Middle Miocene Cyclic Sedimentation Based on Mollusks Taphonomic Analysis in Beach to Lagoonal Deposit of Nyalindung Formation, Sukabumi, West Java

Abstract

Nyalindung Formation is a Middle Miocene fossil-bearing stratum consisting of fine clastic sedimentary rocks and carbonates. Nyalindung Formation exposed in Ci Galasar River indicates beach to lagoon depositional environment with shell beads. Taphonomy of mollusks shells concentration in each shells bed was studied to understand the sedimentation cycle of Nyalindung Formation in the research area and further understand the number of cycles in the deposition of Nyalindung Formation, as well as the suspected event affecting the sedimentary cycle. According to the taphonomy classification, four types of shell beds were identified. Early Transgressive Systems Tract (Early TST) was characterized by sandstone and siltstone with poor shells fragment or none. Late Transgressive Systems Tract (Late TST) was characterized by an articulated specimen in its life position, with a low level of fragmentation. In this type of shell bed, adult specimens were found complete with high variation of other taxa. Maximum TST was recognized through high diversity of taxa, sizes and without certain orientation. Early Highstand Systems Tract (Early HST) was indicated through adult taxa articulated individually in life position accompanied by the juvenile specimen. In the Early HST sequence, shell density and species diversity are relatively low, and shallower environment shells association was obtained. Multi-event concentrations identified late Highstand Systems Tract (Late HST) with the domination of disarticulated shell and carbon streaks. Based on this classification, around ten incomplete cycles can be identified and suspected to happen at the time of deposition. These cycles are presumed to be correlated with Milankovitch Cycle, as represented by the thickness of each sedimentary cycle, which rarely exceed 2 m.