Figure 12: (A and B) Plots comparing total height to pit crater long axes (A) and aspect ratio (B) for those located above dykes, dyke-induced faults, and tectonic faults (Supporting Table 1). Data is also shown from two other 3D seismic surveys (Io-Jansz and Thebe) from the Exmouth Plateau, although the exact relation of each pit crater to underlying structure is unknown (Velayatham et al., 2019). (C) Log-log plot of deflection and cone height, one of which is expected equivalent to pit crater depth (e.g., Wyrick et al., 2004), compared to pit crater long axis. Insets show a zoomed-in view of our data and the power-law best-fit trendlines for each plotted dataset. The power-law best-fit trendline shown (black line) with standard errors was calculated from all literature data plotted, with the exception of data from Frumkin and Naor (2019) and Kling et al., (2021) as these pit craters show evidence of post-formation modification.
Discussion
Pit crater structure
Seismic reflection data provides unique opportunities to image and quantify the entire 3D geometry of pit craters. Like pit craters observed elsewhere on Earth and other planetary bodies, those on the Exmouth Plateau have manifest as the as quasi-circular depressions, which in plan-view are commonly arranged in chains (e.g., Figs 3A and 5). Offset of stratigraphic reflections within most mapped pit craters indicate that these structures form because of spatially restricted host rock subsidence (Figs 3A, 4A, and 6-8). This subsidence is confined to cylindrical pipe-like structures with sub-vertical walls, which towards their tops commonly widen and become inwardly inclined, broadly describing an inverted conical shape (i.e. they are funnel-like) (Figs 3A, 4A, 6-8, and 9C). These observations are consistent with the inferred subsurface geometry of pit craters recognised elsewhere and those modelled using physical or numerical approaches (e.g., Ferrill et al., 2011; Kettermann et al., 2019; Wyrick, 2004; Wyrick et al., 2015). We note that the subsided strata within the studied pit craters typically have a lower amplitude and/or higher variance expression compared to reflections in the flanking host rock (Figs 2D, 3A, 4A, and 6-8). These seismic attribute changes indicate less seismic energy was reflected from within the pit crater, perhaps due to increased scattering of seismic energy from and/or local decreases in acoustic impedance across disrupted beeding (Brown, 2011). Both controls on the amount of seismic energy reflected could be linked to disaggregation of and fluid infiltration through rock during or after subsidence, which are likely common processes during pit crater formation (e.g., Frumkin & Naor, 2019; Halliday, 1998; Velayatham et al., 2018).
Pit crater age
Where data resolution allows, some reflections we observe above the pit craters appear thickened (e.g., F13; Fig. 7B) and/or onlap onto the conical walls of pits (e.g., G10a; Fig. 7D). These seismic-stratigraphic relationships imply that the strata represented by these reflections were deposited within the pit craters; i.e. the pit craters were surficial features, similar to those recently formed on Earth (Abelson et al., 2003; Frumkin & Naor, 2019; Okubo & Martel, 1998; Whitten & Martin, 2019). Having established that the shallowest expression of the pit craters likely marks the contemporaneous surface to their formation, we can use biostratigraphic data from local boreholes to estimate their age.
The youngest pit craters (B1-3, D1-2) formed coincident with the Base Cretaceous unconformity at ~148 Ma (Figs 6B and D). Critically, the uppermost expression of pit craters across the study area and along individual chains (e.g., A–C, E, F, H, and I) often occur at different stratigraphic horizons above the ~165 Ma Top Athol Formation (Figs 6-7, 10A, and B). These observations indicate the pit craters developed periodically during deposition of the marine Dingo Claystone in the Late Jurassic (Figs 2B, 6-7, 10A, and B). For some pit crater chains, specifically D1-D2 and G1-G13, the tops of individual pits occur along the same stratigraphic horizon suggesting they formed near-synchronously (Figs 6D, 7B, 10A, and B).
Our inferred Late Jurassic timing of pit crater formation is consistent with seismic-stratigraphic constraints on the age of the Exmouth Dyke Swarm and associated dyke-induced faults (Fig. 2) (Magee & Jackson, 2020a). As most pit craters are found below the ~148 Ma Base Cretaceous unconformity, but all dyke-induced faults offset this horizon, it seems likely that pit crater formation generally ceased before dyking and associated faulting ended (Figs 4B and C) (Magee & Jackson, 2020a). Overall, we suggest that the oldest pit craters occurred during the early development of the Exmouth Dyke Swarm and associated dyke-induced faults, with dyking and fault growth occurring (periodically) up until ~148 Ma.
Pit crater origin
Numerous processes involving underlying cavity collapse or the volumetric reduction of a subsurface body have previously been proposed to generate space for overburden subsidence and pit crater formation (Fig. 1B) (see Wyrick et al., 2004 and references therein). We identify no clear cavity-like structures at the pit crater bases (Figs 3A, 4A, and 6-8), and disregard the following possible mechanisms of pit crater formation: (i) carbonate or salt dissolution (Fig. 1C [i]) (Abelson et al., 2003; Spencer & Fanale, 1990), because the Triassic-to-Late Jurassic strata hosting the pit craters contains no (or only very little) carbonate rocks and no salt (Fig. 2B) (e.g., Exon et al., 1992; Stagg et al., 2004; Tindale et al., 1998); (ii) evacuation of lava tubes (see Sauro et al., 2020 and references therein), as there is no evidence for high-amplitude, sinuous, strata-concordant reflections that could be attributed to lava flows or tubes (Figs 3A, 4A, and 7-9) (e.g., Sun et al., 2019); and (iii) magma migration from a reservoir (e.g., Mège et al., 2003; Poppe et al., 2015), as there is no evidence for underlying tabular igneous intrusions (e.g., sills or laccoliths) (Figs 3A, 4A, and 6-8), which are typically expressed as high-amplitude, positive, sub-horizontal-to-inclined reflections (e.g., Planke et al., 2005). Instead, we find that some pit craters directly emanate from either the upper tips of dykes (n = 6), dyke-induced fault planes (n= 6), or tectonic faults (n = 5) (Figs 6-8). Where we observe pit crater bases situated some distance above dyke upper tips or dyke-induced faults (Figs 6-8), we consider it plausible that seismically unresolved or obscured portions of their pipes extend down to these underlying structures. Our observations allow for the generation of these pit craters in response to the opening of vertical tensile fractures (Fig. 1C [iv]) (e.g., Ferrill et al., 2011; Smart et al., 2011; Tanaka & Golombek, 1989), which cannot be imaged in seismic reflection data.
The generation of pit craters linked to dyke tips may be driven by (Fig. 1C [vi]): (i) a volume reduction of the dyke itself, perhaps as magma pressure wanes or volatiles escape (e.g., Patterson et al., 2016; Scott & Wilson, 2002); (ii) escape of heated pore fluids from the tip-adjacent host rock and subsequent porosity collapse (e.g., Schofield et al., 2010); (iii) phreatic eruption (Hughes et al., 2018); and/or (iv) subsidence of material into a tensile fracture that opens above an inflating and widening dyke (similar to Fig. 1C [iv]) (e.g., Ferrill et al., 2011; Smart et al., 2011; Tanaka & Golombek, 1989). We show that within some pit crater chains, the tops of individual pits located directly above dykes occur at deeper stratigraphic levels and are thus older towards the north of our study area (e.g., above dykes A and I; Figs 10A and B). These occurrences of pit crater tops at multiple stratigraphic levels above single dykes indicate that periods of sediment deposition separated pit crater formation (Fig. 13); this interpretation is consistent with fault displacement data, which reveals the dyke-induced faults likely grew incrementally via segment linkage (Magee & Jackson, 2020b). As the dykes are part of a radial dyke swarm that intruded laterally northwards (Fig. 2A) (Magee & Jackson, 2020a), the apparent southwards decrease in age of the pit craters suggests that they did not develop above propagating dyke tips (Fig. 13A). Dyke thickness also decreases gradually northwards (Magee & Jackson, 2020a), so it seems unlikely that pit crater formation occurred in response to dyke widening and tensile fracturing of the overburden; i.e. we should expect areas where dyke width is greatest to generate pit craters first (Fig. 13B). We suggest that the possible southwards decrease in age of pit craters directly above dykes may have occurred due to localised volume reductions of the intrusion as driving pressure periodically waned (Fig. 13C). Such volume reductions could have been driven by magma backflow and retreat (e.g., Philpotts & Asher, 1994; Philpotts & Philpotts, 2007), or solidification and contraction (e.g., Caricchi et al., 2014). Cyclical periods of intrusion and driving pressure waning could create complex trends of pit craters along individual dykes.
Pit craters linked to faults observed elsewhere on the Exmouth Plateau have been attributed to local reduction of confining pressure and fluid escape from an overpressured horizon in the Mungaroo Formation during faulting (e.g., Fig. 1C [ii]) (Velayatham et al., 2019; Velayatham et al., 2018). We show that some pit craters link to steep-to-sub-vertical fault portions, suggesting their formation may also be associated with the collapse of dilatational jogs (Figs 1C [v], 7B, 8B, and C) (e.g., Ferrill & Morris, 2003; Ferrill et al., 2011; Ketterman et al., 2015; Kettermann et al., 2019; Smart et al., 2011; Von Hagke et al., 2019).