The modern Mahakam Delta has been acknowledged as a typical example of a tropical mixed fluvial and tide-dominated delta. Its subaerial delta plain has a fan-shaped morphology and is dissected by numerous distributary and estuary channels. As distributaries branch seaward out from the delta apex, they can be grouped into the northern and southern areas, while the central area is only occupied by estuaries. Some morphological evidences suggest that distributaries can be abandoned and converted into estuaries.
This study investigates the distributary abandonment processes in the northern area which has 3 distributaries and 3 estuaries. Channels morphology was studied by 85 high resolution echo-sounding profiles, bottom sedimentary facies was reconstructed by 143 grab sediment samples and hydrodynamics data were systematically measured at 10 locations for complete spring and neap tidal cycles. Distributaries exhibit relatively straight channels with flaring mouths while estuaries are more sinuous and have deeper and smaller mouths. Sand is more distributed in the bottom of the active distributaries than in the abandoned distributaries and gradually fines seaward but does not extend to the channel mouths while mud dominates the estuaries and the distal reaches of the distributaries. Hydrodynamic data indicates that the interaction of fluvial and tidal processes is dynamically active along the entire length of the distributaries. Fluvial processes dominate the sandy reaches of the distributaries, while tides are the most important process in the lower reaches of active distributaries, the inactive distributaries and estuaries.
This study suggests that the channel abandonment processes in the Mahakam Delta is sequential. As the tidal processes become more dominant, fluvially-supplied sand is being stored onshore in the distributaries and tidal processes are modifying distributary morphology and dominating sedimentary facies progressively further landward. These processes influence the distribution of the potential reservoir in the delta plain.
Author : Salahuddin Husein