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