Three-dimensional analogue modelling of an alluvial basin margin affected by hydrological cycles: processes and resulting depositional sequences
The dynamics between sediment erosion and accumulation at an alluvial basin margin affected by changes in the surface hydrology are explored using scaled analogue models produced in a flume. The presented results differ from previous counterparts in that accumulation or erosion has not been forced at a spreading outlet, but occurred at a slope change produced by previously accumulated sediment. Cyclical upstream incision produced by increased stream discharge generated incised valleys, and these were subsequently filled by sediment carried by less efficient streams generated during the low discharge period. High resolution mapping using 2.5 mm contour maps allowed the study of sediment accumulation and terrain modelling. The results of three selected experiments are analysed. The only variable explored was discharge. The basin margin was simulated by a ramp inserted in a low sloping flume, consisting of two segments of different slopes selected to emulate high and low efficiency flume fans produced elsewhere. Water and fine-medium sand entered the ramp along a narrow (0.1 m) channel and flow expanded but without occupying the complete 1.2 m flume width. Flows were highly concentrated and noncohesive. Fan-like accumulation (slope: 0.11) began during low discharge (LO) periods at the ramp slope break, and proceeded upstream, onlapping quickly at first, but shifting to mostly progradation at the end of the period. High discharges (HD) usually generated two or three incised channels at the beginning of the period, but one of them prevailed and rapidly eroded parts of the LO fan and moved the sediment to a more distal low-sloping fan (slope: 0.045). Both LO and HO fans passed downstream into a system of small parallel channels resembling a braided alluvial plain ending in sediment lobes. The mapping of the accumulated sediment during the various periods allowed calculation of sediment budgets for the entire flume. The stratal architecture of the deposits was investigated along five parallel trenches cut after experiment termination. The regression analysis of depositional profiles at fan-like features (expanding flow) and at braided plains (parallel flow) indicated that these fan-like systems are linear and dependent on applied discharge, while the latter showed an exponential decrease of slope downstream, with a starting value set up by the fan slope. Two main types of stratigraphic units were generated, the LDST and HDST (system tracts). The LDST has a nonerosive base over \\\'bedrock\\\' and the previous HOST, filling proximal erosional topography and prograding as well, generating an onlap-downlap array. Its geometry is highly variable and dependent on pre-existing topography. The HOST base is an important erosive surface comparable to sequence boundaries. However, there are places without erosion due to a marginal position with respect to the main stream. Indeed, the results suggest that the three-dimensional variability of erosion and depositional processes might produce very different architectures along the same basin margin.