Quantitative analysis of a footwall-scarp degradation complex and syn-rift stratigraphic architecture, Exmouth Plateau, NW Shelf, offshore Australia

Abstract

Interactions 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 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 an individual fault block in the northern Carnarvon Basin, NW Shelf, Australia. Fault throw, and vertical and headward erosion of the complex are measured. Seismic-stratigraphic and seismic facies analyses allow us to constrain the along-strike architectural variability of the footwall-derived fans 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 for nine quadrants along the fault block, and compare this to the volume of material eroded from the immediately up-dip fault-scarp. This analysis highlights areas of sediment bypass (excess erosion) and areas fed by sediment sources beyond the degraded fault scarp (excess hangingwall-fill). Exposure of the border fault footwall and adjacent fault terraces produced small catchments located beyond the fault crest that fed the hangingwall basin. One source persisted nearly throughout the syn-rift episode, and its location coincided with: (i) an intra-basin topographic high; (ii) a local fault throw minimum, possibly representing a breached relay ramp; (iii) increased vertical and headward erosion within the fault-scarp degradation complex; and (iv) sustained clinoform development in the immediate hangingwall. Our novel quantitative volumetric approach to identify through-going sediment input points and areas dominated by sediment bypass could be applied to other rift basin-fills. We also emphasise the need to incorporate interaction of multiple sediment sources and their resultant architecture in tectono-stratigraphic models for rift basins.

The effect of breached relay ramp structures on deep‐lacustrine sedimentary systems

“…Previous studies have addressed the effect of early‐stage relay ramps in syn‐rift settings on subaqueous sediment gravity flows (Anderson et al, 2000; Athmer & Luthi, 2011; Barrett et al, 2020; Bruhn & Vagel, 2005; Fugelli & Olsen, 2007; Ge et al, 2018; Soreghan et al, 1999). Early‐stage relay ramps are observed to direct sediment down the ramp most effectively when there were pre‐existing channels present on the ramp, the ramp had a landward tilt, or the direction of flow was oblique to the ramp orientation (as opposed to perpendicular; Ge et al, 2018).…”

“…Bell et al, 2018; Covault & Romans, 2009; Cullen et al, 2020; Cunha et al, 2017; Gervais et al, 2004; Lomas & Joseph, 2004; Prather et al, 1998; Tagliaferri et al, 2018; Winker, 1996). In complex settings, such as relay‐ramps, structure has the potential to greatly influence sediment transport mechanisms and routing pathways ( sensu Anderson et al, 2000; Athmer & Luthi, 2011; Barrett et al, 2020; Bruhn & Vagle, 2005; Serck & Braathen, 2019).…”

“…Fault relay ramps are important sediment delivery points along rift margins and often serve as persistent flow pathways in deepwater sedimentary basins (Athmer & Luthi, 2011; Bruhn & Vagle, 2005; Crossley, 1984; Gawthorpe & Hurst, 1993; Jackson & Leeder, 1994; Wells et al, 1999; Young et al, 2000; and references therein; Barret et al, 2020; Elliott et al, 2017; Hemelsdaël & Ford, 2016; Henstra et al, 2016; Serck & Braathen, 2019; Zhang & Scholz, 2015). Relay structures initially comprise a tilted or deformed volume of rock (the ramp) that forms between two overlapping, en‐echelon normal faults of comparable dips.…”

“…In the sub‐aqueous setting (marine or lacustrine), the effect of relay ramps on sub‐aqueous sediment gravity flows has been studied using numerical modelling techniques (Athmer et al, 2010; Athmer & Luthi, 2011; Carmona et al, 2016; Ge et al, 2018; Zhang et al, 2020), through the analysis of seismic data (Barret et al, 2020; Bruhn & Vagle, 2005; Fugelli & Olsen, 2007; Gawthorpe & Hurst, 1993; Serck & Braathen, 2019; Soreghan et al, 1999), and in outcrop (Henstra et al, 2016). Previous studies have focused on understanding syn‐rift subaqueous gravity flows in early‐stage relay ramp configurations (Athmer et al, 2010; Athmer & Luthi, 2011; Bruhn & Vagle, 2005; Fugelli & Olsen, 2007; Gawthorpe & Hurst, 1993; Kim & Paola, 2007) and suggest that the structural influence may have limited effect on syn‐rift sub‐aqueous gravity flow and deposition.…”

Cool deltas: Sedimentological, geomorphological and geophysical characterization of ice‐contact deltas and implications for their reservoir properties (Salpausselkä, Finland)

“…Given the scale of the system, the above detail is unlikely to be imaged in the subsurface by standard seismic exploration methods (e.g. Barrett et al ., 2020).…”