Deccan Continental Flood Basalts Eruption triggered end Cretaceousearly Palaeocene Floral Change: evidence from the Indian subcontinent - a Case ...
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Deccan Continental Flood Basalts Eruption triggered end Cretaceous-
early Palaeocene Floral Change: evidence from the Indian
subcontinent - a Case Study from Nand-Dongargaon Basin
It is still a matter of debate whether the Global Mass Extinction at the Cretaceous-Tertiary
Boundary (KTB) was related with some Extra-terrestrial impact or Deccan Volcanism. The
Deccan Continental Flood Basalt (DCFB) Volcanism that occurred during 69-61 ma, outpoured
millions of km3 of lava and greenhouse gases resulting in a spread of lava field over an area of
500, 000 km2.
The Geological Survey of India, in collaboration with the Department of Geology, BHU,
Varanasi, carried out detailed palynological investigation at different stratigraphic levels of the
intertrappean sediments within the DCFB sequence. The inter-trappean sediments were
deposited during the period of repose in the volcanic activity and record the history of
environmental transition across the Deccan volcanism.
As a case study, the DCFB sequence exposed along the Nand-Dongargoan (N-D) basin and
adjoining areas to the west and northwest in Yeotmal and Wardha districts, was selected. The
DCFB is nearly 500 m thick, classified as Sahyadri Group under Ajanta, Chikhli, Buldana and
Karanja Formations in ascending stratigraphic orders. The Infratrappean (Lameta Formation)
sediments and Intertrappean sedimentary beds were studied as different stratigraphic levels
within the sequence.
The lithofacies and biofacies analyses of sediments deposited prior to the arrival of volcanic
flows have indicated deposition in low-land, open alluvial-limnic environments under semi-arid
climate having strong seasonality (dry-spell dominated). The sediments are characterized by
strong pedogenic modification (calcretised). Such sedimentary beds were formally designated
as Lameta Formation (Infratrappean) deposited in different inland basins including the Nand-
Dongargaon (N-D) basin in Central India. The Lameta sediments of N-D basin were deposited
during C30N-C29R in Maastrichtian.
2
Within the Deccan volcanism province (DVP), the depositional environments were mostly
closed, freshwater shallow lakes and water bodies. The sediments designated as Intertrappean
are devoid of any associated calcrete profiles and are dominated by chert interpreted as silica
mobilization in the province by volcanic induced acid rains. A wet-spell, dominated by increasing
humid condition, is envisaged within the DVP. The Intertrappean beds at different stratigraphic
levels within the DVP were studied for their palynofloral analysis. Further, the palynofloral
studies were supplemented with magnetostratigraphy and stable organic carbon isotope and
time series analysis.
Studies were extended to the DVAS of other sectors including Kutch-Saurashtra to the west
and Chhindwara-Mandla-Jabalpur (CMJ) to the north for establishing spatio-temporal
relationship.
The megafloral record from Lameta Formations are poor. Lameta Formation in the N-D basin, a
sole megafloral bearing locality, has yielded Araucarites, Brachyplyllum, Cycades, a couple of
small varieties of palmae and dicots. The plant bearing coprolites (dung mass) attributed to
titanosaurimorph dinosaurs with comminuted plant tissues have also provided some insight into
the vegetation that these reptiles preferred as their solid diet.
2
3 3
4
Palynoflora yielded by Lameta sediments of N-D basin and associated coprolites show
presence of sizeable concentration of gymnosperms (Araucariacites, Cycadopites, Classopollis,
Podocarpidites) and angiosperm (Palmaepollenites, Longapertites, Graminidites,
Retimonosulcites, Sapotaceoidaepollenites, and Periporopollenites, Compositaepollenites and
other tri and tetra colporate grains) pollen grains. In addition to pollen grains, phytoliths of
grasses (Poaceae) and dicots; fungal spores and mycorrhizal fungi, bacterial and algal remains
are also recorded. Overall Gymnosperm-Angiosperm dominated the floral scenario during the
Lameta time. This time (Maastrichtian, C30N) was also marked by the First Appearance of non-
marine diatoms Aulacoseira in freshwater ecosystem of the Indian subcontinent.
4
5
Lake sequence with well developed diatom bearing varves from Lamata Formation
at Bhatali. The diatoms Aulacoseira make first appearance in the freshwater
ecosystem within Indian Maastrichtian.
Diatom bearing varved clays within lake sequence at Bhatali.
5
6
Following the initiation of volcanism, marked palynological changes are observable in the
intertrappean sediments associated with the basal flows. The first phase of volcanic eruption is
indicated to be more fatal and it almost devastated the existing plant communities, both
terrestrial and aquatic. Bloom of dinoflagellates along with appearance of new palynotaxa such
as Ariadnaesporites sp., Aquilapollenites bengalensis, Azolla cretacea, Gabonisporis
vigourouxii, Triporoletes reticulatus, Normapolles group pollen, etc. within the intertrappean of
first level in the basin is observed. In the new emerging scenario the palynoflora is indicated to
be dominated by Pteridophyte-Angiosperm dominated pollen grains with marked decline in the
pollen grains of gymnosperms.
67
Freshwater Dinoflegellates Peridiniales appear for the first time in the
sediments of the lowest Intertrappean bed within DCFB sequence at Panjurni.
At the higher stratigraphic levels within the DVS, this palynoflora continues vis-à-vis with
appearance of some new forms such as Scabrastephanocolpites spp. and diversification of
Normapollis group.
The earliest palynofloral change commiserate with inititation of Deccan
volcanism. Appearance of dinoflagellates in Daiwal Intertrappean bed.
78
Further, at still higher stratigraphic levels, a sharp decline in the recovery of pollen/spores and
proliferation of mycorrhizal fungi is observed.
Such palynofloral changes are also observable in the adjoining Deccan volcanic subprovince of
CMJ sector to the north and Kutch-Saurashtra region to the west. The latter include the iridium
bearing intertrappean sediments (C29R) associated with dinosaurs, diatoms (Aulacoseira) and
sponge spicules at Anjar.
The change in flora is in response to increasing stress induced by volcanic activity that affected
the ecosystems and environments both directly and indirectly, not only through vitiating
atmospheric condition with influx of volcanic gases, etc. but also the ecosystem coming in direct
contact with the lava fields and associated ash falls.
Sediments at higher stratigraphic level at Sindhi with appearance of Norma pollens
The study indicates a floral change in the Maastrichtian sediments associated with DVS. The
earliest floral change in the N-D basin coincides with the initiation of the volcanic activity in the
Late Cretaceous. Further palynofloral change at higher stratigraphic levels is strategically
associated with the increasing volcanism that erupted periodically. A strong relationship exists
between the volcanic flows and the sediments which have recorded the history of palynofloral
changes during the Deccan volcanism. It is indicated that the floral change is related to the
volcanic activity and, possibly in response to the environmental stress, created by the
volcanism. The existing vegetation came in direct contact with the lava fields and flows, ash fall
and volatile acids in the atmosphere. It also experienced indirect influence of the magma
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outpour and volcanic effusions which brought physiographic changes and the atmospheric and
climate perturbations.
The findings indicate that the floral change towards the KTB, at least in the Indian continent,
was initiated in terminal Maastrichtian (C 29r) and it was induced by Deccan volcanism. During
the Late Cretaceous–Early Paleocene time India was strategically located at the active eruptive
centers of the Deccan continental flood basalts. As such, it experienced the maximum impact of
Deccan volacanism. Globally, the effect of volcanism close to the KPB is expected to be indirect
and relatively late.
In the Indian subcontinent disappearance of dinosaurs 350 ky before the KTB (Hansen et al.,
2005) with advent of Deccan volcanic activity and the palynofloral changes commiserating with
the initiation of Deccan volcanic activity provide a strong evidence for Deccan Volcanism as
strong candidate for biotic crisis towards the KTB.
The work was carried out under Joint Collaborative Programme involving Dr. D. M. Mohabey of
Geological Survey of India, Palaeontology Division, Central Region and Dr. Bandana Samant,
Formerly of Dept. of Geology, BHU, Varanasi and presently RTM, Nagpur University.
1 2
(Contributed by D.M.Mohabey and Bandana Samant)
1. Geological Survey of India, Paleontology Division, Nagpur-440 006, India
Email d.mohabey@gsi.gov.in
2. Department of Geology, RTM, Nagpur University, Amravati Road, Nagpur-440 010
Email bandanabhu@rediffmail.com
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