Lower Oligocene Bivalves of Ramanian Stage from Kachchh, Gujarat, India
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Lower Oligocene bivalves of Ramanian Stage from Kachchh, Gujarat, India R P Kachhara∗, R L Jodhawat and K Bigyapati Devi Department of Geology, M.L. Sukhadia University, Udaipur 313 001, India. ∗Corresponding author. e-mail: [email protected] Marine Oligocene sequences in India outcrop only in western part of Kachchh. Earlier researchers have recognized the Oligocene strata under the Nari Series (Nagappa 1959; Chatterji and Mathur 1966). The Nari Series has a type area in Pakistan. It has two subdivisions – the Lower Nari (Lower Oligocene) and the Upper Nari (Upper Oligocene). It seems that there is no valid proof about the age of the Lower Nari due to lack of proper fauna (Eames 1975), and according to Pascoe (1962), the Upper Nari slightly transgress into Aquitanian (Lower Miocene), therefore, one has to be very cautious. Biswas and Raju (1971) reclassified the Oligocene strata of Kachchh and lithostratigraphically clubbed them as the Maniyara Fort Formation with type section along the Bermoti stream. This Formation has four members. The lower three members correspond to the Ramanian Stage (Lower Oligocene, Biswas 1971, 1973) while the uppermost to the Waiorian Stage (Upper Oligocene, Biswas 1965, 1971, 1973). The Ramanian Stage is characterized by large forams especially Nummulites fichteli, Nummulites fichteli intermedius, Lepidocyclina (Eulepidina) dialata and Operculina sp. Several ostracods are also known to occur. Megafauna include bivalves, gastropods, echinoids, corals, mammals and reptiles. Concerning bivalves earlier researchers have recorded a few taxa namely Trisidos semitorta (Lamarck), Cubitostrea angulata (J de C Sowerby), Pecten (Amussiopecten) labadyei d’Archiac and Haime, Periglypta puerpera (Linne’) var. aglaurae Brongniart, Ostrea fraasi Mayer Eymer and listed Pecten laevicostatus JdeC Sowerby, Callista pseudoumbonella Vredenburg and Clementia papyracea (Gray) from Kachchh as against overall 42 forms from the Nari Series as a whole (Vredenburg 1928). This tempted us to make an attempt to collect bivalve fauna systematically which are occurring prolifically in the Ramanian Stage. In the present work, for this purpose, sections are worked out around Lakhpat (23◦50N; 68◦47E), Maniyara Fort (23◦2805N; 68◦37E) Rakhdi Dam (23◦2726N; 68◦4010E) and Waior (23◦2505N; 68◦4137E) with a view to highlight the entombed bivalve taxa. Authors have encountered 53 species of which 23 are restricted to the Ramanian Stage. 1. Introduction Group, Argillaceous Group and Recent–Upper Tertiary. Of these, Nummulitic Group has been Wynne (1872) for the first time reported Tertiary equated by the subsequent researchers with the deposits in Kachchh. He classified these deposits Nari Series of Pakistan and the term Nari has into six groups namely Sub-Nummulitic Group, been adopted for the Oligocene strata of Kachchh. Gypseous Shales, Nummulitic Group, Arenaceous Sen Gupta (1959, 1964) has considered Oligocene Keywords. Bivalves; Ramanian Stage; Lower Oligocene. J. Earth Syst. Sci. 121, No. 2, April 2012, pp. 405–438 c Indian Academy of Sciences 405 406 R P Kachhara et al beds of Lakhpat area as part of the Nari Series Mathur (1988) has reviewed the work on marine with characteristic Nummulites fichteli Michelotti Tertiary larger invertebrate fauna of India and in abundance. Biswas (1965) proposed a new reported occurrences of Oligocene molluscs in classification of Tertiary rocks of Kachchh and the Maniyara Fort Formation (Bermoti Series) grouped Oligocene beds into the Lakhpat Series, with a check list – Pecten laevicostatus, Cytherea characterized by reticulate Nummulites, well cor- (Callista) pseudoumbonella, Ostrea angulata, relatable with the Nari Series of Sind–Baluchistan. Cerithium sp., Lunatia sp. (Sen Gupta 1964), Tewari and Singh (1966) while working on Kanoj– Venus (Clementia) papyracea, Pecten labadyei, Sehe area of western Kachchh considered their Amusium sp., Trochus sp., Trochus cf. comulans, Bed Nos. 4 and 5 as Lattorfian (L. Oligocene) Natica sigaratina (Chatterji and Mathur 1966), and Rupelian (M. Oligocene) respectively, the and Ostrea angulata, Ostrea latimarginata, Ostrea latter one on the basis of the occurrence of sp., Pecten sp., Turritella sp., Cypraea sp.,? Lepidocyclina (Eulepidina) dialata. Raju (1969) Scalaria sp. (Raju 1974). Khosla and Pant (1988) has shown that in Ramania area of Kachchh have studied three sections in Kachchh (i) near the Oligocene strata start with appearance of the Rakhdi Dam, (ii) between the Waior Dam and Nummulites fichteli. In addition, Raju et al Waior village, and (iii) outside Lakhpat Fort. They (1970) have recognized three biozones, viz., Num- have recorded Oligocene ostracod assemblages mulites intermedius-fischteli assemblage zone, with two biozones one each in the Ramania and N. intermedius-fichteli-Lepidocyclina assemblage the Waior Stages of the Bermoti Series namely zone and Miogypsinoides complanata zone of Actinocytheries ramaniaensis and Actinocythereis Lower, Middle and Upper Oligocene, respectively kutchensis with an age of Lattorfian to Rupelian in Walasara and Goyela areas of Kachchh. They and Chattian, respectively. also favoured the view expressed by Chatterji Biswas (1992) has given more detailed account and Mathur (1966) that the Bed No. 2 of Tewari of Tertiary stratigraphy of Kachchh in which (1959) in Waior area containing Spiroclypus ran- he retained earlier proposed nomenclature for all janae which was assigned to Lower Miocene lithostratigraphic formations and the stage names should be taken as Chattian (Upper Oligocene) were rectified to proper ending. Following him, in the absence of typical Lower Miocene fossils. there are five biozones in the Maniyara Fort For- While writing a note, Biswas (1971) modified mation, viz., Nummulites fichteli zone of Lower his earlier proposed classification by chang- Oligocene in Basal Member, Nummulites fichteli- ing the nomenclature of the Lakhpat Series to Lepidocyclina (Eulepidina) dialata zone of Lower the Bermoti Series with two stages namely the Oligocene (Rupelian) in Coral Limestone Member Ramania (Lattorfian to Rupelian) and Waior and Planolinderina freudenthali zone, Miogypsina (Chattian). It may be recalled that the Waior (Miogypsinodes ) bermudezi zone, and Miogypsina Stage in his earlier classification was part of the (Miogypsinoides) complanata-formosensis zone in Khari Series assigned to Miocene. Later on, Biswas the ascending order of Upper Oligocene (Chattian) and Raju (1971) proposed a rock-stratigraphic in the Bermoti Member. Same scheme is being classification for Tertiary rocks of Kachchh in followed by Pandey and Dave (1998). It may be which Oligocene strata were included in the Mani- noted that the Nari Series were deposited during yara Fort Formation with four members namely Oligocene time and has been divided into Lower Basal, Lumpy Clay, Coral Limestone and Bermoti. and Upper. However, regarding the age of the Of these, first two are Lower Oligocene, third one Lower Nari, there is no fool proof evidence (Eames as Middle Oligocene and the last one as Upper 1951) and the Upper Nari seems to transgress Oligocene. Mohan and Pandey (1971) have also into Lower Miocene (Pascoe 1962). Only a few preferred to retain the name the Maniyara Fort molluscan fauna from Kachchh Oligocene strata Formation for the Oligocene beds of Kachchh. are reported by earlier researchers as compared to Raju (1974) on the other hand, favoured the overall 42 forms of the Nari Series. In view of it, a chronostratigraphy classification for the Oligocene systematic collection is being made from different strata of Kachchh as the Ramania and the Waior sections of the Maniyara Fort Formation, viz: Stages in the ascending order. He recognized two biozones in the Ramania Stage namely Nummulites • outside northern wall of the Lakhpat Fort, fichteli-intermedius zone and Nummulites fichteli- • the Rakhdi Dam, intermedius/Lepidocyclina (Eulepidina)zoneof • the Bermoti stream around Bermoti village, and Lower to Middle Oligocene (Lattorfian to • a section between Waior village and the Waior Rupelian), and on the basis of Miogypsinidae Dam of lower three members namely Basal, assemblage especially Miogypsina (Miogypsi- Lumpy Clay and Coral Limestone of Lattor- noides) complanata Schlumberger, assigned the fian to Rupelian time, i.e., Lower to Middle Waior Stage to Upper Oligocene (Chattian). Oligocene. Lower Oligocene bivalves of Ramanian Stage 407 2. Stratigraphy among bivalves Pectinids and Oysters are quite conspicuous as also foraminifers. Middle part is Oligocene strata of Kachchh indicated earlier are solely characterized by corals and in the upper grouped lithostratigraphically under the Maniyara part molluscs again made their appearance but Fort Formation divisible into four members namely with very thin population. The Bermoti stream Basal, Lumpy Clay, Coral Limestone and Bermoti. has a full section of the Ramanian Stage. The Leaving aside the last one, others chronostrati- succession starts with brownish to yellowish glau- graphically equated to the Ramanian Stage of conitic siltstone followed by calcareous claystone, Lattorfian to Rupelian time. Assignment of age is highly fossiliferous cement coloured claystone and based on the presence of reticulate Nummulites dirty white coral limestone with peculiar weather- namely Nummulites fichteli and Lepidocyclina ing at the top. At the Rakhdi Dam, lithic units (Eulepidina) dialata. It may be mentioned that are in the form of glauconitic sandstone, shale, hard otherwise also Nummulites do not occur beyond