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Neogene Carbonate Growth and Drowning Offshore Central Vietnam: Consequences of South China Sea-Opening and Onshore Uplift

Fyhn, Michael B.1; Giang, Tran C.2; Nga, Le H.2; Hong, Nguyen T.3; Nguyen, Nguyen D.4
1Geological Survey of Denmark and Greenland, Copenhagen K, Denmark.
2Vietnam Petroleum Institute, Hanoi, Viet Nam.
3Hanoi University of Science, Hanoi, Viet Nam.
4Hanoi University of Mining and Geology, Hanoi, Viet Nam.

Neogene carbonate platforms cover a large fraction of the Central Vietnamese South China Sea margin. The margin is little explored, and so far, exploration has focussed on Miocene carbonates with a number of oil and gas discoveries made. Carbonate growth initiated during the Early Miocene following Eocene-Oligocene continental rifting. Early carbonate deposition took place on two regional platforms separated by a narrow depression that marks the trail of the East Vietnamese Boundary Fault Zone (EVBFZ).

West of the EVBFZ, the Tuy Hoa Carbonate Platform fringes the Vietnamese margin between Da Nang and Nha Trang. In the west, platform growth initiated during the Early Miocene and continued until Middle Miocene time when regional uplift along the Vietnamese margin led to sub-aerial exposure and karstification.

Seawards from the EVBFZ, carbonate deposition similarly commenced during Early Miocene time (Burdigalian). Platform growth took place on the Qui Nhon Ridge flanking the EVBFZ and farther seawards on the Triton Ridge. From the latest Early Miocene, stepwise partial drowning resulted in platform split-up and seaward retreat of carbonate growth. Hemipelagic drapes cap drowned platforms and indicate protracted periods of sediment starvation after drowning. Continued drowning is evidenced by platforms subcropping the present seafloor at few hundred meters depth, and modern platform growth only remains on a number of seafloor cuestas around the Paracel Islands far from the Vietnamese mainland.

The onset of widespread carbonate deposition largely reflects the establishment of the South China Sea. The mid-Neogene shift in carbonate deposition is interpreted as a consequence of regional uplift of central and south Indochina starting around Middle Miocene time when the Qui Nhon Carbonate Platform became sub-aerially exposed. Stressed carbonate growth resulted from increased inorganic nutrient input derived from the uplifted mainland, which promoted platform drowning farther offshore.


AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012