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

Abstract:

Kilometre-scale synsedimentary faults associated with well-preserved sedimentary successions on both hanging-wall and footwall sides are rarely seen in outcrop. However, one such example is found in a Middle Miocene deltaic sand-shale sequence exposed along the Jerudong Anticline in onshore Brunei Darussalam. Integration of detailed outcrop information with regional geophysical subsurface data documents that: (1) initiation of the Jalan Tutong growth fault was related to delta-front instability and failure; (2) sediment loading under shallow-marine conditions maintained syndepositional faulting; (3) growth-fault abandonment coincided with a significant basinward shift of deposition and subaerial exposure. High-resolution analysis of facies characteristics, stratal expansion and throw development within the studied growth sequence indicates that differential fault movement strongly influenced facies composition, sedimentary geometry and bedding characteristics: sandstone-poor shelf to lower-shoreface successions developed during periods of rapidly increasing fault movement, whereas sandstone-rich, amalgamated upper-shoreface deposits preferentially formed during intervals of constant or decreasing throw development. The data and interpretations presented may help to focus attention on the complex interplay between tectonics and sedimentation associated with the growth and decay of deltaic faults by providing for the first time an outcrop-based view on a feature documented in similar scale only by geophysical subsurface data.

Keywords: Brunei Darussalam, delta, growth fault, correlation, footwall, hanging wall.

A large progradational clastic system centred on Brunei Darussalam (Figs 1 and 2) has been present on the NW Borneo margin since the early Mid-Miocene (as reviewed by Sandal 1996). This system has many sedimentary and structural similarities to major deltaic provinces such as the Niger and Nile (e.g. McClay et al. 1998), but differs from these systems by being affected in the hinterland by contemporaneous compressional tectonics (Sandal 1996; Morley et al. 1998, 2003). Uplift partially forced strong progradation of the system, but also folded older deltaic units (James 1984; Levell 1987; Hutchison et al. 2000; Morley et al. 2003). Erosion and exhumation of folded strata in the area of the Jerudong Anticline resulted in the exposure of large-scale prograding clinoforms and growth faults in a natural cross section several tens of kilometres in extent (Back et al. 2001; Fig. 3). Because mud-rich deltaic systems are rarely well preserved in outcrop, and the early history of large deltas is commonly buried too deep to be well imaged by seismic data, this setting provides a unique opportunity to analyse the development of the base of a major delta in outcrop and to quantify parameters controlling clinoform and growth-fault development.

The study area in Brunei Darussalam covers the youngest part of a stack of northward prograding clinoforms that is terminated by a large seaward-dipping growth-fault system (Fig. 3). As a result of post-depositional folding in Late Miocene and Pliocene times (Morley et al. 1998, 2003), the clinoforms, the fault system and its associated strata on both the upthrown and downthrown sides dip westward between 70� near the axis of the Jerudong Anticline and 30� a few kilometres into the western flank of the anticline (Fig. 3). The field data presented in this study include detailed structural, sedimentological, biostratigraphic and sequence-stratigraphic information on both footwall (southern) and hanging-wall (northern) sides of the growth-fault system. Well data and 2D seismic reflection data add structural and stratigraphic subsurface control. The integration of all field and subsurface information permits: ( 1 ) differentiation between tectonic signatures resulting from early synsedimentary faulting and postsedimentary folding, and reconstruction of the structural and stratigraphie setting prior to folding; (2) documentation and analysis of the internal architecture of synkinematic footwall and hanging-wall units from a scale of several tens of metres (seismic) to centimetre scale (outcrop data); (3) establishment of links between high-resolution field infomation and lower-resolution geophysical data; (4) delineation of parameters controlling the initiation and subsequent infill of large-scale gravity-driven growth structures.

Geological framework

The oldest rocks exposed in NW Borneo are Late Cretaceous to Eocene metasediments of deep-water origin associated with tectonized ophiolites and other ultramafic rocks (as reviewed by Hutchison 1996; Hutchison et al. 2000; Fig. 1). This assemblage is interpreted as the accretionary prism of an active continental margin (e.g. James 1984). Progressive consumption of oceanic crust resulted in the collision of South China Sea continental crust with the accretionary margin of NW Borneo (e.g. Hamilton 1979; Holloway 1982; James 1984; Hinz et al. 1989; Hazebroek & Tan 1993; Hall 1996; Hutchison 1996; Sandal 1996; Milsom et al. 1997). Intense folding, thrusting and uplift in the Oligocene and Early Miocene led to a complex interplay of tectonics and sedimentation, displayed on the western side of the Crocker Range in turbiditic flysch units of the West Crocker Formation (Hutchison 1996; Back & Lambiase 2001), and the partly time-equivalent Temburong and Meligan Formations of Brunei and vicinity (e.g. James 1984). Towards the end of the Early Miocene, significant uplift and erosion resulted in the development of a major unconformity (the Deep Regional Unconformity of Levell 1987). In Mid-Miocene to Recent times, major clastic systems prograded into the South China Sea basin (Fig. 2). Thick sedimentary successions were rapidly deposited while episodic compression affected the NW Borneo margin (Bol & van Hoorn 1980; James 1984; Levell 1987; Sandal 1996; Morley et al. 2003).