TY - JOUR

T1 - Re-examining models of shallow-water deltas

T2 - Insights from tank experiments and field examples

AU - Winsemann, Jutta

AU - Lang, Jörg

AU - Fedele, Juan J.

AU - Zavala, Carlos

AU - Hoyal, David C.J.D.

N1 - Funding Information: We thank Travis Swanson, an anonymous reviewer and editor Catherine Chagu? for constructive comments, which helped to improve the manuscript. We are grateful for the support by ExxonMobil Upstream Research Company for providing access to the experimental facility and supplying the consumables for the experiments. We thank T. Bover Arnal, C. Brandes, G. Cole, R. Jerrett, M. Ghinassi, M. Rajchl, P.P. Rodriguez Lopez, Y. Spychala, R. Tinterri, D. Uli?n? and M. Watkinson for discussion, providing photographs and regional literature. Tim Hartmann and Jan Redeker helped with artwork. Special thanks go to the landowners, who granted access to open-pits and outcrops. Partial funding of the research work by the German Research Foundation (DFG; Grant LA 4422/1-1) is greatly appreciated.

PY - 2021/7/15

Y1 - 2021/7/15

N2 - Shallow-water deltas remain enigmatic in terms of placing the observed facies within a coherent process-based depositional model. Here we report tank experiments on mouth-bar formation from shallow water pure and stratified jets that, combined with recent flume experiments on bedforms, suggest new interpretations of field observations from shallow-water delta outcrops. Our experiments imply that the height, geometry and bedforms of the mouth bars depend on the jet properties and grain size of the supplied sediment. Pure jets with very coarse-grained sediment formed a high and steep mouth bar that is characterised by steep angle-of-repose cross bedding with related avalanche processes (grain flows) on the lee side. The experiments with stratified jets imply that mouth-bar deposition and growth are dominated by supercritical density flows that evolve from the initial jets on the lee side of the growing mouth bar. In stratified jets with very coarse-grained sediment, deposition on the mouth-bar lee side was both from grain-flow avalanches and density flows. While deposition on the upper lee slope was dominated by grain flows, a concentric field of low relief, asymmetric, downflow-migrating bedforms evolved on the lower slope and beyond the mouth bar. In the stratified jet with medium-grained sediment a very low relief mouth bar formed within a concentric field of low, asymmetric, downflow-migrating bedforms covering the entire lee slope and the area beyond. Many previous field studies show that mouth bars deposited from dense stratified jets (hyperpycnal flows) are characterised by a distinct facies assemblage of coarse-grained cross-stratified or low-angle cross-stratified sandstone passing downslope into finer-grained plane-parallel, or “quasi-parallel” laminated sand and into climbing-ripple cross-laminated sandstone. Comparison to flume and tank experiments suggests that the proximal coarse-grained planar and trough cross-stratified sandstones could represent deposition by supercritical dunes that pass downslope into antidunes, characterised by sinusoidal stratification and/or low-angle cross stratification. The repeated vertical transition between antidune deposits and climbing-ripple cross-laminated sandstone may indicate the superposition of ripples onto antidunes in finer-grained sediments, indicating ripple formation under supercritical flow conditions. Similar bedforms/sedimentary structures have previously been interpreted as hummocky cross-stratification or swaley cross-stratification and attributed to combined flows in storm-dominated settings, which probably in some cases must be revised.

AB - Shallow-water deltas remain enigmatic in terms of placing the observed facies within a coherent process-based depositional model. Here we report tank experiments on mouth-bar formation from shallow water pure and stratified jets that, combined with recent flume experiments on bedforms, suggest new interpretations of field observations from shallow-water delta outcrops. Our experiments imply that the height, geometry and bedforms of the mouth bars depend on the jet properties and grain size of the supplied sediment. Pure jets with very coarse-grained sediment formed a high and steep mouth bar that is characterised by steep angle-of-repose cross bedding with related avalanche processes (grain flows) on the lee side. The experiments with stratified jets imply that mouth-bar deposition and growth are dominated by supercritical density flows that evolve from the initial jets on the lee side of the growing mouth bar. In stratified jets with very coarse-grained sediment, deposition on the mouth-bar lee side was both from grain-flow avalanches and density flows. While deposition on the upper lee slope was dominated by grain flows, a concentric field of low relief, asymmetric, downflow-migrating bedforms evolved on the lower slope and beyond the mouth bar. In the stratified jet with medium-grained sediment a very low relief mouth bar formed within a concentric field of low, asymmetric, downflow-migrating bedforms covering the entire lee slope and the area beyond. Many previous field studies show that mouth bars deposited from dense stratified jets (hyperpycnal flows) are characterised by a distinct facies assemblage of coarse-grained cross-stratified or low-angle cross-stratified sandstone passing downslope into finer-grained plane-parallel, or “quasi-parallel” laminated sand and into climbing-ripple cross-laminated sandstone. Comparison to flume and tank experiments suggests that the proximal coarse-grained planar and trough cross-stratified sandstones could represent deposition by supercritical dunes that pass downslope into antidunes, characterised by sinusoidal stratification and/or low-angle cross stratification. The repeated vertical transition between antidune deposits and climbing-ripple cross-laminated sandstone may indicate the superposition of ripples onto antidunes in finer-grained sediments, indicating ripple formation under supercritical flow conditions. Similar bedforms/sedimentary structures have previously been interpreted as hummocky cross-stratification or swaley cross-stratification and attributed to combined flows in storm-dominated settings, which probably in some cases must be revised.

KW - Bedforms

KW - Hyperpycnal flows

KW - Pure jets

KW - Shallow-water deltas

KW - Stratified jets

KW - Supercritical flows

KW - Tank experiments

UR - http://www.scopus.com/inward/record.url?scp=85110185840&partnerID=8YFLogxK

U2 - 10.1016/j.sedgeo.2021.105962

DO - 10.1016/j.sedgeo.2021.105962

M3 - Article

AN - SCOPUS:85110185840

VL - 421

JO - Sedimentary Geology

JF - Sedimentary Geology

SN - 0037-0738

M1 - 105962

ER -