Geomorphological processes of the coastal wetlands in Indonesian Archipelagoes

Abstract

Immense tracts of coastal wetland exist in the east coast of Sumatra and around the Borneo coast. I present hereby the geomorphological history of the coastal wetlands which I elucidated in the Batanghari river basin of East Sumatra.
Due to the rising sea level in the post-glacial period, which reached its peak in c. a. 6000 years ago, by 1.9m higher than present one, Plio-pleistocene terraces were drowned by the Sunda Sea. Under humidified climate, tropical rain forest expanded to the wetlands, and peat deposition started to take place on the former terraces. In the coast, brackish water sediments deposited in the lagoons occluded by sand ridges and on tidal flats. After the final sea regression which occurred c.a. 1500 years ago, the exposed brackish sediments were covered by relatively thin peat of 2 to 3 meters depth.
Zonation of the coastal wetlands was proposed, depending on microtopography and stratigraphy. (1) Flood zone: River water flowing down from the steep slopes of the middle reach have repeated floodings and caused the river meanderings when it enters the low-lying and level lower reach. On the basement of Plio-pleistocene terraces, silt and clay have deposited on the natural levees, and eutrophic woody peat have developed in the river-affected backswamp. (2) Central zone: In the farther terrains of the backswamp, ombrogenous peat have developed with remarkable peat-dome microtopography. The basal peat rests on the Plio-pleistocene terraces, and is enriched in the fern spores and Gramineae pollens, while the upper peat is predominantly composed of swamp forest vegetation with varying degree of decomposition. (3) Tidal zone: Several sand ridges which are parallel to the present coast line have remarkably been developed, with inter-ridge level plains composed of mud clay. Inter-ridge level plains are classified to the recent lagoons and very recent tidal flats, depending on the different degrees of the ripening of the mud clay. This zonation itself represents the geomorphological processes.