The modern Mahakam Delta has long been classified as a mixed fluvial and tide-dominated prograding delta. The delta plain has a lobate, fan-shaped morphology with a network of distributaries and estuaries. Previous studies which were mainly focused on sedimentary characteristics indicate that tidal features are not restricted to the channel mouths.
This study focuses on the dynamics of the tide and its influence on the sediment distribution. Hydrodynamic measurements were systematically taken at 22 locations for complete spring and neap tidal cycles, along with 328 bottom grab sediment samples and a few shallow cores. Sand covers the bottom of the distributaries at the delta apex and gradually fines seaward but does not extend to the channel mouths. Mud dominates the offshore, the estuaries and the distal reaches of the distributaries. Sand and mud couplets are common upstream to at least the delta apex. An abundant, diverse assemblage of benthonic marine organisms was recovered as much as 20 km upstream in the distributaries. The hydrodynamic measurements indicate that tidal processes strongly control sedimentation throughout the distributaries and even upstream of the delta apex. Tidal stratification occurs dynamically and influences the bedload transport that commonly takes place during spring tide but not during neap tide, which later suggests that the sand and mud couplets reflect spring-neap variations.
This study suggests that the Mahakam Delta is indeed a mixed fluvial and tide-dominated system but has been recently transgressed. Fluvial dominance is constrained to the upper reaches of the active distributaries and tides are the most important processes on the delta. The tidal processes control the distribution of the potential reservoir in the delta plain and significantly decrease the reservoir quality.
Author : Salahuddin Husein