Foraminiferal Stratigraphy and Paleoenvironments of Late Jurassic to Early Cretaceous Deposits in Thakkhola, Nepal

Abstract

This is the first detailed account of Late Jurassic, deeper marine, southern hemisphere, agglutinated benthic faunas, and their close taxonomic affinity to coeval assemblages from northern high latitudes. Over 1100m of dark shale, siltstone and sandstone in Thakkhola (Nepal) are dated by means of ammonites, dinoflagellate cysts and foraminifera as Oxfordian through latest Albian. The succession was deposited along the northern Gondwana margin, bordering Tethys, while Thakkhola lay at mid-latitudes (30-41?S). A highly diversified agglutinated faunal record in the Oxfordian lower Nupra Formation, with 44 taxa, is segmented (in ascending order) into the Eomarssonella paraconica assemblage, Vemeuilinoides graciosus assemblage and Spiroplectammina suprajurassica assemblage. The assemblages are of restricted deeper shelf nature, in agreement with geochemical, sedimentary and ammonite-based depositional interpretation. The Tithonian upper Nupra Formation is attributed to a prodelta depositional setting, and contains a low-diversity, mainly agglutinated assemblage named after Trochammina annae. The Early Cretaceous Tangbe Formation shows a change to coarser terrigenous clastics deposited under shallow shelf conditions characterized by the Trochammina aff. schaimica assemblage of agglutinated taxa with extremely low diversity. The Late Jurassic assemblages of Nepal have a majority of taxa in common with those of Western Siberia, Svalbard and the Canadian Arctic Archipelago, which suggests that these contain many cosmopolitan taxa, suitable for global stratigraphic correlations. INTRODUCTION During 1988 and 1991, international field excursions were carried out to the Mesozoic strata of the Himalayan mountain range (Gradstein et al. 1989, 1992). These passive continental margin sediments are preserved immediately north of the central Himalayan range in the Thakkhola region of northern Nepal. The present report describes new paleontologic findings, with emphasis on biostratigraphic, paleoecologic and paleogeographic interpretations from studies of foraminifera. Field observations and sediment samples originate from the above-mentioned excursions. The field area is delineated by the villages Jomosom-TangbeChhukgaon-Muktinath, at approximately 28?N and 84?E. Here, relatively easily accessible Late Jurassic and Early Cretaceous strata, over 1100m thick, are exposed at altitudes of 2800 to 5000m. The sediments were deformed to varying degrees by folding and faulting during Himalayan collision in Cenozoic time. The geological literature refers to this area as Thakkhola and we follow this usage. The location of the field sections studied is shown in text-figure 1. Samples are designated according to field section, e.g. J02, followed by the sample number. The Late Jurassic through Early Cretaceous formations of Thakkhola contain exclusively or predominantly agglutinated assemblages. An exception is the topmost (Late Albian) part of the succession, which contains a calcareous assemblage mainly of planktonic species. Preliminary accounts of these assemblages were published previously in Gradstein et al. (1989, 1992). The Late Jurassic agglutinated assemblages are new to southern Asia, and show unexpected affinities to the so-called boreal faunas of the northern hemisphere, which may be more cosmopolitan than known so far. PALEOGEOGRAPHIC SETTING Paleomagnetic reconstructions indicate that Australia, Antarctica and Greater India, which included northern Nepal, formed part of the southern landmass of Gondwana in Jurassic time (Sengor 1985; Powell et al. 1988). The Thakkhola area lay on the continental margin of northern Gondwana, bordering the Tethys Ocean to the south, until at least the Early Cretaceous (text-fig. 2). Consequ ntly, the Jurassic and Early Cretaceous succession of Thakkhola shows marked similarities with the sequences drilled at ODP sites off northwestern Australia (Gradstein and von Rad 1991; Gibling et al. 1994). Northerly paleoflow directions measured in Early Cretaceous fluvio-deltaic deposits of Thakkhola are in accordance with deposition on the subsiding northern Gondwana continental margin. Paleomagnetic data confirm that the Thakkhola area lay at mid latitudes, 30-41?S, in Late Jurassic to Early Cretaceous times (Ogg in Gradstein et al. 1992). Early Jurassic carbonates (Jomosom and Bagung formations) reflect Thakkhola's position in the subtropical zone, and correspond to the most northerly paleolatitude recorded. In the Late Jurassic to Early Cretaceous the area drifted to higher latitudes, reflected by deposition of terrigenous clastics composing the Nupra, Chukh and Tangbe formations. A return to hemipelagic carbonate sedimentation in Late Albian time may reflect in part a reversal of drift (with movement northward), and in part a global rise in sea level, which drowned the shelves and restricted influx of terrigenous clastic sediments. STRATIGRAPHIC AND DEPOSITIONAL FRAMEWORK The Late Jurassic to Early Cretaceous succession of Thakkhola comprises deltaic, marine shelf and upper slope sediments. The micropaleontology, vol. 41, no. 2, pp. 143-170, text-figures 1-12, plates 1-5, tables 1-4, 1995 143 This content downloaded from 157.55.39.163 on Fri, 18 Nov 2016 04:19:11 UTC All use subject to http://about.jstor.org/terms Jeno Nagy et al: Foraminiferal stratigraphy and paleoenvironments of Late Jurassic to Early Cretaceous deposits in Thakkhola, Nepal