Depositional Environment and Sequence Stratigraphy of Miocene Deltaic Cycles Exposed along the Jerudong Anticline, Brunei Darussalam

Abstract

FIG. 1.-A) Landsat satellite composite image of NE Brunei Darussalam (after Sandal 1996) and B) geological map (after Wilford 1961). The Belait Formation is dominated by shallow-marine shoreface and tidal sandstones. The Setap Shale Formation (Setap Shales) is mainly composed of shelfal mudstones and shales. Note pronounced progradational geometries at the base of the Belait Formation on the eastern margin of the Belait Syncline. Black boxes indicate locations of Figures 2 and 4.ABSTRACT: Kilometer-scale prograding clinoforms associated with deltas are rarely seen in outcrop; however, one such example is found in a Miocene sand-shale sequence exposed along the Jerudong anticline in Brunei Darussalam. Regional sequence stratigraphic interpretation shows that large clinoforms at the base of the Miocene Belait delta represent a succession of at least three major sand-shale sequences. The stratigraphically highest and best exposed sequence exhibits large slumps and sharp-based detached sand bodies at its base. Accumulation of these units most likely occurred during a relative sea-level lowstand. An overlying 1-1.5 km thick shale unit is interpreted to have developed during subsequent transgressive and early highstand conditions. Rapid progradation of thick sand-dominated shoreface deposits characterizes the late highstand systems tract. The clinoforms below show similar depositional geometries: slumps and thin blankets of shallow-marine sandstones mark the individual bases, shales and mudstones succeed, and progradational shoreface and tidal deposits form the top of each clinoform. New sedimentological and micropaleontological data document that all sediments (regardless of whether sand-or shale-dominated) formed in a shoreface to shelfal setting in front of a mud-rich delta. This differs from previous studies interpreting a continental-slope to deepmarine depositional environment for all shale-dominated units, and 914 S. BACK ET AL. FIG. 2.-Three large clinoforms at the base of the Belait delta system interpreted as a stack of three individual sequences. A) Map view of clinoform succession, approximate ages (after Sandal 1996), and strike and dip of strata. B) Cross-sectional view along strike of section and sequence stratigraphic interpretation; cross section is flattened on 12.0-Ma surface and strata are restored to horizontal. For location of Section A-AЈ see Figure 4. FIG. 3.-Progradation of the Belait delta. Curve A shows progradation along a SE-NW section through the center of the Belait syncline. Curve B shows progradation from south to north along the Jerudong anticline. Gradient changes between 12 and 9 Ma coincide with a major eustatic lowstand. However, changes in progradation rates are also influenced by structural features that either provide (e.g., growth-faults) or diminish (e.g., anticlines) accommodation. Progradation pattern and timing are after Sandal (1996). Eustatic curve is after Haq et al. (1987). indicates that kilometer-scale clinoforms can develop entirely on the continental shelf in water ...

Deltaic, mixed and turbidite sedimentation of ancient foreland basins

“…Hyperpycnal flows, formed in fan-delta and river-delta systems during catastrophic flood events, are able to carry sand and gravel over considerable distances (up to tens of km) across shelfal regions-a character of hyperpycnal flows that is difficult to perceive from modern settings where these flows are essentially viewed as suspensions of mud and fine sand. The typical deposits of these flows are flood-generated delta-front sandstone lobes a category of deposits which is still unfortunately mistaken, in most recent literature, either for storm-dominated shoreface and shelfal deposits due to the common occurrence of HCS (Back et al, 2001) or for basin-floor turbidites, where they develop in relatively deeper shelfal regions at the toe of prograding deltaic clinoforms (Plink-Bjorklund, Mellere, & Steel, 2001).…”

The deep-water fold-and-thrust belt offshore NW Borneo: Gravity-driven versus basement-driven shortening

“…Following complete subduction of the proto-South China Sea oceanic crust, continental crust of the Dangerous Grounds region was partially subducted beneath the Crocker Formation basin of NW Borneo in the latest early Miocene before its buoyancy locked the system (James, 1984;Levell, 1987;Hazebroek and Tan, 1993;Hutchison, 1996aHutchison, , 1996bSandal, 1996;Hall, 1996;Milsom et al, 1997). Subsequently, northern Borneo experienced signifi cant compressional deformation, as documented onshore in folded sedimentary units of late early Miocene to middle Miocene ages (e.g., Sandal, 1996;Morley et al, 2003;Back et al, 2001Back et al, , 2005Back et al, , 2008, as well as offshore in folded and thrusted middle Miocene to present-day shelf and slope sequences (e.g., Levell, 1987;Hinz et al, 1989;Hazebroek and Tan, 1993;Morley et al, 2003;Ingram et al, 2004).…”

“…(B) Regional geological cross section across NW Borneo along line A-A′. The section is based on offshore seismic-refl ection data (Sandal, 1996; Van Rensbergen and Morley, 2000), onshore seismic-refl ection and well data , geological maps (Sandal, 1996;Back et al, 2001Back et al, , 2005, and models for the tectonic development of northern Borneo (e.g., James, 1984 attention to the similarities of structures in their "Outboard Belt" with thrusts at the toe of the Niger Delta, suggesting that deep-water folding and thrusting offshore NW Borneo primarily developed in response to gravitational delta tectonics. Between these two end-member interpretations (basement-driven versus gravity-driven), Ingram et al (2004), for example, proposed a compressional regime for deep-water NW Borneo dominated by crustal shortening in the range of 4 cm/yr until recent times (sensu Hinz et al, 1989) and a small contribution by gravityrelated deltaic toe thrusting (sensu Hazebroek and Tan, 1993).…”

The structural evolution of folds in a deepwater fold and thrust belt – a case study from the Sabah continental margin offshore NW Borneo, SE Asia

“…Subsequently, northern Borneo experienced further compression as documented onshore in folded sedimentary units of Middle to Late Miocene ages (e.g. Sandal, 1996;Back et al, 2001;Morley et al, 2003;Back et al, 2005;Back et al, 2008), as well as offshore in folded and thrusted Late Miocene to present-day shelf and slope sequences (e.g. Levell, 1987;Hinz et al, 1989;Hazebroek and Tan, 1993;Morley et al, 2003;Ingram et al, 2004, Hesse et al, 2009).…”