Stratigraphy and sequence stratigraphy of the Neoproterozoic (Cryogenian–Ediacaran) Stuart Shelf, South Australia

Abstract

The Stuart Shelf is part of the Adelaide Superbasin overlying the Gawler Craton and Cariewerloo Basin in South Australia. The basin is of interest for sediment-hosted copper mineralisation known to be hosted in numerous stratigraphic intervals across the region. Therefore, facies analysis and understanding of spatial distribution of host units are essential for exploration targeting and mineral systems research. Our study presents improved and new definitions of Cryogenian and Ediacaran Stuart Shelf stratigraphy, and a detailed, regional-scale sequence stratigraphic analysis. The Cryogenian non-glacial interlude was of particular interest, as it includes the Tapley Hill Formation, a known host for copper mineralisation. The succession of Tapley Hill Formation (including Sturtian cap carbonates), Brighton Limestone and Angepena Formation represents a third-order depositional cycle, an equivalent to the lowermost cycle of the Cryogenian non-glacial interlude in the adjacent Adelaide Rift Complex. The Ediacaran post-glacial succession includes the Nuccaleena Formation and Tent Hill Formation and is interpreted as a second-order depositional cycle. The Sturtian and Marinoan cap carbonates form regional stratigraphic marker horizons and were deposited in a transgressive systems tract. Variations in facies and thickness can be linked to development of localised depocentres and topographic highs, such as the Pernatty High. Modelling of 3D surfaces reveals a shift in the basin orientation between the Cryogenian and Ediacaran sedimentation from an NNW–SSE axis towards a N–S axis. The Stuart Shelf sequence stratigraphic framework provides new insights in the basin sedimentology and evolution, which aids sediment-hosted mineral systems analysis and improves the understanding of Cryogenian and Ediacaran basin evolution in Australia.

KEY POINTS

1. The Cryogenian non-glacial interlude on the Stuart Shelf represents a third-order depositional cycle.

2. The Pernatty High develops during deposition of the Cryogenian successions.

3. The Stuart Shelf basin orientation shifts from NNW–SSE during the Cryogenian towards N–S in the Ediacaran.

4. 3D stratigraphic surfaces aids copper exploration targeting in sediment-hosted mineral systems.

Keywords:

• Sequence stratigraphy
• Neoproterozoic
• copper
• mineral systems
• Cryogenian
• Ediacaran

Introduction

Sedimentary basins host economically valuable metals, such as copper, lead, zinc, uranium, and critical minerals like cobalt, REE and lithium. The understanding of basin stratigraphy, architecture and evolution is fundamental in mineral exploration targeting using a sediment-hosted mineral systems approach. Facies analysis is one of the first steps in evaluation of prospectivity and comprises descriptions of depositional environments based on sedimentary structures and lithology.

The stratigraphic framework and understanding of depositional environments allow prediction of the spatial extent of favourable host units. Thus, establishing such a framework is of essence for any further research in understanding and locating copper mineralisation within the basin using a mineral systems approach. Here we present a description of stratigraphy, lithology, depositional environments and show their distribution across the Stuart Shelf.

The Neoproterozoic Stuart Shelf in South Australia hosts several copper deposits with most notable mineralisation at Mount Gunson, Myall Creek, Emmie Bluff and Sweet Nell (Bockmann et al., 2022; Tonkin & Creelman, 1990). Together with the Adelaide Rift Complex, its eastern neighbour, it represents one the of the most prospective sediment-hosted copper provinces in Australia. There are 662 known copper occurrences hosted within the Neoproterozoic strata within the Adelaide Superbasin (including Stuart Shelf, Torrens Hinge Zone, Adelaide Rift Complex; Bockmann et al., 2022). Noticeable historical mines outside the Stuart Shelf are Kapunda and Blinman (Dentith & Stuart, 2003; Selley, 2000).

Given the potential economic importance, there are only a few detailed sedimentological and no sequence stratigraphic studies of the most prospective host sequences, including the Tapley Hill Formation, available in the literature (Coats & Preiss, 1987b; Cowley & Martin, 1991; Preiss, 1987, 1993; Preiss et al., 1998; Tonkin, 2019). In this study, we present a detailed facies analysis of the Cryogenian and Ediacaran stratigraphy on the Stuart Shelf based on drill-core logging. The facies analysis aids interpretation of lithostratigraphy and sequence stratigraphy, focussing particularly on the Cryogenian non-glacial interlude. Furthermore, we discuss basin evolution and aspects of copper on the Stuart Shelf.

Regional geology

The Stuart Shelf (Figure 1) is a segment of the Adelaide Superbasin, South Australia (including the Adelaide Rift Complex and Torrens Hinge Zone; Lloyd et al., 2020). The Adelaide Superbasin was initiated as a rift during the breakup of Rodinia accompanied with intrusion of the mafic Gairdner Dyke Swarm at ca 827 Ma (Wingate et al., 1998). The Stuart Shelf extends over ∼40 000 km² and overlies varied magmatic, sedimentary and metamorphic Paleo- and Mesoproterozoic rocks of the Gawler Craton and Cariewerloo Basin (Preiss, 1993). The Stuart Shelf is the westward platform extension of the deformed Torrens Hinge Zone, and the Adelaide Rift Complex being the eastward depocentre (Johns, 1968; Sprigg, 1952). The Torrens Hinge Zone, a major crustal shear zone along the Gawler Craton margin, is the main basin-bounding structure of the Adelaide Superbasin and plays a fundamental role in the basin architecture and evolution (Parker, 1993). The 520–490 Ma Delamerian Orogeny is the main driver for deformation in the Adelaide Rift Complex and Torrens Hinge Zone (Foden et al., 2006). On the Stuart Shelf, regional deformation phases (syn- and post-deposition) left the flat-lying strata largely undeformed owing to its placement on the relatively stable Gawler Craton (Drexel et al., 1993; Lambert et al., 1987). However, basin subsidence on the Stuart Shelf occurred during reactivation of basement faults and development of local horst structures (e.g. Pernatty High; Preiss, 1993; Figure 1). There is also evidence that post-deposition fault reactivation and significant erosion may have been linked to the Cambrian Delamerian Orogeny with basin inversion and contraction at 514 ± 4 Ma (Foden et al., 1999). Recent Lu–Hf dating (ca 502 Ma) of Cu-bearing fluorite veins in sedimentary rocks on the Stuart Shelf indicate hydrothermal fluid mobilisation during the Delamerian Orogeny (Glorie et al., 2023).

Figure 1. (a) Location map of the Stuart Shelf showing distribution of drill holes (pink dots), copper mines or deposits (black cross), studied drill holes (with label), background map gravity in colour, magnetic TMI-VRTP-1VD in relief shading, and (b) location map of the Stuart Shelf within Australia.

The Adelaide Superbasin depositional processes comprise numerous transgression and regression cycles and two glaciations, which led to a thick sedimentary succession in its depocentre (Plummer, 1983). However, the Stuart Shelf was only subject to deposition during times of maximum transgression (Plummer, 1983), resulting in a condensed succession of ∼1000 m of Neoproterozoic strata, dominantly of Cryogenian and Ediacaran age (Figure 2). In contrast, Cryogenian strata in the Adelaide Rift Complex has a thickness of up to ∼4.5 km (Lloyd et al., 2020; Preiss, 2000).

Figure 2. Stratigraphy, dominant lithology and geochronological constraints of the Stuart Shelf and sequence stratigraphic sequences described in this study. *Williams & Schmidt, 2021; **e.g. Hoffman et al., 2017; Prave et al., 2016; ***Prave et al., 2016; ****Fanning & Link, 2006.

The age of the Sturtian glaciation in the Adelaide Superbasin is indirectly dated based on global correlations and started between 717.5 and 716.3 Ma and ceased with the onset of transgression and formation of cap carbonates between 663 and 658.2 Ma (Cox et al., 2018; Fanning & Link, 2006; Hoffman et al., 2017; Lloyd et al., 2023; Prave et al., 2016; Rooney et al., 2020; Zhou et al., 2019). Cox et al. (2018) reported a CA-ID-TIMS U–Pb zircon age of 663.03 ± 0.11 Ma for a tuff horizon within the Wilyerpa Formation, ∼80 m below the contact with the Tapley Hill Formation. Kendall et al. (2006) provides a younger Re–Os age for the cap carbonates in the drill hole SCYW-79 1 A on the Stuart Shelf, which was recommended by Rooney et al. (2013) to be rejected for global correlation owing to sample heterogeneity. A cessation age of the glaciation was reported by Fanning and Link (2006; Cox et al., 2018) with a SHRIMP U–Pb zircon age of 659.7 ± 5.3 Ma from a tuffaceous horizon in the Wilyerpa Formation (Figure 2). The age is similar to high-precision dating of U–Pb zircon in tuffaceous layers within the Sturtian cap carbonates in China, which yielded an age of 658.23 ± 0.57 Ma (Zhou et al., 2019).

The onset of Marinoan glaciation is less well constrained and has been postulated to have started between ca 657 and 639 Ma (e.g. Nelson et al., 2020; Prave et al., 2016; Rooney et al., 2020). The minimum duration time of the Marinoan glaciation is 6 Myr based on tuff beds in the upper part of the glacial successions in Namibia (639.29 ± 0.26 Ma U–Pb ID-TIMS age; Prave et al., 2016) and China (641.69 ± 0.17 Ma U–Pb CA-ID-IRMS; Lan et al., 2022). The maximal depositional age of 651.69 ± 0.64 Ma (U–Pb ID-TIMS) of the glacial succession was reported from a tuffaceous interval within the non-glacial units near the erosive and disconformable contact (Nelson et al., 2020). Nelson et al. (2020) pointed out that it is uncertain whether there was a significant erosional or non-depositional gap above the layer dated within the non-glacial units and the base of the glacial rocks. Bao et al. (2018) estimated a minimum of ca 9.8 Myr for the non-glacial interlude in a ∼275 m-thick shallow marine succession in South China using cyclostratigraphic analysis. The authors (Bao et al., 2018) recognise that the non-glacial strata were likely to have been subject to glacial erosion and thus the onset of glaciation may have occurred at a younger age, and focus should be on offshore marine successions. Based on the various geochronological and cyclostratigraphic evidence, the duration of the non-glacial interlude ranged between 10 and 21 Myr. In Australia, the non-glacial interlude in the Adelaide Rift Complex comprises three transgressive–regressive cycles of shallow marine to offshore marine successions with an average thickness of 2000 m (McKirdy et al., 2001; Preiss, 2000).

The Marinoan glaciation terminated with the onset of transgression and formation of cap carbonates between 636.0 and 634.7 Ma at the base of the Ediacaran (e.g. Hoffman et al., 2017; Prave et al., 2016). In Australia, a similar age was reported to 636.41 ± 0.45 Ma in the top Marinoan glacial succession in Tasmania (Calver et al., 2013).

The youngest Ediacaran strata encountered on the Stuart Shelf is the Yarloo Shale in drill hole SCYWY-79 1 A. A significant regional chronostratigraphic marker within the Yarloo Shale (and equivalent to the Adelaide Rift Complex—Bunyeroo Formation) is the Acraman impact ejecta. Hill et al. (2007) recognised this layer 10 m above the base of the Yarloo Shale and contact to the underlying Simmens Quartzite Member of the Tent Hill Formation. The Acraman impact ejecta has an inferred age of ca 580 Ma based on chemostratigraphy (Williams & Schmidt, 2021). The duration of the Ediacaran succession including the basal Nuccaleena Formation cap carbonates and the Acraman impact ejecta marker horizon is at least ca 56 Myr (636 to 580 Ma).

Stratigraphy

The Stuart Shelf stratigraphy comprises mainly the Cryogenian Umberatana Group and the Ediacaran Wilpena Group (Preiss, 1987, 1993; Preiss et al., 1998; Figure 2). The Beda Basalt and associated Backy Point Formation are part of the Tonian Callanna Group and present along the northwestern (drill hole WC05D001) and eastern margin of the Stuart Shelf towards the Torrens Hinge Zone. The Tonian Burra Group (drill hole SR13/2) was described in the northern and southern basin margin. The Tonian stratigraphy is described in Coats and Preiss (1987a). The definition of stratigraphic units and descriptions of depositional environments is limited on the Stuart Shelf (Preiss, 1987; Tonkin, 2019) although correlations can be drawn to the Adelaide Rift Complex, e.g. Tapley Hill Formation (Ambrose & Flint, 1980), or Nuccaleena Formation, Tent Hill Formation (Cowley & Martin, 1991).

The Umberatana Group (Cryogenian) comprises the Sturt Formation, Tapley Hill Formation, Brighton Limestone, Angepena Formation, Wilmington Formation and Whyalla Sandstone on the Stuart Shelf (Lloyd et al., 2023; Preiss et al., 1998). The Sturt Formation (previously known as Appila Tillite) was first described by Coats and Preiss (1987b) as green, pebbly mudstone and conglomerate, sparsely distributed across the Stuart Shelf. The Tapley Hill Formation extends across the Adelaide Superbasin and transgresses onto the Stuart Shelf. Preiss (1993) describes the unit as dark grey, carbonaceous, calcareous to dolomitic shale varying in thickness from a few metres to up to 300 m owing to the irregular paleo-topography on the Stuart Shelf. The Brighton Limestone was included in the Stuart Shelf stratigraphy by Preiss et al. (1998) but no further description was provided. However, the Woocalla Dolomite Member of the Tapley Hill Formation was suggested as equivalent (Preiss, 1993) based on outcrop observations. The Woocalla Dolomite Member is locally described on the western shore of the Pernatty Lagoon in two locations, but there were differences in depositional environments between both locations. In one location, the unit comprises a 15 m massive to laminated dolostone interbedded with grey, dolomitic shale deposited under offshore marine conditions. At the second location at Mount Gunson, the dolostone is characterised by frequent ooids, stromatolites, oncolites and cross-bedding suggesting a shallow marine depositional environment. Based on our observations and interpretations, it is suggested that the two locations are not representative of the same stratigraphic unit. Along the western shore of Pernatty Lagoon, descriptions resemble the basal Tapley Hill Formation composed of highly dolomitic/calcareous mudstone, whereas in the Mount Gunson region, facies described are considered part of the Brighton Limestone. The Angepena Formation is dominantly found on the eastern Stuart Shelf and described as grey to red fine-grained silt- and sandstone, with ripple marks, desiccation mud cracks and red shale rip-up clasts (Coats & Preiss, 1987b). The Wilmington Formation has been described in drill holes near Mount Gunson as red–green dolomitic siltstone and sandstone. The contact to the underlying Angepena Formation is gradual. The succession is interpreted as potential tidal channels with onset of transgression. The sparse occurrences of Wilmington Formation are the youngest, preserved, non-glacial sedimentary unit before onset of Marinoan glaciation (Preiss, 1993). The Whyalla Sandstone is a fine- to coarse-grained, poorly cemented, well-rounded sandstone with sedimentary structures such as cross-bedding. The unit unconformably overlays Mesoproterozoic basement and various Neoproterozoic successions (Preiss, 1993). At Mount Gunson (in outcrops in the Cattlegrid pit) the Whyalla Sandstone is characterised by large-scale cross-beds indicative of an eolian depositional environment.

The Ediacaran Wilpena Group on the Stuart Shelf comprises the Nuccaleena Formation, Tent Hill Formation and Yarloo Shale (Preiss et al., 1998). The Nuccaleena Formation is described as a pink or white, massive to laminated dolostone locally intercalated with the Seacliff Sandstone, or Tregolana Shale Member of the Tent Hill Formation (Cowley & Martin, 1991). The Tregolana Shale Member of the Tent Hill Formation was described by Cowley and Martin (1991) as red brown, finely laminated shale and siltstone with occasional cross-bedding and load casts. Cowley and Martin (1991) described the Corraberra Sandstone Member of the Tent Hill Formation as pink to brown fine- to medium-grained, micaceous sandstone with a gradational contact with the underlying Tregolana Shale Member and characterised by increased abundance of sandstone beds. The Simmens Quartzite Member of the Tent Hill Formation gradationally overlays the Corraberra Sandstone Member and is composed of pink, fine- to coarse-grained sandstone with cross-bedding, and ripple marks (Cowley & Martin, 1991). The Yarloo Shale, also referred to as Bunyeroo Formation, is only recognised in drill hole SCYWY-79 1 A and described as thick successions of red and green shales (Calver, 2000). The unit is interpreted to have been deposited during transgression in an offshore environment (Dyson, 1996).

Methods

Core logging

Core logging was carried out on 23 drill cores intersecting the Cryogenian and Ediacaran stratigraphy at the Department for Energy and Mining ‘South Australia Drill Core Reference Library’ in Tonsley, South Australia. The aim of the core logging was to refine the existing stratigraphy of historical drill core, sedimentological logging of core, and collection of auxiliary data (geochemistry, gamma ray). The data for each drill core can be individually from SARIG.

Gamma-ray data acquisition

Gamma radiation was measured directly on all cores at 50 cm (for Tapley Hill and across stratigraphic boundaries) to 1 m resolution using a Radiation Solutions RS-230 handheld gamma-ray scintillometer. Individual measurements were automatically averaged over a 10-second interval. The data for each drill core can be individually from SARIG.

Facies analysis

Facies analysis is based on decimetre-scale logging of 23 drill cores intersecting Cryogenian and Ediacaran stratigraphy and includes interpretation of lithofacies and facies associations. Lithofacies were characterised by a specific combination of lithology and sedimentary structures that varies from rocks above, below and laterally adjacent (Dalrymple, 2010). Facies associations were interpreted based on compositional and textural properties, and the occurrence of distinct sedimentary structures that are genetically related to one another and described in the lithofacies.

Sequence stratigraphy

Unlike lithostratigraphy, which is based on the identification and the correlation of similar sedimentary facies, sequence stratigraphy aims at identifying depositional sequences (Mitchum et al., 1977) through the analysis of the stacking pattern and spatial distribution of depositional environments. In a sequence stratigraphic framework, stratigraphic units, either sequences or systems tracts, are bounded by surfaces considered to be events controlled by variations of the base-level. This implies that sedimentary strata deposited in the same stratigraphic unit are genetically related and have similar ages of deposition and thereby provide insights into the paleogeographic evolution of sedimentary basins.

Applying sequence stratigraphy in this study aims to highlight shoreline movements based on facies description and sedimentary environment identification. We then infer model-independent surfaces related to base-level rises and falls. Furthermore, we discuss these trends in a model dependent framework following the terminology of depositional sequence IV (Catuneanu et al., 2009), which follows the terminology proposed by Hunt and Tucker (1992) and Helland-Hansen and Gjelberg (1994). In this framework, the sequence boundary (SB) is defined at the end of the base-level fall. This marker is followed by the deposition of the lowstand systems tract (LST) during a normal regression. The top of the LST is marked by the end of regression that is defined as the transgressive surface (TS). Above this marker, the transgressive systems tract (TST) is deposited and bounded at its top by the maximum flooding surface (MFS) defined at the end of transgression. This marker is followed by the deposition of the highstand systems tract (HST) during a normal regression. In the present depositional framework, the top of the HST is marked by the basal surface of forced regression (BSFR) happening at the onset of base-level fall. This event is followed by the deposition of the falling stage systems tract (FSST), which is bounded at its top by the end of the base-level fall that is defined as the sequence boundary.

Here it is important to acknowledge that the definition of these time markers and sedimentary packages (represented on the Stuart Shelf by distinctive stratigraphic units) postulates constant sedimentary supply to the basin and does not account for local structural movements capable of influencing stratigraphic trends (see discussions in Schultz et al., 2020).

Results

Core logging

As part of the sedimentological study, existing stratigraphy for drill holes was updated and revised where required. Stratigraphic boundaries were picked based on visual observations and acquired handheld gamma-ray measurements. An example presented from drill hole SLT 101 (Figure 3) shows that the original logs did not recognise all stratigraphic units or misidentified some. The diamictites of the Sturt Formation were not recognised, and the Brighton Limestone and Angepena Formation only summarised as Umberatana Group. Also, the thin Nuccaleena Formation cap carbonates were not identified in the original logs. This example shows how improved stratigraphy helped sequence stratigraphic interpretations.

Figure 3. Example of drill-core logging with historical stratigraphy, revised stratigraphy, facies associations (FA), lithofacies (LF) and gamma-ray log for drill hole SLT 101.

Facies analysis

On the Stuart Shelf, we distinguished 20 lithofacies (LF; Table 1) grouped into seven facies associations (FA; Table 2) and lithofacies descriptions are provided for the facies associations. It should be noted that the level of detail between facies associations varies as the Ediacaran and glacial units were logged at a lower resolution and represent dominant lithofacies only, as shown in brackets in Table 1. The different facies associations represent a section from offshore marine, offshore transition, shoreface, foreshore, backshore and terrestrial depositional environments (Figure 4).

Figure 4. Profile of depositional environments and their typical facies associations in marginal marine to marine settings. The coloured bars represent the facies distribution on the Stuart Shelf.

Table 1. Description of the lithofacies present on the Stuart Shelf. Units logged at lower resolution shown in brackets.

Lithofacies	Composition	Sedimentary structures	Depositional processes	Stratigraphic distribution
LF1: carbonaceous mudstone	Dark grey to black, laminated claystone and siltstone, mostly carbonaceous (organic matter rich), commonly pyritic and/or calcareous/dolomitic; minor grey, very fine-grained calcareous sandstone intercalations	Massive to laminated; base and top either sharp or gradational; fining-upward	Suspension; low-density turbidity currents	Lower Tapley Hill Formation
LF2: laminated siltstone (mm thick)	Dark grey, massive to laminated, calcareous siltstone with rare sandstone intercalations, pyritic layers, intercalations of grey parallel- to cross-bedded dolomitic siltstone with black mm-thick, partly convoluted, mudstone layers	Massive; laminated; cross-bedding, soft sediment deformation; fining-upward	Suspension; low-density turbidity currents; minor higher-density turbidity currents	Lower Tapley Hill Formation
LF3: intercalated carbonaceous siltstone and dolomitic siltstone (‘ZEBRA’ facies) (mm to <2 cm thick)	Dark grey, parallel-bedded carbonaceous siltstone and light grey dolomitic siltstone; intercalation of fine-grained sandstone; partly pyritic	Parallel-bedding; cross-bedding; wavy-bedding; slumping; convoluted bedding; bases of light grey siltstone in places have small scours or load casts	Gravity flow or high-density carbonate turbidity currents; minor low-density turbidity currents; suspension	Lower and upper Tapley Hill Formation
LF4: grain flow deposit – intercalated dolomitic siltstone and minor sandstone (>2 cm; av. 10 cm, max 30 cm)	Dark grey to light grey, parallel-bedded siltstone with some mm-thick sandstone beds; calcareous/dolomitic siltstone intercalation	Parallel-bedding; cross-bedding; soft sediment deformation; rip-up clasts; erosive bases	Gravity flow or high-density carbonate turbidity currents; minor low-density turbidity currents; suspension	Lower and upper Tapley Hill Formation
LF5: massive to parallel-bedded sandstone to siltstone	Light grey, massive to parallel-bedded, fine- to medium-grained sandstone to dark grey, parallel-bedded siltstone; pyritic layers, rarely carbonaceous, dolomitic	Massive; parallel-bedding; somel cross-bedding; flaser-bedding; sharp-based; fining-upward	Suspension; minor low-density turbidity currents or unidirectional currents	Lower Tapley Hill Formation
LF6: debris flow – conglomeratic siltstone	Light grey to dark grey, parallel-bedded, fine- to medium-grained siltstone with angular to subrounded conglomerate (edgewise conglomerate) at the base as debris flows (2–30 cm thick); rip-up clasts common	Parallel-bedding; rare cross-bedding; rip-up clasts; slumping; convolute-bedding; fining-upward; sharp base	Gravity flow; turbidity currents	Upper Tapley Hill Formation
LF7: slumped siltstone	Dark grey, planar-to cross-bedded siltstone, light grey planar-bedded dolomitic siltstone and conglomerates; rare mud drapes	Parallel-bedding; cross-bedding; intense slumping – soft sediment deformation; fining-upward	Gravity flow; turbidity currents	Upper Tapley Hill Formation
LF8: rip up clast conglomerate (edgewise conglomerate)	Dark grey, planar-bedded pebble conglomerate in siltstone matrix with floating and imbricated rip-up clasts	Rip-up clast beds (up to 12 cm thick, average 2–4 cm); parallel-bedding; cross-bedding; convoluted-bedding	Gravity flow; turbidity currents	Upper Tapley Hill Formation
LF9: conglomerate	Dark grey, convolute-bedding, matrix-supported conglomerate with some pyritic layers, carbonaceous intervals; conglomerate composed of granite, vein quartz, volcanics (0.2–3 cm)	Convolute-bedding; coarsening-upward	Gravity flow; turbidity currents	Upper Tapley Hill Formation
LF10: stromatolite	Light- to medium grey, dolomitic limestone with stromatolitic features, mud drapes, in places wavy-bedding, gypsum pseudomorphs, slumps and parallel-bedded, dolomitic, very fine-grained sandstone and dark grey siltstone	Parallel-bedding; wavy-bedding; slumping	Suspension; oscillatory currents; tidal currents	Brighton Limestone
LF11: massive dolostone	Light grey, massive to vuggy dolostone interbedded with dark grey, carbonaceous siltstone with rip-up clasts and conglomeratic layers at their base; partly microbiolaminites present; pyritic in vugs	Massive; rip-up clasts; slumping	Suspension; rare unidirectional currents	Brighton Limestone, basal Tapley Hill Formation
LF12: laminated silty dolostone	Dark grey, massive to parallel-bedded siltstone to silty dolostone with reworked floating dolostone rip-up clasts	Massive; parallel-bedding; convolute-bedding; rip-up clasts; erosive base; coarsening-upward	Suspension; rare unidirectional and tidal currents,	Nuccaleena Formation
LF13: sandstone	Light grey, pink to purple grey, massive to parallel-bedded, fine- to very fine-grained sandstone with some rip up clasts; mm–cm scale dolomite veins; rare medium- to coarse-grained sandstone beds; convolute-bedding; flaser-bedding; mud drapes; cross-bedding; soft sediment deformation; hummocky cross-bedding, erosive base	Massive; parallel-bedding; convolute-bedding; flaser-bedding; cross-bedding; soft sediment deformation; hummocky cross-bedding, erosive base	Unidirectional currents; tidal currents; suspension	Cox Sandstone Member, [Brighton Limestone], [Simmons Quartzite Member]
LF14: microbialaminite	Light grey, parallel- to wavy-bedded dolomitic siltstone (microbiolaminites) to sandstone with mud drapes, carbonate nodules, vugs with pyrite and gypsum and rare dark grey, massive carbonaceous siltstone; rare rip-up clasts	Parallel-bedding; wavy-bedding; rare massive; convolute-bedding; some cross-bedding; rare rip-up clasts	Suspension; oscillatory currents; tidal currents; rare unidirectional currents	Brighton Limestone
[LF16]: massive sandstone	Light pink to grey, massive to parallel-bedded, poorly to moderately sorted, subrounded to rounded, medium- to coarse-grained, polymictic (calcareous) sandstone (or conglomerate) with isolated well-rounded quartz granules; and grey, polymictic clast-supported conglomerate; soft sediment deformation, convolute-bedding, cross-bedding, rip-up clasts	Massive; parallel-bedding; convolute-bedding; cross-bedding; rip-up clasts; soft sediment deformation; fining upward	Unidirectional currents	Whyalla Sandstone, Appila Tillite
[LF17]: parallel-bedded sandstone	Light purple to grey, massive, planar-bedded to cross-bedded, moderately to well-sorted, medium- to coarse-grained (calcareous) sandstone interbedded with light grey to dark purple, massive to parallel-bedded (dolomitic and/or carbonaceous) siltstone intervals; rip-up clasts, convolute-bedding, cross-bedding, flaser-bedding	Massive; parallel-bedding; cross-bedding; convolute-bedding; flaser-bedding; rip-up clasts, coarsening upward and fining upward	Unidirectional currents	Whyalla Sandstone, Appila Tillite
[LF18]: diamictite	Purple to light grey conglomerate, matrix supported, subangular to well-rounded lithic clasts (quartzite, jasperlite, Gawler Range Volcanics, minor granite, siltstone); and grey to dark grey sandy diamictite, massive to parallel-bedded with subangular to rounded clasts (Gawler Range Volcanics, sst, granite, schist, basalt) in silty matrix, convolute-bedding	Massive; parallel-bedding; convolute-bedding	Suspension	Appila Tillite, [Whyalla Sandstone]
[LF19]: massive to parallel-bedded sandstone and siltstone	Light pink, grey, parallel- to cross-bedded, fine-grained sandstone with rip-up clasts; massive in some basal parts; with carbonaceous black mud drapes on bedding surface; and dark grey, purple, massive to parallel-bedded (carbonaceous) siltstones; rip up clasts; slumping; convolute-bedding; sandstone beds have mainly sharp to erosive base	Cross-bedding; parallel-bedding; massive; rip-up clasts; slumping; convolute-bedding; sharp to erosive base; fining upward	Unidirectional currents; oscillatory currents; rare tidal currents	Corraberra Sandstone Member, Simmons Sandstone Member
[LF20]: intercalated cross-bedded to parallel-bedded sandstone to siltstone	Light purple to greenish-grey, cross- to planar-bedded, fine-grained sandstone and purple parallel-bedded siltstone; individual fining upward layers 0.2–3 cm thick with erosive base; soft sediment deformation; rare rip-up clasts; convolute-bedding	Cross-bedding; parallel-bedding; rare rip-up clasts; convolute-bedding; soft sediment deformation; erosive base; fining upward	Unidirectional currents; oscillatory currents	Tregolana Shale Member
[LF21]: cross-bedded sandstone	Light purple, massive, parallel- to flaser-bedded, or cross-bedded, very fine- to fine-grained, sandstones and purple siltstone with rip up clasts; minor massive mm–cm thick medium- to coarse-grained; calcareous sandstone lenses or beds with rip up clasts; soft sediment deformation; gypsum pseudomorphs; sand injection dykes; convolute-bedding; dewatering; erosive or sharp base	Massive; parallel-bedding; flaser-bedding; cross-bedding; rip-up clasts; soft sediment deformation; sand injection dykes; convolute-bedding; dewatering; erosive or sharp base, fining upward	Unidirectional currents; tidal currents; suspension; evaporation	Angepena Formation, Wilmington Formation

Table 2. Distribution of the lithofacies (LF) in the facies association (FA) on the Stuart Shelf.

	LF1	LF2	LF3	LF4	LF5	LF6	LF7	LF8	LF9	LF10	LF11	LF12	LF13	LF14	LF16	LF17	LF18	LF19	LF20	LF21
FA1	x	x	x	x	x
FA2			x	x		x	x	x	x
FA3																			x
FA4																		x
FA5										x	x	x		x
FA6													x							x
FA7															x	x	x

FA1: offshore

The offshore sedimentary environment is defined as being composed of deposits being accumulated below the storm-weather wave-base (Figure 4). In this environment, laminated, fine-grained deposits are the dominant facies. However, some sandy siltstone beds may be present and reflect the occurrence of gravity-flow systems into the basin (Macquaker et al., 2010). In offshore environments, pelagic settling or hemipelagic deposition is the main sedimentary process (Stow et al., 2001; Stow & Piper, 1984). Lithofacies associated with this facies association comprise black mudstone, laminated siltstone, intercalated siltstone and dolomitic siltstone, grain flow deposits and sandstone (Figure 5a–e).

Figure 5. Drill core photographs of various lithofacies from the Tapley Hill Formation: (a–e) FA1 offshore marine and (f–i) FA2 offshore transition (turbiditic). (a) LF1 (SLT 102 614 m) black, calcareous, massive, carbonaceous mudstone; (b) LF2 (HODD3 701 m) grey to dark grey, finely laminated, calcareous siltstone; (c) LF3 (SLT 102 542 m) intercalated dark grey, calcareous, laminated siltstone and light grey, calcareous, laminated siltstone; (d) LF4 (SLT 101 799 m) dark grey, laminated siltstone intercalated with sharp-based 5–10 cm thick, light grey, massive, calcareous siltstones with carbonaceous rip-up clasts of various size; (e) LF5 (SLT 101 749 m) light grey, cross-bedded, fine-grained sandstone. Lithofacies from FA2 offshore transition (turbiditic; all Tapley Hill Formation); (f) LF6 (BLANCHE 1 594 m) dark grey siltstone with siliciclastic carbonate clasts; (g) LF7 (SAE 22 409 m) slumped, laminated, dark grey siltstone and light grey, dolomitic siltstone with carbonate clasts; (h) LF8 (HODD3 490 m) light grey, carbonate clast ‘edgewise’ conglomerates within interbedded dark grey and light grey siltstones, edgewise conglomerate 3–20 cm thick layers; and (i) LF9 (BLANCHE 1 630 m) grey-orange-pink, poorly sorted conglomerate composed of angular to subrounded granite clasts varying in size from 0.1 to 4 cm and reworked carbonate clasts.

FA2: offshore transition (turbiditic)

Located in between the storm- and fair-weather wave-base (Reading & Collinson, 2009; Figure 4), the offshore transition environment is mainly composed of shale and siltstone and commonly presents wave-induced features such as hummocky cross-stratification (Dott & Bourgeois, 1982) and coarse-grained beds related to gravity flow currents induced by storms. In this environment, oscillatory currents are the main process affecting sedimentation. Lithofacies associated with this facies association comprise slumps, debrite, rip-up beds and edgewise conglomerate (Figure 5f–i).

FA3: lower shoreface to pro-delta

The lower shoreface is generally described as a silt-rich environment with swaley cross-stratification (Dott & Bourgeois, 1982), whereas the pro-delta is mainly composed by fine-grained facies and is interpreted to represent the transition between the delta-front and deeper environments (Figure 4). Here, sedimentary processes in the lower shoreface are interpreted to be driven by oscillatory currents whereas the processes active in the pro-delta are controlled by unidirectional currents. Lithofacies associated with this facies association are cross-bedded siltstone, erosional surfaces and finning-upward trends (Figure 6a).

Figure 6. Drill core photographs of various lithofacies. (a) FA3 lower shoreface to pro-delta – LF20 (BLANCHE 1 411 m) interbedded, greenish grey to brown grey, planar to ripple cross-bedded, fine-grained sandstones to siltstone (Tregolana Shale Member); (b) FA4 shoreface to delta-front – LF19 (BLANCHE 1 282 m) planar to cross-bedded, moderately to well sorted, fine- to medium-grained, light to red brown sandstone (Simmons Quartzite Member); (c) FA6 intertidal to supratidal – LF13 (SLT 106 193 m) light grey, massive to cross-bedded, medium-grained sandstone with shale rip-up clasts (Simmons Quartzite Member); (d) FA6–LF21 (SLT 101 566 m) greyish green fine- to very fine-grained sandstone and purple brown siltstone with dewatering structures (Angepena Formation); (e) FA5 shallow subtidal to subtidal – LF10 (GY13 17 m) light grey to purple stromatolite (Brighton Limestone); (f) FA5–LF11 (SAE 22 392 m) light grey, massive dolostone with minor carbonaceous laminae (Brighton Limestone); (g) FA5–LF12 (SLT 102 164 m) interbedded dark purple, laminated siltstone and light brown, to light greyish-green dolomitic siltstone (Nuccaleena Formation); (h) FA5–LF14 (HODD3 469 m) light grey, dolomitic siltstone (microbialaminites) with mm-thick mudstone drapes (Brighton Limestone); (i) FA7 glacial to fluvio-glacial – LF16 (SLT 101 325 m) grey, massive, well-sorted, medium-grained sandstone (Whyalla Sandstone); (j) FA7–LF17 (SCYW-79 1A 1385 m) light grey, matrix supported, diamictic pebbly conglomerate composed of angular to well-rounded clasts ranging in size between 0.2 and >5 cm, clasts dominantly quartzite, shale, granite and carbonaceous rip-up clast (Sturt Formation); and (k) FA7–LF18 (SCYW-79 1A 1393 m) grey diamictite, matrix supported, silty matrix; clasts subangular to well-rounded, dominantly shales, quartz, quartzite, Gawler Range Volcanics; clast size 0.2 > 5 cm.

FA4: shoreface to delta-front

On the Stuart Shelf, terrigenous-dominated, coarse-grained sedimentary facies are uncommon, which is why pro-deltaic and lower shorefaces environment are grouped into a single facies association (Figure 4). In general, the upper shoreface is expressed as low-angle cross-stratification in a sand-rich environment (Dott & Bourgeois, 1982), whereas delta-front environments are dominated by massive to planar- to cross-bedded sandstone lithofacies. The occurrence of rip-up clasts attests to the presence of intermittent high-energy currents, whereas the occurrence of rare mud drapes highlights the presence of tidal currents (Figure 6b).

FA5: shallow subtidal to subtidal

In parts of the sedimentary systems with little to no detrital input, carbonate rocks are the dominant lithology. In these shallow waters, microbial carbonates progressively build algal domes and mats, and records rare sedimentary structures, suggesting the influence of tidal and/or wind driven currents (Southgate, 1989; Figure 4). Lithofacies associated with this facies association comprise stromatolites, microbialaminites, massive dolostone, laminated silty dolostone and intercalated silt- and sandstone (Figure 6e–h).

FA6: intertidal to supratidal

The shallowest part of the marine environment present on the Stuart Shelf succession is composed of intertidal to supratidal environments (Figure 4). Supratidal environments are defined as the zone above high tide that is flooded only at spring tide or during storms. Intertidal environments are defined as the zone between the mean high and low tides (see Flemming & Bartholomä, 1995 for details). Lithofacies associated with this facies association comprise massive to cross-bedded sandstone presenting flaser-bedding, mud-drapes, sand injections and soft sediment deformation. Furthermore, the supratidal environments tend to present gypsum pseudomorphs related to the evaporation of seawater in areas that were occasionally flooded (Figure 6c, d).

FA7: glacial to fluvio-glacial

Ice sheets are known to affect the deposition of sediments in numerous ways (Evans, 2017). In South Australia, the Stuart Shelf was affected by both the Sturtian and Marinoan glaciations. This resulted in a wide range of glacial to fluvio-glacial facies ranging from glacial valley fills to diamictites, including low- and high-energy glacial outwash plains and fans (Figure 4). Lithofacies associated with this facies association comprise massive- to planar-bedded sandstone with conglomerate layers, cross-bedding, rip-up clasts and matrix supported conglomerate interpreted as diamictite (Figure 6i–k). In the logged drill core, no evidence for eolian deposits has been encountered; however, eolian deposits are exposed in the pit walls of open cut mines in the Mount Gunson area.

Model-independent sequence stratigraphy

Facies correlation across an ∼700 km N–S cross-section is presented in Figure 7 illustrating the relations between the main sedimentary environments and the main lithostratigraphic units. To represent accurately the sedimentary architecture of the Stuart Shelf from the deposition of the Sturt Formation to deposition of the Tent Hill Formation (Simmens Quartzite Member), the sedimentary succession is split in two sedimentary packages flattened on two different datums and both starting with a basin-wide unconformity. The first package, extending from the base of the Sturt Formation to the top of the Angepena Formation, is flattened on the maximum flooding surface (MFS) identified in the lower Tapley Hill Formation. The second package, ranging from the base of the Whyalla Sandstone to the Simmens Quartzite Member, is flattened on the MFS identified on top of the Nuccaleena Formation or lower Tregolana Shale Member. The two datums were chosen, as each of these packages starts with deposits from a glacial period during which widespread glacial erosion occurred in some regions including deeply incised glacial valleys.

Figure 7. Facies distribution and stratigraphy along a north–south section across the Stuart Shelf for the Cryogenian and Ediacaran cycles correlated along the maximum flooding surface, respectively. Note the emergence of the Pernatty High during deposition of the sedimentary succession following the deposition of early Tapley Hill Formation. Inset: location map of transect on the Stuart Shelf of the studied drill core.

First package

By studying the distribution of the sedimentary facies along the 2D section (Figure 7), movement of the paleo-shoreline controlled by base-level variations is inferred. The base of the studied interval is marked by a widespread erosional surface on which glacial deposits (Sturt Formation) are present. In this section, the glacial deposits are mainly represented by diamictite, interpreted as subaqueous deposits, and bedded to massive sandstone, interpreted as fluvio-glacial outwash sedimentary deposits. Furthermore, the shape and the distribution of this first interval suggest that it was deposited in two large valleys present on each side of the Pernatty High (Figure 7). Based on these observations, the surface at the base of this first interval is interpreted to have formed during base-level fall, and the deposition of the overlying succession was enabled by the creation of accommodation space created by the onset of the base-level rise. Therefore, the basal surface present in Figure 7 is interpreted as the end of base-level fall.

Above this first interval, the sedimentary system records a deepening, with the deposition of offshore marine sedimentary rocks of the lower Tapley Hill Formation. The base of this second interval is therefore interpreted as the end of regression. In some parts of the basin, this second interval includes carbonate-rich layers that are interpreted as the first record of a progressive deepening (Figure 7). Further up-section, all the drill holes included in this study record sedimentary successions deposited in an offshore environment. At this time, the Pernatty High is interpreted to be flooded, as evident by deposition of the Tapley Hill Formation over the high. This horizon theoretically presents the deepest part of the sedimentary system and is interpreted as the end of transgression (Figure 7). Until the erosion that is present at the base of the Whyalla Sandstone, the sedimentary system displays an overall regressive trend. The upper Tapley Hill Formation shows signs of shallowing with the deposition of the ‘edgewise conglomerate’ beds, interpreted to be deposited by gravity flows resulting from the destabilisation of a carbonate platform (Figure 7). This depositional event is interpreted to be related to the early stage of base-level fall. The surface associated with the ‘edgewise conglomerate’ is therefore interpreted as the onset of base-level fall. The section preserved above the ‘edgewise conglomerate’ displays a gradual increase in the influence of platform carbonate influx in the basin towards the bio-constructed Brighton Limestone. On top of this carbonate-rich package, an erosional surface is commonly observed, and its generation is interpreted as a result of successive base-level drops associated with a forced regression. The top of the Brighton Limestone is therefore interpreted to represent the end of base-level fall (Figure 7). On the Stuart Shelf, the top of this carbonate unit is overlain by the Angepena Formation, which commonly includes coarse-grained sedimentary rocks interpreted to be deposited in shallow- to intertidal environments. In only a few locations, the Angepena Formation progressively transitions into the Wilmington Formation, interpreted as the most proximal part of the first package, deposited in a nearshore environment. Based on observations in this study, the top of the Angepena Formation is almost everywhere truncated by a second regional erosional surface representing the start of the Marinoan glaciation (Figure 7).

Second package

The first sedimentary strata recorded above the second basin-wide unconformity is part of the Whyalla Sandstone. This unit is mainly diamictic, coarse-grained sandstone and conglomerate with dropstones interpreted to be deposited in a glacial-dominated environment. Similar to the Sturt Formation, the basin-wide unconformity surface at the base of the Whyalla Sandstone is interpreted to represent the end of the base-level drop (Figure 7).

Overlying the Whyalla Sandstone, the Nuccaleena Formation records the deposition of a marine carbonate-rich succession (Figure 7). The marine incursion on top of the continental sedimentary succession of the Whyalla Sandstone suggests a rise of the sea-level. The base of the Nuccaleena Formation is therefore interpreted as the end of the regression. The Nuccaleena Formation is interpreted as deposited in a subtidal environment. It is followed by the deposition of the Tregolana Shale Member, which is composed of a mix of shale, siltstone and sandstone deposited in shallow water settings. It is uncertain if the transition from the Nuccaleena Formation to the Tregolana Shale Member is marked by a shallowing-upward trend or if the deepening that started at the top of the Whyalla Sandstone continues in the lower deposits of the Tregolana Shale (Figure 7). Regardless, the end of transgression will be located either at the top of the Nuccaleena Formation or within the first metres of the Tregolana Shale Member. Following the end of the transgression, the sedimentary system only records a shallowing-upward trend with the deposition of the interval from the Tregolana Shale Member to the Simmens Quartzite Member deposited in a nearshore environment. Although this interval is not the focus of this work, it is inferred that the base of the Corraberra Sandstone Member is associated with the onset of the base-level fall (Figure 7).

Discussion

Updated stratigraphic description

Here we present an updated stratigraphic description based on our detailed drill-core logging across the Stuart Shelf. This study extended the geographic distribution of each unit.

Umberatana Group (Cryogenian)

Sturt Formation

The Sturt Formation is characterised by light grey to purple, matrix-supported conglomerate with subangular to well-rounded lithic clasts (quartzite, Gawler Range Volcanics, jasperlite, granite, siltstone), and grey to dark grey, massive to parallel-bedded, sandy diamictite with subangular to rounded clasts (Gawler Range Volcanics, granite, schist, basalt, sandstone) in a silty matrix. Sedimentary structures include parallel-bedding and convolute-bedding with a mechanism of suspension-driven deposition that took place in glacial to glacio-fluvial to fluvial environments. Intercalated are light grey, massive to parallel-bedded, fine-grained sandstone in places with flaser-bedding, which was deposited by unidirectional currents in a glacial to glacio-fluvial setting. The Sturt Formation is up to 273 m thick (drill hole SR6), whereas the average thickness is 44 m (n = 28).

Tapley Hill Formation

The Tapley Hill Formation comprises two distinct subunits. The lower Tapley Hill Formation is characterised by dark grey to black, massive to laminated, carbonaceous mud- and siltstone with a sharp contact to the underlying Tonian or unconformably on older successions. The succession is commonly calcareous or dolomitic and pyritic. Intercalation of grey, dolomitic siltstone with fine-grained sandstone or mudstone is common. Sedimentary structures include laminated to parallel-bedding, cross-bedding, soft sediment deformation (slumping, convoluted bedding), rip-up clasts, flaser-bedding and erosional bases. Individual cycles are dominantly fining-upward sequences. The basal tens of metres are composed of dark grey mudstone interbedded with a dominantly calcareous to dolomitic mud- to siltstone. The lower Tapley Hill Formation is interpreted as deposited in a marine-offshore (below wave storm base) environment from suspension, low-density turbidity currents, high-density currents and gravity-flows. The basal part of the succession has been recognised as the deep-water dominated, globally recognised Sturtian cap carbonate rocks.

The upper Tapley Hill Formation is light- to dark grey, fine- to medium-grained, dolomitic siltstone with distinct angular to subrounded conglomeratic interbeds (edgewise conglomerate). Sedimentary structures within this unit include parallel- and cross-bedding, soft sediment deformation (slumping, convolute-bedding) and rip-up clasts. The succession is interpreted to have been deposited as gravity flows, and turbidity currents in an offshore transition (between storm-wave base and fair-weather-wave base) marine environment. The edgewise conglomerate is a significant marker horizon across the Stuart Shelf and was deposited as gravity flows. On the Stuart Shelf, the Tapley Hill Formation is up to 331 m thick (drill hole BUTE DDH 7), whereas the average thickness is 60 m (n = 695).

Brighton Limestone

The Brighton Limestone is composed of dolostone and limestone with abundant stromatolites and microbialaminites with a sharp contact to the underlying Tapley Hill Formation. The stromatolitic units comprise light- to medium grey, silty, dolomitic limestone with gypsum pseudomorphs, and soft sediment deformation (slumping) that are intercalated with fine-grained sandstone and dark grey siltstone. The microbialaminites are composed of light grey, laminated dolomitic silt- to sandstone with carbonate nodules, pyritic and gypsum-bearing vugs, and rare dark grey, massive carbonaceous siltstone interbeds. Sedimentary structures within the Brighton Limestone include lamination, wavy-bedding, soft sediment deformation (slumping, convolute-bedding), rip-up clasts and rare cross-bedding. The succession was deposited through suspension, oscillatory currents, tidal currents and rare unidirectional currents in shallow subtidal to shoreface environments. The Brighton Limestone is up to 51 m thick (drill hole SCYW-79 1 A), whereas the average thickness is 37 m (n = 18).

Angepena Formation

Cox Sandstone Member

The Cox Sandstone Member has not been described previously on the Stuart Shelf. The contact with the underlying Brighton Limestone is sharp. The unit comprises light grey, pink to purple grey, very fine- to fine-grained sandstone with rip-up clasts and rare coarse-grained sandstone interbeds. Sedimentary structures include parallel-bedding, soft sediment deformation (convolute-bedding), flaser-bedding, cross-bedding, hummocky cross-bedding and erosive bases. The succession is interpreted to have been deposited in intertidal to supratidal depositional environments through processes of unidirectional currents and tidal currents, and from suspension. The Cox Sandstone Member is up to 15 m thick (drill hole SLT 107), whereas the average thickness is 6.5 m (n = 5).

Angepena Formation – main

The Angepena Formation has a gradual contact with the Brighton Limestone and is composed of light purple, very fine- to fine-grained sandstone and interbedded purple siltstone with rip-up clasts, and minor massive, calcareous sandstone lenses or beds. Sedimentary structures include parallel-bedding, cross-bedding, flaser-bedding, rip-up clasts, soft sediment deformation (convolute-bedding), dewatering features, erosional or sharp bases and fining-upward cycles. Deposition is interpreted as intertidal to supratidal through unidirectional currents, tidal currents, suspension and evaporation processes. The Angepena Formation is up to 171 m thick (drill hole SLT 101), whereas the average thickness is 68 m (n = 12).

Wilmington Formation

The Wilmington Formation is characterised by light pink to grey, well-rounded medium- to coarse-grained sandstone with sedimentary structures such as convolute-bedding, cross-bedding, parallel-bedding and erosional bases. The depositional environment is interpreted as intertidal to supratidal. The Wilmington Formation is up to 44 m thick (drill hole SLT 106), whereas the average thickness is 18 m (n = 10).

Whyalla Sandstone

The Whyalla Sandstone unconformably overlies older stratigraphy and is mainly composed of a light pink to grey, poorly to moderately sorted, subrounded- to rounded, medium- to coarse-grained, polymictic (calcareous) sandstone and minor grey, polymictic, clast-supported conglomerate. Sedimentary structures are abundant and include parallel-bedding, cross-bedding, soft sediment deformation (convolute-bedding), flaser-bedding, rip-up clasts, coarsening-upward and fining-upward cycles. The depositional environment is glacial to glacio-fluvial with rare eolian deposits around the Pernatty High. The Whyalla Sandstone is up to 155 m thick (drill hole SAE 22), whereas the average thickness is 46 m (n = 509).

Wilpena Group (Ediacaran)

Nuccaleena Formation

The Nuccaleena Formation is composed of a dark grey, dolomitic siltstone to silty dolostone with reworked dolostone rip-up clasts. The unit has a sharp contact with older strata. The contact with the overlying Tregolana Shale Member is gradational and less distinct than in the Adelaide Rift Complex. Sedimentary structures include lamination, soft sediment deformation (convolute-bedding), rip-up clasts, erosive bases and common coarsening-upward cycles. The processes of suspension settling, rare unidirectional and tidal currents suggest deposition in shallow subtidal to shoreface environments. The succession has been recognised as equivalent to the globally occurring Marinoan cap carbonate rocks. The Nuccaleena Formation is up to 17 m thick (drill hole MSDP02), whereas the average thickness is 6 m (n = 115).

Tent Hill Formation

Tregolana Shale Member

The Tregolana Shale Member is defined as light purple to greenish-grey, fine-grained sandstone and purple laminated siltstone. Sedimentary structures include soft sediment deformation (convolute-bedding), parallel-bedding, cross-bedding, rare rip-up clasts, erosive bed bases, fining-upwards cycles. The depositional environment is dominated by a lower shoreface to pro-delta setting with unidirectional currents, oscillatory currents and rare tidal currents. The contact with the underlying Nuccaleena Formation is gradational. The Tregolana Shale Member is up to 525 m thick (drill hole FHD 1), whereas the average thickness is 128 m (n = 497).

Corraberra Sandstone Member

The Corraberra Sandstone Member comprises light pink to grey, fine-grained sandstone and dark grey to purple, massive to parallel-bedded siltstone. Sedimentary structures include cross-bedding, parallel-bedding, rip-up clasts, soft sediment deformation (slumping, convolute-bedding) fining-upward sequences and sharp to erosive bed bases. Deposition by unidirectional currents, oscillatory currents and rare tidal currents are interpreted to have taken place in shoreface to delta-front environments. The Corraberra Sandstone Member is up to 223 m thick (drill hole HWD 1), whereas the average thickness is 40 m (n = 187).

Simmens Quartzite Member

The Simmons Quartzite Member is composed of light grey, fine-grained, sandstone with dark grey to purple, siltstone beds. Sedimentary structures include parallel-bedding, cross-bedding, soft sediment deformation (convolute-bedding), abundant rip-up clasts and fining-upward cycles. The depositional environment is interpreted as a shoreface to delta-front setting with deposition as unidirectional currents, oscillatory currents and rare tidal currents. The Simmens Quartzite Member is up to 345 m thick (drill hole SCYW-79 1 A), whereas the average thickness is 96 m (n = 216).

Model-dependent sequence stratigraphy

Relying on the base-level variations observed from drill cores across the Stuart Shelf, model-dependent packages (or stratigraphic sequences) can be defined that are separated by four main stratigraphic surfaces: the end of base-level fall, the end of regression, the end of transgression and the onset of base-level fall. In the depositional model IV from Catuneanu et al. (2009), these horizons correspond to the correlative conformity (CC; or unconformity) considered as a sequence boundary (SB), the transgressive surface (TS), the maximum flooding surface (MFS) and the basal surface of forced regression (BSFR). These four stratigraphic surfaces define the four systems tracts that compose a stratigraphic sequence, the lowstand, transgressive, highstand and falling stage systems tract.

Based on previous observations of transgressive and normal or regressive trends, at least two stratigraphic cycles starting with the Sturtian and Marinoan glaciation can be defined (Figure 8). In each cycle, the basal sequence boundary is interpreted as a subaerial exposure and is followed by the deposition of glacial deposits, which can be interpreted as the lowstand systems tract (LST). It is followed by the transgressive systems tract (TST) during which only thin sedimentary deposits accumulate (Figure 8). In both cycles, the TST is followed by a relatively thick high-stand systems tract (HST). In the first cycle, the onset of the falling stage systems tract (FSST) is marked by the onset (or first occurrence) of the basin-wide ‘edgewise conglomerate’ in the sedimentary succession, which differs from the second cycle where only terrigenous material has accumulated (Figure 8). In the first cycle, the FSST is followed by a normal regressive unit, which is interpreted as the LST. In the second cycle, the uppermost part of the stratigraphic architecture is poorly constrained, but the Simmens Quartzite Member is tentatively interpreted to represent the LST.

Figure 8. Sequence stratigraphic architecture along a north–south section across the Stuart Shelf for the Cryogenian and Ediacaran cycles correlated along the maximum flooding surface, respectively. Note the emergence of the Pernatty High during HST, FSST and LST. Inset: location map of transect on the Stuart Shelf of studied drill core.

The first cycle preserves the Cryogenian non-glacial stratigraphic succession of the Tapley Hill Formation (TST, HST, FSST), Brighton Limestone (FSST) and Angepena Formation (LST) (Figures 2 and 8). It represents the lowest of three cycles within the non-glacial interlude reported in the adjacent Adelaide Rift Complex. Based on the thickness of mean ∼2000 m non-glacial (shallow marine to offshore marine) succession in the Adelaide Rift Complex, we use the maximum estimated duration of the non-glacial interlude of 21 Myr for three cycles in total. The sequence stratigraphic analysis and correlation with the Adelaide Rift Complex suggest that the non-glacial Cryogenian succession on the Stuart Shelf represents a time interval of ca 7 Myr and a third-order cycle based on Catuneanu (2019). Deposition would have taken place between ca 660 Ma (cap carbonates, Tapley Hill Formation) and ca 653 Ma (top Angepena Formation). The thickness, depositional environment and estimated duration are similar to intervals reported in South China (Bao et al., 2018), who also suggested that studying shallow marine successions might not represent the entire stratigraphic record between the Sturtian and Marinoan glaciations.

The following Ediacaran second cycle above the Marinoan glacial succession includes the Nuccaleena Formation (TST), Tregolana Shale Member (TST, HST), Corraberra Sandstone Member (FSST) and Simmons Quartzite Member (LST). The inferred duration of the cycle is ca 56 Myr, which, based on Catuneanu (2019), would represent a cycle of second order.

Basement architecture

The crystalline basement underlying the Stuart Shelf is dominated by the Mesoproterozoic Gawler Range Volcanics (ca 1590 Ma) and Hiltaba Suite (1595–1575 Ma; Hand et al., 2007). Minor formations include Neoarchean units of the Mulgathing Complex in the north, Paleoproterozoic rocks of the Donington Suite (ca 1850 Ma) and Wallaroo Group (ca 1760 Ma; Hand et al., 2007). Iron oxide copper–gold (IOCG) style mineralisation is known within several of these basement units with structural intersections (commonly orientated SW–NE and SE–NW) being an important control on deposit location such as at Olympic Dam (Gloyn-Jones et al., 2022). Many of these basement faults may affect the deposition of subsequent sedimentary sequences.

Overlying the igneous and metasedimentary basement units is the Mesoproterozoic Pandurra Formation of the Cariewerloo Basin, underlying large parts of the Stuart Shelf succession. The Pandurra Formation is a sedimentary succession composed of unmetamorphosed fluvial terrestrial strata deposited ca 1490 Ma based on ⁴⁰Ar/³⁹Ar dating of diagenetic illite (Beyer et al., 2018). The present-day topography of the Pandurra Formation (Figure 9) shows it outcropping along the western part of the Stuart Shelf and most noticeably as a prominent N–S-trending ridge within the central part of the Stuart Shelf, referred to as the Pernatty High (Drexel et al., 1993; Figures 1 and 7–9). The main orientation of deepening is in a SSW-to-NNE direction in concordance with the overall geometries of the overlying Stuart Shelf successions. The thickness map of the Pandurra Formation shows three depocentres divided by SW–NE-striking ridges (Figure 9). The most noticeable difference in comparison with the present-day topography is the absence of a prominent basement high beneath the Pernatty High during the deposition of the Pandurra Formation. Instead, the high marks a depocentre in which a thick sedimentary succession is preserved (Figure 9b).

Figure 9. Modelled surfaces based on drill core and outcrop data: (a) topography surface of the Pandurra Formation showing the Pernatty High and nearby copper deposits (Emmie Bluff, Mt Gunson, Oak Dam West) and (b) thickness surface of the Pandurra Formation showing the main depocentre trending NW to SE.

Basin evolution

The first regionally widespread stratigraphic unit on the Stuart Shelf is the Tapley Hill Formation. The deposition of the lowermost Tapley Hill Formation, including deepwater cap carbonates, marks the westernmost extent of marine transgression onto the Stuart Shelf. At the present day, the Tapley Hill Formation outcrops or is near surface in the western parts of the Stuart Shelf and around the Pernatty High, but deep interceptions are noted towards the east and north (Figure 10a). The distribution pattern reflects similar orientations in deposition to the overall geometry of the Adelaide Rift Complex and Torrens Hinge Zone. The thickness of the Tapley Hill Formation on the Stuart Shelf ranges from tens of metres to ∼300 m with thickening towards the east. As such, the Tapley Hill Formation in its eastern extension reaches up to 3000 m in thickness in the Adelaide Rift Complex (Preiss, 1993; Figure 10b). Most notable is the stark contrast of thickness variations across the Pernatty High. During transgression (MSF) in the lower Tapley Hill Formation, areas of the Pernatty High were flooded and marine successions deposited. However, the thicknesses of the Tapley Hill Formation remained only a few tens of metres near the Pernatty High, which suggests that during deposition, the Pernatty High formed a N–S-trending horst structure (Figure 11). The thickness variations of the Cryogenian non-glacial intervals between adjacent drill holes around the Pernatty High infer a vertical displacement of at least ∼100 m during the first transgressive–regressive cycle (Figures 7 and 8).

Figure 10. Modelled surfaces based on drill core and outcrop data: (a) topography surface of the Tapley Hill Formation showing a NW–SE axis of the basin with deepening towards the east and (b) thickness surface of the Tapley Hill Formation showing deepening towards the Torrens Hinge Zone east of the Pernatty High. Contour line thickness of the Tapley Hill Formation in the Adelaide Rift Complex showing progressive deepening eastward (extracted from Preiss, 1993).

Figure 11. Simplified cross-section showing the interpreted basement architecture (faults not to scale), basement copper deposits (Emmie Bluff, Olympic Dam) and basin fill represented as stratigraphic packages. The Mt Gunson deposit sits on the Pernatty High, interpreted as a horst structures. Cryogenian non-glacial includes Tapley Hill Formation, Brighton Limestone and Angepena Formation. Ediacaran non-glacial includes Nuccaleena Formation and Tent Hill Formation. Inset map shows location of cross-section in red, outline of Stuart Shelf (yellow) Tapley Hill Formation (black line) and Tent Hill Formation (dashed black line).

The glacial Whyalla Sandstone deposited during the Marinoan glaciation is mainly recorded in the central Stuart Shelf regions with present-day increasing depth of the top of succession towards the NE (Figure 12a). Ongoing subsidence of depocentres adjacent to the Pernatty High are evident by thickened units along the flanks of the Pernatty High and relatively little deposition on the top of the high (Figure 13). The thickness map of the Whyalla Sandstone suggests that the mostly glacio-fluvial, terrestrial successions were deposited in two S–N-trending depocentres divided by the Pernatty High, where out- and sub-crop eolian sedimentary rocks are present on the paleo-high (Figure 12b). Another location that suggests local tectonic uplift is around Olympic Dam (drill hole BLANCHE 1), where the upper part of the Cryogenian succession has been eroded, and Whyalla Sandstone is absent.

Figure 12. Modelled surfaces based on drill core and outcrop data: (a) topography surface of the Whyalla Sandstone showing deepening towards the NE and (b) thickness surface of the Whyalla Sandstone showing two depocentres NW and SE of the Pernatty High.

Figure 13. Modelled surface based on drill core and outcrop data: (a) topography surface of the Ediacaran showing deepening towards the N and NE, noticeable is the topographic breaks coinciding with basement structures trending SW–NE and NW–SE; and (b) drill holes locations of studied drill core showing stratigraphic formations with anomalous copper concentrations and their relative location to basement structures, basement geology, copper mines and deposits (SARIG online portal). Note black lines – interpreted basement structures, grey lines – Gairdner Dyke swarm, red lines – interpreted selected structures active during Stuart Shelf basin evolution.

The post-glacial Ediacaran succession marks the most westward transgression on the Stuart Shelf marked by the depositional limit of the Nuccaleena Formation (cap carbonates) and Tregolana Shale Member (Tent Hill Formation). The present-day topography shows infilling of previous depocentres and deposition in areas previously being part of the northern Pernatty High (Figure 13a). The Ediacaran also marks a shift in the general basin orientation from a NNW–SSE axis towards a N–S axis that deepens towards the north (Figure 13a). Most noticeable are the general breaks in topography following SW–NE-trending basement structures. The lack of Ediacaran units in the southern Pernatty High suggests further tectonic activity in this region with ongoing subsidence in the northern Stuart Shelf after the deposition of Ediacaran strata (Figure 11). The shift in depocentre towards the north is also exhibited by the present-day topography of the Cambrian succession overlying the Stuart Shelf.

Copper on the Stuart Shelf

Basement rocks underlying the Stuart Shelf, especially in the Olympic Domain of the Gawler Craton, provide one of the most economic copper provinces globally with dominantly iron-oxide copper–gold (IOCG) type mineralisation (Figure 13b; Olympic Dam, Prominent Hill, Emmie Bluff, Carrapateena, Oak Dam West; Reid, 2019). The Stuart Shelf strata host known copper mineralisation and were mined historically near surface in the Mount Gunson area (Figure 13b; Bockmann et al., 2022). The host of the Mount Gunson orebodies is associated with the brecciated interface of the basement Pandurra Formation and Whyalla Sandstone (Selley, 2000). The Tapley Hill Formation is host to mineralisation at Myall Creek, Emmie Bluff, Windabout and Sweet Nell (Selley, 2000). Copper sulfide mineralisation is either stratabound as replacement of framboidal pyrite, or along fractures, breccias, and hosted within sandstones (e.g. Johns, 1974; Lambert et al., 1984, 1987; Selley, 2000; Tonkin, 2019; Tonkin & Creelman, 1990; Tonkin & Wallace, 2021). Although there is no direct age on mineralisation within the Stuart Shelf, Glorie et al. (2023) suggested a link between basement copper mineralisation and mineralisation found in the overlying Neoproterozoic sedimentary rocks based on Lu–Hf dating of fluorite–calcite–sulfide veins. Age dating of these veins indicates hydrothermal fluid transport of Cu during the ca 500 Ma Delamerian Orogeny.

Numerous new anomalous Cu-bearing intervals throughout the Stuart Shelf strata (18 out of 23 drill holes contain intervals >300 ppm Cu; Figure 13b) have been identified. Although this study does not cover investigations around timing of mineralisation, the following observations can be made: (1) anomalous intervals occur in almost every stratigraphic unit but are dominant in the Tapley Hill Formation and Whyalla Sandstone; (2) six drill holes show mineralisation in several stratigraphic intervals; (3) the youngest stratigraphic interval with anomalous Cu is the Tregolana Shale Member; (4) the oldest interval with anomalous Cu is within the uppermost Pandurra Formation; (5) the shallowest mineralisation is known at surface; and (6) the deepest encountered in this study is at ∼900 m (Sturt Formation, SLT 106) and ∼840 m (Tapley Hill Formation, HWD1). Given that Cu-bearing fluids circulated throughout its basin evolution, and evidence for ongoing tectonic activity after the basin formation (e.g. uplift, erosion, reactivation of old structures), we suggest that the sedimentary-copper mineral system was intermittently active over a long period from deposition to post-depositional tectonic events.

The geophysical data are dominated by signatures associated with the Gawler Craton structures, making it difficult to identify structures relevant to the Stuart Shelf basin evolution. However, a combination of sequence stratigraphic interpretations, airborne electromagnetic (AEM) processing and interpretation, and 3D surface modelling of stratigraphic horizons (Schmid et al., 2024) led to interpretation of basement faults that were reactivated throughout the basin evolution (Figure 13b). As this integrated dataset is limited to the extent of selected AEM lines, further interpretations would be warranted by better geophysical data coverage.

Overall observations are that the shift and orientation of the depocentre can be associated with a preferred basement structural orientation. Furthermore, the SW–NE-orientated faults are interpreted to have influenced the geometry of the Ediacaran successions. Interestingly, the faults controlling the basement-hosted IOCG mineralisation at Olympic Dam played a role in a thickness offset within the Stuart Shelf Ediacaran sedimentary rocks, suggesting reactivation of this fault system during and/or after deposition. Therefore, it might be plausible that periods of fluid migration and potential associated metal transport have occurred between basement and basin as suggested by Glorie et al. (2023). Further studies are necessary to better understand the spatial and temporal association between basement and basin mineralisation.

Conclusions

This study details the Cryogenian and Ediacaran Stuart Shelf stratigraphy, and a detailed, regional-scale sequence stratigraphic analysis.

The logging of drill core led to an improved definition of the Stuart Shelf stratigraphy and a spatially greater extent of the Sturtian cap carbonates (basal Tapley Hill Formation) and Marinoan cap carbonates (Nuccaleena Formation). A facies analysis reveals that the Stuart Shelf succession can be grouped into seven facies associations and 20 lithofacies, ranging from offshore marine to backshore and fluvio-glacial depositional environments. The Sturtian cap carbonates were deposited in an offshore marine setting, whereas the Marinoan cap carbonates were deposited in a shallow subtidal to intertidal setting.

The Cryogenian non-glacial interlude, the main focus of this study, represents a third-order depositional sequence on the Stuart Shelf, which represents the first third-order depositional sequence of the Adelaide Superbasin and is estimated to have been deposited between ca 660 and 653 Ma.

The Ediacaran post-glacial succession represents a second-order depositional cycle related to transgression after glaciation beginning with deposition of the Nuccaleena Formation cap carbonates, a globally recognised chronostratigraphic event.

The facies and thickness variations are interpreted as related to the development of localised depocentres and topographic highs, such as the Pernatty High, during basin evolution. Modelling of 3D surfaces reveals a shift in the basin orientation from a NNW–SSE axis to a N–S axis between the episodes of Cryogenian and Ediacaran sedimentation. Faults are interpreted as related to reactivation of Meso- and Paleoproterozoic basement structures. Metal-carrying fluid migration may have occurred along the structures, leading to Cu-enrichment in numerous stratigraphic intervals on the Stuart Shelf. Our study shows that sequence stratigraphy can be used to gain a better understanding of sediment-hosted mineral systems and potentially creating regions of interest for exploration targeting.

Acknowledgements

This study was made possible through a collaboration between CSIRO Mineral Resources and the Department for Energy and Mining, South Australia (Geological Survey of South Australia). We would like to express our appreciation to the dedicated team at the Tonsley core library for their assistance in logistics. Additionally, we extend our special thanks to Clive Foss, Marcus Kunzmann (formerly CSIRO), Teagan Blaikie (formerly CSIRO) and Helen McFarlane for their contributions to initial discussions and evaluation of project data. Reviewers are thanked for their constructive comments and suggestions.

We respectfully acknowledge and pay our respects to the Australian First Nation peoples, on whose ancestral lands this research was conducted. We recognise their deep connection to this land and their enduring contributions as first scientists.

Disclosure statement

The authors report there are no competing interests to declare.

Data availability

Additional data related to this publication can be downloaded via SARIG at https://sarigbasis.pir.sa.gov.au/WebtopEw/ws/samref/sarig1/wci/ResultSet?w=NATIVE%28%27TITLE+ph+is+%27%27drillhole+report%27%27%27%29&sid=27691def23254d72aed5a0e580dd8286&upp=0&order=NATIVE%28%27DISP_CALL%281%29%2Fascend%27%29&rpp=25&r=1&set=3&reorder=1.

Additional information

Funding

The project was co-funded by the CSIRO and the Department for Energy and Mining, South Australia.

The project was co-funded by the CSIRO and the Department for Energy and Mining, South Australia.