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Petrologic analysis and geochemistry of the Late Neogene-Early Quaternary hardpan calcretes of Western Rajasthan, India
Quaternary International 106–107 (2003) 3–10
Petrologic analysis and geochemistry of the Late Neogene-Early Quaternary hardpan calcretes of Western Rajasthan, India Hema Achyuthan Centre for Geoscience and Engineering, Anna University, Chennai 600 025, India
Abstract Calcretes formed over bedrock such as Precambrian hornblende-chlorite schist, rhyolite and carbonaceous phyllites in Western Rajasthan, India, were studied for their mineralogical composition and geochemistry. They form a distinct element of the Quaternary landscape of Western Rajasthan, India. The study area is located within the Nagaur–Churu–Jaipur tract of Western Rajasthan. The purpose of this study was to determine the processes governing the development of hardpan calcretes, and to evaluate the local and regional controls on their formation. Micromorphology included pedogenetic and groundwater features within the hardpan calcretes. Thickening of calcite laminae downward and tapering at the sideward edges around the unweathered minerals of quartz and feldspars indicated cumulative and compound pedogenesis, which probably occurred locally, and downward movement of carbonate solution and pore water. Carbonate solutions were probably derived from the upper horizons or surfaces bringing about the process of dissolution and recementation of individual laminae. Occurrence of fibrous palygorskite as coatings around the detrital grains and siderite (oolitic and pisolitic in shape) points to a subalkaline–subacidic process of pedogenesis in a semi-arid to arid climate. Stable d13C and d18O isotope data of the hardpan calcrete laminae vary between 0.3% to 1.5% and 5.9% and 1.5%, respectively indicating their formation at or near surface (capillary fringe), probably supporting a thin column of soil. The source of most of the calcite is groundwater; however, calcite nodule formation was largely dependent on pedogenic processes associated with evaporation, evapotranspiration and /or microenvironmental changes in pH and CO2 partial pressure. Dust is also a major source for carbonate precipitation. Although it is commonly assumed that the powdery calcretes are younger in age compared to the more complex forms, the occurrence of Middle Palaeolithic tools below the hardpan calcrete at Roopangarh and Dayalpura, and above the hardpan calcretes at Mitri, Genana and Rol, indicates that the morphology of calcretes is not a reliable indicator of age. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction Calcretes and calcic soils provide information about the age of a Quaternary deposit and the climate under which the soil developed (Gile et al., 1966; Machette, 1978; McFadden and Tinsley, 1985). The usefulness of calcic soils for stratigraphic interpretation was described by Gile et al. (1966), who introduced the concept that carbonate morphology in soils changes with time and can be described by a sequence of morphologic stages. Machette (1978, 1985) quantified carbonate development by using the total amount of secondary carbonate in a soil profile as a soil-age indicator. The use of total mass of carbonate in soils as an age indicator is based on the condition that carbonate is introduced to soils as solid carbonate in dust and Ca2+in rainwater. Calcretes occurring in the Torripsamment soils and Psammentic Camborthid
(Dhir et al., 1992) have also been used for palaeoclimatic interpretation (Achyuthan et al., 2002). Other data on calcretes occurring as calciorthids or paleorthids (Dhir, 1977) also play an important part in geologic studies of active tectonics, geomorphology, palaeoclimatology and stratigraphy, requiring age control on young deposits. Calcretes and their types from the Thar Desert have been used in geochemical, sedimentological and stratigraphic studies (Courty et al., 1987; Raghavan, 1987; Achyuthan and Rajaguru, 1997, 1998). In these studies many important fundamentals such as geochemistry, types of alteration of minerals, and formation of clays regarding their genesis have been neglected. Such fundamental information is essential if the calcretes are to be used for examining an important period in the evolution of Quaternary landscape and palaeoclimate of the Thar Desert.
1040-6182/02/$ - see front matter r 2002 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 2 ) 0 0 1 5 8 - 1
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H. Achyuthan / Quaternary International 106–107 (2003) 3–10
In this paper, calcretes formed over the parent rock with distinct contact and underlying the aeolian/fluvial sediments are termed as hardpan calcretes with no genetic connotations. They are important lithological units as they are form a distinct element of the Quaternary landscape. These calcretes remain undated due to the paucity of dateable material and unsuitable dating methods. The purpose of this study is to determine (a) the distribution and primary controls of calcrete development, (b) morphological features and internal structures within the calcrete, (c) mineralogy of the clay and non-clay fraction of the hardpan calcrete and (d) stable carbon and oxygen isotopic values and processes governing the development of hardpan calcretes.
2. Study area The study area is located within the Nagaur–Churu– Jaipur tract (Fig. 1), which formed a part of the arm of the Tethys Sea during the Tertiary period and structurally bears evidence of Precambrian uplifted blocks of the Aravalli range overlain by Quaternary deposits. Hardpan calcretes are associated with the bedrock such as the Precambrian hornblende gneiss, rhyolite, and hornblende-chlorite schist in the area between Sambhar–Narena tract near Jaipur to Mitri and
Dayalpura near Didwana, Churu–Ladnun near Sujangarh, and occuring as well rounded boulders and gravel along the Jayal–Katoati area. In the Talchappar salt lake, the lacustrine sediments lie over the hardpan calcrete, which in turn overlies the Precambrian hornblende-chlorite schist. Around Sujangarh, rhyolites of Precambrian age are overlain by the hardpan calcrete horizon. CaCO3 also cements the weathered basement rock. Laminated hardpans also occur within large fractures parallel to bedding (Fig. 2). The beginning stages of calcrete formation have taken place along bedding and joint planes. The calcrete unit is generally reddish orange (10R 6/6,10R 6/8) to red (10R 5/6) in colour nearly a metre and a half-thick exhibiting pisolitic and oolitic textures. The thickness of calcrete in vertical and lateral sequence is variable, and this unit is observed thinning out towards the pediment surfaces. Archaeological material in the form of Middle Palaeolithic tools occurs below the hardpan calcrete at Roopangarh and Dayalpura, and above the hardpan calcretes at Mitri, Genana and Rol (Misra et al., 1982; Raghavan, 1987). Based on geomorphology, structural configuration of the study area and occurrence of archaeological material, formation of these calcretes has been dated to Late Neogene–Early Quaternary period (Achyuthan and Rajaguru, 1997, 1998). The present day climate is continental semiarid, characterised by long hot summers and cool winters
Fig. 1. Location map of the study area in Western Rajasthan.
H. Achyuthan / Quaternary International 106–107 (2003) 3–10
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(Bullock et al., 1985) and staining test following Dickson (1965) were applied. Major oxides were determined following Snell and Snell (1963). CaO and MgO were determined by titration method with EDTA following Heald (1976) and are presented in Table 2. These analyses were determined using colorimeters and compared with BG rock standard samples. Clay mineralogy was determined using XRD analyses (Table 2). XRD analyses were made of oriented and unoriented samples of most clay concentrates, and the clay concentrates were also analysed after glycolation. Samples of matrix and calcite cement rims and phases were analysed for d13C and d18O values, both reported with reference to PDB. Stable isotopes of the individual laminae were cryogenically measured and the results are presented in Table 3. Stable isotope analyses were conducted at the Desert Laboratory, University of Arizona, Tucson.
(Bshw of Thornthwaite (1948). The annual precipitation in the area around Didwana varies from 281 to 503 mm from year to year with a mean average annual rainfall of 367 mm. Average rainfalls for six stations in the study area are presented in Table 1. The daytime temperature during summer reaches 48–501C whereas the winter temperature is as low as 2–41C. Except during the monsoon, high diurnal temperature variations with amplitude of 25–301C occur at the surface, and around 201C at 5 cm depth in the soil. Dust storms of severe intensity and of long duration are common during the summer. 3. Materials and methods In order to understand the mineralogical composition of the hardpan calcretes micromorphological analyses
4. Results Chemical composition of calcretes reveals that they are predominantly composed of low-magnesium calcite. The ratio of CaO to MgO is almost 2 to 1. The low MgO content in the calcretes reflects the original carbonate and Mg phase. The second most important constituent of the calcretes is silica. The ratio of SiO2/CaCO3 is not equal as a result of calcite accumulation and void filling. Reeves (1976) made similar observations. Minor constituents include Al2O3 and Fe2O3, which are also low in content (Table 2). Low iron content in calcite is consistent with removal of available Fe2+ by Hydrogen sulphide. The slight ferroan composition of matrix calcite in individual calcrete nodules within the hardpan calcretes could have resulted from earlier and shallower precipitation during microbial reduction of Fe3+. Low proportion of insoluble residue in the hardpan calcretes indicate that they formed by more than a simple process of dissolution, filling, reprecipitation, recementation and recrystallisation. CaCO3 values vary from 56.37% to 74.2%. Mineralogical analyses of the non-clay fraction (>2 mm fraction) of the hardpan calcretes show that
Fig. 2. An exposure of hardpan calcrete. Quarried exposed section at Mitri, illustrating very thinly laminated calcitic texture around the quartz grain. The pen as a scale bar is B12 cm.
Table 1 Rainfall distribution parameters of annual rainfall for different stations in Western Rajasthan Station
Mean (mm)
Median (mm)
Standard deviation (mm)
Coefficient of variation %
Nagaur Didwana Merta City Parbatsar Nawa
315.3 359.6 406.7 389.0 468.4
303.6 333.3 400.0 381.3 472.2
157.5 181.5 186.8 174.1 199.0
50 51 46 45 43
Pearsonian Coefficient of skewness 0.21 0.45 0.12 0.12 0.06
H. Achyuthan / Quaternary International 106–107 (2003) 3–10 Palygorskite, dehydrated illite, vermiculite Kaolinite, illite, vermiculite Kaolinite, illite, vermiculite, hematite Kaolinite Palygorskite, dehydrated illite, vermiculite, hematite Illite, vermiculite, hematite Palygorskite, dehydrated illite, vermiculite, hematite Illite, vermiculite, hematite, kaolinite Illite, vermiculite, hematite Palygorskite, dehydrated illite, vermiculite, hematite, kaolinite 2.0 1.6 1.8 1.9 2.0 1.7 2.2 2.1 2.0 1.9 25.7 27.6 23.4 25.2 22.3 30.4 22.3 24.3 23.4 26.2 Talchappar Narena Sambhar Ladnun Mitri Jayal Ringan Genana Rol Didwana 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
3.1 2.87 2.62 3.2 3.68 1.39 3.88 2.39 3.15 2.1
1.39 1.27 0.92 1.73 1.38 0.64 1.55 1.10 1.76 0.82
2.2 2.2 2.8 1.8 2.6 2.2 2.5 2.1 1.7 2.5
0.93 0.54 0.48 0.64 0.72 0.52 0.68 0.68 0.56 0.48
74.2 62.3 68.33 61.76 62.54 60.94 66.11 61.76 62.54 56.37
53.4 46.7 44.2 48.3 46.26 51.87 49.07 51.87 47.60 48.7
Clays CaO/MgO MgO% CaO% CaCO3% Fe2O3% SiO2/Al2O3 Al2O3% SiO2% Site Sl.No.
Table 2 Major oxides and clay mineralogy of the hardpan calcretes from the sites studied.
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Table 3 Stable Isotope data of the hardpan calcretes Sl.No.
Site
d13C%
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
Kataoti crust (a) Kataoti crust (b) Kataoti 2a Kataoti 2b Kataoti 3a Kataoti 3b Mitri crust a Mitri crust b Mitri 2a Mitri 2b Mitri 3a Mitri 3b Jayal crust a Jayal crust b Jayal 2a Jayal 2b Jayal 3a Jayal 3b PBMDa crust a PBMD crust b PBMD 2a PBMD 2b PBMD 3a PBMD 3b
+1.2 +1.2 +1.1 +1.1 +1.6 +0.9 +1.1 +1.1 +1.5 +1.6 +1.3 +1.3 +1.1 +0.7 +0.8 +0.8 +1.2 +1.3 +0.4 +0.3 +0.9 +1.1 +1.6 +1.7
d18O % (PDB) 2.0 2.2 3.7 3.6 3.5 4.7 2.5 2.4 5.9 5.9 2.7 2.5 2.7 3.9 3.9 4.0 1.7 1.8 5.1 5.3 1.7 1.5 4.1 4.0
a
PBMD is Pir Baba Masjid site at Didwana. Crust a,b,2a,2b,3a,3b are crusts and inner laminae of hardpan calcrete nodule. Stable isotope values increases with depth 3a and 3b are the inner most laminae surrounding the mineral grain.
low-magnesium calcite and lesser amounts of quartz are the two dominant minerals present within the calcrete with trace amounts of Ferroan dolomite [Ca (Fe, Mg, Mn)(CO3)2 (Ankerite) or Ca(Mn, Mg, Fe)(CO3)2 (Kutnohorite)], and non-ferroan dolomite (Ca Mg(CO3)2), feldspar, hematite, altered biotite, and siderite. Traces of hematite are indicative of an oxygenated environment (Jaynes and Chafetz, 1997). The quartz, feldspar, and biotite are interpreted as being lithogenic and/or detrital in origin. Quartz and feldspars are common to the dust, Torripsamments soils, Camborthids, Calciorthids, and limestone residues within the region, and are considered to be locally derived. Micromorphological analyses of the hardpan calcrete exhibit polycyclic and polyphase development of calcite (micrite 2–4 mm, microsparite 4–8 mm, sparite 8–12 mm and above) around the detrital grains of quartz, chlorite, feldspar, hornblende, mica, and rock fragments of chlorite hornblende schist, rhyolite, and quartzite (Achyuthan and Reddi, 1993). Petrographic analyses show a variety of features such as meniscus cement, clay coating around the quartz and feldspar grains within the individual calcic nodules, alteration of mica, and iron oxide impregnation that can be interpreted as the result of groundwater action and pedogenesis. Therefore, these hardpan calcretes are not
H. Achyuthan / Quaternary International 106–107 (2003) 3–10
singularly ‘‘alpha-type or beta type’’ calcretes but are a combination of the two (Wright, 1990). Groundwater features analysed in this study include recrystallised mottles consisting of ferruginous impregnations, cracks related to desiccation and fractures, clay mineral replacement such as dehydrated illite, palygorskite, concretionary laminations and etched non-carbonate grains. Biotic features observed in the field and within the thin sections are roots and rootlet filaments coated with powdery calcite (micrite). The combination of groundwater, biotic and pedogenesis of Torripsamment and Camborthid soils has produced many macrofeatures. The soils have slightly higher contents of clay and silt (7–9% clay and 4–7% silt) (Dhir et al., 1992). The groundmass is microcrystalline to crystalline and strongly cemented with calcite. Channels and voids within the nodules are filled with microsparite and sparite. The individual nodules within the calcretes (0.1–0.5 mm across) are spherical or ellipsoidal with 2–3 concentric rims (10–15 mm thick) of clear micrite. Individual nodules consists of 0.3–0.5 mm thick carbonate rims (3–4 in number), rims around the detrital grains and nodules of calcium carbonate and siderite. The alternate light and dark brown rims (10–15 mm thick) of microcrystalline calcite were clearly visible on staining microcrystalline calcite with Alizarin red ‘S’. The dark laminae can be correlated to the Fe/Mn oxides and micrite and the lighter laminae to the pale pink microsparite. Darker laminations occur by the breakdown of primary iron-bearing minerals from the hornblende gneiss and schist. Some of the intergranular fractures within the laminations were cemented with either micrite or sparite. Floating texture was also observed. Exfoliation of biotite grains with iron oxide (hematite) halo is common within the individual nodules. Between the interface cement (micrite/microsparite) and the pore space, meniscus cement has formed at localised grain contacts. At the point contact, the cement consists of fine clear microspar or micrite, formed by the process by which freshwater films around the grains dissolve carbonate to allow original point contacts between the grains to become flatter (Knox, 1977). This cement is evidence of fluctuating ground water activity (Umargikar, 1983). Palygorskite as identified in diffractograms is a minor constituent in Talchappar, Mitri, Jayal, and Genana. Palygorskite clays (Fig. 3) occur as incomplete lining around the detrital quartz, siderite grains and also individual calcic nodules within the hardpan calcretes. Palygorskite in calcretes has been documented by Vanden Heuvel (1966), Gardner (1972), and Millot et al., (1977). The formation of palygorskite has been attributed to the alteration of detrital montmorillonite and mixed layer montmorillonite-illite (Gardner, 1972). Palygorskite differs in composition from sepiolite in having a higher content of Al and a lower ratio of Si to
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Fig. 3. Thin section photomicrograph of hardpan calcrete with Fe oxide coating and discontinuous Palygorskite (py), plane polarised light.
Mg. Mg can be selectively captured at relatively shallow depths by clay minerals such as montmorillonite and chlorite (Folk, 1976). Palygorskite would be expected to form by reaction and replacement of aluminous detritus such as montmorillonite or aluminous bedrock (Gardner, 1972, Millot et al., 1977). In the study area the hardpan calcretes have formed over the bedrock, which is composed of aluminosilicate minerals. Hardpan calcretes display calcified rootlets and filaments, which is indicative of some rooted vegetation overlying the hardpans during the formation. Micronodules within the hardpan calcretes have been interpreted as being formed by either cyanobacteria or bacteria (Verrecchia et al., 1995; Wright et al., 1996).
5. Discussion Low magnesium calcite and quartz are two dominant minerals present within the fraction >2 mm of the calcrete with traces of feldspar, hematite, rhyolite, and quartzite grains. Occurrence of fibrous palygorskite as coatings around the detrital grains and siderite (oolitic and pisolitic in shape) points to a subalkaline–subacidic process of pedogenesis (Folk, 1976) in a semi-arid to arid climate (Courty et al., 1987; Raghavan, 1987). Palygorskite has formed due to the dissolution of silica grains and reaction and replacement of aluminous detritus such as montmorillonite or aluminous bedrock. Lamination of calcite rim around the quartz grain (Fig. 4) indicates cumulative and compound pedogenesis, which probably occurred locally. Stable isotope data of the hardpan calcrete laminae (Table 3) vary between d18O 5.9% and 1.5%. Wideranging d18O values of the matrix and the calcite rims are typical of carbonate minerals precipitated under shallow burial conditions during the anaerobic microbial oxidation of organic matter (Irwin et al., 1977; Coleman, 1993). The negative d18O values of most matrix calcite with non-ferroan composition suggest
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Fig. 4. Thin section photomicrograph of hardpan calcrete revealing micritic lamination and coating around the quartz grain, crossed nicols.
precipitation during sulfate reduction (Coniglio et al., 2000). In this study, the most significant negative shifts in the d18O values are up to 4.4% which is indicative of increased concentration of plant derived CO2 below the soil surfaces (Cerling and Quade, 1993). The enriched 13 C values could be indicative of either: (a) isotopic mixing with atmospheric CO2, (b) degassing of 12C enriched CO2 closer to the ground surfaces or (c) an increase over time in the abundance of C4 plant biomass (Jaynes and Chafetz, 1997). The higher d13C values within the hardpans may reflect higher rates of CO2 degassing closer to the ground surfaces (Salomons and Mook, 1986; Rossinsky and Swart, 1993). Lowering of PCO2 can occur rapidly along cracks and voids open to the ground surface, and laminar coatings occur associated along the walls of the cracks (Rabenhorst and Wilding, 1985). In this study many of the cracks and fractures in the hardpan calcretes were observed using thin sections, cemented by micrite or sparite. Jaynes and Chafetz (1997) and Rabenhorst and Wilding (1985) made similar observations. Wide-ranging d13C values indicate their precipitation at or near surface (capillary fringe), probably supporting a thin column of Camborthid soil. The two variables that effect d18O in hardpan calcretes are: (a) the d18O value of soil water, and (b) temperature (Cerling, 1984; Quade et al., 1989). The d18O of calcrete forms in equilibrium with the local soil water, and the oxygen isotopic composition of the soil water is related to the local meteoric water (Cerling, 1984). Within the soil zone, the oxygen isotopic composition of soil water can be different from the meteoric water. This results from seasonal variations in the isotopic compositions of meteoric water. The d18O value of the meteoric waters can be enriched in 18O during evaporation as they percolate into the soil zone (Cerling, 1984; Quade et al., 1989). In this study, the d18O values range between 5.9% and 1.5% and indicate d18O formed from mixing with meteoric water. The contribution of oxygen from the host rock is
negligible because of the very high-water/rock ratios in soil systems (Jaynes and Chafetz, 1997). The broad range of d18O compositions of the calcretes reflect fluctuations possibly related to climate, addition of fresh water, etc. Presence of micronodules within the hardpan calcrete indicates that these calcretes were formed at the ground surface with minimal soil cover (Verrecchia et al., 1995). The concentric fabric of calcite and iron oxide nodules is indicative of episodic growth. Diffuse spherical iron oxide-rich appear to be incipient nodules. The iron may have been supplied, in large part, by alteration of heavymineral and iron rich mineral grains within the calcrete horizon. Iron capping present on some detritus grains and small calcite nodules in lower parts on noncalcareous horizons indicate downward movement of water (Bullock et al., 1985). The source of Mg2+ is probably the chlorite hornblende gneiss. The source of most of the calcite is groundwater. However, calcite nodule formation was largely dependent on pedogenic processes associated with evaporation, evapotranspiration, and/or microenvironmental changes in pH and CO2 partial pressure. Calcium carbonate in atmospheric dust may be one of the main sources of carbonate found in desert soils that have non-calcareous parent materials (Gile et al., 1966). For example, the maximum amount of soil calcite derived from the weathering of silicate minerals was calculated as 1% in comparison with 99% from dust sources in the Whipple Mountains (McFadden, 1982, p.182). Hardpan calcretes in the study area overlie diverse rock types such as shale, hornblende-chlorite schist, phyllites, and limestone. This does not preclude local sources of calcium carbonate from bedrock. Structures such as individual calcitic nodules in the hardpan calcretes also offer evidence of calcium carbonate source. These individual nodules are enlarged downward side due to the thickening of individual laminae. The downward movement of carbonate solution derived from upper horizons or surfaces could explain this thickening (Reeves, 1976) and this also brings about the process of dissolution and re-cementation of successive laminae.
6. Age of hardpan calcretes It is commonly assumed that powdery calcretes are younger in age compared to the more complex types such as the hardpan calcretes. Archaeological material associated with calcretes implies that the morphology of calcrete is not a reliable indicator of age. For example Acheulian tools, which are, at least of the Late Middle Pleistocene (Gaillard et al., 1983, Achyuthan et al., 1991) are associated with the powdery and nodular calcrete while Upper Palaeolithic tools in the 16R dune
H. Achyuthan / Quaternary International 106–107 (2003) 3–10
section dated to 26,000 yr. BP (Raghavan and Courty, 1987; Achyuthan et al., 1989) are associated with pedogenic nodular calcretes. Middle Palaeolithic tools (150,000–40,000 yr. BP) are found to occur within the pedogenically transformed dune sediments and nodular calcretes as at the 16R dune section (Achyuthan et al., 1989), transported nodular calcretes as at Mangalpura and Shyampura, and below the hardpan calcrete at Roopangarh. The evidence discovered at Roopangarh indicates that the development of hardpan calcrete can take place rapidly as compared to the hardpan calcretes of Mitri, Genana, and the boulder calcretes of Jayal. In the study area, all the calcrete types, ranging from the simple form of powdery calcrete to the hardpan variety, reveal similar chemical and mineralogical composition (Raghavan, 1987). However, these are not the reliable parameters for understanding the age of the calcretes. Since all the calcretes form by the complex process of dissolution, reprecipitation, and recementation, local soil microenvironment plays an important role in the development of calcrete types. The numerical dates on pedogenic carbonates indicate only minimum ages. Dating methodologies have limitations and more so with the calcretes in the study area, as they are formed in open systems. In the Nagaur–Jodhpur tract, based on archaeological and radiometric dating evidence, calcrete forming processes must have operated since the Late Neogene–Early Quaternary period for the low lying valley hardpan calcretes such as those of Genana, Mitri, Ringan, and Didwana, and middle to late middle Pleistocene for the hardpan calcrete at Roopangarh. Thus, hardpan calcrete formed on various bedrock in the present day arid–semiarid tract of Western Rajasthan is, therefore, not merely a reflection of local topography but host material, provenance of sediment, time and rates of accretion, vegetation, soil microenvironment and carbonate source.
7. Conclusions Caliche occurs on various Precambrian bedrock such as hornblende-chlorite schist, rhyolite, and limestone throughout the tract. The mineralogy of the non-clay fraction is low-magnesium calcite and quartz, with traces of feldspar, biotite, hematite, and siderite. Both groundwater and biotic processes have played a role in the development of the calcrete and the combination of these processes has produced many of the macrofeatures. The d13C values within the calcretes reveal C4 biomass during calcrete development, which formed at the near surface horizon with a thin soil cover.
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Acknowledgements The author is grateful to Dr. Jay Quade, Dept. of Geosciences, University of Arizona, Tucson, USA, for discussions and help in interpreting the isotopic data on calcretes.
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preliminary report on 1982 excavation. Man and Environment 7, 112–130. Gardner, L.R., 1972. Origin of the Mormon mesa caliche, Clark county, Nevada. Geological Association of America Bulletin 83, 143–156. Gile, L.H., Peterson, F.F., Grossman, R.B., 1966. Morphological and genetic sequences of carbonate accumulation in desert soils. Soil Science 101, 347–360. Heald, R.W., 1976. Calcium and magnesium. In: Black, C.A., Evans, D.D., White, J.L., Ensminger, L.E., Clark, F.E. (Eds.), Methods of Soil Analysis. Part II. Madison, American Society of Agronomy, Inc. Publishers, pp. 999–1009. Irwin, H., Curtis, C.D., Coleman, M., 1977. Isotopic evidence for the source of diagenetic carbonates formed during burial of organicrich sediments. Nature 269, 209–213. Jaynes, R.S., Chafetz, H.S., 1997. A petrographic analysis of caliche within the central Texas region. Gulf Coast Association of Geological Societies Transactions 67, 239–249. Knox, G.J., 1977. Caliche formation, Saldhana Bay, South Africa. Sedimentology 24, 657–674. Machette, M.N., 1978. Geology of the San Acacia Quadrangle, Socorro County, New Mexico. United States Geological Survey Geologic Quadrangle Map GQ-1415. Machette, M.N., 1985. Calcic soils of the South-western United States. In: Weide, D. (Ed.), Soils and Quaternary geology of the southwestern United States. Geological Society of America Special Paper 203, pp. 1–21. McFadden, L.D., 1982. The impacts of spatial and temporal climatic changes on alluvial soils in southern California, Ph.D. Thesis: Tucson, University of Arizona, p. 182. McFadden, L.D., Tinsley, J., 1985. Rate and depth of Pedogeniccarbonate accumulation in soils: formulation and testing of a compartment model. In: Weide, D. (Ed.), Soils and Quaternary geology of the Southwestern United States. Geological Society of America Special paper 203, pp. 23–42. Millot, G., Nahon, D., Paquet, H., Ruellan, A., Tardy, Y., 1977. L’ Epigenie calcaire des roches silicates dans les encroutements carbonates en pays subaride Antiatlas. Maroccoan Science Geological Bulletin 30, 129–152. Misra, V.N., Rajaguru, S.N., Raju, D.R., Raghavan, H., Gaillard, C., 1982. Acheulian occupation and evolving landscape around Didwana in the Thar Desert, India. Man and Environment 6, 72–86. Quade, J., Cerling, T.E., Bowman, J.R., 1989. Systematic variations in the carbon and Oxygen Isotopic composition of pedogenic carbonate along elevation transects in the southern Great Basin,
United States. Geological Society of America Bulletin 101, 464–475. Rabenhorst, M.C., Wilding, L.P., 1985. Pedogenesis on the Edwards Plateau, Texas: II Formation and Occurrence of Diagnostic Subsurface Horizons in a climosequence. Soil Science Society of America Journal 50, 687–692. Raghavan, H., 1987. Quaternary Geology of Nagaur District, Rajasthan. Ph.D. Thesis, Poona University, Pune. Raghavan, H., Courty, M.A., 1987. Holocene pedosedimentary and Pleistocene pedo-sedimentary environments in the Thar desert (Didwana, India). In: Federoff, N., Bresson, L.M., Courty, M.A., (Eds.), Proceedings VII Soil Micromorphology. Association Francaise pour l’etude du Sol, Paris, pp. 639–646. Reeves, Jr., C.C., 1976. Caliche origin, classification, morphology and Uses. Estacado Books, Lubbock. Rossinsky, Jr., V., Swart, P.K., 1993. Influence of climates on the formation and Isotopic composition of calcretes. In: Swart, P.K., Lohmann, K.C., Mckenzie, J., Savin, S., (Eds.), Climate change in continental Isotopic Records. Geophysical Monograph 78, 67–75. Salomons, W., Mook, W.G., 1986. Isotope geochemistry of carbonates in the weathering zone. In: Fritz, P., Fontes, J. Ch. (Eds.), Handbook of Environmental Isotope Geochemistry 2, Elsevier, Amsterdam, pp. 239–269. Snell, F.D., Snell, C.T., 1963. Colorimetric Methods of Analysis. Vol. II. Toronto, D. Van Nostrand Company. Thornthwaite, C.W., 1948. An approach towards a rational classification of climate. Geographical Review 38, 55–94. Umargikar, S.V., 1983. Quaternary Geology of the Upper Krishna Basin. Ph.D. Thesis, Poona University, Pune. Vanden Heuvel, R.C., 1966. The occurrence of sepiolite and Palygorskite in the calcareous in the calcareous zone of a soil near Las Cruces, New Mexico. Proceedings, 13th National Conference on Clays and Clay Mineralogy, pp. 193–207. Verrecchia, E.P., Freytet, P., Verrecchia, K.E., Dumont, J.L., 1995. Spherulites in calcrete laminar crusts: Biogenic CaCO3 precipitation as a major contributor to crust formation. Journal of Sedimentary Research 65, 690–700. Wright, V.P., 1990. A micromorphological classification of fossil and recent Calcic and petrocalcic microstructures. In Douglas, L.A. (Ed.), Soil Micromorphology: A Basic and Applied Science. Developments in Soil Science, Vol. 19, Elsevier, Amsterdam, pp. 401–407. Wright, V.P., Beck, V.H., Sanz-Montero, M.E., 1996. Spherulites in calcrete laminar crusts: biogenic precipitation as a major contribution to crust formation-Discussion. Journal of Sedimentary Research 66, 1040–1041.
Petrologic analysis and geochemistry of the Late Neogene-Early Quaternary hardpan calcretes of Western Rajasthan, India Hema Achyuthan Centre for Geoscience and Engineering, Anna University, Chennai 600 025, India
Abstract Calcretes formed over bedrock such as Precambrian hornblende-chlorite schist, rhyolite and carbonaceous phyllites in Western Rajasthan, India, were studied for their mineralogical composition and geochemistry. They form a distinct element of the Quaternary landscape of Western Rajasthan, India. The study area is located within the Nagaur–Churu–Jaipur tract of Western Rajasthan. The purpose of this study was to determine the processes governing the development of hardpan calcretes, and to evaluate the local and regional controls on their formation. Micromorphology included pedogenetic and groundwater features within the hardpan calcretes. Thickening of calcite laminae downward and tapering at the sideward edges around the unweathered minerals of quartz and feldspars indicated cumulative and compound pedogenesis, which probably occurred locally, and downward movement of carbonate solution and pore water. Carbonate solutions were probably derived from the upper horizons or surfaces bringing about the process of dissolution and recementation of individual laminae. Occurrence of fibrous palygorskite as coatings around the detrital grains and siderite (oolitic and pisolitic in shape) points to a subalkaline–subacidic process of pedogenesis in a semi-arid to arid climate. Stable d13C and d18O isotope data of the hardpan calcrete laminae vary between 0.3% to 1.5% and 5.9% and 1.5%, respectively indicating their formation at or near surface (capillary fringe), probably supporting a thin column of soil. The source of most of the calcite is groundwater; however, calcite nodule formation was largely dependent on pedogenic processes associated with evaporation, evapotranspiration and /or microenvironmental changes in pH and CO2 partial pressure. Dust is also a major source for carbonate precipitation. Although it is commonly assumed that the powdery calcretes are younger in age compared to the more complex forms, the occurrence of Middle Palaeolithic tools below the hardpan calcrete at Roopangarh and Dayalpura, and above the hardpan calcretes at Mitri, Genana and Rol, indicates that the morphology of calcretes is not a reliable indicator of age. r 2002 Elsevier Science Ltd and INQUA. All rights reserved.
1. Introduction Calcretes and calcic soils provide information about the age of a Quaternary deposit and the climate under which the soil developed (Gile et al., 1966; Machette, 1978; McFadden and Tinsley, 1985). The usefulness of calcic soils for stratigraphic interpretation was described by Gile et al. (1966), who introduced the concept that carbonate morphology in soils changes with time and can be described by a sequence of morphologic stages. Machette (1978, 1985) quantified carbonate development by using the total amount of secondary carbonate in a soil profile as a soil-age indicator. The use of total mass of carbonate in soils as an age indicator is based on the condition that carbonate is introduced to soils as solid carbonate in dust and Ca2+in rainwater. Calcretes occurring in the Torripsamment soils and Psammentic Camborthid
(Dhir et al., 1992) have also been used for palaeoclimatic interpretation (Achyuthan et al., 2002). Other data on calcretes occurring as calciorthids or paleorthids (Dhir, 1977) also play an important part in geologic studies of active tectonics, geomorphology, palaeoclimatology and stratigraphy, requiring age control on young deposits. Calcretes and their types from the Thar Desert have been used in geochemical, sedimentological and stratigraphic studies (Courty et al., 1987; Raghavan, 1987; Achyuthan and Rajaguru, 1997, 1998). In these studies many important fundamentals such as geochemistry, types of alteration of minerals, and formation of clays regarding their genesis have been neglected. Such fundamental information is essential if the calcretes are to be used for examining an important period in the evolution of Quaternary landscape and palaeoclimate of the Thar Desert.
1040-6182/02/$ - see front matter r 2002 Elsevier Science Ltd and INQUA. All rights reserved. PII: S 1 0 4 0 - 6 1 8 2 ( 0 2 ) 0 0 1 5 8 - 1
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In this paper, calcretes formed over the parent rock with distinct contact and underlying the aeolian/fluvial sediments are termed as hardpan calcretes with no genetic connotations. They are important lithological units as they are form a distinct element of the Quaternary landscape. These calcretes remain undated due to the paucity of dateable material and unsuitable dating methods. The purpose of this study is to determine (a) the distribution and primary controls of calcrete development, (b) morphological features and internal structures within the calcrete, (c) mineralogy of the clay and non-clay fraction of the hardpan calcrete and (d) stable carbon and oxygen isotopic values and processes governing the development of hardpan calcretes.
2. Study area The study area is located within the Nagaur–Churu– Jaipur tract (Fig. 1), which formed a part of the arm of the Tethys Sea during the Tertiary period and structurally bears evidence of Precambrian uplifted blocks of the Aravalli range overlain by Quaternary deposits. Hardpan calcretes are associated with the bedrock such as the Precambrian hornblende gneiss, rhyolite, and hornblende-chlorite schist in the area between Sambhar–Narena tract near Jaipur to Mitri and
Dayalpura near Didwana, Churu–Ladnun near Sujangarh, and occuring as well rounded boulders and gravel along the Jayal–Katoati area. In the Talchappar salt lake, the lacustrine sediments lie over the hardpan calcrete, which in turn overlies the Precambrian hornblende-chlorite schist. Around Sujangarh, rhyolites of Precambrian age are overlain by the hardpan calcrete horizon. CaCO3 also cements the weathered basement rock. Laminated hardpans also occur within large fractures parallel to bedding (Fig. 2). The beginning stages of calcrete formation have taken place along bedding and joint planes. The calcrete unit is generally reddish orange (10R 6/6,10R 6/8) to red (10R 5/6) in colour nearly a metre and a half-thick exhibiting pisolitic and oolitic textures. The thickness of calcrete in vertical and lateral sequence is variable, and this unit is observed thinning out towards the pediment surfaces. Archaeological material in the form of Middle Palaeolithic tools occurs below the hardpan calcrete at Roopangarh and Dayalpura, and above the hardpan calcretes at Mitri, Genana and Rol (Misra et al., 1982; Raghavan, 1987). Based on geomorphology, structural configuration of the study area and occurrence of archaeological material, formation of these calcretes has been dated to Late Neogene–Early Quaternary period (Achyuthan and Rajaguru, 1997, 1998). The present day climate is continental semiarid, characterised by long hot summers and cool winters
Fig. 1. Location map of the study area in Western Rajasthan.
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(Bullock et al., 1985) and staining test following Dickson (1965) were applied. Major oxides were determined following Snell and Snell (1963). CaO and MgO were determined by titration method with EDTA following Heald (1976) and are presented in Table 2. These analyses were determined using colorimeters and compared with BG rock standard samples. Clay mineralogy was determined using XRD analyses (Table 2). XRD analyses were made of oriented and unoriented samples of most clay concentrates, and the clay concentrates were also analysed after glycolation. Samples of matrix and calcite cement rims and phases were analysed for d13C and d18O values, both reported with reference to PDB. Stable isotopes of the individual laminae were cryogenically measured and the results are presented in Table 3. Stable isotope analyses were conducted at the Desert Laboratory, University of Arizona, Tucson.
(Bshw of Thornthwaite (1948). The annual precipitation in the area around Didwana varies from 281 to 503 mm from year to year with a mean average annual rainfall of 367 mm. Average rainfalls for six stations in the study area are presented in Table 1. The daytime temperature during summer reaches 48–501C whereas the winter temperature is as low as 2–41C. Except during the monsoon, high diurnal temperature variations with amplitude of 25–301C occur at the surface, and around 201C at 5 cm depth in the soil. Dust storms of severe intensity and of long duration are common during the summer. 3. Materials and methods In order to understand the mineralogical composition of the hardpan calcretes micromorphological analyses
4. Results Chemical composition of calcretes reveals that they are predominantly composed of low-magnesium calcite. The ratio of CaO to MgO is almost 2 to 1. The low MgO content in the calcretes reflects the original carbonate and Mg phase. The second most important constituent of the calcretes is silica. The ratio of SiO2/CaCO3 is not equal as a result of calcite accumulation and void filling. Reeves (1976) made similar observations. Minor constituents include Al2O3 and Fe2O3, which are also low in content (Table 2). Low iron content in calcite is consistent with removal of available Fe2+ by Hydrogen sulphide. The slight ferroan composition of matrix calcite in individual calcrete nodules within the hardpan calcretes could have resulted from earlier and shallower precipitation during microbial reduction of Fe3+. Low proportion of insoluble residue in the hardpan calcretes indicate that they formed by more than a simple process of dissolution, filling, reprecipitation, recementation and recrystallisation. CaCO3 values vary from 56.37% to 74.2%. Mineralogical analyses of the non-clay fraction (>2 mm fraction) of the hardpan calcretes show that
Fig. 2. An exposure of hardpan calcrete. Quarried exposed section at Mitri, illustrating very thinly laminated calcitic texture around the quartz grain. The pen as a scale bar is B12 cm.
Table 1 Rainfall distribution parameters of annual rainfall for different stations in Western Rajasthan Station
Mean (mm)
Median (mm)
Standard deviation (mm)
Coefficient of variation %
Nagaur Didwana Merta City Parbatsar Nawa
315.3 359.6 406.7 389.0 468.4
303.6 333.3 400.0 381.3 472.2
157.5 181.5 186.8 174.1 199.0
50 51 46 45 43
Pearsonian Coefficient of skewness 0.21 0.45 0.12 0.12 0.06
H. Achyuthan / Quaternary International 106–107 (2003) 3–10 Palygorskite, dehydrated illite, vermiculite Kaolinite, illite, vermiculite Kaolinite, illite, vermiculite, hematite Kaolinite Palygorskite, dehydrated illite, vermiculite, hematite Illite, vermiculite, hematite Palygorskite, dehydrated illite, vermiculite, hematite Illite, vermiculite, hematite, kaolinite Illite, vermiculite, hematite Palygorskite, dehydrated illite, vermiculite, hematite, kaolinite 2.0 1.6 1.8 1.9 2.0 1.7 2.2 2.1 2.0 1.9 25.7 27.6 23.4 25.2 22.3 30.4 22.3 24.3 23.4 26.2 Talchappar Narena Sambhar Ladnun Mitri Jayal Ringan Genana Rol Didwana 1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
3.1 2.87 2.62 3.2 3.68 1.39 3.88 2.39 3.15 2.1
1.39 1.27 0.92 1.73 1.38 0.64 1.55 1.10 1.76 0.82
2.2 2.2 2.8 1.8 2.6 2.2 2.5 2.1 1.7 2.5
0.93 0.54 0.48 0.64 0.72 0.52 0.68 0.68 0.56 0.48
74.2 62.3 68.33 61.76 62.54 60.94 66.11 61.76 62.54 56.37
53.4 46.7 44.2 48.3 46.26 51.87 49.07 51.87 47.60 48.7
Clays CaO/MgO MgO% CaO% CaCO3% Fe2O3% SiO2/Al2O3 Al2O3% SiO2% Site Sl.No.
Table 2 Major oxides and clay mineralogy of the hardpan calcretes from the sites studied.
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Table 3 Stable Isotope data of the hardpan calcretes Sl.No.
Site
d13C%
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.
Kataoti crust (a) Kataoti crust (b) Kataoti 2a Kataoti 2b Kataoti 3a Kataoti 3b Mitri crust a Mitri crust b Mitri 2a Mitri 2b Mitri 3a Mitri 3b Jayal crust a Jayal crust b Jayal 2a Jayal 2b Jayal 3a Jayal 3b PBMDa crust a PBMD crust b PBMD 2a PBMD 2b PBMD 3a PBMD 3b
+1.2 +1.2 +1.1 +1.1 +1.6 +0.9 +1.1 +1.1 +1.5 +1.6 +1.3 +1.3 +1.1 +0.7 +0.8 +0.8 +1.2 +1.3 +0.4 +0.3 +0.9 +1.1 +1.6 +1.7
d18O % (PDB) 2.0 2.2 3.7 3.6 3.5 4.7 2.5 2.4 5.9 5.9 2.7 2.5 2.7 3.9 3.9 4.0 1.7 1.8 5.1 5.3 1.7 1.5 4.1 4.0
a
PBMD is Pir Baba Masjid site at Didwana. Crust a,b,2a,2b,3a,3b are crusts and inner laminae of hardpan calcrete nodule. Stable isotope values increases with depth 3a and 3b are the inner most laminae surrounding the mineral grain.
low-magnesium calcite and lesser amounts of quartz are the two dominant minerals present within the calcrete with trace amounts of Ferroan dolomite [Ca (Fe, Mg, Mn)(CO3)2 (Ankerite) or Ca(Mn, Mg, Fe)(CO3)2 (Kutnohorite)], and non-ferroan dolomite (Ca Mg(CO3)2), feldspar, hematite, altered biotite, and siderite. Traces of hematite are indicative of an oxygenated environment (Jaynes and Chafetz, 1997). The quartz, feldspar, and biotite are interpreted as being lithogenic and/or detrital in origin. Quartz and feldspars are common to the dust, Torripsamments soils, Camborthids, Calciorthids, and limestone residues within the region, and are considered to be locally derived. Micromorphological analyses of the hardpan calcrete exhibit polycyclic and polyphase development of calcite (micrite 2–4 mm, microsparite 4–8 mm, sparite 8–12 mm and above) around the detrital grains of quartz, chlorite, feldspar, hornblende, mica, and rock fragments of chlorite hornblende schist, rhyolite, and quartzite (Achyuthan and Reddi, 1993). Petrographic analyses show a variety of features such as meniscus cement, clay coating around the quartz and feldspar grains within the individual calcic nodules, alteration of mica, and iron oxide impregnation that can be interpreted as the result of groundwater action and pedogenesis. Therefore, these hardpan calcretes are not
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singularly ‘‘alpha-type or beta type’’ calcretes but are a combination of the two (Wright, 1990). Groundwater features analysed in this study include recrystallised mottles consisting of ferruginous impregnations, cracks related to desiccation and fractures, clay mineral replacement such as dehydrated illite, palygorskite, concretionary laminations and etched non-carbonate grains. Biotic features observed in the field and within the thin sections are roots and rootlet filaments coated with powdery calcite (micrite). The combination of groundwater, biotic and pedogenesis of Torripsamment and Camborthid soils has produced many macrofeatures. The soils have slightly higher contents of clay and silt (7–9% clay and 4–7% silt) (Dhir et al., 1992). The groundmass is microcrystalline to crystalline and strongly cemented with calcite. Channels and voids within the nodules are filled with microsparite and sparite. The individual nodules within the calcretes (0.1–0.5 mm across) are spherical or ellipsoidal with 2–3 concentric rims (10–15 mm thick) of clear micrite. Individual nodules consists of 0.3–0.5 mm thick carbonate rims (3–4 in number), rims around the detrital grains and nodules of calcium carbonate and siderite. The alternate light and dark brown rims (10–15 mm thick) of microcrystalline calcite were clearly visible on staining microcrystalline calcite with Alizarin red ‘S’. The dark laminae can be correlated to the Fe/Mn oxides and micrite and the lighter laminae to the pale pink microsparite. Darker laminations occur by the breakdown of primary iron-bearing minerals from the hornblende gneiss and schist. Some of the intergranular fractures within the laminations were cemented with either micrite or sparite. Floating texture was also observed. Exfoliation of biotite grains with iron oxide (hematite) halo is common within the individual nodules. Between the interface cement (micrite/microsparite) and the pore space, meniscus cement has formed at localised grain contacts. At the point contact, the cement consists of fine clear microspar or micrite, formed by the process by which freshwater films around the grains dissolve carbonate to allow original point contacts between the grains to become flatter (Knox, 1977). This cement is evidence of fluctuating ground water activity (Umargikar, 1983). Palygorskite as identified in diffractograms is a minor constituent in Talchappar, Mitri, Jayal, and Genana. Palygorskite clays (Fig. 3) occur as incomplete lining around the detrital quartz, siderite grains and also individual calcic nodules within the hardpan calcretes. Palygorskite in calcretes has been documented by Vanden Heuvel (1966), Gardner (1972), and Millot et al., (1977). The formation of palygorskite has been attributed to the alteration of detrital montmorillonite and mixed layer montmorillonite-illite (Gardner, 1972). Palygorskite differs in composition from sepiolite in having a higher content of Al and a lower ratio of Si to
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Fig. 3. Thin section photomicrograph of hardpan calcrete with Fe oxide coating and discontinuous Palygorskite (py), plane polarised light.
Mg. Mg can be selectively captured at relatively shallow depths by clay minerals such as montmorillonite and chlorite (Folk, 1976). Palygorskite would be expected to form by reaction and replacement of aluminous detritus such as montmorillonite or aluminous bedrock (Gardner, 1972, Millot et al., 1977). In the study area the hardpan calcretes have formed over the bedrock, which is composed of aluminosilicate minerals. Hardpan calcretes display calcified rootlets and filaments, which is indicative of some rooted vegetation overlying the hardpans during the formation. Micronodules within the hardpan calcretes have been interpreted as being formed by either cyanobacteria or bacteria (Verrecchia et al., 1995; Wright et al., 1996).
5. Discussion Low magnesium calcite and quartz are two dominant minerals present within the fraction >2 mm of the calcrete with traces of feldspar, hematite, rhyolite, and quartzite grains. Occurrence of fibrous palygorskite as coatings around the detrital grains and siderite (oolitic and pisolitic in shape) points to a subalkaline–subacidic process of pedogenesis (Folk, 1976) in a semi-arid to arid climate (Courty et al., 1987; Raghavan, 1987). Palygorskite has formed due to the dissolution of silica grains and reaction and replacement of aluminous detritus such as montmorillonite or aluminous bedrock. Lamination of calcite rim around the quartz grain (Fig. 4) indicates cumulative and compound pedogenesis, which probably occurred locally. Stable isotope data of the hardpan calcrete laminae (Table 3) vary between d18O 5.9% and 1.5%. Wideranging d18O values of the matrix and the calcite rims are typical of carbonate minerals precipitated under shallow burial conditions during the anaerobic microbial oxidation of organic matter (Irwin et al., 1977; Coleman, 1993). The negative d18O values of most matrix calcite with non-ferroan composition suggest
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Fig. 4. Thin section photomicrograph of hardpan calcrete revealing micritic lamination and coating around the quartz grain, crossed nicols.
precipitation during sulfate reduction (Coniglio et al., 2000). In this study, the most significant negative shifts in the d18O values are up to 4.4% which is indicative of increased concentration of plant derived CO2 below the soil surfaces (Cerling and Quade, 1993). The enriched 13 C values could be indicative of either: (a) isotopic mixing with atmospheric CO2, (b) degassing of 12C enriched CO2 closer to the ground surfaces or (c) an increase over time in the abundance of C4 plant biomass (Jaynes and Chafetz, 1997). The higher d13C values within the hardpans may reflect higher rates of CO2 degassing closer to the ground surfaces (Salomons and Mook, 1986; Rossinsky and Swart, 1993). Lowering of PCO2 can occur rapidly along cracks and voids open to the ground surface, and laminar coatings occur associated along the walls of the cracks (Rabenhorst and Wilding, 1985). In this study many of the cracks and fractures in the hardpan calcretes were observed using thin sections, cemented by micrite or sparite. Jaynes and Chafetz (1997) and Rabenhorst and Wilding (1985) made similar observations. Wide-ranging d13C values indicate their precipitation at or near surface (capillary fringe), probably supporting a thin column of Camborthid soil. The two variables that effect d18O in hardpan calcretes are: (a) the d18O value of soil water, and (b) temperature (Cerling, 1984; Quade et al., 1989). The d18O of calcrete forms in equilibrium with the local soil water, and the oxygen isotopic composition of the soil water is related to the local meteoric water (Cerling, 1984). Within the soil zone, the oxygen isotopic composition of soil water can be different from the meteoric water. This results from seasonal variations in the isotopic compositions of meteoric water. The d18O value of the meteoric waters can be enriched in 18O during evaporation as they percolate into the soil zone (Cerling, 1984; Quade et al., 1989). In this study, the d18O values range between 5.9% and 1.5% and indicate d18O formed from mixing with meteoric water. The contribution of oxygen from the host rock is
negligible because of the very high-water/rock ratios in soil systems (Jaynes and Chafetz, 1997). The broad range of d18O compositions of the calcretes reflect fluctuations possibly related to climate, addition of fresh water, etc. Presence of micronodules within the hardpan calcrete indicates that these calcretes were formed at the ground surface with minimal soil cover (Verrecchia et al., 1995). The concentric fabric of calcite and iron oxide nodules is indicative of episodic growth. Diffuse spherical iron oxide-rich appear to be incipient nodules. The iron may have been supplied, in large part, by alteration of heavymineral and iron rich mineral grains within the calcrete horizon. Iron capping present on some detritus grains and small calcite nodules in lower parts on noncalcareous horizons indicate downward movement of water (Bullock et al., 1985). The source of Mg2+ is probably the chlorite hornblende gneiss. The source of most of the calcite is groundwater. However, calcite nodule formation was largely dependent on pedogenic processes associated with evaporation, evapotranspiration, and/or microenvironmental changes in pH and CO2 partial pressure. Calcium carbonate in atmospheric dust may be one of the main sources of carbonate found in desert soils that have non-calcareous parent materials (Gile et al., 1966). For example, the maximum amount of soil calcite derived from the weathering of silicate minerals was calculated as 1% in comparison with 99% from dust sources in the Whipple Mountains (McFadden, 1982, p.182). Hardpan calcretes in the study area overlie diverse rock types such as shale, hornblende-chlorite schist, phyllites, and limestone. This does not preclude local sources of calcium carbonate from bedrock. Structures such as individual calcitic nodules in the hardpan calcretes also offer evidence of calcium carbonate source. These individual nodules are enlarged downward side due to the thickening of individual laminae. The downward movement of carbonate solution derived from upper horizons or surfaces could explain this thickening (Reeves, 1976) and this also brings about the process of dissolution and re-cementation of successive laminae.
6. Age of hardpan calcretes It is commonly assumed that powdery calcretes are younger in age compared to the more complex types such as the hardpan calcretes. Archaeological material associated with calcretes implies that the morphology of calcrete is not a reliable indicator of age. For example Acheulian tools, which are, at least of the Late Middle Pleistocene (Gaillard et al., 1983, Achyuthan et al., 1991) are associated with the powdery and nodular calcrete while Upper Palaeolithic tools in the 16R dune
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section dated to 26,000 yr. BP (Raghavan and Courty, 1987; Achyuthan et al., 1989) are associated with pedogenic nodular calcretes. Middle Palaeolithic tools (150,000–40,000 yr. BP) are found to occur within the pedogenically transformed dune sediments and nodular calcretes as at the 16R dune section (Achyuthan et al., 1989), transported nodular calcretes as at Mangalpura and Shyampura, and below the hardpan calcrete at Roopangarh. The evidence discovered at Roopangarh indicates that the development of hardpan calcrete can take place rapidly as compared to the hardpan calcretes of Mitri, Genana, and the boulder calcretes of Jayal. In the study area, all the calcrete types, ranging from the simple form of powdery calcrete to the hardpan variety, reveal similar chemical and mineralogical composition (Raghavan, 1987). However, these are not the reliable parameters for understanding the age of the calcretes. Since all the calcretes form by the complex process of dissolution, reprecipitation, and recementation, local soil microenvironment plays an important role in the development of calcrete types. The numerical dates on pedogenic carbonates indicate only minimum ages. Dating methodologies have limitations and more so with the calcretes in the study area, as they are formed in open systems. In the Nagaur–Jodhpur tract, based on archaeological and radiometric dating evidence, calcrete forming processes must have operated since the Late Neogene–Early Quaternary period for the low lying valley hardpan calcretes such as those of Genana, Mitri, Ringan, and Didwana, and middle to late middle Pleistocene for the hardpan calcrete at Roopangarh. Thus, hardpan calcrete formed on various bedrock in the present day arid–semiarid tract of Western Rajasthan is, therefore, not merely a reflection of local topography but host material, provenance of sediment, time and rates of accretion, vegetation, soil microenvironment and carbonate source.
7. Conclusions Caliche occurs on various Precambrian bedrock such as hornblende-chlorite schist, rhyolite, and limestone throughout the tract. The mineralogy of the non-clay fraction is low-magnesium calcite and quartz, with traces of feldspar, biotite, hematite, and siderite. Both groundwater and biotic processes have played a role in the development of the calcrete and the combination of these processes has produced many of the macrofeatures. The d13C values within the calcretes reveal C4 biomass during calcrete development, which formed at the near surface horizon with a thin soil cover.
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Acknowledgements The author is grateful to Dr. Jay Quade, Dept. of Geosciences, University of Arizona, Tucson, USA, for discussions and help in interpreting the isotopic data on calcretes.
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