Geol. Mag. 155 (4), 2018, pp. 907–920. c Cambridge University Press 2016. This is an Open Access article, distributed 907 under the terms of the Creative Commons Attribution licence (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted re-use, distribution, and reproduction in any medium, provided the original work is properly cited. doi:10.1017/S0016756816001114 The first record of the Permian Glossopteris flora from Sri Lanka: implications for hydrocarbon source rocks in the Mannar Basin ∗ ∗ G. EDIRISOORIYA , H.A. DHARMAGUNAWARDHANE & STEPHEN MCLOUGHLIN† ‡ ∗ Department of Geology, University of Peradeniya, Peradeniya, Sri Lanka †Department of Palaeobiology, Swedish Museum of Natural History, Box 50007, S-104 05, Stockholm, Sweden (Received 8 July 2016; accepted 9 November 2016; first published online 13 December 2016) Abstract – Strata exposed near Tabbowa Tank, Tabbowa Basin, western Sri Lanka have yielded the first representatives of the distinctive Permian Glossopteris flora from that country. The assemblage includes gymnosperm foliage attributable to Glossopteris raniganjensis, roots referable to Ve r t e b - raria australis, seeds assigned to Samaropsis sp., sphenophyte axes (Paracalamites australis)and foliage (Sphenophyllum emarginatum), and fern foliage (Dichotomopteris lindleyi). This small mac- roflora is interpreted to be of probable Lopingian (late Permian) age based on comparisons with the fossil floras of Peninsula India. Several Glossopteris leaves in the assemblage bear evidence of ter- restrial arthropod interactions including hole feeding, margin feeding, possible lamina skeletonization, piercing-and-sucking damage and oviposition scarring. The newly identified onshore Permian strata necessitate re-evaluation of current models explaining the evolution of the adjacent offshore Man- nar Basin. Previously considered to have begun subsiding and accumulating sediment during Jurassic time, we propose that the Mannar Basin may have initiated as part of a pan-Gondwanan extensional phase during late Palaeozoic – Triassic time. We interpret the basal, as yet unsampled, seismically reflective strata of this basin to be probable organic-rich continental strata of Lopingian age, equi- valent to those recorded in the Tabbowa Basin, and similar to the Permian coal-bearing successions in the rift basins of eastern India and Antarctica. Such continental fossiliferous strata are particu- larly significant as potential source rocks for recently identified natural gas resources in the Mannar Basin. Keywords: Glossopterids, plant–insect interactions, petroleum systems, Gondwana, continental break-up. 1. Introduction isolated parts of low-palaeolatitude Gondwana (e.g. Morrocco; Broutin et al. 1998) have been assigned to Glossopteris was formally defined by Brongniart Glossopteris, but these records need thorough taxo- (1828), and the genus has been emended subsequently nomic re-appraisal. Leaves attributed to Glossopteris by several workers. This fossil-genus of tongue-shaped in areas remote from Gondwana, for example the Rus- leaves with reticulate venation characterizes contin- sian Far East (Zimina, 1967), together with examples ental strata throughout the middle- to high-latitude re- from much younger strata, such as Triassic (John- gions of the Gondwanan Permian (McLoughlin, 2001, ston, 1886) or Jurassic (Harris, 1932; Delevoryas, 2011). Indeed, the distribution of the genus was cent- 1969), undoubtedly belong to other plant groups that ral to early arguments that the Southern Hemisphere evolved morphologically similar leaves (Harris, 1937; continents were formerly united into a supercontin- Delevoryas & Person, 1975; Anderson & Anderson, ent (Gondwana) during late Palaeozoic time (du Toit, 2003). 1937). Glossopteris has so far been recorded through- Apart from its palaeobiogeographic importance, the out the core regions of Gondwana (India, Africa, Glossopteris flora contributed to the accumulation Arabia, Madagascar, South America, Australia, Ant- of vast economic coal resources across the South- arctica and New Zealand) with the notable excep- ern Hemisphere that are exploited and distributed tion of Sri Lanka (McLoughlin, 2001). In addition, a globally for the generation of electricity, metal refin- few leaves in peri-Gondwanan terranes, for example ing and other industrial purposes (Balme, Kershaw Thailand (Asama, 1966), Turkey (Archangelsky & & Webb, 1995; Maher et al. 1995; Rana & Al Hu- Wagner, 1983), Laos (Bercovici et al. 2012) and in maidan, 2016). Buried Permian plant remains (princip- ally wood, spore/pollen and leaf material converted to ‡Author for correspondence: [email protected] type II and III kerogens) also constitute a major source Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.139, on 28 Sep 2021 at 06:45:08, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756816001114 908 G. EDIRISOORIYA, H.A. DHARMAGUNAWARDHANE & S. MCLOUGHLIN of both conventional and coal-seam gas (McCarthy Permian-Mesozoic Pedro 1 Cauvery et al. 2011), which are exploited widely in nations a basin Basin Exploration well (dry) occupying the ancient Gondwanan landmass (Flores, Delft 1 Exploration well (gas) 2014). Palk Bay 1 Precambrian Records of plant macrofossils from Sri Lanka are Pesalai 1-3 crystalline rocks sparse. Previous studies have identified only Juras- 9° Pearl 1 Wanni N sic plants (Sitholey, 1944; Edirisooriya & Dharma- Mannar Complex gunawardhane, 2013a, b), and these have been Basin Dorado & Dorado N. recovered from outcrops in small isolated graben sys- Miocene limestone Barracuda tems in the northwestern part of the island. No Per- Tabbowa Basin mian fossils or strata have been reported from Sri Aruwakkalu 8° Lanka so far, although Katupotha (2013) speculated Andigama Basin Vijayan Pallama Basin that some topographic planation features and isolated Complex erratic boulders might date back to an ancient gla- cial episode, the last of which affected this region Kadugannawa Colombo Complex during early Permian time (Fielding, Frank & Isbell, 7° 2008). 2000 m 1000 m Highland Here we describe the first assemblage of the Glos- India-Sri Lanka Maratime Boundary Complex sopteris flora from Sri Lanka derived from the first 100 km Southern Basin documented exposures of Permian strata in the Tab- 6° bowa Basin. In addition to Glossopteris, the flora con- 79°E 80° 81° 82° tains the remains of ferns and sphenophytes that help b constrain the age of the deposit. We also highlight the N potential significance of this discovery for hydrocar- bon source rocks in the adjacent (offshore) Mannar Basin. 2. Geological setting 55 Crystalline 30 Around 90 % of the bedrock in Sri Lanka consists of rocks Precambrian crystalline rocks. Jurassic, Miocene and 30 Quaternary strata are largely confined to a thin belt of exposures in the north and northwest of the is- Tabbowa Tank land (Herath, 1986; Fig. 1a). The fossiliferous strata Crystalline sampled in this study are exposed close to the spill- rocks way of an irrigation reservoir – the Tabbowa Tank (8° 04ʹ 21ʺ N, 79° 56ʹ E) – within the Tabbowa Basin 8°03'N (Fig. 1a, b). The Tabbowa Basin is a small graben pre- 30 60 viously considered to host only Jurassic strata. The 60 basin extends over an area of only a few square kilo- 20 To Anuradhapura metres in the Northwestern Province of Sri Lanka (Wayland, 1925; Cooray, 1984) and has a surface to- 20 30 pography of slightly elevated but flat terrain. The graben developed in a crystalline basement terrane 30 30 and hosts nearly 1000 m of strata consisting mostly To Puttlum Crystalline rocks of grey to reddish shales, mudstones, siltstones and Strike and dip of bedding Foliation orientation arkosic sandstones (Cooray, 1984). A similar succes- Fault (approximate) Fossil locality sion is developed 30 km to the south of Tabbowa in the 0 4 km Watercourse Andigama–Pallama Basin, which hosts at least 350 m Road Tabbowa Basin strata 79°58'E of strata (Herath, 1986). The surrounding Precambrian (mostly Jurassic) basement rocks comprise mainly granitic gneisses with extensive jointing and faulting (Wayland, 1925;Co- Figure. 1. (Colour online) Maps showing: (a) the major geolo- oray, 1984). Many of these structural features are par- gical provinces of Sri Lanka and (b) the sample location (indic- allel or orthogonal to the local graben systems and may ated by a star) adjacent to Tabbowa Tank (modified after Way- land, 1925 and Ratnayake & Sampei, 2015). have developed in association with Permian–Triassic extension and Mesozoic break-up of Gondwana (Co- oray, 1984). 45 000 km2. This basin contains >6000 m of strata and In the offshore region between northwestern Sri is covered by water to depths generally greater than Lanka and southern India (Tamil Nadu) lies the 1000 m (Premarathne et al. 2013). The sedimentary Mannar Basin (Fig. 1a), covering an area of about development of this basin is poorly understood since Downloaded from https://www.cambridge.org/core. IP address: 170.106.40.139, on 28 Sep 2021 at 06:45:08, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms. https://doi.org/10.1017/S0016756816001114 First Glossopteris from Sri Lanka 909 there have been relatively few seismic surveys and no 4. Systematic palaeobotany hydrocarbon exploration wells have penetrated the full Order Equisetales Dumortier, 1829 stratigraphic succession. To date, the succession has Family uncertain been subdivided into four