PALEOGENE STRATIGRAPHY AND PALEONTOLOGY OF THE BRANI, SANGKAREWANG, AND SAWAHLUNTO FORMATIONS, WEST SUMATRA

Paleogene strata of West Sumatra include coarse grained alluvial fan and fan delta deposits of the Brani Formation interfingering with the Eocene lacustrine sediments of the Sangkarewang Formation and the fluvial sandstones, mudstones and coals of the Oligocene Sawahlunto Formation.

Ichnofossils and invertebrate and vertebrate fossils have been recovered from both the Sangkarewang and Sawahlunto Formations. The lake deposits of the Sangkarewang include 15 species of fish including minnows (Cyprinidae), catfish (Siluriformes), and bony-tongued fishes (Osteoglossidae). One fossil bird (Pelicaniformes) and crocodylian teeth have also been recovered from the formation.

The Sawahlunto Formation near Kandi (northwestern Ombilin Basin) has yielded a low diversity ichnofossil assemblage. The invertebrate trace fossils include 6 ichnogenera attributed to infaunal and epifaunal activities. Arenicolites, Diplocraterion and Monocraterion / Skolithos record the suspension feeding activities of either arthropods (most likely amphipods) or worms. Planolites reflects the presence of an infaunal deposit feeder. Coenobichnus records the walking activities of hermit crabs and their detritus scavenging in a subaerial setting.

Two distinct types of avian footprints, referable to the ichnogenera Aquatilavipes and Avipeda, have been recovered from the Sawahlunto Formation near Kandi. These traces are similar in morphology to those of modern shorebirds such as rails, sandpipers or plovers. They occur in very fine-grained sandstone with low-relief current ripples many of which exhibit mud-draping. This succession is interpreted as intertidal sand flats. Probe and peck marks preserved on the same bedding planes support the interpretation that these birds occupied the Kandi intertidal flats for foraging purposes.

The presence of traces attributable to subaqueous suspension feeders co-occurring with well-preserved avian footprints and surficial traces of invertebrates further implies periodic subaerial exposure. Thus, it is most likely that the Sawahlunto Formation records deposition within an intertidal flat setting. This evidence of marine influences in the Oligocene interval of the Ombilin Basin implies a more complex tectono-stratigraphic history than has previously been suspected.