, ;

GENESIS- OF CAR~ONA TE PAN IN ARID REGION OF RAJASTHAN

B. B. ROY, P. C. CHATTERJI AND S. PANDEY

Central; Aria Zone Research Institute, Jodhpur.,t!_,

INTRODUCTION

The carbonate pan ',or 'the' 'kankar' pan known as 'Rotha' in local language is,;a very common feature In ~he soil profile of the arid region or Rajasthan.Generally, it isconce~~ed unqer alluvj1l1 or blown sand deposit but sometimesthe calcium carbonate pa~ ,is e~p'osed at the surface, when the soil is removed by,erosion. Megascopically, tQe pan)s very thick, hard and compact, and when dryit is whitish grey toreddis~ }Vhite in co;lour. In appearance it resembles neithera rock nor the alluvium or sand. La Touche (1902) believed that the carbonater .•. 1 ::

pan has developed from .t~e ~?ra,rp.inifera brought by storms from· the Kutchregion. Auden (1952) observed., ,that these pans had no observed source of lime

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other than the narrow outcrops; ~f ~!n~liyan limestone at Gotan. Krishnan (1952)attributed the cause for such., development to marine transgression upto Mes-ozoic period. Raychaudhuri and Sen (1952) are of the opinion that the pan hasdeveloped due to the weathering of minerals in the soil under humid conditionsrelated to the earlier phase of the desert: Taylor et al (1955) suggested four factorsresponsible for'the development of pan, namely, (i) rainfall less than 200 mm withnet excessive evaporation, (ii) relatively stable or slowly degrading land surface,(iii) deep weathering of parent rocks~-'and (iv) effect of rain water containingcarbonic acid on weathered rocks. In this paper the authors have discussed themode of occurrence of these pans, and the role they play in land use of theregion.

ENVIRONMENT

For detailed study a small area of about 3,000 sq. km. representative ofentire arid tract of Western Rajasthan was taken. This region forms partsof Jodhpur, Barmer; Jalor and Pali districts and is bounded by latitudes 25°30'to 26° north and longitudes 72°30' to 73° east. The depth at which the carbonatepan is encollntered at various locations is shown in Fig. 1. The mean annualrainfall varies from 250 mm in the west to about 500 mm in the east. Themaximum summer temperature exceeds 40°C, and the avt"rage minimum tempera-ture is 9°C which often goes below freezing point. Humidity is generally low andstrong south-westerly winds blow during April-October. Physiographically the

MORPHOLOGICAL CHARACTERISTICS

Two soil profiles. one over rhyolite and the other over granite, are des-cribed below.

region is practically a plain having an elevation of 152 ± 8 m above M. S. L.with a general slope towards the west. The principal drainage system consistsof the e'i1hemer.alLuni and Sukri rivers which flow only during high st0rfus. 'Fher'eare a few residual domes of volcanics and granite which cover less than one percent of the region. These residual domes have radial drainage channels whichare also ephemeral and, dry up or get suffocated due to piling up of blown sand,after flowing for short distance. The soils of the region are generally sandy tosandy loam and calcareous of different degrees. There is very little horizon differ-entiation in the profile. The tllickness of the soils over 'kankar' pan varies froma ew centittleters to over a meter near the ephemeral channels. The vegetationoflhe region consIsts of sparse and scattered thorny trees, shrubs and grassesana is classed as northern thorn forest. The trees are generally confined to thehills establishing in runnels, flood plains or in regions where there is enough soilcover; shrubs and grasses are usually confined to the area of shallow soilcover. A few'small- trees and shrubs have come up on the carbonate pan wheresand has been depogited. These associations are shown in Fig. 2. The litholog-ical formation of this region belong to Palaeozoics, composed of Malanivolcanics, Siwana granite and Jalore granite, and Quaternaries composed of olderalluvium and grit (above carbonate pan). younger alluvium and blown sand. Theolder alluvium is derived from erosion of bed rock debris and also developedin situ. Ground water is generally associated with Quaternary formationsand is found' at d'epths ranging from 3 to 30 m below surface. The wateris generally mineralized to different degrees. The environmental association ofdie carbonate pan is shown in Fig. 3.

B. B. ROY, P. C. CHATTERJI AND S. PANDEY182

1. Profile description at Garah over rhyolite surface.0-75 em Sandy loam.75-120 cm Coarse sand, calcareous.120-180 em Friable, loose, carbonate nodules and lime

coated gravels.

180 em + Weathered rock mantle, calcareous.

2. Profile description at Paoawa over granite surface.

0-100 cm Sandy loam, calcareous, gritty.100-230 em Carbonate nodules and lime coated gravels.230 em + Weathered rock mantIe, calcareous, segregation of lime in joints.

B. B. ROY, P. C. CHATTEIUI AND S. PANDEY

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GENESIS OF CARBONATE PAN IN ARID REGION OF RAJASTHAN 183·

It can be seen from the profile descriptions that in tbis region below thethin soil cover an abrupt zone of friable, poorly cemented, rounded tosubroUnded carbonate nodules appear wbich gradually become more compactand hard with' depth. Generally, the pan of one. to one and a half metersthickness is impervious and confines the regional water-table. This pangenerally overlies granite surface but at some places it has been obs-

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erved overlying r the volcanics. The pan has layers sub-parallel to the landsuiface •. 'Thekankar zone is thin in piedmont sloping plains. but the thick-ness increases' considerably in the alluvial plains and depressions .

. Fig. 4 Illustrates the occurrence of the hard pan in relation to thegeomorphic etivironment. Pan formarion is not encountered in inselberg zone.In-the piedmont plains the pan is found at about 45 cm depth which grad-ually increases in depth towards the lower plains, where it is found moreor less at' uniform depth ranging from 60 to 100 cm. In the alluvial 'plainsthe pan lies at 45 to 100 cm depth and at 120 cm depth in the shallow salinede'pressions of internal drainage.

DISCUSSION

(a) Origin oj pan: The origin of carbonate pan is essentially due to theepigenic changes of physico--chemical reactions. Under such influences theparent rock is first mechanically disintegrated, and then the primary and sec-ondary products which result from the chemical reactions either remain insitu. or are transported elsewhere. Although such changes are slow, they dotake place even under normal climatic conditions.

The region under consideration is mostly covered with granitoid (granitelike) rocks. The granitoid rocks are composed of stable basic silicates. Undermetamorphism some of the weakly held base metals are released when reacted upon.by carbonic acid of the rain water. Infiltrated rain water charged with carbonic acidstarts chemical reactions in the weathered zone. When the carbonic acid becomesstronger than silicic acid, under the katamorphic condition some reactionsdevelop new salts, predominantly calcium carbonate. The oxidation and·leaching processes due to imensified action of oxygen and carbon dioxide resultin rock solution (crest) in the zone of weathering. These processes saturate thesub-surface zone with calcium and magnesium bicarbonates, ferric oxide, sulphatesand chlorides of the same metals, and with salts of sodium and potassium. Thecalcium and magnesium carbonates precipitate from the solution. In the deeperz~nes along with carbonat~s. suJphlites and chlorides also precipitate. Thisprocess leads to the cementation of sub-surface formations by filling intersticesand crevices, and coating rock surface by these salts.

18 B. B. R(i)Y, P. C. CHi\:TTER:JI AND S. PANDE¥'

In tlie piedmont zone layers of. the pan ate faund. In be'ween libese la~rssoi<ldepos-i~sare aiso faund. The roc~ £ragments which have been ~ashed awa~from the highlands have been deposited in bancls. bater on, tbese fragmentsha¥e been cemented with lime. The lime nodules of the pan found in lihe allu ialplaiPls inyarilibly. consist ot: fragments of rock-s of the locality.

The origin of the gan due to foraminifera requires a veryextensiv.e coastalarea for these shells which could have contributed so much of lime to makethick deposits of calcium carbonate. Moreover, some undissolved shells sliouldhave been found in the deposits.

The limestones of Sojat, Gotan and the Ara-vallis are an dolomitic, Ifthese outcrQRSare taken as the soutce of. lime as suggested by .~uden (1952)for, the Ran, then this should have been found along the stream courses ~his:horiginate from them and magnesium should have been the predominant constitu~nt.But carbonate pan is found throughout the aJ;id tract of Rajasthan and is Ivery,rich in calcium. The dominance of magnesium should ha:ve been in the chemi~~l '., .'constituents of the pan if the idea of Krishnan (1952) is taken that the pan .consists of sea-salts due to transgr.$ssion of, sea over this part of Rajasthan;Moreover, the sea never covered the entire arid part of Rajasthan where thepan is found.

\rule opinion ofoRa~eha>1:ldhuciand Sen (1952) aJa'ants a'~ery thick: depQ,s',o soils go as to aceoun f'Or the accumulation of so huge an Mlle:unt ot] lime di!~le1fChingunder very humid climatic conditions. 'I:his would have also simulta-.neously resulted in the formation ot' a tbature soil pr,ofile. Neither the remnant ~fsuch mature soil is found anywhere in the region, nor there was any powerfulerosive agency which could transRort them to the sea. Over and above these, therewas never sucli climatic conditions of the region in the past which could provi\tesuch situations. The older alluvium below wliich the pan is feundconsists ofcearse grained immature soils. Tlie thickness of alluvium issmall.

lit is iiiore plbbaBle, theref0re, that the calcium carbonate pim lias· beenformed in the arid region by the weathering of c(i)~ntrS'rock.:s-,predominaRtlygra"ilit~and rhyolite, and by the release of calcium through. reaction withc'lubonic. '

aaiEl as stated ati<lve. '¥he soils above the pan are .alltranspor.tediand as such,haiVeeonti!ibufed very little to the formation of thick'~kankaf" zone.

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GENESIS OF CARBONATE PAN IN ARID REGIONS OF RAJASTHAN 185

This type of the climate in this part of the country has been existingsince very long during the Quaternary era or even earlier. It is knownthat during this era four glaciations occurred upto the height of 6000ft in the Himlhyas. Tilese glaciations must have considerably influencedthe climatic conditions of Western Rajasthan situated in the tropical belt. Inbetween these glaciations there were three interglacial intervals. The glacial andinterglacial periods have been characterised as pluvial (warm and moist) and non-pluvial (hot and dry) periods. It is surmised that the glaciations were ofdifferent intensities. The last two glaciations 5,00,000 years ago were of lowintensities. As a result of this, the climate started declining towards morearidity In between the two glaciations or pluvials of low intensities dunes of oldsystem were formed. In the last pluvial period these dunes were stabilized andsome of them were dissected. During the present period of extreme ariditynew dunes are being formed. It is also supported by the presence of alternatelayers of pan in the piedmont zones and dunes.

The palaeobotanical studies done by Jacob et al (1952) and Singh (l967)also support this. Voute (1964) has concluded that calcareous crusts or calicheof tbe Mediterranean area represented a deposit of interpluvial periods of theQuaternary era. Verstappen et al (1969) stated that the formation of the"kankar" concretions was most likely to be associated with past changes inclimate, alternately towards less arid and more arid conditions.

Role of carbonate pan on land use : The "kankar" pan is very hard andimpervious and the penetration of roots through it is very difficult. Accordingto Kaul and Ganguli (1964) plantation trials in shal10w soils over pan in the aridzone of Rajasthan did not succeed unless a groove was made Into the pan. Thenatural vegetation of this region is thus very poor. Litchfield and Mabbutt(1962) while discussing the silicate hard pan of Australia have indicated that thevegetation comes up only in the grooves fil1ed with al1uvia or sand or on thebanks of sands (Wandarrie banks), where the roots can penetrate. In otherparts of the region only those shrubs and grasses come up which have lateralexpansion and can establish in the shallow soils or sand cover. Due to thecontinued process of pan formation and calcareous nature of the soils thegroundwater is generally saline. Where the pan formation is closer to the surfacethe static water level is also close to the surface. Groundwater associated withthe older alluvium i!\ more mineralized as this formation overlies the pan(Chatterji, 1965). This indicates that the mineralization of groundwater is morerelated to the lithology of the aquifer.

In general, this region has calcareous and saline soil, mineralized ground-water, sparse and scattered vegetation of thorny trees, shrubs and grasses and

poor land use due to tbe presence of pan at shallow depth. These factors modifiedthe micro-climatic conditions which in turn enhanced the arid environment ofthis region.

186 B. B. ROY, P. C. ,CHATTERJI AND S. PANDEY

CONCLUSIONS

It is concluded that the carbonate pan has developed due to the katamorphicprocesses in the weathered zone of regional geological formations, i e., granite andvolcanics. The salts, of which the pan is composed. have developed from therocks due to the physico-chemical reactions. The very thickness of the panindicates that this process has been active since long in the geological past ratherthan in the last few thousand years. The sparse and scattered vegetation, calcareousand saline soils, and mineralized groundwater of this region are mainly due tothe development of the carbonate pan. Moreover, cultivation is generallypractised either on' the flood plains of the existing ephemeral channels or overblown sand covers or sand dunes giving rise to a type of land use which is ultim-ately due to the presence of the carbonate pan. In general, the development ofthe carbonate pan' which is still in process of formation, is enhancing thearid conditions.

ACKNOWLEDGEMENTAuthors are grateful to the Director, Central Arid Zone Research Inst-

itute, Jodhpur, for providing facilities and for going through the manuscript.

REFERENCES

Auden, J. B. 1952. Some geological and chemical aspects of the Rajasthansalt problem. Bull. N at. I nst. Be. India. 1: 53-67.

Chatterji, P. C. 1965. Some chemical type of ground water from SiwanaDevelopment Block region (Barmer dist.), Western Rajasthan. Quart.J. Geol. Min. Met. Soc. India. 39: 193-207.

Jacob, K., Sastry, M. V. A. and Sastry, V. V. 1952. A note on the microfos-sils of the impure gypsum from the Jamsar mine, Bikaner and on thepossible origin of the gypsum. Bull. N at. Inst. Bei. Ind. 1: 68-69.

Krishnan, M.S. 1962. Geological history of Rajasthan and its relation topresent day conditions. Bull. Nat. Inst. Bei. India. 1: 19-31.

Kaul, R. N. and Ganguli, B. N. 1964. Afforestation studies in the arid zoneof India, Symp. Probl. Indian Arid Zone, (in press).

La Thouche, D. 1902. Geology of Western Rajputana. Geol. Sur. lnd. Mem.XXXV : 24-41.

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GENESIS OF CARBONATE PAN IN ARID REGION OF RAJASTHAN 187

Litchfield, W. H. and Mabbutt, J. A. 1962. Hardpan in soils of semi-arid westernAustrala. J. Soil Sei. 13: 148-159.

Raychaudhuri, S. P. and Sen, N. 1952. Certain geomorphological aspects ofRajputana desert, Bull. Nat. In'lt. Sei. Ind. 1: 249-253.

Singh, G. 1967. A palynological approach towards the resolution of someimportant desert problems in Rajasthan. Geohyd. 3: 111-128

Taylor, G. C. , Roy, A. K., Seth, D. N. and Sen, B. M. 1955. Groundwatergeology of Pali region, Jodhpur Division, Western Rajasthan. Bull. Geol.Sur. Ser. B. 6. : 23-26.

Verstappen, H. Th., Ghose, B. and Pandey, S. 1969. Landforms and resourcesin Central Rajasthan (India) Results of the Jalore pilot survey. Int.In8t. Aerial Sur. &, Earth Se'. B 51 : 1-20.

Voute, C. 1964. Engineering characteristics of some soils in the semi-arid and,arid zones (unpublished). Int; Inst. Aerial Survey and Earth Be••,(Delft.)