TY - JOUR

T1 - Lateral and vertical facies relationships of bedforms deposited by aggrading supercritical flows

T2 - From cyclic steps to humpback dunes

AU - Lang, Jörg

AU - Winsemann, Jutta

N1 - Funding Information: Financial support by the Niedersächsisches Ministerium für Wissenschaft und Kultur (MWK) is gratefully acknowledged (PRO Niedersachsen Project No. 11.2-76202-17-3109 ). We would like to thank the owners of the sand pits for the permission to work on their property. Borehole data were generously provided by E.ON-Kraftwerke GmbH, Niedersächsisches Landesamt für Bergbau, Energie und Geologie, Brinkmeyer Kieswerk GmbH & Co. KG. and E.ON Westfalen Weser AG. C. Brandes, J. Meinsen, L. Pollok, K. Skupin and D. Steinmetz are thanked for discussion and help with field work. Critical comments by P. Carling and J. Venditti on an earlier version of this manuscript helped to sharpen our ideas. Constructive reviews by M. Cartigny, P. Fralick and editor J. Knight are highly appreciated.

PY - 2013/10/15

Y1 - 2013/10/15

N2 - The preservation of bedforms related to supercritical flows and hydraulic jumps is commonly considered to be rare in the geologic record, although these bedforms are known from a variety of depositional environments. This field-based study presents a detailed analysis of the sedimentary facies and stacking pattern of deposits of cyclic steps, chutes-and-pools, antidunes and humpback dunes from three-dimensional outcrops. The well exposed Middle Pleistocene successions from northern Germany comprise glacilacustrine ice-contact subaqueous fan and glacial lake-outburst flood deposits. The studied successions give new insights into the depositional architecture of bedforms related to supercritical flows and may serve as an analogue for other high-energy depositional environments such as fluvial settings, coarse-grained deltas or turbidite systems.Deposits of cyclic steps occur within the glacial lake-outburst flood succession and are characterised by lenticular scours infilled by gently to steeply dipping backsets. Cyclic steps formed due to acceleration and flow thinning when the glacial lake-outburst flood spilled over a push-moraine ridge. These bedforms are commonly laterally and vertically truncated and alternate with deposits of chutes-and-pools and antidunes. The subaqueous fan successions are dominated by laterally extensive sinusoidal waveforms, which are interpreted as deposits of aggrading stationary antidunes, which require quasi-steady flows at the lower limit of the supercritical flow stage and high rates of sedimentation. Humpback dunes are characterised by downflow divergent cross-stratification, displaying differentiation into topsets, foresets and bottomsets, and are interpreted as deposited at the transition from subcritical to supercritical flow conditions or vice versa. Gradual lateral and vertical transitions between humpback dunes and antidune deposits are very common.The absence of planar-parallel stratification in all studied successions suggests that the formation of these bedforms is suppressed in flows characterised by hydraulic jumps under highly aggradational conditions. The large-scale lateral and vertical successions of bedforms are interpreted as representing the temporal and spatial evolution of the initial supercritical flows, which was strongly affected by the occurrence of hydraulic jumps. Small-scale facies changes and the formation of individual bedforms are interpreted as controlled by fluctuating discharge, bed topography and pulsating unstable flows.

AB - The preservation of bedforms related to supercritical flows and hydraulic jumps is commonly considered to be rare in the geologic record, although these bedforms are known from a variety of depositional environments. This field-based study presents a detailed analysis of the sedimentary facies and stacking pattern of deposits of cyclic steps, chutes-and-pools, antidunes and humpback dunes from three-dimensional outcrops. The well exposed Middle Pleistocene successions from northern Germany comprise glacilacustrine ice-contact subaqueous fan and glacial lake-outburst flood deposits. The studied successions give new insights into the depositional architecture of bedforms related to supercritical flows and may serve as an analogue for other high-energy depositional environments such as fluvial settings, coarse-grained deltas or turbidite systems.Deposits of cyclic steps occur within the glacial lake-outburst flood succession and are characterised by lenticular scours infilled by gently to steeply dipping backsets. Cyclic steps formed due to acceleration and flow thinning when the glacial lake-outburst flood spilled over a push-moraine ridge. These bedforms are commonly laterally and vertically truncated and alternate with deposits of chutes-and-pools and antidunes. The subaqueous fan successions are dominated by laterally extensive sinusoidal waveforms, which are interpreted as deposits of aggrading stationary antidunes, which require quasi-steady flows at the lower limit of the supercritical flow stage and high rates of sedimentation. Humpback dunes are characterised by downflow divergent cross-stratification, displaying differentiation into topsets, foresets and bottomsets, and are interpreted as deposited at the transition from subcritical to supercritical flow conditions or vice versa. Gradual lateral and vertical transitions between humpback dunes and antidune deposits are very common.The absence of planar-parallel stratification in all studied successions suggests that the formation of these bedforms is suppressed in flows characterised by hydraulic jumps under highly aggradational conditions. The large-scale lateral and vertical successions of bedforms are interpreted as representing the temporal and spatial evolution of the initial supercritical flows, which was strongly affected by the occurrence of hydraulic jumps. Small-scale facies changes and the formation of individual bedforms are interpreted as controlled by fluctuating discharge, bed topography and pulsating unstable flows.

KW - Antidunes

KW - Chutes-and-pools

KW - Cyclic steps

KW - Humpback dunes

KW - Hydraulic jumps

KW - Supercritical flows

UR - http://www.scopus.com/inward/record.url?scp=84883705296&partnerID=8YFLogxK

U2 - 10.1016/j.sedgeo.2013.08.005

DO - 10.1016/j.sedgeo.2013.08.005

M3 - Article

AN - SCOPUS:84883705296

VL - 296

SP - 36

EP - 54

JO - Sedimentary geology

JF - Sedimentary geology

SN - 0037-0738

ER -