REGIONAL UNCONFORMITIES AND MEGASEQUENCES
Four regional unconformities were identified in the study area (Figure 3). The seismic units bounded by these unconformities are megasequences that generally represent deposition during tectonically controlled phases of basin development and or deformation. The geologic ages of the unconformities or megasequence boundaries are based on the data from the Bach Ho 1 and Mia 1 wells. Stacking velocities from the seismic data were used to construct two-way traveltime vs. depth relationships at the wells; these relationships were used to tie stratigraphic tops from the wells to the seismic data. The megasequence boundaries cor-respond to (1) the top of the acoustic basement (pre-
Tertiary?), (2) the top of the Oligocene (ca. 24 Ma), (3) the late Miocene (ca. 8 Ma), and (4) the early Pliocene (ca. 4 Ma). The megasequence boundaries (MB) and the megasequences (MS) are referred to as MB1 to MB4 and MSI to MS4, respectively, from oldest to youngest. Each MS rests on its corresponding MB.
A series of cross sections of the study area illustrating characteristic rift basins developed from rifting to regional subsidence is shown in Figure 4. Cross sections shown in Figure 4A—D are northwest-southeast— oriented, parallel, and spaced at intervals ranging from 45 to 50 km. Figure 4E is northeast-southwest— oriented, approximately perpendicular to other cross sections.
Regional Unconformities
MBl (Pre-Tertiary)
MB1 is the deepest traceable unconformity (Figure 4).
It represents the top of the acoustic basement, which,
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