WITHIN A CLASTIC DELUGE: DATING MIOCENE BIODIVERSITY DEVELOPMENT IN INDONESIA

Today, Borneo is home to record-breaking biodiversity. Located in the middle of the Indo-Pacific biodiversity hotspot the seas around Borneo contain the most biodiverse coral reef communities in the world. Strong deltaic sedimentation within the basin initiated sometime within the Miocene and continues presently. New data from Renema et al. (2008) suggests that this was concurrent with the arrival of the global biodiversity hotspot to Borneo. Contrary to its importance as a living laboratory for biodiversity studies today, Borneo’s geologic and paleontological history remains poorly documented. Flourishing natural-resource industries and urban development around the city of Samarinda produced many large exposures of Miocene bedrock. Over three kilometers of well exposed stratigraphy depict an offshore to deltaic progradational succession. Moreover, the presence of two patch-reefs, separated by perhaps millions of years within the succession, also provides important insight into evolutionary development of biodiversity around Borneo. Preliminarily studies have dated the section to Middle Miocene, but sediment cannibalism and the scarcity of diagnostic fossils make dating difficult. The extent of new exposure allows magnetostratigraphy possible, currently the most accurate dating technique in this geological setting. With a combination of magnetostratigraphy, sequence stratigraphy, nannofossil, planktonic foraminifera and larger benthic foraminifera biostratigraphy an integrated stratigraphic age model has been constructed for the Middle Miocene of the Samarinda region. This age model helps to pinpoint the initiation of the biodiversity hotspot and quantify biodiversity changes in Indonesian reef communities. In addition, it helps to better understand the timing and rate of uplift and sedimentation in the Miocene.