EFFECTS OF BASIN SUBSIDENCE ON EXPERIMENTAL DELTA SEDIMENTATION PATTERNS AND SURFACE MORPHOLOGY

Tectonic subsidence has long been recognized as an important factor affecting surficial patterns of delta morphology. However, the link between subsidence and its manifestation in surface morphology is still unclear. Fluvial systems actively undergoing subsidence have an uncanny ability to fill in the newly created space. The rate at which sediment is deposited is adjusted to the rate of subsidence, on average. Here we present an analysis of two experiments conducted using the Experimental EarthScape (XES) basin. The XES basin allows for the study of sedimentological and geomorphological characteristics under controlled conditions of sediment/water supply, base level, and subsidence rate and geometry in an area of ~17 m^2. We observed depositional expansion structures, avulsion patterns, and depositional bar formation in response to two varying types of subsidence; XES-02 experienced passive margin style subsidence, while XES-10 underwent foreland style subsidence, both of which had temporally constant subsidence. Depositional events in the system primarily occurred via two features; through point bars and expansion structures, both of which are related to avulsion characteristics. Expansion structures are defined as regions transitioning from channelized flow to sheet flow. Differential subsidence geometries appear to have a significant effect on the frequency and spatial distribution of expansion structures across the system. XES-02 expansion structures occurred most often (22.5%) between linear distances of 0.76 m – 1.00 m from the sediment source. XES-10 expansion structures were found most frequently (22.6%) between linear distances of 1.76 m – 2.00 m. In both experiments, the largest expansion structures formed between 0.5 m and 1.0 m away from the sediment source. Sheet flow of the expansion structure would appear to retreat towards the sediment source, subsequent bifurcations would occur, new channels would emerge leading to the formation of new expansion structures and the cycle would repeat. During channelized flow, we saw an increase in bar formation when compared to sheet flow. We show that, experimentally, varying types of subsidence geometries express themselves differently at the surface, and an understanding of surface process can provide insight to subsidence geometry.