Distributary Channel Geometry and Sediment Distribution in a Modern, Monsoonal, Mixed-Process Delta, Gulf of Carpentaria, NE Australia

Nanson, Rachel A.; Ainsworth, Bruce; Dashtgard, Shahin E.; Lane, Tessa I.; Vakarelov, Boyan

The Mitchell River is constructing a mixed-process delta (tide-dominated, fluvial-influenced, wave-affected; Tfw) on the east coast of the Gulf of Carpentaria. The delta exhibits a suite of distributary channels at various stages of the active to abandonment continuum. Paleodistributary channels on the upper delta are classified as fluvial-dominated (F) and their channel belts are predominately sand filled. Channels on the lower delta are classified as either fluvial-dominated, tide-influenced (Ft) or tide-dominated, fluvial-influenced (Tf) and are of variable mud and sand fill. Some abandoned distributary channel belts are now occupied by smaller tidal (T) channels, with no fluvial input or sand bars. The geometries of active and abandoned lower delta distributary channels are distinctly different; active channels have higher width/depth (w/d) than their abandoned counterparts. Two scales of process variation cause these differences: 1) seasonal fluctuations in fluvial discharge, and 2) longer time frame progressive reductions in fluvial discharge, as the channels evolve from active to abandoned. During the peak of the wet season, active distributary channels are Ft and bedload is supplied from the catchment. During the dry season, however, fluvial flows are minimal and the relative importance of tidal processes increases proportionately such that they can become Tf. Over longer timescales, channels may receive reduced volumes of catchment sediment as they are progressively abandoned. To investigate channel geometries and fill, bathymetric and sediment distribution maps were produced for the active (Ft), semi-active (Tf) and abandoned (T) channels. The results indicate that while the beds of active distributary channels (high w/d) predominantly comprise coarse sediment, fluid mud accumulates in the thalweg during the dry season when channels become more tide-influenced. In contrast, abandoned distributary channels (lower w/d) have fewer sand bars and are more mud rich. The relative proportion of sand in active and abandoned channels is attributed to fluvial connectivity. The extensive fluid mud deposits in both distributary channel types are attributed to dry season tidal flows reworking fluvial-sourced washload. These results have important implications for understanding and predicting the geometry and composition of distributary channel belt fills across similar monsoonal delta systems, where seasonal variations in process dominance are pronounced.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013