Dinoflagellate Cysts from the Lakadong Sandstone, Cherrapunji
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Palaeobolall;l/ 49 (2000) . 461-484 0031-0174/2000/461-484 $2.00 Dinoflagellate cysts from the Lakadong Sandstone, Cherrapunji area: biostratigraphical and palaeoenvironmental significance and relevance to sea level changes in the Upper Palaeocene of the Khasi Hills, South Shillong Plateau, India RAHUL GARG AND KHOWAJA-ATEEQUZZAMAN Birbal Salll1i Institute ofPalaeobotany, 53 University Road, Lucknow 226 007, India. Entail: [email protected]/1 (Received 26 June 2000: revised version accepted 6 November 2000) ABSTRACT Garg R & Ateequzzaman K 2000. DinoOagellate cysts from the Lakadong Sandstone, Cherrapunji area: biostratigraphical and palaeoenvironmental significance and relevance to sea level changes in the Upper Palaeocene of the Khasi Hills, South Shillong Plateau, India. Palaeobolanist 49(3) : 461-484. The biostratigraphical and palaeoenvironmental signi ficance ofdiagnostic Late Palaeocene (Thanetian) dinoOagellate cyst assemblages (dominated by ApeclOdilliuJ1l and WiLsollidiniwn species) recovered from Lakadong Sandstone exposed around CherrapunJi, Khasi Hills (Meghalaya) is discussed. Based on the occurrence of the cosmopolitan markers, the dinoOagellate cyst assemblages are assigned to the combined Apeclodinium hyperacalllhwn and A. augus/wII (Ahy-Aau) Biozones. Integration of dinoOagellale cyst evidence with larger foraminiferal data from the underlying Lakadong Limestone suggests close correspondence of the Apec10dillium acme with the Rallikolhalia IIUl/aLLi- Miscellania misceLLa Assemblage (SBZ 5-SBZ6). Estuarine and coastal swamp depositional setting is favoured for the coal-bearing Lakadong Sandstone which is considered to be the lateral facies equivalent of the upper Lakadong Limestone, developed during sea level highstand. TheTherria limestone/sandstone - Lakadong limestone/sandstone succession of the Khasi Hills is interpreted (to be deposits of transgressive and high stand system tracts) as representing a single depositional sequence between two minor sea level falls, possibly corresponding to the 3'" order cycles TA2·I·TA2·3 of the Global Sea Level Cycle CharI. Key-words-DinoOagellate cyst biostratigraphy, Palaeoenvironmenl. 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INTRODUCTION coal in the Lakadong Sandstone has also evinced keen interest for interpretations of its depositional environment. According HE Shillong Plateau is situated in Meghalaya. northeast to Raja Rao (198 J) and Misra (1992), the coal-beari ng T India, bordering the plains of Bangladesh (Fig. I). It lies Lakadong Sandstone was laid down in estuarine or lagoonal on the southwestern edge of the Assam Shelf, which is or coastal swampy conditions. However, Pandey and considered to be the northeastern extension of the Indian Ravindran (1988) favoured a fresh water origin for this unit Peninsular shield. The Assam Shelf includes the Shillong considering it to be devoid of marine biota. Recovery of Plateau, Garo, Khasi, Jaintia and Mikir Hills and the Upper dinoflagellate cysts from Lakadong Sandstone, especially from Assam Valley (Murty, 1983). The southern fringes of the horizons underlying and overlying the coal measures are, Shillong Plateau, commonly referred to as the South Shillong therefore. considered significant in this context. Plateau, form the Khasi-Jaintia Hills. This is characterised by The objectives of the present study are: (l) to investigate extensive development of carbonates alternating with the dinoflagellate cyst assemblages for their stratigraphical sandstones ranging in age from Upper Palaeocene to Middle and palaeoenvironmental significance. (2) to integrate the Eocene, belonging to the Sylhet Limestone Formation. The information with larger foraminiferal data for the purpose of geology of the South Shillong Plateau has been dealt in detail stratigraphical precision and facies interpretation. and (3) to by several workers (MedlicolI, 1869; Evans, 1932; Ghosh, understand the microfossil evidence in terms of the 1940; Wilson & Metre, 1953; Nagappa, 1959; Biswas, 1962: depositional setting and changes in relative sea level. Mathur & Evans, J964; Chakraborty, 1972; Murthy et al.. Interpretations of the depositional environment have been 1976; Dasgupta, 1977; Samanta & Raychaudhury, 1983). made with the support ofdata provided by Jauhri (1994, 1997) The present paper deals with the dinoflagellate cyst who made a detailed study of the larger foraminifera and the assemblages recovered from the Lakadong Sandstone, the carbonate bui Idup in this region (Jauhri 1988, 1996, 1998). coal-bearing sandstone unitofthe Sylhet Limestone Formation The inferred relative sea level changes, supplemented by (Wilson & Metre. 1953; Raja Rao, 1981), exposed around dinoflagellate cyst and larger foraminiferal bioevents, are Cherrapunji in the East Khasi Hills. It is sandwiched between interpreted in a sequence stratigraphic context. well-dated larger foraminiferal-algal rich sediments of the Lakadong Limestone and the Umlatdoh Limestone. Although GEOLOGICAL SETTING terrestrial palynofossils (spores and pollen) have been documented (Sah & Dutta, 1966; Dulla & Sah, 1970; Kar & The stratigraphic framework of the Upper Cretaceous Kumar, 1986), datable benthic or planktonic microfossils are Lower Tertiary shelfsediments exposed in the Khasi and Jaintia yet to be recorded from the Lakadong Sandstone. In the Hills has been the subject of intensive study by several workers. absence ofany direct evidence, age interpretations have been Evans (1932) redefined the "Nummulitic Series" of Medlicott based primarily on its stratigraphical position and have ranged (1869) as Sylhet Limestone stage. Wilson and Metre (1953) from Upper Palaeocene (Nagappa, 1959; Jauhri, 1994, 1996. assigned separate names to the four limestone bands and three 1998) to Lower Eocene (Pandey, 1972). The association of clastic interbands. The classification of Wilson and Metre GARG & KHOWAJA-ATEEQUZZAMAN- DINOFLAGELLATE CYSTS FROM UPPER PALAEOCENEOF INDIA 463 0 26 -----7 r/ V" l/------ .... ---' -I\'","" \ \ ) EAST / I I IGARO HILLS / . SHILL9NG. I JAINTIA HILLS : ,.,./ . Nongsto.n /_ I: .,""" • Tura I 1""- , Jowai e ~!"illiamna.garJwEST KHASI HILLS I KHAST~~LS I\.e eBapung ----" , \ GARO HILLS '-- \ esohrarom~_~"Jaraln Khlehnat .... Ch~uapun)le 1 eMynkre . -'I (. ,Therrl2. .£>auk. eLakadong '-'~'~._." _._.".,."'...... _._. - - - -, .....\... ""_ .... ""'\. -" -- '\.v ... ~Okin·BANGLADESH 9,0 920 -l JAINTIA &. DISANG GROUPS I~ I~~~I ALLUVIUM (Recent) [[]] I SYLHET TRAP ~ ~ (Upper Palaeocene - Eocene) (Lower Cretaceous) DIHING &. DUPITILA GROUPS LANGPAR-MAHADEO FM. ~ METAMORPHIC &. IGNEOUS ROCKS I:=:::::=J (Mio - Pliocene) ~ (Up. Cret. - Lower Palaeocene + ( Archaean) f7'7/l TIPAM &. SURMA GROUPS ~ (Miocene) t===1 LOWER GONDWANA o THRUST &. FAULT [:::::::.:~i BARAIL GROUP (Oligocene) Fig l-Localion Map showing main geological fealurcs of the Soulh Shillong Plaleau (afler Malhur & Evans. 1964). (1953) has been used widely with modifications by later & Metre, 1953; Nagappa, 1959; Raja Rao, 1981). The Sylhet workers (Nagappa, 1959; Biswas, 1962; Mathur & Evans, Limestone Formation ranges in age from Upper Palaeocene 1964; Murthy et aI., 1976; Dasgupta. 1977; Raja Rao, 1981). to Middle Eocene (Thanetian - Bartonian). All three limestone Biswas (1962) and Raja Rao (1981) subsequently gave the units contain characteristic and datable larger foraminiferal formal lithostratigraphical status to the chronostratigraphical assemblages. The Lakadong Sandstone contains thin but nomenclature introduced by Wilson and Metre (1953) The workable coal seams in parts of the Khasi and Jaintia Hills. lithostratigraphical classification followed in the present work The Sylhet Limestone Formation overlies the Upper is given in Fig. 2. Palaeocene Therria Formation which is characterised by hard, The Palaeogene carbonate succession exposed on the compact, well-bedded sandstones with an extensively South Shillong Plateau belongs to the Sylhet Limestone bioturbated sandy horizon, rich in ThaLlasinoides burrows, Formation, which includes three foraminifera rich limestone towards the top. This ichno-horizon, which is termed as the units interbedded with thick sandstone - shales units (Wilson Therria Ichllo-horizon, is well exposed south of Cherrapunji 464 THE PALAE0130TANIST ~ ~ ~Vl 0 FORMATION/ UVl~ -< 0