Mid Cenozoic freshwater wetlands of the Sunda region

The Sunda region was the scene of widespread rifting during the mid-Cenozoic, resulting in the development of numerous large lake-filled rifts, analogous in scale to the rift valley system of East Africa. The Tonle Sap in Cambodia forms the closest modern analogue for these lakes in the Southeast Asian region. Many of the palaeolakes were long lived, continuing uninterrupted as open lakes for several millions of years during the Oligocene. Smaller rift systems infilled with fluvial sediments, but the larger ones remained as lakes, and with Late Oligocene subsidence, were transformed by brackish, and in the earliest Miocene, by marine incursion, into large inland seas. These seas reached their greatest extent at the time of the mid Miocene thermal maximum. This paper describes the development and eventual demise of these lakes following marine transgression, and, based on their rich content of pollen and spores, illustrates the variety of fresh and brackish water swamp communities which developed around their margins. The marginal swamps can be divided into: i) seasonally inundated swamps, mainly during the Oligocene, characterised by Barringtonia, Lagerstroemia and grasses/sedges; ii) fern swamps and iii) from the Late Oligocene onward alluvial swamps, often characterised by Pandanus; and iv) peat swamps. The latter can be differentiated into kerapah peat swamps, first occurring during the Oligocene, and basinal peat swamps, becoming widespread from the Early Miocene onward.