Paleogeographical evolution of the Mahakam delta in Kalimantan, Indonesia during the quaternary and late pliocene

Abstract

The Mahakam delta, which covers about 1200 km 2 on the eastern coast of Kalimantan in Indonesia, is situated at the mouth of the Mahakam river which drains a basin of about 75,000 km 2 and is presently covered with dense humid evergreen rain forest. The MISEDOR well, drilled in the delta to a depth of 638.56 m, reaches the upper Pliocene. The records of three stratigraphic markers — Phyllocladus hypophyllus, Podocarpus imbricatus, and Stenochlaena laurifolia —mmake it possible to locate the Plio-Pleistocene boundary at a depth of about 400 m. This limit is confirmed by the contrast between the uniformity of the paleogeographical features below this depth and the great variability of the environmental conditions indicated above it. The paleoenvironmental reconstruction is based on the good match between the present vegetation of the delta and its palynological record from marine and onshore surface samples. Several aspects of the paleoenvironment were noted in this study. First, a general uniformity, with a noteworthy continuance of deltaic conditions, the study site always remaining located in the subaerial or shallow parts of the delta. Second, there was a general variability in the detail of the paleoenvironments. Finally, during the upper Pliocene and Quaternary, relative sea level changes as deduced from MISEDOR data fit with those on a worldwide scale. Climatic changes appear to have left little mark on the local vegetation. A better record could have been expected from changes in the more climate-sensitive hinterland mountain vegetation, but its representatives are only minimally present as airborne grains in the spectra of the Mahakam delta. Indications of climatic changes do occur, however, in several periods of low sea level and accompanying rise of detritus, when high frequencies of grass pollen, probably due to savanna development, suggest a response to colder climatic conditions.