Microbial carbonates in Miocene reefs in the Mahakam Delta in East Kalimantan, Borneo, Indonesia

Coral patch reefs grew in shallow marine turbid waters in the Miocene Mahakam Delta in East Kalimantan (Borneo, Indonesia). These patch reefs developed from delta front to deeper (prodelta) settings in areas with temporary reduced siliciclastic input. These coral reef deposits are well exposed in limestone quarries in the Batu Putih area in Samarinda and locally include microbial carbonates.
Two different types of microbial carbonates have been found in two localities around Samarinda, 2 km apart from each other. In the northern section, microbialites occur as low-relief domes, up to 2 m wide and 0.5 m high, with internal lamination, developed around large coral fragments. They appear at the transition from bioclastic sediments with coralline algae and corals to fine-grained siliciclastic deposits in a deepening succession.
The second type of microbialites can be found in the southern locality as decimetre-scale nodules formed around nuclei of large coral fragments. Microbial micrite with laminated to digitated fabrics intergrew with coralline algae to form the thick covers of these "megaoncoids". The large nodules are the main components of carbonate breccias deposited on the flanks of a patch reef.