Incision of rivers in Pleistocene gravel and conglomeratic terraces: Further circumstantial evidence for the uplift of Borneo during the Neogene and Quaternary

Incised Pleistocene gravel beds and conglomerates are a common feature of the Baram, Limbang and Temburong drainage systems in NW Sarawak and Brunei. Incision values vary from 9 to 76 m, a likely result of strong temporary precipitation, in combination with ongoing uplift. Conglomerates and gravels are monomictic, almost exclusively formed by the Lower Miocene Meligan sandstone, and deposited in nested fluvial terraces. The described pattern differs from coastal terraces of the Miri area; the latter do not contain conglomerates other than locally-derived and recycled quartz clasts. The available age and uplift data, when plotted log-normal, suggest that the mountainous central Borneo uplift was continuous of some 6-7 mm a-1, whilst low-lying coastal wetland areas in Brunei and Sarawak may only have seen a very minor uplift in the order of 0.8 mm a-1. The uplift may be ongoing at the present day.