Modern Mahakam Delta, Indonesia: Sand Distribution and Geometry in Mixed Tide and Fluvial Delta: ABSTRACT

Abstract

A core study was made of a mixed, tide and fluvial, low-wave delta formed in the humid tropics. Little subsurface data were previously available for this type of delta, in which morphology and sediment distribution reflect both fluvial and tidal characteristics. The delta plain is a tidal marsh, with bioturbated organic clays, incised by separate networks of distributary and tidal channels. In contrast to the mud-filled meandering tidal channels, distributaries are linear and filled with sand accumulating as lateral accretion bars. Distributaries form narrow ribbons of channel-fill sands of variable thickness (5 to 11 m), eroded or superimposed onto underlying delta front clays and sands. Facies and vertical sequences are fluvial, erosive based and fining-up in the upper delta plain, and tidal and coarsening-up in the lower delta plain. The lack of fluvial levees and splays reflects the tidal influence in the distributaries. On the delta front, sand occurs as numerous bars, forming a spectrum of distinct types which reflect the local river-tide ratio. Major distributaries form thick (7 m) localized arcuate mouth bars, while off smaller distributaries, triangular middle ground bars occur, forming more sheet-like, thinner sand bodies. In areas of low river input, tidal ridges predominate. The bars are separated by organic clay, but lateral coalescence and stacking can locally increase sand continuity. Sequences are coarsening-up, and facies show tidal and marine characteristics. A general seaward bar thickening exists owing to the more distal position of the thicker arcuate bars; this results in a vertical bar thinning-up progradational sequence. Seaward of the delta front bars, prodelta sediments composed of massive clays form the base of a 50-m thick regressive sequence composed of multiple bar deposits, followed and incised by sandy distributary fills. Sedimentation rates range between 0.2 and 1.3 cm per year-1. End_of_Article - Last_Page 889------------