Deltas or Marine-Influenced Distributive Fluvial Systems? Predicting Facies Distributions in Fluvio-Deltaic Systems
Hartley, Adrian; Weissmann, Gary; Scuderi, Louis; McNamara, Kelsey
Remote sensing analysis of a dataset of modern deltas from different climatic and tectonic settings allows quantification of processes that are operative from delta apex to marine termination. Combining these observations with recently published work on the relationship between the upstream apex location and the backwater effect (the upstream point at which the base of a channel intersects sea-level) allow predictive models to be made for facies distribution within the fluvial dominated subaerial part of the deltaic systems including sand body extent and connectivity.
Our datasets support previous work indicating that the scaling relationship between channel depth, apex elevation and the back water effect can be used to predict the apex location and therefore the size and subaerial extent of the deltaic system. The key control being that the upstream distance of the apex scales to the depth of the fluvial channel, such that apex of larger rivers is located at higher elevations with the subaerial extent of the deltaic system being larger than that associated with smaller rivers.
The apex point of a delta occurs where a fluvial channel distributes sediment as it encounters the backwater effect, this leads to sand deposition and development of amalgamated fluvial channel sands. Downstream of the apex area there is a progressive decrease in sand deposition and sandstone body size together with an increase in soil development, overbank sediment accumulation and preservation potential. Downstream of this low net area marine processes dominate, with the prevailing process (e.g. wave or tide) controlling the shoreline morphology and extent of marine influence at the downstream end of the system. Marine influence in the majority of the large deltaic systems we have examined forms a relatively minor proportion of the subaerial part of the delta. With the exception of the marine-influence toe of the deltaic system, all the features we observe are characteristic of distributive fluvial systems in non-marine settings. We suggest that as the largest proportion of the subaerial part of a delta is a distributive fluvial system, this area could more accurately be referred to as a marine-influenced distributive fluvial system (MIDFS) and then qualified by the dominant basinal process such as tide or storm-dominated. It is important to note that all DFS and MIDFS are distributive but not all systems are distributary (i.e. all channels are active at the same time).
AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013