Figure 1: (A) A pit crater chain in the Natron Basin, Tanzania (Magee et al., 2019; Muirhead et al., 2015). (B) Pit crater chains and graben-bounding fault traces in the Noctis Labyrinthus area of Mars (Kling et al., 2021). Basemap is THEMIS Daytime-IR. (C) Conceptual models of pit crater formation (modified from Kettermann et al., 2019; Sauro et al., 2020; Velayatham et al., 2019; Velayatham et al., 2018; Wyrick et al., 2004).
If the surface expression of pit craters reflects their formation mechanism(s), we could use their morphology to interrogate inaccessible subsurface processes and structures on Earth, as well as other planetary bodies and asteroids (e.g., Kling et al., 2021; Korteniemi et al., 2010; Martin et al., 2017; Mège & Masson, 1997; Smart et al., 2011; Whitten & Martin, 2019). However, imaging pit craters on other worlds is difficult (Wyrick et al., 2004), and even our knowledge of the subsurface geometry of pit craters on Earth remains limited (cf. Abelson et al., 2003; Frumkin & Naor, 2019; Halliday, 1998; Wall et al., 2010). We thus rely on physical experiments and numerical modelling to predict how pit crater formation may translate into surface deformation (e.g., Poppe et al., 2015; Smart et al., 2011). Identifying pit craters on Earth, the subsurface structure of which we can study, is crucial to validating these models and thus determining whether pit crater surface expressions can be used to distinguish formation mechanisms.
Reflection seismology provides unprecedented insight into Earth’s subsurface and can uniquely image the three-dimensional structure of pit craters (Abelson et al., 2003; Magee et al., 2019; Magee & Jackson, 2020a; Wall et al., 2010). Here, we use seismic reflection data imaging a sedimentary basin on the Exmouth Plateau, offshore NW Australia to conduct the first-ever quantitative, 3D analysis of pit craters and their underlying structure. Seismic-stratigraphic analyses reveal the now-buried pit craters developed during a phase of Late Jurassic igneous dyking (Magee & Jackson, 2020a). Our data allow us to: (i) quantify the palaeosurface expression and subsurface structure of pit craters in 3D; and (ii) identify underlying structures that may be related to pit crater formation. Overall, we can thus test whether the (palaeo)surface expression of pit craters are diagnostic of subsurface structure and processes.
We map 59 pit craters, which typically have a funnel-like appearance comprising an upper inverted cone that is underlain by a pipe, and are commonly arranged in linear chains. Most (54) pit craters occur in chains located along the floor of ≲2 km wide, buried graben that are bound by dyke-induced faults and underlain by what were northwards-propagating dykes (Magee & Jackson, 2020a); five other pit craters are connect to steeply dipping portions of tectonic normal faults. The link between pit craters and dykes and faults confirms that magmatic processes and overburden collapse into dilatational fault jogs. By recognising that some pit craters above dykes occur at different stratigraphic levels and broadly get younger southwards, we specifically suggest these pit craters may have formed when a waning of magma pressure and potential backflow led to a local volume reduction of the dyke. In addition to obtaining insight into pit crater formation mechanisms, we also show that pit crater depths are variable across the study area. Pit crater depth are one of the few morphological properties of pit craters that can be measured from surficial data, and have been suggested to equal and thus be a proxy for regolith thickness on other planetary bodies (e.g., Whitten & Martin, 2019; Wyrick et al., 2004). The variability in pit crater depths we record suggest this characteristic may not always control regolith thickness. Overall, our work demonstrates that seismic reflection data provides a unique insight into the structure and growth of pit craters on Earth. Further seismic-based studies will help us understand extraterrestrial pit craters and, thus, probe the subsurface structure and composition of other planetary bodies for which direct, in situ data are not yet available.
Geological setting
The North Carnarvon Basin, offshore NW Australia (Fig. 2A) formed during periodic rifting between Australia and Greater India in the Late Carboniferous-to-Early Cretaceous (e.g., Direen et al., 2008; Longley et al., 2002; Stagg et al., 2004). The Exmouth Plateau, where the studied pit craters are situated, lies within the North Carnarvon Basin and itself began in the Rhaetian (Late Triassic) to Callovian (Middle Jurassic) rift phase, which produced an array of ~N-striking, large-throw (up to ~1 km) normal faults (Figs 2B and C) (e.g., Bilal et al., 2018; Black et al., 2017; Gartrell et al., 2016; Marshall & Lang, 2013; Stagg & Colwell, 1994; Tindale et al., 1998). These tectonic faults displace a thick pre-rift succession, including the fluvio-deltaic Mungaroo Formation, and accommodated deposition of a relatively condensed (≲100 m thick), clastic-dominated, syn-rift succession (i.e. the Brigadier and North Rankin formations, the Murat Siltstone, and the Athol Formation; Figs 2B and C) (Hocking, 1992; Hocking et al., 1987; Stagg et al., 2004; Tindale et al., 1998).
The Callovian unconformity caps the Athol Formation and underlies the Oxfordian-to-Tithonian, marine Dingo Claystone, marking the end of major Late Triassic-to-Middle Jurassic rifting (Figs 2B and C) (e.g., Tindale et al., 1998; Yang & Elders, 2016). Renewed rifting in the Tithonian (Late Jurassic) to Valanginian (Early Cretaceous) involved (Figs 2B and C): (i) sub-aerial development of the regionally developed, Base Cretaceous unconformity at ~148 Ma (latest Tithonian); (ii) rapid and significant subsidence to accommodate deposition of the ≲3 km thick, fully marine Barrow Group; and (iii) relatively limited upper crustal faulting, which was restricted to minor reactivation of older faults and generation of an array of N-S to NE-SW striking, low-throw (<0.1 km) normal faults (e.g., Driscoll & Karner, 1998; Magee et al., 2016; Paumard et al., 2018; Reeve et al., 2016). Continental break-up occurred along the western and southern margin of the Exmouth Plateau in the Valanginian-to-Hauterivian (~135–130 Ma; Fig. 2B) (e.g., Direen et al., 2008; Reeve et al., 2021; Robb et al., 2005). Following break-up, thermal subsidence accommodated deposition of a thick post-rift succession that hosts several tiers of polygonal faults (e.g., Paganoni et al., 2019; Velayatham et al., 2019).
During the Late Jurassic, at ~148 Ma, a radial dyke swarm, up to ~170–500 km long and ~300 km wide, was emplaced across much of the Exmouth Plateau (Figs 2A and B) (Magee & Jackson, 2020a). Associated with this dyke swarm is an array of dyke-induced faults that extend up from and dip towards the upper tips of dykes, offsetting Late Triassic-to-Late Jurassic strata and bounding dyke-parallel graben (e.g., Figs 2D and 3A) (Magee & Jackson, 2020a; Magee & Jackson, 2020b). Within these graben, linear chains of sub-circular depressions are recognised (e.g., Figs 2D and 3A) (Magee & Jackson, 2020a; Velayatham et al., 2019; Velayatham et al., 2018). These depressions, which are interpreted to have formed at the contemporaneous free surface, are underlain by pipe-like features that extend down towards dykes, dyke-induced normal faults, or tectonic normal faults (e.g., Figs 2D and 3A) (Magee & Jackson, 2020a; Velayatham et al., 2019; Velayatham et al., 2018). The depressions have previously been interpreted as pockmarks formed by fluid escape from an overpressured horizon when faulting locally reduced the overburden pressure (e.g., Fig. 1B [ii]) (Velayatham et al., 2018). However, the spatial and temporal association between the depressions and underlying dykes suggests they may be analogous to pit craters observed elsewhere on Earth and other planetary bodies (Magee & Jackson, 2020a); henceforth we refer to these depressions as ‘pit craters’.