Asymmetrical condition of the Bogor Basin (West Jawa, Indonesia) during the Middle Miocene to Pliocene based on taphonomic study of shellbed and its sequence architecture

Abstract

Examples of lithology, fossils and taphonomic features of shellbeds and intervening less-fossiliferous intervals are presented from two Middle Miocene to Pliocene successions of the Bogor Basin in West Jawa, Indonesia: the Nyalindung Formation in the Sukabumi area which was deposited in the western part of the basin and the Kaliwangu Formation in the Sumedang area, which is the eastern extent of sediments in the basin. As for Miocene to Pliocene 6th order depositional sequences, the two successions contain a variety of shellbeds which are often associated with surfaces or intervals that are characterized by sedimentary condensation, omission or erosion (e.g., sequence boundaries, ravinement surfaces, downlap surfaces and condensed sections). Stratigraphic patterns in the shellbeds tend to be similar and repetitive within a basin with respect to position and age. For example, shellbeds formed in the context of toplap are common only in the Nyalindung Formation, and are characterized by thicker units of trangressive systems tracts (TST) and thin units of highstand systems tracts (HST). In contrast, sedimentary cycles from the eastern part of the basin in the Kaliwangu Formation can be recognized as formed under conditions of marine backlap and downlap, where the TST units are thin but the HST units are thicker. Such contrasting cycle architectures are interpreted to reflect an asymmetrical shelf depositional setting between the western and eastern edges of the Bogor Basin since Miocene to Pliocene time.