1068figure 6. Continued. (C) Isochr

1068
figure 6. Continued. (C) Isochron map of MS3 (late Miocene-early Pliocene). MS3 is characterized by shelf environment over the Cuu Long Basin and Con Son swell and the western Nam Con Son Basin; in the eastern Nam Con Son Basin, MS3 is characterized by complex prograding clinoforms or lowstand deltaic sequences and a distinct seaward shift of the shelf edge. Continued.
sediments, based on comparison with potential analogs in other rift basins in the southeast Asian region (Sladen, 1993). In the Nam Con Son Basin, late Oligocene sediments have been penetrated by wells located on structural highs and are characterized by high net-to-gross sandstones that have mudstones and coals, suggesting fluvio-deltaic facies (Matthews et al., 1997), Wedge-shaped geometries that have variable internal reflections (Figure 7) have been identified in some places immediately adjacent to graben or half- graben bounding faults. We interpret these wedge- shaped sediment bodies as a series of alluvial fans and or fan deltas that prograded into the interior of graben and half-graben systems. The wedge-shaped geome-tries generally pass to a seismic facies characterized by more-or-less parallel reflections that have variable amplitude and low to moderate continuity. Very low angle progradation (Figure 8) is locally recognizable along

1070
Figure 8. Seismic reflection profile showing lake deltas along hanging-wall slope in the Cuu Long Basin. See Figure 2 or Figure 6A for location.
area than MSI as subsidence expanded (Figure 6B). The thickness of MS2 in the Cuu Long Basin exceeds 2.5 s in two-way traveltime. In the Nam Con Son Ba¬sin, MS2 forms two depocenters in the central part of the basin; it gradually thins toward the Con Son swell where it attains a thickness of a few reflectors or almost pinches out.
In the Cuu Long Basin, the base of MS2 forms a shallow, concave onlap surface; the gradual onlap of the lowermost reflections of MS2 suggests that a large lake may have occupied the basin (Figure 9). These lowermost reflections are overlain by zones of variable- amplitude, low-continuity reflections that become moderately continuous upward (Figure 9). The vari-able-amplitude, low-continuity reflections probably represent fluvial environments, whereas the moderately continuous reflections suggest a paralic or mixed nonmarine and shelf environment. In the Nam Con Son Basin, the deeper part of MS2 is characterized by reflections of variable amplitude and low continuity, suggesting nonmarine sediments; wedge-shaped or mounded sediment bodies, depending on the line orientation, are also seen adjacent to faults.
The variable-amplitude, low-continuity reflections in the deep part of MS2 in the Nam Con Son Basin grade upward into a moderate-amplitude, continuous,