Indian Journal of Geology Vol. 66, No. 2, p. 124-147, 1994

STRATIGRAPHY OF THE LATE PROTEROZOIC SULLAVAI GROUP, PRANHITA-GODAVARI VALLEY, ANDHRA PRADESH

T a p AN C h a k r ABORT Y Geological Studies Unit, Indian Statistical Institute, 203 B. T. Road, Calcutta-700 035

ABSTRACT Mapping of about 800 sq kms in the southwestern outcrop belt and study of selected sections has led to the subdivision of the Late Proterozoic Sullavai Group of the Pranhita-Godavari Valley into three formal lithostratigraphic units, each of the status of a formation : (a) Ramgiri Formation, a coarse-grained, purple to red arkosic sandstones and conglomerates, (b) Mancheral Quartzite, a red coarse-grained quartzare- nite and (c) Venkatpur Sandstone, a fine-grained, salmon red quartzose to subarkosic sandstone. Venampalli Sandstone Member and Kapra Sandstone M em ber are defined as two formal members within Ramgiri Formation Paleocurrent pattern indicates that the alluvial fan-braided river deposits of the Ramgiri Formation and Mancheral Quartzite were derived from the opposite flanks of the basin sampling different source terrain. Ramgiri Formation was derived from Archean (?) granite gneiss complex cropping out to the south. Chert layers of under­ lying Pakhal and Penganga Group, and a quartzose sedimentary rock (Pranhita Sand­ stone of Penganga Group ?) fed the Mancheral river system. Mancheral and Ramgiri alluvium appear to have been the major source for the aeolian Venkatpur Sandstone that shows persistent northeasterly flow direction. In contrast to traditional “layer-cake” concept, regional lithocorrelation reveal a complex mosaic of different stratigraphic units. Consequently, it is difficult to propose a ••generalised” stratigraphic succession for the Sullavai Group. Initial coarse-grained alluvial fan-braided river deposit and overlying fine-grained aeolian deposit are believed to represent a distinct phase of climatic and tectonic evolution of the basin during the Late Proterozoic time.

Key-words: Sullavai Group, Pranhita-Godavari 'Valley, Proterozoic, stratigraphy, sedimentation.

INTRODUCTION in the Pranhita-Godavari Valley (P. G. In terms of their huge thickness, exten­ Valley) represent one such major succession sive occurrence and gross similarity, the in India (Figs. 1, 2). The Purana rocks of Purana successions (Holland, 1906 ; Radha- this basin crop out in two northwest-south­ krishna, 19B7) comprise a distinctive horizon east trending belts separated by an axial belt in the Indian stratigraphy. Purana rocks of Permo-Jurassic Gondwana rocks. The Purana sediment of the P, G. Valley range in age from ~ 1400 to ~800 Ma (Vinogra­ dov et al., 1964 ; Chaudhuri and Howard, 1985). This paper focusses attention on the regional stratigraphy of the Sullavai Group, the youngest unit of the Proterozoic succes­ sion (Table I) in the south-western Protero­ zoic belt of the P. G. Valley (Fig 1). King (1881) first studied the Proterozoic rocks of the P. G. Valley and subdivided them into lower 'Pakhal series’ and upper ‘Sullavai series’. Since King, a number of workers have studied the Proterozoic sedi­ ments of the P. G. Valley (Johnson, 1967; Basumallick, 1967a, b ; Chaudhuri, 1970, 1985 ; Subba Rajuet al., 1978 ; Sreenivasa Rao etal., 1987 ; Chaudhuri et al., 1989). However, m ost of the earlier publications deal with the details of the Pakhal Group, and the stratigraphy of the Sullavai Group has been discussed only in very broad out­ line. Sullavai Group forms a regionally extensive sheet and is the only Proterozoic unit that crops out both in the southwestern Fig. 1. Regional geological map of the Pranhita- and northeastern belts. The study of the Godavari Valley, India. Inset shows the Sullavai Group therefore has important bea­ distribution of the major Proterozoic ring on the correlation of the Proterozoic basins of India.

Fig. 2. Map showing the distribution of the major groups of the Proterozoic rocks in the study area ; sectors defined for the convenience of the study and the positions of the measured lithologs. Table 1 Generalised succession of Proterozoic sequences of (he

Ramgundam area Stratigraphic Units Lithology and primary structures

GONDWANA SUPERGROUP (Permian to Jurassic) ------Unconformity ------Venkatpur Sandstone Salmon red, medium to fine, quartzose/ subarkosic sandstone; well rounded grains ; alternation of large planar cross beds and horizontal beds. M G Sullavai Mancheral Quartzite: Medium to coarse I 0 Group pebbly quartzose sandstone ; thin lenses D of conglomerate and well sorted fine D D sandstone ; profuse planar and trough L A cross-beds ; channel forms ; locally E Mancheral ? Ramgiri abundant adhesion structures. V Quartzite ? Formation T (23m) (456m) Ramgiri Formation : Medium to coarse A O (K-Ar date arkosic sandstones and conglomerates ; R 871 ±14my) profuse trough cross beds. L 1 Unconformity ------A Rajaram Limestone Micritic and intraclastic limestone, cal­ T S (735m) careous shale ; cross-beds and channels. E u Ramgundam Arkosic to subarkosic sandstone with P Sandstone minor shale ; locally abundant glau­ p conite ; well rounded grains ; well deve­ R Mulug (120m) O, E Subgroup loped trough cross-beds, wave ripples, mud cracks. T R E Damla Gutta Pebbly arkose, chert pebble conglo­ G R Conglomerate merate with graded beds ; trough cross­ O R (90m) beds. Z O P akhal, ------U n c o n f o r m l t y— ------0 Group Pandikunta Limestone Flat-bedded limestone and dolomite; 1 U (340m) lenses of shale and glauconitic sand­ C P (K-Ar date 1330±53my) stone ; abundant stromatolites; silcrete Mallampalii horizon on top. Subgroup Jonalarasi Bodu Interbedded limestone and quartzare- Formation n ite ; abundant well rounded grains; (50m) salt pseudomorphs ; graded beds, trough cross-beds, mudcracks. ------— — — Unconformity ------ARCHE AN (?) BASEMENT COMPLEX Pranhita-Godavari Valley in the southwestern belt

Mancheral area Stratigraphic Units Lithology and primary structures

GONDWANA SUPERGROUP ------Unconformity ------Venkatpur Sandstone

Mancheral Quartzite (76m) Sullavai Group

Ramgiri Formation (250 m +)

-Unconformity Sat Nala Shale Reddish brown shale with very presistent thin lamination. Chanda Limestone Limestone with minor shale ; thin, presistent Penganga bedding; matrix supported limeclast conglo­ Group merates.

Pranhita Sandstone Medium grained subarkosic sandstone; locally glauconitic; profuse trough eross- beds.

------Unconformity ------ARCHEAN (?) BASEMENT COMPLEX successions of the two belts and also on the 1970, 1985). ‘Purana formations’ is an general question of the origin of cratonic informal term, whereas the usage of a for­ sandstone sheets (cf. Dott and Byer, 1981 ; mal term ‘Purana Supergroup’ contradicts D o tt et a l , 1986 ; Fedo and Cooper, 1990). its original sense. The term “Puranas” as Classification, characterisation and correla­ used by Holland (1906) refers to the sedi­ tion of different lithounits of the Sullavai mentary successions intervening between Group, as presented here, is based on map­ the Archean and the Gondwana rocks in ping of an area of 800 sq kms in 1 : 31,680 different basins that are widely apart, and scale and detailed logging in 11 sections evidently does not carry any lithostratigra- spread between Mancheral (18-52'N, 79'28'E) phic connotation. By implication, Purana and Ramappa (18°15'N, 79°56'E) (Fig. 2). may qualify for the status of a diachronic Measurement of actual thickness is difficult unit (cf. NACSN, 1983, article 91). Subba due to presence of large number of intrafor- Raju et al. (1978) and Sreenivasa Rao (1987) mational faults. The thicknesses were mea­ raised the status of the well established sured in blocks not affected by such faults Pakhal Group (Basumallick, 1967 ; Johnson, and the measurements, therefore, represent a 1967 ; Chaudhuri, 1985 ; CSNI, 1977, article minimum estimate and may be less than the no. 11.04) to the higher stratigraphic rank actual preserved thickness of any particular of “Supergroup” without any overriding unit. The area under consideration has consideration. Further, designation of a been divided into four sectors for con­ part of the Purana sequence of the Valley as venience of study and the description has “Supergroup”, as has been done by Sree­ been organised accordingly. These sectors, nivasa Rao (1987), violates the sense of for­ from northwest to southeast, are the Man­ mal stratigraphic ranking (CSNI, 1977; cheral sector, Ramgundam sector, Begampet article 10 05) and such usage also contra­ sector and the Ghanpur sector (Fig. 2). dicts the original connotation of King Longitude and latitude of the place names (1881) who clearly accorded similar stra­ and other geographic features are given at tigraphic status to his carbonate dominant the end (Appendix I). “Pakhal series” and the siliciclastic-domi- nant “Sullavai series”. PROBLEM OF NOMENCLATURE OF THE Chaudhuri and Chanda (1990) have PURANA ROCKS OF THE P. G. VALLEY redefined the sequence as the “Godavari A. survey of the literature indicates that Supergroup” after the name of the river the Purana stratigraphy of the P. G. Valley Godavari which drains the basin. The is plagued by the use of names with varying Godavari Supergroup is a lithostratigraphic stratigraphic connotations. King (1881) unit defined in conformity with the codes designated the sequence as the “transition of stratigraphic nomenclature (CSNI, 1977 ; series”. Basumallick (1963, 1967a) described NACSN, 1983), and brings the stratigraphic them as “Purana formations” ; whereas status of the Purana rocks of the P G. Valley others designated the sequence as “Purana at par with the well established Purana Supergroup” (Johnson, 1967 ; Chaudhuri, sequences of several other basins, namely, Vindhyan Supergroup, Cuddapah Super­ afresh a pebbly to conglomeratic arkosic group or Delhi Supergroup. In the present sandstone as Ramgiri Formation of the work the term “Godavari Supergroup” has Sullavai Group from the Ramgiri hill been used. section. Mapping on a regional scale and exami­ LITHOSTRATIGRAPHY OF THE nation of the Timapuram Gutta section, SULLAVAI GROUP the Ramagundam section, the Ramgiri hill Classification and Nomenclature section and the Encharani hill section indi­ The Proterozoic succession overlying cates that the ‘Encharani quartzite’ of King the Pakhal Group (sensu Chaudhuri, 1985) (1881) may be classified into two lithostrati- and unconformably overlain by the Talchir graphic units of formation status, as has boulder beds o f the Gondwana Supergroup been attempted by Chaudhuri (1985). in the southwestern belt is generally referred Since the term Encharani quartzite has to as “Sullavais”. It consists essentially of been used with different connotations by red or reddish purple sandstones and con­ different workers, use of this term should glomerates with minor am ount o f shale. be discontinued. A new term “Mancheral Detailed m apping of the sequence reveals Quartzite” is being formally introduced that it can be grouped into three units map- here for the quartzose unit, whereas the term pable on 1 : 31,680 scale on the basis of Ramgiri Formation of Chaudhuri (1985) is lithology and texture : retained for the arkosic unit. The definition (a) a conglomerate and coarse-grained of “Ridge Sandstone” (Johnson, 1965) is sandstone of arkosic or subarkosic compo­ not based on specific type locality and as sition, such is inappropriate for formal usage (b) a pebbly to coarse-grained quartz- (CSNI, 1977 ; NACSN, 1983). Further, arenite with m inor conglomerate, and both the arkosic sandstone and the quart­ (c) a typically salmon red, well sorted, zose sandstone form similar high ridges, medium to fine grained quartzarenite/subar- and designating one of them as “Ridge kosic sandstone. Sandstone” may be misleading. King (1881) subdivided his ‘Sullavai The term “Mancheral Quartzite” has series’ into the ‘Encharani quartzite’, the been coined after Mancheral, the type area ; ‘Venkatpur sandstone’ and the ‘K apra sand­ several excellent sections of the quartzite stone’. King (1881) described Encharani are exposed in the hills north-west of quartzite from Ench ani Gutta as well as Mancheral. from the Tim apuram G utta near M ancheral. The fine-grained salmon red sandstones Johnson (1965) described the quartzites of Timapuram G utta as the Ridge Sandstone. had been designated as Venkatpur Sand­ Chaudhuri (1985), on the other hand, des­ stone by King (1881) and Chaudhuri (1985), cribed a quartzose sandstone from Rama- and as Kistampet Sandstone by Johnson Eundam as Encharani quartzite and defined (1967). The term Venkatpur Sandstone has been retained here in preference to Kistam- Chaudhuri (1985) defined the Ramgiri hill pet Sandstone on the ground of priority. section in the Begampet sector as the type Critical examination reveals that “Kapra section for this formation. Nalla Gutta Sandstone” of King (1881) is a part of the section south of Aklaspur in the Ramgun­ Ramgiri Formation and has been formally dam sector is an additional reference section, designated here as Kapra Sandstone Mem­ where all the major sandstone and conglo­ ber. Another unit of the Ramgiri For­ merate facies are well exposed. Thickest mation in the Begampet sector has been occurrence of the Ramgiri Formation has accorded the status of a formal member and been noted around Ramgundam (more than has been designated as Venampalli Sand­ 445 m) and it thins out both to the north­ stone Member. west and southeast where the unit is about 200 m thick. CHARACTERIZATION OF THE FORMATIONS Kapra Sandstone Member and Venam* Ramgiri Formation palli Sandstone Member are two mappable The Ramgiri Formation comprises but areally restricted units occupying speci­ pebbly to pebble-free arkosic sandstone, fic stratigraphic positions within the Ramgiri conglomerate and minor mudstone. This Formation. The rest of the formation, a is the thickest and most extensive unit of complex of highly variable facies and facies the Sullavai Group and is characterised by assemblages has not been assigned to any purple to red colour with white blotches, formal member (cf. NACSN, 1983, article arkosic composition, coarse grain size and 25). ubiquitous trough cross-beds. The typical red colour with white blotches is absent Trough cross-beds of varying scale are over a considerable stretch in the Begampet the most abundantly developed primary sector, where the sandstone and conglo­ structures in the sandstones of the formation. merate are dirty white or white. However, Cosets of troughs generally forms sheet-like the facies and the paleocurrent pattern units bounded by extensive planar as well as remain the same irrespective of their colour. concave up erosional surfaces. Large planar

Fig. 3. Conglomerate of the Ramgiri Formation. Note low-angle and horizontal stratification, small scours (below the scale), sharp base of the beds and interbedded sandstones. Nalla Gutta section. Scale = 30 cm. Fig. 4. Intense penecontemporaneous deformation in pebbly, coarse-grained sandstone of the Ramgiri Formation ; Ramgiri Hill section, Begampet sector. Fig. 5. Spectacular flat-topped, vertical-sided ridges of Kapra Sandstone Member; Nasaram Gutta, Ghanpur sector. Human figure for scale. Fig. 6. Coset of planar cross-beds in the Mancheral Quartzite, Ramgundam Gutta. Fig. 7. Typical sequence of Venkatpur Sandstone showing alternating large planar cross-beds and flat-beds ; stone quarry west of Bellampalli Railway Station. Figs. 3-7

cross-beds, 0.30 to 1.72 m thick, are locally stone matrix (Fig. 3) ; pebbles range in size abundant. from 20 to 100 mm. Clasts are dominantly The conglomerates of the Ramgiri For­ of vein quartz with rare fragments of chert, mation are mostly clast-supported with sand­ granite or metaquartzite. The conglomerate beds normally have slightly scoured irregular base ; they vary in thickness from 0.1 to 3.0 m, and show planar, trough or low-angle cross-beds. Several 20 to 30 cm thick ma­ ssive conglomerate beds have flat non-eros- ional basal contact, matrix-supported texture and inverse grading. Generally individual beds pinch out within a short distance but conglomeratic zones occurring at several stratigraphic levels extend over several hun­ dreds of metres to a few kilometres. Thin sequences of red or greenish grey mudstone or muddy fine sandstone with well developed ripple lamination and thin parallel laminae occur interlayered with other facies of this formation. Low-lying areas east of the Encharani hill (Ghanpur sector) show Fig. 8. The log through the Venampalli Sandstone Member, north of Venampalli. Note a extensive development of the mudstone wave-rippled lower shale dominated part. facies. The Ramgiri Formation is at places affected by intense soft sediment deforma­ Venampalli in the Begampet sector and com­ tion. In the Ramgiri hill section, lower prises a thin basal green shale sequence and 100 m of the scrap section is pervasively overlying thicker white, buff or light greenish deformed and the sequence gradually passes coarse-grained arkose (Fig. 8). The maxi­ upward into undeformed facies The inten­ mum preserved thickness of the sequence is sity of deformation varies from total des­ 126 m. The greenish colour is due to the truction of the original structures by lique­ presence of authigenic ferricillite (Dasgupta faction of the beds to well developed dishar- et al., 1990). A peculiar massive weathering monic folds (convolute laminations ; Fig 4) makes identification of the primary sedimen­ to gentle slumping and oversteepening of the tary structures very difficult. Large planai foresets. bounding surfaces, indistinct but ubiquitous Paleocurrent direction of this formation medium or small trough cross-beds, and is highly variable though a strong south­ locally fine-grained wavy laminated beds easterly mode is discernible in many sections. characterise the sequence. However, the paleocurrent direction abruptly The green mudstone and interlayerec changes to south and southwest in the Ghan­ coarse sandstone beds show abundance o pur sector. wave ripple cross-lamination and large Venampalli Sandstone Member: It is best symmetrical megaripples. Trough cross-bei exposed in the low-lying ridges north of data indicate strongly unimodal paleocurren Fig. 9. Geological cross-section along the line A7-A'7 shown in Fig. 2. Note the low-angle discordance of the Kapra Sandstone Member with the underlying part of the Ramgiri Formation. towards southeast. Lithologlcally, the Kapra sandstone is Kapra Sandstone Member This : member very similar to the small trough cross­ forms the upperm ost part of the Ramgiri bedded facies of the Ramgiri Formation. Formation around Kapuram and Nasaram It has been delineated as a separate member Gutta in the Ghanpur sector. The section because of the angular discordance with the around Nasaram Gutta is defined here as the rest of the Ramgiri sequence and the dis­ type section of this member. In the type tinctive geomorphic form that this unit section it is about 75 m thick. Kapra Sand­ displays. It may be mentioned here that stone Member is reddish to white, coarse­ the criteria used for the definition of the grained, arkosic sandstone with dispersed Kapra Sandstone Member are very similar pebbles th at vary from 10-15 mm in their to those used in defining an allostratigraphic longest dimension. The sandstone shows unit (NACSN, 1983, article 58). abundant trough cross-beds with individual Mancheral Quartzite cross-sets 5 cm to 15 cm thick. Small isola­ This is a deep brown to red, poorly ted channels occur at places. The sandstone sorted, pebbly to fine-grained quartzarenite forms spectacular flat-topped ridges with ver­ with minor amount of conglomerate. In tical sides (Fig. 5). Nearly subhorizontal the Ramgundam area planar cross-beds are beds of the K apra Sandstone overlie the well developed, whereas trough cross-beds gently dipping beds of the rest of the Ram­ are the dominant sedimentary structures at giri Formation with a low-angle discordance Mancheral. Numerous mud clasts, thin (Fig 9). E*posurewise mean of paleocurrent sheets of conglomerate and shallow or deep directions of the member varies from 123° to channels are common in both the places. 156°. Planar cross-beds vary in thickness from 12 cm to 130 cm, averaging around 40-50 cm, mation, and this area is defined as an addi­ and occur as isolated or grouped sets tional reference section. The maximum (Fig. 6). Trough cross-beds are 2 - 15 cm preserved thickness of the formation, as in thickness. Cosets of troughs locally build measured in the Mancheral area, is 77 m up 30-81 cm thick sheet-like sand bodies. and the preserved sequence at Ramgundam Extensive quartz overgrowth, diffuse or section is about 30 m thick. concentrated blotches of iron oxide, and Venkatpur Sandstone infiltrated secondary clay matrix at places The Venkatpur Sandstone consists of characterise the sandstones. At Ramgundam salmon red beds of well sorted, medium to area the mean paleoflow direction is towards fine-grained quartzose to subarkosic sand­ southwest and that at Mancheral area is stone, and subordinate amount of red silt- towards north. stone. The sandstone is characterised by At places, polymict boulder pebble high degree of textural uniformity through­ conglomerate with clasts of quartzite, chert, out the area. It is a soft, poorly indurated vein quartz and intraformational sandstone sandstone and normally forms low-lying and occurs as 3 - 5 m thick channel-fill lenses. small hillocks. King (1881) first proposed Pebbly conglomerates occur commonly as and defined this unit from Venkatpur in the thin sheets and less frequently as 10 - 30 cm Chelvai area ( a little beyond the south­ thick horizontally stratified beds. Muddy eastern end of the area mapped during this fine sandstone and mudstone interbeds occur study). However, best natural exposures in in units, 5 cm to 23 cm thick, and show the study area occur north of Jala Gutta near well-developed ripples, shallow scour pools Mancheral in the stone quarries near the and mud cracks. Bellampalli railway station and in the creeks north of the Laknavaram Lake. These Salmon red, well sorted fine to medium sections are proposed here as additional grained quartzarenite occurs as tabular reference sections for the Venkatpur Sand­ sheets at several stratigraphic levels at Ram­ stone. The thickest observed section in gundam and Bellampalli areas. Some of the Laknavaram area measures 60 m. At the beds are characterised by adhesion Mancheral this unit is about 50 m thick and structures, whereas others are massive or thickness at the Ramgundam Gutta is less trough cross-bedded. than 30 m. Mancheral Quartzite does not occur Meter-scale planar tabular or planar south of Ramgundam, whereas it forms wedge shaped cross-beds (up to 3 m thick) prominent ridges around Mancheral and is the hallmark of this sandstone. The further north. It is very well exposed in the scrap section of Timapuram Gutta designa­ cross-beds normally alternate with flat- ted here as the type section of the formation. bedded facies (Fig. 7) locally displaying Excellent exposures in the north-eastern intense soft sediment deformation. Aeolian slope of the Ramgundam Gutta exhibit translatent strata (Hunter, 1977) and adhe­ all the characteristic features of this for­ sion structures are common. Channel-fi)] sand bodies and small scale aqueous cross­ * 906 t Faul t beds are locally present. Wave or current ripples and mud cracks may be locally abundant. As Paleocurrent measurements from the large cross-beds show remarkably persistent north-easterly flow direction. Fig. 10. Geological map of tbe Mancheral area.

LOCAL STRATIGRAPHIC SEQUENCES Mancheral area (Fig. 10) ganga rocks and the overlying Sullavai rocks This is the only area where, in a few is also seen in the small hillock 3 km north sections, all the three Sullavai formations of the village Kasipet where coarse and occur in a few sequence. In the southern poorly sorted Mancheral Quartzite overlies part of Jala Gutta, 5 km NNW of Mancheral, the deformed Sat Nala Shale. In all these the Ramgiri Formation is developed as the sections small fragments of brown Sat Nala basal unit of the Sullavai Group. The shale (Penganga Group) occur in the sand­ subhorizontal Ramgiri Formation uncon- stones of both the Ramgiri Formation and formably overlies the deformed beds of the the Mancheral Quartzite. Penganga Limestone. M ancheral Q uartzite conformably overlies the Ramgiri Formation. Ramgundam area (Fig. 11) The contact between the two is marked at In this area Mancheral Quartzite and several places by lenticular conglomerate Ramgiri Formation do not occur together units that at one place attain a maximum in any single section, but crop out separate­ thickness o f 18 m. The Venkatpur Sand­ ly in isolated fault-bounded blocks. Chau­ stone overlies the Mancheral Quartzite with dhuri (1985) reported intertonguingrelation­ a gradational contact marked by an 18 m ship between the quartzose and arkosic thick transitional zone. The rocks of the units of the Sullavai Group from the north­ Permo-Jurassic Gondwana Supergroup over­ western corner of this area. However, no lies the Venkatpur Sandstone in most of the such intertonguing relationship has been sections in the area. observed by this author. The quartzose In a small outlier near Goddapur, in the sandstones referred to by Chaudhuri (1985, Mancheral sector, the Ramgiri Formation Fig. 2) has been reinterpreted on the basis unconformably overlies steeply inclined beds of lithology, primary structures and field of the Penganga Group and in the hill range, disposition as part of the Mulug Subgroup 6 km north-northwest of Bellampalli Rail­ of the Pakhal Group. North of the Ram­ way Station, Mancheral Quartzite uncon- gundam Railway Station and west of the formably overlies the Sat Nala Shale of the railway line, a narrow strip of the Manche­ Penganga Group with a low angular discor­ ral Quartzite dips north-easterly and overlies dance. The unconformity between the Pen- gently folded beds of the Pandikunta Lime­ Fig. 11. Geological map of the Ramgundam area.

stone and southeasterly dipping beds of the glomeratic zones are present a t different Damala Gutta Conglomerate, both of the stratigraphic levels. Pakhal Group. In Ramgundam Gutta it South of Nalla Gutta and north of overlies the Rajaram Limestone in the Taklapalli a thick sequence o f Penganga southern part, whereas in the n orth it overlies limestone and sandstone occurs in a trian­ successively older formations of the Pakhal gular fault-bounded block, as also reported Group. As also noted earlier (Chaudhuri, by Sreenivasa Rao (1987) and Chaudhuri 1970, 1985 ; Chaudhuri and C hakraborty, et al. (1989). However, due to its faulted 1987) the contact between the Mancheral contacts with the adjacent Sullavai rocks the Quartzite and underlying Pakhal Group is relationship between the Penganga and the erosional and the fragments of the under­ Sullavai Groups in not discernible in this lying Pakhal rocks abound in the conglo­ region. merate lenses in the Mancheral Quartzite. The Venkatpur Sandstone is juxtaposed The contact is clearly an unconformity. above the Mancheral Quartzite along a fault South of Kundanapalli Ramgiri Forma­ contact in the Ramgundam Gutta (Fig. 12), tion overlies highly weathered and ferricriti- whereas it conformably overlies the Man­ sed Rajaram Limestone, whereas further cheral Quartzite in the section north of Ram- west near Aklaspur it overlies the Damala gundam Railway Station. But its contacts Gutta Conglomerate or the Pandikunta with the Ramgiri Formation is faulted every­ Limestone. Therefore, Ramgiri Formation where in the Ramgundam area. also overlies the Pakhals unconformably. Ramgiri Formation is well exposed in the Begampet area (Fig. 13) Nalla Gutta hill ranges, where several con­ Ramgiri Formation alone is exposed ir Upper Pakhal quartzite, because its correla­ tions either with the Mulug Orthoquartzite or the Ramgundam Sandstone is uncertain. The contact between the Ramgiri arkose and the underlying granite is marked by pro­ nounced topographic irregularity. In Pedda Gutta within a horizontal distance of 1.6 km the contact comes down by about 135 m. Similar change in elevation is also observed in the section south-southeast of Jaffar Khanpet which indicates an undulating pre- Sullavai topography and the erosional nature of the sub-Sullavai surface. The facies assemblage varies widely in different localities. The Venampalli Sand­ stone Member occupies a large area north- northeast of Jaffar Khanpet and Venam­ palli. In its area of occurrence the member occurs as the uppermost unit of the Ramgiri Formation. The contact of the basal shale- sandstone alternation unit of the Venam­ palli Sandstone Member with underlying sandstones of the Ramgiri Formation is gradational in the western side of the main outcrop (Fig. 8), whereas in the eastern side of this outcrop the contact is sharp (Fig. 13). Ghanpur area (Fig. 14) In this area, between the Maner river and the Ramappa tank, the Sullavai rocks overlie mainly the Pakhal rocks. They Pig. 12. Geological cross-section along the line appear to be structurally conformable A,-A', shown in Fig. 2. though the Sullavai rocks overlie different Pakhal units at different localities and the contact between them is erosional and his area, and unconformably overlies the locally undulating. Immediately to the east 'ranitic basement as well as the quartzose of the Maner river the Sullavai rocks overlie andstone of the Pakhal Group. The quart- the limestones of the Mallampalli Subgroup, ose Pakhal sandstone around Adwi Sri- whereas near the Ramappa tank or further ampur and Edlapur has been mapped as the southeast beyond the mapped area, they Fig. 13. Geological map of the Begampet area.

Fig. 14. Geological map of the Ghanpur area.

overlie deformed Mulug Shale, or Mulug Ghanpur tank, and at a place <~3.5 km Orthoquartzite of the Mulug Subgroup (cf. southeast of Ghanpur where this zone is Basumallick, 1967a). about 50 m thick, and a few tongues of Ven­ katpur Sandstone occur within the Ramgiri The Sullavai sequence in this area con­ Formation (Fig. 15, section VIII). sists of Ramgiri Formation and the Venkat­ pur Sandstone. The latter directly overlies The Kapra Sandstone Member overlies the former through a transitional zone, or the rest of the Ramgiri Formation with a sli­ the two intertongue with each other. The ght structural discordance around Nasaram transition zone is marked by intercalation of Gutta (Fig. 9). A small outlier of the thin tongues of Sat bedded fine to coarse­ Venampalli Sandstone occurs near Donga- grained Venkatpur Sandstone with Ramgiri mantimala. arkose. The intercalated zone is well deve­ The Sandstone of Ramgiri Formation loped at the northwestern corner of the in this sector are less felspathic compared ic 1 * X -'O

j~’ -| Venkatpur Sandstone FTTT'l Transitional Unit between Vwkafpgr San«t»«*e r ::::l and fiamgin'PoreiafiBn/Martcltefal Ouorfwe

Kapra SamHtont Neuter Penganga Group

Vencmpalli Sandstone Member Pakhal auar^riK

[a 7] Rotngiri Formation Pakhal limestone L&S Mancheral Quartzite GranHlc 9o»e*ent

Fig, 15. Correlation of measured vertical logs indifferent sections of the study area. I, II, III.... denotes the section numbers whose positions are indicated in the Fig. 2. Asterisks in section III and VII refer to respective geological cross-sections ( see Fig, 9 & 12 ).

those in the Ramgundam or Begampet sec­ though locally, the overlying and under­ tors. In two exposures, near Papan Bodu lying successions may be structurally confor­ (hill) and Dongamantimala, they become mable. The present investigation does not quartzose, and feldspar content, in general, support an earlier suggestion that Sullavai is decreases southeastward. a facies variant of Pakhals (Heron, 1949 ; Pascoe, 1950, p. 494). The considerable REGIONAL CORRELATION AND difference between the K-Ar age dates ob­ STRATIGRAPHY OF THE SULLAVAI GROUP tained from the glauconitic minerals of the lower Pakhal Pandikunta Limestone (1300 The Pakhal Sullavai contact in different ± 53 Ma) and from the Venampalli Sand­ sectors is an irregular, undulating, erosional surface marked by angular discordance and stone Member of the Sullavai Group overstepping of the Sullavai sandstones over (871 ± 14 Ma ; Chaudhuri and Howard, successively older Pakhal formations and on 1985) lends support to this interpretation. to the Basement Complex. The contact is The gently dipping undeformed beds of the clearly an angular unconformity surface, Sullavai Group, in a number of sections in the Mancheral-Bellampalli area, overlies the tion. In M ancheral area the depositional deformed and folded limestone and shale regime of the quartzite replaced that of the sequences of the Penganga Group. At seve­ Ramgiri arkose with time. In Ramgundam ral places the Sullavai sandstones incorpo­ area, on the other hand, occurrence of both rate fragments of the Penganga shale. The the formations on the same units of the Sullavai-Penganga relationship, therefore, is Pakhal Group at different places and their also that of an angular unconformity. The markedly different mean paleotransport field relations contradict the earlier sugges­ direction indicate different depositional loci tion by Chaudhuri et al. (1989), on the basis of the two systems. It is, however, difficult of the radiometric age data, that the Pen­ to comprehend the temporal relationship ganga Group is younger than the Sullavai between them in Ramgundam area. The G roup. depositional system of the Mancheral In Mancheral area, the Mancheral Quartzite did not develop in the Ghanpur Quartzite conformably overlies the Ramgiri and Begampet areas. In these sectors the Formation with a fairly thick conglomerate Venkatpur system started to develop over­ marking the contact at places. At Ramgun­ lapping in time with the Ramgiri system. It dam, on the other hand, both the units is difficult to comprehend the temporal unconformably overlie different Pakhal for­ significance of the absence o f Mancheral mations in separate fault blocks. Mancheral Quartzite because of the Jack of any key Quartzite is not developed southeast of Ram­ bed or time marker. gundam. The Venkatpur Sandstone overlies Sedimentation and paleocurrent the Mancheral Quartzite in Mancheral and Ramgundam sectors either gradationally or The distinctive lithology, sedimentary sharply. But over a long stretch in the structures and their organisation and distinc­ Begampet and Ghanpur sectors, where Man­ tive paleodispersal pattern of each of the cheral Quartzite is absent, it either inter­ Sullavai formations are the imprints of their tongues or gradationally overlies the Ramgiri respective depositional regime. Large scale Formation. Figure 15 attempts to correlate geometry of the lithounits and their inter­ the measured sections. Clearly, a generalised relationship in space are the product of inter­ order of vertical superposition of litho- action between depositional processes and stratigraphic units can not be proposed. The basin tectonism (cf., Galloway, 1981 Miall,; variable relationship between the three for­ 1984 ; Prave et al., 1991). mations in different parts of the area under Laterally extensive, sheet-like units of consideration indicates that the depositional small trough cross-beds and lenticular to systems were partly overlapping in time in wedge-shaped conglomerate units of the different parts of the basin. Ramgiri Formation have been interpreted to represent a distal alluvial tan setting The Mancheral Quartzite is more res­ (Chakraborty, 1988, 1991b). Sim ilar sheet- tricted in occurrence compared to the Ven­ flood deposits with local lenses of fan-head katpur Sandstone and the Ramgiri Forma­ channel conglomerates characterise these Fig. 16. Summary of the paleocurrent data from the Ramgiri Formation.

settings (cf., Heward, 1978 ; Blair, 1987 ; DeCelles et al., 1991). On the other hand, pebbly sandstone facies of the Ramgiri Formation, characterised by large planar cross-beds and cosets of trough cross-beds and frequently organised in fining-upwards sequences, is interpreted to represent an Fig. 17. Paleocurrent pattern of the Mancheral alluvial plain setting dissected by braided Quartzite in the Ramgundam area. streams ( Chakraborty, 1988, 1991b ). Ex- posurewise mean paleocurrent direction of the Ramgiri Formation shows wide scatter. 1991b ). Thin units of fine-grained, salmon However, facies-wise measurements show red sandstones within Mancheral Quartzite that fan-related strata consistently yield a represent wind blown sands which in places north-east to eastward flow direction, covered up the distal part of the Mancheral whereas the data from alluvial plain-braided alluvial plain (Chakraborty and Chaudhuri, stream deposits show a south-eastward 1993). Data collected from the Mancheral paleoflow direction in the Ramgundam and fluvial deposits in the Ramgundam area show Begampet area (Fig. 16). paleocurrent direction towards southwest In the Ramgumdam area, poorly sorted (Fig. 17). pebbly sandstones of M ancheral Quartzite Venkatpur Sandstone records deposition show several large-scale concave-up channel- in an extensive aeolian sand sea (Chakra­ form erosion surfaces that are overlain by borty, 1988, 1991a). M easurements from fining-upwards sequences. These sequences locations scattered across the study area indi­ have been interpreted as high gradient brai­ cates a very consistent paleowind direction ded fluvial deposits ( Chakraborty, 1988, to the northeast. The paleowind pattern of the Venkatpur Sandstone is believed to indi­ stone crops out extensively in the Ramgun- cate presence of a stable high pressure cell in dam-Mancheral areas, which are also the the area during the Late Proterozoic time. areas where the Mancheral Quartzite is well developed. Although these evidences are Notwithstanding the broad similarities suggestive of Pranhita Sandstone as a possi­ of depositional environment (alluvial fan- ble source rock, a definitive correlation braided river), caliber of sediment load (con­ would require systematic patrographic study. glomerate - coarse-grained sandstone ) and occurrence in correctable stratigraphic posi­ Stratigraphic relationship of the Verkat- tion, the remarkable contrast in the litho- pur Sandstone with both the Mancheral logy of Ramgiri Formation and Mancheral Quartzite and the Ramgiri Formation indi­ Quartzite is somewhat confusing. However, cates that these intrabasinal alluvium were opposing paleocurrent patterns of the Ram­ probably the major source of the aeolian sys­ giri fan system and the Mancheral fluvial tem. A regional wind direction to the north system indicate presence of opposing paleo- east and subarkosic composition (Chau­ slopes on the two flanks of the basin. Two dhuri, 1970) of the sandstone support its systems, therefore, sampled two different derivation from the Ramgiri arkosic sand­ source terrains. Abundance of fresh felspar stones. and granite fragments and flow from the A semi-arid climate has been inferred for southwest indicate that the Archean (?) gra­ the underlying Middle Proterozoic Pakhal nite gneiss complex exposed to the south of Group on the basis of occurrence of sabkha the Sullavai outcrop belt was the source rock sequences ( Chaudhuri et a l, 1987 ). Red for the Ramgiri Formation. Preliminary colouration of Ramgiri arkoses, interlayered petrographic study of the Mancheral Quart­ aeolian or sabkha sequences within the Man­ zite shows abundance of chert and sedimen­ cheral Quartzite indicate similar set-up for tary quartzite fragments possibly derived the basal Sullavai rocks. Regional climate from the chert beds and nodules which are might have evolved to a more arid one common in both the Pakhal and Penganga during the Venkatpur time. Presence of ex­ limestones unconformably underlying the tensive alluvial fan-derived coarse-grained, Mancheral Quartzite. It is difficult to pin immature sediment in the lower part of the point the source rock for the quartzite frag­ Sullavai Group indicate presence of granitic ments. There is at present no outcrop of highlands along the southwestern margin of older quartzite north, northeast of the Man­ the Sullavai basin. Basin margin highlands cheral exposures and the area is now covered can locally attract rainstorms and generate by younger Gondwana rocks. Mulug Ortho- coarse-grained alluvial sedimentation (Blac- quartzite of the Pakhal Group (Basumallick, key, 1986 ; Stewart, 1991). D enudation o f 1967a) and Pranhita Sandstone of the Pen­ these highlands can therefore increase the ganga Group are potential candidates for severity of the climate aiding thereby esta­ the Mancheral source rock. Mulug Ortho- blishment of a large erg system in the basin. quartzite crops out only in the southeastern Fig. 18 shows the postulated paleogeogra- part of the area. In contrast, Pranhita Sand­ phic set-up of the Pranhita-Godavari Valley Fig. 18. Diagram showing schematically the reconstructed paleogeography for the closing phase of the Sullavai sedimentation.

during the Late Proterozoic time. stone. This broad characteristic seems to persist throughout the studied area and is DISCUSSION AND CONCLUSIONS believed to represent a near complete sequ­ Thickness variation (notably that of the ence of basin initiation and filling-up. Ramgiri Formation) and lateral pinching Exclusively continental sedimentation history out of lithologic units indicate differential appears to usher in the grand finale of subsidence during deposition as well as dia­ Proterozoic sedimentation in the Pranhita- chronous change in depositional environ­ Godavari Valley and distinguishes it from ment. However, in most of the complete the marine Pakhal and Penganga Group of stratigraphic sections, a fluvial sandstone, sediments. either Ramgiri Formation or Mancheral As is evident from the figure 15, it is Quartzite, occurs unconformably above the not possible to erect a generalised strati­ underlying basement and is followed grada- graphic succession of the Sullavai Group, as tionally upward by aeolian Venkatpur Sand­ the formations in a regional scale are not homotaxial but are marked by pinching out, of ‘layer cake arrangement’, correlation of thickness variation, and vertical and lateral Sullavai formations in a regional scale reveals gradation in all stratigraphic levels. It is a complex stratigraphic architecture. It is evident that the lithounits were partly coeval difficult to propose a ‘generalised’ succession with each other. of the Sullavai Group of rocks. The Sullavai Group, bounded between (iv) Ramgiri and Mancheral sandstones two basin-wide unconformities and charac­ were laid down by alluvial fan-braided river terised by continental elastics, represents a system coming down from two opposing separate pulse of extension and basin-fill flanks of the basin. Venkatpur erg system during the Late Proterozoic time. Initial covered up the basin during its terminal phase was characterised by rapid subsidence phase. Change in depositional regime is and development of basin margin highlands believed to reflect a change from semi-arid that contributed a large amount of coarse­ climatic set-up. grained clastics. Subsequent decrease in the rate of subsidence and denudation of (v) Sullavai Group represents a near basin margin highlands restricted supply of complete sequence of basin filling. Bounded coarser material to the basin-fill. Basin con­ by two regional unconformities at the base tinued to subside more slowly, possibly under and at the top, the Group represents a sepa­ the influence of sediment load as the fine­ rate phase o f basin extension and filling-up grained aeolian and sabkha sediments in the Late Proterozoic history of the Pran- accumulated. hita-Godavari Valley. Following conclusion can, therefore, be drawn from the present study of the Sulla­ ACKNOWLEDGEMENTS vai Group : The infrastructural facilities for this (i) The Sullavai Group unconformably work was provided by the Indian Statistical overlies both the Pakhal and the Penganga Institute. Findings presented here comprise Groups in the southwestern Proterozoic belt part of the author’s Ph. D. thesis carried of the P-G Valley and is unconformably out under the supervision of Prof. A. K- overlain by Gondwana rocks. Chaudhuri, to whom the author is grateful. (ii) The Sullavai Group can be subdivi­ An earlier version of the manuscript bene- ded into three formations, namely, Ramgiri fitted from the critical comments of Dr. S. Formation, Mancheral Quartzite and Ven­ K. Chanda of Jadavpur University and katpur Sandstone. Venampalli Sandstone Dr. S. N. Sarkar of ISI. S. N. Das assis­ and Kapra Sandstone are two formal mem­ ted me in the field and drafting was done bers within the Ramgiri Formation and by A. K. Das. Paleocurrent diagram s were occur as distinctive lenticular units. prepared using software developed by Kutty (iii) Contrary to the traditional concept and Ghosh (1992). APPENDIX— 1 (Latitude & Longitude of the localities and other geographic features referred to in the text)

Adwi Srirampur 18°30'N 79°38'E Kistampet 18°51'N 79°45E Aklaspur 18°47'N 79°21'E Kundanapalli 18°45'N 79°26'E Ancharami 18°28'N 79°42'E Laknavaram 18°09'N 80°05'E Begampet 18°36'N 79°34'E Mancheral 18°52'N 79°26'E Bellampalli 19°03'N 79°26'E Nalla Gutta (Hill) ... I8°46'N 79°22'E Bellampalli Railway Station ... 19°03'N 79°26'E Nasaram Gutta (Hill) 18°27'N 79°47'E Chelvai 18°l 5’N 80°12‘E Papan Bodu 18°27'N 79°49'E Dongamantimala 18°I9'N 79°49'E Pedda Gutta (Hill) ... 18°38'N 79°26'E Edlapur 18°32‘N 79°34'E Ramappa 18°15'N 79°56'E Encharani Gutta (Hill) 18°29'N 79°44'E Ramgiri Fort Hill ... 18°34'N 79°33'E Ghanpur 18°18'N 79°53'E Ramgundam 18°48N 79°27'E Goddapur I8°57'N 79°22’E Ramgundam Gutta (Hill) 18°47'N 79°27'E Jafar Khan Pet 18°33'N 79°31'E Ramgundam Railway Station 18°46'N 79°26'E Jala Gutta 18°54'N 79°25'E Timapuram Gutta (Hill) 18°58'N 79°24'E Kapuram 18°30’N 79°45'E Venampalli 18°45'N 79°33'E Kasipet 19°02’N 79°26'E

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