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Citation: Srivastava G, Mehrotra R. Low latitude floral assemblage from the Late Oligocene sediments of Assam and its palaeoclimatic and palaeogeo-

graphic significance. Chin Sci Bull(Chin Ver), 2013, 58(Suppl.Ⅰ): 156–161, doi: 10.1360/tb-2013-suppl011

《中国科学》杂志社 SCIENCE CHINA PRESS Article

Low latitude floral assemblage from the Late Oligocene sediments of Assam and its palaeoclimatic and palaeogeographic significance

SRIVASTAVA Gaurav* & MEHROTRA Rakesh Chandra Birbal Sahni Institute of Palaeobotany, 53 University Road, Lucknow 226007, India

*Corresponding author (email: gaurav_jan10@yahoo.co.in)

Received April 11, 2013; accepted August 8, 2013

There are many outcrops of the Tertiary sediments in northeast India and a large number of plant megafossils have been described from there. Among all, the Late Oligocene sedimentary basin of Makum Coalfield is very important because there is no other exposure having such a rich palaeofloral assemblage not only from northeast India but also from the whole country. The assemblage is also important to confirm whether the suturing between the Indian and Eurasian plates was complete to facilitate plant migration or not. All the reported plant fossils clearly indicate the prevalence of tropical climate in the region during the deposition of the sediments. As the majority of taxa occur in tropical evergreen to moist deciduous and littoral and swampy forest, a warm and humid climate may be envisaged in Upper Assam during the Late Oligocene. The abundance of palms and pantropical megathermal plant families in the Makum Coalfield indicate that the cold month mean temperature (CMMT) was not less than 18°C. The quantitative palaeoclimate reconstruction indicates a monsoonal climate during the period with the same intensity as that of the modern day. The absence of Southeast Eurasian elements in the fossil assemblage provides clear evidence that suturing between the Indian and Asian plates was not complete till the Late Oligocene. Several modern analogues of the fossil taxa are now endemic to the Western Ghats which lies in the same palaeolatitude.

northeast India, Makum Coalfield, megafossil, monsoon

The Late Oligocene (Chattian 28.4–23 Ma) is considered as the last significant global warmth period on the earth [1]. In India, the Makum Coalfield (27°15′–27°25′N; 95°40′– 95°55′E) is the only coalfield which contains an exposure of Late Oligocene (Chattian 28.4–23 Ma) sediments [2]. It is situated in the Tinsukia District of Assam in northeast India [2] and is an important basin because of its rich and diversified plant assemblage [3–8]. In fact, there is no other Oligocene sedimentary basin in India which con-tains such a rich assemblage. The basin was situated at a low palaeolatitude, i.e., ~10°–15°N [9] at the time of sediment deposition, and during the period the land con-nection between the Indian and Eurasian plates was not sufficiently complete to facilitate the plant migration [10]. The basin shows a deltaic, mangrove or lagoonal deposi-tional environment [3,5,11].

The Makum Coalfield is made up of six collieries, namely Baragolai, Ledo, Namdang, Tikak, Tipong and Tirap which are lying in between the latitudes 27°15′– 27°25′N and longitudes 95°40′–95°55′E and are located

along the northern flank of the Patkai range.

1 The Makum Coalfield and its geological setting

The Makum Coalfield belongs to the Tikak Parbat For-mation considered to be Late Oligocene (Chattian 28.4–23 Ma) in age on the basis of regional lithostratigraphy [12], remote sensing [13] and biostratigraphic controls [2,11]. The formation has five coal seams confined within the basal 200 m section [14]. Of the five seams only seam nos 1 (18 m thick) and 3 (6 m thick) have been exploited throughout the field.

The Tikak Parbat Formation consists of alternations of sandstone, siltstone, mudstone, shale, carbonaceous shale, clay and coal seams [14], while the plant remains are mainly restricted to the grey carbonaceous and sandy shales. The formation is underlain by 300 m of predominantly mas-sive, micaceous or ferruginous sandstone that forms the Baragolai Formation, which is successively underlain by 1100–1700 m of Naogaon Formation which consists of

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thin-bedded fine-grained quartzitic sandstones with thin shale and sandy shale partings [15]. Together the three formations represent the Barail Group (Table 1). In the Barail Group, there occurred an upward trend of marine to non-marine palaeoenvironment indicating the infilling of a linear basin on the eastern edge of the Indian plate. The detailed sedimentary information of the Tirap mine sec-tion was provided by Kumar et al. [11].

2 Plant fossil assemblage

The Makum Coalfield is very important in view of its di- versified and rich assemblage of plant fossils and there is no other comparable Oligocene sedimentary basin in India. The modern plant taxa comparable to the fossils recorded from the Makum Coalfield are Holigarna, Lannea, Man- gifera, Parishia and Semecarpus of the Anacardiaceae, Alphonsea, Saccopetalum and Uvaria of the Annonaceae, Alstonia of the Apocynaceae, Nypa and several other palms belonging to the Arecaceae, Avicennia of the Avi- cenniaceae, Santiria of the Burseraceae, Calophyllum, Garcinia, Kayea, Mesua and Poeciloneuron of the Clusi- aceae, several species of Terminalia of the Combretaceae, Equisetum of the Equisetaceae, Bridelia and Macaranga of the Euphorbiaceae, Butea and several species of Legu- minocarpon of the Fabaceae, Apollonias of the Laura- ceae, Barringtonia of the Lecythidaceae and Pterygota and Sterculia of the Malvaceae. A complete list of the fossils, along with their modern analogues and localities is given in Table 2 and Table S1.

3 Discussion

3.1 Palaeofloristic analysis

The Makum Coalfield is important in view of its high di-versity of plant fossils. The Late Oligocene was the time of last significant globally warm period and the fossil lo-

cality was situated at 10°–15°N palaeolatitude [9]. The diversity bar graph (Figure 1) indicates that Fabaceae was the most dominant family followed by Anacardiaceae, Clusiaceae, Combretaceae, Arecaceae, Annonaceae, Lau-raceae and Sapindaceae etc. Most of the aforesaid families have pantropical distribution, while the abundance of palms indicates high water availability.

3.2 Palaeoclimate analysis

(1) Qualitative reconstruction. For the palaeoclimate reconstruction qualitative as well as quantitative analysis has been done. Qualitative analysis is based on the pal-aeofloristic assemblage and depends on the principle of Nearest Living Relative (NLR) approach. The NLR ap-proach is purely taxonomical in nature and it assumes that a very little or insignificant change has taken place in the studied fossil taxa in relation to climate and is frequently used by many palaeobotanists [16–21]. The NLR taxa of the fossil assemblage from the Makum Coalfield have been enlisted in Table 2 showing their distribution in dif-ferent modern forest types. Among all the families re-ported from the Makum Coalfield (Figure 1), the families like Annonaceae, Burseraceae, Clusiaceae, Combretaceae, Lecythidaceae, Myristicaceae, and Rhizophoraceae are typical pantropical megatherm families [22] whose pres-ence in palaeoflora provides an evidence that the CMT (mean temperature of the coldest month) was at least not less than 18°C. Similarly, the presence of most dominant family Fabaceae [6] whose abundance and richness cova-ry with the temperature [23], indicates warm climate. The occurrence of families Avicenniaceae and Rhizophoraceae is also very significant in terms of the depositional envi-ronment. These families are highly indicative of deltaic, mangrove or lacustrine deposition of coal seams and asso-ciated sediments in the Makum Coalfield. The abundance of palms like Nypa [4] provides clear evidence of a coastal

Table 1 Showing lithostratigraphic sequence in the Makum Coalfield [36]

Age/Group Formation Lithology

Recent/Pleistocene Alluvium Alluvium and high level terraces

Dihing/Pliocene Alternating pebble beds, coarse bluish green felspathic and ferruginous sandstone and grey to brown clay

Tipam/Miocene Girujan Clay Variegated clay, silt clay, bluish green and grey sandstone, coarse, gritty and massive bluish green to grey felspathic and micaceous sandstone

Tipam Sandstone

Variegated clay, sandy clay, shale, coaly streaks, silicified woods and conglomeratic sandstone

Barail/Oligocene Tikak Parbat Hard and light coloured quartose sandstone. Alternations of siltstone, sandy shale, shale, mudstone, car-bonaceous shale and thin impersistent coal seams. Alternation of siltstone, mudstone, carbonaceous shale, clay and workable coal seams. Occasional clayey sandstone, clay and sandy shale

Baragolai Alternation of buff, bluish green and grey coloured sandstone, sandy shale, carbonaceous shale, sandy clay, clay and thin impersistent seams of coal and shaly coal. Hard massive and floggy, grey micaceous and ferruginous sandstone and oilsand

Naogaon Hard compact and floggy dark grey fine-grained sandstone and interbeds of grey splintery shale

Disang/Eocene Dark grey splintery shale with interbeds of dark grey and fine grained sandstone

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Table 2 Distribution of the modern comparable species in different forest types

Taxa Tropical wet

evergreen forest Tropical semi-

evergreen forest Tropical moist

deciduous forest Littoral &

swamp forest Tropical dry

deciduous forest Tropical dry

evergreen forest Holigarna arnottiana + + +

Mangifera indica + + + +

Lannea coromandelica + +

Parishia insignis +

Semecarpus anacardium + + +

S. kurzii +

Alphonsea +

Saccopetalum longiflorum + +

Uvaria hamiltonii +

Alstonia + +

Avicennia officinalis +

Calophyllum +

Garcinia + + +

Kayea floribunda +

Mesua +

Poeciloneuron indicum +

Combretum + +

Terminalia catappa +

T. chebula + + + +

T. coriacea +

T. crenulata + +

Bredelia retusa + + +

Macaranga + +

Butea frondosa + +

Entada phaseoloides + +

Apollonias arnottii +

Neolitsea +

Barringtonia racemosa +

Firmiana colorata + +

Sterculia urens + + +

Pterygota alata + + +

Heynea trijuga +

Memecylon amplexicaule + +

Myristica sylvestris +

Rhizophora mucronulata +

Nephelium + +

Paranephelium xestophyllum + + +

Sapindus emarginatus + + Tetramerista glabra +

Figure 1 Bar diagram showing the diversity of families in the Makum Coalfield, Assam.

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plain environment where both temperature and humidity remain high throughout the year [24]. The Figure 2 also supports the dominance of humid climate during the dep-osition of the sediments.

(2) Quantitative palaeoclimate reconstruction. The qualitative analysis has given some idea about the palaeo-climate of the Makum Coalfield, however, seasonal varia-tions do play a major role in the climate system that is controlled by land-sea distribution, general atmospheric circulation and ocean currents [25]. At present there are two main approaches in practice for the quantitative reconstruc-tion of land palaeoclimate using fossil plants (1) Coexist-ence approach (CoA) [26–28] and (2) Climate leaf analy-sis multivariate program (CLAMP) [2,29–32]. CLAMP and CoA are the two best land plant based palaeoclimate proxies to reconstruct the seasonal variation in tempera-ture and precipitation.

Recently, quantitative palaeoclimate reconstruction of the Makum Coalfield palaeoflora was made by Srivastava

et al. [2]. As CLAMP was not previously calibrated for monsoonal type of climate, Srivastava et al. [2] have cali-brated it for India by covering nearly all types of modern forests, including wet evergreen to semi-evergreen forest from Western Ghats and northeast India, moist deciduous forest from northern India, dry deciduous forest from cen-tral India and mangrove type of forest from Sunderbans. They have used 80 different leaf morphotypes, from the Tirap mine of the Makum Coalfield, which were numeri-cally scored similar to those of the modern forests. Out of 31 leaf physiognomic characters two are the most domi-nant i.e. entire margin (score 100%) and elliptic shape (97.5%) (Figure 3). These two characters indirectly suggest high temperature and rainfall [29]. The CLAMP analysis indicates that mean annual temperature (MAT) 28.3± 3.7°C, warm month mean temperature (WMMT) 34.2± 5.2°C and cold month mean temperature (CMMT) 23.6± 5.5°C. The precipitation estimates suggest a marked sea-sonality in rainfall pattern showing a wet season with 20

Figure 2 Showing different types of forest elements in the assemblage of Makum Coalfield, Assam.

Figure 3 Spidogram showing percentage score of 31 leaf physiognomic traits of the fossil leaves from the Makum Coalfield, Assam.

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times the rainfall of the dry season. The similar pattern can be seen in Sunderbans lying in the modern Ganges/Brah- maputra/Meghna delta. Therefore, it is suggested that the South Asian Monsoon was already established by the Late Oligocene time at an intensity similar to that of today [2].

3.3 Palaeophytogeography

Some of the taxa like Holigarna and Poeciloneuron now endemic to the Western Ghats were present in the Makum Coalfield during the Late Oligocene. They migrated to the Western Ghats due to increased seasonality in temperature

caused by uplifting of the Himalayas and the recession of the Tethys sea during the Neogene [33,34]. Other taxa like Mangifera, Semecarpus, Alphonsea, Uvaria, Combretum, Macaranga, Butea, Pterygota, and Sterculia etc. are still growing in the vicinity of the fossil area suggesting their broad climatic tolerance.

Presence of Mangifera indica in the Late Oligocene sediments of the Makum Coalfield gives an additional support to the earlier view that the oldest fossil record of Mangifera is from India, particularly from its northeastern part [35].

The authors are thankful to the Director, Dr. N.C. Mehrotra, of the Birbal Sahni Institute of Palaeobotany, Lucknow, for giving permission to attend the 3rd Sino-India International Conference. They are also thankful to the organizers for inviting them to attend the conference.

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Supporting Information

Table S1 Showing plant megafossils from the Makum Coalfield, Assam, India

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