Morphology, Sedimentology and Architecture of Megafan-Shoreline Systems: A Case Study of the Mitchell River Delta, Gulf of Carpentaria, Australia

Abstract

Megafan-shoreline systems are understudied, despite their common occurrence in modern settings. The Mitchell River delta is a superbly exposed, architecturally complex megafan-shoreline system. The delta is fed by a radial network of modern and palaeo-distributary channels that drain a wet-dry monsoonal catchment into the low gradient Gulf of Carpentaria. The Holocene deposits of the Mitchell River delta can be considered a natural laboratory for examining the lateral relationships between architectural units within one reservoir flow unit in a complex mixed-influence megafan-shoreline reservoir analogue. Detailed mapping, hand auger and vibracoring, trenching and topographic surveying have been used to characterise the palaeodistributary channel belts on the megafan and the depositional elements that comprise the delta plain. Chronologic investigations have refined the timing for development of the megafan and delta to discrete periods within the last 6 ka. The delta has prograded up to 20 km in response to a 1.5 m sea level fall. Changes in the relative influence of wave, tide and fluvial energy during the last 6,000 years have had a major impact on delta morphology and sediment distribution. Coastal processes, including waves and tides, cause reworking of sediment at the coastline, while fluvial processes cause shifts in depositional loci through avulsion. Calculations of backwater length demonstrate that avulsion nodes are mostly concentrated in the upstream portion of the backwater zone; however, the hydraulic apex of the megafan is located well above direct base-level influence. Avulsions upstream of the backwater limit are attributed the lateral restriction of channel adjustment caused by indurated surfaces combined with the flashy monsoonal discharge regime. Thus, two distinct types of avulsions have been characterised: (1) avulsions on the fan, which are associated with large scale subdelta development, and (2) backwater avulsions on the upper delta plain that cause local cycles of lobe development, abandonment and minor coastal erosion. These results have important implications for understanding the distribution of reservoir units in similar ancient (reservoir) deposits.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015