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

Figure 12 shows a high-resolution correlation of the footwall and hanging-wall sides of the Jalan Tutong Fault within the boundary conditions of the large-scale depositional model. The correlation is supported by an analysis of stratal expansion, throw development, and data on the sandstone ratio of corresponding footwall and hanging-wall intervals. The correlation of hanging-wall units B1, B2 and B3 to footwall intervals A4-A7, A8 and A9, respectively, results in a 1:1 thickness ratio between footwall and hanging-wall units, indicating no growth at the Jalan Tutong Fault. The absence of faulting during the basal intervals is clearly illustrated in the throw/unit plot of Figure 12. In the same period, the sandstone content per outcrop unit shows corresponding bow-trends on footwall and hanging wall, supporting an interpretation of contemporaneous, structurally undisturbed across-fault sedimentation.

The initial fault movement of 50 m and associated hanging-wall expansion is observed during deposition of parasequence pair A10 and B4. Faulting and expansion decreased to around 10m during deposition of correlative units A11 and B5, followed by an increase to 20 m during deposition of units A12 and B6. This period of initial synkinematic sedimentation exhibits a steady increase of the sandstone content on the hanging wall between B4 and B6, in contrast to a bow-trend between A10 and A12 on the footwall (Fig. 12). Significant fault growth and expansion characterizes the succeeding depositional intervals. A comparison between the footwall succession A13-A17 (Fig. 7) and hanging-wall intervals B7-B14 (Fig. 9) suggests that layer and parasequence addition (sensu Hodgetts et al. 2001) controlled stratal expansion in the hanging wall. The presence of additional sedimentary units on the hanging-wall side of the fault, probably indicating missing section on the footwall side, makes a high-resolution correlation across the fault difficult. To overcome the problems associated with unidentified, but possibly existing footwall unconformities, a simple prediction of decreasing fault throw to constrain a high-resolution footwall to hanging-wall linkage has been used. This approach is in line with the ideal kinematic development of a synsedimentary fault close to abandonment (Fig. 12). The resulting correlation links hanging-wall unit B7 to footwall interval A13, associates hanging-wall deposits B8-B13 with footwall units A14-A16, and shows fault abandonment during the accumulation of unit pair A17 and B14, as documented in outcrop. This interpretation places periods of significant layer and parasequence addition on the hanging wall into intervals B8-B13, suggesting stratigraphic incompleteness of the correlative footwall units A14-A16.

Discussion

The study presented contrasts with previous work on the development of deltaic growth faults (e.g. McCulloh 1988; Edwards 1995; Cohen & McClay 1996; Hodgetts et al. 2001; Hooper et al. 2002; Rouby et al. 2002; Imber et al. 2003; Saller & Blake 2003) by providing for the first time hands-on outcrop data for a kilometre-scale regional growth structure. It is clear that in terms of data continuity and multidimensional prediction the outcrop-based work cannot compete with recent analyses of high-quality 3D seismic data (e.g. Hodgetts et al. 2001; Hooper et al. 2002; Saller & Blake 2003) and high-resolution 2D data (Hiscott 2003). However, the great advantage of an approach integrating regional 2D seismic data with classic geological mapping, high-resolution outcrop analysis and well-log interpretation is the possibility of documenting synsedimentary fault development from regional scale down to centimetre scale with the constant support of rock analogues.