This review examines the history of the Sarawak “Cycles” and their application in subdividing offshore Cenozoic stratigraphy. The Cycles are widely but inconsistently used, at least in part because important reports and data collections were never published (e.g. Geiger, 1964; Hageman et al., 1987; Hageman maps, 1985, eventually reproduced in Madon, 1999 and Hutchison, 2005; and Taylor et al., 1997) or were published in highly abridged summaries only (Ho Kiam Fui, 1978; Hageman, 1987; Mansor et al., 1999). The lack of a data-audit trail left open possibilities for ambiguity and confusion, as has been commented on by several workers (e.g. Snedden et al., 1995; Ismail & Tucker, 1999). This account reviews the major contributions, published and unpublished, and the concepts of the Cycles, especially over times of geological change. The data behind the model is cited in order to give confidence when integrating Sarawak stratigraphic data into a regional geological model. The Cycles began with an assumption that transgressions over regressive surfaces were distinct and approximately synchronous events for correlation. By including biostratigraphic data these transgressive events could be traced into clay-dominated areas, where lithological and seismic contrast was weak. This integrated approach was carried out through the 1970’s and 80’s, during which time the data pushed the model towards a three dimensional view of sedimentation, with the basin shape evolving and changing through time. However, development of a full tectono-stratigraphic model paused during the period of accelerated seismic capabilities of the 1990’s and early 21st Century. While geophysical data coverage increased, application of geological analyses decreased, and the integrated approach lost momentum. This review aims to re-establish the role of the Cycles as a part of a large scale geological model. An initial integration with regional geological events is attempted, linking some of the Cycle boundaries with times of known tectonic change.

/pdf/a-review-of-the-sarawak-cycles-history-and-modern-4y78klwgh8.pdf

A Review of the Sarawak Cycles: History and Modern Application

This review combines a wide range of onshore and offshore data from Oligocene to Pliocene sediments of northern Borneo to address the following topics: (i) the stratigraphic conditions before and after the Top Crocker Unconformity [TCU], (ii) Early Miocene palaeogeography, (iii) events during later Early to mid Middle Miocene times, including the Deep Regional Unconformity [DRU], and (iv) the waning of the Sabah Orogeny in Early Pliocene times. Emphasis is placed on dating the key events, in order to consistently identify the stratigraphic and tectonic changes observed in the different data sets. The data shows a period of uplift and deformation in the north, perhaps focussed in southern Palawan, during the Oligocene then its sudden cessation at the TCU, with strong contrast in both facies and deformation style of Late Oligocene outcrops on the Kudat Peninsula compared to the Early Miocene sediments drilled offshore. This termination of tectonic deformation coincides with published estimates for the onset of ridge-jump and sea-floor spreading in the South China Sea northwest of Sabah. Following this unconformity the deposition of sediments during the Early Miocene appears to have consisted of a fluvio-deltaic high in the south (from central Borneo) and a broad deep marine basin in the north, from west of the Kudat Peninsula to at least the western part of the Sulu Sea. For the subsequent DRU, its timing and character appears to coincide with large scale sedimentary changes in eastern Sabah and this suggests a pause in regional compression during the early part of the long lasting Sabah Orogeny. The stratigraphic description of these events has important implications in plate tectonic reconstructions for northern Borneo and the South China Sea. The role of subduction on the eastern side of Sabah may be a key component of revised tectonic models and further evaluation of the area from the Dent Peninsula south to the Indonesian border is recommended.

/pdf/onshore-to-offshore-correlation-of-northern-borneo-a-1b86egfpi8.pdf

Onshore to Offshore Correlation of Northern Borneo; A Regional Perspective

Supercritical geothermal resource is a renewable and carbon-free energy with enormous potential. Quantitative assessment of the geochemical processes involved in the future operation of this resource requires a comprehensive understanding of quartz solubility in subcritical and supercritical water. However, quartz solubility behaviors from subcritical to supercritical conditions are unclear, and the quartz solubility model requires refinement by improving the accuracy of experimental measurements. In this study, a series of in-situ sampling and measurements are conducted to investigate quartz solubility at 300–500 °C and 25–50 MPa by exploiting a novel and specifically designed experimental apparatus. The results indicate that the use of in-situ sampling and resistance continuous monitoring procedures can bring an accurate and reliable estimation of quartz solubilities near the supercritical point and in the low-density region. It also emerged that the quartz solubility in water is significantly affected by temperature and pressure both under subcritical and supercritical conditions, showing a positive correlation with pressure at isothermal and a retrograde behavior near the supercritical point. These features are interpreted by the thermodynamic theory, quartz hydrolysis process, and mathematical model. By combining our experimental results with high-quality data from the literature, a five-parameter density model is provided for the calculation of the silica concentrations in aqueous fluids up to supercritical geothermal conditions. To overcome the current limitations in the modeling of supercritical geothermal fluids, future experimental work needs to collect additional data on silica in real geofluids (e.g., saline fluid) to develop an advanced model. The findings of this study can help for a better understanding of geochemical processes in supercritical geothermal systems, as well as ore deposits and vent structures. 

An experimental study on quartz solubility in water under supercritical geothermal conditions

It has been accepted for many years that eastern Borneo and western Sulawesi were close together in the Late Cretaceous but the mechanism and age of formation of the Makassar Straits, which now separate them, have been the subjects of much debate. Geological studies on land show that the straits formed by Eocene rifting. However, the nature of the crust beneath the straits remains controversial. The southern parts are likely to be underlain by extended continental crust but, in the northern Makassar Straits, it is more difficult to decide. Water depths are up to 2500 m, there is a very thick sedimentary cover, the basement is not well imaged on seismic lines and there is no way of directly sampling it. Field studies from the Borneo and Sulawesi margins have provided the basis for reconstructing the development of the straits, and suggesting they are underlain by oceanic crust. The rift and its margins are asymmetrical and wide, with up to 400 km of stretched crust on the Borneo side and about 200 km on the Sulawesi side, separated by about 200 km of the deepest crust in the northern Makassar Straits. Gravity data and flexural modelling on the Borneo side suggest a junction between continental and oceanic crust beneath the Mahakam delta. The oceanic crust is inferred to be of Middle Eocene age, similar to the Celebes Sea to the north; apparent conical structures on seismic lines have been interpreted as volcanic edifices. However, the earliest backstripping studies suggested thinned continental crust in the central straits and this has been supported by interpretations of new seismic data from the offshore area west of Sulawesi. Half-graben and graben are interpreted beneath thick sediments, there are low-angle extensional faults, and lineaments crossing basement can be traced into the deepest parts of the straits. These structures suggest an origin by oblique rifting of continental crust in which the apparent conical structures are interpreted as carbonate build-ups on tilted fault blocks.

/pdf/the-north-makassar-straits-what-lies-beneath-5cyqcv3dlf.pdf

The North Makassar Straits: what lies beneath?

Abstract The Kutai Basin, Kalimantan, is the deepest sedimentary basin in Indonesia. It contains both substantial reserves of hydrocarbons and minerals. The basin formed in the middle Eocene as a result of extension linked to the coeval opening of the Philippines Sea, the Celebes Sea and the Makassar Straits. Seismic profiles across the northern margin of the Kutai Basin reveal inverted half-graben of middle Eocene age oriented north-northeast-south-southwest and north-south. Further extension during the late Oligocene occurred on northwest-southeast trending extensional faults, reactivating basement structures. Restoration of deformed half-graben suggests that the syn-rift sequence, which contains important coal horizons, was sufficiently deeply buried to generate hydrocarbons prior to inversion. Shortening from the early Miocene to the present day resulted in breaching of some structural and stratigraphic traps, and also in the generation of new traps, in particular folds in the hanging walls of inverted half-graben and as a result of short-cut faults. Middle Eocene, late Oligocene-early Miocene and Plio-Pleistocene volcanic activity was focused at the intersection of basement weaknesses and set up hydrothermal systems which formed several mineral deposits. Reactivation of northwest-southeast and northeast-southwest trending basement structures as a result of a variation in the stress regime has controlled the location of hydrocarbon and mineral deposits in the Kutai Basin. This study highlights the importance of a regional understanding of varying stress regimes in controlling the orientation and location of structures active in a basin at a particular time, and in predicting the location of mineral deposits.

Structural controls on the hydrocarbon and mineral deposits within the Kutai Basin, East Kalimantan

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Structural controls on the hydrocarbon and mineral deposits within the Kutai Basin, East Kalimantan