0016-7622/2017-90-4-437/$ 1.00 © GEOL. SOC. INDIA | DOI: 10.1007/s12594-017-0737-1 JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.90, October 2017, pp.437-446 Biostratigraphy and Depositional Environment of the Miocene Limestone Bed of Baripada, Mayurbhanj District, Odisha: Foraminiferal, Sedimentological and Bulk Organic Geochemical Evidences Ajoy K. Bhaumik 1* , Joydeep Paikray 1 , Jagadish Dutta 1 , Arijit Mitra 1 , Devleena Tiwari 2 and Dattatray J. Patil 2 1 Department of Applied Geology, Indian Institute of Technology (Indian School of Mines), Dhanbad – 826 004, India 2 Petroleum Geochemistry and Microbiology Group, National Geophysical Research Institute, Hyderabad – 500 007, India *E-mail: ajoyism@gmail.com ABSTRACT Baripada Marine Beds (BMB) have been studied extensively in terms of its mega fossil content. However, not much has been discussed about the foraminiferal content and the organic matter assemblage in these beds. The fossiliferous sequence of BMB consists of sandstone, shale and limestone units. The present study is persued on the limestone unit of BMB. Foraminiferal, thin section and rock-eval pyrolysis studies were performed on the 33 surface samples collected from five sections of Jamdapal and Mukurmatia region along the Budhabalang river bank. Foraminifera are less abundant in the samples of Jamdapal, whereas Mukurmatia is comparatively rich. First appearance datum of Globorotalia menardii[ranges since middle Miocene (12.6 Ma); FAD at planktic foraminiferal zone N12] and last appearance datum of Neogloboquadrina continuosa [ranges between early Miocene (23.2 Ma) to late Miocene (8.3 Ma); FAD at planktic foraminiferal zone N4B and LAD at N16] together suggest that the limestone unit was deposited in between 12.6 to 8.3 Ma within the upper Miocene. Also, the association of shallow water benthic foraminifera (Species of Ammonia, Asterorotalia, Bolivina, Buliminella, Cibicides , Challengerella, Criboelphidium, Cribononion, Elphidium, Hanzawaia, Nonionella) and planktic foraminifera ( Globigerina falcoensis , Globigerina woodi , Globorotalia menardii, Neogloboquadrina continuosa) together with oyster bank and shark teeth suggest deposition of limestone within well oxygenated, tropical, shallow water, open marine condition (within 40m water depth). Lithological and thin section studies together with global sea level fluctuation history advocate that upper Miocene marine transgression promoted the formation of this unit. Bulk organic geochemical data obtained by the rock-eval pyrolysis studies on selected samples indicate a low total organic carbon (TOC), with low hydrogen index (HI), high oxygen index (OI). The organic facies is characterised by type-IV kerogen with major contribution from near shore terrestrial plants. This also suggests deposition in shallow, oxygenated environments that did not promote significant accumulation and preservation of organic content in sediments. INTRODUCTION Fossiliferous Tertiary Baripada Marine Beds (hereafter, BMB), Mayurbhanj district, Odisha, India are famous for their mega fossil content, particularly Ostrea. However, these Tertiary beds have been less attended by the researchers in comparison to the other Tertiary beds exposed in the country, possibly owing to the scattered and restricted exposure with lesser thickness of beds. Exposures of fossiliferous clay, yellow shale and impure limestone beds are mostly restricted in and around Budhabalang river section, particularly in Mahulia, Satpatua and Mukurmatia area. These beds were first described by Bose (1904), who assigned the Eocene-Oligocene age to it, based on the Ostrea. Studies on the Ostrea in BMB have also been done by several other workers (Eames, 1936; Jena, 1942; Modak, 1952; Sarma, 1957; Tewari and Aswathi, 1960). Besides Ostrea, abundance of the other lamelli branches (Nuculana, Arca, Cardium, Pectin etc.), fragments and moulds of gastropods (Turritella, Natica, Conus, Dentalium), arthropods (fragments of crab, prawn), fragments of chondrichthyes (parts of Mylobite, shark teeth), teeth of some rodents, snakes, molar teeth of bovine (?), canine teeth of carnivores, remains of turtle and crocodiles have also been reported by several workers (Ghosh, 1956; Bhalla and Dev, 1975a,b; Sahni and Mehrotra, 1981; Mondal et al., 2009; Sharma and Patnaik, 2010, 2013a,b; Rath and Mishra, 2013). However, the foraminiferal investigation in this area has been less attended and only few reports are available (Sarma, 1957; Bhalla and Dev, 1975c; Dev, 1975; Singh et al., 1976a,b). Singh et al. (1976a,b) and Dev (1975) recorded presence of benthic foraminiferal genera like Ammonia, Buliminella, Brizalina, Cribroelphidium, Elphidium, Florious, Nonion, Valvulineria, Cribononion, Lagena and Cibicides. On the contrary, planktic foraminiferal abundance within these sequences are relatively scare and the dominant species are Globigerina cf. G. quadrilobata, Globorotalia sp., Orbulina suturalis, Turborotalia obesa and Turborotalia continuosa (Sarma, 1957; Bhalla and Dev, 1975c; Singh et al., 1976a,b). It is considered that the sediments of the BMB deposited in sub- tropical, shallow water, warm condition close to land, in a lagoonal or back reef or protected environment near to the shore line (Bhalla and Dev, 1975a,c; Singh et al., 1976a,b; Paikra, 1990; Vaidyanadhan and Ramakrishnan, 2010; Sharma, 2013; Sharma and Patnaik, 2010; 2013a,b). However, there are a lot of disagreements regarding the age of the BMB. The assigned ages by different workers show a wide range from Eocene to lower Pleistocene. The suggested ages are Eocene to Oligocene (Bose, 1904), Miocene (Jena, 1942,1943; Tewari and Aswathi, 1960), lower Miocene (Eames, 1936; Modak, 1952; Sarma, 1957; Mehrotra et al., 1973; Singh et al., 1976a,b; Sahni and Mehrotra, 1981; Vaidyanadhan and Ramakrishnan, 2010), middle Miocene (Bhalla and Dev, 1975a,b,c; 1984, 1988; Paikra, 1990;Sharma and Patnaik, 2013a), upper Miocene (Sharma and Patnaik, 2010; 2013b; Sharma, 2013) and Tertiary to lower Pleistocene (Ghosh, 1956, 1959; Chaudhuri, 1958).