Rapid grain size coarsening at sandstone/conglomerate transition: similar expression in Himalayan modern rivers and Pliocene molasse deposits

摘要: Radical grain size changes between two main units of a sedimentary megacycle in foreland basin are commonly interpreted to result from tectonic activity or climate the adjacent mountain range. In central Nepal, Cenozoic Siwalik molasse deposits exposed frontal Himalayan folds characterized by such radical transition. Locally gravel completely replace sands vertical succession over approximately hundred metres, median size(D50) displaying sharp increase factor ca. 100. Such rapid gravel-sand transition (GST) is also observed present-day river channels about 8–20 km downstream outlet Range. The passage gravel-bed channel reaches (proximal alluvial fans) sand-bed (distal occurs within few kilometres on Gangetic Plain Nepal,and D50 ratio types equals We propose that dramatic and remarkably similar Neogene series along modern rivers basin, results hydraulic process, i.e. sorting process during selective deposition sediment load. sudden appearance gravels upper would be related crossing this progressive southward migration front, response continuous orogen construction. And as consequence, GST diachronous nature. This study demonstrates an abrupt change does not necessarily relate climatic forcing, but can simply arise internal adjustment piedmont run out coarse bedload. It illustrates, addition, genesis quartz-rich conglomerates Himalayan foreland through associated with differential weathering, abrasion processes recycling thrust wedge advance shortening basin.