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southern parts of the Nam Con Son Basin, the uppermost part of MS2, representing the late stage of the second phase rifting, contains channels or channel-like features. These features appear to form valley systems that extend eastward into a deeper marine environment (Matthews et al., 1997) (Figure 6B). The channels are up to more than 5 km wide and have a relief of as much as 0.1 s in two-way traveltime, except for one channel whose relief is about 0.3 s in two-way traveltime. Because no shelf-slope transition is evident in MS2, these channels may represent valleys incised into the shelf surface during sea level lowstands. Toward the end of the second phase of rifting in the Nam Con Son Basin, the fault-induced subsidence diminished, and slower thermal subsidence began. Therefore, eustatic sea level falls probably began to exceed the tectonic subsidence, resulting in the channel incision.
MS3 (Late Miocene-Early Pliocene)
MS3 (ca. 8 Ma-ca. 4 Ma) completely fills the Cuu Long Basin and covers the Con Son swell, forming a broad shelf (Figure 6C). It gradually thins and, in places, almost pinches out toward the Con Son swell. MS3 comprises the postrift unit in the Nam Con Son Basin, representing the deposition after the second phase of rifting. MS3 does not have a distinct depocenter in the Cuu Long Basin, whereas in the Nam Con Son Basin
thick sediment accumulation formed two large depo- centers, outlined by the 1.0 s isochron, and several subbasins. This indicates that much of the subsidence was restricted to the Nam Con Son Basin. MS3 depocenters in the Nam Con Son Basin appear to have shifted eastward compared with those of MS2.
MS3 is characterized by continuous, parallel, moderate-amplitude reflections, typical of a shelf environment over the Cuu Long Basin and Con Son swell and in the western Nam Con Son Basin. In the eastern Nam Con Son Basin, distinct, broad features that have relatively strong top reflections (Figure 10J, inter¬preted as carbonate buildups, occur over structural highs near the lower boundary of MS3. Onlap of the overlying units onto these distinct top reflections is further suggestive of carbonate buildups. Their characteristics closely resemble those of drowned carbonate platforms, which commonly appear as unconformity surfaces in seismic profiles (Schlager and Camber, 1986). Rising sea level caused by slow but continued subsidence probably provided favorable conditions for carbonate buildups on the structural highs. Influx of clastic sediments from the west and north eventually buried the carbonates (Figure 4A) and resulted in a gently inclined slope, marking the transition from a ramplike setting to a shelf-slope setting in the eastern¬most part of the study area. Here, MS3 is characterized

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the crustal blocks at the onset of the drifting resulted in the breakup unconformity.
A second phase of rifting began in the Nam Con Son Basin in the Miocene while the postrift subsidence continued in the Cuu Long Basin (Figure 13D). The synrift unit corresponding to the second rifting phase in the Nam Con Son Basin and the early postrift unit in the Cuu Long Basin are characterized by a deepening-upward succession, grading upward from nonmarine to paralic or shelf sediments (Figure 12). The second phase of rifting in the Nam Con Son Basin was terminated by the inversion in the middle to late Miocene that locally resulted in structural highs, which later provided favorable conditions for carbonate development (Figure 13E). The erosional surface over the structural highs and its correlative conformity
formed the second breakup unconformity, separating the early synrift and late synrift units corresponding to the first and second rifting phases, respectively. The late postrift unit in the Cuu Long Basin grades upward from nonmarine to paralic and to shelf sediments; the postrift unit in the Nam Con Son Basin is composed of shelf and deeper marine sediments (Figure 12). The slowed subsidence, together with the decreased sediment influx, resulted in the distinct shelf-slope transition in the eastern Nam Con Son Basin (Figure 13F).
Trends of Hydrocarbon Occurrences
Oil and gas production in the Cuu Long and Nam Con Son basins has proven that there are active petroleum systems in the southern Vietnam continental margin.