Slumping and sliding in Miocene and Recent developing arc basins, onshore and offshore Sumba (Indonesia)

Neogene slidemasses in eastern Sumba are discussed and compared to Neogene and Quaternary analogues in seismic profiles from the offshore Lombok and Savu basins. The onshore examples were deposited in deep marine base-of slope environments, within the reach of large amounts of clastics derived from a volcanic arc, i.e. in a setting comparable to that of the recent slides. Onshore, three types of slides are distinguished, ranging from mud clast-rich debris flows (type 1; their possible seismic origin is discussed), elongate lenticular slumps (type 2), to over 100 m thick slide masses of intimately mixed and folded strata together with rafts of less deformed sediments (type 3). The offshore analogues may be twice as thick. Both in the field and in the seismic profiles such slides are found at various, superimposed levels. Five superimposed units can be distinguished in a seismic profile in the Savu basin. Near the westernmost extension of the offshore Sumba Ridge a giant blocky slide occurs. All slide masses are located on slopes with increasing gradients, due to differential vertical movements. Tectonically induced oversteepening is therefore considered a main cause of failure. In outcrops the upper surfaces of the slides are almost flat; this contrasts with a hummocky topography seen in some of the offshore examples.