MODERN SEDIMENT DYNAMICS OF THE MAHAKAM DELTA

摘要

The present-day Mahakam Delta has long been considered as a mixed fluvial and tide-dominated delta. It lies within a low energy marine environment and has a fan-shaped morphology. There are eight principal distributaries in the NE and SE that are separated by an interdistributary area with numerous tidal inlets. The adjacent subaqueous delta plain has an extremely gentle slope (～ 0.6 m/km) to 5 m water depth where the slope abruptly increases to 1° - 2.5° on the delta front. Several subaqueous channels are offshore extensions of adjacent distributaries. Three hundred and twenty-eight bottom sediment samples and 5 shallow cores indicate that sand covers the bottom of the distributaries at the delta apex and gradually fines seaward, but sand does not extend to the channel mouths. Sand also occurs as elongate intertidal bars; most are perpendicular and attached to the shoreline, although a few are detached. Mud dominates the offshore, the abandoned distributaries and the distal reaches of the active distributaries. Hydrodynamic data collected at 22 locations indicates that fluvial processes dominate the sandy reaches of the distributaries and waves are important at the coastline. Tides are the most important coastal process and dominate the distal ends of the active distributaries, the inactive distributaries and interdistributary area. Tides strongly affect the fluvial-dominated and wave-influenced areas as well. This study suggests that the Mahakam Delta is a drowned delta that is being modified by marine processes. Approximately 5 m of ongoing subsidence has shifted the shoreline landward, submerging part of the lower delta plain and the distal reaches of the distributaries. Sand is being stored onshore in the distributaries, waves are reworking the SE shoreline, and tidal processes are modifying distributary morphology and dominating sedimentary facies progressively further landward.