Indian Journal of Geology Vol. 70, No. 3, p. 217-230, 1998
LITHOSTRATIGRAPHY O F D E F O R M E D P R O T E R O Z O IC R O C K S F R O M AR O U N D TH E C O N F L U E N C E O F TH E G O D A V A R I A N D
IN D R A V A T I R IV E R S , S O U T H IN D IA D. Sa h a1 a n d G . Gh o s h*
1Geological S tudies Unit, Indian S ta tistica l In stitu te, 203, B. T. R oad, C alcu tta - 35 2D urgapur G overnm ent C ollege, D u rgapu r, Dist. B u rd w an , W est B en g a l
A B S T R A C T
Six form ation s can be fo rm a lly r e co g n is e d w ith in th e d e form ed se q u e n c e o f th e S om a n p a lli Group occu rrin g arou nd the con flu en ce o f the In d ra v a ti and G od av ari riv e rs in th e ea stern P roterozoic (Purana) belt in th e P ra n h ita -G od a v a ri V a lle y , S ou th In dia. S ta ted in g e n e r a l a s ce n d in g order, they are : i) the B od ela V agu F orm ation , d o m in a n tly a ca rb o n a te u n it; ii) th e Soifrnur F o rm a tio n , m ainly terrigenous; iii) & iv) th e T aru r N ala F o rm a tio n , th ick seq u en ce o f ash b ed s an d g ra y w a ck e -sh a le . and contem poraneous but la te ra lly sep a ra te d P edda G u tta C h ert F o rm a tio n ; v) th e K op e la Sh ale Formation; vi) th e Po G utta F orm ation , sa n d sto n e -sh a le a lte rn a tion s. T h e B o d e la V a gu F orm ation and the S om n u r F orm a tion d efin in g th e lo w e r p a rt o f th e S om a n p a lli G rou p a p p e a r to h ave been formed in a d join in g fa cies belts on a m ix ed silic ic la s t-c a r b o n a te p la tfo rm an d in d ic a te a fa ir degree of progradation o f th e la tter over th e form er. T h e T aru r N a la F o rm a tio n is lim ite d to a fau lt- bounded trou gh w h ich sep arates th e B od ela V a gu F orm a tion from th e S om n u r F orm a tion . T h e fau lt- bounded tro u gh s re su lted from ex ten sion an d su b sid e n ce o f th e p la tfo r m on w h ich th e se d im en ts o f the Bodela V a gu F orm ation and S om n u r F orm a tion w ere deposited.. T h e tro u g h s a cted as r e p o si
tories for the T aru r N ala F orm ation and the P edda G u tta C h ert F orm a tion . T h e u p p er p a rt o f the Somanpalli G rou p con sists o f K opela S h a le F o rm a tio n and th e u n c o n fo r m a b ly o v e rly in g Po G utta Formation. T h e T a ru r N ala F orm ation and th e P ed d a G u tta C h ert F o rm a tio n are p r o d u c ts o f deep water sed im en ta tion . A ll o th er form a tion s are m a rk ed by sh a llow w a te r d e p o s itio n a l fea tu res. The stratigraphic p ositio n o f the S om an p alli G rou p sa n s th e Po G utta F orm a tion see m s to be below the Albaka S a n d ston e w hich seem s to lie u n co n form a b ly below th e S u lla v a i G rou p in th e eastern Proterozoic belt. T h e S om an palli G roup, th erefo re, has a stra tig ra p h ic p o s itio n s im ila r to th at o f the Pakhal rock s o f th e type area in the w estern P rote ro zo ic belt.
K ey-w ords : A sh b ed s an d g r a y w a c k e , fa u lt -b o u n d e d tr o u g h s , lit h o s t r a t ig r a p h y , m ixed silicicla s t-ca rb o n a te pla tform , P ak h al, P roterozoic.
IN T R O D U C T IO N
With th e exception o f some inliers, the P roterozoic rocks o f the Pranhita- Godavari (P-G ) V alley are deposited along two parallel N W -SE trending belts with an intervening strip o f Gondwana rocks.
The s t r a t i g r a p h y o f th e w e s t e r n Proterozoic belt has. been reasonably well
docum ented (K ing, 1881; B asum allick, 1967; C haudhuri, 1985; C hau dh u rie? a l. , 1989). A form al cla ssifica tion schem e for the P roterozoic sequ en ce o f the eastern belt outcropped in the A lbaka Ranges in the southeastern part is available (S rin i
vasa Rao et a l., 1979), but a form al stratigraph y is yet to be established for a
large part o f the b elt n o rth w e st o f th e A lbaka R anges. T h e p resen t w ork is in tended to be a com p reh en siv e stu d y o f the geology o f an area n o rth w e st o f the A lbaka R anges in th e eastern P roterozoic b e lt a r o u n d th e c o n f l u e n c e o f th e Indravati and G odavari rivers (in set in F ig. 1). T h e P r o te r o z o ic rock s o f th e Pranhita-G odavari V a lley w ere origin ally g r o u p e d t o g e t h e r u n d e r th e P a k h a l Series (King, 1881) and w ere devided into a low er P ak h al stage, develop ed m ain ly in the w estern belt and an upper A lbak a stage, largely form in g the eastern belt.
Later w orkers cla ssified the rocks o f the w e s t e r n b e l t u n d e r P a k h a l G r o u p (B asum allick, 1967) and defined A lbak a G roup from th e eastern b e lt (C h a u dh u ri et al., 1989; cf. A lb a k a S a n d ston e o f Srinivasa Rao et al., 1979). T h ou gh the P akhal G roup in the type area is g en er
ally regarded as a n u n m etam orph osed sedim entary sequence w ith little d efor
m ation, th e sou th ern extrem ity o f the w estern belt in the Y allan dlapad area is c h a r a c t e r is e d b y a h ig h ly d e fo rm e d an d m e t a m o r p h o s e d r o c k s e q u e n c e (Ram m ohana Rao, 1971). D etailed strati
graphic work is n eeded to iden tify the probable equivalents o f the Pakhal G roup in the deform ed P roterozoic strata o f the eastern belt.
U N IT N A M E S A N D T H E IR T Y P E S E C T IO N S
T h e P r o t e r o z o i c s e q u e n c e o f S om anpalli show s fau lted bou n daries against the G ondw ana rocks on the west
and A lbak a S an d ston e and basement gn eisses in the east (Fig. 1). Deformation o f the sequence has given rise to large scale folds, cleavage, and h igh and low angle faults. The style o f deformation is ch aracteristic o f the extern al zones of a m ountain belt (Saha, 1989, 1990). Low grade m etam orphism h as given rise to chlorite, m uscovite and chloritoid in the pelitic rocks o f the sequence.
The follow ing are the six stratigraphic units o f form ation rank th a t have been recogn ised in the P roterozoic sequence of the area, arranged in a scen d in g order:
6. P o Gutta Form ation (sandstone-shale alternations)
5. K opela Shale F orm ation 4. Pedda G utta C hert F orm ation 3. Tarur N ala Form ation (includes thick
ash beds and la t e r a lly p e r s is te n t grayw acke shale intercalations) 2. Som nur Form ation (m ixed siliciclast-
argillite dom inant).
1. Bodela V agu F orm a tion (c a rb o n a te dom inant)
A higher ran k in g u n it which in eludes the above six fo rm a tio n s has been recognised and nam ed as th e SomanpaH' Group (see also G hosh, 1986; Saha and Ghosh, 1988).
Table 1 gives the lo ca tio n o f thetyp‘
sections o f the stra tig ra p h ic units ami their thicknesses.
Since the strata are m o r e often than not vertical or steeply d ip p in g , the typ>
sections incorporate th e d a t a obtain^
Fig. 1. Geological map of' Lhe study area. Inset shows the geographic location of the mapped area. C.P. = M a h a r a sth r a , M .P. = M adhya P radesh. A.P.
=Andhra Pradesh. At = Atkapalli, Bd = Bodela.
Jg= Jinganur, Kp = Kopela.
Kr = Korla, PI = Soman
palli, Po = Po Gutta, Pt = Patagudem, So= Somnur, TI=Tarlagura, Tr=Tarur.
"Y" for s tra tig r a p h ic younging direction. 1 = basement gniess (? Arch
aean), 2= Bodela Vagu Forma-tion (BVFm), 3 = b reccia b o d ie s w ith in BVFm, 4 = Somnur For
mation, 5 = Tarur Nala Formation, 6 = Pedda Gutta Chert Formation, 7=Kopela Shale Formation, 8=Po Gutta Formation.
9 = Era'nna Gutta Sand
stone, 10=Albaka Sand
stone, ll=C hikiala Forma
tion (G ondw ana); 12 = gneissic foliation, 13 = bed
ding, axial traces, 14 = .inti form, 15 = synform.
Table 1
F o rm a tio n n a m es and their type sections
Form ation Type section and equivalent
Po Gutta Formation Po Gutta, .south of Kopda -
(c. 155 ml Patagudem Hoad . 18"40'N:80'’ 15'07"E) AH-: C .ita and Ko.«a Gutta north of Kopi'! 1 I.itagudem Road
Kopela Shale Formation Nala sections cast of Kopela <18,>50'10"N:80°13'40 E (c. 450 m) traverse between points (18o49'50"N:80o14l45”Ei
and (18°49'43"N:80"13,40"E)
Pedda Gutta Chert Formation Northern slopes of Pedda Gutta, 374 m peak (c. 250 m) (18o40'N:80o15'07"E); Sarvaipet Nala mouth ontbe
southern bank of the Godavari River, off Sarvaipet village (18°42'N.80')14'30"E)
Tarur Nala Formation Tarur Nala (Daddalsan Nala) between longitudes (c. 1250 m) 80°19,30,,E:80°18’ 10"E ; Godavari River sections near
Tarlagura, between longitudes 80°21'20"E and 80°21'25"E
Somnur Formation Godavari River bank opposite Somnur, between
(c. 1.100 m) longitudes 80°15'50"E and 80°16'20"E;
Godavari River bank around Gangaram between longitudes 80°20'30"E and 80"21'40"E
Bodela Vagu Formation Bodela Vagu (nala) and its tributaries from (c. 220 m) Jinganur (18H54'05"N : 80°11'55"E) through
Bodela (18°49'50"N : 80°11'55"E) to the northwest of Somanpalli (18°47'15"N : 80° 11'55"E);
Indravati River section southeast o f Pendlaya between longitudes 80°19'10"E and 80°19‘30"E.
Table 2
Relative stratigraphic position o f the formations belonging to the Somanpalli Group
Pedda Gutta Chert Form ation
Som nur Formation
Po Gutta Form ation Kopela Shale Form ation Tarur Nala Formation
Bodela Vagu Form ation
West geographic shift o f depocentre
east
SOMNUR FORMATION
m-
algal laminite LLH stromatolite 1st. with shaly &
sandy parting , trough cross bed planar cross bed ripple, Jlaser bed
Duddeda Limestone Member Sangam Quartzite Member
plane laminae
Gomalkonda Sandstone-shale Member
a
BODELA VAGU FORMATION mud crack
trough cross bed ripple
Tarlagura Quartzite
Member
plane laminae
Gangaram Shale Member
biostrome
ripple drift laminae salt cast, mud crack Ist./dolomite slump, trough cross bed, Jlaser bed syneresis cracks calcareous shale
Fig- 2. G eneralised Lithologs o f the Rodela Vagu Form ation (a) and the Som nur Form ation (b).
from rep resen ta tiv e grou nd tra v e rse s across the resp ectiv e units in places, which are so selected as to be as free from structural discon tin u ities as p o ssi
ble- U n le s s otherw ise indicated, the esti
mated thicknesses are the m inim um for the respective units and not corrected for any tectonic strain. Subdivision o f the
Un'ts into m em bers has been possible
lvhere the original stratigraphic relations can be discerned, and lithostratigraphic
^stinctiveness o f certain portions o f the
Un't;s dem ands fin er classification.
B O D E L A V A G U F O R M A T IO N The B odela Vagu F orm ation is an a sso cia tio n o f a rg illa ce o u s lim eston e, d o l o m i t i c lim e s t o n e an d d o lo m it e . Siliciclastic intercalations are com m on in th e lo w e r h a lf o f the fo rm a tio n and d o lo m itic a lga l la m in ite -s t ro m a to lite assem blage con stitu tes the other h a lf [Log (a) in Fig. 2], Carbonate breccia com plexes varying in size from a few tens o f m eters to a few kilom eters and having roughly circular to elliptical outline show
cross cutting relation w ith the su rrou n d
ing bedded carbonate rocks. These breccia bodies are often associated w ith sm all patches o f q u artzofeldspathic m aterial.
The siliciclastic interca lation s range from breccia-conglom erate through gritty quartzite to fine grained quartz-arenite and are co m m o n ly in te rb e d d e d w ith dolom itic lim estone. P artings o f calcare
ous m udstone are occasion ally present and salt peudom orphs are often abun
dant. The sandstone beds are com m only trough, cross stratified to ripple lam i
nated, w hile the breccia-conglom erate or gritty units are thick and m assive. C al
careous m udstone or siliceous lim istone units often show asym m etric ripples, p o
lygonal shrinkage cracks, and w ater es
cape features in the B odela Vagu section.
The top o f the B odela V agu Form ation consists o f a 5-6 m eter thick unit o f algal lam inite-strom atolite, w hich is m arked by developm ent o f m olar tooth structure, stylolites, syneresis cracks filled by sparry calcite, and strom atolite heads (m ostly partially linked hem i-spheroids, LLH) in abundance.
SO M N U R F O R M A T IO N
The rocks o f the Som nur Form ation are disposed in a num ber o f N W -trending la rg e a n tifo r m s an d sy n fo r m s w ith faulted lim bs. Five m em bers have been recognised within the form ation, namely, G an garam S h ale M em b er, T a rla g u ra Q uartzite M em ber, G om alkonda Sand- stone-shale M em ber, Sangam Q uartzite
M em ber and D uddeda Lim estone Mem
ber, stated in ascending order [Log (b) in Fig. 2]. The G angaram Shale Member is a 460 m thick plane-lam inated dark shale unit w ith sparse and thin sandstone in
tercalations. The sandstone layers are laterally continuous for a few tens of m eters and vary in th ickness from a few m illim eters to a few decim eters. Shales are locally m etam orphosed into phyllites o f green sch ist facies and show a cleavage cutting across the bedding lamination.
The basal part o f the G angaram Shale M em ber in Som nur section is calcareous and light green in colour.
T he T a rla g u ra Q u a rtzite Member (thickness c. 320 m) consists o f coarse to fine grained quartz-arenites showing well rounded to well sorted terrigen ou s mate
rial and occasional thin shale partings.
Trough and herrin gbon e cross stratifica
tion, asym m etric to in terferen ce ripples, and desiccation cracks are characteristic o f the quartzarenites.
T he G om a lk on d a Sandstone-shale M em ber is about 120 m th ick and con
sists o f in terstratified sandstone-shale.
which often display w avy, flaser bedding and syneresis features. E xcellen t expo
sures o f this m em ber are available along the banks o f the G odavari R iver between Som nur and G om alkonda.
The Sangam Q u a rtzite Member i*
200 m in thickness and show s nearly the same facies association as the Tarlagura Member. Planar cross beds and flaser?.
T A R U R N A L A F O R M A T I O N
Tarligura b Atk»p«III-T«rur
plane laminated ckerl/black skate occasional coarse lapiUi
plane laminated 700- rippled
l— massive lSo*nurFin
C Kotturu-Soraanpalli Lithnrenite Member
slump structure
reverse to normal graded Utkarenite
plane laminated occasional coarse lapiUi In shale
Tarur Nala normal graded limestone flat pebbles o f
shale/limestone
plane laminated
shale with thin partings o f fin e sandstone
normal graded graywackeshale occasional ripple ump structure
massive to normal graded
0 PEDDA GUTTA CHERT FM.
bedded chert occasional shale parting
L. - ^
black siliceous shale occasional lapilli
calcareous and siUceous shale
Fig. 3. Lithologs o f the Tarur Nala Form ation from Tarur Nala and other sections, and generalised log for the Pedda Gutta Chert Formation. Arrow s indicate fault contacts.
however, are m ore a b u n d a n t in the Sangam Q uartzite M em ber. Lateral and vertical gradation into the Duddeda Lim e
stone Member is distinct.
The D uddeda Lim estone M em ber is 100 m in th ickn ess and consists o f a number o f th in (< 1 m ) s tr o m a to lite biostromes altern ating w ith argillaceous and siliciclastic sedim ents. Algal lam ina
tion and sm all dom al strom atolites are locally abundant. M ention m ay be made
°f the river bed sections o f G odavari w est
■t'•village D uddeda.
T A R U R N A L A F O R M A T IO N
The T aru r N ala Form ation consists dom in an tly o f ash beds and grayw acke- shale grading to intercalated shale-lim e- s t o n e (F ig . 3 ). A r o u n d K o t t u r u Som anpalli, the basal part o f the form a tion is defined by a 50 m thick sequence o f volcaniclastics, litharenite and slum ped shale borderin g the argillaceous lim e stone o f the Bodela Vagu Form ation (Log (c) in Fig. 3). The sandy units contain pebb les o f a rgilla ceou s lim eston e and tu ffaceous lava in the basal part, and are generally very coarse to m edium grained.
Fig. 4. A sh bed s, In d ra va ti R iver bed o fl'K ottu ru -S on ia n p alli. (a) Field p h otog ra p h sh ow in g plan vie'' o f n ear v e rtica l b ed s;tra ce o f be d d in g la m in ae is E SE -W N W . B ar scale is 2 0 mm.
M icrop h otog ra p h (X P L ) sh ow in g re la tiv e ly coarser lam in a o f altered cry sta l t u ff correspond in g to a b rig h ter la m in a in "a); a d join in g fin er la m in a consists o f m icro cy sta llin e quartz ant sericite. (<) T e ss e lla te d m u scovite flak es rep la cin g rectan gu lar to sq u a re sectio n s o f feldspa crystals in altered cry stal t u ff la m in a; m icroph otograp h , X PL. B a r scale is 100 m icron in (b
& (c).
They are ch aracterised by m assive or norm al to reverse graded beds and are topped by slum ped shales grading later
ally to ash beds. T he form al nam e of K otturu Som anpalli Litharenite M em ber
is proposed for the basal part o f th e Taru Nala Form ation.
The ash beds o f K otturu Somanpal continue southw ards and a tta in a max
mum thickness o f 800 m in the A tk apalli- Tarur Nala section [Log (a )-(b ) in Fig. 3].
The ash beds are altered fine crystal tu ffs with occasional relicts o f coa rser lapilli (Fig. 4) and have the appearan ce o f la m i
nated shales. Blue, green or red colou red laminae are traceable for tens o f m eters.
The ash beds are overlain by a sequence of m assive to n o r m a lly g r a d e d fin e grained dirty san d ston e-sh ale in te rca la tion passing upw ard to plane lam ina ted calcareous shales w ith n orm ally graded thi;k bedded pyritiferou s calcaren ite. A lateral facies varian t o f the ash beds is represented by bedded chert in ter-stra ti
fied with siliceous black sh ales, locally with thin interbeds o f lim eston e exposed around T a r u r N a la m o u t h . In th e Tarlagura section, about 150 m o f chert- black shale sequence overlies a breccia facies show ing w avy to irregu lar m assive beds and con sistin g o f silicified pebble size fragm en ts in a q u a rtz o se san dy matrix. The latter grades upw ard through a fine sandstone to siliceous shale [Log(d) in Fig. 3],
PEDDA G U T T A C H E R T F O R M A T IO N The Pedda G utta C hert Form ation 'ies between the Som nur Form ation in east and the P ro te ro zo ic E ranna Gutta Sandston e-con glom erate sequence 0r the M esozoic C hikiala Sandstone in the west and defin es a fa u lt-b ou n d ed Outcrop. E xcellen t exposures are avail- able on the n orth ern slopes o f Pedda Gutta (374 m peak) and the southern
^ank of the G odavari R iver near the con
flu en ce w ith S arvaipet N ala. T he seq u ence is gra d a tion a l from ca lca reou s shale to bedd ed ch ert th rou gh v a ry in g p ro p o r
tion o f in te rstra tifie d sh a le and chert [Log (e) in F ig .3]. The th ick n ess o f the form a tion m easu red in the ou tcrop is about 250 m. S tra tigra ph ic repetition on v a ryin g scales has been ascribed to in
ternal folds and thrusts (Saha, 1990). The form ation has a strik in g sim ila rity in lith olog ic ch aracters w ith a pa rt o f the T aru r N ala F orm ation exposed near the m outh o f the T aru r N ala and around T arlagura.
K O P E L A S H A L E F O R M A T IO N The K opela Shale F orm ation consists largely o f gray-green shale w ith o cca sional in terstra tified flat len ses o f cross bedded san dston e in the low er part, and is capped by interbedded sandstone shale u nit disp la yin g w avy and flaser bedding, asym m etric ripples and shrinkage cracks [Log (a) in Fig. 5]. Ferruginous con cre
tions are com m on in the shales in the nala section s w est o f K opela. The ou t
c r o p p a t t e r n is c o n t r o lle d by E -W - tren din g folds, w hich are associated w ith pencil cleavages.
P O G U T T A F O R M A T IO N
T he Po G u tta F orm ation con sists m ainly o f sandstone-shale intercalation gradin g to qu artzite in the upper part. A thin basal conglom erate is o f local deve
lopm ent and contains pebbles o f shale, sandstone, vein quartz, and feldspar set in a m icaceous sandy m atrix [Log (b) in
/ f / iOQ
a
KOPELA SHALE FORMATION/ laser bed ripple
b PO GUTTA FORMATION
p c b b I coarse taod fine i« id 1
mm6
| I
3 0 0
200
100
plane laminae
load cast
lenticular bed, low angle cross bed
B o d a l a V a g u F m
Fig. 5. G eneralised lithologs o f the K opela Shale Form ation (a) and the Po Gutta Formation (b).
Fig. 5], The com ponents o f th e in terca la tion are gritty san dstone and m icaceous siltstones w hich com m only display w avy bedding and a sym m etric ripples. The quartzites to w hich th ey are gradational are m assive to trough cross bedded. The basal conglom erate facies overlies the basem ent gneisses and contain gneissic fr a g m e n ts . E x c e lle n t e x p o s u r e s are
a v a ila b le arou n d th e v illa g e K orla c
th e ban k o f the In d ra v a ti R iver. T1 co n g lo -m e ra te - sa n d sto n e se q u e n ce ; E ran n a G utta, ex p osed discon tin u ou s alon g th e w estern m a rg in o f th e Pedc G utta C h ert F o rm a tio n , re p re se n ts lith o fa cie s a ss o cia tio n sim ila r to th o f Po G utta, th o u g h b e in g p o o re r in tl sh ale con ten t.
D ISCU SSIO N Form ation B o u n d a ries The m ixed s ilic ic la s t ic - c a r b o n a t e strata o f the Bodela Vagu F orm ation and the Somnur Form ation rep resen t a facies association ch a ra cte ris tic o f p e r itid a l environment (Saha and Ghosh, 1988). The rocks of the Tarur N ala F orm ation occu r in a fault-bounded trou gh and sep arate the outcrops o f the B odela V agu F o rm a tion from the Som nur F orm ation (Fig. 1).
The Pedda Gutta C hert F orm a tion lyin g west o f the S o m n u r F o r m a tio n a lso occurs in a fault- bounded trough. The bounding faults appear to have originated as extensional stage norm al fau lts and reactivated during the con traction o f the belt (Saha, 1992a,b). P rogradation o f the siliciclast-dominant Som nur F orm ation onto the c a r b o n a te -d o m in a n t B o d e la Vagu Form ation is dem on strated in the upfaulted blocks o f siliceou s dolom ite facies of the B odela Vagu F orm ation, which defines inliers w ithin the Som nur Formation at G angaram and also w ithin the Tarur N ala F orm ation outcrop at the In d ra v a ti River bed betw een Som anpalli and Regudani Gutta. It seem s th at the Somnur F orm ation and the Bodela Vagu Formation w ere deposited as tw o adjoin-
facies belts, although a progradation
°f the Som nur F orm ation over to the bodela Vagu Form ation is dem onstrated by the Gangaram and the Indravati River Actions.
The basal part o f Tarur N ala Forma- tlQn in the K otturu Som anpalli section
con ta in s pebb les w hich appear to have been derived from the B odela V agu F or
m a tion . T h e B odela V agu F orm a tion , th erefore, w as deposited e a rlier than the T a ru r N ala Form ation. In the T arlagu ra section ,th e base o f the T arur N ala F or
m ation is ju xtaposed against the Som nur F orm a tion alon g a fault and con tain s m a trix supported debris flow con g lom er
ate. T he association suggests con trol o f dep osition by syn sedim en tary faulting.
T h e T a ru r N a la F o rm a tio n is, th u s, y ou n ger than both the Som n u r F orm a tion and the Bodela Vagu Form ation.
T he Pedda Gutta C hert F orm ation show s a lith ofacies association com p a ra ble to a part o f the T arur N ala F orm ation and occurs w est o f outcrop o f the Som nur F orm ation across a fault contact. A fter the deposition o f the Bodela Vagu F or
m a tio n and th e S o m n u r F o r m a tio n , extension and subsidence o f the basin floor led to the formation o f fault- bounded troughs, w hich acted as repositories for the Tarur N ala Form ation and the Pedda G utta C hert Form ation.
The Kopela Shale Form ation conform ably overlies the Tarur Nala Form ation and onlaps the Bodela Vagu Form ation.
The Po Gutta Form ation abuts against the Precam brian basem ent gneisses and onlaps the Bodela Vagu Form ation and the Kopela Shale Form ation. A sum m ary o f the stratigraphic relations described above is given in Table 2.
R eg io n a l S tr a tig r a p h ic F ra m e w o rk A two fold cla ssifica tio n o f the P urana (P roterozic) se q u e n ce o f th e P ra n h ita - G odavari V a lley in to a lo w e r P a k h a l S e ries and an u n c o n fo r m a b ly o v e r ly in g Sullavai S eries w as p ro p o se d b y K ing (1 8 8 1 ). F u r th e r c la s s if ic a t io n o f th e Pakhal Series led to th e reco g n itio n o f a low er P akhal su b d iv isio n con sistin g o f rock strata w h ich is co n fin e d to the w e st
ern part o f the basin and an u pper A lbaka subdivision , w h ich o ccu p y th e eastern part. T he P akhal su b d iv ision w as later redefined as th e P a k h a l G rou p and w as further classified into low er M ulug S u b
group and upper M a lla m p a lli S ubgroup, on the b a sis o f th e o c c u r r e n c e o f a regional unconform ity sep arating the two su b g rou p s (B a su m a llick , 1 9 67; C h au - dhuri, 1985). A sequence o f lim estone and shale occu rrin g n orth o f th e G odavari River in the w estern b elt and "traceble to the Pakhals o f the type area" w ere d esig nated as the Penganga Beds (A. M . H eron, quoted in P ascoe, 1973) and la ter re
n a m e d as t h e P e n g a n g a G r o u p (Chaudhuri e£ al. , 1989). The Pakhal rocks around Yellandlapad in the southern part o f the w estern belt is differen t from the rest o f the belt in b ein g deform ed and m etam orphosed, and h a v e been assigned to a separate unit o f group status— the Y ella n d la p a d G roup (C h a u -d h u ri and
C handra, 1990).
The sequence consisting o f the Cherla Form ation, Som andevra Q uartzite and
T ippapu ram Shale, and lyin g below the A lbaka Sandstone in the A lbaka Range, was correlated with the P ak h al Group.
A ccordin gly, the Albaka Sandstone came to be regarded as an equ iva len t of the Su llavai G roup (Srinivasa R ao et al., 1979). Chaudhuri and C handra (1990), h ow ever, questioned the above correla
tion and proposed that all th e rock for
m ation o f the A lbaka Ranges be grouped togeth er to define a new stratigraphic unit. The name A lbaka G roup has been assigned to this unit.
Intrabasin al C orrelaion o f the Som an p alli G roup
The Tippapuram Shale occurring be
low the A lbaka Sandstone in the Albaka R anges is lith ologically sim ila r to the K opela Shale Form ation. A n outlier of the latter is the D epali shales which is laterally traceable to the type outcrops of th e Tippapuram Sh ale in th e western footh ills o f the A lbaka R anges. There
fore, the Som anpalli G roup sa n s the Po Gutta Form ation lies stratigraphieall}' below the A lbaka San dston e. The Po G utta S a n d sto n e is c o n s id e r e d to be equivalent to the A lbaka S an d ston e. W e prop ose th a t th e term A lb a k a Group (C haudhuri and C handra, 1990) should be dropped, as the A lbak a S a n d ston e is a form ally establish ed stra tig ra p h ic unit of form ation rank.
In view o f the fa ct th at th e Sullavai Group overlies the A lbak a S a n d ston e in
the eastern belt, the S om a n p a lli G rou p maybe considered to have the sam e p o s i
tion in the stratigraphic seq u en ce as the Pakhal Group in the w estern belt. R ock sequences o f the P ra n h ita -G od a v a ri V a l
ley older than the Sullavai G roup w it
nessed varying degrees o f con tra ction a l deformation. E xam ples are p rovided by the deformed sequence at Y ella n d la p a d in the southern sector o f the w estern belt, which are com p a ra b le to th e P a k h a l Group in lit h o lo g ic a l m a k e u p , th e Penganga G r o u p ( S a r k a r , 1 9 9 0 ), deformed rock sequences o f the A lbak a Ranges (Srinivasa Rao et al. ,1979; Saha, 1988), and the d e fo rm e d S o m a n p a lli Group of the eastern belt.
A C K N O W L E D G E M E N T S
The present w ork is an outcom e o f a research project supported by Indian Sta
tistical Institute. Som e im provem ents on earlier versions of the m anuscript w ere suggested by Prof. A. K. C h audhuri and an anonymous reviewer. P ainstaking edi
torial work by Prof. A m aljyoti Sengupta has rendered the text presentable.
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