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).