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 of Palaeobotany, 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. Sea level changes, Sequence stratigraphy, Late Palaeocene,
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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 of data 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 unit of the 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 shelf sediments 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 of any 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 /_ Tura I ., """ • 1""- , Jowai e ~!"illiamna.garJwESTKHASI HILLS I KHAST~~LSI: I e eBapung ----" , \ GARO HILLS '-- \ esohrarom~_~"Jaraln\. Khlehnat .... Ch~uapun)le1 eMynkre . -'I (. ,Therrl2. .£>auk. eLakadong '-'~'~._."_._.".,."' ...... _._. - - - -, .....\... ""_ .... ""'\. -" -- '\.v ... ~Okin·BANGLADESH9,0 920 -l JAINTIA &. DISANG GROUPS I~ SYLHET TRAP I~~~IALLUVIUM (Recent) [[]] I ~ ~ (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===1LOWER GONDWANA o THRUST &. FAULT [:::::::.:~iBARAIL 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 i~ LITHOLOGY f-< -< MEMBER f0- CI) Z 0:: ::s W Z KOPILI Alternations of fine to coarse sandstones, grey siltstones and Q., 8 Q., 0 V") a::! FORMATION shales with few limestone bands => .... 0:: Q., 0 PRANG V") Bluish grey massive to thinly bedded foraminiferal limestones u.:l Z LIMESTONE , 'Z - with marl earthy limestone W Z< 0...... ~ Z < ...... l ....z f-< u.:l Q E- Z ~ Grey to pinkish grey foraminiferal limestone with calcareous 0:: 0 W < Z UMLATDOH V3 sandstone partings ~ 0 - W LST -I 0 0::: f-< 0 ...l Q., CI) r-- >- ~ ~ ...... Predominantly buff to whitish, soft, friable, medium to coarse ...J 0 LAKADONG V") Z -, grained arkosic sandstone, often pyriteous with thin grey-black, SST V") frequently burrowed, carbonaceous shales and siltstones and ....< f-< C"') E-< ~ thin coal seams W ::r: u.:l z ...J < 0 Grey to brownish grey foraminiferal-algal limestone, dolomitic Z 0:: >- LAKADONG \0 W ~ CI) , at the base with thin grey shale, silty shale partings in the upper u.:l Q., LST 0 U Q., - part => 0 Hard, buff to brownish, medium to coarse, ferruginous THERRIA Z 0 sandstones with thin bands of pyriteous siltstone & ~ < FORMATION 00, 0 carbonaceous shale; 'Thalassinoides Ichno' horizon near the ~ 0 N z top, thick limestone/calcareous sandstone at the base. 0::0 ::r: V") WW U C"') Massive, fine to coarse grained glauconitic sandstones Q.,u MAHADEO C"') Q.,< C2 I containing grey shales, mudstones and calcareous sandstones in =>E-< E-< FORMATION 0 W ell \0 the upper part;conglomerates and pebbly sandstones at the base 0:: < U < - , ::E ? JURASSIC 8 oo::t, TO LOWER SYLHETTRAP 0 Aa and pahoehoe type Basalts V") CRETACEOUS N GARG & KHOWAJA-ATEEQUZZAMAN- DINOFLAGELLATE CYSTS FROM UPPER PALAEOCENE OF INDIA 465 IN DEX N B Kapil; Formation 1 Sylhet Li mestone ~ Formation ...'· D. . . Therria Formation Mahadeo & La ngpar Formatio Sylhet Trap Shillong Group o 2km -v-o ~ _____~ 'Y"' 0-v- -v- -v- Fig 3-Geological map of Chen'apunji area (Redrawn from Ghosh, 1940) (e.g., near Cherra Circuit House and Naokalikai Falls picnic of lowai near larain (Jowai-Muktapur track). southwards of spot, and Mawsmai Gate on the Cherra-Shella road) in the Pynurkha (Rymbai-Borghat track) and on the hill slopes Khasi Hills. It has been found to extend eastward to the south leading to Khadum (westwards of Jowai-Badarpur Road) in L "Fig. 2-Slraligraphical c1assificalion of Upper Crelaceous- Eocene rocks of lhe Khasi Hills (Modified afler Raja Rao, 1981). 466 THE PALAEOBOTANIST S N RangsanobolMawmJuh Laitryngew I Cherrapunji I Sohrarirn Mahadeo I I II I I I I I Therriaghat I _~~//!:/(If:!:id;i.£/{{'~ ~ ~ I L117lJ1fl1/~:=-i:--:-;;I/ _/ ( ," ,( , I I- .. .- n°. . - / / " 0 2 km. " ,. I I I I ( Vertical Scale Exaggerated) I ~ SylhetLst ~ Langpar vv v v Sylhet Traps ~ Fonnation ~ Fonnation ~v v v Mahadeo fI777/lTheola ~'6:'?: I~1_- ~r Crystalline ~ ~C;., :0. Formation .. Fonnation ;" ,. " Basement Fig. 4-Schematic cross-section of the Cherrapunji- Therriaghat area. South Shillong Plateau. showing distribution of Upper Cretaceous-Eocene rocks (Modified after Ghosh. 1940). the laintia Hi lis (see Fig. 1). It is considered to represent a development near Laitryngew and Sohrarim, northwards of widespread regional transgressive event, which may serve as Cherrapunji, but becomes vastly attenuated towards the south a good marker horizon. The Therria Formation is conformably in the Therriaghat section (Fig. 4). Only carbonaceous shales overlain by the algal-foram rich Lakadong Limestone to be occur in the Lakadong Sandstone and no coal is found here. followed higher-up by coal-bearing Lakadong Sandstone. These lateral facies changes, coupled by theJuxtaposition The Lakadong Limestone and Lakadong Sandstone are nature of the two sandstone units (Medlicott, 1869; Ghosh, exposed over a wide area around Cherrapunji. The overlying 1940), seem to have created a cons idera ble con fusion units of the Sylhet Limestone Formation are confined to the regarding the stratigraphical position of the coal-bearing southern fringes of the plateau. Ghosh (1940) mapped the area sandstones exposed north of Cherrapunji, in the Khasi Hills in detail, which has provided the basis of subsequent studies and also in parts of the laintia Hills. The coal-bearing in this area (Fig. 3). Lakadong Sandstone has often been misinterpreted as the The Lakadong Limestone and Lakadong Sandstone show Therria Formation (including the ChetTa Sandstone of earlier a variable development on the Cherrapunji Plateau and along workers) in the Cherrapunji Plateau (Sen, 1948 in Biswas, its southern slopes towards the plains of Bangladesh (Ghosh, 1962; Sith & Dutta, 1966; Dutta & Sah, 1970, J 974). However, 1940)..Towards the south, the Lakadong Limestone has the these coal-bearing sandstones of the Khasi-laintia Hills have thickest development as seen at the Therriaghat (Um long been considered to constitute the lower part of the Sylhet Sohryngkew) section. It is attenuated northwards near Limestone Formation (Medlicott, 1869; Ghosh, 1940; Wilson Mawmluh, Rangsanobo and Nongthymmai with a considerably & Metre, 1953; Nagappa, 1959; Biswas, 1962; Das Gupta, reduced thickness at Cherrapunji. North of Cherrapunji, the 1977; Raja Rao, 1981; Samanta & Raychaudhuri, 1983) Lakadong Limestone rapidly pinches out towards Laitryngew Biswas (J 962) had categorically mentioned that there is no (Medlicott, 1869). Near 29 MP (mile post) on Shillong coal associated with the "Cherra Formation" (Therria Cherrapunji Road section, Lakadong Limestone is completely Formation). In the 1aintia Hills, exposures of the coal-bearing absent and the Lakadong Sandstone directly overlies the Lakadong Sandstone around Bapung, north ofMynkre along Therria Formation. There is a corresponding increase in the the 10wai-Badarpur Road (see Fig. 1), have been mapped as thickness of the Lakadong Sandstone, which shows its thickest the Therria Formation which has been mistaken as the main /'...... Fig. S-5tudied Sections exposed North of Cherrapunji along the Shillong-Cherrapunji Road (M Prefers 10 'milepost' as marked on the Survey of India toposheet no. 78 0/ II). I. Section c.a300 m west of road across a rivulet (31/2 MP). II. Section on a hillock along the eastern side of the road between 31 & 32 MP (sample NG7 was collected from the other side of the hillock after a travcrse to Nongpriang Valley. The uppcr part of the hillock exposing more than 30 m of the Lakadong Sandstone. containing a minor coal scam at the lOp. was not sampled due to slumping.) III. Composite section along the road from north of Sohrarim lip to Mawmithied (between 24 & 26 MP). IV Section near Laitryngew, samples CS8 & CS9 collected from the road side section just south of Laitryngcw, overlying the thick succession of coal bearing sandstone exposed in the coalfield. GARG & KHOWAJA-ATEEQUZZAMAN- DINOFLAGELLATE CYSTS FROM UPPER PALAEOCENE OF INDIA 4(,7 ..· .. "0'.0... '.,' .' .' ...<.{y:-~ ~:··:-O··~:O·''.- ,0.. ",' " _ ••• -• ,-. "0· ',-:.' "0 '0 '0 •••••••• * ·.~S:":'.·":"'·.'-'-:· -----\ '..,..::~.'c:-·.S."-: ---- NG-7 * ...... CS-9 ~~\ ... ' . -: --: -.!5-;'..-:- CS-8 * :....---~CH-8 •.. ~--:-.~~.~. \ ·.. CH-7 * :.:.~·s~:I ·.:.:.,s:...;...;..~., ~.'~':;::'~:~.~~.\ F .. , CH-6 CH-5 .$'.'$:.:' ~"~:T-::-":"'""=}CS-7 * · .... · . . ... \ ·,'s.'.s·...... ·.S.: · . . ..~ SCH-9 ··...... 9._ " 1·-::.",-":"'~i::'~1 \ -- -- •• \ .----_.-.-.-''-'-.-.-'-- -- CH-4 \ CS-6\ CS- 5 b - SCH 8 CS 5a I' '.-"-~'--:r - CS- 5 \ CH-3 \ ::...... !i ....'.::::....:. I;?;-v~q :-; SCH -7 * ~I ., I CRJ (~::::.]CS-4 \ ~s CIP_l s CH-2* \ CS-3 H • _l::'._;SCH-6 ~ :--;-':::'~.-=--:s.:.~CS-2* 1_'- -,~lSCH-5 illlli: \ ----- SCH-4 1-----',0,'. '0" '0' \1::-;':-:·"»; ·· .. \ ~:~..:..:.:-:.. CS-1 ~ '.:-'~.~'':''': ... ~ SCH 3 · . ~::-:' :~".~ · . \ · .:. ·.... '':tl ------=..... SCH-2 :--:--:--:--:--:--:-. CH-1 ·· .. III · . .. . · ...... ·. ..· . .. ·s s:s·.s,.·:tSCH-1 : ·.S:5:5·.5; : ~coal IV • •• 0 •••• ,0, ',' . ·'s.··.S:.:5··.· .. · · ··S· .'. ·s····'. · . ••.....,0 ••••••••• 5m -_._._._. 1"9',"0';1Overburden ··...... . ~ III •••••.. ·...·...... 1 F.-.3Carbonaceous ··...... II Streaks I Partings ··...... Productive · . * sample I--©IHard Sandy ~ ® - Shale Nodules I Carbonaceous ~Shale s s s s Burrows ,$" b Algae ~ @/@I Foraminifera Fine Sandstone Limestone o ~ V 1"\ Bivalves ~, § t. - - - .1 OL .. - "h ..__ 1=_=_=-_:.1Shale I Sandy >' Gastropods ------Shale Q Q Leaf Impression 468 THE PALAEOBOTANIST "" 1 2 3 \'-. ...," 4 5 6 / '".' :\ 7 8 ...... 9 . - ...... ",. ....~- " 10 11 .. 12 PLATE 1 GARG & KHOWAJA-ATEEQUZZAMAN- DINOFLAGELLATE CYSTS FROM UPPER PALAEOCENEOF INDIA 469 coal measure unit in the area (Saxena & Tripathi, 1982). The with often-high numerical abundance of individual species. Therria Formation is exposed near Mynkre (Dutta & Jain. The assemblages contain 30 species of dinoflagellate cyst and 1980) and coal occurrences near Bapung and other areas in 3 species of acritarch (see checklist; Plates 1-3). A the Jaintia Hills have been found to be restricted to the preponderance of peridinioid dinoflagellat'e cysts Lakadong Sandstone (Raja Rao, 1981; Misra. 1992). The age (ApeclOdil/iulIl and Wilsol/idiniulJI) is strikingly evident over significant findings of the dinoflagellate cyst assemblages all the other species. reported from the supposedly Therria Formation (Tripathi & The assemblages contain many species of APCClOdil/il/lI1 Singh, 1984; Singh & Tripathi. 1986; Tripathi. 1989) have. including A. liolJloJ1lorpliUlII, A. Ilypcracallllllllll, A. parVIIIJI, therefore. to be viewed in this context. A. pal/iculalulIl, A. quil/que/aill/n and ApcClodinilll1/ cr. A. allgl/slllIlI. Another species of Apcclodinilllll, recorded here MATERIAL AND METHODS as ApcclOdinil/lIl sp. A. is also very common and is an important component of the assemblages. APCClOdil/ill1ll sp. Samples for the present study were collected from A is characterised by a shol1er than broad cyst lacking an apical sections exposed north of Cherrapunj i. The sections Iie along horn, much reduced or absent antapical horns and long and the Cherrapunji-Shillong Road in vicinity of Cherrapunji. broad lateral horns. Transitional forms between A. Laitryngew, Sohrarim and Mawmithied (Fig. 5). Out of 29 pal/iculafUin and A. allgusflllll, characterised by very well samples, 8 proved to be producti ve of dinoflagellate cysts and developed horns and much longer processes were also recorded acritarchs. Additional material collected from the Um and documented here asApeciodillilllll sp. B. The assemblages Sohryngkew (Therriaghat) section, Nongthymmai and also show common Wilsonidinilllli species. Two of the most Mawmluh limestone quarries (south ofCherrapunji) and from commonly occurring species of Wilsol/idiniulIl are documented the Jowai-Badarpur Road section in Jaintia Hills has also been here as Wilsol/idil/iUlI/ sp. A and Wi/sol/idillilllli sp. B. W studied for comparison and correlation purposes. l/igcricllSc and W. cclllnosllluralUlll occur rarely. As detailed For the recovery of dinoflagellate cysts, the samples were morphologic and taxonomic study of dinoflagellate cysts are processed using standard palynological techniques. After HCI not intended here. a taxonomic account of these significant and HF treatment. the macerate was treated with 40% HN03 morphotypes of Apeciodinilllll and WilsOllidil/illll1 will be for oxidation and washed using 25 11msieve. After staining pubIished elsewhere. with safranin, the water-free residue was mixed with polyvinyle Some other important species occurring rarely in the alcohol and spread evenly on the glass cover slides. Permanent assemblages include ljccysla pacllyderllla, MUralOdil/iulIl slides were prepared by fixing the oven dried cover slides filllbriarlllli and Lal/lcrnosphaeridiulIl lanoslllli. The remaining using Canada balsam as the mounting medium. Study and species in the assemblages (PolyspliaeridiulIl slIblilc, photography was carried out on Olympus Vanox -AH2 Cordosplwcridiulil il/odcs, C. cxililllllrtlJ1l, AcholllOsplwcra microscope with Nomarski Interference Contrast and sp., Spinijcrilcs spp., Adnalosphacridilllll lIlullispinos/llIl, automatic photo attachments. The illustrated dinoflagellate OpcrclIlodil/illll/ spp., ThalassipllOra pclagica and cysts are provided with the England Finder positions on the AlIlphorosphaeridiulJIlllllllispil/oslll/l) are longer ranging and respective slides. The slides have been registered and deposited occur rarely. in the repository of the Museum, Birbal Sahni Institute of Out of all the productive samples. richest dinoflagellate Palaeobotany, Lucknow. cyst assemblages are recorded from three levels (samples NG7, CS7 and CS8; see Fig. 5). Occurrence of dinoflagellate cysts DINOFLAGELLATE CYST ASSEMBLAGES above and below the coal measures is significant as it helps to date precisely the coal occun'ences in this region. Assemblages The organic-walled phytoplankton assemblages below the coal measures display high frequencies of both recovered from the Lakadong Sandstone of Cherrapunji area APCClOdil/ill/1i and Wilson idil/iulII , while those overlying the are well preserved, and show low to moderate species richness coal measures (Sample CS8) show abundance of ApcclOdiniulII PLATE 1 /''0: (All figures in Nomarski Interference Conlrasl; magnified x 500) l, 4. Ape('lOdilliulli panint/alllln (Cosla & Downie) Lenlin & Williams Slide no. £3SIP 12482 (V271J). 1977. Slide nos BSIP 12482 (W4412): 12481(E4611). 9. Ape('/odil1iulI/ pan'll/ll (Albeni) Lenlin & Williams 1977, Slide no. 2, 3. Ape('lodilliwn !lyperal"Q/lIlwlII (Cookson & Eiscnack) Lenlin & 12482 (R60) Williams 1977. Slide nos. BSIP 12483 (K59): 12482 (Y52/3) 10-12. ApwodilliwlI 1101/lOII/O/p!l/ll/l (Deflandre & Cookson) Lenlin & 5,6. Ape('lodinilllll quinque/alulli (Williams & Downie) COSL:l& Downie Williams 1977. Slide nos BSIP 12487 (G60): 12490(R5111): 12481 J979, Slide nos. BSIP 12480 (N45/4), 12481 (K56) (W52). 7,8. Apeclodiniulli cf A augll.ll/llil (Harland) Lcnlin & Williams 1977, ... o • GARG & KHOWAJA-ATEEQUZZAMAN- DINOFLAGELLATE CYSTS FROM UPPER PALAEOCENE OF INDIA 471 only. Wilsonidillium was not recorded at this level but 17. Eocladopyxis pellicu!ata Morgenroth, 1966 MuratodiniulII fimbriatum is rarely present. Ifecysla 18. Fibrocysta sp. pachyderma and Lallternosphaeridiulll !anosum (solitary 19. Ifecysla pachyderma Jan du Chene & Aderi9an, 1985 occurrences) are confined to lower levels (Sample NG7) only. 20. Lantemosphaeridium lanosum Morgenroth, 1966 A few samples taken from the Lakadong Limestone also 21. Muratodiniulll fimbriatum (Cookson & Eisenack) yielded dinoflagellate cyst assemblages represented only by Drugg, i970 rare Operculodiniulll, Polysphaeridium and Adllatosphaeri 22. Opercu!odillium sp. dium. Samples of Lakadong Sandstone collected from 23. Polysphaeridium subtile Davey & Williams, 1966 southerly exposures at the Nongthymmai and Mawmluh 24. Spinidinium macmurdoellsis (Wilson) Lentin & limestone quarries and the Therriaghat section yielded rare Williams, 1977 unrecognisable dinoflagellate cysts and mainly contain 25. Spillijerites sp. acritarchs including Collulllosphaera spp. 26. Thalassiphora pelagica (Eisenack) Eisenack & Gocht 1960 emend. Benedeck & Gocht 1981 Checklist 27. Wilsonidillium echillosulUratulll (Wilson) Lentin & Williams, 1976 For taxonomy of genera and species and complete 28. Wilsonidillium sp. cf. W nigeriense Jan du Chene & bibliography, reference is made to that cited in Williams et al. Aderidan, 1985 (1998) for dinoflagellate cysts and Fensome et al. (1990) for 29. Wilsonidinium sp. A acritarchs. 30. Wilsonidillium sp. B Dinoflagellate cysts: Acritarchs: I. Achomosphaera sp. I. CollUinosphaera cf. Cfruticosa Jain & Dutta in Dutta 2. AdnatosphaeridiulIl multispinosulIl Williams & & Jain, 1980 Downie, 1966 2. Cyclopsiella sp. 3. Amphorosphaeridiulll ltIultispillosum (Davey & 3. Pterospermopsis sp. Williams) Sarjeant, 1981 4. Apectodinium cf. A. augustum (Harland) Lentin & Previous dinoflagellate cyst records from South Shillong Williams, 1977 Plateau 5. A. homomorphum (Deflandre & Cookson) Lentin & Williams, 1977 In the South Shillong Plateau, Palaeocene dinoflagellate 6. A. hyperacamhUln (Cookson & Eisenack) Lentin & cysts have been recorded from the Cherrapunji-Therriaghat Williams, 1977 areas of the Khasi Hills and the Jowai-Badarpur Road section 7. A. palliculalum (Costa & Downie) Lentin & Williams, in the Jaintia Hills (Sah el at., 1970; Jain et al., 1975; Dutta & 1977 Jain, 1980; Jain & Garg, 1982; Tripathi & Singh, 1984; Singh 8. A. parvUln (Alberti) Lentin & Williams, 1977 & Tripathi, 1986; Garg, 1986; Tripathi, 1989; Garg & Jain, 9. A. quillquelatul/I (Williams & Downie) Costa & 1993, 1996). Upper Palaeocene assemblages are known only Downie, 1979 from the Jowai-Badarpur Road section (Dutta & Jain, 1980; 10. A. longispinosum (Wilson) Bujak & Davies, 1983 Tripathi & Singh, 1984; Singh & Tripathi, 1986; Tripathi, II. Apeclodillium sp. A 1989; authors' unpublished data) and contain species of 12. Apectodillium sp. B Apectodiniulll. 13. Cordosphaeridiulllfibrospillosum Davey & Williams, Dutta & Jain (1980) recorded seven species of 1966 dinoflagellate cyst from the uppermost part of the Lakadong 14. C gracile (Eisenack) Davey & Williams, 1966 Sandstone near its contact with the overlying Umlatdoh 15. C inodes (Klump) Eisenack, 1963 Limestone and assigned Late Palaeocene age. Except for 16. Diphyes colligerum (Deflandre & Cookson) Cookson Hystrichokolpoma rigaudae, all the other species are common 1965 ./ ..... PLATE 2 (All figures in Nomarski Interference Contrast: magnified x 500) 1-3, 5. Apeclodiniwn sp. A. Slide nos. BSIP 12483 (FM): 12480 (U49 . 7, 10. WilsOllidiulIl echillOSU[Urarulll (Wilson) Lentin Williams 1976. M50): 12481 (W39/4). Slide no. BSIP 12486 (N34/4). 4. ApeclOdilliullllollgi.'pilwslIIn (Wilson) Bujak & Davies. 1983, Slide 8. II, 12. WilsOllidiwl1 sp. A. Slide nos. BSIP 12480 (K41/4): 12485 (J431l): no. BSlP 12489 (V61). 12483 (US 111). 6.9. ApeClodilliulIl sp. B. Slide nos. BSlP 12487060/2): 12488 (N39/ I) 472 THE PALAEOBOTANIST ' .. ',lor..._ 1 2 4 5 6 7 8 9 ~ 10 11 PLATE 3 GARG & KHOWAJA-ATEEQlJZZAMAN- DINOFLAGELLATE CYSTS FROM UPPER PALAEOCENEOF INDIA 473 to the Cherrapunji area also. ApeclodilliulIL parllum is found DISCUSSION to be the most abundant species whi Ie A. 1101I1O//lOrphu/1land A. hyperacanlltum are only rarely recorded. Tripathi & Singh (1984), Singh & Tripathi (1986) and Stratigraphic Significance Tripathi (1989) documented dinoflagellate cyst assemblages from the so-called Then'ia Formation (Lakadong Sandstone The genus Apeclodillilllil is the oldest representative of as discussed elsewhere in this paper). These studies are the sub family Wetzelielloideae (Family Peridiniaceae), and significant as datable dinotlagellate cysts assemblages were is one of the most characteristic and stratigrap'hically important reponed from strata associated with coal seams in this area. genera in the Palaeogene dinoflagellate cyst assemblages The assemblages contain 13 species and show dominance of worldwide. Standard biozonation schemes based on their Apeclodinilll1l, represented by A. h0ll101ll0rphUill and A. species have been proposed and precisely calibrated with parvlIIN only. Other species belong to Adilalospltaeridilllll, Standard planktonic foraminifera and calcareous nannofossil HomOI rybl i 11111, Cordospltae ridi 11111. Ope rClilod il1 ill III, biozonation schemes. Powell (1992, pp. 176-178) re-defined Polysphaeridiulil and T!wlassipltora (recorded as Codoniella two Late Thanetian biozones ApeClodilliull1 hyperacalllltwil langparellsis by Tripathi, 1989) and show a close similarity (Ahy) and A. aliguslwl1 (Aau) for the Palaeocene of NW with the Cherrapunji assemblages. This has been aptly Europe. The older Ahy Biozone is defined between the FAD corroborated by dinoflagellate cyst evidence from the of A. 1101I101IIOrphwil and FAD of A. augus!tllil whereas the Lakadong Sandstone exposed between Bapung and younger Aau Biozone covers the interval between the FAD of Lumshnong in the Jowai-Badarpur Road section (authors' A. augllslwll and that of P magl1ijiclII/1. However, Brinkhuis unpublished data). However, no dinoflagellate cysts were el at. (1994) reported A. hyperacallllllllll from the early part recovered from upper levels of the Therria Formation of Late Palaeocene close to the Danian - Selandian boundary underlying the Lakadong Limestone, during the course of the corresponding to the base of the planktonic foraminifer Zone present study. P3 in the low -latitude EI Kef Section. It represents the earliest Comparable assemblages domi nated by Apeclodillillill are stratigraphic occurrence of the genus ApeclodilliulIl. This is widely recorded from India including Vriddhachalam (Jain & agreed with our unpublished investigations as part of a wider Garg, 1986) and subsurface of Karaikal (Jain & Garg. study encompassing Late Maastrichtian to Palaeocene unpublished data) in the Cauvery Basin, subsurface of the dinoflagellate cysts from Meghalaya. In these studies. A. Krishna-Godavari Basin (Garg el al., 1995) and subsurface Ityperacalltltlllll has been recorded in the uppermost part of of the Kutch Basin (authors' unpublished data). Direct the Langpar Formation. containing P3 Zone planktonic calibration of these assemblages with the Late Palaeocene foramini fera. A worldwide Apeclodilliulll Acme nevertheless nannofossil DiscoaSler IllUltiradiallis Zone (NP9) has also occurs close to the Palaeocene-Eocene Boundary (Harland, been established in the Gopurapuram area, Vriddhachalam in 1979; Powell, 1992; Brinkhuis el al., 1994). the Cauvery Basin (Jain el al., 1983; Jain & Garg, 1986). The Aau biozone is characterised by the presence of Recent findings of ApeclOdillill/ll and WilsOllidillil/ll1 Apeciodilliulil augusluill. A. IlyperacalllltulIl, A. pmzicu{alillu, assemblages, closely comparable to those recorded in this A. parlllllll, A. quinquelalUlII and A. SUlIllI1isSUIll, besides the report, from the subsurface of the Upper Assam Valley rather long ranging A. h0ll101llOrplllllN. It has been calibrated (authors' unpublished data) may be extremely useful in with calcareous nannofossil Zone NP9 (pars) of Martini identification of equi valent sediments in the subsurface of the (1971), planktonic foraminifera Zones P5 (pars) plus P6a Assam Shelf. (Berggren, 1969; Berggren & Miller. 1988) equivalent to Zone P5 of Berggren el al. (1995), and the dinoflagellate cyst sub Biozone D5a of Costa & Manum (1988). The age of the Aau ./ PLATE 3 " (All figures in Nomarski Inlerference Contrast: magnified x 500) I WilsllnidiulII sp. B, Slide no BSIP 12480 (Q6311) 7. SpiliidiniwllllWI'lIIurdoen.l'e (Wilson) Lenlin & WilliamsI977. Slide 2, Alllphorospllaeridiuul? IIIU/lispiIlO,1'II1I7(Davey & Williams) Sarjeanl no. BSIP 12483 (T25). 1981 Slide no. BSIP 12482 (V23). 8, Laillemosphaeridilllll lallosLllu Morgenroth 1966. Slide no, BSIP 3 Adualosplweridilllllllllllli.l'piliOSlIlII Wiliams & Downie 1966. Slide 12484 (V60). no. BSJP 12483 (K58/ I). 9. 12. CordlJ.\phaeridiulII fibrosJiilloSlI1II Davey & Williams 1966. Slide 4 Poly.'phaeridilllll.l'ublile Davey & Williams 1966 emend. Bujak ('1 no BSIP 12484 (039). aI, Slide no. BSIP 12491 (T60). 10, Co//ulI7o"plwera cf, C../i'lIlim.lo Jain & Dulla in DUlla & Jain 1980, 5 Eocladopyxis peniculata Morgenroth 1966, Sliele no. BSIP 12484 Slide no. BSIP 12492 (V45/3). (Q62/2) II l(eevsla p{/thyderllla Jan elu Chene & Aderidan 1985. Slide no. 6. Cordo.l'plweridiulII gratile (Eisenack) Davey & Williams 1966. BSIP 12485 (W64/I). Slide no. BSIP 12484 (H61/2), LATE PAL AE O::t::~,:N'E",EARLY EOCENE EPOCH -' -- --, -, "1' T H A N E T IA N. -.". - . .". :.'::: .". AGE ',I L I A hyperacanthum A homomorphum o A augustum en A summisum "1j -z A. parvum tTl o (j ~ A paniculatum ...... r- A. quinquelatum tTl en > Alongispinosum ------o"1jC':l l.pachyderma ------~ trl ~r 5, macmurdoense .. '"-r M. fimbriatum .. ~>N..., trl '"0----0 e; -+>- o o (j C/l VI 0\ to ...... " '-< oN V'J o ..., Z V'J :> :> :> C) ~~ tTl ::r ~ en :3 o., ~ ~ '< c:: ~:3 .LSIN'v'.L0803'v'1'v'd 3H.L PLP GARG & KHOWAJA-ATEEQUZZAMAN- DINOFLAGELLATECYSTS FROM UPPER PALAEOCENEOFINDIA 475 Biozone ranges from Latest Thanetian to base Ypresian dinoflagellate cyst and larger foraminiferal species is presented (Powell, 1988, 1992). A. augustUlIi has its LAD at the in Figure 6. The evidence indicates that a major part of the Palaeocene-Eocene Boundary (Powell. 1988; Williams et af., Lakadong Sandstone (including the coal measures), at 1993) Cherrapunji is coeval and the lateral facies equivalent of the The presence of rich and diverse Apectodi/lillill upper part of the Lakadong Limestone which was laid down assemblages in the Lakadong Sandstone exposed in the in more basinal areas to the south. It is interesting to note that Cherrapunji area suggests its placement in the Aau Biozone Biswas (1962) had suggested that coal-bearing sandstones in indicating Late Thanetian age. Occurrence of Wifsonidiniu/ll areas north ofCherrapunji were most likely deposited at the nigeriense and /fecysra paclzyderllla, known from Late time of limestone deposition (Sylhet Limestone Formation) Thanetian-Early Ypresian (NP9 or NP9 - NP 10 zones) of of the southerly sections. However, interpretations regarding Nigeria (Jan du Chene & Adediran, 1985), further supports their precise age and correspondence with any specific unit of assignment of the Lakadong Sandstone assemblages to the the Sylhet Limestone Formation were left unresolved by Late Thanetian, close to the Thanetian- Ypresian Boundary. Biswas (1962). The Cherrapunji assemblages also contain Spinidinilllll A suggested correlation of the Cherrapunji Plateau macmurdoense, Wilsonidiniu/ll eclzinosuturarlllll and sections with the Then'iaghat section, based on integration of Apectodiniul11 fongispinosuill in common with New Zealand dinotlagellate cyst and larger foramini fera (Jauhri, 1994. 1996, Late Palaeocene assemblages (Wi Ison, 1968, 1988). However, 1998), is presented in Figure 7. The Glo/l/afveo/ina primael'a in the basal part of the Lakadong Sandstone, immediately Zone (SBZ3), identified on the evidence of Miscellanea above the Lakadong Limestone but below sample NG7, a yveffae and M. julietfae in the ChelTapunji section, is cOITelated slightly older age (possibly equivalent to Ahy Zone) is to the lower part of the combined G. prillweva- G. levis Zone preferred due to the occurrence of a meagre assem blage (SBZ3-SBZ4) of the Therriaghat section (con-esponding to containing only A. hOlllOlllOlphwlI and Apecrodiniwll sp. A. the upper part of the planktonic foraminifera P4 Zone). The The dinoflagellate cyst assemblages assigned to the Ahyl upper part of the G. primaeva-G. lel'is Zone (latest P4 Zone), Aau biozones are found to overl ie directly the Gfolllalveolina characterised by total range of G. levis in the Therriaghat primaeva Assemblage Zone (SBZ3 = Late P4) within the section, is equated with the lower part of Ahy-Aau Zone at the Lakadong Limestone dated on the evidence of Miscel/[Illia base of the Lakadong Sandstone in ChelTapunji section (which yvettae and M. jlllietfae from the Cherrapunji section (Jauhri, directly overlies the G. prililaeva Zone). G. levis is not recorded 1994, 1998; Serra-Kiel er al., 1998). Dinoflagellate cyst from the Cherrapunji section. The Apectodillilill/ acme in the assemblages from levels overlying the coal measures, although Cherrapunji section is tentatively correlated to the first less diverse, are sti IIquite rich in Apectodiniul1l, and in addition appearance of Ranikorllalia Iwualli in the lower part of R. contain M.jimbriatulIl and A. longispinosul1l. This is indicative Ill/tfalli-M. lIIiscella Zone (SBZ5-SBZ6) corresponding to the of a slightly younger age assignable to the upper part of the P5 Zone. Occun-ence of Muratodinillllljilllbriatlllll, supporting Aau Biozone. a comparatively younger age in the assemblage overlying the In the Therriaghat section, the interval overlying the coal measures, is probably correlatable with the uppermost SBZ3-SBZ4 assemblage, is represented by the Ranikothalia part of the combined SBZ5-SBZ6 Zones identified at lZuttalli-Misceliania miscella Assemblage (SBZ 5 to SBZ6), Therriaghat. Overlying strata, equivalent to the Lakadong corresponding to the planktonic foraminiferal Zone P5. It Sandstone exposed in the Therriaghat section, are apparently extends from the upper part of the underlying Lakadong not present in the ChelTapunji section or may have been eroded Limestone to the base of overlying Umlatdoh Limestone and away. Samples from this part in Therriaghat section show a is dated as "Early Ilerdian I Late Thanetian" (Jauhri, 1994). preponderance of acri tarchs (CollulI1ospllaera spp.), observed The combined evidence of dinoflagellate cysts and larger in the Mawmluh Limestone quarry section also which lies foraminifera from Chen'apunji and Therriaghat areas are not between Therriaghat and Cherrapunji (see Fig. 4). The larger only useful in terms of integrated biostratigraphical resolution, foraminifera from the basal part of the overlying Umlatdoh but also permit inferences to be drawn upon lateral facies Limestone at Therriaghat show the appearance of Daviesilla development and interrelationship ofLakadong Limestone and ruida in association with R. nl/ffalli, indicative of an 'early Lakadong Sandstone. A comparative range chart of selected Ilerdian'ILate Thanetian age, corresponding to late P5 Zone ./,,~-- Fig. 6--Comparative Range Chart of selected species of dinoflagellate cysls and larger benthic foraminifera recorded in Cherrapunji- Therriaghat area (i) The Dinoflagellale Cyst Zones and Shallow Benthic Zones (SBZ) are directly integrated with Standard Foraminiferal Nannofossil Zones by respective authors; their correlation shown here is by implication; (ii) The Planktonic Foraminifer P5 Zone of Berggren el at. 1995 incorporates Subzone P6a of Berggren 1969 and Berggren & Miller, 1988 (iii) Shaded part in Ihe Epoch/ Age column shows the Palaeocene/ Eocene "Boundary Time Span" in Shallow Tethys Region indicated by Molina el al. (1992) spanning the illlerval between - 54.610 55.5 Ma according to Serra-Kiel el al. (1998) & Berggren el at. (2000. p. 45, fig 3). (iv) In Dinocyst Bizonalion Scheme of Powell (1992), the top of NP9 Zone is placed in the basal Early Eocene following Haq £'1 al. (1987a). 476 THE PALAEOBOTANIST 5 N Basinward Landward D. ruida V) 0.. SBZ7 UML LST .. ~ ------.. __.. _------. Aau Biozone /' ~ . ------. .. _------/' Z /'" Z 0 LKD.SST /'" tJ.l ______1. Z -< -E------_. Z Fig. 7-Correlalion of Therriaghal and Cherrapunji Plateau section. (Jauhri, 1998). Significantly, the occurrence of Apeclodiniwn noteworthy that correspondence of the lower part of the A. hyperacamhum is noted in the basal part of the Umlatdoh hyperacanlhwn Zone (equivalent to the combined Ahy-Aau Limestone, (overlying the ApecLOdinium acme horizon in the Biozones) with G. levis Zone has also been noted in the North upper levels of the Lakadong Sandstone), in the Thangski Pyrenees of Europe (Tambareau, 1994). Coffee Garden section. Jowai-Badarpur Road in the Jaintia Hills. ,The evidence from Meghalaya allows integration Palaeoenvironmental aspects between larger foraminifera and dinoflagellate cysts (as Ahy A predominance of Apeclodiniul11 is considered to be Aau Biozones are found to closely calibrate with SBZ4 to indicative of low salinity, estuarine or near shore environments lower part of SBZ7). The close stratigraphic correspondence (Downie el al., 1971; Costa & Downie, 1976). Its acme in of the Apeclodiniul1l Acme with the Ranikolhalia nUllalli NW Europe represents reduced salinity in a depositional Miscellanea miscella Assemblage Zone is suggested. It is setting below wave base with anoxic bottom conditions GARG & KHOWAJA-ATEEQUZZAMAN- DINOFLAGELLATE CYSTS FROM UPPER PALAEOCENEOF INDIA 477 (Heilmann-Clausen, 1985). The occurrence of bacterial accompanied by reduced salinity. In such an environment, infestation and pyrite relic structures (sensu Neves & Sullivan, carbonate precipitation is hampered resulting in considerable 1964) on cyst walls of Apectodiniul1l (pI. 1, fig. 11; pI. 2, fig. loss of carbonate platform environments and the de.struction 6) is suggestive of post depositional biodegradation of cysts of larger foraminiferal populations (Caron & Homewood, in a reducing environment. Similar conditions have been noted 1983; Jauhri, 1997). in the Apectodiniu/11 suite reported from the Krishna-Godavari Phytoplanktons. especially dinoflagellate cysts, are Basin (Garg et at., 1995). The association of Apeclodiniwil important (palaeo) biological indicators of upwelling and rich assemblages with NP9 Zone calcareous nannofossils eutrophication events. A predominance of Peridiniacean cysts (dominated by tiny Braarudosphaerids) has been attributed to (P) over Gonyaulacacean cysts (G) have been used as marker near shore, low salinity, restricted marine conditions (Jain & for palaeo-upwelling based on the studies of the Late Miocene Garg, 1986). to Holocene succession of the Bering Sea and North Pacific The occurrence of Apectodilliunz rich levels in the (Bujak, 1984). and Quaternary sediments off Peruvian Coast Lakadong Sandstone, therefore. suggests low salinity, reducing (Powell et at., 1992). An increase in phytoplankton biomass bottom conditions probably below wave base. The association typically takes place during eutrophication events caused by of pyrite relic structures with dinoflagellate cysts in the upper coastal upwelling (Powell et at., 1992) or river discharge part points towards anoxic bottom conditions. The occulTence (Malone, 1991). of iron sulphide has been noted in the upper horizons of In the context of a prolonged phase of "Expanded Lakadong Sandstone near its contact with the Umlatdoh Oligotrophy", in the Shillong Shelf, it may be possible to record Limestone in the ChelTapunji- Sheila Road section, Khasi Hills a short-lived phase of eutrophic conditions developed as a and the Thangski Coffee Garden section (Jowai-Badarpur consequence of changes in the depositional environment. The Road) in Jaintia Hills. In the Therriaghat and Mawmluh overall predominance of P-cysts and the rarity of G-cysts in sections, the predominance of almost monospecific assemblage the Lakadong Sandstone points towards stressful conditions of acritarchs and rarity of dinoflagellate cysts in the uppermost due to the onset of deposition in estuarine-coastal swamp Lakadong Sandstone may indicate stressful conditions (see environments. Lowered salinity caused by very high terrestrial Tyson, 1995). runoff is indicated by the abundance of land derived plant In a study based on larger benthic foraminifera from the debris, which tends to accompany increased terrigenous flux. Lakadong Limestone in South Shillong Plateau, Jauhri (1994, Sudden replacement of the dinoflagellate cyst assemblages 1996) related the foraminifera-algal rich carbonate buildups by an almost monospecific assemblage of the acritarch to the phase of "Expanded Oligotrophy" in a shallow neritic Collul1losphaera in the thin Lakadong Sandstone wedge selling that began to develop during P4 (Thanetian) times in (between thick foram-rich limestones in Therriaghat section) the Shillong region. This phase is characterised by the may have taken place due to shallowing accompanied by emergence of carbonate platform environments during the increased fresh water flux. Carbonate precipitation was deposition of the Lakadong Limestone and continued up to hampered for a short period. These conditions seem to be the deposition of the Umlatdoh Limestone. The oligotrophic indicative of a short-lived eutrophication event associated with selling, however, seems to have been interrupted for a while a sea level low stand towards the top of the Lakadong when limestone deposition was hampered and the coal-bearing Sandstone sandwiched between a major phase of "Expanded Lakadong Sandstone was laid down. Oligotrophy" towards the end of the Palaeocene. According to Brasier (1995), the marine ecosystem is under prolonged influence of oceanic control which operates DEPOSITIONAL SETTING AND through the interaction of several factors including climate, sea level fluctuations, nutrient influx, terrigenous input, SEQUENCE STRATIGRAPHIC upwelling, ocean mixing, etc. A state of Oligotrophy is CONSIDERATIONS generated during high stands of sea level when the climate is warmer and sea waters are depth stratified with low nutrient A comprehensive study of larger foraminifera and facies supply (Brasier, 1995). Because of decreased terrigenous flux development of the carbonate buildup in the Cherrapunji and increased production of carbonate precipitating organisms, Therriaghat area by Jauhri (1988, 1994, 1996, 1998) has oligotrophy enhances carbonate buildups on shallow shelves provided the basic depositional framework of the studied (HOllinger, 1987). During low stands of sea level, with succession. According to Jauhri (1994; personal relatively cooler climates and highly connective seawaters, communication), the Lakadong Limestone represents deposits eutrophic conditions are likely to develop. Such conditions of a progradational cycle of the carbonate platform laid down lead to considerable mixing of water masses and thereby during sea level high stand. This began after the Late enriched nutrient supply. This is also accompanied by increased Palaeocene marine flooding event, correlative to the Therria telTigenous flux giving rise to stressed environments and often lchno-llOrizon, possibly before the G. prill1aeva Zone MICROFOSSIL ASSOCIATION LITHOLOGY ENVIRONMENT DEPOSITIONAL LARGER DINOFLAGELLATE SETTING FRAMINIFERA CYSTS Influx of Low salinily reducing Cessallon of peal accumulatIon Apeclodinium bOllom conditions below due to Increased marine influence wave base (sediment supply reduced) Z o Coastal swamp inundated by marine waters f Peat accumulation taking a -< (lJ place just landward of the Thin impersistent coal seams Z Z :a Z shoreline in slill waler, low developing in bays or back No «Zc:: energy bottom conditions « water swamps; Peat accumula o dinoflagellate -«0 f- E lion frequently interrupted due cysts E--u. C/l (/) to active channel shifting and a o siliciclastic influx Z UJO:::(lJ o Z :r: -l ZUJZ :x: -< o tT1 «-l0 C/l ;;; -f E ------r - - +---+------1 :r: r :r: C/l o Ape<:lodif1ium » tT1 E-0:::(lJ Z ""u spp -l o '" Carbonate precipitalion hamper co UJ:a :0 o UJ o ~'-7-'''':'''''7' "0 >-- ed in shallow inner shelf due to o o ~e-~ > ;; .. 5:.'S: .' 5:. 'E ~'S .~ Fluctuating low salinity prograding shore line; z -< ----- ~ ~:U~ Poor dinocysls UJ (Coeval carbonate buildups o reducing bollom (/) ::.:: .. .~ -= ~ t: >. -l -l }::'<':::~:'~.~.~ condilions below continue in lower shore -< :g z wave base face/offshore region) ...J ----r~~~ (Partlyanoxic/dysoxic) ~~~' ~'~! ~ ~.~~ ~ ! .; E Increase in siliciclastic influx C/l - -- - .~.~ ~.~ coupled with high terrestrial ~~~~~~E~ ~-e(; ~ organic mailer supply during "' '" '" Poor dinocysls : ,-: ....., ':~ « ~ -5i 0 deceleration of sea level rise - f----- ~~:~: - ~ - - - -- +-----,---j------'------f--+------=------,-,-----1 CJ ...... '0 == - (equivalent to early P4 Zone). Evidence from dinoflagellate Mangroves grow along tropical coasts and estuaries under tidal cysts and other related data have now made it possible to influence with ability to root in an anoxic substratum understand the depositional setting of the Lakadong Sandstone (Chapman, 1977 in Van Campo, 1986). A wet and humid and associated strata and interpret the Upper Palaeocene climate is postulated on palynomorph evidence (Misra, 1992). succession of Khasi Hills in a sequence stratigraphic context The peat accumulation that was taking place just landward of (Figs. 8-9). the shoreline in still water, low energy bottom conditions was The Thaliasinoides -rich Ichno-horizon towards the top frequently interrupted by active channel shifting (Misra, 1992) of the Therria Formation represents a transgressi ve shelf sheet and intermittently increased terrigenous supply which is sand body formed during upward deepening as a consequence suggested by relatively high ash content of these coals (Raja of the rise in relative sea level after the post-Langpar sea level Rao, 1981). The progradational cycle seems to have fall. It is a widespread planar deposit of regional extent, albeit culminated with cessation of peat accumulation during late with some variations in the burrowing intensity (in the Khasi high stand possibly due to a change in sediment supply (which Jaintia Hills) and may serve as a marker bed within the Therria caused inundation of the peat mires by estuarine waters and Formation. Reduced sediment supply during the transgression cessation of coal formation before final withdrawal of the sea). favoured the prolific growth of burrowing organisms and Although the overlying sedimentary succession is missing from extensive bioturbation under subtidal conditions with a well Cherrapunji area, the thin wedge of Lakadong Sandstone oxygenated soft ground substrate. This Ichno-horizon overlying the Lakadong Limestone in Therriaghat section represents a marine flooding event of regional extent and provides evidence of a short-lived relative sea level fall. The indicates the most landward position of the shoreline extending common occurrence of acritarchs and lack of recognisable farthest northward onto the Cherrapunji Plateau (see Fig.4). dinoflagellate cysts, with high influx of terrestrial organic The overlying strata of the Therria Formation are relatively matter, in this unit is indicative of a general shallowing trend. thin bedded, finer sandstones/siltstones containing thin black Consequent upward shallowing in the upper part of the carbonaceous organic-rich partings and burrowed horizons. Lakadong Limestone in the basinal Therriaghat section is also The Therria lehno-horizon could be interpreted as a maximum reflected by the transition from dominantly wackestone facies flooding surface (mfs) or else, together with the overlying to packstone/ boundstone facies (Jauhri, 1994; Nichols, 1999). strata, may either represent a maximum flooding zone (sensu That the shelf area was not exposed completely (as a Emery & Meyers, 1996). consequence of the withdrawal of the sea) is indicated by the Continued decrease in sediment supply supported the lack of fluvial sediments or an erosional contact at the top of growth of algal-foram rich carbonate buildups that began to Lakadong Sandstone. This Late Thanetian relative sea level aggrade during the period of sea level high stand on the shallow fall seems to have lasted only for a short period and was shelves in lower shoreface/offshore region. This occurred followed soon after by a relatively rapid sea level rise during mainly under low-water energy conditions, possibly below fair latest Thanetian- early Ypresian times as evidenced by thick weather wave base, with depths ranging between 10-20 m or carbonate buildups of the overlying Umlatdoh Limestone. The even less (Jauhri, 1994). Sudden increase in terrigenous flux inferred relative sea level changes in South Shillong are during a further deceleration of sea level rise resulted in rapid indicated in Fig. 9. progradation of the shoreline. Carbonate buildups on the The identification of these short term sea level innermost part of the shelf in Cherrapunji Plateau region were fluctuations during the Late Thanetian (close to the obliterated giving rise to estuarine conditions. Alternating Palaeocene-Eocene boundary) supplemented by dinoflagellate sandstone/shale, showing limited burrowing activity (mostly cysts and foraminifer bioevents facilitates correlation of the thin horizontal tubes), and laminated carbonaceous shale Upper Palaeocene sequences of Khasi-Jaintia Hills with the partings (rich in terrestrial organic matter and low diversity Haq el ai. (1988) Sea Level Cycle Chart (see Fig. 9). The but rich peridinioid dinoflagellate cyst assemblages, and a total Langpar Formation exposed in the Um Sohryngkew lack of body fossils) is indicative of fluctuating low salinity (Therriaghat) section presents one of the most complete Upper reducing bottom (?partly anoxic/ dysoxic) conditions below Maastrichtian-Danian dinoflagellate cyst (Garg & Jain, 1993; wave base. Coeval with this, carbonate deposition continued authors' unpublished data), calcareous nannofossil (Garg & uninterrupted under well-oxygenated normal marine conditions Jain, 1995; Garg, unpublished data) and planktonic foraminifer in more basinal areas at Therriaghat. Continuation of the (Pandey, 1981; Pandey & Ravindran, 1988) records in the progradational cycle during sea level high stand gave rise to Indian region. The dinoflagellate cyst and planktonic coastal swamp conditions leading to the development of thin foraminiferal rich Danian shales/limestones of the Langpar impersistent coal seams in bays or back water swamps (Misra, Formation in Therriaghat area are overlain by a sandy-shaly 1992) enhanced by periodic cutoff of clastic supply. succession (the uppermost part of which contains A. Occurrence of mangrove pollen in associated sediments (Kar hyperacanlhum along with a sudden flux of terrestrial plant & Kumar, 1986) also favours proximity to the shoreline. debris but lacks planktonic foraminifera). It overlies the P ~ o00 SEQUENCE CHRONOSTRATIGRAPHIY SIGNIFICANT STRATIGRAPHIC EVENTS Haq el al. 1988 IN SOUTH SHILLONG REGION ::c u o Q. RELATIVE CHANGE OF COASTAL ONLAP I P3B f--- A. hyperacanthum " "- - 1.4 \ "\ P3A \ 1------I------~60 LANGPAR \ < 1--11-----1 SHALES \ z P2 NP4 < 1.3 Z - < C PIC GARG & KHOWAJA-ATEEQUZZAMAN- DINOFLAGELLATE CYSTS FROM UPPER PALAEOCENE OF INDIA 481 pussila bearing calcareous shales/limestones, indicating P3 coastal swamp depositional environments for the coal Zone (Pandey, 1981). The shallowing upward trend at the top bearing Lakadong Sandstone. of the Langpar Formation is indicative of a fall in relati ve sea 4. The close stratigraphical corresponden~eof the level. The hard sandy limestones (recrystallised and dolomitic Apectodiniu/11 Acme (Aau Biozone) with the at places) at the base of the Therria Formation, containing Ral1ikothalia nuttalli - Miscellanea miscella porcellaneous and arenaceous foraminifers, with a few Assemblage Zone of Larger Foraminifera (SBZ 5 calcareous perforate forms (Nagappa, 1959), represents a SBZ6) is suggested. marine flooding event and deposits of the early transgressive 5. The Lakadong Sandstone on the Cherrapunji Plateau phase. The relati ve sea level rise subsequent to the post region is the lateral facies equivalent of the upper part Langpar sea level fall in the late Selandian (late P3Zone) of Lakadong Limestone in the more basinal areas of matches with the base of the 3r~order cycle TA 2.1. The 77lerria Therriaghat area. fchllo-horizon that predates the G. primaeva Zone (late P4) 6. The Therria Limestone/Sandstone-Lakadong may correspond with the maximum flooding surface within Limestone/Sandstone sequence of Khasi hills TA 2·1 at 56·5 Ma (although precise age control is missing). represents deposits of transgressive and highstand The Therria Formation represents transgressive deposits while system tracts, representing a single depositional the aggradational-progradational succession of the Lakadong sequence between post-Langpar and post-Lakadong sea Limestone and Lakadong Sandstone are representatives of level falls corresponding broadly to the 3'd order cycles highstand system tract deposits (with extremely short-lived TA2·1-TA2·3. period for coal formation). This brief phase of coal formation AcknowLedgements-The aU/hors are grateflll to Prof Anshu K seems to be a widespread coeval event during a sea level Sinha, DirectOl; BSIP for facilities and constant encouragemel1l. highstand in the entire Garo-Khasi-Jaintia hilly tract. The sea Sincere appreciation is expressed to KP Jain, BSIp, AK Jauhri, De level fall at the top of Lakadong Sandstone apparently partment of Geology, University of Lucknow and Late SK Dutla, corresponds to the top of TA23 on the evidence of Dibrugarh University for help during fieldwork and for many sug dinoflagellate cysts (Aau Biozone) and larger foraminifera gestions. We are especially grateful to IB Singh and AK Jauhri, (SBZ 6) that may be calibrated with the late P5 Zone of Department of Geology, Universiry of Lucknow,for sri/llularing dis cussions and advice, which considerably improved our interpreta planktonic foraminifera. Thus, the combined Therria tions. Constructive commel1lS by AJ Powell, Surrey, UK, who criti Li Li mestone/Sandstone-Lakadong mestone/Sandstone cally reviewed the manuscript, are highly appreciated. Avinesh K succession of Khasi Hills represents a single retrogradational Srivastava, Pramod K Bajpai and Pradeep Mohan are thankedfor progradational depositional sequence, that possibly processing the material, drafting the figures and photoprillling re corresponds to combined )'d order cycles TA2'1-2'3 (despite spectively. the lack of more precise plankton dating). With the available data, it is not yet possible to find signatures of sea level changes REFERENCES that may have been associated with the cycles TA2·2 and TA2·3. A detailed analysis involving sedimentological and Berggren WA 1969. Rates of evolution in some Cenozoic planktonic biostratigraphical data from additional sections may foraminifera. Micropaleontology 15 : 351-365. considerably enhance our understanding of the Upper Berggren WA, Kent DV, Swisher CC & Aubury M-P 1995. A revised Palaeocene sequences of the South Shillong Plateau. Cenozoic geochronology and chronostratigraphy. 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