Abstract The geodynamic evolution of the Palaeo-Tethyan Ocean is key to understanding the development of Pangaea. The main continents of SE Asia were welded together in response to the closure of the Palaeo-Tethyan Ocean. However, the precise location of the sutures and timing of the collisions are poorly constrained, especially the southward continuation from Peninsular Malaysia into west Indonesia. This study presents new zircon U–Pb geochronological, petrological, elemental and Sr–Nd–Pb–Hf–O isotopic data in order to investigate the origin and tectonic setting of the granitoids from the Bangka and Belitung islands, west Indonesia. Although possessing different mineral compositions, the two groups of samples yield similar crystallization ages of c. 230–220 Ma and enriched Nd–Hf isotopic compositions (whole-rock ε Nd ( t ) = −9.6 to −2.4 and zircon ε Hf ( t ) = −12.1 to −0.1), and probably originated from melting of a mixed source of metagreywacke and metapelite with a subordinate meta-igneous component. Both granitic groups formed during post-collisional processes, and when this interpretation is combined with regional investigations on the granitic magmatism, the Bangka–Belitung granitoids may represent the southward continuation of the Main Range granitoids. If correct, the distribution of granitoids with distinct petrogenesis across both islands implies a location for the Palaeo-Tethyan suture zone between Bangka–Belitung and West Kalimantan.

This paper describes the deposition of Miocene carbonates around Sarawak in a tectono-stratigraphic framework. The onset, termination, and location of the two main carbonate units, the Subis or Lower Cycle II limestones and the Luconia limestone, were controlled by tectonic processes, each beginning with a subsidence event, and terminated by influxes of siliciclastic sediments due to hinterland uplift. New data are presented on the intra–late Miocene decline of Luconia Limestone platforms that is correlated to the uplift of onshore Sarawak (Tinjar Province) and renewed siliciclastic sedimentation, which is dated as being at the same time as major uplift in northern Borneo. Miocene sedimentation around Sarawak was controlled mostly by extensional tectonics with several rapid subsidence events, which produced transgressive unconformities with mappable focal areas. Away from these focal areas, the contrast in facies, before and after the event, gradually diminishes in a predictable manner. This property of the unconformity is governed by Walther’s Law in that one well or field section cannot be exempt from the mappable trends in facies contrast observed in surrounding wells. This relationship constrains the interpretation of seismic, mapping, and analytical data, as illustrated by an example of a misdated unconformity that previously violated this balance of facies change in space and time. The tectono-stratigraphic model is a refinement of an existing empirical scheme devised in the area, with units called “Cycles” (Cycles I to VIII). This evidence-based framework is argued to be a genetic description of depositional units that developed in a dynamically evolving depocenter, subject to geographic rotation and relative variations in sea level that were dependent on location. This shifting basin configuration precludes use of a passive margin sequence stratigraphic approach, which assumes and requires a constant proximal to distal sedimentary direction and steady basement subsidence.

A new compilation of data suggests aragonitic coral reefs were already common in Southeast Asia by the mid-Oligocene. A gradual change from calcite to aragonite seas through the Oligocene and early Miocene appears to be related to a gradual expansion of the importance of scleractinia, along with green algae and mollusks, and an associated decline in the abundance of calcitic larger foraminifera. The larger foraminifera had been important rock-forming bioclasts in the early part of the early Miocene, but were a minor component of carbonate faunas by the end of the middle Miocene. This gradual decline in abundance included a few extinction events that reduced diversity, and these extinctions appear to correlate with periods of tectonic change. The K-selection evolutionary pressure impacted carbonate facies, but foraminifera maintained their taxonomic diversity until the abrupt faunal extinctions. Changes in sea-surface temperature, or the regional change from seasonal to ever-wet climate, do not appear to have impacted larger foraminiferal diversity or caused extinctions, only modified their latitudinal range. Some extinction events can be recognized across the whole Tethys Ocean, as can some of the times of tectonic activity and possible climate change. These correlations tentatively point to a link between large-scale changes in plate motion, oceanography, and foraminiferal extinctions. In contrast, the change from seasonal to ever-wet conditions around the Oligo–Miocene boundary around the South China Sea does not appear to have been caused by a wider tectonic event, and this event does not impact larger foraminifera diversity. A combined tectonic unconformity and mass extinction of larger foraminifera in middle middle Miocene times might have been due to the plate tectonic constriction of a throughflow between the Pacific and Indian Oceans.

Isolated carbonate platforms are abundant and widespread in Cenozoic strata and in the present-day oceans of Southeast Asia. The purposes of this article are (1) to describe the basic oceanographic setting of present-day Southeast Asia oceans; (2) to synthesize, compare, and contrast observations of the character of extant platforms in the context of fundamental oceanographic controls; and (3) to leverage these insights to develop a more complete understanding of older isolated platforms, especially the Miocene systems of Central Luconia. The data, presented to mimic an offshore-to-nearshore transect, illustrate Holocene platforms with a spectrum of sizes, depositional relief, facies abundances, and water depths. Although the first-order patterns of relief, size, and orientation are controlled by the geologic setting and Pleistocene history, the results demonstrate the influences of physical processes (waves, tides, currents), siliciclastic sediment, and chemical oceanography (nutrients, salinity, temperature) on the sedimentologic and geomorphic character of these platforms. Careful and critical application of these concepts to Central Luconia reservoirs in isolated carbonate platforms provides actualistic examples and process-response analogs. Although these perspectives offer understanding into controls on horizontal and vertical reservoir heterogeneities, they also emphasize that any one modern system can only be a partial analog for an ancient reservoir in an isolated carbonate platform.

Remote-sensing analysis of high-resolution satellite imagery of modern carbonate platforms in the Celebes Sea, east of Sabah, Borneo, Malaysia, was used to map geomorphology and sediment. Unsupervised classification of satellite images was interpreted in the context of environmental facies of seven isolated carbonate platforms and calibrated using analyses of surface sediments. In total, 140 sediment samples were collected and analyzed for grain-size and sorting. The grain-size analysis showed that sediment varied among the geomorphic elements, which included island, island/volcano, reef complex, carbonate sand shoal, grass-covered sand shoal, shallow lagoon, and deep lagoon. To generate carbonate sediment texture maps, the proportion of mud and the grain-size attributes (mean grain size and sorting) of each sediment sample provided a basis to classify samples into rock-equivalent textures. Integration of remote-sensing, field, and sedimentological data provided a means to characterize texture distribution maps and depositional facies maps. These maps suggest that mudstone to wackestone occurs mainly in the deep lagoon; wackestone to packstone is dominant in the shallow lagoon; the carbonate sand shoal is characterized by packstone to grainstone; and the reef complex is made up of boundstone to rudstone. These results facilitate estimates of the proportions of potential reservoirs on these platforms and the heterogeneity in facies distribution, based on the size of various recent carbonate platforms. Diagenesis notwithstanding, ancient analogs indicate the Selakan and Maiga platforms could be potential reservoirs, whereas the Selakan and Gaya platforms display more facies classes and represent poor potential reservoirs.