07 October 2009

North East Borneo, Bulungan Stratigraphy

GENERALIZED BULUNGAN STRATIGRAPHY

Bunyu Beds (Late Pliocene)

Sandy and clayey beds containing numerous intercalation of lignite, lying unconformably on older Tertiary. Either monoclinally dipping 1,5-2o E, or gently folded. Overlain by young alluvial deposits. No distinctive fossil. Sometimes these beds contain fossiliferous layers with brackish water arenaceous foraminifera and shallow marine Rotalias and Elphidiums.

Sajau-Tarakan Beds (Late Pliocene)

The Sajau Tarakan Formation is composed of two parts: an upper green clay and tuffaceous sandstone member and a lower light blue-green plastic, homogeneous clay member containing some lignite beds. Total thickness about 360 m. The lower division contains some economically important lignite seams in its upper part. These beds are overlain by the Bunyu Beds, and overlie with an angular unconformity the miocene Antjam and Tabul formation.

Antjam Beds (Early Pliocene)

Blue gray firm clay, rich in mollusca and corals, a few limestone beds and thin beds of coal (between half-shiny coal and lignite). Thickness 150 m. They are unconformably overlain by the Sadjau-Tarakan Beds (Group) and in turn disconformably overlie the Kundjang Beds (Group).

Kundjang Beds (Late Miocene)

Reef limestone and fossiliferous marls and clay. Thickness 200 m. The formation is overlain with a slight disconformity by Antjam Beds, and in turn slightly disconformably overlies the Sitam beds. They are correlated with the lower part of the Tabul Beds and the upper part of the Meliat Sandstone Formation, the upper part of the Taballar Limestone Formation and the Menkrawit Beds, etc.

Tempilan Beds (Late Oligocene)

Alternating thin-bedded sandstones, marls and shales, the marls increasing upward, so that the top of the beds consists of pure marl. The thickness is about 1000 m. The larger Foraminifera included in the formation indicate an Upper Oligocene age. The formation overlies the Seilor Beds (Formation) of Lower Oligocene age, conformably in Tidung Districts, disconformably in Salimbatu-Antjam-Bulungan, where the Mankabua marls are intercalated. They are overlain by the Mesalai Marls (Formation) of Lower Miocene age.

Mankabua Marls (Early Oligocene)

Neritic sediments with Camerina fichteli, indicating a Oligocene age. No thickness or composition is mentioned. The beds are disconformably overlain by the Tempilan Beds, and in turn overlie the Seilor Beds. Possibly they are lateral equivalent of at least part of the latter formation.

Marah Beds (Late Eocene)

Some thousands of meters of mica sandstone with intercalations of limestone and marl with larger Foraminifera. The age is Upper Eocene. The Marah Beds can be correlated with the Lower Taballar Marls in the Mankalihat Peninsula, and with the Tulit Beds in the Tidung Districts.

Sungai Orang Beds (Early Eocene)

Several thousands of metres clastic sediments, formed under geosynclinal circumstances, and strongly folded. They consist mainly of mica sandstone with some intercalations of marl and limestone containing larger foraminifera, indicating a lower Eocene age. They may be correlated with the Tikung Beds (formation) in the Tidung districts and the Sandstone Stage (formation) in Mangalihat Peninsula. The S. Orang Beds overlie unconformably the pre-tertiary of Borneo, and they are overlain by the Marah layers (formation) of Upper Eocene age.

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