Cenozoic magmatism in East Kalimantan is believed relate to the two separated periods of Eocene-Oligocene and Late Oligocene -Miocene subductions. The influence of volcanic activity during sedimentation processes within the Upper Kutai Basin indicated by presence of volcanic materials in the sedimentary rock along Belayan and Ritan Rivers. This evidence is properly preserved within the Late Eocene Batu Ayau and Late Oligocene -Early Miocene Marah Formations.
The Batu Ayau Formation in the Belayan area is composed by polymict conglomerates, cross-bedded sandstones, shale, tuff with coal intercalation, and Marah Formation comprises andecitic conglomerates, lahar deposits, sandstone, shale, tuff with coal intercalation. Both formations are deposited in the fluvial-deltaic environments in association with volcanic activity. Petrographic analysis on sandstones indicates that origins of the volcanic material are both primary sedimentation of pyroclastic as well as mixing of re-sedimented volcanic material and terrigenous sediment. The pyroclastic rock consist of predominantly vitric tuff, vitric crystal tuff and lithic crystal tuff. Origin of volcanic material is possibly resulted by explosion of Nyaan and Kelian volcanoes during Eocene-Early Miocene Times.
Keywords: volcanic material, Batu Ayau and Marah Formations, fluvial-deltaic environments.
Authors : Sugeng Sapto Surjono, Herning Dyah Kusuma Wijayanti, Debby Irawan