The Gomantong Limestone of eastern Borneo: a sedimentological comparison with the near-contemporaneous Luconia Province
Abstract The Luconia Province, lying to the North of Borneo, is an important hydrocarbon-bearing province. Faulting in the Oligo–Miocene led to the uplift of a series of blocks, upon which at least 240 carbonate build-ups were developed in the late Lower to Middle Miocene. As part of the search for appropriate analogues, an extensive field study was undertaken on the slightly older to contemporaneous Gomantong Limestone, which outcrops in eastern Sabah, some 800 km to the east of the Luconia Province. In eastern Sabah, southeast of the Crocker Range, Eocene to Miocene clastic and subordinate carbonate sedimentary rocks were deposited in a large east–west trending foreland basin. The Gomantong Limestone outcrops in the eastern third of this basin, the Central Sabah Basin, and is interpreted as having been deposited in shallower shoaling portions of the basin. Exposure is limited to isolated outcrops of deformed, often overturned or steeply dipping, carbonates. The studied localities form a linear trend extending some 50 km in a SSW–NNE orientation. The bulk of the limestone consists of coral-dominated framestones and boundstones and associated fore-reef and open marine facies. Coral morphology, foraminiferal/coralline red algal content and detailed sedimentological observations enabled an interpretation of the depositional settings of these carbonates to be made. The varied carbonate deposits were then interpreted in terms of palaeobathymetry. A very rich fauna is present throughout the formation, with some intervals dominated by large benthic foraminifera, particularly Lepidocyclina. These indicate a Chattian to Burdigalian age. Interpreted depositional environments range from open marine shelfal marls to a variety of reef-associated deposits. These have been subdivided on the basis of the dominant component: corals, coralline algae, foraminifera or siliciclastic components. Limited burial of these carbonates has resulted in excellent preservation of these sediments with very little diagenetic alteration. The Gomantong Limestone is interpreted to have been deposited in an open shelf setting, adjacent to an east–west trending shoreline. This shoreline bordered a palaeohigh within the foreland basin, and the basin is interpreted to have deepened to the north. This is in extreme contrast to the isolated carbonate build-ups in the Luconia Province to the west. Similarities in primary facies can be observed, and both the Gomantong Limestone and Luconia Province exhibit evidence of drowning events, but the facies are affected by very different diagenetic histories, which have left the two provinces with completely different reservoir characteristics. The disparity in secondary alteration shows the tremendous importance of diagenesis on the development of porosity in carbonates.