Figure 2: (A) Map of the southern portion of the North Carnarvon Basin highlighting principal tectonic elements, including the: ESB = Exmouth Sub-basin, CT = Carnarvon Terrace, BSB = Barrow Sub-basin, DSB = Dampier Sub-basin, and PS = Peedamullah Shelf. The location of the Exmouth Dyke Swarm is also shown (Magee & Jackson, 2020a). Elevation data are based on the 2009 Australian Bathymetry and Topography grid (Geoscience Australia). Inset: Location map of the North Carnarvon Basin (NCB) relative to Australia. (B) Stratigraphic column for the Exmouth Plateau summarising the age, dominant lithology, and generalised depositional environment for key units (after Hocking et al., 1987; Partington et al., 2003; Tindale et al., 1998). Key tectonics and magmatic events are also shown for (see Reeve et al., 2021 and references therein). (C) Interpreted seismic section, showing generalised stratigraphic architecture of the Exmouth Plateau and Exmouth Sub-basin (modified from Reeve et al., 2016). See Figure 2A for location. (D) Interpreted time-structure map and seismic section showing pit crater-like features arranged in chains and associated with normal faults in the western portion of the Exmouth Plateau (modified from Velayatham et al., 2018). See Figure 2A for map location.
Dataset and Methods
The zero-phase, time-migrated Chandon 3D seismic reflection survey covers an area of ~951 km2, has a bin spacing of 25 m, and a record length of 6 seconds two-way time (s TWT). We present seismic images where a trough (black) reflection has a positive polarity and corresponds to a downward increase in acoustic impedance, and a peak (white) reflection has a negative polarity and represents a downward decrease in acoustic impedance (e.g., Fig. 3A). To help identify and map the pit craters, their underlying dykes, and spatially associated faults, we extract a variance volume attribute; this accentuates spatial variations in waveform similarity (e.g., Chopra & Marfurt, 2005; Marfurt & Alves, 2015).
We use data from five boreholes (Chandon-1, Chandon-2, Chandon-3, Mercury-1, and Yellowglen-1) within the survey area to establish estimates for: (i) the age and intervening lithology of three mapped stratigraphic horizons, which correspond to the Top Mungaroo Formation (TM; ~209 Ma), Top Athol Formation (TA; ~164 Ma), and Base Cretaceous unconformity (BC; ~148 Ma); (ii) an average time-depth relationship for the subsurface (see Supporting Figure 1), allowing us to depth-convert (from ms TWT to metres) measurements taken from the top 4 s TWT of the survey; and (iii) the seismic velocity (v ) range of the interval of interest within which the pit craters are imaged. Seismic velocity increases with depth, from 2.64(±10%) km s-1 to 3.66(±10%) km s-1, as the dominant frequency (f ) of the data broadly decreases from ~33 Hz to ~23 Hz; with these data we calculate the wavelength (λ =v /f ) of the data and estimate that its vertical seismic resolution can be characterised by limits of separability (λ/4) of 37(±7)–17(±3) m and limits of visibility (λ/30) of 5(±1)–3(±1) m (e.g., Brown, 2011). The limits of separability and visibility correspond to the minimum vertical distance between two features required for them to be imaged, respectively, as: (i) two discrete reflections; and (ii) a tuned reflection package, created by convolution of the two reflections on their return to the surface (Brown, 2011). The horizontal resolution of the time-migrated seismic reflection data is estimated to be at least 25 m, which is equivalent to the bin spacing, and likely increases to 37(±7) m (i.e. λ/4) with depth.
Plan-view measurements
We identify 59 pit craters that we label An –In if they overlie a dyke, with A–I corresponding to the dyke name (Magee & Jackson, 2020a; Magee & Jackson, 2020b), or Xn if they appear spatially related to tectonic normal faults; n denotes the pit crater number, which increases along each chain from south to north. We map the plan-view outline of each pit crater at: (i) their uppermost expression in the seismic reflection data; and (ii) where they intersect the Top Athol Formation; this allows us to characterise how pit crater plan-view morphology changes with depth. For each mapped pit crater outline, we use image analysis software (ImageJ) to define a best-fit ellipse and centroid position; where pit craters appear to have merged, we define an encompassing best-fit ellipse but manually determine centroid positions (Fig. 3B). We use the outline of each pit crater, or merged pit craters, to measure their area and the best-fit ellipses to determine their long axis lengths and azimuths, and short axis lengths (Fig. 3B). From these values, we calculate the aspect ratio of the pit crater long and short axes.