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

On a regional scale, our field data constrain the initiation of the Jalan Tutong Fault to a simple stratigraphic mechanism, where (1) a major slope failure in front of prograding clinoforms results in the development of a listric slump scar, and (2) progradation of shallow-water clastic deposits onto the slump scar and underlying prodelta shales initiates hanging-wall collapse and fault movement along the pre-existing listric plane. At this point, predominant sedimentary control on fault development turns into faulting influencing sedimentation, as accommodation increase by high hanging-wall subsidence and landward and upward growth of the Jalan Tutong Fault slows overall regional progradation down. The following establishment of a balance between sediment loading and fault movement leads to the development of a stable nearshore sedimentary system around the Jalan Tutong Fault, ultimately controlling the accumulation of significant quantities of reservoir-quality sandstones on both sides of the fault.

A parasequence-scale analysis of expansion, throw and sandstone content indicates a close relationship between fault movement and sandstone development. Figure 12 shows that, independent of correlation problems at the top of the growth succession, periods of rapid increase of fault growth generated hanging-wall units with relatively low sandstone content, whereas sandstone-rich successions formed during intervals of constant or decreasing throw development. The sedimentological consequences of rapid faulting and high tectonic subsidence are clearly archived in hanging-wall successions B4 and B7-B8 (Fig. 8d), where sand is dominantly stored in isolated storm beds within incomplete shelf to lower-shoreface successions. The lack of major accumulations of sandstone-rich upper-shoreface units is interpreted to reflect the high rate of tectonic subsidence exceeding sediment supply. In turn, the highest sandstone content in the hanging wall of the Jalan Tutong Fault is found in those intervals that are linked to periods of slow fault growth or final fault abandonment, e.g. documented in hanging-wall units B2, B6 and B14 (Fig. 12).

For the mechanism of growth-fault abandonment, a number of possibilities exist including the loss of mobility of the underlying prodelta shales as a result of either thinning or dewatering, or increased resistance of fault movement under changing stress conditions. In the Jalan Tutong example, abandonment is abrupt and associated with an apparently regional event that resulted in a drop in sea level forcing deposition basinwards. Apparently, once deposition was renewed above the 10.6 Ma unconformity (Fig. 5), the fault did not reactivate, but instead more basinward faults had taken over. This appears to be a very typical sequence as documented for regional growth faults, for example, in the onshore Niger Delta region (Doust & Omatsola 1990).

The presented scenario of stratigraphic initiation, syndepositional growth and abandonment of the Jalan Tutong Fault is probably too simple to be directly applied as analogous to the large number of much more complex growth systems along the NW Borneo margin that, for example, involve deep-seated tectonics (Sandal 1996; Hodgetts et al. 2001), or other deltaic settings with much larger growth-fault systems (e.g. Morley & Guerin 1996: Hooper et al. 2002). However, regardless of the mechanisms for fault initiation and abandonment, most basinward-dipping deltaic growth faults appear to establish at some time a nearshore balance between sediment loading and fault growth similar to that of the Jalan Tutong Fault, and it is very likely that associated growth successions show rock characteristics similar to those documented in this study. Thus, use of the outcrop data presented in this study should be considered when developing detailed reservoir models for shoreface-dominated growth sequences.