Abstract

East of the Mamberamo River the low, hilly, broken terrain is underlain dominantly by Late Miocene to Pleistocene clastic sedimentary rocks of the Mamberamo Formation. This was deposited in a predominantly deep, marine basin. Aerial photograph interpretation, field observations, and palaeontological data show that continuing shale diapirism caused much of the surface deformation.

Deformed recent river gravel overlying pervasively sheared mud which contains hard, angular siltstone kernels of variable size is clear evidence of currently upwelling viscous diapiric mud. Active mud-volcanoes, oil seeps and mud flows are also surface manifestations of diapirism at depth. Many landslides consist of material identical to that present in diapirs, and are inferred to have been caused by diapirism.

Substantial parts of the succession are overturned, but preliminary evidence suggests that this occurred prior to current diapiric intrusion. Palaeontological data indicate ages of blocks within the diapirs ranging from probable Eocene to Middle Miocene and help define the large scale of diapiric disruption to the surface geology.

Diapiric mud is similar to "scaly mudstone" or "scaly clay", which are often associated with arc-continent collision or trench-subduction melange. However, it is probable that diapirism in this area was initiated because of over-pressuring in the sequence due to rapid deposition and tectonic compression. Scaly clay formation in the area is not related to collision or subduction, but to diapirism in the transcurrent fault system now operating at the plate boundary.