Saturday, 19 October 2019

Evidence for a giant submarine landslide in the South China Sea at the Oligocene-Miocene boundary.

Submarine landslides are one of the most important processes for global sediment fluxes and tsunami generation. Such events can be massive, far larger than subaerial landslides; the largest known subaerial landslide, the Markagunt Gravity Slide, occurred about 22 million years ago in what is now Utah, and had a lateral extent of roughly 90 km. For comparison, the largest submarine landslide in the last decade was triggered by the 2011 Tōhoku Earthquake, had a lateral extent of 27.7 km, and far larger such events are recorded in the geological record; the Storegga Slide on the coast of Norway, which happened between 6225 and 6170 BC, had a lateral extent of about 290 km, and the Halibut Slide, which occurred between 62 and 64 million years ago on the west coast of Scotland was of similar size. 

In a paper published in the Journal of Marine Science and Engineering on 17 May 2019 Chaoqi Zhu of the Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering at the Ocean University of China, and the Laboratory for Marine Geology at the Qingdao National Laboratory for Marine Science and Technology, Sheng Cheng, also of the Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering at the Ocean University of China, Qingping Li of the Research Center of the China National O ffshore Oil Corporation, Hongxian Shan, also of the Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering at the Ocean University of China, and the Laboratory for Marine Geology at the Qingdao National Laboratory for Marine Science and Technology, and the Key Lab of Marine Environment and Ecology, Jing’an Lu of the Guangzhou Marine Geological Survey, Zhicong Shen again of the Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering at the Ocean University of China, and Xiaolei Liu and Yonggang Jia, once again of the Shandong Provincial Key Laboratory of Marine Environment and Geological Engineering at the Ocean University of China, the Laboratory for Marine Geology at the Qingdao National Laboratory for Marine Science and Technology, and the Key Lab of Marine Environment and Ecology, present evience for a gigantic submarine landslide in the Pearl River Mouth Basin of the South China Sea.

The Pearl River Mouth Basin has formed by continental rifting and sea floor spreading in the South China Sea area since the Early Cretaceous. The Baiyun Sag area of the Basin contains a succession of sediments laid down on the continental shelf edge on the outer opening of this basin, which contains the main reservoir of oil, gas, and gas hydrate in the northern South China Sea. However the area also contains numerous submarine landslide structures, many of which are buried or otherwise obscured, making them difficult to map accurately. At least 142 such mass transport complexes have been found in the submarine canyons of the Pearl River Mouth Basin.

 Geological map showing the Pearl River Mouth Basin. Zhu et al. (2019).

The present shelf break is aligned with the boundary between the Panyu Low Uplift and the Baiyun Sag. Sequence stratigraphy of the Baiyun Sag reveals that the shelf break has been located in its present position since about 23.8 million years ago, and that the Baiyun Sag was in a deep-water slope environment at that time. Previous studies of the evolutionary history of the shelf break showed that from about 23.8 million years ago swung back and forth around the boundary, although it stayed quite close to the present location. Continental shelf deltaic deposition is found in the Zhuhai Formation (laid down between 32 and 23.8 million years ago, and is characterised by southward progradational reflections with sigmoid-oblique configurations seen in seismic surveys. These deltaic deposits extend to the Baiyun Sag, where deepwater fan structures can be seen.

Delta structures contain sediment and other material from rivers, which is dumped when it reaches the sea forming the delta. Over time the organic material within the sediment forms pockets, which are then heated and crushed by the overlying sediment (increasing pressure also increases temperature, and vice versa, which is why aerosol sprays, undergoing rapid decompression, are cold), forcing water out of more complex organic compounds (dehydration) and forming pockets of gas and oil.

The main cause of shelf break migration is usually changes in sea level, combined with changes in sediment flow and other geological processes. The progressive seaward migration of the shelf break in the Baiyun Sag between 30 and 23.8 million years ago can be attributed to sedimentary supply and sea level changes, but at 23.8 million years ago (the Oligocene-Miocene boundary) there is a sudden shoreward shift from the south to north of the Baiyun Sag, which Zhu et al. attribute to a submarine landslide. This marks a change in the depositional environment on the Sag, before 23.8 million years ago it was in a shallow, shelf-edge environment, after 23.8 million years ago in a deep-water depositional environment, and the shelf break migrated from south to north of the Baiyun Sag.

 Multi-beam submarine geomorphology shadow map showing the Baiyun–Liwan submarine slide, the Baiyun slide, the Dongsha creep zone, and the shelf break line (SBL). The white dotted line is a scarp of the Baiyun–Liwan submarine slide. The brown dotted lines are the scarp of the Baiyun slide and range of the Dongsha creep zone, respectively. Zhu et al. (2019).

This change in depositional environment is marked in drill cores by a sharp decrease in the deposition rate, from a relatively high rate in the Oligocene to a very low rate in the Miocene, separated by what appear to be slumping related fault structures. The Oligocene and Miocene sediment were differentiated on the basis of microfossil analysis. The landslide structure extends laterally for over 250 km, and appears to show the movement of about 35 000 to 40 000 cubic kilometres of sediment.

Submarine landslides can be causes by a variety of phenomena, but very large events are almost always associated with major tectonic events. Zhu et al. suggest that the Baiyun Sag landslip is associated with slip-strike faulting (faulting in which the areas of land on either side of a fault move sideways relative to one-another) along the Red River Fault, around the Oligocene-Miocene boundary. This was associated with the movement of Southeast Asia about 500 km to the south, relative to South China, driven by the penetration of the Indian Plate into the Eurasian, and the formation of the Himalayan Mountains.

Simplified map showing the triggering mechanism of the Baiyun–Liwan submarine slide. Zhu et al. (2019).

See also...

https://sciencythoughts.blogspot.com/2018/12/hundreds-dead-after-krakatau-eruption.htmlhttps://sciencythoughts.blogspot.com/2018/08/monitoring-turbidity-currents-on-margin.html
https://sciencythoughts.blogspot.com/2016/08/using-zircon-uranium-lead-geochronology.htmlhttps://sciencythoughts.blogspot.com/2016/04/assessing-impact-of-land-reclamation-in.html
https://sciencythoughts.blogspot.com/2015/11/queensland-sinkhole-linked-to-submarine.htmlhttps://sciencythoughts.blogspot.com/2014/10/interpretting-turbidite-deposits-on-eel.html

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