Figure 3: (A) 3D view of a funnel-like pit crater (F4), the top of
which occurs at a horizon between the Base Cretaceous unconformity (BC)
and Top Athol Formation (TA) (Magee & Jackson, 2020a). The pit crater
occurs with a graben and extends down to the top of Dyke F, below the
Top Mungaroo Formation (TM). (B and C) Schematics showing the
measurements of pit crater plan-view (B) and cross-sectional (C)
morphologies.
Cross-section measurements
The only cross-section attributes commonly available to measure for most
pit craters on Earth and other planetary bodies are the depth and slope;
together these can be used to estimate volume, of their surface
expression (e.g., Gwinner et al., 2012; Sauro et al., 2020; Scott &
Wilson, 2002; Wyrick et al., 2004). Most (54; i.e. 92%) pit craters we
study display a funnel-like morphology, comprising an upper, inwardly
inclined conical surface, which we refer to as an ‘inverted cone’,
underlain by a sub-vertical cylindrical ‘pipe’ (e.g., Fig. 3A). Some pit
craters lack an inverted cone and we refer to their morphology as
pipe-like. Stratigraphic reflections within many of the inverted cone
sections appear continuous with in the flanking host rock, although they
are often locally deflected downwards within the pit crater (e.g., Fig.
3C). Note that for some pit craters it is difficult to correlate
internal and external stratigraphic reflections. Where possible, we
measure both the maximum ‘cone height’ and ‘deflection height’ of pit
craters (Fig. 3C). By integrating these measurements with those of pit
crater long axis lengths, we estimate slopes and volumes of the inverted
cone sections and deflected reflections, assuming they are axisymmetric
(Fig. 3C) (Wyrick et al., 2004). We also measure ‘pipe height’, which
for funnel-like pit craters we define as the vertical distance from the
base of inverted cones down to where the pit crater structure is no
longer seismically resolved (Fig. 3C). We calculate pipe volume from the
pipe height and the pit crater long axis length as measured at the Top
Athol Formation. To aid comparison between pit craters with and without
a funnel-like morphology, we describe each as having a ‘total height’;
for pipe-like pit craters their total height is equal to pipe height,
but for funnel-like pit craters their total height includes the cone
height and pipe height (e.g., Fig. 3C). Our measured pipe and total
heights are minimum estimates as a reduction in imaging with depth and
the presence of other structures (e.g., faults), limits our ability to
determine the true base of pit craters (Velayatham et al., 2019;
Velayatham et al., 2018; Wall et al., 2010). We measure the depth of
dykes or normal faults directly beneath the top of each pit crater (Fig.
3C).
Sources of error
The primary error source in our study relates to uncertainty in the
seismic velocities used to depth-convert the seismic data from seconds
two-way time to depth in metres. For example, our depth conversion
assumes that seismic velocities do not vary laterally across the study
area. The similarity in the time-depth relationships for all four
boreholes wells within the Chandon 3D survey supports our assumption
that there is little lateral variation in seismic velocity (Supporting
Fig. 1), but we conservatively consider that calculated velocities and
measured dominant frequencies may vary by up to ±10% (Magee & Jackson,
2020a). Measurements of limits of separability and visibility,
deflection height, cone height, pipe height, and the depth to underlying
dykes thus have assumed depth conversion-related errors of ±10%. Manual
mapping and measurement can introduce human errors (e.g., in defining
pit crater pipe bases) that cannot be quantified; however, we consider
that these errors may be up to ±5% (Magee & Jackson, 2020a). Overall,
plan-view measurements are presented with ±5% errors as they do not
require depth-conversion, but may be susceptible to human error. For
measurements of pit height, cone height, pipe height, and the depth to
underlying dykes, we estimate cumulative errors of ±15%; this based on
the uncertainty in calculated seismic velocities and potential human
errors.
Results
Stratigraphic and structural framework
Stratigraphic reflections within the interval of interest typically have
a moderate amplitude and laterally continuous character (Fig. 4A). In
places these reflections are: (i) offset by ~N-striking,
linear-to-curvilinear, moderate-to-high throw (up to
~500 m) tectonic normal faults (e.g., Figs 4A-D); (ii)
cross-cut by sub-vertical, ~N-trending dykes (named A to
I) that are expressed as >100 m wide zones of disrupted,
low-amplitude reflections (Figs 4A and E); (iii) displaced by
graben-bounding, low-throw (≲0.1 km), dyke-induced normal faults that
dip towards and converge upon the upper tips of the underlying dykes at
depths of ~3–3.5 km (Fig. 4A); or (iv) intersected by
pit craters (see below) (Figs 3A and 4A). Below the Base Cretaceous
unconformity, pit craters and dykes are marked by high variance, as are
areas where strata are folded and/or offset by tectonic faults (e.g.,
Fig. 4A).