Stratigraphic development of synkinematic deposits in a large growth-fault system, onshore Brunei Darussalam

Journal of the Geological Society, Mar 2005 by Back, S, Tioe, H J, Thang, T X, Morley, C K

The synoptic view of the stack of footwall outcrops and the radioactivity log of well A (Fig. 7) supports a subdivision of well A into four major units corresponding to outcrop stacks A1-A3, A4-A9, A10-A12 and A13-A17, respectively. The basal 600 m of well A are characterized by high radioactivity readings, and radiation generally decreases upsection with increasing sandstone content. Even though projected over more than 2 km, the overall radioactivity trend and the interpreted sandstone content of footwall well A corresponds well to the observed outcrop signatures between locations Al and A3. At 1200 m true vertical depth (TVD), radioactivity readings at well A increase, and correspond to a significant shale unit located at the base of outcrop A4. This sharp sandstone-shale transition is also recorded in the morphology profile of the outcrop projection line. Further towards the top of well A, the radioactivity pattern exhibits alternating coarsening-upward and fining-upward trends, with pronounced fining-upward intervals at 100Om TVD and above 800 m TVD. These intervals possibly represent signatures of distributary channels in a coastal-plain environment. Correlative outcrops A7 and A10 are situated 2.2 and 1.5 km north of the well A projection-line and are interpreted as shelf to shoreface facies successions. Coastal plain sediments are not observed in outcrop, making a clear-cut linkage between the field locations and well A difficult. At 300 m TVD, a massive sandstone unit with low radioactivity readings starts, corresponding in its top part to amalgamated upper-shoreface sandstones observed at outcrop A17. The top of this sandstone unit can be traced without offset into the massive sandstones forming hanging-wall outcrop B14 (Fig. 8e); thus it marks cessation of faulting along the Jalan Tutong growth fault.

Hanging-wall succession

On the hanging-wall side of the Jalan Tutong Fault, over 1800 m of interbedded sandstones and shales form a stack of dominantly coarsening-upward parasequences, each of which has an average thickness of 30-50 m (Figs 8 and 9). In correspondence to the sandstone-dominated upper part of the footwall, individual facies successions exposed on the hanging wall are also interpreted as progradational shelf to shoreface units. Figure 8 provides two examples of typical hanging-wall parasequences, with outcrop B2 illustrating a complete upward-shallowing cycle from offshore-transition shales to upper-shoreface sandstones (Fig. 8a-c), and outcrop B8 showing a shelf to lower-shoreface succession (Fig. 8d).

At outcrop B2, clean, medium-grained, well-sorted, rippled shoreface sandstones 4m thick characterize the lowermost 10m of section. Above, interbedded sandstones and shales succeed. Individual sandstone beds are 0.1-1 m thick, and locally amalgamate into bedsets up to 2m thick. Most sandstones arc characterized by erosional bases, hummocky cross-stratification and intensely burrowed tops. Microfossil analyses of the interbedded shales indicate deposition in a shelfal to offshore-transition zone environment. At 50 m in the section, the sandstone content increases (Fig. 8a). The sandstone beds display erosional bases, occasional rip-up mud clasts at the base of individual hummocks (Fig. 8b) and frequent Ophiomorpha burrows (Fig. 8c). These sandstones are interpreted as storm sands deposited in a lower-shoreface setting. Above 60 m height, low-angle cross-bedded and wave rippled sandstones amalgamate to bedsets up to 7 m thick, and are interpreted as upper-shoreface deposits. These sandstones are capped by a transgressive shale forming the base of a succeeding offshore-transition to shoreface parasequence.