Early stages of foreland basin evolution in the Lesser Himalaya, N India

Abstract

Abstract In this paper we report preliminary results of an integrated sedimentological and structural study of the Late Cretaceous-Early Tertiary facies exposed in the Lesser Himalaya. The Precambrian-Palaeozoic basement of the Indian plate was initially transgressed in Late Cretaceous times (after c. 100 Ma), with the deposition of shallow shelf carbonates, (50 m thick Singtali Formation). These are depositionally overlain by green ferruginous mudstones and thin limestones of the 150 m thick Palaeocene-Eocene ( c. 65–40 Ma) Subathu Formation. During Late Eocene-Oligocene times ( c. 40–30 Ma), clastic sediment input increased and red lenticular sandstones, thinner planar-bedded sandstones, mudstones and caliche of the 350 m thick Dagshai Formation were deposited. A semi-arid, meandering fluvial/floodplain setting is envisaged, with thick channelised and overbank sands. Palaeocurrent evidence indicates a general SE progradation. Our detailed field mapping shows that where the entire Subathu and Dagshai Formations are intercalated, this is the result of post-depositional tectonics, rather than primary intertonguing as described in some previous reports. The relative abundance of sandstones greatly increased in upper Dagshai times, continuing into the Early-Mid-Miocene ( c. 30–10 Ma) when the 250 m thick Kasauli Formation was laid down. This formation is characterized by lenticular and planar-bedded grey sandstones, rich in plant material including occasional logs (but without caliche) interbedded with minor grey mudstones. A rapidly prograding, braided fluvial environment is proposed, with the petrography (e.g. presence of garnets) suggesting erosion and derivation from deeper levels of the Himalayan mountain belt to the north. The climate had by then changed from semi-arid to humid, possibly in response to the onset of the monsoon, initiated when the mountain belt had reached sufficient height to interfere with the jet-stream. During Late Miocene-Quaternary times (younger than c. 15 Ma), the Tertiary foreland basin was deformed and incorporated into three structural levels of a southward migrating thrust stack. At the lowest level, thick successions of the Tertiary Subathu, Dagshai and Kasauli Formations are structurally underlain by fluvial sediments (Siwalik Gp.) and overlain by the Krol Precambrian-Palaeozoic sedimentary nappe. At the intermediate structural level, Early Tertiary sediments unconformably overlie the stratigraphy within this nappe and comprise, often thin (<50 m), successions of mainly Subathu and Dagshai Formation sediments. The highest structural level sediments form small imbricates, highly dismembered units and melange (including evaporites) beneath the major metamorphic Jutogh Nappe. We interpret the Late Cretaceous-Palaeocene Singtali Formation as pre-collisional transgressive sediments related either to flexural downwarping of the Indian plate due to the obduction of the Spontang ophiolite onto the north Indian plate margin, or to extensional tectonics related to the detachment and drift of India from Gondwana and Neo-Tethyan subduction beneath Asia. This would have been coupled with the Late Cretaceous global eustatic sea-level high stand. The marine Subathu Formation is interpreted as having been deposited between initial and terminal India-Eurasia continental collision, with the overlying fluvial/floodplain Late Eocene-Oligocene Dagshai and Miocene Kasauli Formations as southward prograding foreland basin successions related to progressive stages of India-Eurasia continental collision.