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.

浅水三角洲在将观察到的相置于基于过程的连贯沉积模型中方面仍然是个谜。在这里，我们报告了浅水纯和分层喷射口坝形成的水池实验，结合最近对河床进行的水槽实验，提出了对浅水三角洲露头野外观测的新解释。

我们的实验表明，口坝的高度、几何形状和床型取决于所提供沉积物的喷射特性和粒度。具有非常粗粒沉积物的纯射流形成了一个高而陡峭的口坝，其特征是陡峭的休止角交叉层理以及背风侧的相关雪崩过程（颗粒流）。分层射流实验表明，口坝沉积和生长受超临界密度流的支配，超临界密度流从生长口坝背风侧的初始射流演变而来。在具有非常粗粒沉积物的分层射流中，口坝背风侧的沉积物来自颗粒流雪崩和密度流。虽然上背风坡上的沉积以颗粒流为主，但低地势、不对称、下流迁移的床型在较低的斜坡和口坝之外演化。在具有中粒沉积物的分层射流中，在覆盖整个背风坡及其以外区域的低、不对称、下流迁移床型的同心区域内形成了一个非常低的浮雕口坝。

许多先前的实地研究表明，由密集层状喷流（超重心流）沉积的河口坝的特征是粗粒交叉层状或低角度交叉层状砂岩下坡进入细粒平面平行的不同相组合，或“准平行”层合砂岩和攀爬波纹交叉层合砂岩。与水槽和水槽实验的比较表明，近端的粗粒平面和槽交叉层状砂岩可以代表超临界沙丘的沉积，这些沙丘通过下坡进入反沙丘，其特征是正弦分层和/或低角度交叉分层。反沙丘沉积物和攀爬波纹交叉层状砂岩之间的重复垂直过渡可能表明，在细粒沉积物中，涟漪叠加在反沙丘上，表明在超临界流动条件下形成波纹。类似的床型/沉积结构以前被解释为丘状交叉层状或 swaley 交叉层状，并归因于风暴主导环境中的组合流，在某些情况下可能必须修改。