Carbonate banks and ramps on the northern shore of Palaeogene and Early Neogene Borneo: Observations and implications on stratigraphy and tectonic evolution

Following the intense Palaeocene-Early Eocene Sarawak Orogeny (around 40-36 Ma), the South China Sea engulfed the northern shore of Borneo in present-day NW Sarawak, enveloping both the Luconia/Tinjar terrains and also rimmed the recently emerged and eroding Rajang Group hinterlands on the northern Borneo shore. With prevailing inner neritic depositional environment at that time, benthic foraminiferal limestone banks and ramps developed on sheltered shoals, separated from each other by clastic fairways with turbiditic channel deposits. By Early-Middle Oligocene times, carbonate deposition slowed as a consequence of increased subsidence and or, less likely, of a strongly global rising sealevel. After a pause in which clastics dominated the area, a second carbonate system formed during the Early-Middle Miocene times. These carbonates contain the first hard evidence of small bioherms, mainly corals and coralline algae. However, in the study area, there is not a single outcrop or well which shows an uninterrupted carbonate sequence from the Palaeogene to the Neogene. In addition, it is believed that the palaeo-edge of the platforms today lies somewhat masked by tectonic events, in particular, by a Late Miocene to Early Pliocene fold and thrust belt. Consequently, we believe, that both the Eo-Oligocene and Early-Middle Miocene carbonate systems are independent, not linked or vertically interconnected. Arguably, the presence of carbonates in two distinct systems points to a deepening, and later shallowing in a mega cycle. Within the mentioned hypothesis, the Eo-Oligocene carbonate system was formed during the deepening of the NW Borneo foredeep, whereas the Lower-Middle Miocene carbonates originated as the foredeep shallowed. The latter eventually disappeared with the establishment of a shallow, clastic shelf.