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The Geometry and Heterolithic Fill of Tide Influenced Channels in the Gulf of Carpentaria, Australia

Abstract

The Gulf of Carpentaria (GoC) in northern Australia is a modern example of a low accommodation epicontinental seaway. Tidally influenced channels have developed along this shoreline and these channels demonstrate significant geometric variability. Abandoned channels are generally filled with inclined heterolithic strata (IHS), and thus represent excellent petroleum reservoir analogues. This research investigates the effects of variations in tide, wave and fluvial influences and shoreline morphology on the fill and geometry of over 150 channels. Tidal range increases from microtidal (< 2 meters) to mesotidal (2-4 meters) towards the south-eastern corner of the GoC. Higher tides are associated with more pronounced funnelling. Significant wave height also decreases in a southerly direction, such that the ratio of wave to tide energy influencing channel systems generally decreases from north to south. Wave reworking of channel mouths is demonstrated to partially obscure the tide-funnel relationship. The amount of fluvial flow received by partially abandoned channels also influences the funnel geometry of channels and further complicates the tide-funnel geometric relationship. Two contrasting systems in the SE of the Gulf of Carpentaria are being used to refine our understanding of the fill of these channels; the Gilbert and Mitchell Rivers. These systems are both tide dominated, however, the ratio of tide to wave energy is slightly higher in the more southern Gilbert River. New core and ground penetrating radar datasets from the Mitchell River delta indicate that channel fills downstream of the maximum Holocene backwater limit are comprised of IHS, however, the proportion of sand fill is dependant of the evolutionary history of the channel. As channels are progressively abandoned they become increasingly tidally influenced. Tidal flows can transport some sand, however, grab samples from their thalweg in the dry season indicate that tidal flows transport predominantly mud. Downstream and cross-sectional fills in a single channel can therefore progress from F (sand) through Ft and Tf to T (mud) deposits. Higher tidal ranges in the Gilbert River system have resulted in the backwater effect extending further upstream than in the Mitchell River delta. These results indicate that only modest increases in tidal range (< 1 meter) can have a significant impact on the extent of the backwater effect and thus on proportion of mud in their IHS channel fills.