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ARAVALLI- BUNDELKHAND CRATOn
Introduction and Distribution:- The Aravalli-
Bundelkhand craton is divided by the boundary fault of the Himalaya in the north and the Narmada son lineament in south and southeast. West of craton is covered by the recent deposits. The best exposures are along Aravalli mountain ranges and in Bundelkhand massif and son valley regions. The Aravalli ranges and Bundelkhand son valley area is separated by the Great Boundary Fault (GBF).
The Aravalli supra crustal rocks are mostly Proterozoic and consist of phyllites and greywacke with abundant orthoquarzites and minor maffic/ultramaffic schist. Another difference is shown distribution of iron. Banded iron formation (BIF)are about absent from the Aravalli craton.
The oldest rocks of west of GBF in Aravalli is Banded Gneissic Complex (BGC). East of the GBF is the Bundelkhand region, it have salic rocks including granites and gneisses in Bundelkhand complex. (Fig 1)
Fig.1 Aravalli Bundelkhand Craton
ARAVALLI BUNDELKHAND
Marwar supergroup
BHILWARA SUPERGROUP:
Banded Gneissic Complex (BGC) of Heron(1935) included a series of gneisses and granulites that were considered as basement to the Aravalli-Delhi orogen. Later geological synthesis after extensive mapping by the Geological survey of India (Gupta et al. 1980) led to the division of BGC, renamed as Bhilwara Supergroup, into three tectono-stratigraphic units (Fig.2) viz. Hindoli Group, Mangalwar Complex together with Isolated Mineralised Belts and Sandmata Complex.
Hindoli Group:- Hindoli group consisting essentially of turbidites and minor
volcanics is named after Hindoli near Bundi. (Fig.3)
This group consists mainly of greywacke and phyllites, besides minor cross-bedded quartzite, dolomitic limestone and algal dolomite. Turbiditic greywacke preserve ripple marks, load casts and cross bedding. Recently, a suite of volcanics and volcanoclastic are reported from the greywacke sequence.
Hindoli dolerites occur as sills and dykes within the metasediments of Hindoli and Jahazpur groups.
Mangalwar complex:- Mangalwar complex encompasses the vast sretch of
varied gneisses occurring between the Banas Lineament bounding the Hindoli belt in the east and the Delwara Lineament bounding sandmata complex in the west. (Fig.3) The complex is divided into Lasaria, Potla, Suwana and Kekri Formations by Gupta et al.(1997). Lasaria Formation occurring in the south consists of migmatites and composite gneisses. Potla Formation comprises garnet-staurolite- biotite schist, kyanite-sillimanite-mica schist, crystalline limestone, amphibolites. Suwana Formation essentially made up of migmatised amphibolites with minor bands of garnet-biotite-kyanite schist and quartzites. Kekri Formation, exposed in the north consists of migmatites and composite gneisses and contains enclave of garnet-biotite schist and calc-silicate rocks.
Fig.2 Map of Bhilwara Supergroup
(after Roy and Jakhar, 2002)
Fig.3 Geological map of Hindoli belt(after Sinha-Roy et
al.,1998)
Sandmata complex:- The boundary between the granulites belt of Sandmata
complex and the gneissic terrain of Mangalwar complex is marked by Delwara Lineament. Sandmata complex(Fig.4) consists of large areas of migmatitic gneisses in amphibolites facies containing enclaves similar to those of Mangalwar complex. It spreads in the area of 300km from south (amet) to north (khetri).
BUNDELKHAND GRANITES AND GNIESSES:- Bundelkhand
granites represents a large plutonic complex of batholithic dimension. Three types of granites were recognized (Basu,1986), namely (i) coarse grained porphyritic granite (ii) medium grained porphyritic granite and (iii) leucogranite , each with numerous subtypes.
Fig.4 Geological map of Sandmata complex(after Gyani and
kataria, 2002).
AGE(2500-2000 ma)
Introduction and distribution :- Rocks of the Aravalli Supergroup,
which occur as the oldest cover succession of Palaeoproterozoic age overlie the Archaean basement rocks with profound unconformity. The listhostratigraphic succession of the Aravalli supergroup is characterized by the presence of stromatolitic phosphorites, extensive development of uraniferous black shales facies , occurrence of lead-zinc mineralization, and the lead and iron isotopic signature in the rocks (Deb, 1986). All the features confirm the Proterozoic age of the Aravalli Supergroup.
The Aravalli Supergroup occurs over wide areas in southeastern Rajasthan lying south and east of the younger Delhi Folds Belts. The Aravalli Supergroup is delimited by two types of faults. One fault is present between the contact of Vindhyan and Aravalli it is known as Great Boundary Fault. Another fault is found in lower part of Aravalli known as Narmada Son Fault. The type area of Aravalli rocks is Udaipur. The large number of bodies of serpentine occurs in the Aravalli rocks of the type area. Group of such bodies form a linear trend.( Fig.5)
(Fig.5) MAP SHOWING ARAVALLI
SUPERGROUP
LITHOLOGY: - In this section, detailed description of different lithogical
units of the Aravalli Supergroup of the type area. This will help not only in understanding relationship of different lithological units of the type area, but also in attempting correlation of rocks of different sectors. Detailed lithological study of rocks successions will also help in inferring the depositional enviornment of these Palaeoproterozoic rocks, which in turn may be used in establishing their tectonostratigraphic evolution.
Aravalli Supergroup is divided into three lower, middle and upper.
Delwara Formation – This formation is best exposed between Nathdwara
and Pipalkhunt for over 300km with an average width of 500m.
The basement cover interface is marked by a significant occurrence of discontinuous pockets of paleosol. The paleosol is represented by pockets of white mica having sharp boundary with the overlying quartzite. A thin layer of meta- arkose or ‘granite wash’ locally underlies the white mica pockets.
Delwara Group starts with pebbly orthoquartzite (QPC). Arkosic quartzite is rare. The quartzite band is offset along its strike by several transverse faults. A thin vaneer of metavolcanics intervenes rarely between the gneiss and quartzite. The quartzite is overlain by a major volcanic unit called informally as Delwara volcanics consisting of amygdular basic flows intercalated with pyroclastic and epiclastic deposits, including volcanic conglomerates. Localized occurrences of barite mineralization are reported in the bands of metavolcanics near to the basal succession of the Delwara Formation (Shekhawat, 1999). Conglomerates with pebble and boulder size clasts embedded in a matrix of chlorite schist and amphibolites occur as discontinuous lenticular bodies at several places, near the top of the succession. The following sequence of the Delwara Formation has been recorded (Paliwal, 1988):
- Metavolcanics
- (^) Quartzite, ferruginous at the base (locally arkosic)
- Banded-hematite-quartzite
- Conglomerate with quartzitic and arkosic matrix
- Conglomerate with chlorite-schist matrix
- Metavolcanics
- White-mica deposit(palaeosols)
- Basement rocks.
Jhamarkotra Formation – The basal bed of Jhamarkotra Formation in the
Jhamarkotra basin is quartzite, which, at places, is represented by quartz-pebble conglomerate. Occurrence of thin lenses of dolomite within the thin vitreous- looking basal quartzite suggests that atleast a part of this unit is a silicified dolomite.(Fig.6)
The Jhamarkotra basin constitute the type locality for the development of the carbonate succession. Here the carbonate rocks show development of various facies variants and alternation products. Among the most commonly occurring lithofacies types are (1) grey and grayish-white dolomite grading to marble; (2) brownish, ferruginous dolomite; (3)dark-grey carbonaceous dolomite grading to carbonaceous phyllites; (4) argillaceous dolomite and dolomitic phyllites;(5) brecciated dolomite and (6)silcrete-ferricrete rocks.
Udaipur Formation- A sequence of (meta-) greywacke showing rhythmic alternation with phyllites ( Pandya, 1965; Paddar and Mathur, 1965) overlies the carbonate rocks of the Jhamarkotra Formation with a prominent unconformity. The stratigraphic relationship is abundantly clear in several sections around Udaipur and around Zawar Mines area. It ranges in composition from feldspathic to lithic types. Coarser varieties of greywacke are pebbly whereas the finer ones are siltstones. A characteristic feature of thick beds of greywacke is the presence of graded bedding (Fig.7)
Banerjee and Bhattacharya(1994) recognized four different facies associations of greywacke in the Udaipur region.
- Facies A: fine to medium grained quartzofelspathic greywacke.
- Facies B: these are of three basic types. A) Coarse grained feldspathic greywacke facies. B) Coarse to medium grained quartzo-feldspathic grey- wacke facies and C) conglomeratic greywacke facies.
Fig.6 Jhamarkotra Formation
Fig.8 Gritty dolomite(Mochia Formation) showing regular rows
of quartz-feldspars grains in dolomitic matrix.
Bowa Formation-^ The Bowa Formation includes quartzite and quartzose phyllite, stratigraphically overlying the carbonates of the Mochia Formation. Conglomerate lenses occur locally at the base of this formation. Rocks are off- white and brownish white to brown and dark grey in colour, characteristically hard and compact. Bedding and other primary depositional structures such as cross- bedding, ripple marks, cut-and-fill structures, are well preserved in the quartzite.
Tidi Formation- The formation consisting of thinly bedded slate and phyllite
with interbeds of dolomite and quartzite, constitutes the youngest formation of the Middle Aravalli Group. Because of the very low grade of metamorphism, bedding is usually very well preserved in slates and phyllites, displayed by rhythimic alteration of argillaceous and quartzite layers. Phyllite have well developed cleavage planes. Crenulation cleavage has developed locally on the early-formed slaty cleavage. More than two sets of cleavage planes are present at places.
Debari Formation- The sequence starts with poorly sorted, polymictic conglomerate beds containing peebles, cobbles and boulders of grey, white light green coloured quartzite and vein quartz (Shukla and Sharma, 1970). Larger clasts of other lithologies include granite and granitic gneiss, marble, amphibolites, tourmaline-rich rock and mica schist (Sinha-Roy et al., 1993). According to Shukla and Sharma (1970), the pebbles of quartzite and quartz vein are mostly elongate and rod-like, whereas those of tourmaline-rich rock granite are rounded.
There is a general decrease in clast size in the upward (in stratigraphic sense) direction. The matrix is composed either of arkosic quartzite or of schistose quartzite. Thin lenses of dolomitic limestone and phyllite occur within conglomerate horizon, particularly near the basal part(Fig.9) In the
best exposed sections, quartzite occurs as uniformly thick beds showing profuse development of cross bedded units.
Fig.9 Large elongate boulders and pebbles of quartzite in
meta-arkosic matrix, Debari Formation
