Sedimentation pattern in a trans-Himalayan Quaternary lake at Lamayuru (Ladakh), India

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Located on the Indo-Tsangpo Suture Zone, Lamayuru Lake in western Ladakh was created by neotectonic instability around 35–40 ka. The valley fill sequence, about 110 m thick, reveals an interplay of lacustrine to fluvio-deltaic to colluvial processes operating in response to changing climate and tectonic conditions. Based on integrated approach of sedimentology and palaeontological analysis, we propose a sedimentation model for Lamayuru Lake with reference to changing climatic conditions and neotectonics. The coarsening upward succession was divided into five lithofacies associations (Associations A–E). Each association, comprising one or more lithofacies, represents a specific depositional environment, and marks an independent episode of sedimentation. A dominantly muddy sequence of Facies Association A, a product of a lacustrine environment, is followed by coarsening upward cycles (Association B) composed of silt to sand that implies laterally shifting deltas. Thereafter, thick sand and gravel deposits constituting fining upward (FU) unit cycles (Associations C and D) represent deposition in a fluvial domain. The upper part of Association D, composed of matrix-supported gravel horizons, was deposited as debris flows and marks the termination of sedimentation in the valley. The clastic lithofacies (Facies Associations A to D) with plant fragments and charcoal presumably indicate warm–humid climatic conditions. In contrast, Association E, comprising marl lithofacies occurring at four different levels is biogenic and indicates arid to semi-arid climatic phases. Five main types of trace fossils (including rhizoliths) seem to be representatives of Scoyenia ichnofacies and animal burrows indicating a thin population of soft-bodied annelid and arthropod communities living under low energy condition. Nine fossiliferous horizons have yielded characteristic ostracods, charophytes and gastropods indicating shallow, low energy and cold-water ecological conditions. Coarsening up lake succession, extensive synsedimentary deformation, tilting of lake beds and the entrenchment of the Lamayuru drainage indicate basin evolution under an effective role of neotectonics in the area.