Environmental Signal Propagation in the Transfer Zone: Implications for Sequence Stratigraphy

Abstract:

In a simple linear alluvial reach, upstream perturbations of grain size, sediment concentration and water discharge control river aggradation/degradation through their effect on river slope. At the downstream end of the system, base level also controls aggradation/degradation of the riverbed, but without change of river slope. We show that this dichotomy is important as it implies different timescale of adjustment to perturbations. Minimum aggradation time can be computed as a function of slope change or base level rise and sediment supply. Degradation time is more delicate to estimating as it depends partly on poorly constrained parameters such as the erodibility of the substratum. We use an analytical solution of alluvial slope to explore the aggradation problem: how likely is aggradation to record upstream versus downstream signals? For the degradation problem, we use a numerical model of water flow with erosion and deposition. We investigate a range of natural variations in grain size, sediment concentration and water discharge for rivers with similarity with known large fluvial systems such as the Brahmaputra and Mississippi. First results show that aggradation due to base level rise is in most cases faster than aggradation due to grain size increase or water discharge decrease, thus suggesting that such upstream perturbations will likely be overshadowed by base level changes in an icehouse climate, except in the upstream segments of a fluvial system. Perturbations of sediment concentration however are usually transmitted faster, and may thus propagate fully during periods of base level rise. This work emphasizes the competing effects of source area signals versus basin signals in determining aggradation-degradation of fluvial systems and sediment delivery to the basin margin. It confirms in particular the important disconnection between the marine and the fluvial environments during low sea level and decreased rainfall, with implications for sediment delivery to the basin margin and sequence stratigraphy.