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Tuff beds in Kurnool subbasin, southern India and
implications for felsic volcanism in Proterozoic
Dilip Saha*, Vikash Tripathy1
Geological Studies Unit, Indian Statistical Institute, Kolkata 700108, India
Received 25 October 2011; received in revised form 17 January 2012; accepted 19 January 2012
Available online 10 February 2012
southern India is presented. The rhyolitic to rhodacitic tuffs, overlying shelfal limestones formed at
depths below storm wave base, have rheomorphic features indicative of viscoplastic flow, and geochem-
ical signatures of rhyolitic to rhyodacitic unwelded to welded tuffs, similar to those described from other
Proterozoic intracratonic basins like Vindhyan and Chhattisgarh basins in India. Fragmentary nature of
altered glass with perlitic cracks and local admixture with intrabasinal sediments suggest phreatomag-
matic reactions. The widespread and repeated occurrences of felsic tuffs in these basins, possibly derived
from low degree melting of continental crust, suggest intermittent tectonothermal instability which likely
influenced basinal topography and cyclic development of the carbonate platforms.
ª 2011, China University of Geosciences (Beijing) and Peking University. Production and hosting by
Elsevier B.V. All rights reserved.
A first report on tuff beds from the Owk Shale in the Proterozoic Kurnool sub-basin in
aresituated away from known plate boundaries, andtheir originand
subsidence has been modeled as linked with thermal anomalies
beneath thick continental lithosphere (e.g. Kaminski and Jaupart,
2000). These basins show near symmetric disposition of basin in-
fill with maximum thickness of preserved sedimentary sequences
in central part of the basin, possibly caused by basin centered
subsidence. Various transgression-regression cycles have also been
studied from facies analysis in such basins (e.g., Shao et al., 2011).
Proterozoic intracratonic basins differ in many respects from their
Phanerozoic counterparts (Friedman et al., 1992; Eriksson et al.,
1998), and shallow epeiric seas constitute a major depositional
setting for the Proterozoic intracratonic basins including the Prote-
rozoic (Purana) basins in India (Chakraborti, 2006). Traditionally,
* Corresponding author.
E-mail address: firstname.lastname@example.org (D. Saha).
1Present address: A-1/79, Sector-18, Rohini, New Delhi 110089, India
1674-9871 ª 2011, China University of Geosciences (Beijing) and Peking
University. Production and hosting by Elsevier B.V. All rights reserved.
Peer-review under responsibility of China University of Geosciences
Production and hosting by Elsevier
available at www.sciencedirect.com
China University of Geosciences (Beijing)
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GEOSCIENCE FRONTIERS 3(4) (2012) 429e444
Author's personal copy
a sag basin model or initiation through passive continental rifts
followed by subsidence has been invoked in understanding the
tectonic evolution of these basins. However, there is a growing
tendency to interpret the development of these plate-interior basins
as linked to plate margin processes as well (e.g. Shaw et al., 1991),
and global tectonic events of supercontinent break-up and assembly
(e.g. Dharma Rao et al., 2012). For example, the Late
of the Rodinia supercontinent, and of far field effects of horizontal
stresses related to subsequent compressional regimes reactivating
central Australia led to segmentation of the initial much larger sag
basin into a number of sub-basins with asymmetric distribution of
the sediment thickness patterns. Thus evolution of the even the
interior basins can be complex, and study of the intercalated
important clues on the tectonic evolution of the Proterozoic intra-
cratonic basins with a protracted history.
A number of publications in recent years highlight occurrence
of felsic tuffs from the Proterozoic basins in India, and their
tectonic implications. While tuff beds from the Sindreth Group in
western Rajasthan are interpreted to be associated with Cry-
ogenian (666e644 Ma) subduction related volcanic arcs (Dharma
Rao et al., 2012), those from ca. 1400 Ma old Singhora tuff in the
Chhattisgarh Basin are also considered to be of volcanic arc
setting (Das et al., 2009). The 990e1020 Ma old Sukhda Tuff
from the upper part of the Chhattisgarh Supergroup has rhyolitic
affinity (Patranabis-Deb et al., 2007), but there is no detailed
information on the tectonic setting of the magmatic activity except
a broad hint that the felsic eruption toward closure of the basin
may be related to Rodinia amalgamation (Patranabis-Deb and
The peninsular India hosts a number of large Proterozoic
basins with cumulative sediment thickness up to 8e10 km as in
the Cuddapah basin occurring in the Eastern Dharwar craton
(Nagaraja Rao et al., 1987; Chaudhuri et al., 2002; Ramakrishnan
and Vaidyanadhan, 2008). Unconformity bound sedimentary
successions in these intracratonic basins have recently been
interpreted to have links with sea-level changes (Patranabis-Deb
et al., 2012), possibly influenced by more than one cycle super-
continent break-up and assembly through the Paleoproterozoic to
the Eocambrian (Saha and Tripathy, 2012). Felsic volcanics/vol-
caniclastics from the lower part of the Vindhyan Supergroup in the
Bundelkhand craton, from the Chhattisgarh Supergroup in the
Bastar craton, from the Somanpalli Group in the Pranhita-
Godavari basin are well documented (e.g. Saha and Ghosh,
1987, 1998; Patranabis-Deb et al., 2007; Mishra and Sen, 2008).
Although mafic flows, sills and dykes are common in the western
part of the Cuddapah basin (Nagaraja Rao et al., 1987; Anand
et al., 2003), felsic tuffs are rare in the lower Cuddapah succes-
sions of the Papaghni Group and the Chitravati Group, except that
the middle to upper part of the Tadpatri Formation (Chitravati
Ramakrishnan and Vaidyanadhan, 2008). The kimberlites and
lamproites in and around the Cuddapah basin were emplaced over
a protracted period of time (1400e1100 Ma, Chalapathi Rao,
2007), and the deposition of the Kurnool Group took place after
the emplacement of kimberlites in the Wajrakarur and associated
fields, with age cluster around 1100 Ma. While a single hot spot
model is unlikely for the protracted emplacement of these
kimberlites and lamproites, multiple events of lithospheric
stretching and consequent partial melting beneath the Eastern
Dharwar craton is likely to have important bearing on the devel-
opment of volcano-sedimentary successions within the polyhistory
Cuddapah basin (Kingston et al., 1983; Saha and Tripathy, 2012).
We report here for the first time the occurrence of felsic tuffs from
the Kurnool Group which unconformably overlies the ca. 1900 Ma
old Chitravati Group in the western part of the Cuddapah basin.
Documentation of the nature of these felsic volcanics/volcani-
clastics, their geological association and influence on the ongoing
sedimentation is important in constraining how intracratonic
sedimentary basins respond to passive rifting triggered by litho-
spheric stretching, mantle melting and possible continental
underplating by mafic magmas. The NeoarcheaneEarly Paleo-
proterozoic amalgamation of the cratonic blocks of India was
followed by widespread development of the Proterozoic basins in
all the major blocks (Meert et al., 2010). These basins have
a prolonged history of sedimentation, extending over at least
1000 Ma as in the case of the Cuddapah basin which shows three
major unconformity bound sequences and maximum flooding
events (Saha and Mazumder, 2012). The repeated opening and
inversion of the basins (or sub-basins) as indicated by the basin-
wide angular unconformities within an overall intracratonic
framework needs cyclic tectonothermal instability. In this paper
we highlight the possible connection between felsic volcanism,
pulsed development of carbonate platforms, and the overall
tectonic framework of the large intracratonic Proterozoic basins in
India, particularly with reference to the Cuddapah basin.
2. Geologic background
The polyhistory Cuddapah basin (area w45,000 km2) occurring in
the eastern part of the Eastern Dharwar craton, southern India,
represents the second largest Proterozoic basin in India as far the
outcrop extent is concerned (Ramakrishnan and Vaidyanadhan,
2008), and recent studies link the origin of this basin to the
separation between Napier Complex and Dharwar Craton
(Mohanty, 2011). Three major unconformity bound sequences
constitute the Cuddapah Supergroup which is unconformably
overlain by the Kurnool Group (Meijerink et al., 1984; Nagaraja
Rao et al., 1987; Chaudhuri et al., 2002, Table 1). While the
Pulivendla mafic sills are dated at >1817 Ma (Rb-Sr age, Bhaskar
Rao et al., 1995) and more recently the Tadpatri mafic/ultramafic
sills are dated at 1900 Ma (Ar-Ar ages, Anand et al., 2003; French
et al., 2008), the Kurnool Group is traditionally regarded as
Neoproterozoic (Ramam and Murty, 1997; Ramakrishnan and
assignment for the Kurnool Group is based on (i) the major basin
wide angular unconformity between the Cuddapah Supergroup
and the Kurnool Group; (ii) the diamondiferous Banganapalle
conglomerate at the base of the Kurnool Group; w1090 Ma old
Kimberlite pipes (Chalapathi Rao et al., 1999) at the western
margin of the basin being the supposed source of the diamonds in
the basal conglomerate of the Kurnool Group; (iii) a phase of
older regional contractional deformation in the Nallamalai fold
belt, which does not affect the unconformably overlying Kurnool
Group (Saha and Chakraborty, 2003; Chakraborti and Saha, 2006).
The Proterozoic Kurnool Group with a cumulative thickness of
?500 m has two cycles of sediments which grade from coarse
siliciclasts through carbonates to shales deposited on a passive
margin (Saha et al., 2006). The Banganapalle Quartzite (Table 1)
2011). The Neoproterozoic
D. Saha, V. Tripathy / Geoscience Frontiers 3(4) (2012) 429e444430
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with a basal polymict conglomerate points to development of an
alluvial fan on a basement of the Peninsular gneiss and tilted
(Chaudhuri et al., 2002; Patranabis-Deb et al., 2012). Following
a major break in sedimentation represented by the angular
unconformity at the base of the Kurnool Group, a major intra-
cratonic basin with extensive carbonate platform was established
in southern India (Saha et al., 2006; cf. Chalapathi Rao et al.,
2010). Synsedimentary extension in the Kurnool subbasin is
evident from recently acquired fault slip data (Tripathy and Saha,
2008). These data from the Banganapalle Quartzite and the Pan-
iam Quartzite suggests extensional regime affecting these forma-
tions. Moreover, the increased thickness of the Nandyal Shale
north of the Gani-Kalva fault line is consistent with episodic north
and eastward deepening of the subbasin with the fault line acting
as a hinge (Tripathy, 2011).
In the western part of the Cuddapah basin, an angular uncon-
formity separates the Kurnool Group and the Cuddapah Super-
group. In sections around Tamarajupalle, about 25 km west of
Paniam, the moderately dipping strata of the Tadpatri Formation
are unconformably overlain by the flat lying Banganapalle
Formation. The latter is conformably overlain by the Narji
Limestone, Owk Shale and Paniam Quartzite formations in
ascending order (Fig. 1, Table 1). In these sections the topmost
part of the Tadpatri Formation consists of coarse to fine tuff beds
with cherty interlayers and stromatolitic dolomite/limestone.
The Owk Shale is traditionally described as consisting of plane
laminated yellow-ochre to white shale occurring in between the
Narji Limestone and the Paniam Quartzite (e.g. Nagaraja Rao
et al., 1987, Fig. 2). The present work shows that a significant
part of the Owk Shale with thickness varying between 3 and 10 m
consists of a tuffaceous zone with odd cm-dm thick altered
unwelded to welded tuffs. The detailed physical stratigraphy,
petrography and geochemical characters of the tuff beds in the
Owk Shale are described, followed by a discussion on the related
issue of influence of submarine volcanism on a Proterozoic
carbonate platform and episodic extension within a long lived
intracratonic basin (polyhistory basin).
the Cuddapah Supergroup
3. The Owk Shale in Narnuru-Loddipalli section
The upper part of the Kurnool Group including the Owk Shale and
the succeeding Paniam Quartzite has attracted considerable
attention due to reported occurrence of fossils (impressions and
trace) having Late Proterozoic/Edicaran affinity, and the impor-
tance of such paleobiota in the ProterozoiceCambrian boundary
stratigraphy (e.g. Gururaja et al., 2000; Sharma, 2011). In spite of
its limited thickness not exceeding 10e12 m at the maximum, the
Owk Shale is laterally persistent in the Kurnool district, and often
marked by clayey horizon near its contact with underlying and
overlying units. The present observations are based on outcrops
around Narnuru-Loddipalli about 60 km south of Kurnool, north
of the ENE trending Gani-Kalva fault (GKF in Fig. 1a). The upper
part of the Narji Limestone in the Narnuru-Loddipalli consists of
a black, thin bedded to parallel laminated limestone with occa-
sional pyrite framboids or cubes. The top 1e2 m consists of
intercalations of thin (1e2 cm) black limestone beds and green
parallel laminated shale. This grades upward into the brown to red
weathering thinly laminated Owk Shale (Fig. 3). The latter
includes a 2 m thick tuffaceous horizon consisting of intercalation
of ochreous shale and more compact cm thick tuff layers.
The upper 1 m of the tuffaceous strata appears as a more
compact, weather resistant horizon (Fig. 2b) with alternate coarse
and fine tuff, with outcrop features and textural characters in some
of these cm-dm thick strata similar to welded tuff (Fig. 4). The
horizon can be traced laterally for about 50 m and is marked by
ferruginous enrichment, particularly in the coarser tuff layers.
Occasional dm size non-cylindrical disharmonic folds affecting
the lamination in the compact tuff are observed in the outcrops
near Loddipalli; broken compact layers with small misorientation
between adjoining fragments as in a jig-saw are also seen (Fig. 5 ).
These features, together with imbricate centimeter scale fiammes
and stretched layers are comparable to flow structures in high
grade ignimbrites, described as rheomorphic features (e.g.
Branney and Kokelaar, 1992, 1994). The variable orientation of
folds across the section is comparable to those related to late
viscoplastic flow in high grade ignimbrites (Andrews et al., 2003).
The tuffaceous horizon grades upward into a rippled to wavy
bedded white coarse to medium grained quartzite belonging to the
base of the Paniam Quartzite.
The description below is based on optical microscopy of thin
sections from the top of relatively compact layer in the Narnuru-
Loddipalli section (Fig. 1), supplemented by limited EPMA anal-
yses in the GSI CHQ Laboratories, Kolkata. Alternate fine and
coarser tuff layers are clearly distinguishable by the relative
abundance of larger crystal fragments (median size ? 500 mm) in
the latter. The crystal fragments include quartz (including those
resembling pyramidal form), biotite, muscovite, and rare opaque
(mainly iron oxides), set in a devitrified groundmass (Fig. 6).
Secondary limonite, and veins containing siderite rhombs are not
uncommon in the coarser layers. Subangular to subrounded quartz
grains often show embayments. Some coarse quartz grains are
undulose and rarely polycrystalline. Some elongate biotite grains
are slightly crumpled. More compact layers show lensoid to flame
like altered glass shards with frayed or forked margin as in
resembling fiammes (aspect ratio varying between 1:3 and 1:15,
Fig. 5). Usually a very fine chloritic to micaceous alteration is
dapah basin (modified after Saha and Tripathy, 2012).
Abridged stratigraphy of the western part of the Cud-
D. Saha, V. Tripathy / Geoscience Frontiers 3(4) (2012) 429e444 431
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common. Discontinuous pressure solution seams, locally with
sutured appearance, are common in the coarser layers. In some
samples (e.g. location 7J-3-07, Fig. 1), the altered glass shards
partly retainvesicular texture as expected in a pumice (Fig. 6). The
fiammes are set in a devitrified matrix and are strongly aligned
parallel to overall layering/bedding parallel foliation. Locally an
swerving around partly resorbed larger subangular to subrounded
quartz phenocrysts (Fig. 6c).
Tuffaceous samples (location 7J-3-07, Fig. 1) from around
Ujjalavada contain gray colored finer grained laminations alter-
nating with relatively coarser mm scale laminations. Glassy
micro-beads (?570 mm) with onion skin cracks (cf. perlitic crack)
are common in the relatively coarser layer (Fig. 5). Subangular to
subrounded pumiceous fragments also occur (Fig. 6e). Secondary
electron images show remnant vesicles in them (Fig. 7a). Locally,
platy glass shards show concave outward margin (Fig. 7b). These
fragments show some degree of devitrification and siliceous
replacement as apparent from patches of microcrystalline quartz
along perlitic cracks or as vesicle fills.
In some samples the finer layers have floating subangular to
subrounded quartz silt with weak alignment of white mica grains
with aspect ratios ?1:10. Clayey alterations are common and so
are spots (cavities) filled with quartz fibers in the groundmass.
Occasional mm size lithic fragments with biotite and altered
feldspar fragments set in a devitrified groundmass are also seen in
5. Geochemistry of tuff beds in the Owk Shale
Relatively fresh and compact samples of tuff beds from the upper
part of the Owk Shale were collected avoiding as far as possible
weathered soil zones and pockets of clay/ochre which are not
uncommon in the Kurnool Group outcrops. As evident from the
petrography of the tuff described earlier, the glass fragments were
altered due to rock-water interactions during the eruption and
subsequent deposition and diagenesis. The analyzed samples were
collected from near surface excavations, and possibly were sub-
local ferruginous (limonitic) stains and thin encrustations. Both
rock chips at Activation Laboratories Limited, Ontario, Canada,
and the results are given in Table 2. Fused samples are diluted and
(package 4Litho, details in www.actlabs.com). FeO was estimated
shown boxed. GKF Z Gani-Kalva Fault; KF Z Kona Fault; AF Z Atmakur Fault. b: Geological map of the study area. Red asterisk for sample
a: Simplified geological map of the Cuddaph basin, southern India. Study area in the northwestern part of the Kurnool subbasin is
D. Saha, V. Tripathy / Geoscience Frontiers 3(4) (2012) 429e444432
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separately by conventional titration (package 4F-FeO). Detection
limits are of the order of 0.01% for major oxides, except Mn and Ti
where it is of the order of 0.001%. For REE the detection limit is
0.04e0.1 ppm, for other trace elements it varies between 0.5 and
5 ppm, except for Cr (20 ppm), Ni (20 ppm), Zn (30 ppm).
Whilethemajor elements(e.g.Si,Al,Fe, Mg, Ca,Na,K, etc.)in
igneous rocks are usually mobile in the widespread zones of low-
earth elements (REE) appear to be immobile (e.g. Campbell et al.,
1983). In the following discussion on the significance of the
geochemistry of the analyzed samples, emphasis is put on the trace
and REE abundances and their comparison with reference materials
in the Owk Shale are likely to contain non-volcanic clastic grains of
quartz. One may argue dilution of geochemical signatures due to
sediment admixture. In a subsequent section, we enumerate the
rationale behind our use of the trace and REE geochemistry of the
analyzed samples as representative of the parent magma.
5.1. Major elements
Major element chemistry of the tuff samples from the Owk Shale
are shown in Table 2. SiO2 content in the analyzed samples
(locations in Fig. 1) of tuff beds from the Owk Shale varies
between 67 and 73 weight percent indicating that the bulk of the
tuffaceous material was derived from rhyolitic to rhyodacitic
magmas. The TiO2varies between 0.3% and 0.4%. The Al2O3
content is also noticeably low (6%e8%), while the total iron
(FeO þ Fe2O3) varies between 12% and 14%. A small (2%e3%)
amount of FeO is possibly due to odd biotite, diagenetic chlorite,
ferroan dolomite/calcite and siderite. The unusually low abun-
dance of Na2O, K2O and CaO in the analyzed samples shows that
these were subjected to leaching of easily soluble alkalis.
Secondary ferruginous enrichment is suggested by relatively high
5.2. Trace and rare earth elements
In the chondrite normalized multi-element spider diagrams
(Pearce et al., 1984), the analyzed tuff samples from the Owk
Shale show marked enrichment of Th and U among the HFSE
(Fig. 8). There is also enrichment of Ba (LFSE) and marked
negative anomaly for P. The strong enrichment (w100 times
corresponding value in primordial mantle) of incompatible
elements Cs, Rb, Ba and Th compared to primordial mantle, is
suggestive of continental crust source for the parent melt
(Hoffman, 1988). Note also relative higher relative abundance of
Nb and Ta. It may be noted that compared to North American
Shale Composite (NASC) values (Gromet et al., 1984), the Ba
value is much higher in the tuff samples, and there is marked low
values of Ti; Sr is also lower than NASC value.
In the chondrite normalized rare earth element plot, LREE
shows significant enrichment compared to HREE (SLREE/
(SHREE varies between 14.8 and 22.5). There are also small Eu
and Ce anomalies. However, LREE values are consistently lower
than the chondrite normalized Post-Archean Australian average
shale (PAAS) values or North American shale composite (NASC)
values (Gromet et al., 1984; Taylor and McLennan, 1985;
McLennan, 1989), indicating that trace element geochemistry of
the Owk Shale tuff samples is distinct from average shales
(Fig. 8a). Chondrite normalized REE pattern for the analyzed
samples show more marked enrichment of LREE, compared to
HREE (Fig. 8c). In all the samples, normalized slope (La/Gd)nis
steeper than that of (Sm/Lu)n. Comparison of REE chemistry in
the samples with that of some other known Proterozoic felsic tuff
samples e.g. Chopan Porcellanite in the Semri Group of the
Vindhyan Basin (Mishra and Sen, 2008), or the Singhora tuff at
the base of the Chhattisgarh basin (Das et al., 2009) clearly show
close similarity of REE pattern of the analyzed samples with felsic
tuff in the other large Proterozoic intracratonic basins in India.
Compared to shales of the Rabanpalli Formation in the Bhima
basin, which are derived from felsic provenance of the Dahrwar
craton (Nagarajan et al., 2007), the LREE values are distinctly
lower (Fig. 8c).
In the trace element based discriminant plots for felsic rocks
(Pearce et al., 1984), the studied samples again show affinity with
rhyolitic to rhyodacitic magmas. In the SiO2vs. (Zr/TiO2)?10?4
diagram the Owk Shale tuff samples plot in the rhyolite field;
these samples plot in the rhyodacite-dacite field in the Nb/Y vs.
Zr/Ti diagram (Fig. 9). Tectonic discrimination based on ratio of
trace element pairs (Pearce et al., 1984), show that all the studied
samples plot in the Volcanic Arc Granite (VAG) field (Fig. 10).
This is in contrast to the general expectations of broadly intra-
cratonic (intra-continental) setting of these Indian Proterozoic
basins interpreted by various workers (e.g. Chaudhuri et al., 2002;
stone and the Paniam Quartz, south of Belam, Kurnool district. b:
Close-up view showing the transition from pale green argillaceous
limestone (Narji) to plane, parallel laminated tuff beds of the Owk
Shale, south of Loddipalli.
a: The Owk Shale sandwiched between the Narji Lime-
D. Saha, V. Tripathy / Geoscience Frontiers 3(4) (2012) 429e444 433
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Chakraborti, 2006; Chakraborty et al., 2010). However, Pearce
et al. (1984) considered that dilution effect of plagioclase
crystal accumulation may shift the WPG granites to VAG gran-
ites. Based on studies of Bushveld granites and their associated
felsite lavas, Twist and Harmer (1987) concluded that Nb-Y and
Rb-(Y þ Nb) plots can be utilized for uncontaminated felsitic
lavas, and that the difference in trace element chemistry could be
influenced as much by variation in source chemistry as differences
in tectonic setting.
6.1. Pyroclastic rocks and welded versus unwelded tuff
As described above the upper part of the Owk Shale host a distinct
1e2 m thick horizon, which has recognizable characters of
pyroclastic rocks, with rhyolitic to rhyodacitic composition.
Discriminating between welded and unwelded tuff is important in
understanding the depositional environment and eruptive style (e.g.
Ross and Smith, 1961; Fisher and Schmincke, 1984; Gifkins et al.,
2005). Generation of welding textures requires high temperature
during or shortly after deposition, and are thought to be more
common with subaerial deposits. However, welding in pyroclastic
flow deposits is possible under shallow submarine condition also
(Smith, 1960; Fisher and Schmincke, 1984) even though only a few
convincing examples are known (e.g. Wright and Coward, 1977;
Yamazaki et al., 1973). Fisher and Schmincke (1984) consider
fiammes to be rare in submarine welded tuffs. Further, uncertainty
arises fromthefactthatapparentweldingtexturesmayresult dueto
various processes e for example, diagenetic alteration and
compaction of pumice clasts (e.g. Branney and Sparks, 1990);
Smith, 1961); diagentic alterationandcompaction ofvesicularflow
bands in lava; ‘viscous shear’ accompanying vesiculation and
autobrecciation (Gifkins et al., 2005).
tuff beds. Red asterisk for sample position.
Simplifed litholog (left panel) showing the stratigraphic set up of the Kurnool Group, and the detailed log from the Owk shale with
D. Saha, V. Tripathy / Geoscience Frontiers 3(4) (2012) 429e444434
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As described earlier, lenticular to flame like geometry of
altered glass similar to fiammes (Figs. 4e6) are not uncommon in
the tuff beds of the Owk Shale south of Narnuru in the Kurnool
subbasin. As apparent from secondary electron images (Fig. 7a)
some of the shards retain vesicularity, increasing the probability
that the pyroclastics in the Owk Shale contained pumiceous
material. However, there are other types of altered shards which
apparently lack any vesicularity (Fig. 7b), suggesting variation in
eruptive style and/or composition of the magma pulses and
volatile contents. Unlike recent subaerial ash fall deposits and
their Phanerozoic counterparts, the preserved thickness of the
tuffaceous horizon in the Owk Shale is small, and the layering is
only on a cm-dm scale. In the Ordovician Capel Curig Formation
of Snowdon in Wales submarine welded tuff is much thinner than
the correlative subaerial unit (Howells et al., 1979). As we discuss
further below, the Owk Shale is marine, so is the overlying Paniam
quartzite (Saha et al., 2006; Patranabis-Deb et al., 2012). Thus, in
our understanding, the tuff beds in the Owk Shale most likely
represent submarine tuff retaining some welding features.
6.2. Admixture with non-volcanic clastic material
A few thin sections from the Owk Shale samples from the
Narnuru-Loddipalli section show angular to subrounded poly-
crystalline quartz, undulose quartz with subgrains or cherty frag-
ments embedded in a fine clayey/sericitic to ferruginous matrix.
The angularity and irregular shapes favor fragmentation due to
explosive eruption. Shock induced, high stress episodic defor-
mation are known to produce highly localized brittle and plastic
deformation in quartz leading to undulosity and localized subgrain
formation (e.g. Trepmann et al., 2007). While possible source of
odd polycrystalline quartz could be pre-existing metamorphic or
plutonic rocks caught up in the erupting magma as it traveled
through the basement conduit, it is difficult to exclude the possi-
bility that some may have come from the disrupted sedimentary
stratum near the eruptive vent/fissure. The cherty fragments may
be products of silicification of lithic clasts (carbonate or tuffaceous
protoliths). Slender sericitic mica (dimensions of 200:10 mm)
within the fine grained laminae also suggests some admixture of
detrital silt. As the the Owk Shale overlie the Narji Limestone, the
apparent absence of any admixed carbonate clast likely generated
during supposed explosive eruption associated with the tuff beds,
require an explanation. The absence of recognizable carbonate
clasts, may be due to complete replacement of such clasts, if any,
by microcrystalline silica. Noticeably, the bulk composition of the
analyzed tuff samples shows very low CaO (<0.4%, Table 2). This
and other trace and REE signatures as detailed below lead us to
consider that bulk of the quartz grains in the corser tuff are
phenocrysts and/or xenocrysts, mainly derived from the parent
magma, and to some extent from the conduit that passed through
the granitic basement.
6.3. Influence of sediment admixture on trace element
Based on a large suite of Proterozoic to Triassic sediments, Nance
and Taylor (1976) observed that there is hardly any secular vari-
ation in REE abundances in these sediments and that post-Archean
sediments usually show a near constant SLREE/SHREE ratio
(9.7 ? 1.8). The tuff samples from the Owk Shale have a much
higher SLREE/SHREE ratio varying between 14.8 and 22.5
(Table 2). The higher values in tuff samples of the Owk Shale do
not exclude the rare possibility of minor dilution of the source
magmatic REE concentration by mixing with sediments in the
Kurnool subbasin. Particularly, quartz/carbonate admixture is
known to result in lowering of total REE content (e.g. Rollinson,
1993). However, known Proterozoic rhyolitic tuffs from the
Chhattisgarh basin (Singhora tuff samples of Das et al., 2009) and
lenses and flames of dark altered glass (black arrows in upper left of
photo) set in light gray matrix, suggesting fiamme geometry. Note also
light colored altered glass shard.
Outcrop photo of compact tuff bed showing mm-cm scale
coarse-fine layers, and jig-saw fragments as in a broken viscous lava
flow (upper part of photo). b: Disharmonic folds indicating visco-
plastic deformation as in ignimbrites.
Rheomorphic feature in the Owk Shale tuff. a: Alternate
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the Vindhyan basin (Chopan Porcellanite samples of Mishra and
Sen, 2008) show average SLREE/SHREE ratio of 10.8 and 14.3
respectively. Thus, geochemical characters of tuff beds in the Owk
Shale reflect to a reasonable degree that of the source rhyolitic to
rhyodacitic magma. One may note though that SHREE in the
Owk Shale tuff samples are relatively lower (Fig. 8c) compared to
Singhora tuff or Chopan Porcellanite.
We also exclude the possibility of any significant influence of
chertification on the REE pattern due to the following reason.
Nodular chert samples from Deep Sea Drilling Project (DSDP)
and Ocean Drilling Program (ODP) show Lan/Ybnvalues varying
between 0.8 and 1.0 in case of the Pacific Ocean, and intermediate
between the PacificOcean sample valuesand inherited
(terrigenous) values of 1.2e1.4 in case of the Atlantic Ocean
(Murray et al., 1992). Lan/Ybnvalues in tuff samples of the Owk
Shale vary between 9 and 13, an order of magnitude higher than
the DSDP and ODP samples.
6.4. Submarine volcanism and episodic extension
The coarse welded tuffs in compact meter thick horizons with
rheomorphic features have been identified only at a few localities
so far within the Owk Shale. The Owk Shale has a maximum
thickness of 10e12 m, and the formation is widely exposed in the
western part of intracratonic Kurnool subbasin in southern India.
The formation has been mapped over a geographic distance of
forming darker streaks within light gray glassy matrix. Note subangular to subroudned quartz phenocrysts and/or xenocrysts. Bedding parallel
foliation is enhanced by discontinuous wavy solution seams with secondary concentration of ferruginous matter. b: Lensoid geometry with frayed
end of an altered fiamme (‘fiam’ on photo) and broken quartz phenocrysts with possible pyramidal termination (upper left of photo). Note late
diagenetic vein with siderite rhomb growing into altered glassy matrix. c: Anvil shaped quartz grain with bedding parallel foliation swerving
around it. d: Pale green subrounded glass shard with onion skin fractures in a coarse layer. Note clear quartz grain in the centre with resorbed
margin and embayment. e: Subangular glass shard with vesicles (compacted pumice); note large muscovite grain and platy shard (bottom right).
f: Finer laminae in tuff with floating angular to subangular quartz silt in clayey to micaeous matrix; note two sericite flakes with large aspect ratio.
Petrography of the Owk Shale. Microphotographs, plane-polarized light. a: Lensoid to flame like altered glass fragments (fiammes)
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over 100 km from the banks of Tungabhadra/Krishna River in the
north to Penner River in the south (King, 1872). In all the exposed
sections the buff to white shales (locally clayey) are sandwiched
between the laminated to flaggy, gray to dark colored Narji
Limestone below, and the wavy bedded, well sorted Paniam
Quartzite above. In the Narnuru-Loddipalli area, the top of the
Narji Limestone is plane laminated black limestone with occa-
sional pyrite and without any shallow water features. The top of
the limestone represents deposition of an extensive carbonate
platform below storm wave base and without any siliciclastic
input (Saha et al., 2006). The transition zone through green shale
is also plane laminated (Fig. 2b) and wave features are absent.
Thus shelfal carbonates were being deposited below wave base
immediately preceding the eruptive pulses through vents in the
sediment substrate. The stratigraphic relationship is a clear indi-
cation of submarine emplacement/deposition of the Owk Shale
and associated tuff horizons.
The Nurnuru-Loddipalli tuff horizon within the Owk Shale has
common rheomorphic features and a planar fabric seen in thin
sections under optical microscope. The features are indicative of
b: BSE image of glass shard.
Textural details of glassy material in the tuff bed, Owk Shale. a: Secondary electron image showing compacted pumice fragment.
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Table 2 Major and trace element composition of the Owk Shale tuff.
Sample Nos.7J-1-07 7J-2-07 7J-2-07-ii7J-3-07
Major elements (wt%)
Transition metal (ppm)
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unwelded to welded tuff (ignimbrite). Alternate coarse tuff and
fine tuff layer on cm scale with variation in crystal fragments
suggest eruptive pulses, rather than en masse emplacement of the
welded tuff horizon. Elutriation within the collapsed eruptive
column could have also produced coarse-fine sorting into cm-scale
layers. Explosive eruption of a gas charged viscous magma and/or
phreatomagmatic fragmentation (Fisher and Schmincke, 1984;
Gifkins et al., 2002) of a siliceous lava at sediment water
sample abundances normalized with respect to corresponding abundances in primitive mantle (PriM). c: Chondrite-normalized REE abundances.
Normalizing values are after Pearce et al. (1984) for chondrite and Hoffman (1988) for primordial mantle. See text for details.
Geochemistry of the Owk Shale tuff. a: Chondrite-normalized multi-element spider diagram. b: Multi-element spider diagram,
D. Saha, V. Tripathy / Geoscience Frontiers 3(4) (2012) 429e444439
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interface close to erupting vent probably produced the glass
shards. Hot rock-water interaction within a highly turbulent and
charged mass in the erupting column led to clayey alteration of
glass and crystal fragments, bleaching of biotite phenocrysts, and
some degree of chemical/mechanical rounding of the fragments.
The widespread occurrence of the Owk Shale and the asso-
ciated tuff call for voluminous eruption and substantial height of
the eruptive column. While ash fall tuff is more likely to be
unwelded, particularly if deposited under water, and widespread
dispersed sheets of pumice and ash are derived from high erup-
tion columns that result from high eruption rate gas-rich erup-
tions as in the 79 AD, referred to as Plinian deposits (Pliny the
Youger quoted by Fisher and Schmincke, 1984). As recognized
by Fisher and Schmincke (1984), pyroclastic flows and surges are
an integral part of Plinian deposits where fallout pumice is fol-
lowed by pyroclastic surges and flows. In the western part of the
Cuddapah basin the Owk Shale is spread over 1000 km2.
Preliminary estimate of the compacted volume of the tuffaceous
horizon is of the order of 3e10 km3. The geochemistry of the
felsic tuff samples (Table 2) and the limited volume of
the welded tuff and volcaniclastics is consistent with low degree
of partial melting (1%e2%) expected under continental crust
(Hoffman, 1988). Considerable fluidization of the subjacent
carbonate horizon (gray limestone in lower to middle part of the
Narji Limestone) is attested by brecciation and isolated vent
features cutting bedding laminae in the limestone. The cumula-
tive effect of emplacement of non-welded to welded tuff and
deposition of volcaniclastics possibly modified the submarine
vs. Nb/Y plot.
Geochemical discriminatory plots for the Owk Shale tuffs, plots after Winchester and Floyd (1977). a: Zr/TiO2vs. SiO2plot; b: Zr/Ti
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topography, as the succeeding Paniam Quartzite shows shallow
water features in the form of common wavy bedding and even
locally desiccation features in the lower part (Patranabis-Deb
et al., 2012). Thus, a submarine siliceous volcanism led to the
degradation of a carbonate platform and rapid but transient
decrease in the accommodation space.
The tuff beds of the Owk Shale separate two cycles of silici-
clastic to carbonate sediments within the Kurnool Group (Saha
et al., 2006). The lower and upper carbonate platforms are rep-
resented respectively by the Narji Limestone and the Koilkuntala
Limestone (Table 1). Based on analysis of fault slip data, Tripathy
and Saha (2008) argued that syn-sedimentary extensional stresses
affected the Banganapalli Formation at the base of the Kurnool
Group, and the Paniam Quartzite which overlies the Owk Shale.
These data from the Banganapalle Quartzite and the Paniam
Quartzite suggests extensional regime affecting these formations
(Tripathy, 2011; Saha and Tripathy, 2012). It remains to
be examined further whether tectonic extension of a passive
margin during the deposition of the Kurnool Group and submarine
felsic volcanism over the Narji carbonate platform were geneti-
6.5. Wider implications for the Proterozoic basins of India
Thick carbonate to shale transitions are common in the Proterozoic
sequences of India, particularly in the Vindhyan, Chattishgarh,
Pranhita-Godavari, Cuddapah basins (e.g. Ramakrishnan and
Vaidyanadhan, 2008; Chakraborty et al., 2010; Saha and
Tripathy, 2012). Recently, Saha and Tripathy (2011) have shown
that volcanic debris are rather common in the shaley units
immediately overlying platformal carbonates in the above basins,
and such sharp transitions occur at more than one levels within
these sequences. These basins are thought to be intracratonic and
putative stable passive margin depositional signatures are common
in the preserved sedimentary sequences (Chaudhuri et al., 2002;
Saha et al., 2006; Saha and Mazumder, 2012). However, the
occurrence of repeated felsic volcanism as in the Vindhyan basin
from the Chopan and equivalent porcellanites from the Semri
Group (Mishra and Sen, 2008; Paikaray et al., 2008), from the
Singhora Group (Das et al., 2009) and Sukdah Tuff in the Chhat-
tisgarh basin (Patranabis-Deb et al., 2007), from the Pranhita-
Godavari basin (Saha and Ghosh, 1987, 1998; Patranabis-Deb,
2003), or from the Cuddapah basin, suggest intermittent tectono-
the above felsic tuffs not necessarily represent contemporaneous
eruptions, one cannot fail to notice the widespread occurrence of
felsic volcanicity in these basins. It remains to be examined further
whether such widespread felsic volcanicity can be linked to deep
level processes affecting supercontinent break-up and assembly in
which Indian Proterozoic cratonic provinces were involved.
Moreover, inview of the relative paucity of precise geochronologic
dates from these Proterozoic basinal sequences with internal
unconformities, and prolonged depositional history spanning over
1000 Ma, the reported or hitherto undiscovered felsic volcanic and
volcaniclastics at several levels within these basins may provide
new material for more precise geochronologic dating. More precise
dates may resolve some of the emerging debates on the age of
Proterozoic sedimentation in India (e.g. Basu et al., 2010; Bickford
et al., 2011; Ray et al., 2011).
Widespread occurrence of mafic flows, sills and dykes within
the lower part of the Cuddapah Supergroup (Table 1) has been
ORG Z orogenic granites; syn-COLG Z syncollisional granties; WPG Z within plate granites. a: Yb-Ta plot. b: Y-Nb plot. c: (Yb þ Ta) - Rb
plot. d: (Y þ Nb) - Rb plot. Note all the samples plot in the VAG field. See text for further details.
Tectonic discrimination plot for the Owk Shale tuff samples, after Pearce et al. (1984). VAG Z volcanic arc granites;
D. Saha, V. Tripathy / Geoscience Frontiers 3(4) (2012) 429e444 441
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interpreted in terms of up to 10%e15% of melting of subconti-
nental mantle material associated with ca. 1900 Ma old litho-
spheric stretching beneath the intracratonic Cuddapah basin
(Anand et al., 2003). Several younger episodes of mafic and
alkaline melt generation are attested by phased emplacement of
lamproites and kimberlites in the neighborhood of the Cuddapah
basin (Chalapathi Rao, 2007). The geochemistry of the Owk Shale
tuff on the other hand suggests generation of felsic melts, and the
strong enrichment of incompatible elements Cs, Rb, Ba and Th
compared to primordial mantle (Fig. 8), favoring continental crust
source. Intermittent felsic melt generation in the Proterozoic
basins in India, e.g. the Vindhyan basin with the Chopan Porcel-
lanites, the Chhattishgarh basin with several felsic tuff horizons,
and the newly reported tuff beds from the Owk Shale in the
Kurnool subbasin, are possible indicators of passive rifting in
these intracratonic basins, possibly triggered by intermittent lith-
ospheric stretching, mafic melt generation and underplating of the
Rhyolitic to rhyodacitic tuff beds in the upper part of the Owk
Shale in the Proterozoic Kurnool subbasin, southern India and
their geochemistry including REE and trace element abundances
are reported here for the first time. The rheomorphic features of
the tuff beds and textural characters of the altered vesicular glass
are interpreted to represent welded as well as unwelded felsic tuff.
Association with submarine sediments below and above the tuff
beds raise the possibility of contribution from submarine ash flow
and ash fall. However, minor admixture with clasts of non-
volcanic origin cannot be excluded altogether, and may explain
slightly lower ƩLREE compared to other known rhyolitic/rhyda-
citic tuffs from the Chhattisgarh and Vindhyan basins in India.
Chondrite-normalized REE abundances of the analyzed
samples from the above tuff beds from the Kurnool subbasin show
abundances broadly comparable with that of the Chopan Porcel-
lanite from the lower part of the Vindhyan basin (Mishra and
Sen, 2008), and the Singhora tuff from the Chhattisgarh basin
(Das et al., 2009).
There is strong enrichment of incompatible elements Cs, Rb,
Ba and Th compared to primordial mantle, and following Hoffman
(1988) a continental crust source for the parent melt is suggested.
The tuff samples from the Owk Shale have SLREE/SHREE ratios
varying between 14.8 and 22.5, much higher than Post-Archean
sediments with an average of 9.7 ? 1.8 (Nance and Taylor,
1976). We therefore, argue sediment admixture had minimal
influence on the trace and REE geochemistry of the tuff samples
from the Owk Shale.
In view of the putative Neoproterozoic age of the Kurnool
Group (e.g. Ramakrishnan and Vaidyanadhan, 2008; Gururaja
et al., 2000; Sharma, 2011), a straight forward comparison
between tuff beds in the Owk Shale and those in ca. 1400 Ma
Singhora tuff from the basal part of the Chhattisgarh basin or ca.
1600 Ma old Chopan Porcellanite in the lower part of the
Vindhyan basin is fraught with uncertainty, particularly with
reference to the details of the tectonic setting that facilitated the
felsic magma generation and emplacement. However, the broad
intracontinental setting and periodic development of extensive
carbonate platforms is known from all the three Indian Proterozoic
basins viz. Cuddapah, Chhattisgarh and Vindhyan basins. In view
of the common transitions from thick carbonate to shale, and
stable passive margin depositional signatures in the sedimentary
sequences in these basins (e.g. Chaudhuri et al., 2002;
Chakraborti, 2006; Saha and Mazumder, 2012), it would be
a worthwhile future exercise to examine in detail whether and how
the widespread felsic volcanic eruption can be linked to deep level
processes affecting supercontinent break-up and assembly in
which Indian Proterozoic cratonic provinces were involved.
This work is supported by the Indian Statistical Institute (ISI),
Kolkata in the form of several research grants to DS during the
past decade. VT acknowledges a senior research fellowship
granted by ISI during the initial stage of the work. Lively
discussions with Sarbani Patranabis-Deb, Abhijit Basu and Mar-
ion Bickford helped in refining our ideas. However, the authors
only are responsible for the views expressed here. We also
acknowledge a DST grant (SR/S4/ES-307/2007) which partly
supported this work. Critical but constructive comments from Dr.
C. V. Dharma Rao and an anonymous reviewer are thankfully
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