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

Abstract

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.

Two new important outcrops along the Pan-Borneo Highway in the Lambir Hills, Miri area, Northern Sarawak

“…There was an exchange of ideas with the late Prof. Tija in 2016 (Kessler & Jong, 2016b). Fieldwork by Kessler (2009) plus seismic and gravity interpretation (Kessler & Jong, 2016c, 2017bJong et al, 2016Jong et al, , 2017 suggest the Baram Line is making a turn to the East, crossing Bukit Lambir, heading towards Gunung Mulu and eventually aligning with other fault systems in Sabah, separating the NW Borneo shelf and turbidite basin from the Crocker Basin. If we follow this interpretation, the Baram Line is a crustal detachment fault system which encloses areas of thinned continental crust of the South China Sea, overlain by some Oligocene but mainly Neogene sediments.…”

The Setap Shale Formation on either side of the Baram Line Divide: Facies aspects and tectonic implications

“…This transition points to a gradual change of sedimentary environment, possibly a change of sea level and/or water chemistry, which eventually stopped carbonate production. It is also noted that the rocks are fossiliferous, and detailed investigation of the marine species might reveal further interesting paleo-environmental indications during this time of carbonate growth in northern Sarawak (e.g., Kessler & Jong, 2017a).…”

“…The green sand appears to indicate fully marine conditions just before clastic deposition of the Lambir Formation (Cycle IV) started. • Possibly, a carbonate buildup offshore, drilled with mixed success by Sarawak Shell Berhad called A1 (Kessler & Jong, 2017a marine fauna within a platform margin environment of deposition. The BD Setap above the carbonate contains fluvio-marine influences, and shallow-up from inner neritic to lower coastal plain.…”

“…In addition to the availability of published data by Wannier et al (2011), fieldwork carried out in the greater Miri area from 2004 to 2010, and a recent visit to a fresh outcrop in 2018 lead to the documentation of several new outcrops mainly along roadsides (Kessler, 2005(Kessler, , 2006(Kessler, & 2009aKessler & Jong 2016a, 2017a. This field studies sparked a hypothesis that the Setap Shale south of the Baram Line, located in the Tinjar/Luconia Block (now called TL Setap) was markedly different in terms of facies and depositional environments from the Setap Shale found in the Baram Delta Block (now called BD Setap) ( Figure 3).…”

“…This field studies sparked a hypothesis that the Setap Shale south of the Baram Line, located in the Tinjar/Luconia Block (now called TL Setap) was markedly different in terms of facies and depositional environments from the Setap Shale found in the Baram Delta Block (now called BD Setap) ( Figure 3). It was noted at that time by Kessler (2009a) that the Setap Shale south of the Baram Line contained turbidite sequences, and slumped coastal sandstone olistoliths are pointing towards a neritic depositional environment (Kessler & Jong, 2016a, 2017a. The Setap Shale north of the Baram Line, however, hosted carbonate bodies such as shallow water carbonate shoals and oyster reefs (Kessler & Jong, 2018).…”