Petrology of the Tista and Rangit river sands (Sikkim, India)

Giovanni Vezzoli (*), Bruno Lombardo (**) & Franco Rolfo (**,***)
(*) Dipartimento di Scienze dell'Ambiente, del Territorio e di Scienze della Terra, Università Milano-Bicocca, Piazza della Scienza 4, I-20126 Milano (Italy). Corresponding author e-mail: giovanni.vezzoli@unimib.it.
(**) CNR, Istituto di Geoscienze e Georisorse, Sezione di Torino. Via Valperga Caluso 35, I-10125 Torino, (Italy).
(***) Dipartimento di Scienze della Terra, Via Valperga Caluso 35, I-10125 Torino, (Italy).

The Tista River originates from glaciers in the North Sikkim (India) and flows towards southwest draining most of the Himalaya tectonic units up to Triveni locality, where it is joined by the Rangit River (its larger tributary). Tista fluvial profile has moderate concavity (θ = 0.39) and high steepness indices (ksn = 380). In Sikkim, Tista sand is characterized by quartz, feldspars and high-rank metamorphic rock fragments from the Higher Himalayan Crystallines. After Triveni, Tista River leaves Sikkim and enters in the Indian (West Bengal) and in Bangladesh floodplain to meet the Brahmaputra River after ~ 400 km from its source. In the alluvial plain, Tista carries sand rich in quartz, feldspars and low-rank metamorphic lithic grains. The Rangit River sourced from a glacier of Mt. Kabru in the southern Kangchendzonga region (West Sikkim). Its fluvial profile display high concavity (θ = 0.75) and high steepness indices (ksn = 408). Rangit sand from the upper catchment (Chokhurang Chu and Prek Chu) includes quartz, feldspars, and biotite. Fluvial detritus from the metacarbonates of the Pandim Massif includes high-grade metamorphic lithic grains, diopsidic pyroxene and amphiboles. Sand from the Higher Himalaya Crystallines (Kalej and Ramam Khola) is characterized by high-grade gneissic rock fragments. Before the confluence with the Tista River at Triveni, Rangit sand is enriched in low rank metamorphic grains deriving from the Daling successions of the Lesser Himalaya. Quantitative provenance analysis (integrated bulk-sand petrography, statistical analysis and river-profile analysis) indicates that the Rangit River in Sikkim contributes ~ 60% to the total Tista sand flux in Bangladesh.

Sand petrology, river profile, Tista and Rangit rivers.