Paleoposition of the Cambay, Kutch and Barmer basins in India during the
Various Eocene hyperthermal events have been recorded from lignite sections
of western India in the past decade. To infer the paleoenvironment, during a
warm paleotropical climate of India, we have assessed multiple microfossil
groups like pollen/spores, dinoflagellates and foraminifera from Early Eocene
lignite mine sections from the Cambay (Surkha) and Barmer (Giral) basins and
Middle Eocene sections from the Kutch Basin (Matanomadh and Panandhro mines) of
western India. The Surkha and Giral sections exhibit a dominance of
rainforest elements (
Paleoposition of the Cambay, Kutch and Barmer basins in India during the
The Early and Middle Eocene exhibited a very warm climate, but towards the
Late Eocene, the atmosphere cooled and shifted from hothouse to icehouse
conditions (Zachos et al., 2001). The tropical temperatures during the
Early Eocene increased by 5 to 6
The Indian subcontinent (Fig. 1) has well-preserved records of Paleogene marine and continental sediments deposited along its western sedimentary basins ranging from the Eocene (shallow benthic zone: SBZ 5/6–SBZ 11 and SBZ 17; age: 56–49 Myr) to the Oligocene (SBZ 22b–SBZ 23; age: 28.1–23 Myr) (Saraswati et al., 2018). The humid hothouse during the Early Paleogene enhanced the development of tropical rainforests and gave rise to extensive development of lignite deposits in the Indian subcontinent. The sea level highstands which co-occur with these high temperatures, such as that in western India (Samanta et al., 2013; Prasad et al., 2013), probably facilitated the development and preservation of the lignite deposits (Sluijs et al., 2008). These lignite deposits were formed in the restricted marine basins and are coeval with the warming events in the Paleogene. The Early Eocene lignite deposits were developed in the Cambay and Barmer basins, while the Middle Eocene lignite deposits are observed in the Kutch and Barmer basins. These lignite mine sections have recorded some of the warming events like Paleocene Eocene Thermal Maximum (PETM), Eocene Thermal Maximum 2 (ETM 2), Early Eocene Climatic Optimum (EECO) from the Cambay Basin and Middle Eocene Climatic Optimum (MECO) from the Kutch Basin (Samanta et al., 2013; Khanolkar et al., 2017; Clementz et al., 2011). The earlier works on these lignite mines emphasized the determination of paleoenvironmental conditions focusing on one microfossil group like pollen and spores or dinocysts or foraminifera (Sharma and Saraswati, 2015; Garg et al., 2008; Khanolkar et al., 2017). A lacuna exists in using an integrated approach to decipher the paleodepositional and paleoenvironmental conditions across the Early and Middle Eocene sections from paleotropical sites in India.
Eocene Stratigraphy of the Barmer, Cambay and Kutch basins in India.
In this work, we have addressed the similarities and differences observed in the paleodepositional and paleovegetational conditions from the Cambay, Kutch and Barmer basins during the tropical climate of the Early and Middle Eocene. For this, we have used an integrated approach by quantitatively analyzing various microfossil groups like foraminifera, pollen and spores, dinoflagellates and fungi. A holistic approach has made it possible to better determine the paleoenvironmental conditions from a restricted marine setting which depicts transitional environments and is comprised of various groups of microfossils belonging to both continental and open marine ecosystems. For our present study, we have chosen four lignite mine sections: (1) Surkha lignite mine section (Cambay Basin), (2) Giral lignite mine section (Barmer Basin), (3) Matanomadh (Kutch Basin) and (4) Panandhro lignite mine section (Kutch Basin).
The mineable lignite deposits of Eocene age are found along the three sedimentary basins of western India viz. Barmer, Kutch and Cambay. Various well-preserved vertebrate fossils belonging to perissodactyls, artiodactyls, insectivores, proteutherians, apatotherians, marsupials, rodents and microfossils like dinoflagellates, pollen and spores, and foraminifera have been discovered from these lignite mine sections, rendering them essential from a stratigraphic point of view (Bajpai et al., 2005; Khanolkar and Saraswati, 2015; Garg et al., 2008).
The Rajasthan Basin consists of a geological record from Archean to recent times.
Within the Rajasthan Basin, three sub-basins, namely the Barmer, Jaisalmer and
Bikaner–Nagaur (K. Kumar et al., 2007), exist and cover an area of
approximately 120 000 km
The Cambay Basin is comprised of Paleogene sediments overlain by Quaternary alluvium. The Paleogene sequence rests unconformably over the Deccan flood basalts, ranges in age from Paleocene to Plio-Pleistocene and has been deposited by various transgressive–regressive cycles. The oldest Paleogene rocks are comprised of the Olpad Formation, which is unconformably overlain by the Cambay Formation (Paleocene–Early Eocene) (Fig. 2) (Chandra and Chowdhary, 1969). The Cambay Formation is best exposed along the open-cast lignite mines and has yielded rich and diverse fauna of various fossil groups. The lignite mine sections of Vastan and Valia have reported the presence of hyperthermal events including PETM and EECO (Samanta et al., 2013; Clementz et al., 2011). The Surkha lignite mine section is examined in this study.
Percentage distribution of palynomorphs in the Giral lignite mine section, Barmer Basin.
The Kutch Basin is a pericratonic rift basin trending laterally beside the western margin of India and developed during the separation of India from Gondwanaland in the Late Triassic (Biswas, 1992). A complete stratigraphic succession from Paleogene to Quaternary, with records of marine transgression and several stratigraphic breaks, was recorded in the basin (Biswas, 1992). A significant part of the Tertiary sequence is observed in the offshore region of the Kutch Basin, whereas on land, these Tertiary sections are restricted to the western part. The Eocene sections mainly constitute shales, claystones and limestones deposited in lagoonal to shallow marine environments. These Eocene sediments have preserved large groups of faunal assemblages including foraminifera, dinocysts, pollen and spores (Khanolkar and Saraswati, 2015; Sharma and Saraswati, 2015; Garg et al., 2008). The Paleogene succession of the Kutch is comprised of five formations. These include Matanomadh Formation, Naredi Formation, Harudi Formation, Fulra limestone and Maniyara Fort Formation) (Fig. 2) (Biswas, 1992; Saraswati et al., 2018).
Percentage distribution of palynomorphs in the Surkha lignite mine section, Cambay Basin.
The Cambay, Kutch and Rajasthan basins have been previously investigated by various workers to decipher the age of the mine sections.
Dolson et al. (2015) updated the stratigraphy of the Barmer Basin in Rajasthan using the data acquired from 420 wells, 4 km of conventional core and 2-D and 3-D seismic mapping techniques. The age of the lignite mine section of the Giral was earlier thought to be of Late Paleocene–Early Eocene (Rana et al., 2005). However, the recent work by Dolson et al. (2015) has updated it to be of Early Eocene age.
The Early Eocene (Ypresian) palynomorph taxa from the Surkha mine consist of
The age of lignites from the Kutch Basin was earlier thought to be Early Eocene
(Biswas, 1992). However, recent work has revised the stratigraphy of the Kutch Basin,
and the current understanding states that the lignites belong to Middle Eocene age (Saraswati et al., 2018, 2014). In a marginal
marine environment, it is essential to infer the age of the sequence using an
integrated approach which involves the study of various group of microfossils
like palynomorphs and foraminifera (Sharma and Saraswati, 2015). The shales
deposited above the lignite seams of the Panandhro and Matanomadh mines consists
of foraminifera such as
The field area is comprised of lignite mine sections from three different sedimentary basins (Cambay, Kutch and Rajasthan) alongside the western margin of India (Fig. 1; Sect. S1 in the Supplement).
Percentage distribution of palynomorphs in the Matanomadh lignite mine section, Kutch Basin.
For the current study, we collected 33 samples from 26 m exposure of the open-cast lignite mine of the Giral from the Barmer sub-basin. The profile constitutes alternations of shale and lignite beds. The sample positions are marked in the lithocolumn illustrated in Fig. 3 (refer to Sect. S1 in the Supplement).
The Surkha lignite mine section is equivalent to the subsurface Cambay Shale Formation and belongs to Early Eocene. We have collected eight samples from the bottom to top of the mine section, and their positions are marked within the lithocolumn (Fig. 4; Sect. S1 in the Supplement). The mine section is comprised mainly of lignites and shales. Around 5 m thick brown colored friable lignite horizon is overlain by marine shale, which gradually grades into silty shale at the top. The whole lignite-bearing sequence is covered by 20 m thick bentonite clay.
The palynomorph
assemblage from Early Eocene sections: panels
The palynomorph
assemblage from Middle Eocene lignite mine sections of the Panandhro and
Matanomadh: panel
From the Kutch Basin, we have collected samples from the Matanomadh mine section (23 samples from 10 m exposure of lignites, shales and mudstones; Fig. 5; Sect. S1 in the Supplement) and Panandhro lignite mine (47 samples from 55 m exposure of lignite and shale sections; Fig. 6; Sect. S1 in the Supplement).
The
foraminiferal assemblage from the Panandhro and Matanomadh lignite mines.
Percentage distribution of palynomorphs in the Panandhro lignite mine section, Kutch Basin.
For palynomorph analysis, we processed samples from all four mine sections.
Initially, we treated 20 g of the dry sample with dilute HCl (30 %) for
removal of carbonates. The residue left was consecutively treated with
hydrofluoric (HF) acid (38 %) for removal of silica and
We have used around 20 g of sample (shales and mudstones) for processing.
The sample was treated with
Distribution of a group of palynomorph taxa in the Giral lignite mine, Barmer Basin.
The age of lignite mines of the Surkha (Cambay), Panandhro and Matanomadh (Kutch) has already been established earlier. However, the age of the Giral mine section of the Barmer Basin was determined in this study using palynomorphs.
For the Rajasthan Basin, the palynological analysis of the Giral mine section (Barmer
Basin) indicates an Early Eocene age. Dinocysts are present in a few shale
layers within the Giral lignite section. The dinocyst assemblage is comprised of
Distribution of a group of palynomorph taxa in the Surkha lignite mine, Cambay Basin.
Distribution of a group of palynomorph taxa in the Matanomadh lignite mine, Kutch Basin.
Overall, 33 samples from the Giral lignite mine were processed and studied for
the palynomorph assemblage (Fig. 3). The mine section is rich in both
terrestrial and marine flora. Some of the palynoflora recovered are
Distribution of a group of palynomorph taxa in the Panandhro lignite mine, Kutch Basin.
In the stratigraphic order, they are
The lowermost lignite and intervening shale units. Thickness is about 7 m.
From sample no. RG-12/1 to RG-12/10.
In relation to the overlying assemblage, the litho-unit of this cenozone is
poor in pollen yield. All the samples were studied but only sample number
RG-12/1 yielded enough palynomorphs for quantification.
The lower limit is not traceable.
The upper contact is the overlying 70 cm thick shale bed.
Lignite- and marine-fossil-bearing shale unit. Thickness is about 8 m.
From sample no. RG-12/11 to RG-12/22.
Percentage distribution, diversity indices and RBF morphogroup of
foraminifera in samples from the
This cenozone is comprised of marine and terrestrial palynomorphs. The
cenozone is named after the first occurrence of stratigraphically important
dinocyst
First appearance of
Significant increase in the percentage of
Alternate lignite and shale beds. Thickness is about 10 m.
From sample no. RG-12/22 to RG-12/33.
This cenozone is comprised of a high percentage of different species of
Lignite seam above the marine-fossil-bearing shale unit.
The overlying bentonite just above the last lignite seam in the exposure.
Out of the eight samples, only samples from five horizons yielded
palynomorphs. Palynomorphs ascribed as pteridophytic spores have been
assigned to six genera and eight species, while 12 genera and 22 species have
been identified as angiosperm pollen. Spores, hyphae and microthyraceous
fruiting bodies constitute the fungal remains. The angiosperms dominate the
assemblage with a low percentage of pteridophytes. The gymnospermous pollen
grains are absent in the assemblage. The assemblage has been divided into two
cenozones viz. (i)
Lignite with shale intercalations at 3 m from the base. Thickness is about 4 m.
From sample no. BN 12/1 to BN 12/3.
This cenozone is marked by a high percentage of
The lower limit is not traceable.
Significant decrease in the percentage of
Comprised of carbonaceous shales. Thickness is about 4 m.
From sample no. BN 12/5 to BN 12/7.
This cenozone is marked by an appreciable decrease in the percentage of
Appearance of dinoflagellates.
Bentonite layer.
Palynological analysis of 23 samples from the Matanomadh mine section
was accomplished. The lower part of the mine section includes pollen and
spores with sparse dinocysts. Well-preserved and abundant dinocysts
characterize the top part of the section. The entire lithocolumn is dominated
by dinocysts, followed by the pollen of the
The lowermost lignite and intervening shale unit. Thickness is about 8 m.
From sample no. KM-12/19 to KM-12/23.
In relation to the overlying assemblage, the litho-unit of this cenozone is
poor in pollen yield. All the samples were studied but the pollen diagram
shows the presence of palynomorphs only at sample number KM-12/23, as the
yield was not enough for quantitative analysis from other samples. In the
samples at other depths,
The lower limit is not traceable.
The upper contact is the overlying green shale bed.
Green shales just above the second lignite seam. Thickness is about 3 m.
From sample no. KM-12/17 to KM-12/7.
The cenozone is characterized by the absolute dominance of dinocysts with the
presence of
The second lignite seam.
The upper contact is the overlying calcareous mudstone.
Calcareous mudstone. Thickness is about 4 m.
From sample no. KM-12/11 to KM-12/16.
This cenozone lies above the green shale layer.
Here, mangrove pollen,
The unfossiliferous zone just above the green shales.
The upper contact is not discernible.
We have analyzed a total of 16 samples; however, only few yielded good
results. The overall assemblage consists of angiosperms, pteridophytes,
dinocysts and fungal remains. Only one or two specimens of dinoflagellate
The profile has been divided into three cenozones for the convenience of
study (Fig. 6). The cenozones are as follows:
Lower lignite is having a thickness of about 8 m.
From sample no. KP-12/1 to KP-12/8.
This cenozone is poorly fossiliferous. The cenozone is named after the
occurrence and abundance of
The basement is not exposed.
The upper contact is
Lignite and overlying highly carbonaceous shale thickness of about 12 m.
From sample no. KP-12/10 to KP-12/18.
The angiosperm pollen predominantly present in the assemblage is comprised of
Pteridophytes and fungal fruiting bodies were also observed in this cenozone.
This zone lies above the thin band of dark grey carbonaceous shale.
The upper contact is the overlying carbonaceous shale bed with sand lenses.
Carbonaceous shales and calcareous green shale, about 15 m thickness.
From sample no. KP-12/20 to KP-12/31.
The cenozone is characterized by the presence of both terrestrial and marine
palynomorphs. The assemblage is comprised of the taxa
The carbonaceous shale with sand lenses. Dinocysts were found from this level.
The upper contact is not discernible.
The lignite mine sections of the Giral and Surkha are devoid of foraminifera. However, the Matanomadh and Panandhro mine sections are comprised of remarkably well-preserved foraminifera. The characteristics of foraminifera from these marginal marine settings are described below.
A bloom of planktic foraminifera
The diversity of foraminifera (planktic plus benthic) remains low in
the Matanomadh (Fig. 11b; Fisher alpha:
The RBF percentage in the Matanomadh mine section is quite high throughout (
The Indian subcontinent began its northward journey after its separation from the Gondwanaland around the Cretaceous (P. Kumar et al., 2007). The subcontinent carried flora and fauna from Africa to Asia. The Deccan volcanism covered the western and central portions of the subcontinent with basaltic lava flows, rendering it inhospitable for the growth of flora and fauna on the mainland. Following this, during the Late Paleocene, extensive weathering occurred, which gave rise to the soil cover in the subcontinent. The phase of sediment deposition occurred around the Early Eocene, coinciding with the marine transgression along the Kutch, Cambay and Rajasthan basins. Major parts of the mainland were covered with marine sediments ranging from glauconitic shales to limestones; however, some restricted marine areas along the coast gave rise to lignite deposits comprised dominantly of angiosperms (Sarkar et al., 2012; Saraswati et al., 2012).
In the Early Eocene, India was positioned at the Equator (Ocean Drilling
Stratigraphic Network, 2004), and the tropical climate enhanced the
production of mangrove forests along the western coast of India (Fig. 1a).
The modern analog of these are the Sunderbans
mangrove forests, along the Ganges–Brahmaputra Delta on the eastern coast of
India. The Early Eocene sections of the Surkha and Giral mines consist of
palynofloral assemblage belonging to the
The lignite seams from both the Giral and Surkha mines are dominant in tropical
rainforest elements (
Similarities and differences between the Early and Middle Eocene lignites from western India based on diversity, abundance of various groups of palynomorphs and foraminifera.
The Indian continent was placed within the tropical belt during the Middle Eocene
(Fig. 1b). The basal part consisting of lignites in both mine
sections of the Kutch Basin exhibits a dominance of
The Early Eocene sections from Tasmania, Tanzania, Africa and Egypt have
exhibited a tropical mangrove forest domination (Contreras et al., 2014).
Similarly, the Early Eocene sections of the Cambay and Barmer basins in India
also indicate an overall domination of mangrove vegetation
(
A gap in sedimentation exists from the late Early Eocene to late Middle
Eocene and is observed across basins in India, Pakistan and Australia,
referred to as the Lutetian gap, and is also recorded in the Kutch Basin
(McGowran et al., 2004; Saraswati et al., 2018; Fig. 2). The mangroves
continued to flourish in the tropics in the Middle Eocene; however, in the
high latitudes, the tropical vegetation which prevailed in the Early Eocene
was replaced by temperate flora, e.g., the growth of temperate
The Early and Middle Eocene palynomorph assemblages from lignite mines in
western India represent an overall tropical vegetation pattern; however, our
current study further pointed out the following similarities and differences
in terms of paleovegetation and paleodepositional conditions (Table 1).
During the Early Eocene, angiosperms like
The data is provided in Sect. 2 of the Supplement; the micropaleontological slides comprising the foraminifera are in the microscopy lab, Department of Earth Sciences, Indian Institute of Technology Kanpur; the palynomorph slides are housed in the Department of Geology, K. J. Somaiya College of Science and Commerce, University of Mumbai.
The supplement related to this article is available online at:
SK wrote the manuscript and carried out foraminiferal analysis. JS carried out palynomorph analysis.
The authors declare that they have no conflict of interest.
Sonal Khanolkar and Jyoti Sharma thank the Department of Science and Technology, India (project nos. DST/INSPIRE/04/2016/002525 and DST-WOSA PSR/WOS A/ES_31/2011) for providing the funding for carrying out this research work. Jyoti Sharma and Sonal Khanolkar are grateful to the Indian Institute of Technology Bombay and Indian Institute of Technology Kanpur for providing the facilities to carry out this work. We are thankful to the two anonymous reviewers and the handling editor Francesca Sangiorgi for the constructive comments which helped us improve the manuscript. We would like to dedicate this paper to Pratul Kumar Saraswati , IIT Bombay, who has been a motivation for our work and has played an important role in revising the Paleogene biostratigraphy of western India. Sonal Khanolkar would like to thank Manudeo Singh for help with the preparation of figures. Edited by: Francesca Sangiorgi Reviewed by: two anonymous referees