Evolution of lower proterozoic epicontinental deposits: Stromatolite-bearing Aravalli rocks of Udaipur, Rajasthan, India

Precambrian Research, 14 (1981) 49--74

49

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

EVOLUTION OF LOWER PROTEROZOIC EPICONTINENTAL DEPOSITS: STROMATOL1TE-BEARING ARAVALLI ROCKS OF UDAIPUR, RAJASTHAN, INDIA

A.B. ROY and B.S. PALIWAL

Department of Geology, University of Rajasthan, Udaipur 313 001 (India) Department of Geology, Government Bangur College, Didwana 341 303 (India) (Received January 7, 1980; revision accepted August 8, 1980)

ABSTRACT Roy, A.B. and Paliwal, B.S., 1981. Evolution of Lower Proterozoic epicontinental deposits: stromatolite-bearing AravaUi rocks of Udaipur, Rajasthan, India. Precambrian Res., 14: 49--74. The Aravalli rocks (> 2060 Ma old) which crop out around Udaipur, Western India, comprise a thick sequence of metasediments with stromatolites and basal volcanics resting unconformably over a peneplained basement, known as the Banded Gneissic Complex (ca. 2585 Ma old). The rocks have undergone a very low grade of metamorphism, and display a complex structure resulting from two major and several minor episodes of folding. There are two distinctly different 'facies sequences' in the Aravalli rocks, indicating deep-sea and nearshore shelf environments. The stratigraphic sequence of the rocks deposited under the shelf environment starts with basic volcanics and tuffs (altered to greenschists) and quartizites with arkosic conglomerate. In the next sequence carbonates predominate in association with orthoquartzites, carbonaceous phyllites, phyllites, and stromatolitic rock-phosphate. The carbonate sequence passes upward into greywacke-phyllite--lithic arenite in the distal parts and conglomerate--arkose--orthoquartzite in proximal areas. The youngest sequence comprises orthoquartzite with silty arenite. The distribution of different facies, particularly that of dolomite with stromatolitic rock-phosphate, is controlled by sea-floor topography suggesting the presence of an epicontinental sea bounded by a landmass to the west and a series of islands and shoals. Sedimentation in the shelf and epicontinental sea was presumably triggered by development of fault-controlled troughs along craton margins. Terrigenous debris was deposited in newly-developed troughs with contemporaneous volcanicity along trough margins. With the erasing of the ephemeral relief in the provenance, carbonate deposition was initiated. The environment encouraged algal growth and formation of stromatolitic rock phosphate. Carbonaceous phyllites developed in areas of restricted circulation. Rapid influx of terrigenous detritus with renewed tectonism in the next phase resulted in the deposition of a turbidite sequence of greywacke--phyllite and lithic arenite in the deeper parts of the epicontinental sea, and conglomerate--arkose--orthoquartzite in the marginal areas. The final phase of sedimentation was presumably under fluvial conditions which marked the completion of epicontinental trough filling. The nature of the terrigenous clasts indicates a predominantly granitic source of sediments. Supply of sediment was mainly from the continent to the east and partly from a landmass to the west. The cycle of sedimentation noted in the epicontinental AravaUi sea is broadly similar to the model of tectonic stages suggested by Krynine (1942). 0301-9268/81/0000--0000/$02.50 © 1981 Elsevier Scientific Publishing Company

50 INTRODUCTION The Aravalli rocks, exposed over a wide area in central and southern Rajasthan (Fig. 1), are perhaps the most ancient group of sediments in India in which evidence of life (stromatolites) has been recorded. Resting on the gneissic basement, the Banded Gneissic Complex, with a " p r o f o u n d " erosional unconformity (Heron, 1953), the Aravallis form a thick pile of predominantly pelitic sediments with some arenites, carbonates and volcanics. Metamorphism and migmatization have altered these rocks in certain regions, erasing their sedimentary character (Naha et al., 1966). In some parts, however, the rocks show very little metamorphism (Udaipur and its surrounding area, for example), thus many interesting depositional features are preserved (Damle and Sharma, 1970). These are therefore the areas, where a correct stratigraphic order can be established, and the process of deposition and basin development can be worked o u t with reasonable precision. Heron (1953) has given an excellent account of the Aravalli and other Precambrian rocks of the region in his classic memoir, and has suggested the following stratigraphic succession of the main rock units: Delhi System Raialo Series AravaUi System Banded Gneissic Complex The regional correlation between the major stratigraphic units has recently been reviewed by a n u m b e r of workers (Sen, 1970; Raja Rao et al., 1971; Naha and Halyburton, 1974a). All these studies notwithstanding, the basic framework of the stratigraphy erected by Heron (1953) for Rajasthan as a whole remains unchanged, although certain revisions in local stratigraphic sequence have been proposed. On the basis of Sr/Rb age determination by Crawford (1970), Naha and Halyburton (1974a) suggested that the closing age of the earliest folding in the Aravalli rocks is ca. 2060 Ma. Assuming that the Berach Granite is the time equivalent of the Banded Gneissic Complex, the basement of the AravaUi metasediments would be around 2585 Ma old (Crawford, 1970). The nature and the distribution of AravaUi metasediments around Udaipur indicate their deposition under two contrasted environmental conditions. The rocks exposed to the east of the Madar-Kharpina line (Fig. 2) Fig. 1. Regional distribution of the Aravalli and the other Precambrian rocks in central and southern Rajasthan. Based on the map published by the Geological Survey of India, 1969. (1) pre-Aravalli granites, gneisses, and schists (Banded Gneissic Complex and Berach Granite); (2) AravaUi Group; (3) post-Aravalli--pre-Delhi granite; (4) Raialo Group; (5) Delhi Group; (6) post-Delhi granites; (7) Vindhyans; (8) alluvium. (U) Udaipur City; (C) Chittorgarh; (A) = Ajmer. The area in Fig. 2 is shown in the frame;

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52 show diverse lithological associations. The prevalence of carbonate rocks showing profuse development of stromatolites in this part of the Aravalli rocks indicates a shallow-water shelf environment. On the other hand, the rocks to the west of the Madar-Kharpina line, comprising a very thick and monotonous sequence of micaschists and phyllites with some beds of quartzites, represent a deep-sea depositional environment (cf. Poddar, 1966). In the present account an attempt has been made to study the evolutionary history of the sediments deposited in the shelf sea. This assumes significance because of the presence of stromatolitic rock-phosphate in this region. STRUCTURE The metasedimentary rocks belonging to the Aravalli Group (the term Group here replaces Heron's "System") including the associated volcanic effusives, were involved in two main phases of folding, generally referred to as F, and F2 folding phases (Roy et al., 1971; Roy, 1972; Naha and Halyburton, 1977). The structure of the rocks of the region is primarily the result of superposition of folds on different scales produced during these two phases of folding. Later fold movements (F3 and F4) were less penetrative in nature and produced mostly small-scale folds and kink bands (Roy, 1973; Naha and Halyburton, 1974b). The earliest deformation which was the most penetrative in nature produced a series of isoclinal folds with variable attitudes of axial planes. The trend of these folds was roughly east-west. A penetrative schistosity was impressed on the rocks during this folding movement. In addition, the rocks suffered a large amount of stretching along the direction of fold axes. The early formed folds were refolded on all scales by open to isoclinal folds of the second generation (F2) with steep to vertical north-south-striking axial planes. The interference of these two phases of folding produced different outcrop patterns depending on the angles between the axial planes and hinges of two fold systems. The most commonly developed outcrop pattern is a mirror-image type showing non-plane, noncylindrical fold geometry. There are also some closed outcrops representing dome and basin structures. Coaxial refolding of earlier folds producing cylindrical non-plane folds has also been recorded at places. The F2 folds show an interesting pattern in the attitude of their axial planes over the whole region. Around Udaipur City the F2 folds are strictly upright. Eastward these folds show westerly overturning. Thus, all the rocks east of Udaipur structurally underlie the basement rocks. Similarly, the axial planes of F2 folds west of Udaipur dip westerly and consequently there is a predominance of westerly dip of the beds. The trend and amount of plunge of F 2 folds also show a wide range of variation (Roy, 1972}, which is difficult to explain merely assuming the development of these F~ folds on straight limbs and hinges of early isoclinal

53 folds. It has been suggested that F1 isoclinal folds were coaxially refolded into open folds before the superimposition of the second generation folds (cf. Naha and Majumdar, 1971; R o y et al., 1980). The axial planes and form surfaces of F~ folds (bedding and schistosity corresponding to the axial planes of F1 folds) have been deformed sporadically by a set of small-scale recumbent and reclined folds (Roy, 1973; also see Naha and Halyburton, 1974b). These folds (F3), which are often in the form of kink bands, show a rough parallelism of trend with F 2 folds. In all probability the F3 folds were formed during the same continuous movement which produced the F2 folds. The last phases of folding (F4) in the Udaipur region produced kink bands, conjugate folds and open upright folds with subvertical axial planes with roughly east--west strike. Like the F3 folds, the F4 folds are usually on a small scale. It is only in the Jhamarkotra Mines area that some largescale folds of F4 generation have effected the outcrop pattern of the rocks (Roy et al., 1980). On the basis of studies in the area north of Udaipur, Naha and Halyburton (1974b) suggested that the F¢ structures represent longitudinal shortening and may be a type of " a c c o m m o d a t i o n structures" in relation to Fz deformation. It is, however, possible that some of the large-scale F4 folds m a y be the result of a late phase of deformation unconnected with F2. LITHOLOGIC ASSOCIATIONS AND THEIR STRATIGRAPHIC CORRELATION Several lithological associations (cf. Knill, 1960) can be recognized in the rocks of eastern belt. The characteristics and stratigraphic relationship of different lithologic associations are stated below.

Quartz ite--greensch ist association The rocks belonging to this association crop out along two sub-parallel belts (Fig. 2). The eastern belt of rocks forms a narrow strip between the outcrops of pre-Aravalli basement rocks to the east and a thick sequence of conglomerate--arkose--orthoquartzite (Debari quartzites, mapped by Heron, 1953, as outliers of the Delhi Group) to the west. The sequence starts with 'clean-washed' orthoquartzites which locally grade into conglomerate (Fig. 3). Arkosic rocks are comparatively rare, some of the arkosic-looking quartzites are in reality feldspathized quartzites. The quartzites are n o t everywhere in direct contact with the basement rocks; a thin veneer of amphibolites of chlorite--biotite schists locally intervenes between the two. Even within the quartzite units there are some thin layers of greenschists. Westward from the contact of the basement rocks, the greenschists and amphibolites become the more d o m i n a n t members of the association, showing intercalations of thin beds of calcarenites, quartzites and phyllites. The amygdular character of some of the amphibolites and greenschists clearly reveals their

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Fig. 3. Quartzose conglomerate with slightly flattened pebbles at the contact with preAravalli (Formation A). volcanic parentage. Some of the greenschists and chloritic phyllites m ay represent ash-fall tuffs (Heron, 1953). Th e basal character of this association is abundant l y clear from Heron's description of these rocks (Heron, 1953, p. 138). In the J h a m a r k o t r a region, the gneisses, granites and amphibolites representing the pre-Aravalli basem e n t rocks are directly overlain by a ferruginous quartzitic horizon. Here the quartzite horizon (which is gritty at places) can be traced continuously for a b o u t 10 km. It truncates the foliation planes in the basement rocks ( R o y et al., 1980). There is an a br upt change in grade of m e t a m o r p h i s m from the basement t o the quartzites which in m a n y areas hardly show any trace o f metamorphism. T h e r e are also rare cross-beds in these quartzites which indicate younging away from the basement rocks. The second belt of greenschists runs to the west o f Udaipur, between Madar and Kharpina (Fig. 2). There is a strong lithological similarity Fig. 2. Geological map showing distribution of different lithological units around Udaipur (modified from Heron 1953, plate 38). (1) basal volcanics and quartzite--grit-conglomerate; 2 = dolomites with rock phosphate; (3) carbonaceous phillite; (4) phyllite and micaschist; (5) greywacke--phyllite and lithic arenite; (6) conglomerate--arkose-orthoquartzite (Debari quartzites of Heron); (7) orthoquartzite--silty arenite; (8) micaschist--phyllite with bands of quartzite; (9) Delhi parametamorphics; (10) postAravalli granites; (11) pre-Aravalli granites and gnelsses.

56 between the basic volcanics of the two belts (cf. Heron, 1953). in both these cases the basic volcanics are associated with quartzites. In the western belt however, the quartzite is commonly arkosic in character, and has a polymictic conglomerate near its base. Assuming that the basic volcanic outcrops of the Madar-Kharpina belt are effusives erupted during the post-Delhi period, Heron (1953) correlated the quartzite (arkose)--conglomerate sequence here with the thick sequence of Debari Quartzites outcropping to the east. However, if lithological similarity is considered a basis for stratigraphic correlation, then the sequence of basic volcanics--quartzite (arkose)--conglomerate should be correlated with a similar sequence which outcrops in the area between the Debari Quartzites and the basement rocks to the east of Udaipur. In fact, detailed mapping of the rocks of the Madar-Kharpina belt by the present authors has revealed that the quartzite (arkose)--conglomerate sequence associated with the basic volcanic rocks (amphibotites and greenschists) constitutes the oldest rocks over which the dolomites (often phosphatic) and argillaceous phyllites lie. The different primary sedimentary features present in the rocks helped in erecting the following stratigraphic sequence of the rocks exposed between Madar and Kharpina: Argillaceous phyllite Dolomitic limestone (commonly with stromatolitic rock-phosphate) and carbonaceous phyllite Quartzite (locally arkosic) Conglomerate and grit Greenschist and amphibolite The top part of the greenschist horizon underlying the conglomerate often contains big pebbles and boulders of vein quartz and quartzite (Figs. 4 and 5). The nature of the pebbles continues to be the same in the overlying conglomerate horizon, in which the matrix is quartzitic. The pebbles show a fairly high degree of sphericity and roundness, and vary in size from a few millimeters to about 0.5 m in diameter, The arkosic beds are mostly confined to the regions adjacent to the outcrops of granite (mapped by Heron, 1953, as post-Aravalli and pre-Delhi granite). Carbonate association

The passage from the basal formation of quartzite--greenschist association to the next higher dominantly carbonate sequence appears gradational in nature. The carbonates are dominantly dolomitic and show rapid changes in facies to orthoquartzite, ferruginous dolomite, manganiferous dolomite, carbonaceous phyllite and phyllitic dolomite, besides passing to argillaceous phyllite containing intercalations of dolomitic limestone. In addition, in many places there is a persistent horizon of stromatolitic rock-phosphate within the dolomitic sequence.

57

Fig. 4. Pebbles and angular blocks of vein quartz and quartzite in greenschist matrix, west of Udaipur (Formation A).

Fig. 5. A large boulder of quartzite in greenschist matrix. Schistosity in the matrix curves round the boulders. The longest dimension of the boulder is 25 cm (Formation A).

58 The dolomitic rocks virtually encircle the valley in which the city of Udaipur is situated. Another interesting feature regarding the distribution of carbonate-group rocks is the presence of a north--south linear belt of carbonaceous phyllite running along the eastern margin of a dolomitic limestone band (which also contains a rock-phosphate horizon) to the east of Udaipur. There are reported occurrences of copper--uranium minerals in the carbonaceous phyllite, whereas manganese minerals and rock-phosphate (stromatolites) are known in the dolomitic limestones. The presence of stromatolites in dolomitic limestone lends a characteristic appearance to the rocks which can be successfully utilized as stratigraphic markers. Assuming a constant westerly direction of younging of the rocks, Banerjee (1971a) proposed that the stromatolites outcropping to the west of Udaipur represent a younger horizon compared to the stromatolites which occur to the east. The studies made by the present authors have however, shown that the repetitive sequences of stromatolitic rock-phosphate actually represent a single horizon. In many instances, a physical continuity could be established between the supposedly younger and older horizons. On the other hand, some apparent discontinuities in outcrops have been proved to be due to plunge variation resulting from fold interference. Several workers have described the different aspects of stromatolites of the Udaipur region (Muktinath and Sant, 1967; Raja Rao et al., 1968; Banerjee, 1971a,b; Chauhan, 1979; Paliwal, 1980). The main unit is composed of columnar stromatolites (Fig. 6). Stratiform types have also been locally recorded. Where the rocks have not suffered any bedding-parallel shearing, the columns stand perpendicular to the beds. The angle between the stromatolitic columns and the bedding plane has changed greatly because of deformation (Paliwal, 1980). At several places, columns have been totally flattened and rotated parallel to bedding. Many of these types may simulate laminated or stratiform stromatolites. Brecciation, both diagenetic and post, diagenetic, has caused further complication in the form of stromatolites. The presence of reworked fragments of stromatolites has also been recorded (Banerjee, 1971a; Chauhan, 1979). The stromatolites in the Udaipur region are predominantly phosphatic in composition. Non-phosphatic stromatolites occur sporadically. In the Jhamarkotra region, where stromatolites are best developed, non-phosphatic varieties are common in the dolomitic limestone beds underlying the main phosphatic horizon. Greywache--phyllite--lithic arenite association A sequence of greywacke showing rhythmic alternations with phyllite (Fig. 7) is exposed around the Udaipur Valley (Pandya, 1965; Poddar and Mathur, 1965). The greywackes commonly show graded bedding and range in composition from feldspathic to lithic types. Some of the coarser varieties of greywackes are pebbly, whereas the finer ones are siltstones. In

59

Fig. 6. Columnar stromatolites in dolomite (Formation B). Diameter of coin 2.5 cm. thin sections, the coarser grains of ill-sorted and sub-angular quartz, feldspar (oligoclase) and r o c k fragments are e m b e d d e d in a matrix o f fine-grained quartz, feldspar, chlorite and sericite. Carbonate minerals appear t o have locally replaced the matrix materials. The r h y t h m i c sequence of greywacke-phyllite passes into a more massive t y p e o f lithic arenite ~Williams et al., 1954), w i t h o u t phyllitic partings. The, greywackes o f the J h a m a r k o t r a region are of this type, with bedding planes difficult to recognize. In m a n y exposures around Jhamarkotra,. the lithic arenites appear as a structureless mass w i t h o u t bedding, cleavage or even

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Fig. 7. Rhythmic alternations of greywacke and phyUite (Formation C). Diameter of coin 2.5 cm. joint planes. The m o n o t o n o u s texture is broken by scattered large rock fragments of phyllite (Fig. 8), carbonate and rock-phosphate. Because of metamorphism, the detrital character of minerals has been greatly altered in these rocks. However, the mineralogical composition suggests that these are (meta)lithic arenites with significant basic components. Several isolated lenses o f pebbly and bouldery phyUite occur within the outcrops of greywacke of the Udaipur Valley (Fig. 9). The pebbles are mostly of quartzites and vein quartz. There are also some pebbles o f granite,

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Fig. 8. Angular fragments of phyllite embedded in lithic arenite (Formation C). carbonates and phyllite. Pebbles and boulders show an opeD framework in a phyllitic matrix. Physical continuity of the pebbly lenses with the type greywackes, and similarities in deformation structures in both rock types precludes any possibility of their being of recent age (cf. Heron, 1953). Poddar and Mathur (1965) considered these lenses as polymictic greywacke conglomerates of " w i l d f l y s c h " type. Excellently preserved sedimentary features in the greywacke beds show that the greywackes overlie rocks of carbonate facies of the Udaipur region.

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Fig. 9. Conglomerate with assorted pebbles and boulders enclosed in phyllitic matrix (Formation C).

Usually a thin band of phyllite occurs between the greywackes and the carbonates. However, in m a n y outcrops greywackes lie directly above the carbonates (stratigraphically). Besides the overstepping relationship, the presence of angular blocks of rock-phosphate and dolomite in greywackes and lithic arenites clearly indicates their age to be younger than that of the carbonates.

Conglomerate--arkose--orthoquartzite association (Debari quartzites) The rocks of this association crop out along a fairly wide belt between carbonate--carbonaceous phyllite--phyllite sequence to the west and basal sequence to the east. The sequence starts with a polymictic conglomerate horizon in which pebbles of quartzite and vein quartz greatly outnumber those of granite, carbonate, amphibolite and phyllite. The pebbles are of various sizes and have been deformed to ellipsoids. The matrix is composed either of pebbly arkosic quartzite or of schistose quartzite. The conglomerates grade upward through arkosic quartzites into orthoquartzites which are highly ferruginous in the upper part. There are also some phyllitic intercalations between arkosic quartzites and orthoquartzites. The entire sequence shows profuse development of cross-lamination. The younging direction is persistently towards the west, and the beds, which dip easterly, are in an overturned attitude.

63 Heron (1953) mapped the conglomerate--arkose--orthoquartzite sequence as outliers of Delhi metasediments. On the other hand, several later workers (Poddar and Mathur, 1965; Damle and Sharma, 1970; Banerjee, 1971a; Chauhan, 1979) described this unit as a basal association of the Aravalli Group. Heron's (1953) correlation of the unit with the Delhi Group was primarily on the basis of lithological similarity and on a supposed unconformable relation between this unit and the other rocks of the Aravalli Group. By contrast, those who placed this unit at the base of the Aravalli sequence argue that all the metasediments constitute a continuous sequence starting with a basal conglomerate (Poddar, 1965, 1966; Damle and Sharma, 1970). That the conglomerate--arkose--orthoquartzite sequence outcropping to the east of Udaipur is really a part of the Aravalli sequence is evident from the pattern of deformation in these and other Aravalli rocks. All the rocks here show the same stages of superposed deformation, and there is no decipherable structural break which would warrant placing an angular unconformity between the supposed 'Delhi outliers' and the 'true' Aravalli rocks as presumed by Heron (1953). The Delhi Group of rocks exposed in the 'main Delhi synclinorium' is reported to exhibit a different structural style from that in the Aravallis (Naha and Halyburton, 1974). This correlation notwithstanding, the exact position of this unit in the stratigraphic succession of the Aravalli rocks is n o t very clear. To the east of Udaipur this unit lies stratigraphically above the rocks of the quartzite-greenschist association. The same unit around Umra (also showing westerly younging) lies above the rocks of carbonate association. It is, therefore, evident that the conglomerate--arkose--orthoquartzite sequence does not represent the basal Aravalli formation, but occupies a higher position in the stratigraphic column. No definite age relationship between this unit and the greywackes could be established, because these two units are never found in contact. In the J h a m a r k o t r a region outcrops of the two rock units occur close to each other. The relationship established at this place is one of contemporaneity (Roy et al., 1980).

Quartzite--silty arenite association The greywacke--phyllite sequence passes upward (stratigraphically) into a quartzite--silty arenite sequence, the outcrops of which fringe the western limits of the valley of Udaipur. The passage beds between the greywackes and the n e x t higher horizon are a thin sequence of argillites showing lens bedding and rhythmic change in clay and mudstone. The quartzite--silty arenite sequence starts with lenticular thin beds of conglomerate comprising rounded and poorly-sorted pebbles of quartzite, vein quartz, dolomite, greywacke and phyllite. This is overlain by a thickly bedded orthoquartzite which grades upward into massive, locally cross-bedded orthoquartzite. The uppermost beds are of siltstone and silty mudstone arranged in fining-

64

upward cycles (cf. Allen, 1965; Visher, 1965; Reineck and Singh, 1975). There are abundant desiccation cracks in this unit (Fig. 10) and the sedimentary structures such as parallel lamination, small ripple bedding (Fig. 11) trough-shaped cross-bedding, wave ripple cross-bedding, convolute bedding (Fig. 12), recumbently folded cross-bedding (Fig. 13), climbing-ripple lamination (Fig. 14), flaser and lenticular bedding, and slump features like flame structure, slump roll and pillow structure, are well preserved, particularly in the silty arenite (siltstone) unit. S T R A T I G R A P H I C SUCCESSION

The stratigraphic succession of the Aravalli metasediments and associated rocks is given in Table I. Field relations suggest that there is an overlap between the top member of Formation A and the basal member of Formation B. Some outcrops of Formation B were earlier mapped by Heron (1953) as Raialo Marbles. Detailed mapping by the authors has clearly shown that these marbles are parts of the AravaUi dolomites, showing a slightly higher grade of metamorphism than the others. The increase in metamorphic grade in these dolomites can be directly correlated with granite emplacement in these areas. The greywacke--phyllite sequence of Formation C transgresses the boundaries of the phyllite horizon of the underlying Formation B and in some places lies directly over the lower horizon (dolomitic member with rock-phosphates). This relationship may indicate a hiatus between the two formations. A similar hiatus probably exists between Formations C and D. S E D I M E N T A R Y HISTORY AND TECTONIC F R A M E W O R K OF SEDIMENTATION

Peneplanation of the craton prior to Aravalli sedimentation is clearly implied in Heron's (1953) description of 'erosional unconformity' between the Banded Gneissic Complex and the Aravalli Group. The uniformly low relief in the source area is indicated by the development of thin continuous bands of 'cleanwashed' orthoquartzite in the basal formation, overlying the basement gneisses. A feature of particular interest is the occurrence of pockets of pyrophyUite and rarely of sillimanite--kyanite along the plane of unconformity. The intriguing possibility that these deposits represent metamorphosed products of bauxitic clays (palaeosols) would also indicate base levelling and protracted weathering in the continental basement. Sedimentation in the Aravalli Sea, in this region, was probably triggered by the development of fault-controlled troughs along craton margins. Such a situation explains satisfactorily the presence of volcanic rocks (flows and tufts) along two linear belts. Possibly because of faulting, the margins of the troughs stood high and allowed the land-derived debris to be dropped into the floor already paved by volcanic rocks. Differences in the nature of

65

Fig. 10. Polygonal desiccation cracks in silty arenite (Formation D).

Fig. 11. Load-casted asymmetric ripples in orthoquartzite (Formation D).

66

Fig. 12. Convolute-bedding in quartzite (Formation D).

Fig. 13. Recumbently folded cross,bedding in silty arenite (Formation D).

67

Fig. 14. Climbing ripple-laminations in silty arenite (Formation D). TABLE I Stratigraphic succession of Aravalli Rocks around Udaipur F 2 folding (Delhi orogeny, ca. 1660--1200 Ma) Dolerites and ultrabasic bodies Aplitic granites, pegmatites and quartz veins F~ folding (Aravalli orogeny, ca. 2100--1900 Ma) Deep-sea deposits

Aravalli Group

Micaschists, Phyllites and Quartzites

Epicontinental and shelf deposits

Formation D:

Quartzites of Udaipur

F o r m a t i o n C:

Greywackes, lithic arenites, phyllites, pebbly and bouldery phyllites of Udaipur valley and Jhamarkotra Conglomerate--arkose--quartzite of Debari

Formation B:

Dolomites, orthoquartzites, stromatolitic rock phosphates, carbonaceous phyllites, silicified rocks, phyllites with carbonate lenses

F o r m a t i o n A:

Quartzites (locally arkose), conglomerates, amphibolites and green schists (altered basic volcanic flows and ash falls)

Unconformity Pre-Aravalli basement rocks (ca. 2585 Ma)

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sediments in the t w o belts (orthoquartzites in the eastern belt, and arkose and conglomerates in the western belt) probably reflect difference in topographic relief caused by faulting. The restoration of more stable tectonic conditions in the next phase caused a sharp decrease in the supply of terrigenous detritus and heralded the deposition of carbonates. A b r u p t changes in facies in the carbonates and the distribution of stromatolitic carbonates and carbonaceous phyllites clearly reflect the control of sea-floor topography. Depositional subenvironments included subtidal, tidal, and supratidal flats (Chauhan, 1979), extensive shelves, banks and si~oals. The linear belt .ff carbonaceous pl~yllites flanking a stromatolitic dolomite band to the east of Udaipur probably represents a silled trough. Besides the development of stromatolites, a shallow-water depositional environment is also indicated by the frequent changes in lithofacies from carbonate to orthoquartzite, and by the presence of such depositional features as ripple marks (Fig. 15) and cross-bedding, The basin in which stromatolites grew, probably had little communication with the open sea. The distribution of dolomites, with stromatolite horizons encircling the Udaipur Valley (Fig. 2) defines the limits of the stromatolite-growing epicontinental sea (Fig. 16). The following evidence suggests the presence of a land barrier separating the epicontinental sea from the open sea.

Fig. 15. Asymmetric ripple marks in dolomite (calcareous arenite) (Formation B).

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Fig. 16. Conceptual palaeogeographic model showing distribution of land and sea of the "Udaipur" basin. The figure is not true to scale. (1) T h e d e v e l o p m e n t o f basal sequence ( F o r m a t i o n A; see Fig. 2) on b o t h sides o f t h e U d a i p u r V a l l e y is d i f f i c u l t t o explain w i t h o u t a s s u m i n g a l a n d m a s s o v e r t h e area, p a r t o f w h i c h is n o w o c c u p i e d b y t h e ' p o s t - A r a v a l l i ' granites. T h e r e is n o evidence t o suggest t h a t t h e Aravalli r o c k s have b e e n s u b j e c t e d to a n y significant long d i s t a n c e t r a n s l a t i o n . (2) T h e p r e s e n c e o f c o n g l o m e r a t e s , a r k o s e , a n d o r t h o q u a r t z i t e s in the basal a s s o c i a t i o n o f the w e s t e r n b e l t i n d i c a t e s s u p p l y o f t e r r i g e n o u s d e t r i t u s f r o m p r o x i m a l s o u r c e rocks. T r a n s p o r t o f large b o u l d e r s ( o f a r o u n d 1/2 m e t r e d i a m e t e r ) c a n n o t be f r o m a n y d i s t a n t s o u r c e * . H e r o n ( 1 9 5 3 , p. 3 1 2 ) suggested t h a t t h e arkose a n d c o n g l o m e r a t e s o f this region w e r e derived f r o m t h e granite w h i c h o u t c r o p s t o the east. (3) T h e e x i s t e n c e o f e a s t e r l y sloping tidal fiats, a t t e s t e d t o b y t h e s t r o m a t o l i t e s in d o l o m i t e s b o r d e r i n g t h e e a s t e r n m a r g i n o f the abovem e n t i o n e d granite, suggests t h e p r e s e n c e o f positive areas t o t h e west. I t is t e m p t i n g t o a s s u m e t h a t besides t h e l a n d m a s s to t h e w e s t t h e r e were also chains o f islands a n d d i s c o n t i n u o u s shoals w h i c h virtually s h e l t e r e d the epic o n t i n e n t a l sea f r o m the high sea (Fig. 16). M e n t i o n m a y be m a d e in this c o n n e c t i o n o f t h e close a s s o c i a t i o n o f d o l o m i t e s w i t h granitic bodies. T h e r e are m a n y instances o f rafts o f m y l o n itized granitic r o c k s at t h e c o n t a c t of, or w i t h i n d o l o m i t e bodies. S o m e o f t h e granitic b o d i e s b e a r a p p a r e n t intrusive r e l a t i o n s h i p w i t h the d o l o m i t e s . B u t t h e r e are also instances w h e r e earlier granites c o n s t i t u t i n g the b a s e m e n t *It is difficult to assume the boulders to be glacial erratics. There is no evidence to prove that the Aravalli sedimentation was initiated under glacial conditions. On the other hand, the possible evidence of bauxitization of the basement rocks, and the growth of stromatolites in the next higher formation are indicative of a warm climatic regime (cf. Young, 1974).

70 have been remobilised (Roy et al., 1980). Thus it seems most likely that the granitic bodies outcropping in this area of very low-grade metamorphism and represent basement rocks, commonly modified along the margins by intense shear movements concentrated along the plane of unconformity, and by folding (cf. Kennedy, 1955). The onset of a period of influx of terrigenous clastics is marked by argillaceous dolomites and purple shales in the upper part of the carbonate sequence. The initiation of greywacke-lithic arenite deposition in the next phase heralded the flooding of the shelf sea by a copious supply of terrigenous debris, consequent upon widening of the area of provenance and unroofing of new sources (cf. Bassett, 1963; Walker and Pettijohn, 1971). The nature of sediments indicates supply from a predominantly granitic source. The 'transport was by cyclic turbidity currents and deposition below an agitated wave base in a fairly stable basin' (Poddar and Mathur, 1965). The rhythmic sequence of greywacke--phyllite probably represents deposition in deeper parts where shale was being deposited as background sediment. The massive varieties of greywacke and lithic arenite without phyllite partings indicate deposition relatively close to the source of turbidity currents (cf. Walker and Pettijohn, 1971). The beds of conglomerate--arkose--quartzite (Debari quartzites) which outcrop to the east of Udaipur (Fig. 2) represent a thick wedge of terrigenous detritus deposited in close proximity of the source rocks. The granitic provenance of the source rocks is proved by the presence of granite pebbles in the conglomerate and the predominance of arkose in the sequence. Further, the presence of carbonate and phyllitic pebbles indicates the immature nature of the sediments. The Debari quartzites are ubiquitously cross-bedded. By contrast, the greywackes and lithic arenites show only graded bedding. The internal organization of bedding in the two major clastic units is mutually exclusive and thus reflects 'contrasting degrees of agitation and modes of transportation of particles' (Dott and Batten, 1971) under different environmental conditions (cf. Bailey, 1930). A distinct change in environment is marked in the deposition of a quartzite--silty arenite sequence coming next to greywacke--lithic arenite-slate--phyllite beds. The repeated occurrence of desiccation cracks in this unit is indicative of periodic emergence of the sediments laid down in very shallow water. The sediments show a fining-upward cycle characteristic of fluviatile deposits (Allen, 1965; Visher, 1965; Pettijohn, 1975; Reineck and Singh, 1975). The topmost silty arenite (siltstone) horizon comprises repeated channel units starting with parallel-bedded sandstone, and completed with finely laminated mudstone. Several bed forms, in different combinations, are observed in the channels. These are trough-shaped cross-bedding, climbing ripple lamination, ripple lamination, flaser and lenticular bedding, convolute bedding and (rarely) recumbently folded cross-bedding. The array of sedimentary structures appears analogous to those of channel bar deposits of modern rivers (cf. Belt, 1968; Coleman, 1969; Reineck and Singh, 1975; Friedman and Sanders, 1978).

71 CONCLUSIONS

The present study suggests that the depositional environment of the Aravalli rocks varied both in space and time. The relationship between environment, facies and time has been shown diagrammatically in a restored stratigraphical east--west cross section (Fig. 17), ignoring subsequent deformation and erosion. Much of the complexity in the sedimentary geometry has presumably been caused by (i) the irregularities in the sea-floor topography, (ii) the disposition of the strand line consequent upon the presence of landmass and islands, and (iii) the nature of continental relief. It has n o t been possible to undertake paleocurrent analysis because of strong and repeated deformation in the rocks. However, the distribution of different facies indicates that the sediments were supplied predominantly from the continent to the east, and partly from the landmass to the west. The nature of clastic sediments suggests a granitic provenance, although around Jhamarkotra, the supply was at least in part from basic rocks. The most important tectonic-sedimentary factor which controlled the growth of stromatolites in this part of the Aravalli sea, is the development of epicontinental conditions within the shelf. The depositional basin remained starved of terrigenous detritus because of uniformly low relief in the area of provenance, whereas restricted circulation of water in the epicontinental sea ensured the reducing conditions necessary for algal growth. The presence of warm water, as well as a slow rate of sedimentation, is implied by the deposition of rock-phosphate in the stromatolitic columns (Bromley, 1967; Logan et al., 1964). "(-DEEP

SEA

<

•

W

....

~

...... I

EPICONTINENTAL

SEA

UDAIPUR ~

~

- ... '....... -:-"-~-":':::

-.-.

E

i PHYLLITE MICASCHIST WITH BANDS OF QUARTZITE :) QUARTZITE- SILTY ARENITE I GREYWACKE-PHYLLITE LITHIC ARENITE

~::i:: i::~;:~ 2 CONGLOMERATE ARKOSE- ORTHOQUARTZITE ,_ _ -.-,..".-.,

DOLOMITE - ORTHOQUARTZlTE- PHYLLITE- CARBONACEOUS PHYLLITE

WITH S T R O M A T O L r T I C

ROCK P H O S P H A T E

BASAL QUARTZITE CONGLOMERATE WITH GREEN SCHIST BANDED GNEISSIC COMPLEX

Fig. 17. Restored, generalized east-west cross-section through Udaipur showing relationship between facies, environments, and time in the epicontinental sea.

72 A continuous change in depositional environment, indicating a strong interplay of tectonism, erosion, and sedimentation is recorded in the stratigraphic succession of the rocks. The sequence of the Aravalli rocks deposited in the shelf and epicontinental sea thus reveals several depositional stages: Stage 1 -Volcanic phase: Deposition of orthoquartzite (locally arkose and conglomerate) with contemporaneous volcanic flows and ash fall. Stage 2 -P e n e p l a n a t i o n phase: Deposition of carbonates, orthoquartzites, black shales with pyrite, siderite and rock phosphates (stromatolitic). Stage 3 ~ Main clastic phase: Deposition of greywackes, lithic arenites and phyllites in the distal parts and conglomerate--arkose-orthoquartzite in proximal parts. Stage 4 ~ L a t e clastic phase: Deposition of quartzite and silty arenite under fluvial conditions. The influence of tectonics is clearly implied in the aforesaid stages of sedimentation. Leaving aside the minute details, the pattern of sedimentation appears broadly similar to Krynine's model of tectonic stages (Krynine, 1942; Pettijohn, 1975) which are (i) a period of low relief following peneplanation, (ii) a period of marginal upwarping with associated strong relief, and (iii) completion of trough filling with eventual return to nonmarine stage (cf. Basset, 1963). The deposition of land-derived clastics with contemporaneous volcanicity at the initial stage, reflects foundering of the shelf sea; because of normal faulting. Periodic subsidence (probably because of block-faulting) is envisioned as the primary factor controlling sedimentation. It would, however, be interesting to note that except for the initial stages there is no evidence of volcanic activity during subsequent stages of sedimentation. The Aravalli sequence described here may appear typically miogeosynclinal sensu Stille (see Aubouin, 1965), and the depositional basins along the rifted continental margin off the east coast of North America (Dietz, 1963; Dickinson, 1971; Hsii, 1972; Rickard and Fisher, 1973) may be envisioned as modern analogues of the Aravalli-type depositional basin. ACKNOWLEDGEMENTS We are grateful to Prof. K. Naha of Indian Institute of Technology, Kharagpur, and Dr. S.K. Chanda of Jadavpur University, Calcutta, for critical review of the manuscript. We also had many useful discussions with Mr. B.C. Poddar of the Geological Survey of India and Dr. P.K. Bhattacharya of Jadavpur University, Calcutta. Mr. P.R. Golani helped us in the preparation of the diagrams. One of us (B.S.P.) had the financial support of the University Grants Commission of India.

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74 Naha, K. and Halyburton, R.V., 1977. Structural pattern and strain history of a superposed fold system in the Precambrian of Central Rajasthan, India. I. Structural pattern in the "main Raialo syncline". Central Rajasthan. II. Strain history. Precambrian Res., 4: 39--111. Naha, K. and Majumdar, A., 1971. Structure of the Rajnagar marble band and its bearing on the Early Precambrian stratigraphy of Central Rajasthan, Western India. Geol. Rundsch., 60: 1550--1571. Naha, K., Chaudhuri, A.K. and Mukherji, P., 1966. The geometry of the "hammer head syncline" and " h o o k syncline" in central Rajasthan: an example of axial plane folding. Quart. J. Min. Met. Soc. India, 38: 71--75. Paliwal, B.S., 1980. Systematics of the Precambrian stromatolites of Rajasthan: a review in the light of tectonic deformation. Proc. Workshop on Stromatolites: Characteristic and Utility. Geol. Surv. India, Misc. Publ., 44 (in press). Pandya, M.K., 1965. On the discovery of greywackes in the Precambrians of Rajasthan from near Udaipur City. Curt. Sci., 34: 287--288. Pettijohn, F.J., 1975. Sedimentary Rocks. Harper and Row, New York, NY, 3rd ed., 628 pp. Poddar, B.C., 1965. Stratigraphic position of the Banded Gneissic Complex of Rajasthan. Curr. Sci., 34: 483--484. Poddar, B.C., 1966. An example of contrasted tectonic regimes from Precambrians of Udaipur district, Rajasthan. Ind. Minerals, 20: 192--194. Poddar, B.C. and Mathur, R.K., 1965. A note on repetitious sequence of greywacke-slate--phyllite in the Aravalli system around Udaipur, Rajasthan. Bull. Geol. Soc. India, 2: 84--88. Raja Rao, C.S., Iqbaluddin and Mathur, R.K., 1968. Algal structure from Aravalli beds near Dakan Kotra, Udalpur district, Rajasthan. Curr. Sci., 37: 560--561. Raja Rao, C.S., Poddar, B.C., Basu, K.K. and Dutta, A.K., 1971. Pre-Cambrian stratigraphy of R a j a s t h a n - - a review. Rec. Geol. Surv. India, 101: 60--79. Reineck, H.E. and Singh, I.B., 1975. Depositional Sedimentary Environments. Springer, Berlin, 439 pp. Rickard, L.V. and Fisher, D.W., 1973. Middle Ordovician Normans Kill Formation, eastern New York: age, stratigraphic, and structural position. Am. J. Sci., 273: 580--590. Roy, A.B., 1972. Significance of variable plunge and trend of small-scale upright folds in the type Aravalli rocks around Udaipur, Rajasthan (Western India). Geol. Soc. Am. Bull., 83: 1553--1556. Roy, A.B., 1973. Nature and evolution of subhorizontal crenulation cleavage in the type Aravalli rocks around Udaipur, Rajasthan, Proc. Ind. Nat. Sci. Acad., 39: 1119--1131. Roy, A.B., Paliwal, B.S. and Goel, O.P., 1971. Superposed folding in the Aravalli rocks of the type area around Udaipur, Rajasthan. J. Geol. Soc. India, 12: 342--348. Roy, A.B., Nagori, D.K., Golani, P.R., Dhakar, S.P. and Choudhuri, R., 1980. Structural geometry of the rock phosphate bearing Aravalli rocks around Jhamarkotra Mines area, Udaipur district, Rajasthan. Ind. J. Earth Sci. (in press). Sen, S.N., 1970. Some problems of Precambrian geology of the central and southern Aravalli range, Raj~than. J. Geol. Soc. India, 11: 217--231. Visher, G.S., 1965. Use of vertical profile in environmental reconstruction. Am. Assoc. Petrol. Geol., Bull., 49: 41--61. Walker, R.G. and Pettijohn, F.J., 1971. Archaean sedimentation; analysis of the Minnitaki Basin, northwestern Ontario, Canada. Geol. Soc. Am. Bull., 82: 2099--2130. Williams, H., Turner, F.J. and Gilbert, C.M., 1954. Petrography. Freeman, San Francisco, CA, 406 pp. Young, G.M., 1974. Stratigraphy, paleocurrents and stromatolites of Hadrynian (Upper Precambrian) rocks of Victoria Island, Arctic Archipelago, Canada, Precambrian Res., 1: 13--41.