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.

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.

In Southeast Asia, Cenozoic carbonates commonly were initiated or terminated at times of tectonic change, and they are bound by unconformities. A review of the timing of several of the largest unconformities shows they correlate over wide areas. These unconformities divide the stratigraphic record of Sundaland into episodes, with times of rapid basinal re-adjustment separated by periods of relative stasis. These generalizations are true not only in the successions deposited during the dominantly extensional later Eocene to early Miocene, but also in the often-compressional middle Miocene and younger deposits, in which at least three phases of rapid tectonic and stratigraphic change can be correlated over multiple depocenters. A detailed review of many areas of Southeast Asia using quantitative biostratigraphic methods indicates that tectonism resulted in relative changes in sea level, with no indication that eustasy was an important driving force. The boundaries between these sedimentary episodes vary in their effect and distribution, depending upon the tectonic cause. As a consequence, there is no uniform subdivision of the stratigraphic record across the region, which precludes a simple Sundaland-wide classification of primary sedimentary sequences. This review concentrates on both event timing and rates of change within and between basins, especially those changes that initiated or terminated carbonate deposition. In addition, some short-lived carbonate buildups that developed during transitional periods are described. This review provides the beginnings of a new stratigraphic framework for the Cenozoic carbonates and related deposits of Southeast Asia. It requires an inductive analytical methodology, in contrast to the model-driven and deductive approaches that have been applied in recent years. This episodic stratigraphy approach includes integration of large amounts of analytical data, for which the Cenozoic tropical sediments of Southeast Asian basins are particularly well suited.

Abstract The purpose of this account is to review the complex terminology of the Tertiary stratigraphic units in Sumatra and propose a revised and a simplified terminology based on the significance of formations for the tectono-stratigraphic development of the island. Formations are classified in terms of Pre-Rift, Horst and Graben, Transgressive, and Regressive tectono-stratigraphic stages. The island of Sumatra lies along the southwestern margin of the SE Asian continent (Sundaland) beneath which the Indian Ocean Plate is currently being subducted at a rate of about 7 cm a -1 in the Sunda Trench (Fig. 7.1 ). The continental margin of SE Asia is of Andean type, with active and inactive Quaternary volcanoes rising to over 3000 m above a Pre-Tertiary basement, exposed towards the west coast of the island in the Barisan Mountains. Tertiary sedimentary basins occur both to the SW and the NE of the mountains and small basins also occur within the mountain range itself. These basins are described with relationship to the present-day subduction system as forearc, backarc and intra-arc or intramontane basins (Fig. 7.1 ). The Barisan Mountains are transected by the Sumatran Fault System, a major dextral transcurrent fault zone which extends along the length of the island from the Sunda Strait to the Andaman Sea. Stratigraphic research in the Tertiary sedimentary basins commenced in the last decades of the nineteenth century when oil was discovered in the Telaga Tiga (1883) and Telaga Said (1885) wells near Pangkalan Brandan in North Sumatra. Initially, wildcat drills were sited near oil

Abstract

Abstract In the early days of mineral exploration on behalf of the Netherlands East Indies Bureau of Mines and of petroleum exploration by the oil companies it was recognized that Pre-Tertiary rocks were extensively exposed in the Barisan Mountains in the western part of Sumatra (Fig. 1.4). These rocks are variably metamorphosed and were termed the “Barisan-Schiefer” and the “Old-Slates Formation” (Veerbeek 1883) in Central Sumatra, and the ‘Crystalline Schists’ in the Lampung area ( Westerveld 1941 ). Locally these rocks contain fossils, and it was recognized that Carboniferous and Permian rocks occur within this Pre-Tertiary basement. Some basement units were defined during the mapping of Sumatra by the Netherlands Indies Geological Survey between 1927 and 1931, but the definition of units according to modern stratigraphic principles began in the early 1970s, with the commencement of systematic mapping by the Indonesian Geological Survey in collaboration with the United States Geological Survey, in the Padang area of West Sumatra (Kastowo & Leo 1973-Padang; Silitonga & Kastowo 1975-Solok; Rosidi et al. 1976-Painan and Muarasiberut). Mapping and the definition of further units was continued in northern Sumatra by the Indonesian Directorate of Mineral Resources/British Geological Survey (DMR/BGS) between 1975 and 1980 as part of the Northern Sumatra Project and was extended into southern Sumatra in the 1980s and 1990s by the Indonesian Geological Research and Development Centre (GRDC), DMR amd BGS. The results of these surveys, which established the distribution of the basement units, are published by GRDC as 1:250 000 Geological Map Sheets covering the