The Mahakam Delta, Indonesia: a Case Study for the Deposition and Preservation of Transgressive Deltaic Successions

Introduction Traditional models characterize the modern Mahakam Delta as a mixed river-dominated and tide-dominated delta that is presently prograding (e.g Galloway, 1975; Allen et al., 1976; Gastaldo et al., 1995; Allen and Chambers, 1998) and are commonly used as analogs to interpret subsurface successions. However, a recent quantitative study that describes the modern delta as transgressive and depositing a transgressive succession with very high preservation potential (Salahuddin and Lambiase, 2013) invalidates the use of the modern delta as a viable analog for progradational subsurface successions and suggests that transgressive successions may be relatively common in the subsurface. The Modern Mahakam Delta Quantitative hydrodynamic and sedimentologic data demonstrate the transgessive character of the modern delta that causes back-filling of the distributaries and relatively minor reworking of pre-transgression sediment. Very low wave energy in the receiving basin, plus rapid subsidence and burial, limits marine reworking to the uppermost pre-transgression strata and preserves the pre-transgression, progradational distributary and inter-distributary morphology. Ongoing back-filling of the distributaries is generating fining-upward successions that become increasingly marine upward. Current speed, and sediment transport capacity and competence decrease seaward so that the sediment flooring the distributaries is progressively finer downstream, which generates a fining-upward succession as transgression continues (Salahuddin and Lambiase, 2013). These successions also become more marine upward and have excellent preservation potential because of rapid subsidence rates and minor marine reworking. Sandy back-filled distributary successions are somewhat thinner and closer together in the upper delta plain than in the lower delta plain. As these sands fill the topographically low distributaries, they are laterally adjacent to slightly older, pre-transgression progradational strata. In contrast, inter-distributary areas are developing relatively thin, sandstones directly above pre-transgression progradational strata and separated from it by a transgressive erosional surface generated by marine reworking. The three dimensional geometry of the sandstones within a transgressive succession is expected to be complex and highly dependent on the pre-transgression delta morphology. The back-filled distributary sandstones are sinuous and oriented quasi-perpendicular to the shoreline while the transgressive shoreline sandstones are shoreline-parallel with a lateral extent that is determined by the distributary spacing. Ongoing transgressive lobe-switching means that the back-filled distributary successions are not exactly contemporaneous and that they probably have highly variable thicknesses and lateral extent.