Facies architecture of gravelly and sandy supercritical bedforms in subaqueous ice. Contact fans 31st IAS Meeting of Sedimentology, Kraków, Poland.

J. Lang, J.H. van den Berg, J. Winsemann

Research output: Contribution to conferenceAbstractOther research output

Abstract

Subaqueous ice-contact fans are commonly deposited by high-energy plane-wall jets from subglacial conduits into standing water bodies. Their deposits are characterised by the rapid expansion, dilution and deceleration of the highly concentrated flows. Hydraulic jumps, which occur in the zone of flow transition, have a fundamental impact on the facies architecture of initially supercritical jets. We present field examples from Middle Pleistocene subaqueous fans, which were deposited in deep ice-dammed lakes at the margins of the Middle Pleistocene Scandinavian ice sheets across Northern Germany. The studied gravelly and sandy successions represent deposits of the zone of flow transition of supercritical jets. The gravel-rich subaqueous fan deposits are characterised by large-scale scour-fills (up to 25 m wide and 3 m deep; Winsemann et al., 2009), displaying both foreset and backset bedding. Scour-fills comprising gravelly backsets are interpreted as deposits of cyclic steps. The scour-fills form the basal part of 2-6 m thick successions, which are bounded by laterally continuous subhorizontal erosional surfaces. Up section, the successions comprise 1-4.5 m thick crudely stratified coarse-tail normally or inversely graded gravel, subhorizontally stratified gravel and trough cross-stratified gravel. These deposits are interpreted as deposits of antidunes and 3D dunes and indicate different stages of flow deceleration and dilution. The gravelly successions may be capped by 0.5-2.5 m thick low-angle cross-stratified pebbly sand, which is interpreted as deposit of stationary antidunes. Downflow and up-section the gravelly successions pass into sandy successions, which include deposits of chutes-and-pools, breaking antidunes, stationary antidunes and humpback dunes (Lang and Winsemann, 2013). The sandy deposits are dominated by laterally extensive 1-4 m thick thinning upwards packages of sinusoidal stratified sand (wavelength: 1-12 m, amplitude: 0.1-0.5 m) deposited by stationary aggrading antidunes, forming under quasi-steady flows at the lower limit of the supercritical flow stage. The stationary antidune packages are separated by erosionally based deposits of chutes-and-pools, breaking antidunes and humpback dunes. Farther downflow the succession passes into deposits of large 3D dunes and climbing ripples. The large-scale lateral and vertical successions of bedforms are interpreted as representing the temporal and spatial evolution of the highly concentrated supercritical meltwater jets, which were characterised by depletive flows due to expansion, deceleration and hydraulic jumps. Small-scale facies changes and the formation of individual bedforms are controlled by fluctuating discharge, pulsating unstable flows, bed topography and the related hydraulic roughness. References: Lang, J., Winsemann, J. (2013) Lateral and vertical facies relationships of bedforms deposited by aggrading supercritical flows: from cyclic steps to humpback dunes.
Original languageEnglish
Publication statusPublished - 2015
EventIAS Meeting 2015 - Krakow, Poland
Duration: 23 Jun 2015 → …

Conference

ConferenceIAS Meeting 2015
Country/TerritoryPoland
CityKrakow
Period23/06/15 → …

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