted to the Crosby pedons, reflecting the high-lime parent sediments and prevailing Holocene aridity. No traces of an argillic horizon were detected. The partial dissolution of a former calcrete has taken place where the pedon was described, judging from the intact calcrete in much of the quarry exposure and fragments of calcrete in places where it has been partially dissolved.

One unusual feature of Pedon H was the presence of the fibrous mineral sepiolite as a major component of the clay fraction in the Bkk3 horizon (Hirmas, 2003). Although sepiolite is common in lacustrine sediments in the arid and semiarid Southwest, it only rarely has been reported to have been pedogenically formed (e.g., Vanden Heuvel, 1966; Casby-Horton, 1997). Palygorskite was also detected in the horizon, but it is relatively common in calcretes of the Southwest.

Micromorphology
A total of 26 thin sections from selected horizons were examined for micromorphology. Their properties are summarized in Table 3 .


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  		Table 3. Micromorphological summary of the Crosby and Hudspeth pedons.

 
The Bt horizons in all pedons showed that appreciable clay illuviation has occurred. Grain argillans with sharp outer boundaries were common. Channel argillans with diffuse boundaries within the matrix were rare. The fabric in most of these horizons was classified as mo-skelsepic (Table 3). There was sufficient oriented clay to justify an argillic horizon designation immediately under the surface horizon in all pedons except C1, which was described as having a cambic horizon. Traces of thin, discontinuous grain argillans, however, were present in the calcic horizon of Pedon C1, suggesting a former argillic horizon that has been engulfed by CaCO3.

Thin sections of all Bk and Bkk horizons were described as having crystic fabric composed of micrite (e.g., Fig. 8a ). Some Btk and Btkk horizons also had a crystic fabric, but most had some parts with a mo-skelsepic fabric. Crystic fabrics of Bk and Bkk horizons were generally porous (Fig. 8a). Sparite was often observed lining channels and as domains within the micrite (Table 3, Fig. 8b and 8e). In addition, channel argillans were commonly observed in these calcic horizons (Fig. 8e and 8f).


Figure 8
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  		Fig. 8. Photomicrographs of (a) and (b) the Bkk horizon of Pedon C2, (c) and (d) a calcrete fragment in the Btkb3 horizon of Pedon C4, (e) the Bkk6 horizon of Pedon C5, and (f) the Bkk2 horizon of Pedon C6 showing micrite (m), sparite (s), argillans (a), voids (v), pores (p), and framework grains (f) using crossed polarizers. Bar length = 100 µm.

 
Many thin sections of calcrete fragments (Fig. 8c and 8d) showed evidence of partial quartz and feldspar dissolution. This was consistent with previous studies (e.g., Reheis, 1988; Monger et al., 1991). The remaining part of the primary mineral was often surrounded by sparry calcite, as contrasted with micrite, in the matrix. Domains of sparite in the micrite were perhaps former voids. Sparite also lined the walls of channels of calcrete fragments and completely filled some apparent former channels (Fig. 8c and 8d). Calcrete fragments contained argillans that were observed coating grains within pores (Fig. 8f).


   DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
CONCLUSIONS
 REFERENCES
 
Crosby Site
A sharp contrast in well-developed continuous clay films and matrix rubification justified buried horizon designations in Pedons C1 and C4 (Table 1). The increased organic C content of the A horizon in Pedon C1 (Table 1) and the lack of an argillic horizon below the epipedon (Fig. 4) suggest that this burial may be due to relatively recent (Holocene) sediment movement from upslope positions. Pedon C1 may have previously had an argillic horizon in the upper part of the profile instead of the present cambic horizon (Fig. 4 and 5). Confirming evidence, however, of an earlier argillic horizon in the upper 102 cm has been destroyed by the addition of carbonate, apparently because of downwearing of the playa slope on which this pedon occurs. Few to common coarse calcrete fragments were present below 48 cm, suggesting that the underlying calcrete, which outcrops a few meters downslope, has been an important parent material contributor. Given the common occurrence of krotovinas observed at the site, however, bioturbation cannot be ruled out in explaining the position of these fragments.

Pedon C4 was designated a buried horizon primarily because of an abrupt color change (Table 1). Buried horizons instead of lithologic discontinuities were designated in this pedon because a clear stratigraphic marker was not reached within the depth excavated at the site. Other pedons were not designated as buried horizons or lithologic discontinuities because no morphological or textural difference was detected to distinguish them as either a previous pulse of eolian Blackwater Draw sediments or part of the Ogallala Formation. Lower horizons of the Crosby transect, however, are probably developed in the Ogallala Formation since they occur below the upper boundary of the escarpment and playa slope outcrop. The absence of detectable buried soils in Pedons C2, C3, C5, and C6 was somewhat surprising, since they are evident in many exposures on the Llano Estacado, especially in the Blackwater Draw Formation (e.g., Allen and Goss, 1974). Assuming that these pedons represent multiple pulses of eolian sedimentation, as are reported in the Blackwater Draw Formation (Holliday, 1989, 1990), there was not a sufficient time period of stability for associated argillic and calcic horizons to develop before another depositional episode occurred. The high amounts of CaCO3 in lower portions of the argillic horizon may also be a factor in obscuring detection of buried soils.

Hillslope position appears to control profile distributions of calcrete expression. On backslope positions (Pedons C1 and C6), the calcrete was strongly expressed with depth and seen as a sharp increase in CMI values below 150 cm (Fig. 5). Expression of calcrete in Pedons C2 and C5, which were located on shoulder positions, increased more gently below approximately 250 cm. The first peak in calcrete expression in Pedon C2 occurs at a similar depth to Pedons C3 and C4, which occur on summit positions. This is not particularly surprising since Pedon C2 occurs on an upper shoulder position (Fig. 4) and has a similar slope to these pedons (Table 1). Calcrete morphology index profiles for summit pedons C3 and C4 terminate with a decrease below 200 cm (Fig. 5). Properties chosen to calculate CMI values (Table 2) primarily reflect the horizon expression and lateral continuity of calcrete (or lack thereof). The presence and nature of buried horizons in Pedon C4 suggest that the lower depths of all pedons are in the Ogallala Formation. Terminal ends of these distributions, therefore, are probably situated in past expressions of a laterally continuous caprock calcrete. Evidence for this includes the presence of a dense interpore mass of coalescent micrite, the reduced number of framework grains per volume of soil (compared with overlying horizons), and dissolution of quartz and feldspar grains embedded within the micrite matrix (Fig. 8a and 8f). Similar micromorphological observations were made of a mature calcrete in southern New Mexico (Monger et al., 1991).

Although an occasional void in calcic and petrocalcic horizons with indications of incipient calcite formation, such as needle (acicular) forms, was present in some pedons, the overall evidence suggests that dissolution of the former calcrete is still occurring. Sparite and oriented clay bodies lining channel walls observed in thin sections from this study suggest that pores and voids postdate the micrite matrix. Evidence that sparite represents recrystallization from micrite in these pedons (e.g., Fig. 8e) includes the abrupt contact between sparite and micrite and the irregular distribution of crystal size (Bathurst, 1975). Argillans lining the walls of a void in the Bkk2 horizon of Pedon C6 (Fig. 8f) indicate that the clay was emplaced after the void was formed by dissolution in the calcic horizon. Clay films in this horizon were not apparent in the field. In general, argillans are not observed in horizons of high carbonate accumulation (Gile and Grossman, 1968; Nettleton et al., 1969). The occurrence of channel argillans in our study, even in the presence of high CCE, can be explained as follows. Pore formation occurred by the increased deep percolation of meteoric waters (especially on summits) during pluvials. Subsequently, clay was illuviated into these horizons and oriented along channel walls. High CCE values in these horizons present a problem to an eluviation theory, however, since free Ca ions would favor flocculation of clays. Perhaps this problem may be resolved, however, by chemical differences between pores and the carbonate matrix. Larger pores in the carbonate-dominated matrix would preferentially transmit water carrying suspended clays. Channel walls would favor the deposition of clay through flocculation (Chadwick and Graham, 2000). As these channels were further coated with silicate clay, the dissolution of calcite would be slowed and the channels would become more effective conduits for clay translocation. Oriented clay has been reported in thin sections of indurated calcretes (e.g., Jiménez-Espinosa and Jiménez-Millán, 2003).

In addition to the destruction of calcrete continuity, thin sections indicate that internal structures of individual calcrete fragments have been somewhat degraded. Observations of this include the presence of dissolution pores and neoformation of sparite (Fig. 8c and 8d). Argillans coating grains within pores support this conclusion (Fig. 8d).

Calcrete fragments were concentrated at the ends of the transect near exposures of the caprock. The interaction between surface slope and water movement within the profile appears to explain this distribution. Greater amounts of water have infiltrated the soil more on level surfaces than sloping ones, resulting in greater carbonate dissolution. The distribution of calcrete fragments among the pedons strongly suggests a former continuous indurated layer. Additionally, the presence of the thickest argillic horizons under the least sloping surfaces (<1%) of Pedons C3 and C4 supports the conclusions based on the distribution of calcrete fragments. Greater water infiltration into these soils (due to their nearly level slope) has facilitated the translocation of clay, yielding thicker, better developed argillic horizons than in those soils on steeper slopes.

The first bulge in calcrete expression in these pedons (Fig. 5) represents the upper boundary of the calcic horizons (Fig. 4). Ignoring Pedon C1, which appears to have inherited parent sediments from upslope positions, upper calcic horizon boundaries parallel the topography of the land surface. This association suggests that the upper portions of calcic horizons reflect the latest period of soil development for this site. This is supported by enhanced argillan development in lower Bt horizons compared with the uppermost Bt horizons, indicating the formation of the former under more humid conditions (Nettleton et al., 1969). In addition, upper calcic and Btk horizons appear to be argillic horizons that have been increasingly engulfed by carbonates with depth. Evidence includes grain argillans embedded in a carbonate matrix, lack of oriented clay bodies lining dissolution pores, and domains of mo-skelsepic within crystic fabrics (Btk horizons). Alternatively, this latter observation may reflect dissolution of carbonates, permitting translocation of clay into the carbonate-dominated matrix.

The positive relationship between CCE and total clay content (below 40% CCE) largely reflects processes of carbonate precipitation in Bt and Btk horizons and the uppermost calcic horizons (Fig. 7). The depth-weighted average CCE value for these horizons was 15.7%. Evidence from thin sections indicates carbonate engulfment of silicate clay enriched horizons for the upper parts of Pedons C2 to C6. Increasing clay content with CCE in these horizons makes sense in view of this. The initial increase in clay with CCE probably reflects an increase in silicate clay that has been eluviated from overlying horizons. More dominantly, however, the increase in carbonate content is itself probably adding clay-sized carbonate and increasing the total clay contents. The negative correlation observed above 40% CCE may reflect the degradation of calcrete in lower portions of these pedons. Original silicate clays are rarely preserved within horizons containing high carbonate contents and the high values of carbonate may be adding to coarser sized fractions, especially since these horizons were probably indurated at one time. These lower horizons probably represent a calcrete that has been degrading rather than a calcic horizon that is aggrading.

Figure 9 summarizes our interpretation of calcrete degradation at the Crosby site. The distribution of increased water percolation during pluvials was controlled by geomorphic position and surface slope. The result was greater clay illuviation and degradation of the caprock under surfaces with decreasing slope gradient. Downwearing of the playa slope resulted in a wedge of calcrete fragments in which Pedon C1 has developed. The latest period of soil development has resulted in carbonate precipitation and engulfment of argillic horizons that makes the caprock an unreliable stratigraphic marker for the Ogallala Formation at this site.


Figure 9
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  		Fig. 9. Diagram showing idealized process of caprock degradation and the resulting difficulty in using the degraded caprock in recognizing the upper boundary of the Ogallala Formation at the Crosby site. Illustration not to scale.

 
Spatial Extent of Degradational Features
Calcrete degradation appears to be widespread in West Texas. Degradation observed in the Crosby pedons could be attributed to effects associated with the eastern margin of the Llano Estacado. The lack of a continuous caprock calcrete in deep road cuts and quarries across the Llano Estacado, however, suggests that calcrete degradation is not limited to our study area.

Comparison between the eastern edge of the Llano Estacado and the Diablo Plateau is difficult because of different parent materials and soil moisture regimes in the two areas. Despite these differences, the Hudspeth site showed evidence of calcrete degradation in the form of partially degraded calcrete fragments, pronounced piping of the calcrete, and occasional dissolution channels in thin sections. These features suggest a widespread occurrence of calcrete degradation.

Our studies have important implications for soil mapping and interpretations on the Llano Estacado and perhaps other areas of the Southwest where calcretes are extensive. Implications are that the calcrete may not be continuous although it is exposed, or occurs at relatively shallow depths, on the more sloping parts of the landscape. This suggests that more, and perhaps deeper, observations should be made to determine the continuity of the calcrete in the more level segments of the landscape.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
REFERENCES
 
Calcrete degradational features were observed in many exposures such as road cuts and pits excavated for road base material. These suggest that calcrete degradation is widespread on the Llano Estacado. It is also common in the Trans-Pecos region although we did not study that area extensively.

The absence of a continuous caprock on the Llano Estacado questions its use as a stratigraphic marker to distinguish the Ogallala and Blackwater Draw Formations in some areas. The problem is further compounded by the textural similarity of the sediments and the presence of multiple paleosols, often with similar morphologies, in both formations.

The lack of a continuous calcrete capping the apparent Ogallala Formation along the transect we studied suggests that much of it has been degraded, especially on nearly levels areas. Evidence for this includes the concentration of calcrete fragments near the ends of the transect, geomorphic relationships to carbonate horizon expression, slope gradient correlation to depth of free carbonates, dissolution channels in the crystic fabrics of calcic horizons and calcrete fragments, and the presence of argillans lining dissolution channels. Our findings indicate a period of increased degradation of the caprock, probably during pluvial periods, followed by a shallower calcification forming the upper portions of the soil column.


   ACKNOWLEDGMENTS
 
This research was funded in part by the Rockwell Foundation, Houston, TX. We thank Corey Moffet for field assistance and Necip Güven, Geosciences Dep., Texas Tech Univ., for the use of laboratory equipment.


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication October 11, 2006.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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