Hinterland setting and composition of an Oligocene deep rift-lake sequence, Gulf of Tonkin, Vietnam: Implications for petroleum source rock deposition

Factors controlling accumulation of organic carbon in a rift-lake, Oligocene Vietnam

“…Nevertheless, even the intervals where proxy data point to most limited nutrient remobilization (e.g., 475 m, ~ 20 m) or strongest pycnocline (e.g., ~ 300 m) produce rich SR, suggesting that external nutrient input was sufficient for sapropel formation. Recently, Rizzi et al 20 documented that the hinterland sourcing the lake basin comprised phosphate-rich metasediments and granites, which resulted in high input of this nutrient into the graben stimulating primary production and source rock development in the rift lake.…”

“…Strong deformation associated with Neogene inversion hampers seismic based analyses of sediment transport directions and gross-depositional facies. However, slump-fold axis orientation and cross-lamination based paleocurrent measurements, combined with the geochemical and seismic data, suggest that the provenance was stable and that the sediments were delivered from a relay-ramp sediment entry point at the southern end of the graben, and that a secondary, nearby sediment source probably existed along the border fault towards the East 20 (Fig. 1 b).…”

“…Both, the core and Bach Long Vi Island are marked with red dot (20° 07′ 39.4″ N, 107° 43′ 49.4″ E). (b) Reconstruction of the graben (light blue) with inferred paleohighs (grey) and sediment delivery system (modified from Rizzi et al 20 ). BLV-Bach Long Vi Island.…”

“…Previous studies have indicated that the studied deposits represent a deep, freshwater, oil-prone rift lake system 15 – 18 , deposited during the upper Oligocene 18 , 19 . The rift basin was relatively sediment starved and intermittently, throughout its existence, received phosphate-rich sediments from a single footwall block 20 .

Figure 1 ( a ) Geographical map with outlines of the depositional basins.

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