Luan, X., Lunt, P., 2021. Latest Eocene and Oligocene tectonic controls on carbonate deposition in eastern Java and the south Makassar Straits, Indonesia. Journal of Asian Earth Sciences
Once again in order to present a synthesis of reefal carbonate development across Java I had to correct, or suggest modifications for many existing papers, hence its length and somewhat tedious style, and the large numbers of figures with original data on.
I began the paper to illustrate how carbonate formations were initiated and terminated by the major tectonic events, including those already covered in Luan and Lunt (2021). This required tedious age data and the jargon of fossils. Then I applied quantitative techniques such as geohistory plots to assess the speed and geographic variation of these tectono-stratigraphic changes. For this reason I included some data on the intra Late Miocene event in the upper section of some of the study wells, which has good age data and, once again, triggered a carbonate; either reefal Karren Limestone or in deep water the Mundu Formation globigerine chalk.
Having established the ages of the main unconformities it was noted that in some areas specific times showed uplift, while in others there was rapid subsidence at the same time. This was thought to be indicative of transpressional and transtensional strain along a wrench zone.
A correlation summary from a top journal that assumed eustasy was an important part of the stratigraphic story, but the data indicates tectonism controlled sedimentation. While they share the same ‘brick’ symbol, reefal limestones and hemipelagic chalks were deposited in settings with considerable vertical offset. This happened twice, once at the end of the Eocene to elevate the JS53B reef above the extinct 53A reef, but offset then reversed at end Oligocene times. The cores from the top of JS53B contain Te4 larger foraminifera that, like many others, were suddenly subsided, to be overlain by Miocene bathyal clays.
The paper describes several observations that previous local studies had forced into an expected eustatic sea-level sequence stratigraphic framework. This is confirmation bias, enabled by poor age dating as well as underestimating the environmental contrast across the unconformities. The new data in this paper almost certainly proves active tectonic, not eustatic or epeirogenic control. Evidence-based stratigraphy is beginning to replace model-driven interpretations. This is important, as it is in the zone of complex stratigraphy that the biggest oil and gas fields have been found.
The paper gathered data on the reefs that survived until terminal Oligocene times (the Oligo-Miocene boundary or top J70 event). These are located near the axis of extension in the Makassar Straits as well south of the Rembang Line, under the modern island of Java. Multiple large and high relief reefs abruptly terminated near the end of Letter Stage Te4 times. This was not associated with an influx of clastics, in contrast to the top J30 event in the area, when early Late Eocene reefal carbonates were abruptly overlain by thick late late Eocene siliciclastics. Instead the Oligo-Miocene J70 event saw the onset of both siliciclastic and micritic clastic starvation. The implication, dealt with in a subsequent paper on Miocene reefs in the Java / Makassar Straits area, is an exceptional fast rate of subsidence so that the reefal carbonate deposition could not keep up.
Some environmental background
In this paper, as in a separate review of globigerine chalks in Java, I had to introduce some background to the use of planktonic to benthic foraminiferal ratios (P:B) as a good guide to palaeo-water depths. In addition there are some notes on the planktonic foraminiferal species that require open oceanic conditions to reproduce, and as a result are found more in deep marine sediments. As mentioned elsewhere on this blog it is a shame there is no text book summarising this useful data. Excellent modern texts such as McGowran (2005) are dominated by age and correlation considerations. This is not unexpected in such a broad reference. The tropical foraminiferal response to environment of deposition is different from that in boreal conditions, the Mesozoic different from the Palaeogene, and the Miocene faunas are again different. No global text book can do justice to each realm and period. In the absence of such a guide the workers on the Jambaran Field made, I think, some very poor assumptions.
By not having this basic and reliable (but imprecise) P:B index, the people describing the core samples from Jamabaran-2 only noted the core from the very top of the Jambaran-2 limestone to be “rich in planktonic foraminifera”. It would have been nice to have a better observation (the P:B ratio, and whether certain species were present). The term “rich in planktonic foraminifera” to me suggests a P:B ratio greater than 50%, and this would mean an environment of deposition significantly greater than 150m. The Jambaran workers, from a company locked into eustatic stratigraphy mind-set, had interpreted a “middle to outer neritic” setting (i.e. water depths of 30 m to 150 or 200 m; the shallower range being within the range of fluctuations of later Oligocene eustatic variation). I would say I was probably right as the core sample, at the top of the carbonate in J-2, is located 500m below the reef crest drilled by the J-1 well. An un-tilted vertical offset that must be close to depositional topography.
Evidence for the mid-Oligocene unconformity grows
Finally, one other observation in this paper was the recognition of the mid-Oligocene tectonic event described in Lunt (2019), and found by me when, many years ago, I worked on the Baluran-1 well (with supporting strontium isotopes ages). The same event is also recognised by a quantitative review of the Sultan-1 well data. This is an important event for hydrocarbon exploration as it shows how complete transgressive sections, containing source rock, reservoir and seal, had been deposited in some areas, but then these areas were subject to mid-Oligocene uplift before Late Oligocene reefs formed over these structures. The Late Oligocene carbonates are a barrier to seismic imaging and drilling of wells, before rotating BOPs and on-balance drilling was available. Miocene burial could have caused maturation of source rocks and expulsion of hydrocarbons into traps formed in mid-Oligocene times. This play has never been pursued in the area as “everybody knows” reefs grow on old basement highs.
References
Luan, X., Lunt, P., 2021. Eocene to Miocene stratigraphic controls in the far East Java Sea: Implications for stratigraphic studies. Marine Geology 436, 106479
Lunt, P., 2019. The origin of the East Java Sea basins deduced from sequence stratigraphy. Marine and Petroleum Geology 105, 17-31
McGowran, B., 2005. Biostratigraphy: microfossils and geological time. Cambridge University Press.
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