A reappraisal of the Cenozoic stratigraphy of the Malay and West Natuna Basins
Published in the Journal of Asian Earth Science, 2021
This review of the Malay and West Natuna basin covered several topics.
Firstly, the review took the original data from AQ-4x, one of the key wells in the early stratigraphic studies of the region (Armitage and Viotti, 1977) and re-examined it in light of drilling history (casing points etc; not considered by original report) and modern knowledge of fossils ranges – which can now be used to identify caving. This is part of my emphasis on precision, accuracy and trueness in data observation that, as illustrated here, can considerably change final conclusions.
The updated 1977 formations scheme correlates very well with the latest knowledge of the tectono-stratigraphic development of the Malay and West Natuna Basins; the formations, and Esso’s “Groups” can be correlated with a relative sea-level curve set to a modern time scale. This relative sea-level curve also directly correlates to larger crustal movements observed in Nam Con Son and the western end of the South China Sea. Very clearly the major packages of sedimentation are part of a regional tectonic model controlled by plate tectonics, not eustasy.
Two other topics of stratigraphy and sea-level are discussed.
1) It is noted that the Early Miocene of the study area (usually called Group I) remained at a lower coastal plain to occasionally brackish marine setting for about six or seven million years. There are no marine floods, which would stand out in the microfauna as well as other facies indicators. Reports of “marine glauconite” are challenged, and noted to be coated with sphaerosiderite, which is a much more reliable indicator of brackish to freshwater conditions. No wells over a very wide area report true marine conditions. Some subjective hints of shallow marine conditions, such as trace fossils in cores, are discussed. However Walther’s Law says that if such marine facies were present in some reservoir cores, then some of the many dozens of wells across Malay and West Natuna would see objective marine indictors – which they do not (as also noted by Shoup et al. 2012). This suggests that for the entire duration of the Early Miocene, eustatic sea-level appears to have had almost no significant change (Morley et al., 2020 suggest some very small scale sea-level changes might be present). Transgression to marine conditions would likely be preserved even if the overlying highstand deposits were partly eroded by later sea-level falls. In many areas, such as the Java Sea, Early Miocene reefs have had sea-level changes invoked as creating sequence boundaries seen on seismic (e.g. Kusumastuti et al. 2002), so the Malay data suggests these boundaries must be the product of local tectonism (see later paper on Miocene carbonates in Java in press)
2) In the mid Pliocene of the Malay Basin section there was a change, and sediments deposited after about 3.5 Ma have mixed microfaunas and lithofacies in cuttings. Samples with middle neritic foraminifera are adjacent to samples rich in coaly fragments and on seismic (Miall, 2002) there are fluvial channels at some levels. This is also seen in the Baram Delta and East Java sediments from the same mid-Pliocene time to the present day (river systems visible on the floor of the modern Java Sea at 60m, Umbgrove 1949). This 3.5 Ma correlates with the onset of northern hemisphere glaciation and repeated glacials and interglacials. It is important to note that this signature is not seen in older sediments in SE Asia, and not in the very consistent Group I Early Miocene coastal plain of the Malay Basin. Again a caveat that Morley et al. (2020) suggests that dense sample intervals and careful examination for traces of slightly marine palynomorphs, nannofossil or foraminifera might yield weakly correlatable events. This contrasts with the post 3.5 Ma changes that are evident even to well-site geologists (coal fragments and few calcareous fossils in cutting samples followed by samples rich in shell fragments and foraminifera), and have meandering streams on seismic enclosed in clays yielding middle neritic faunas.
References
Armitage, J.H., Viotti, C., 1977. Stratigraphic nomenclature – Southern end Malay Basin. Proceedings Indonesian Petroleum Association Convention 6, 69-94
Kusumastuti, A., van Rensbergen, P., Warren, J.K., 2002. Seismic sequence analysis and reservoir potential of drowned Miocene carbonate platforms in the Madura Strait, East Java, Indonesia. AAPG Bulletin 86(2), 213-232
Miall, A.D., 2002. Architecture and sequence stratigraphy of Pleistocene fluvial systems in the Malay Basin, based on seismic time-slice analysis. AAPG Bulletin 86(7), 1201-1216
Morley, R.J., Hasan, S.S., Morley, H.P., Jais, J.H.M., Mansor, A., Aripin, M.R., Nordin, M.H., Rohaizar, M.H., 2020. Sequence biostratigraphic framework for the Oligocene to Pliocene of Malaysia: High-frequency depositional cycles driven by polar glaciation. Palaeogeography, Palaeoclimatology, Palaeoecology 561, 110058
Shoup, R.C., Morley, R.J., Swiecicki, T., Clark, S., 2012. Tectono-stratigraphic Framework and Tertiary Paleogeography of Southeast Asia: Gulf of Thailand to South Vietnam Shelf. Search and Discovery Article #30246,
Umbgrove, J.H.F., 1949. Structural history of the East Indies. Cambridge: University Press.
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