Late Cretaceous (Santonian-Maastrichtian) Integrated Coccolith-Globotruncanid Biostratigraphy of Pelagic Limestones from the Accretionary Prism of Manipur, Northeastern India

Abstract

Employing special preparation techniques, a rich and moderately preserved assemblage of planktonic foraminifera and coccoliths is documented from pelagic limestones of Melange Zone of Manipur ophiolite belt around Ukhrul area in northeastern India. Integrated biostratigraphy of five exotic limestone blocks encompassing a Late Santonian / Early Campanian to Late Maastrichtian time slice is established; the low-latitude calcareous plankton zones of deep-sea facies identified in such a convergent plate margin terrain, are correlated with published new biostratigraphic, geomagnetic and absolute time scale data. Initial rifting and birth of the Indo-Myanmar ocean took place prior to Latest Santonian / Earliest Campanian and suturing was initiated by Latest Maastrichtian, as suggested by our study. INTRODUCTION Along the eastern border between India and Myanmar, the ophiolites and the associated suite of rocks are exposed in thickly forested hills of Nagaland and Manipur (text-fig. 1); these represent a part of northerly extension of Sunda ArchTrench System linking the Alpine-Himalaya and the Andaman-Nicobar Islands (Ranga Rao 1983; Acharyya et al. 1986, 1989). Rock units exposed in the Nagaland-Manipur hills can be broadly grouped from west to east, representing various stages of basin evolution and orogeny into the following: Facies Molasse Flysch Olistostromal deposits Ophiolite suite Metamorphic Lithounits Barail Group Disang Formation Melange Zone Oceanic Pelagic Sediments, Nagaland Ophiolite/Manipur Ophiolite Naga Metamorphic Complex Structurally, each of these units is overthrust by the next from the east and folded all together. The flyschoid Disang Formation gradually merges into the post-orogenic mollassic Barail Group of rocks towards the west. In the northern part of Manipur ophiolite belt in the Ukhrul district, the Melange Zone occurs along linear zones within the flyschoid shales of Disang Formation (text-fig. 1). The Melange Zone is characterised by the occurrence of exotic blocks of varying size (a few centimetres to tens of meters) composed of micritic limestone, marl, sandstone, basic rocks and conglomerate embedded in matrix of flyschoid rocks. The olistolithic blocks of pelagic limestones and chert could be hitherto dated as Late Cretaceous (Late Santonian to Late Maastrichtian) (Acharyya et al. 1986; Mitra et al. 1986, Chungkham 1996 and Chungkham and Caron 1996). Generally, the microfossil assemblages recovered from pelagic limestone blocks are comprised of coccoliths, planktonic foraminifera (plates 1-4), benthic foraminifera, radiolaria, ostracodes and a few shark placoid scales. Only selected index species of coccoliths and planktonic foraminifera are documented in the present work. Since high resolution age determination by using calcareous planktonic microfossils is crucial for understanding the basin evolution involving rifting, disruption and suturing events of ocean-floor in this arch-trench system, the objectives of the present work are: 1. To study the taxonomy and distribution of planktonic foraminifera and coccoliths, for constructing an integrated biostratigraphy based on the examination of the same sample. 2. To correlate the integrated biozonation with more recently published geomagnetic and absolute time scales for transoceanic correlations. 3. To contribute to the dating of the basin evolution. MATERIAL AND METHODS 120 samples were collected by one of us (P. Chungkham) from three main areas namely, Kangkhui, Hundung and Lambui. Five sections from five exotic blocks were studied specifically, Kangkhui, Hundung North Upper Band, Hundung North Lower Band, Hundung South and Mova (text-figs. 1-4). The Kangkhui section lying close to the ophiolite suite, served as the type for local intrabasinal correlations. Only limited number of samples taken from control points were analysed for the integrated coccolith-globotruncanid study. It is well known that olistolithic pelagic limestone blocks are found in chaotic disposition within a highly sheared matrix, thus hampering the sampling in strict stratigraphic order and regular interval. Considering heavy vegetation cover, the positions of sections were clearly marked on each block by driving iron nails. Planktonic foraminifera The limestones were processed with a modified maceration technique earlier developed by Zolnaj (1979). 300 to 500 specimens from each sample were picked from 60 to 100 mesh size (ASTM) fractions and studied in detail under a normal binocumicropaleontology, vol. 44, no. 1, pp. 69-83, text-figures 1-4, plates 1-4, 1998 69 This content downloaded from 157.55.39.78 on Tue, 21 Jun 2016 07:06:32 UTC All use subject to http://about.jstor.org/terms Prithiraj Chungkham and Sved A. Jafar: Late Cretaceous integrated Coccolith-Globotruncanid biostratigraphv, northeastern India