The coal measures of the Tarakan Basin occur in the Berau Formation which contains at
least 19 seams. The thicknesses of the seams range from a few centimetres up to 4.5 m with dips of 5° to 20°. Exposure of the seams is good and they can be traced in three geographic areas which correspond to structural zones formed by post-Miocene folding
Most of the seams were deposited over sandstone which grades downwards into marine
mudstone or siltstone. The seams are commonly overlain by burrowed or bioturbated
mudstone which marks a transgressive marine sequence that terminated the coal swamps.
In some cases, the top of the coal contains sand-filled burrows but sandstone dose not overlie the coal. These features possibly indicate that the Berau Formation represents
subsiding delta lobes and lagoonal deposit