Conglomerate recycling in the Himalayan foreland basin: Implications for grain size and provenance

Abstract

The nature of coarse sediment in rivers emerging from mountain ranges determines rates of downstream fining, the position of the gravel-sand transition, sediment entrainment thresholds, and channel morphologies. Additionally, in the stratigraphic record, clast size distributions and lithologies are used to reconstruct paleo-hydraulic conditions and source area provenance. Using Himalayan rivers, we demonstrate that the signal of first-generation clasts derived from the hinterland of a mountain range can be significantly altered by recycling older, structurally exhumed foreland deposits. The Siwalik foothills of the Himalaya comprise Neogene fluvial sandstones and quartzite-rich conglomerates with well-rounded clasts that were deposited in the Indo-Gangetic foreland basin and later exhumed by erosion, following uplift along the Himalayan mountain front. Mass balance calculations reveal that the Upper Siwalik conglomerate may contribute a significant proportion of the total gravel flux exported from the main Himalayan catchments (up to 100%) despite forming <1% of the catchment geology. Three end-member catchments with variable proportions of gravel flux from Siwalik conglomerates are analyzed to test for the effects of conglomerate recycling. Catchments that recycle the most Upper Siwalik conglomerate form quartzite-rich gravel bars comprising well-rounded pebbles and a narrow grain size distribution, mimicking the characteristics of the Upper Siwalik conglomerate. Conversely, catchments that recycle the least Upper Siwalik conglomerate form gravel bars with a range of Himalayan lithologies, angular quartzite pebbles and a wider grain size distribution. This study highlights that recycling of quartzite-rich conglomerate can dramatically modify the flux, lithology, grain size, and shape of gravel entering the Indo-Gangetic Plain.

Depositional environment of polymictic conglomerate of the Gadag greenstone Belt, Western Dharwar Craton, south India: An insight for Neoarchean marginal sedimentation

“…Representative well‐exposed outcrops were selected throughout the belt for clast counting using the Howard ribbon counting method (Howard, 1993) to understand the spatial and temporal variation in clast composition. Conglomerate clast sizes (long [ a ], intermediate [ b ], and short axes [ c ]) were measured using the photographic method (Figure 2; for detail procedure see Quick, Sinclair, Attal, & Singh, 2019) where each photo overlap on a numeric square grid with 100 nodes assuming to be the b ‐axis is the shortest axis visible on the surface (Attal & Lavé, 2006). However, in the cross‐section, the short axis or c ‐axis of the pebble is more clearly identifiable (Quick et al, 2019).…”

Times Associated With Source-to-Sink Propagation of Environmental Signals During Landscape Transience

“…On land, sediment can be stored on hillslopes due to reduced hillslope-channel connectivity (Figure 5; DiBiase and Lamb, 2013;Hoffmann, 2015;Harries et al, 2021) and within the river system. Overbank flow, as well as lateral and vertical movement of the active channel can result in long-term sediment incorporation in floodplains (Nakamura and Kikuchi, 1996;Wittmann et al, 2011Wittmann et al, , 2020Bradley and Tucker, 2013;Coulthard and Van De Wiel, 2013), alluvial fans (Jolivet et al, 2014;D'Arcy et al, 2015D'Arcy et al, , 2017Guerit et al, 2016;Mason and Romans, 2018;Carretier et al, 2020), fluvial terraces (Blöthe and Korup, 2013;Limaye and Lamb, 2016;Schildgen et al, 2016;Malatesta et al, 2017Malatesta et al, , 2018Tofelde et al, 2017;Quick et al, 2019), or entire valley fills (Hilley and Strecker, 2005). As storage along the fluvial system on continental scale SRSs is of major importance, we discuss storage outside the active river channel in form of ( 1) floodplain deposition due to overbank flow, (2) burial in the channel bed due to sediment deposition during periods of channel aggradation, and (3) deposition due to lateral channel movements (e.g., point bar accretion) (Figures 5A,B).…”

Dynamic flood topographies in the Terai region of Nepal

“…The Karnali basin has a drainage area of ~43 000 km 2 upstream of the mountain outlet at the town of Chisapani (Figure 1), where the channel exits a confined bedrock gorge and flows out onto the alluvial Indo‐Gangetic Plain. In the upper reaches of the alluvial plain, the channel is characterized by a coarse gravel to cobble bed which fines downstream ( D 50 = 46–148 mm between the mountain front and gravel–sand transition; Quick et al ., 2019). The gravel channel is braided with exposed gravel bars (at low flow) and mature, vegetated islands.…”