3D architecture of cyclic-step and antidune deposits in glacigenic subaqueous fan and delta settings: Integrating outcrop and ground-penetrating radar data

Jörg Lang,Julian Sievers,Markus Loewer,Jan Igel,Jutta Winsemann

doi:10.1016/j.sedgeo.2017.10.011

Jörg Lang, Julian Sievers + Show 3 more

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https://doi.org/10.1016/j.sedgeo.2017.10.011

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Abstract

Bedforms related to supercritical flows are increasingly recognised as important constituents of many depositional environments, but outcrop studies are commonly hampered by long bedform wavelengths and complex three-dimensional geometries. We combined outcrop-based facies analysis with ground-penetrating radar (GPR) surveys to analyse the 3D facies architecture of subaqueous ice-contact fan and glacifluvial delta deposits. The studied sedimentary systems were deposited at the margins of the Middle Pleistocene Scandinavian ice sheets in Northern Germany.Glacifluvial Gilbert-type deltas are characterised by steeply dipping foreset beds, comprising cyclic-step deposits, which alternate with antidune deposits. Deposits of cyclic steps consist of lenticular scours infilled by backset cross-stratified pebbly sand and gravel. The GPR sections show that the scour fills form trains along the delta foresets, which can locally be traced for up to 15m. Perpendicular and oblique to palaeoflow direction, these deposits appear as troughs with concentric or low-angle cross-stratified infills. Downflow transitions from scour fills into sheet-like low-angle cross-stratified or sinusoidally stratified pebbly sand, deposited by antidunes, are common. Cyclic steps and antidunes were deposited by sustained and surge-type supercritical density flows, which were related to hyperpycnal flows, triggered by major meltwater discharge or slope-failure events.Subaqueous ice-contact fan deposits include deposits of progradational scour fills, isolated hydraulic jumps, antidunes and (humpback) dunes. The gravel-rich fan succession consists of vertical stacks of laterally amalgamated pseudo-sheets, indicating deposition by pulses of waning supercritical flows under high aggradation rates. The GPR sections reveal the large-scale architecture of the sand-rich fan succession, which is characterised by lobe elements with basal erosional surfaces associated with scours filled with backsets related to hydraulic jumps, passing upwards and downflow into deposits of antidunes and (humpback) dunes. The recurrent facies architecture of the lobe elements and their prograding and retrograding stacking pattern are interpreted as related to autogenic flow morphodynamics.

Highlights

The aim of this study is to present new insights into the 2D and 3D depositional architecture of deposits related to supercritical flows in glacilacustrine environments
Gravel-rich subaqueous ice-contact fan deposits The gravel-rich subaqueous ice-contact fan succession (Fig. 3) is characterised by (i) scours infilled by gravelly backsets (Gbl) (Fig. 3B), (ii) scours infilled by gravelly foresets (Gsc) (Fig. 3C), (iii) subhorizontally and low-angle cross-stratified gravel (Gl) (Fig. 3D), and (iv) trough crossstratified gravel (Gt) (Table 1)
Thicker foresets occur within the larger scours and are characterised by coarse-tail normal grading perpendicular to the set boundaries

Summary

Introduction

Bedforms related to supercritical flows, including antidunes, chutesand-pools and cyclic steps, have recently received much interest and have been identified as important constituents of depositional environments such as deltas (Ventra et al 2015; Dietrich et al 2016; Normandeau et al 2016; Massari 2017), submarine fans (Postma et al 2014; Ventra et al 2015; Covault et al 2017; Lang et al 2017), and subaqueous ice-contact fans (Russell and Arnott 2003; Winsemann et al 2009; Lang and Winsemann 2013). Deposits of supercritical flows are commonly characterised by lateral and vertical transitions between the various bedform types due to spatio-temporal variations of the flow conditions and feedbacks between the flow and the developing bedform (Spinewine et al 2009; Kostic 2011, 2014; Cartigny et al 2014; Zhong et al 2015). Such facies changes may complicate the depositional architecture of the resulting bedforms

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