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’.