Proceedings, the annual meeting of the Sedimentological Society of Japan
Hirosaki, April 2008
A longitudinal turbidite system in the Late Miocene back-arc basin, the Halang Formation, Java
M. Ma’ruf Mukti*, Makoto Ito*
* Chiba University
A sedimentological study in the eastern part of Bogor Trough, a Late Miocene back-arc basin, in Java, revealed the existence of a longitudinal turbidite system developed in the east and southeast downslope direction. This turbidite system is represented by the Halang Formation (up to 1240 m in maximum thickness), which is exposed over 200 km2 in the east-west trending the trough axis. Overall, the formation is a mudstone-dominated turbidite system and the sandstones occupied 22 % –50 % of total section thickness. On the basis of field mapping and observation of centimeter-scale lithofacies features of the formation, six major lithofacies associations were identified: (1) channel-fill deposits, characterized mainly by medium- to thick-bedded, graded and massive sandstones with distinct scour bases and sharp tops. Dewatering structures are common and lateral accretion packages are locally developed in the basal part of some channel-fill deposits; (2) channel-overbank deposits, characterized by thin- to thick-bedded massive sandstones with climbing ripple and convolute laminations, and mudstone clasts. Locally thin-bedded sandstones are intercalated with thin-bedded siltstones and mudstones; (3) sheet-package sandstones, characterized by medium- to thick-bedded massive sandstones, in association with some graded sandstones and muddy sandstones. These sandstone beds exhibit sheet-like geometry; (4) channel-sheet package transition deposits, characterized by mixed types of channel-fill deposits and sheet-package sandstones, in association with poorly sorted muddy sandstones with mudstone clasts; (5) sediment-wave and/or distal overbank deposits, characterized by fine-grained intervals that are dominated by laminated mudstones and massive silty mudstones, in local association with massive siltstones; (6) hemipelagic deposits, characterized by massive mudstones and siltstones, in local association with very fine-grained sandstone laminae and some concretionary horizons. Several volcanic ash beds and hemipelagite-dominated intervals were used as datums for clarifying spatial and temporal variations of the six major lithofacies assemblages in seven measured sections (Sections 54–467 m thick) along the trough axis over 22 km. In general, channel-fill deposits are not laterally continuous. Paleocurrent directions diverge from the general paleocurrents to the east and southeast direction. These features suggest high-sinuosity channel patterns in the muddy turbidite system. Sheet-package sandstones are interpreted to represent both frontal-splay and crevasse-splay deposits in relation to channel-sheet package and overbank deposits in the upslope areas, respectively. Although the two types of the sheet-package sandstones do not exhibit any distinct difference on component lithofacies and geometry, the frontal-splay deposits are commonly intercalated with muddy sandstones (i.e., debrites) and suggest local flow transformation from turbidity currents to debris flows at a mouth of submarine channels. An overall geometry of fine-grained intervals exhibit wave-like geometry and some of the fine-grained intervals are interpreted to represent sediment-waves on a distal overbank environment.