Geomorphic patterns on distributive fluvial systems

Distributive fluvial systems (fluvial fans) have been demonstrated to dominate the planform area of modern sedimentary basins. Due to their dominance in areas where sediments are actively aggrading, it has been argued that they form the bulk of the fluvial rock record. However, there are some key gaps in the documentation of DFS characteristics, particularly the quantification of channel characteristics. Meandering planforms are prevalent across DFS. In recent years, great advancements have been made in understanding processes associated with meandering rivers as well as increasing our understanding of their deposits. However, little work has been done on understanding how, and whether, meander characteristics vary across a fluvial system.

This study assesses geomorphic patterns across a suite of DFS from a variety of climates to; 1) understand how channel characteristics change downstream on DFS; and 2) assess whether meander characteristics (amplitude, wavelength, migration rate and deposit area) change from proximal to distal regions. Observations from modern systems will be compared to the Jurassic Morrison Formation for which channel width and meander deposit area can be measured.

Our studies show that the active channel width and channel belt widths broadly decrease downstream, except in instances where: 1) an axial system truncates the toe of a DFS; 2) a nearby fan contributes sediment laterally into the system; and 3) a significant spring line is present. Interestingly, the active channel and channel belt width do not concurrently decrease downstream. It is postulated that this is due to planforms playing a key role in the width of the channel belt opposed to the active channel width. Migration rates and meander deposit area generally decrease downstream on the measured systems, however, trends on the Wood River DFS are weak with axial truncation interpreted to be a reason for a weak downstream trend. The Jurassic Salt Wash system mirrors patterns observed on modern systems, in particular the Wood River DFS as a downstream decrease in active and channel belt width is observed, with weak to no changes observed in meander deposit area.

These findings contribute to our overall understanding of fluvial processes, which is essential to understand flood risk for large population centres that live on DFS. In addition, a greater understanding of DFS deposits is gained, contributing to our understanding of reservoir characterisation in fluvial deposits.