A synoptic view on the current discordant geo- and biochronological ages of the Vindhyan supergroup, central India

Abstract

We present a synoptic view on current discordant views on the age of the Vindhyan Supergroup (VSG), arising from the recent geo- and biochronological data sets. Against exceptionally long duration of ∼ 1200 million years (∼1800-600 Ma) from late Paleoproterozoic to late Neoproterozoic age based on geochronology, diverse paleontological evidences (metazoan traces, small shelly fossils, fossil embryos, calcareous skeletal algae, sponge spicules, acanthomorphic acritarchs, scolecodont-like structures and metaphytes) suggest a much shorter duration of Vendian - Early Cambrian (∼650-520 Ma) for deposition of the VSG. It is demonstrated that ∼1800 Ma (late Paleoproterozoic) initiation of the Vindhyan sedimentation is not in conformity with the regional geology, whereas the younger age for the VSG is consistent with the regional geology and is also supported by a basal Vindhyan glaciation of correlatable Marinoan (Vendian) age ("Snowball Earth"). We believe that the wide age disparity in the VSG stands sorted out by adhering to the regional geological perspective of the Vindhyan Basin.

Full text

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Himalayan Geology, Vol. 29 (2), 2008, pp.177-191 Printed in India
A synoptic view on the current discordant geo- and biochronological ages of the
Vindhyan Supergroup, central India
R.J. AZMI1, DEEPAK JOSHI1*, B.N. TIWARI1, M.N. JOSHI2, S.S. SRIVASTAVA3
1Wadia Institute of Himalayan Geology, Dehra Dun - 248 001, India
*Present address: Schlumberger-WesternGeco International Ltd., BSEL Technology park, Vashi-Navi Mumbai-400 705, India
2Dept. of Geology, D.B.S. (Pg) College, Dehra Dun- 248 001, India
3Dept. of Earth Sciences, IIT Roorkee, Roorkee- 247 667, India
Abstract: We present a synoptic view on current discordant views on the age of the Vindhyan Supergroup (VSG),
arising from the recent geo- and biochronological data sets. Against exceptionally long duration of ~1200 million years
(~1800-600 Ma) from late Paleoproterozoic to late Neoproterozoic age based on geochronology, diverse paleontological
evidences (metazoan traces, small shelly fossils, fossil embryos, calcareous skeletal algae, sponge spicules,
acanthomorphic acritarchs, scolecodont-like structures and metaphytes) suggest a much shorter duration of Vendian -
Early Cambrian (~650-520 Ma) for deposition of the VSG. It is demonstrated that ~1800 Ma (late Paleoproterozoic)
initiation of the Vindhyan sedimentation is not in conformity with the regional geology, whereas the younger age for the
VSG is consistent with the regional geology and is also supported by a basal Vindhyan glaciation of correlatable
Marinoan (Vendian) age ("Snowball Earth"). We believe that the wide age disparity in the VSG stands sorted out by
adhering to the regional geological perspective of the Vindhyan Basin.
INTRODUCTION
Since 1998, the age of the Vindhyan Supergroup (VSG) has
been in focus with renewed interest because apparently
irreconcilable views have been expressed on the basis of the
latest paleontological and geochronological findings. At
present, three views on the age of the VSG exist: 1, the
traditional view of Mesoproterozoic to Vendian (~1400-550
Ma) or earliest Cambrian age mainly based on Riphean
stromatolites and pre-1998 geochronology; 2, Vendian
(Marinoan glaciation) to Early Cambrian age (~650-520 Ma)
mainly based on small shelly fossils (SSF) reported from the
Rohtas Subgroup, and 3, late Paleoproterozoic to late
Neoproterozoic age (~1800-600 Ma) based on the latest
geochronology and chemostratigraphy. Views 1 and 3 are
more or less the same except that in the latter case the older
age limit of the VSG has been extended from ~1400 Ma to
~1800 Ma, with an onus to demonstrate that the exceptionally
older dates in the Lower Vindhyan are not due to the inherited
components from the older provenance(s). For the acceptance
of view 2, authenticity/reproducibility of Lower Cambrian SSF
should be unequivocal (see reports in Editor JGSI 2000;
Mahadevan 2002).
The VSG comprises about 4000 m thick unmetamorphosed
and mostly undeformed sedimentary succession distributed
in central India in a sickle-shaped outcrop around the
Bundelkhand Massif (Fig. 1), spreading from Agra in the
northwest through southeastern Rajasthan to eastward in Son
Valley up to Sasaram in Bihar, covering an area of about 1,04,000
km2 (Auden 1933; Heron 1953; Soni et al. 1987; Sastry & Moitra
1984; Prasad 1984). The VSG is divided into Lower Vindhyan
(Semri Group) and the Upper Vindhyan (Kaimur, Rewa and
Bhander Groups). It is unconformably underlain by the
lowgrade metamorphics of Bijawar Supergroup (~1800-1600
Ma) and/or Bundelkhand Granite-Gneiss Complex (BGC, ~2500
Ma). The detailed lithostratigraphy of the VSG is shown in
figure 2.
In a major upheaval in the Vindhyan chronostratigraphy
and paleobiology in October 1998, two startling fossil discoveries
were published: 1) trace fossils of ‘triploblastic animals’ by
Seilacher et al. (1998) from the Chorhat Sandstone of the Lower
Vindhyan that were assigned more than 1.1 billion year-old age
as per the traditional age (view 1), and 2) small shelly fossils
(SSF) of earliest Cambrian age (P
ЄЄ-
boundary markers, ~542
Ma) by Azmi (1998a) from the Rohtasgarh Limestone that
conformably lay little above the trace fossils-bearing Chorhat
strata in the Lower Vindhyan (Fig. 2), suggesting far younger
than the traditional age to the VSG . However, the claim by Seilacher
et al. (op. cit.) being revolutionary from the evolution point of
view was instantly celebrated as a major success to molecular
biologists longing for such empirical evidence in Earth’s early
evolutionary record. But the record of the earliest Cambrian SSF
indicating a major upward age revision of the VSG made claim of
‘deep’ metazoan origin in the Vindhyan succession a hot
debatable issue (Azmi 1998a,b; Brasier 1998; Kerr 1998a). During
prolonged discussions, not only occurrence of SSF in the Lower
Vindhyan was questioned on the count of their non-
reproducibility and/or due to their inorganic genesis (Conway
Morris et al. 1998; Brasier 1999; Bhatia in Editor JGSI 2000;
Bagla 2000), but also the Chorhat traces were suspected to be of
abiotic origin (Doser in Kerr 2002; Hofmann 2005). Such
developments made these fossil records a subject of national

178
and international scrutiny/discussions, and there was no
immediate conclusion possible - the age of the VSG, the
biogenicity and/or reproducibility of the reported fossils remained
inconclusive. Nevertheless, these fossil reports did generate
genuine interest, especially among the geochronologists, primarily
to resolve the basic question: How old are the Vindhyans and
whether the claim of deep metazoan origin in the VSG would be
sustainable? As a result, there was a spurt of studies for radiometric
dates (Fig.2) and chemostratigraphic data from the VSG (Banerjee
& Frank 1999; A. Kumar et al. 2001; B. Kumar et al. 2002;
Rasmussen et al. 2002; Ray et al. 2002, 2003; Singh et al. 2002;
Sarangi et al. 2004; S. Kumar et al. 2005). Although majority of
the latest radiometric dates pushed the Lower Vindhyan into the
late Paleoproterozoic (~1800-1600 Ma) – even older than its
traditionally accepted Mesoproterozoic age - there was a lone
date of 617 Ma (Ar-Ar, Banerjee & Frank 1999) that also indicated
a much younger age to the Vindhyans as suggested earlier by
Azmi (1998a, b). However, majority of the new radiometric dates
implied that the burrowing metazoans (Seilacher et al. 1998) and
SSF (Azmi 1998a), if present in the Lower Vindhyan, were already
in existence >1600 Ma ago (Rasmussen et al. 2002; Ray 2006)
rather than their generally accepted <600 Ma origin (during
Ediacaran Period) and at or near the P
ЄЄ-
boundary (542 Ma),
respectively. In a major leap forward Bengtson et al. (2007) while
confirming the occurrence of fossils resembling the Ediacaran-
Cambrian forms in the Lower Vindhyan wrote, “In view of the
strong geochronological evidence for a Paleoproterozoic age of
the Lower Vindhyan we need to consider the mounting
indications that Paleoproterozoic biota was more diversified than
is generally assumed.” For such an eventuality there undoubtedly
has to be a major paradigm shift in understanding the early life
evolution (see also Sankaran 2008). Further, Seilacher (2007, p.176)
in spite of his strong belief in his identification of the burrows
too succumbed to the Paleoproterozoic age and stated, “In
conclusion, Chorhat structures would pass as worm burrows in
later rocks. Only because of radiometric ages a non-biological
origin must be considered”.
In the light of the above startling developments it is amply
Fig. 1. Geological map of the Vindhyan Basin (compiled from Krishnan & Swaminath 1959 and Soni et al., 1987) showing locations of the fossil
yielding sections in the eastern Vindhyan Basin after Azmi et al. (2007).

179
Fig. 2. Conflicting radiometric dates vs evolutionary consistency in the Vindhyan fossil records, latter indicating Vendian - Early Cambrian age for the Vindhyan Supergroup
(updated from Azmi et al. 2007). Note Gangau Tillite at the base indicating basal Vindhyan glaciation of possible Marinoan age. Post-1998 radiometric ages are shown
in blue.

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Fig. 3. Corrected version of figure 2 of Ray et al. (2003). Note
intrusion level of the Majhgawan Kimberlite Pipe (MKP) only
up to the top of the Lower Vindhyan as shown by Ray et al.
(2003). As corrected here, the actual intrusion level is up to
the top of the Kaimur Group, Upper Vindhyan. Stars with E,
T, and S represent reported levels of Ediacaran fossil, trace
fossils, and small shelly fossils respectively; arrows with LS
point positions of major limestone formations. Purported
'reliable' dates of the authors (Ray et al. 2003) are also shown
in toto. Also note that the previously considered 910 ± 39 Ma
date as a 'reliable' one in Kaimur Sandstone has been given up
in Ray (2006).
clear that the core issue that remains to be solved is the age of
the Vindhyans. Here we evaluate below the latest papers on
Vindhyan geochronology and demonstrate that the proposed
late Paleoproterozoic age for the Lower Vindhyan is grossly
inconsistent both with the paleontological content and the
regional geology of the Vindhyan Basin (see also Azmi 2008).
LATEST GEOCHORONOLOGY AND
CHEMOSTRATIGRAPHY
Recent geochronology and chemostratigraphy provide
substantial isotopic data from the silisiclastic-carbonate
sequences, covering major part of the Vindhyan succession
(Banerjee & Frank 1999; A. Kumar et al. 2001; B. Kumar et al.
2002; Rasmussen et al. 2002; Ray et al. 2002, 2003; Singh et al.
2002; Sarangi et al. 2004; S. Kumar et al., 2005; Ray 2006). It is
intriguing that the Porcellanite Formation has been variously
dated as 617 ± 3.5 Ma (Ar-Ar; Banerjee and Frank, 1999), 1628
± 8 Ma [SHRIMP (sensitive, high-resolution microprobe) U-
Pb zircon, Rassmussen et al. 2002] and 1630.7 ± 0.4 Ma [TIMS
(thermal ionization mass-spectrometer) U-Pb zircon, Ray et al.
2002]. There exists about 1 billion year difference in these data
sets of the same formation, having a thickness of ~300 m.
Further, Rasmussen et al. (2002) obtained a date of 1599 ± 8
Ma (SHRIMP U-Pb zircon) from the Rampur Shale, which
conformably underlies the Rohtas Subgroup from which Azmi
(1998) reported SSFs. Ray et al. (2003) and Sarangi et al. (2004)
also obtained Pb-Pb dates of 1601 ± 130 Ma and 1599 ± 48 Ma
respectively, from the Rohtasgarh Limestone. Notably, all these
recent dates from the Lower Vindhyan, between Porcellanite
and Rohtasgarh Limestone, clustered within a short range of
1599 – 1631 Ma. In contrast, however, the Kajrahat Limestone
that apparently lies conformably below the Porcellanite, gave
an age of 1721 ± 110 Ma (Pb-Pb, Sarangi et al. 2004). Based on
this new age of the Kajrahat Limestone and considering the
two underlying siliciclastic formations (viz. Arangi and
Deoland), S. Kumar et al. (2005) surmised that the Lower
Vindhyan sedimentation was initiated ~1800 Ma.
Majority of the geochronologists involved in the
Vindhyan stratigraphy, though have confidence in the recent
late Paleoproterozoic dates (
>
1600 Ma) from the Lower
Vindhyan, Wolfgang Frank seemed quite confident in his Ar-
Ar depositional date of 617 ± 3.5 Ma for the Lower Vindhyan
Porcellanite (Banerjee and Frank, 1999) as he stated (in Kerr
1999, “All these samples gave consistent ages close to 620
million years……. I am absolutely confident we can reject the
very old age of 1.1 billion years”.
In the Upper Vindhyan stratigraphy, however, there have
been only a few additions in the absolute radiometric dates in
recent years: Re-Os date of 1670 ± 60 Ma (Singh et al. 2002)
from the black pyritous Bijaigarh Shale (Lower Kaimur) and a
Pb-Pb date of 650 ± 770 Ma from the Bhander Limestone (Ray
et al. 2003; Ray 2006). However, the Re-Os date of 1670 ± 60
Ma has been ignored by all (geochronologists) as it surpassed
most of the age estimates of Upper as well as Lower Vindhyans.
It may be noted however that like the Lower Vindhyan, this
Upper Vindhyan date too clustered around 1600 Ma.
For the upper age limit of the Upper Vindhyan, Ray (2006)
used an age of ~650 Ma of the Bhander Limestone despite a
large error in this date (650 ± 770 Ma, Pb-Pb, Ray et al. 2003)
and suggested continuation of the Upper Vindhyan

181
sedimentation even after 650 Ma. In a previous publication,
Ray et al. (2003) had shown the uppermost limit of the Upper
Vindhyan as ~550 Ma, erroneously quoting this as a K/Ar
date from the Bhavpura Shale (Rajasthan) by Crawford &
Compston (1970). As pointed out earlier (Azmi et al. 2007),
Crawford & Compston (1970, p.368) had merely suggested
that the topmost Vindhyan succession could range “to
perhaps 550 m.y. or even later”. Gregory et al. (2006), however,
suggested a less than ~1075 Ma age for the Upper Vindhyan
sedimentation (Bhander-Rewa) based on an Ar-Ar age of ‘large
phlogopites from the Majhgawan Kimberlite Pipe that intruded
into the Kaimur Group of the Upper Vindhyan.
In addition to above, the following points are also very
relevant for resolving the age enigma of the VSG.
Intrusion level of the Majhgawan Kimberlite Pipe
(MKP)
Ray et al. (2003) showed the intrusion level of the MKP only
up to the top of the Lower Vindhyan, i.e. the Semri Group (see
Fig. 2 in Ray et al. 2003; reproduced herein as figure 3 showing
corrected level of intrusion). It is well established in the
Vindhyan stratigraphy that the diamondiferous MKP intrudes
up to the topmost unit of the Upper Vindhyan Kaimur Group
(Mathur 1962; Haldar & Ghosh 1978; Babu 1998; Bose et al.
2001; B. Kumar et al. 2002). By restricting intrusion of MKP
into the Lower Vindhyan and showing a few selected
convenient dates, Ray et al. (2003) apparently succeeded in
portraying that all Vindhyan geochronological dates showed
therein were “reliable” and “consistent” including 910 Ma
age of the Kaimur Sandstone. Through this way they attempted
to circumvent a well known geochronological bizarre where
younger Kaimurs dated as 910 ± 39 Ma (K/Ar, Vinogradov et
al. 1964) were shown intruding the older MKP of 1140 ± 12
Ma (Rb/Sr by Crawford & Compston (1970). However Ray
(2006, Table 1) now rectified this error without giving reason
for restricting the intrusion of MKP into the Lower Vindhyan
(Ray et al. 2003, Fig. 2). As a consequence, Ray et al.’s (2003)
“reliable age” of 910 Ma (K/Ar) of the Kaimur Sandstone that
was the main basis of a “long lasting hiatus” between Lower
and Upper Vindhyan has now been given up (Ray 2006). This
probing aspect is discussed below.
Long lasting hiatus between the Lower and Upper
Vindhyans
The inference drawn by Ray et al. (2003) of a “long lasting
hiatus” of ~700 million years duration (Ray et al. 2003, Fig. 2)
separating the Lower and the Upper Vindhyans is a sequel to;
1) the imprecise portrayal of the intrusion level of MKP, and 2)
selection of those seven dates favouring latest
Paleoproterozoic age (~1600 – 1631 Ma) for the Lower
Vindhyan and the Neoproterozoic (~910-550 Ma) age for the
Upper Vindhyan (see assorted large number of available
radiometric dates in figure 2). Since Ray (2006) extended the
lower age limit of the Kaimur Group by about 200 million years
(~1100 Ma instead of 910 Ma mainly due to correct depiction
of the intrusion level of MKP), the span of the hiatus
accordingly sharnk to ~500 million years. However, since Lower
to Upper Vindhyan succession in western Vindhyan Basin is
transitional (e.g., Kota-Chittaurgarh area, southeastern
Rajasthan and Rampura area, western Madhya Pradesh; see
in Prasad 1984, p.38-39; S. Kumar 2003b, p. 286 and S. Kumar
et al. 2005, Table 3), the proposition of a ‘basin-wide’ hiatus
even of ~500 million years duration is not viable.
87Sr/86Sr ratios
B. Kumar et al. (2002) and Ray et al. (2003) used 87Sr/86Sr ratios
from nearly all the Vindhyan carbonates to determine their
relative ages within the Late Paleoproterozoic to Late
Neoproterozoic framework of the VSG. But the available 87Sr/
86Sr data is as inconsistent as that of its geochronology. A
glairing inconsistency worth noting in the eastern Vindhyan
Basin succession is that while Ray et al. (2003) obtained 87Sr/
86Sr value of 0.70599 from the Upper Vindhyan Bhander
Limestone and inferred a mid-Neoproterozoic age (~650 Ma), B.
Kumar et al. (2002) obtained the same value (0.7059) for the
Lower Vindhyan Rohtasgarh Limestone which they considered
as ‘best-preserved’ 87Sr/86Sr compositions and had assigned a
Mesoproterozoic age (~1100 Ma). Although Ray et al. (2003)
have explained such contradictions due to methodology; it
certainly reveals vulnerability of 87Sr/86Sr ratio-based age
estimates, particularly in the absence of good temporal
constraints. Further, it would be highly significant to note here
that the lowest 87Sr/86Sr values of 0.70777 in the Tirohan
Dolomite of Chitrakoot (eastern Vindhyan Basin) and 0.70850 in
the Nimbahera Limestone of Rajasthan (both from the Lower
Vindhyan units equivalent to the Rohtasgarh Limestone), which
Ray et al. (2003) interpreted as reset values, are most likely the
pristine seawater values because they closely correspond with
the earliest Cambrian age that has been assigned to the Tirohan
Dolomite on the basis of SSF and calcareous skeletal algae
assemblages (Joshi et al. 2006; Azmi et al. 2007).
Duration of Vindhyan sedimentation
Ray et al. (2003) initially suggested the deposition of the Lower
Vindhyan, comprising predominantly of carbonates and fine
siliciclastics, within a short duration (‘in a few tens of millions

182
of years’), in contrast to Upper Vindhyan, comprising
predominantly of coarse siliciclastics, spanning for most of
the Neoproterozoic (910-550 Ma = 360 million years), is
apparently irrational (compare lithology in figure 2). As
previously mentioned, since Ray (2006) later expanded the
duration of the Upper Vindhyan to ~500 million years (~1100-
600 Ma), this made the depositional relationship between the
Lower and Upper Vindhyans apparently more irrational. Even
if we consider the extended age-range of the Lower Vindhyan
(~1800-1600 Ma) as per the latest suggestions (Sarangi et al.
2004; S. Kumar et al. 2005; Ray 2006), a duration of ~200 million
years for deposition of the Lower Vindhyan is not in tandem
with the time duration of the Upper Vindhyan (~500 million
years). This is prima facie irrational unless unconformities
within the Upper Vindhyan formations would represent long
hiatuses. At present, however, there is no apparent field
evidence of any major break(s) within the Upper Vindhyan
succession (see also S. Kumar et al. 2005). Thus inappropriate
short and long intervals for the deposition of Lower and Upper
Vindhyans, as per the latest geochronology, too beseech
attention for corrective measures.
LATEST FOSSIL FINDS AND BIOCHRONOLOGY
Our biostratigraphic investigation in the Lower Vindhyan
during last one decade has brought out a much enlarged
assemblage of P
ЄЄ-
boundary transition fossils representing
seven groups of mega- and microfossils comprising metazoan
traces, metaphytes, small shelly fossils including metazoan
embryos, calcareous skeletal algae, sponge spicules,
acanthomorphic acritarchs and scolecodont-like structures
(Figs. 5 and 6), covering southern and northern margins of the
eastern Vindhyan Basin (Figs. 1 and 4) (Azmi et al. 2003; Joshi
2004, 2005; Joshi et al. 2006; Azmi et al. 2007). These additional
Fig. 4. Lithologs with GPS locations showing positions of the fossil yielding samples.

183
Fig. 5. (1) Horizontal burrows on rippled undersurface of the Chorhat Sandstone (CH-F1, WIMF/A 432),
Chorhat (GPS N240 25.331; E810 39.441); (2) Close-up of the burrow fill in 1. This is from
Seilacher et al.'s (1998) Chorhat burrow locality, and (3) Hyolithellus tube and Olivooides embryo
embedded in the Tirohan Dolomite hand specimen, Jankikund Section, Chitrakoot (JNK 4, WIMF/
A 430).
fossil finds have established beyond doubt that the original
claim by Azmi (1998a) for the discovery of Lower Cambrian
SSFs in the Lower Vindhyan was correct which was also
subsequently confirmed by Srivastava (in Editor JGSI 2000).
The Rohtasgarh Limestone and Bhagwar Shale formations in
several localities on the southern margin of the Son Valley
(Figs. 1 and 4) have yielded distinct Lower Cambrian SSFs
besides a new mongolitubulid (Vindhyanitubulus semriensis

184
Fig. 6. (Explanation on facing page)

185
Fig. 6. Lower Vindhyan (Semri Group) small shelly fossils, fossil embryos, sponge spicules, carbonaceous metaphyte compressions, acanthomorphic
acritarch, scolecodont-like polychete remains and calcareous algae from wide apart locations in eastern Vindhyan Basin. Fossils recovered
by techniques other than dissolution and images other than SEM are specified. Scale bars equal 100ìm unless otherwise mentioned. All
illustrated material is deposited in the repository of the Wadia Institute of Himalayan Geology, Dehra Dun under repository number prefixed
as WIMF/A, preceded by their sample numbers. 1-5. Hyolithellus spp. 1, JNK 2, WIMF/A 264; 2, JNK 2, WIMF/A 263; 3, JNK 4, WIMF/
A 261; 4 detail of aperture in 3; 5, JNK 3, WIMF/A 262. 6-7. Anabarites spp. 6, JNK 4, WIMF/A 270; 7, JNK 4, WIMF/A 273. 8 & 15.
Vindhyanitubulus semriensis Azmi, Joshi, Srivastava (2007), 8, BD 1, WIMF/A 288; 15, BD 1, WIMF/A 289. 9, 12 & 18. Spirellus sp. 9,
SJ11, WIMF/A 348, scale bar 50ì m; 12, JNK 4, WIMF/A 345; 18, RJ 33, WIMF/A 352. 10. Obruchevella sp. JNK 2, WIMF/A 342. 11,
19 & 20. Protohertzina spp., 11, JNK 2, WIMF/A 293, scale bar 50ì m; 19, JNK 2, WIMF/A 292; 20 showing inner cavity in 19. 13, 14, 16,
17 & 25. Olivooides sp. (fossil embryos), 13, JNK 4, WIMF/A 340; 14, details of rectangle portion in 13, scale bar 10ìm; 16, NQS 1, WIMF/
A 332; 17, (JNK 4, WIMF/A 319); 25, JNK 2, WIMF/A 334. 21. Flabellophyton sp., hand specimen light photomicrograph, RJQ 1, WIMF/
A 422, scale bar 5mm. 22. Konglingiphyton sp., hand specimen light photomicrograph, RJQ 1, WIMF/A 421, scale bars 5mm. 23 & 24.
Scolecodont-like jaw remains, thin section light photomicrograph, Olive 1, WIMF/A 402, scale bar 10ìm; 24, Olive 1, WIMF/A 417, scale
bar 10ìm. 26, 30 & 31. Platysolenites sp. 26, RQ 10, WIMF/A 434; 30, JNK 4, WIMF/A 305; 31, JNK 4, WIMF/A 301. 27 & 35. Hexactine
sponge spicules, 27, thin section light photomicrograph, GHUR 1, WIMF/A 395, scale bar 10ìm; 35, Pyritized hexactine sponge spicule,
SILPI 1, WIMF/A 400, scale bar 10ìm. 28. Acanthomorphic acritarch Micrhystridium dissimilare, thin section light photomicrograph,
GHUR 1, WIMF/A 378, scale bar 10ìm. 29. Orbisiana sp., hand specimen light photomicrograph, RMD F1, WIMF/A 433, scale bar 1mm.
32. Halkieria sp., JNK 3, WIMF/A 295. 33. Renalcis sp., thin section light photomicrograph,, JNK 2, WIMF/A 367. 34. Korilophyton sp.,
thin section light photomicrograph, JNK 4, WIMF/A 371. 36-38. Girvanella sp. 36, JNK 4, WIMF/A 356; 37 details of tubules in 36, scale
bar 10ìm; 38, cross section of tubules, thin section light photomicrograph, JNK 2, WIMF/A 365, scale bar 20μm.
Azmi et al. 2007), hexactine sponge spicules and small
acanthomorphic acritarch Micrhystridium dissimilare (Fig. 6).
In addition, the Rohtasgarh Limestone has also yielded typical
Late Ediacaran megascopic carbonaceous compressions viz.
Orbisiana, Konglingiphyton and, Flabellophyton (Fig. 6:21,
22, 29). Earlier, from the same stratigraphic horizon
carbonaceous compressions of Tyrasotaenia, Grypania,
Chuaria and Tawuia were recorded (Shukla & Sharma 1990;
S. Kumar 1995, 2001). The phosphatic Tirohan Dolomite, the
uppermost unit of the Chitrakoot Formation in Chitrakoot area
of Madhya Pradesh, on the northern margin of the eastern
Vindhyan Basin, is of special significance as this horizon has
yielded a fairly good assemblage of the earliest Cambrian SSFs
(e.g. Protohertzina, Mongolodus, Halkieria, Hyolithellus,
Anabarites, Cambrotubulus, Bathysiphon, Platysolenites,
Spirellus, Obruchevella, Olivooides- the metazoan embryos,
etc). This SSF assemblage (Fig. 6) typically represents the
Meishucunian Zone I or Nemakit-Daldynian Stage fossils of
earliest Cambrian of China, Siberia, Kazakhstan, Inner
Mongolia Canada, India and elsewhere (Brasier 1989). In
addition, the Chitrakoot SSF assemblage also contains
abundant rock forming calcareous algae assemblage
(Girvanella, Renalcis and Korilophyton) (Fig. 6: 33,34,36-
38), which further strengthen the earliest Cambrian age for the
Tirohan Dolomite. According to these biostratigraphic results
the P
ЄЄ-
boundary (~542 Ma) would clearly lie in the
Rohtasgarh Limestone of the Semri Group (Lower Vindhyan).
The reported occurrence of Cymatiospheroides kullingii, a
distinct Vendian-Cambrian boundary acritarch, from the lower
part of the Chitrakoot Formation of the Lower Vindhyan
(Anbarasu, 2001) also corroborates the earliest Cambrian age
deduction of the Tirohan Dolomite (see discussion in Tiwari
2001). Additionally, the Koldaha Shale (Olive Shale), a distinct
lower unit in the Lower Vindhyan, far below the Rohtasgarh
Limestone but immediately above the Porcellanite, contains
numerous scolecodont-like structures in a petrographic thin
section (Fig.6: 23, 24). These jaw-like structures are akin to the
oldest known scolecodonts from the Middle Vendian
(Redkinian Stage) of the Russian Platform (Sokolov 1977).
Therefore, considering the appearance of the SSFs,
acanthomorphic acritarchs, metaphytes and metazoan traces
in descending stratigraphic order in the Lower Vindhyan, it
appears that the scolecodont-like structures in Koldaha Shale
(N240 31.165; E830 02. 939) would probably correspond with
the reported oldest scolecodonts of the Middle Vendian
(Ediacaran) of the Russian Platform (Sokolov 1990). The Lower
Vindhyan fossils thus impart a plausible chronostratigraphic
picture that suggests that the Gangau Tillite at the base of the
VSG (Fig. 2; see Bose et al. 2001 and references therein for the
basal Vindhyan glacial event) is most likely a signature broadly
correlatable with the global Marinoan glaciation event of ~635
Ma (Condon et al. 2005). The recent confirmation of occurrence
of fossils resembling Edicaran-Cambrian forms in the Lower
Vindhyan by Bengtson et al. (2007) is an independent
validation of our view besides eroding the skepticism
enumerated in the JGSI report (In Editor JGSI 2000).
DISCUSSION
1. The occurrence of the P
ЄЄ-
boundary transition fossils
in the Lower Vindhyan (Figs. 5 and 6) raises intriguing question
on the latest geochronological results (A. Kumar et al. 2001;
Rasmussen et al. 2002; Ray et al. 2002, 2003; Sarangi et al.,
2004). The ~1800-1600 Ma age range obtained for the Lower
Vindhyan succession is almost three times older than the
global appearance time of the small shelly fossils, calcareous

186
skeletal algae, megascopic triploblastic animal traces and
acanthomorphic acritarchs, which are recorded from the Lower
Vindhyan. These fossils are globally known within an interval
of Ediacaran to basal Cambrian (~635–542 Ma), and nowhere
occur in the rocks of Paleoproterozoic age. We therefore
reiterate (Azmi 1998a; Azmi et al. 2007) that based on the
global biotic evolutionary records, with support from presence
of globally correlatable Marinoan glaciation (~635 Ma) at the
base of the succession (Fig. 2) and regional succession of
geological events (e.g., preceding Delhi Orogeny dated ~1450
Ma), the age of the Lower Vindhyan (Semri Group) would
range from the latest Neoproterzoic to the earliest Cambrian
(latest Cryogenian to Nemakit-Daldynian or earliest
Meishucunian). Logically we envisage age of the Upper
Vindhyan to be at least as young as Early Cambrian.
Significantly, none of the reported fossils from the Upper
Vindhyan is inconsistent with this inferred age (Fig. 2). The
presence of triploblastic animal traces (Seilacher et al. 1998)
and the putative Ediacaran fossil Spriggina(?) (Kathal et al.
2000) are in accordance to our proposed Lower Vindhyan
chronostratigraphy. As said earlier (Brasier 1998; Azmi 1998b),
we thus substantively argue that the presence of the trace
fossils (Sarkar et al. 1996; Seilacher et al. 1998) in the Lower
Vindhyan need not be construed as evidence for the deep
origin (Paleoproterozoic/Mesoproterozoic) of the multicellular
animals.
2. In the Vindhyan geochronology the intrusion of the MKP
in the Kaimur Group has invariably been used as an important
magmatic event to constrain age of the Lower and the Upper
Vindhyan successions. The frequently quoted age of the MKP
is 1140 ± 12 Ma (Rb-Sr, Crawford and Compston, 1970) is
further supported by additional radiometric ages of 1067 ± 31
Ma (Rb-Sr, A. Kumar et al. 1993) and 1073 ± 13.7 Ma (Ar-Ar,
Gregory et al. 2006) (Fig. 2). Obviously, these dates would
strongly suggest that the succession of Lower Vindhyan and
the overlying Kaimur Group has to be older than ~1100 Ma.
Nevertheless, Haldar and Ghosh (1981) were the first to
question the date of 1140 Ma as the ‘intrusion age’ of the
MKP on the basis of presence of minimum four generations of
phlogopites. They interpreted that the date of 1140 Ma
possibly reflect the age of the phlogopite crystals of the first
(=earliest) generation that are more abundant and larger in
size and might have been present in the parent magma in the
lower lithosphere or in the upper mantle itself. Thus Haldar &
Ghosh (1981) argued that in such a case the age data of MKP
(1140 Ma) would not correspond with the date of emplacement
of the kimberlite and could be substantially younger. In this
context, the observations of Kelley & Wartho (2000) on
intrusion age of kimberlite is also very relevant: “Because
phlogopite generally retains Ar only below 4800C, K-Ar and
Ar-Ar bulk mineral dating has been applied to small phlogopite
grains in order to measure eruption ages. The ages yielded by
large phlogopites from xenoliths are commonly older than the
eruption, a phenomenon that has been interpreted as the
incorporation of excess radiogenic Ar from a deep fluid
source.” These view points lend support to our
biochronological inference suggesting intrusion of MKP as
considerably later than 542 Ma, i.e. the P
ЄЄ-
boundary age
of the Rohtas Subgroup, and even younger than the overlying
Early Cambrian Kaimur Group to which the MKP has also
intruded. And since the overlying Bhander-Rewa Groups are
possibly not far younger than the Kaimur Group, the close
paleomagnetic directional similarity between the MKP and
the Bhander-Rewa Groups (Gregory et al. 2006) is,
understandably, due to their close ‘real younger’ age
relationship (at least Early Cambrian) and perhaps not due to
any ‘remagnetization or fortuitous’ relationship. With our new
temporal constraints we suggest that the diamond producing
MKP of Panna (M.P.) should be of late Pan-African origin,
which may be correlated with that of the recently dated ~500
Ma diamondiferous Kodomali kimberlite from Chhattisgarh
Basin of the Bastar Craton (Chalapathi Rao et al. 2005;
Lehmann et al. 2006; Fareeduddin et al. 2006). Thus we firmly
believe that all older ages so far obtained from the MKP,
including the latest one (1073.5 ± 13.7 Ma, Ar-Ar by Gregory
et al. 2006; see all MKP ages in figure 2) that is categorically
based on ‘large’ phlogopites, have nothing to do with the
intrusion age of the MKP.
3. Chemostratigraphically, the δ13C profiles from the VSG
(Friedman and Chakraborty 1997; Kumar et al. 2005) also appear
to be consistent with our young age deduction. It is significant
that both profiles are consistent in having a prominent
negative excursion in the Rohtas Formation. It also has a high
positive excursion in the Bhander Limestone and the overlying
Sirbu Shale, with a slight negative trend at the top. Indeed the
negative excursion in the Rohtas Formation coincides with
our biostratigraphically constrained P
ЄЄ-
boundary
transition interval, which has also been globally observed
(Shields 1999). Friedman and Chakraborty’s (1997) suggestion
for placing the P
ЄЄ-
boundary zone at the upper negative
trend within the Bhander Limestone and Sirbu Shale was in
fact guided by the premise that trace fossils (Chakrabarti 1990)
first appeared in the Bhander Group. Interestingly, in a
subsequent discussion (S. Kumar 1998; Chakraborty &
Friedman 1998) trace fossils appearing in the Lower Vindhyan
(Koldaha Shale & Chorhat Sandstone, Sarkar et al.1996) were

187
taken into account as indicative of ‘Vendian to Lower Cambrian’
age in view of Crimes’s (1992) trace fossil-based correlation
scheme across the P
ЄЄ-
boundary. But now with the discovery
of P
ЄЄ-
boundary SSF in the Rohtas Subgroup it is amply
clear that the lower δ13 C negative excursion in the Rohtasgarh
Limestone indicates nothing but the P
ЄЄ-
boundary event.
We draw a parallel of δ
13 C excursions of the Vindhyan
(Friedman & Chakraborty 1997; and Kumar et al. 2005) with
that of the P
ЄЄ-
boundary section of Anti-Atlas mountains
of Morocco where initially the boundary was suggested at
the upper negative trend (Tucker 1986); however later, with
the findings of Lower Cambrian calcareous algae and trace
fossils in the Dolomite Inferieure, the P
ЄЄ-
boundary was
finally delineated at the lower negative δ13C excursion, ~1000
m below of the upper excursion (Latham & Riding 1990).
4. From magnetostratigraphic point of view, even the
Vindhyan Geomagnetic Polarity Time Scale (VGPTS) does not
favour a very long period of sedimentation of the VSG.
Recently, Goutham et al. (2006) based on the recent
geochronology indicating prolong deposition (mid-
Paleoproterozoic to almost the end of Neoproterozoic, ~2000-
550 Ma) for the Cuddapah-Kurnool and Vindhyan
successions, attempted to correlate the Cuddapah-Kurnool
and Vindhyan Geomagnetic Polarity Time Scales (CKGPTS
and VGPTS) with the well constrained Siberian Geomagnetic
Polarity Time Scale (SGPTS) of the Riphean-early Vendian
period (1650-~600 Ma). Their attempt however failed because
they found only few reversals in the KGPTS and VGPTS (two
and four reversals respectively) in comparison to twelve in
the SGPTS (see Fig. 7 of Goutham et al. 2006). So the authors
(Goutham et al. 2006, p. 230) were categorical in conclusion
that correlation of the VGPTS and CKGPTS with that of the
Proterozoic SGPTS is “not possible” (see also comments by
Azmi & Joshi 2007).
5. The ~1800 Ma age (late Paleoproterozoic) ascribed
through latest geochronology to the base of the Vindhyan
Supergroup is not only incongruent to its fossil content but
also to the regional geology and stratigraphy of the northern
Indian Peninsula. This is because it is widely held that the
Vindhyan Supergroup sedimentation followed culmination of
the Delhi Orogeny at ~1450 Ma (Roy & Jahkar 2002); now
with putative ~1800 Ma beginning of the Vindhyan Supergroup
it is beyond ground geological perception to accept that its
early phase of sedimentation witnessed the Delhi Orogeny. It
becomes obvious in view of the fact that the Vindhyan package
is bereft of tectonic and metamorphic signatures that are so
characteristic to the unconformably underlying ~1.6 - 1.8 Ga
old Bijawar, Mahakoshal, Gwalior and Delhi Supergroups
(Crawford & Compston 1970; Rao et al. 2005). Thus it is rational
to conclude that the Lower Vindhyan cannot be of Bijawar ’s
age (i.e. 1.6–1.8 Ga).
6. A recent Pb-Pb date of 1921 Ma, enhanced by 200 to 300
million years on account of resetting, puts the Jharmarkotra
Dolomite in the basal part of the Aravalli Supergroup at ~2200
Ma (Sarangi et al. 2006); this brings to fore through simple
estimates that while Aravalli and Delhi Supergroups (together
far thicker than the VSG) with an intervening unconformity
(marking the Aravalli Orogeny) were laid down during ~300 -
400 million years, contrastingly, the current geochronological
estimate would suggest that the Vindhyan Supergroup was
alone deposited in three to four times longer duration of ~1200
million years (~1800 to 600 Ma). This imbalance indicates that
the geochronological estimate for such an exceptionally long
duration for the Vindhyans is unreasonable.
7. Our estimation suggests Vindhyan deposition within a
short interval of ~ 130 million years (~650 – 520 Ma), during
Vendian to Early Cambrian (Azmi 1998a, b; Azmi et al. 2007),
clearly bringing out a major Proterozoic hiatus of about a
billion-year between the metamorphosed Bijawars (~1800 Ma)
and the ~650 Ma old beginning of the unmetamorphosed
Vindhyans (Azmi 2007b). Similar coeval hiatuses also exist
below the Trans-Aravalli Vindhyan in Rajasthan (Dasgupta et
al. 1988) and throughout in the Lesser Himalaya, below the
Blaini-Krol-Tal and equivalent successions (Azmi & Paul 2004;
Azmi 2007a). It thus appears that this major break in the
Proterozoic sedimentation is of great tectonic significance.
CONCLUDING REMARKS
The above elaboration on the latest Vindhyan biochronology,
geochronology, chemostratigraphy, paleomagnetostratigraphy,
sedimentation rate and hiatus studies in the regional
stratigraphic set up, reveal that the proposed Late
Paleoproterozoic to Neoproterozoic (~1800 – 600 Ma) age
assignment to the VSG needs afresh inspection. The occurrence
of the Early Cambrian fossils in the radiometrically assigned
late Paleoproterzoic age to the Lower Vindhyan (Rohtas
Formation) is no doubt perplexing as such Lower Vindhyan
fossils are nowhere known in the Paleoproterozoic rocks. We
are unable to comprehend that the Vindhyan Basin alone can
be of such an exception where the fairly well known P
ЄЄ-
transition biotic evolutionary sequence (Glaessner 1984; Condon
et al. 2005) would drastically precede the global records!
The synoptic view of the assorted Vindhyan studies presented

188
above makes it clear that the geochronological endeavors
divorced off the available paleontological and regional
geological constraints are the fountainhead of implausible
propositions in the context of age of the Vindhyan Supergroup.
Acknowledgements: Discussions on Vindhyan fossil material with
Professors Franz Fürsich, Hans Hofmann, Hubert Szaniawski, Stefan
Bengtson, Jere Lipps and Yuan Xunlai have been very useful and
encouraging. Authors thank Prof. B.R. Arora, Director, Wadia Institute
of Himalayan Geology, Dehra Dun for providing necessary facilities.
DJ is indebted to the Head, Department of Earth Sciences, IIT
Roorkee for extending various facilities during the progress of his
Ph.D. thesis which forms a part of this work. Insightful comments
of an anonymous reviewer and of Dr. P. Kundal improved the
manuscript. Sincere technical support by Shri Sanjeev Dabral is
greatly appreciated.
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