Abstract
AbstractInteractions between footwall‐, hangingwall‐ and axial‐derived depositional systems make syn‐rift stratigraphic architecture difficult to predict, and preservation of net‐erosional source landscapes is limited. Distinguishing between deposits derived from fault‐scarp degradation (consequent systems) and those derived from long‐lived catchments beyond the fault block crest (antecedent systems) is also challenging, but important for hydrocarbon reservoir prospecting. We undertake geometric and volumetric analysis of a fault‐scarp degradation complex and adjacent hangingwall‐fill associated with the Thebe‐2 fault block on the Exmouth Plateau, NW Shelf, offshore Australia, using high resolution 3D seismic data. Vertical and headward erosion of the complex and fault throw are measured. Seismic‐stratigraphic and seismic facies mapping allow us to constrain the spatial and architectural variability of depositional systems in the hangingwall. Footwall‐derived systems interacted with hangingwall‐ and axial‐derived systems, through diversion around topography, interfingering or successive onlap. We calculate the volume of footwall‐sourced hangingwall fans (VHW) for nine quadrants along the fault block, and compare this to the volume of material eroded from the immediately up‐dip fault‐scarp (VFW). This analysis highlights areas of sediment bypass (VFW > VHW) and areas fed by sediment sources beyond the degraded fault scarp (VHW > VFW). Exposure of the border fault footwall and adjacent fault terraces produced small catchments located beyond the fault block crest that fed the hangingwall basin. One source persisted throughout the main syn‐rift episode, and its location coincided with: (a) an intra‐basin topographic high; (b) a local fault throw minimum; (c) increased vertical and headward erosion within the fault‐scarp degradation complex; and (d) sustained clinoform development in the immediate hangingwall. Our novel quantitative volumetric approach to identify through‐going sediment input points could be applied to other rift basin‐fills. We highlight implications for hydrocarbon exploration and emphasize the need to incorporate interaction of multiple sediment sources and their resultant architecture in tectono‐stratigraphic models for rift basins.
Highlights
IntroductionFault-scarp degradation (e.g. Bilal, McClay, & Scarselli, 2018; Elliott et al, 2012; Henstra et al, 2016; Leeder & Jackson, 1993; Morley, Ionnikoff, Pinyochon, & Seusutthiya, 2007; Mortimer & Carrapa, 2007; Underhill et al, 1997; Welbon et al, 2007) typically feeds sediment into its immediate hangingwall basin to form subaerial and subaqueous fans (Fraser & Robinson, 2002; Leeder et al, 2002; McLeod & Underhill, 1999; Sharp, Gawthorpe, Armstrong, & Underhill, 2000; Stewart & Reeds, 2003; Figure 1)
We require a method for determining the location of footwall-derived depositional systems are most commonly reported (e.g. Backert, Ford, & Malartre, 2010; Ford, Williams, Malartre, & Popescu, 2007; Gawthorpe et al, 1994; Leppard & Gawthorpe, 2006; Turner et al, 2018), in rifts defined by prominent pre-rift topography, and in which antithetic faults, folds, and interacting fault arrays are widespread, hangingwall- and axial-derived systems can be as influential as antecedent footwall-derived systems in controlling the basin infill (Figure 1; Jackson, Larsen, Hanslien, & Tjemsland, 2011; Leeder, Mack, & Salyards, 1996; McArthur, Hartley, Archer, Jolley, & Lawrence, 2016)
A quantitatively-informed interpretation of the tectono-sedimentary evolution of the Thebe-2 fault block on the Exmouth Plateau, NW Shelf, offshore Australia, suggests that the Thebe-2 hangingwall basin evolved through four phases linked to the evolution of the Thebe-2 fault and a number of parallel, antithetic faults, which became active at different times
Summary
Fault-scarp degradation (e.g. Bilal, McClay, & Scarselli, 2018; Elliott et al, 2012; Henstra et al, 2016; Leeder & Jackson, 1993; Morley, Ionnikoff, Pinyochon, & Seusutthiya, 2007; Mortimer & Carrapa, 2007; Underhill et al, 1997; Welbon et al, 2007) typically feeds sediment into its immediate hangingwall basin to form subaerial and subaqueous fans (Fraser & Robinson, 2002; Leeder et al, 2002; McLeod & Underhill, 1999; Sharp, Gawthorpe, Armstrong, & Underhill, 2000; Stewart & Reeds, 2003; Figure 1). A better understanding of these processes is necessary to unravel the complexity of the ultimate depositional architecture of a hangingwall basin-fill
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