Moyra Wilson
Clastic sedimentation on shallow marine shelves is generally thought to inhibit carbonate production. However, delta front reefs are common down to depths of 100 m around the major, active Mahakam Delta in east Borneo, SE Asia. The formation of the modern delta has been influenced by moderate tides of about 3 m, a steady, high fluvial input, with an average sediment input of 8 x 103 m3/yr and very low wave energies.
Outcrops of, and core through, carbonates within the Neogene deltaic succession are analogous to the present day and indicate that delta front reef development is not just a recent phenomenon. Studies of these modern and ancient delta front reefs shows that coral patch reefs developed in less than a few tens of metres water depth, whereas Halimeda dominates in deeper water reefs. The shallow water coral reefs may be up to 2-4 km across, were sited on delta lobes or mouth bars, and in the sedimentary succession have post-compactional thicknesses generally up to 40 m.
Nummulites and heterosteginids deposited at moderate depths in the photic zone on a carbonate platform along the northern margin of the Kutai Basin, Borneo.
The deeper water buildups and carbonate platforms have similar dimensions, formed on ravinement surfaces and are inferred to have developed during periods of transgression. In outcrop, the coral reefs consist of lithologies dominated by platey, branching, head, branching and platey corals passing successively up section from base to top of the carbonate unit. These cycles represent apparent shallowing then deepening of the depositional environment, and subaerial exposure surfaces have not been recognised. Partial and multiple cycles may occur, especially where clastics are interbedded with the carbonates.
There is considerable lateral facies variations across the coral or Halimeda buildups and stratigraphic thicknesses vary laterally within the buildups. Reefs may coalesce or when covered by clastics often form slight highs on which later carbonate buildups preferentially form. The development of these delta-front reefs was controlled by a complex array of factors, including climate, relative-sea level changes, oceanographic factors, sediment and nutrient input. A comparison is made with other deltaic systems in the region and carbonates developed in clastic localities.
Modern and Tertiary carbonate production is and was extensive and diverse in the seas surrounding Borneo, and mirrors the variety of carbonate depositional systems seen in SE Asia. The availability of favourable conditions for carbonate sedimentation around Borneo was related to a combination of factors, including tectonic setting, the formation of large basinal areas, differential subsidence providing shallow marine areas, a tropical climate and a range of local factors, such as currents or limited clastic input. Middle Eocene to Plio-Pleistocene carbonates are present the northern Kutai region and the Mangkalihat peninsula, northeast Kalimantan. Carbonate sedimentation in this area occurred in a range of depositional environments, from mixed carbonate clastic shelves, localised and ephemeral carbonates, a variety of platform top settings to deep water redeposited carbonates.
