Figure 6. Continued. (B) Iso- chron

Figure 6. Continued. (B) Iso- chron map of MS2 (early-late Miocene). In the Cuu Long Basin, MS2 consists of lacustrine deposits overlain by fluvial to paralic sediments; in the Nam Con Son Basin, MS2 is composed of fluvial-lacustrine deposits overlain by paralic to shelfal sediments. Continued.
The maximum thickness of MSI in the Cuu Long Basin is more than 1.5 s in two-way traveltime in the northern depocenter; MSI gradually thins or pinches out toward the Con Son swell (Figure 6A). Depocenters are not obvious in the Nam Con Son Basin except in the south where an elongate depocenter is seen. A thick accumulation of MSI sediments appears to extend beyond the eastern boundary of the study area.
Seismic reflections within MSI are characterized by variable amplitude and low continuity, suggesting nonuniform deposition and thus a nonmarine or fluvial environment; however, the scale of fluvial depositional structures is generally below seismic resolution. The synrift part of the Cuu Long Basin is known to consist of fluviolacustrine to deltaic sediments (WGRA, 1991). Oligocene and older layers in the Nam Con Son Basin may also be composed of fluviatile and lacustrine

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Figure 5. Continued. (D) Time-structure map of early Pliocene unconformity (MEM). M84 is characterized by a broad, shallow platform in the Cuu Long Basin and over the Con Son swell and deepens seaward.
that there is no large area of complex deformation induced by faults. This indicates that there have been no major tectonic movements since, probably, the Pliocene.
Seismic Stratigraphic Interpretation of Megasequences
We constructed isochron maps (Figure 6A-D) for the megasequences to describe thickness variations and distribution patterns of each megasequence. Then, we superimposed on the isochron maps the depositional environments interpreted from seismic facies analysis of each megasequence; the spatial and temporal distributions of seismic facies helped define the distributions of depositional environments within each megasequence. The seismic facies analysis was based on the techniques described by Mitchum et al. (1977) and Sangree and Widmier (1977).