Facies association and depositional environment of fan-delta sequence in southwest Kathmandu Basin, Nepal

Abstract

Lake marginal sedimentation prevailed around the Paleo-Kathmandu Lake. Owing to the difference in local basin conditions; tectonics, source rock types and river systems therein, the lake marginal environments and sedimentary facies associations differ around the Paleo-Kathmandu Lake. In this study, the basin-fill sediments of southwestern margin of the Kathmandu Basin were studied for the sediments recorded in vertical sequences at various localities and facies analysis was made. Mainly eight facies were recognised. They were matrix-supported massive gravel (Gmm), matrix-supported graded gravel (Gmg), gravelly fine or mud (GF), massive silt (Fsm), massive mud (Fm), ripple-laminated silt or laminated silt/mud/clay (Fl), carbonaceous clay (C), and incipient soil with roots (Fr). Four facies associations that were identified were proximal fan-delta facies association (FA1), mid fan-delta facies associaiton (FA2), distal fan-delta facies association (FA3), and gravelly sinuous river facies association (FA4). Remarkably, these facies associations do not contain any sandy facies and foreset bedding of Gilbert-type. The fan-delta region was characterised by flood-dominated flows and vertical accretion of fines in the flood basins, and vegetated swamps rich in organic sediments. The distribution of facies associations suggests extensive lake transgression followed by rapid lake regression. The recent river system then incised the valley against local upliftment due to faulting or lowering of base level of the main river in the Kathmandu Basin probably related to draining out of the lake water. doi: Bulletin of the Department of Geology, Vol. 12, 2009, pp. 1-16

Seismites in the Pleistocene succession and recurrence period of large earthquakes in the Kathmandu Valley, Nepal

“…The sediments of the Gokarna, Thimi, Tokha, and Patan formations show lithological similarities with each other. Alternating sand and silt beds of lacustrine deltaic origin characterize the major part of these formations, while the gravel-dominated sediments of alluvial fan or subaqueous slope origin are observed in the proximal parts of the Gokarna and Tokha formations (Natori et al 1980;Yoshida and Igarashi, 1984;Sakai et al, 2001Sakai et al, , 2006Sakai et al, , 2008Saijo and Kimura, 2007;Tamrakar et al, 2009). The sediments of all formations are horizontal except those near the basin margin, where slope facies has been observed (Sakai et al, 2008).…”

The Sedimentary records of the Hapcheon impact crater basin in Korea over the past 1.3 Ma

“…The low matrix content (e.g., 35.5−35.6 m depth), sharp lower contact, and unoriented gravel to pebble clasts indicate deposition from viscous gravelly debris or hyper-concentrated flows (Maharjan et al, 1970;Horton and Schmitt, 1996;Yu et al, 2019). The Gm1 facies was deposited by a debris flow.…”

“…In contrast, Unit 2b is interpreted to have been deposited in a shallow-water lake. Horizontal laminae and bedding developed on top of the dark gray mud on top of the gravelly mud, indicating that these sediments were deposited by hyper-density flows and turbidity currents (Maharjan et al, 1970;Heward, 1978;Miall, 2006;Talling et al, 2012).…”

Facies analysis and depositional model of the Midcontinent Rift System in Kansas, USA

“…The fine-grained intervals were deposited by suspension settling in a standing water body (Fl) where synsedimentary deformations occurred (Fd). Altogether, interbedding of these high-energy, proximal, gravity-flow deposits with fine-grained, subaqueous deposits suggests a proximal fan delta environment (Blair & McPherson, 2008;McPherson et al, 1987;Benvenuti, 2003;Tamrakar et al, 2009) where the conglomeratic facies represents subaerial debris flow deposits, as has been shown in both modern environments (Blair & McPherson, 1998) and ancient successions (Benvenuti, 2003). Even though the facies association resembles those of modern-day, arid fans, climate reconstruction in pre-vegetation successions is difficult because the lack of land plants may make continental environments appear (Went, 2005).…”