Proterozoic Sedimentary Successions in the Himalayan Orogen: Stratigraphy, Sedimentology and Palaeobasinal conditions

The Himalayan orogen forms a 2500 km long and 250 km wide arc along the leading margin of the Indian plate. The Higher, Lesser, and Sub-Himalaya are thus slices of the old Indian shield that have stacked over one another by transfer along a series of southward verging thrusts. Four prominent lithoevents are recorded in the Late Palaeoproterozoic to Early Palaeozoic sedimentary succession of the Lesser and Tethys Himalayan part: i) Palaeoproterozoic Argillite=Siliciclastic, ii) Mesoproterozoic Calcareous (limestone) >Argillite, iii) Neoproterozic Argillite=Siliciclastic, and iv) Late Neoproterozic-Early Palaeozoic Mixed Siliciclastic-Argillite-Calcareous (limestone). Based on geologic, sedimentologic and petrographic attributes, the central Lesser Himalayan (Garhwal and Kumaun) Proterozoic successions have been segregated into outer (southern) and inner (northern) domains. Of these the pre-Blaini sedimentary succession of the Outer Lesser Himalaya (OLH) is younger (Neoproterozoic) than Inner Lesser Himalaya (ILH) (Palaeoproterozoic-Mesoproterozoic). The gradual coarsening up succession, sub-aerially erupted volcanics, and occurrence of several gravelly beds suggest an unstable shallowing basin. A consistent northwest-southeast trending palaeo-shoreline from Neo- and Palaeoproterozoic successions of OLH and ILH has been visualized. The compositional consistency of OLH and ILH siliciclastics hints towards similar source area lithology (the Aravalli-Delhi Supergroup and less commonly the Banded Gneissic Complex and Bundelkhand Granite-gneiss). The presence of alluvial fan facies along the Tons thrust possibly hint towards the existence of a palaeo-high (horst) region of the rifted basin between OLH and ILH. The thinning and sagging of the basin floor of the basement and development of shallow marine set up is also consistent as some of the mafic bodies present within the studied sediments bear a "within plate basaltic" signature. It is also consistent with the palaeogeographic settings during the Proterozoic time, when no deep oceans existed around the Indian subcontinent. Precambrian stratigraphy of Himalaya has close proximity and resemblance with the Vindhyan basin which rests unconformably on the Bundelkhand and Aravalli cratons. Though the Vindhyan basin is traditionally considered intracratonic but it is in fact, an epicratonic basin located along the hinges of the Archean nuclei of the Indian shield and has a lots of similarities in sedimentation pattern with the Lesser Himalayan basin. Therefore, it will be appropriate to consider the Precambrian succession of Himalaya as a part in the Aravalli- Bundelkhand craton.