Publications

The prolific hydrocarbon resources of East Kalimantan have provoked extensive research on the modern Mahakam delta which is often used as an analogue for its Miocene precursor. An exceptional exposure of Miocene sediments is found in Perjuangan Quarry, near Samarinda, in East Kalimantan, which provides an opportunity to examine distributary channel and mouth bar sandstones in outcrop. Re-examination of this section reveals a wider variety of fluvio-deltaic and paralic facies than previously documented, reflecting different depositional systems. This raises questions about the application of the modern Mahakam delta as an analogue in subsurface studies.The base of the quarry exposes coarsening-upwards mouth bar deposits capped by in-situ coals with palaeosols genetically related to an overlying sandstone. This sandstone comprises a thinningupwards package interpreted as the product of lateral accretion within a distal fluvial-dominated delta system. This mouth bar-channel couplet is truncated by and amalgamated with a proximal fluvio-deltaic distributary channel sandstone above. This overlying channel deposit displays large intra-formational ripup blocks at the base of a thick succession of trough cross-stratified very coarse grained sandstones. The contrast between these two channel sandstones is interpreted to reflect a basinward-shift-in-facies indicative of a Sequence Boundary. Previous interpretations of this section have not differentiated these contrasting styles of deposition and do not recognise this Sequence Boundary.The evolution of Miocene fluvial-dominated delta systems as described from Perjuangan Quarry into the mixed fluvial-tidal delta system of the present day Mahakam River appears to reflect changes in discharge in the (palaeo-) Mahakam drainage basin. The frequency and magnitude of floods is believed to have reduced subsequent to the Miocene reflecting the influence of the Kutei Lakes in regulating discharge.