Foraminiferal biostratigraphy of lignite mines of Kutch, India: Age of lignite and fossil vertebrates

Pratul Kumar Saraswati , Sonal Khanolkar Dalta Surya Narayana Raju, Suryendu DuttaSantanu Banerjee

1.Department of Earth Sciences, IIT Bombay, Mumbai 400076, India

2.M.S.R.Residency, Pragati Marg, Rajahmundry 533103, India

1 Introduction*

The Cenozoic succession of Kutch, West India is known for its commercial deposits of lignite.Stratigraphically,lignite deposits referred to the Naredi Formation (Table 1)were initially assigned to the Early Eocene (Biswas and Raju, 1971)and later revised to the Late Paleocene to the Early Eocene in age (Biswas, 1992).The recent biostratigraphic study refutes the revised age of the formation and suggests that it ranges from the shallow benthic zone SBZ 5/6 to SBZ 11 of the Early Eocene (Saraswatiet al., 2012).In the type locality the Naredi Formation is not associated with lignite and in the mine section of Panandhro no def inite age could be assigned to lignite due to poor faunal control.However, due to the subsurface occurrence of lignite just south of the type locality of the Fulra Limestone (Middle Eocene)and the absence of the characteristic foraminiferNummulites obtususof the Harudi Formation, probably led earlier workers to believe that the lignite belongs to the Naredi Formation and it is of Early Eocene age.

The Eocene period was marked by hyperthermal events in the Early Eocene (Zachoset al., 2001)and the Middle Eocene (Bohatyet al., 2009).Several hyperthermal events lasting for a few tens of thousands of years are recorded in the Early Eocene, prominent among which are the PETM(Palaeocene Eocene Thermal Maximum), ETM2 (Eocene Thermal Maximum 2)and EECO (Early Eocene Climatic Optimum).The hyperthermal event of the Middle Eocene lasted for about 750 kyrs and is known as the MECO (Middle Eocene Climatic Optimum).In western India, lignite deposition corresponding to the Early Eocene warming event is found in the Cambay Basin near Surat (Punekar and Saraswati, 2010).A carbon isotope excursion reported in this section is referred to one of the events, called ETM2(Clementzet al., 2011).Lignite deposition was extensive during this period in western India and the Indus Basin in Pakistan.In Pakistan lignite occurs in the Bara Memberand Lakhra Member of the Ranikot Formation.The former is biostratigraphically assigned to planktic foraminiferal zone P4 and the latter to P4-P7 (Jones, 1997; Afzalet al., 2009).The Early Eocene lignite in western India occurs in Cambay Basin, Barmer Basin, Bikaner Basin and Kutch Basin (assuming the presently held age of lignite).In comparison, the lignite deposits of the Middle Eocene are few.The lignite of Barmer and Bikaner occur in formations that extend to the Middle Eocene in age (Raju and Mathur, 2013).Further detail is not available if it is of Lutetian or continues to the Bartonian.In Pakistan, a minor occurrence of lignite is reported from the upper part of the Domanda Formation (nannoplankton zone NP15-NP16;Afzalet al., 2009).While there are a number of lignite deposits corresponding to the Early Eocene hyperthermal events in this part of the subcontinent, was there lignite deposition during the Middle Eocene warming period in the early Bartonian?

Table 1 The Eocene stratigraphy of Kutch, India

The stratigraphy of the lignite mine section also has implication for the age of the fossil vertebrates recorded from the mine.An assemblage comprising snakes, fishes,turtles, crocodiles and whales was recorded from the strata overlying lignite in Panandhro mine (Bajpai and Thewissen, 2002; Rageet al., 2003).Considering the Early Eocene age of the lignite, the vertebrate fossils in the overlying strata were also assumed to be of the same age and,as a result, some of them were initially regarded to be the earliest representatives of respective fauna in Asia or the Indian subcontinent (Rageet al., 2003).Later, Thewissen and Bajpai (2009)supposed the Panandhro fauna to have been washed down from the overlying Harudi Formation and thus assigned them a Lutetian age.In our recent study of the stratigraphically equivalent Matanomadh mine section we observed the Middle Eocene larger benthic foraminifera comparable with those of the Fulra Limestone(of Bartonian age)directly overlying lignite (Khanolkar and Saraswati, in press).This led us to re-investigate the Panandhro mine section to establish the stratigraphic position of lignite with the aim to (1)determine the period of lignite deposition in relation to the Middle Eocene warming interval and (2)verify the reported age of the vertebrate fossils.

2 Methodology

The rock samples were collected from Panandhro(23°42’N; 68°46’E)and Matanomadh (23°30’N; 68°57’E)(Figure 1)mines and macerated to separate foraminifera by standard procedure.The litholog, sample positions and foraminiferal taxa in the two mine sections are shown in Figure 1.Scanning electron micrographs and oriented sections of the biostratigraphically important species are illustrated in Figure 2.

3 Results

The lignite-bearing succession overlies the Deccan basalt.In Panandhro mine, it consists of sandstone, carbonaceous shale and lignite in the lower part and shale,siltstone and limestone in the upper part.The shale interbedded with lignite is practically barren and did not yield any age-diagnostic foraminifera.The shale immediately overlying lignite consists ofNummulites spectabilis, N.pinfoldiandN.cf.discorbinus.These Middle Eocene species are reported from the upper part of the Harudi Formation in other sections of Kutch (Samantaet al., 1990;Saraswatiet al., 2000).Nummulites obtusus, a marker of Harudi Formation, is absent.The planktic foraminifera includeStreptochilus martiniandJenkinsina columbiana.The co-occurrence of these species suggests planktic foraminiferal zone P12-P13 (Berggrenet al., 1995)for the lowermost beds of the post-lignite succession.Orbulinoides beckmanniis recorded up the section, referring it to the planktic foraminiferal zone P13.The topmost sample of the mine section has yieldedSubbotina gortanii, suggesting zone P14.The upper part also containsNummulites discorbinus, Discocyclina dispansa,Discocy-clina sowerbyi,CalcarinaandLockhartia alveolata.The foraminiferal assemblage corresponds to that of the Fulra Limestone.It is of signif i cance thatNummulites burdigalensis,Assilina laxispiraandAssilina spinosatypical of the Naredi Formation are not present in the mine section.The post-lignite sedimentary succession is thus of Middle Eocene age and referred to the Harudi Formation and Fulra Limestone.The absence ofN.obtususand the occurrence ofN.spectabilisimmediately above the lignite indicate that only the upper part of the Harudi Formation is present in the mine section.As discussed below, it is referred to Bartonian Stage.The lignite is therefore directly overlain by the Bartonian sediments in the Panandhro mine section.In Matanomadh mine, the strata above the lignite containsHalkyardia minima, Linderina kutchensisandLockhartia alveolatathat are typical larger benthic foraminifera of the Fulra Limestone (Khanolkar and Saraswati, in press).It also contains the planktic foraminiferOrbulinoides beckmanni, referring the bed to zone P13.Lignite is thus directly overlain by Bartonian sediments both in Panandhro and Matanomadh mine sections.There is no sedimentological break between the lignite and the overlying sediments and therefore it is interpreted that the upper age limit of the lignite is early Bartonian.In absence of a definite biostratigraphic control, the lower age limit of lignite at best may extend to late Lutetian.

4 Discussion

The lignite-bearing successions of Panandhro and Matanomadh were deposited in restricted and isolated basins(Biswas, 1992).The restricted setting continued till the entire basin was f l ooded at the peak of the Bartonian transgression.A rapid rise of sea-level recorded in the Harudi Formation (Figure 3; Banerjeeet al., 2012), f l ooded the restricted parts and a continued rise of sea-level led to uniform deposition of shelf carbonate across the basin during the zone P13.The carbonates in the upper part of the mine section represent this phase of the sedimentation.The zone P13 is known to represent a major maximum f l ooding surface in several pericratonic basins of India (Raju, 2008).

The age of lignite constrained in the present study is of signi fi cance for the Middle Eocene climate reconstruction.The hyperthermal event of the Middle Eocene (MECO)lasted for about 750 ka, and a minimum δ18O coinciding with increasing abundance ofOrbulinoides beckmanni(Edgaret al., 2010).Biostratigraphically, this period falls in the upper part of the lignite and the lower part of the overlying sediments, implying humid climate at the onset of the warming event in the Middle Eocene.The palynological assemblage of the Matanomadh mine section is dominated by tropical angiospermic pollen (Duttaet al.,2011).It is inferred that trees of humid tropical climate thrived in the region that acted as source materials for lignite (Lakhanpalet al., 1984; Kar, 1985).The occurrence of pollen of Dipterocarpaceae and Bombacaceae in the Matanomadh lignite-bearing sediments indicates proximity of the depositional site to an evergreen rain forest (see Duttaet al., 2011).Biostratigraphically constrained stable isotope data is not available to estimate the temperature in Kutch during this interval but isotopic composition in the overlying Fulra Limestone indicates temperatures ranging from 24℃ to 28℃ (Saraswatiet al., 1993), implying the possibility of a still higher temperature during the MECO.The lignite deposits of Cambay Basin and Kutch Basin correspond to two different hyperthermal events of the Early and Middle Eocene respectively (Figure 4)and suggest that humid climate prevailed during both of the warming periods.Judging by the stratigraphic distribution of lignite beds, it seems that while the humid climate was widespread in this region and continued through the multiple warming events of the Early Eocene, data are limited to infer these characteristics for the Middle Eocene period.

The vertebrates of Panandhro mine were recorded at about 10 m above the lignite (as per Figure 1 of Bajpai and Thewissen, 2002).Among the diverse assemblage of fossil vertebrates, the occurrence of whaleKutchicetus minimusand the snakePterosphinuswere considered as signi fi cant findings.Two of the reported species ofPterosphinuswere regarded as the earliest representatives of the genus.The authors seem to have assumed the beds overlying the lignite to be a continuation of the Naredi Formation and based on the known age of the formation referred the recorded fossils to the Early Eocene.Later Thewissen and Bajpai(2009)stated that the Panandhro mine stratigraphy is more complex and that the previously reported fossil specimens were washed down from the overlying Harudi Formation.The age of the fauna was consequently revised to Lutetian.Both the initial and the revised age of the Panandhro fauna lacked the biostratigraphy of the particular section.Recently, strontium isotopic analysis of mollusc and shark tooth from the mammal-bearing strata of the Harudi Formation, including that of the Panandhro mine, led the authors to propose a late Lutetian age of the fossil vertebrates(Ravikant and Bajpai, 2010).Large errors in strontium isotopic values between 41 Ma and 42.5 Ma in the above study (±2 Ma error in age estimation)and limitations of strontium isotope stratigraphy for the Eocene time interval necessitate biostratigraphic validation.The present study refers the fossil-bearing strata of Panandhro to planktic zone P12-P13 and the Bartonian Stage.This validates the strontium stratigraphy within the limit of the analytical error.Some of the estimated ages (see Table 1; Ravikant and Bajpai, 2010)are close to the base of the Bartonian (41.2 Ma as per Gradsteinet al., 2012).Therefore, the age of the Panandhro fauna is indisputably Bartonian and more so when the authors report them to have been washed down from the overlying beds.

It is highly probable that a major part of the Harudi Formation is of Bartonian age.The formation is readily identif i ed in the field by its characteristic foraminiferN.obtusus, occurring ~8 m above the base.The zonal speciesO.beckmanni(P13)occurs ~2 m above the occurrence ofN.obtususin the section.The stratigraphic section (Figure 2)explicitly shows that the two species occur during the rapid rise of sea level,Nummulitesoccurring expectedly at a relatively shallower depth compared with the planktic foraminiferOrbulinoides.Samanta (1981)had correlated theN.obtusus-bearing bed withGloborotalia lehneriZone(P12).While assigning the zone, Samanta (1981)showedO.beckmannito be restricted to the Fulra Limestone, but its occurrence in the Harudi Formation is now well-established.Racey (1995)foundN.obtususassociated withA.exponensandN.discorbinusin Oman and therefore suggested a stratigraphic range of the species from mid to late Lutetian.But it is of signif i cance that both of the latter species occur in the Fulra Limestone (Zone P13)and thus their ranges extend to the Bartonian.The stratigraphic setting and the associated fauna suggest a Bartonian age forN.obtususin its type locality of Kutch.The previous workers have already provided calcareous nannofossil evidence of Bartonian age for the Harudi Formation (Singh and Singh, 1991; Jafar and Rai, 1994).Although the lower few meters of the formation is devoid of age diagnostic taxa, the presence of zonal species of planktic foraminifera and calcareous nannofossils in the remaining part are strongly in favour of a Bartonian age of the formation.It is of signif i cance that Gingerichet al.(2001)in their review of fossil whales from Pakistan-India region correlated the Harudi Formation of Kutch with the Pir Koh Formation of Bartonian age of Pakistan.The Harudi Formation and the fossil whales reported from this formation should in view of the above be assigned a Bartonian age.

5 Conclusions

The foraminiferal evidence in the present study has constrained the upper age limit of the lignite to the early Bartonian.Its lower age may extend to the late Lutetian.The occurrence of lignite in the early Bartonian, corresponding to the Middle Eocene Climatic Optimum (MECO), is suggestive of humid climate at the onset of warming.The fossil vertebrates of Panandhro mine are of early Bartonian age.The age of the fossil vertebrates of the Harudi Formation may also be revised, because the biostratigraphic evidences and regional stratigraphic setting are strongly in favour of the Bartonian age for the major part of the Harudi Formation.

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