Follow up from AAPG workshop, Oct 2021. See also this link, and this one
In my (October 2021) AAPG presentation I began by heavily stressing a new role for stratigraphy / biostratigraphy “it is more than just correlation and the naming of thing” … “stratigraphy does the testing of sedimentary geology that makes it a science”. In subsequent chats and messages I was asked to explain this.
What I am proposing is a change in scale and focus of biostratigraphy studies, because SE Asia has such good material to work on. Cenozoic biozonations, and Sr dating, as well as diverse tropical and often extant microfaunas for environment of deposition. Age and facies: two of the three variables of Walther’s Law (along with stacking order/geography). There was much discussion about documentation of the currently very dispersed, often unpublished, data on interpreting environment of deposition from microfossils
As mentioned before, in Sundaland sections there are many rapid, non-erosional paraconformities termed non-Waltherian contacts. There are two earlier posts that discuss non-Waltherian contacts (see these links, here and here).
The key property of non-Waltherian contacts is that they are a step-like shift in sedimentary facies, as recorded in the well (or outcrop) succession. At the scale of drill cutting samples and log analyses there are repeated instances of such contacts, especially during the extensional phase of Sundaland from about 40 to 15 Ma. These are frequently seen as very deep marine facies immediately above thick shallow or non-marine beds. They appear to correlate to more modest, but still step-like environmental shifts away from the axes of extension.
The step-like change contrasts with often hundreds of metres of constant sedimentary facies above and below. In spite of the name, this step-like change has a few properties constrained by Walther’s Law. As noted in the linked summaries, this sudden shift in facies architecture and palaeogeography cannot “leave behind” any location in the same basin with the same clastic supply, while the rest of of the sedimentary system has withdrawn due to rapid subsidence. Horsts can remain highs, but the sedimentary system has migrated a long way and can no longer supply the same sediment to these highs. The movement on the faults may continue for some time, and some fault blocks may move after others, possibly in a directed pattern of strain, but the step-like change in the sedimentary system recorded in a well section must translate to an equally sudden shift in sedimentary palaeogeography, as required by Walther’s Law. This property makes these events first-order sequence boundaries.
The example drawn below is based on the extreme movement in the North Luconia to Central Luconia Provinces offshore Sarawak, and it is places like this that the best examples can be studied, before tracking these events into marginal areas with less extreme subsidence. Knowing the primary nature of the sequence boundaries, and that they correlate, means that we can composite age data in wells. For example in the G2-1 and G10-1 (the latter not shown; both in the east of the study area) there is enough shallow marine material in the Oligocene “Cycle I” to date the subsidence as having been on the Te4-Te5 boundary, based on index fossil ranges as well as biometric evolution of larger foraminifera done by Ho Kiam Fui (1973). For most of the Cycle I fluvio-deltaics to the west there is no marine influence and the top Oligocene correlation datum is very hard to pick, as key markers such as Meyeripollis naharkotensis are only sporadically recorded in the section, so the highest occurrence is almost certainly not a true extinction datum. However, with Zone N4 / NN1-lrNN2 marine fossils in the marine beds above, and knowledge of the correlation of the major withdrawal of sediment at the 24 Ma event then, as noted above, this means that no pockets of coastal plain sands with coals can be left behind if the rest of the sedimentary system had migrated a long way south. This would break Walther’s Law. The Oligo-Miocene boundary event is therefore identified at the major environmental shift.
This is a simple example but I know many well interpretations that do not follow this, and in doing so break Walther’s Law. like the First Law of thermodynamics and perpetual motion machine, this is simply not possible. Data from the flanks of the subsiding areas requires better documentation of how micropalaeontologists have determined environment of deposition, especially the contrast of facies across the correlatable conformity. This is a separate subject, already with some posts here.
The point of this post is to explain how the scale and focus of biostratigraphic work has changed from being just correlation and age dating to a more integrated form of stratigraphy. Using age AND environment of deposition we have erected a hypothesis that can be tested. The data proposes an area that can be mapped out on seismic for (in North Luconia) two major tectono-stratigraphic changes. The environmental / facies contrasts over both events must tally with lithofacies (sands, coals, limestones bearing larger foraminifera vs limestones of hemipelagic bioclasts). Accommodation space will have been created with a palaeogeography that must also match seismic thicknesses and the locations and height of reefs. Rates of sedimentation must form a palaeographic pattern (sediment starvation cannot co-exist with progradation except at a very large scale). Consequently if someone insists that have found basal Miocene coastal plain beds with coals in an area adjacent to bathyal clays this is a logical crisis that must be resolved (is the age wrong? is there a new sediment source in the north and a fault with a different time of movement? – Occam’s razor would initially suggest the first). But this problem must be resolved, and in solving it the model will have been tested and improved. As a result biostratigraphy and integrated stratigraphy is playing a much more active role in developing a play scale and regional geological model. As I claim in my talk; stratigraphy is capable of playing a central role, testing and improving the geological model. Not just in offshore Sarawak but in multiple basins, from Eocene to at least Middle Miocene times (after which compression and erosion begin to make things harder, but not impossible)
The old workers used to do this, but we reduced stratigraphy and especially biostratigraphy to an outsourced correlation service. It is time to bring analyses back into a central role.
References
Some geohistory plots for the North Luconia area are linked here
Ho Kiam Fui, 1973. Morphometric trend in Lepidocyclina and its application to time stratigraphy in N.W. Borneo. Shell Report Exp. 529,
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