Palm Leaves from the Late Oligocene Sediments of Makum Coalfield
Total Page:16
File Type:pdf, Size:1020Kb
Palm leaves from the Late Oligocene sediments of Makum Coalfield, Assam, India Gaurav Srivastava 1,∗, RCMehrotra1 and Hugues Bauer2 1Birbal Sahni Institute of Palaeobotany, 53 University Road, Lucknow 226 007, India. 2Bureau de Recherche G´eologiques et Mini`eres (BRGM) Regolith and Reservoir Geology Division, 3 Avenue C. Guillemin, BP 36009, 45060 Orl´eanscedex 2, France. ∗Corresponding author. e-mail: gaurav [email protected] Two new palm leaf impressions, cf. Iguanura wallichiana and Palmacites makumensis sp.nov.are described from the Makum Coalfield, Tinsukia District, Assam. They belong to the Tikak Parbat For- mation being considered as Late Oligocene (Chattian 28–23 Myr) in age. Their presence, along with the other known fossil records indicates that CMMT (cold month mean temperature) was not less than 18◦C with plenty of rainfall, in the region during the period of deposition. 1. Introduction et al 2011) considers palms to be of Laurasian origin based on generic level phylogenetic study. Palms (Arecaceae) are a diverse group of plants Fossil records of palms are less known from the confined to the tropical and subtropical regions tropics in comparison to those from the middle lat- of the world (Henderson et al 1995). The family itudes (Harley and Morley 1995; Dransfield et al comprises 183 genera with 2500 species (Govaerts 2008). In this paper, we have described two new and Dransfield 2005; Dransfield et al 2005, 2008). leaf impressions of palms from the Late Oligocene Palm species richness is highest in tropical Asia (Chattian 28–23 Ma) sediments of Makum (>1200 species) and the Americas (730 species), Coalfield, Assam which was located at 10◦–15◦N while only 65 species occur in Africa (Dransfield palaeolatitude during the period (Molnar and et al 2008). The family has been placed within the Stock 2009). commelinid clade of the monocotyledons (Chase et al 2006; Davis et al 2006) and composed of five sub-families: Arecoideae, Calamoideae, Cerox- yloideae, Coryphoideae and Nypoideae (Dransfield 2. Geological setting et al 2005, 2008). It is speculated that palms have been originated The Makum Coalfield is the most important coal- in Australasia (including New Caledonia, New field of northeast India as far as the resources of Guinea and New Zealand) (Bremer and Janssen coal and infrastructural facilities are concerned. 2005) though the oldest definite megafossil record This field accounts for nearly 90% of the produc- is from the Late Cretaceous (Turonian) of France tion of coal in this part of the country and has very (Dransfield et al 2008). A recent paper (Couvreur well developed mines. This field lies between the Keywords. Arecaceae; Assam; India; Late Oligocene; Makum Coalfield; fossil palms; Tikak Parbat Formation; northeast India. J. Earth Syst. Sci. 121, No. 3, June 2012, pp. 747–754 c Indian Academy of Sciences 747 748 Gaurav Srivastava et al Figure 1. Map of Assam showing the fossil locality in Tinsukia District. Figure 2. Simplified geological map of Makum Coalfield, Assam (after Ahmed 1996). latitudes 27◦15–27◦25N and longitudes 95◦40– (Raj Rao 1981). The formation has five coal seams 95◦55E (figure 1) and is situated along the outer- confined within the basal 200 m section (Misra most flank of the Patkai range. On the southern 1992). and south-eastern side of the field are hills, which • rise abruptly to heights of 300–500 m from the allu- Seam no. 5 vial plains of the Buri Dihing and Tirap rivers. (2.4 m thick) 1.3–2.5 m abandoned These hill ranges are traversed by the Namdang, Parting 30–40 m Ledopani and Tirap rivers, whose courses expose • Seam no. 4 sections of the coalbearing Tikak Parbat Forma- (1.5 m thick) 1.2–1.8 m, often merges with tion. The Makum Coalfield encompasses Barago- seam no. 3 abandoned lai, Ledo, Namdang, Tikak, Tipong, and Tirap Parting 3–18 m collieries (figure 2). This study is concerned with the Tikak Par- • Seam no. 3 bat Formation being considered as Late Oligocene (6 m thick) 2–7.5 m worked out in (Chattian 28–23 Myr) in age based on regional composite seam 2 sections geology and an array of biostratigraphic controls Parting 38–68 m Palm leaves from the Late Oligocene sediments of Makum Coalfield 749 • Seam no. 2 The affinities of the fossils were traced at the Cen- (2.1 m thick) 1.5–2.6 m tral National Herbarium, Howrah and the Forest Parting 5–20 m Research Institute, Dehradun after comparing • them with the herbarium sheets of the extant Seam no. 1 plants. One of our specimens (sp. no. BSIP 39910) (18 m thick ) 10–20.8 m, worked was destroyed during the extraction. The type and composite seam out in 2 or 3 sections figured specimens are housed in the museum of the Of the five seams only seam nos. 1 and 3 have Birbal Sahni Institute of Palaeobotany, Lucknow. been exploited throughout the field. The Tikak Parbat Formation comprises alter- nations of sandstone, siltstone, mudstone, shale, 4. Systematic description carbonaceous shale, clay and coal seams (Misra 1992). However, the occurrence of plant remains Family: Arecaceae Schultz Sch. 1832 is mainly confined to the grey carbonaceous and Fossil specimen cf. Iguanura wallichiana sandy shales. The Tikak Parbat Formation is underlain by 300 m of predominantly massive, micaceous or fer- ruginous sandstones that comprise the Baragolai Formation, which in turn is underlain by 1100– 1700 m of thin-bedded fine-grained quartzitic sand- stones with thin shale and sandy shale partings that make up the Naogaon Formation (Mishra and Ghosh 1996). Together the three formations comprise the Barail Group (figure 2). 3. Methodology Material for the present study was collected from the sediments of the Tikak Parbat Formation of Tinsukia District, Assam. The specimens were first cleared with the help of a fine chisel and hammer and then photographed in natural low angled light using 10 megapixel digital camera (Canon SX110 and Fuji color 9500). The classification proposed by Read and Hickey (1972) has been followed in assigning the fossil leaves. Read and Hickey (1972) have proposed five basic features of extant palm leaves that can be used alone, or in various combinations to distinguish fossil palm leaves: • Leaf blade and segments plicate (not always apparent in fossil fragments of individual seg- Figure 3. Fossil specimen cf. Iguanura wallichiana (a) A fos- ments). sil leaf showing shape, size and fused leaflets and (b)apor- • Leaf blades pinnately veined and either simple or tion of the fossil leaf showing rachis (white arrow) and fused pinnately compound in form, or palmately veined leaflets (pencil size = 15 cm). and palmatifid in form. • Leaf segments with a firm, consistent mid-vein Description: Leaf large, pinnate; coriaceous in bounded on either side by two orders of parallel texture; preserved lamina length and width about veins. 42.3 and 50 cm respectively; apex and base not pre- • A ligule-like structure (hastula) at the apex of served; rachis strong, about 1.2 cm thick; preserved the petiole (commonly on the adaxial surface, leaflets 14, fused adjacently and attached to the rarely on both surfaces) where the radiating rachis by the entire base, about 2 cm wide on an segments are inserted on the palmate blade. average, reaching about 40 cm in length, each with • A well organized primary costa (the attenuated a firm midrib arching away from the rachis, angle of continuation of the petiole into the blade of divergence narrow acute (25◦–41o), numerous sec- costapalmate and simple blades). ondaries present on either side of the midrib and 750 Gaurav Srivastava et al running parallel to it; spines absent on the exposed fossil into Lakhanpal’s (1966) genus. Sabalites surface and rachis. longirhachis described by Kvaˇcek and Herman (2004) from the Cretaceous of Lower Austria is also Figured specimen: Specimen no. BSIP 39910 very similar to the present fossil, but the presence Horizon: Tikak Parbat Formation of costa makes the difference from the present fos- Locality: Tirap Colliery, Tinsukia District, Assam, sil. Under these circumstances, the present fossil India is being described as fossil specimen cf. Iguanura Age: Late Oligocene (Chattian 28–23 Myr) wallichiana. Number of specimens studied: One well preserved The genus Iguanura Bl. consists of eight species specimen distributed in western Malaysia (Mabberley 1997). Iguanura wallichiana with which the present fos- Affinities: The characteristic features of the fos- sil shows some resemblance is distributed in the sil are: large coriaceous leaf with fused segments, Malaysian rainforest. joining strong rachis by their entire bases and par- allel venation. All these features indicate that the Genus: Palmacites Brongniart 1828 fossil represents a part of the palm leaf. A large Species: Palmacites makumensis, Srivastava, number of palm taxa were observed at the Cen- Mehrotra and Bauer, sp. nov. tral National Herbarium, Howrah and the Forest Research Institute, Dehradun. After making com- parisons with them it is concluded that it is nearer to Iguanura wallichiana (Mart.) Becc. (Herbar- ium sheet no. CNH 492342), but as Read and Hickey (1972) have pointed out that it is diffi- cult to identify specimens of modern palms accu- rately from their leaves alone, no attempt has been made to assign the present fossil palm under the modern genus. The fossil was compared with other similar forms described under various names. Lakhanpal (1964) described a palm leaf from the Tura Sandstone Formation of West Darangiri Coal- field in Garo Hills as Phoenicites with its possi- ble affinities with Phoenix. It cannot be compared with the present fossil due to lack of sufficient details. Bose and Shah (1964) also reported a pin- nate leaf as ?Sabalites sp. from the early Tertiary of Laitryngew in Meghalaya, while Mahabale and Rao (1973) recorded a pinnate palm leaf from the Rajahmundry Sandstone of Bommuru.