Foraminifera 1 morphogroups , paleoenvironments and new taxa from Jurassic to Cretaceous strata of Thakkhola , Nepal

The foraminiferal assemblages of the Upper Jurassic Nupra Formation and Lower Cretaceous Tangbe Formation in Thakkhola, Nepal are documented and a morphogroup analysis is carried out. Fifty-five agglutinated and four calcareous species are identified from a shale siltstonesandstone succesii%n which was originally deposited on the northern Gondwana margin (southern hemisphere). Four new species are described: Tolypammina undosa, Thalmannammina glomata, Parvigenerina mucromata and Trochammina nupraensis. The distribution of agglutinated foraminiferal morphogroups and the variations in species diversity are important indicators of environmental changes. The Oxfordian to Kimmeridgian black shales contain well diversified assemblages which are dominated by epifaunal morphogroups, but include also a large infaunal component. These shales are interpreted as slightly dysaerobic deeper shelf deposits. The Tithonian dark silty shales are typified by low diversity assemblages and by the predominance of elongate infaunal morphotypes; these shales are taken to represent a prodelta facies. The Lower Cretaceous consists of alternating shales, siltstones and sandstones with foraminiferal assemblages of very low diversity, dominated by flattened spiral epifaunal morphotypes. These shales are attributed to shallow prodelta shelf to delta plain environments. The study demonstrates that the morphogroup distribution pattern of the Thakkhola succession shows strong analogy with modern distribution trends. The Jurassic assemblages recorded from the prodelta to delta plain deposits of the North Sea Basin and those from corresponding facies in the Thakkhola succession display similar morphogroup patterns. INTRODUCTION The Upper Jurassic to Lower Cretaceous succession of Thakkhola was deposited in the southern hemisphere, on the northern continental margin of Gondwana. Previous studies of agglutinated assemblages have been heavily biased towards northern faunas, and relatively little is known about the agglutinated foraminifera of the southern hemisphere. The Late Jurassic was a period of extensive organic-rich shale deposition which led to the accumulation of important hydrocarbon resources. Agglutinated foraminifera can, therefore, provide very useful tools for palaeoenvironmental and palaeoecological studies of these deposits. The material studied in this paper was collected in 1988 and 1991 in the Thakkhola area of northern Nepal, located immediately north of the central Himalayan range. The field area is accessible through the Kali Gandaki River valley, and is delineated by the villages of Jomosom, Tangbe, Chhukgaon, and Muktinath at approximately 29"N and 84"E (Fig. 1). The Upper Jurassic to Lower Cretaceous succession of the region has a minimum thickness of 1100 m, and consists mainly of shales, siltstones and sandstones deposited under deltaic to deeper shelf conditions. The sediments were strongly lithified during diagenetic processes, and were, to a varying degree, folded and faulted during the Himalayan collision. ' The geology and stratigraphy of the Mesozoic of the Thakkhola region was recorded in detail by Gradstein et al. (1989; 1992). Both papers also include preliminary reports of the Upper Jurassic to Lower Cretaceous foraminifera. The Lower Cretaceous stratigraphy of the region was emended by Gibling et al. (1994). The foraminiferal succession of Thakkhola is discussed by Nagy et al. (in press), with regard to stratigraphy, depositional facies and regional affinities. The main objectives of the present study are: 1) to use a morphogroup analysis In: Kaminski, M.A. Geroch, S., 8 Gasinski, M.A. (eds.) 1995. Proceedings of the Fourth International Workshop on Agglutinated Foraminifera, Krakow Poland, September 12-1 9, 1993. Gaybowski Foundation Special Publication no. 3 , pp. 181 -209. 182 Nagy, Gradstein, Kaminski, and Holbourn Figure 1. Map of the Thakkhola area with location of the studied sections marked. Inset ma shows India and Ne a1 wig position of T h a k d . for the environmental interpretation of the westwards in the Northern Himalayas where the succession, 2) to describe new taxa present at this well-known Spiti Shale represent this facies. mid-latitude site in the southern hemisphere. Depositional setting The Mesozoic deposits of northern Nepal are the remnants of the sedimentary cover of the Tethyan lithosphere consumed at a northward-dipping subduction zone, when oceanic ridge-spreading led to the northward drift of India and the opening of the Indian Ocean. The formations studied are part of a Triassic-Lower Cretaceous sedimentary succession representing a detached sliver of the ancient Gondwana margin, which was obducted onto the Eurasian plate during ocean closure and continental collision in the mid-Tertiary. The deposition of the Thakkhola succession took place on the southern margin of the Tethys Ocean (Powell et al., 1988), when Thakkhola lay at mid latitudes, 30"-41"s (Gradstein et al., 1992). The Thakkhola region was then located on the northern continental margin of India, which was part of eastem Gondwana, also including Madagascar, Antarctica and Australia (Fig. 2). This passive margin formed part of an extensive continental shelf to slope area where organic-rich deposits accumulated in Late Jurassic time. The Nupra Shale belongs to this facies, and similar deposits can be traced Figure 2. Paleo eographic reconstruction of circumNeoTethys lan!masses in Late Jurassic and Early Cretaceous time (modified after Sengor, 1985, and other sources). Th=Thakkhola, Nepal. Stratigraphical framework A compilation of age significant fossil occurrences in the Oxfordian to Albian succession of Thakkhola is given in Table 1 and Fig. 3. Detailed discussions of the stratigraphy and age relationships are provided by Gibling et al., (1994) and Nagy et al., (in press). The lithostratigraphic scheme proposed by Gibling et al. (op.cit.) is followed in this paper. Jurassic Cretaceous foraminiferal morphogroups, paleoenvironrnents, and new taxa from Nepal 183