The Adelaide Fold Belt crops out extensively in central and eastern South Australia, between 28oS in the Peake and Denison Inliers and 36oS on Kangaroo Island (Figure 2.1). It is a Neoproterozoic intra-cratonic rift that was initiated between about 1,000 Ma and 827 Ma, at the onset of the break-up of Rodinia, the Meso- to Neoprotoerozoic supercontinent. Over a period of about 300 Myr, it progressed from an intra-cratonic rift to a passive margin before deposition within the Adelaide Fold Belt ceased in the earliest Cambrian. It underwent basin inversion in the Cambrian Delamerian Orogeny (Powell et al., 1994, Preiss, 2000).
Basement to the Adelaide Fold Belt is the Archaean to Mesoproterozoic Gawler Craton along its western margin and the Curnamona Province in the northeast (Figure 2.1). The contact between the basement and the overlying Adelaide Fold Belt is best observed on the margins of the Mt Painter Inlier (Figure 2.1). On the southern and western margins of the Mt Painter Inlier, the Arkaroola Subgroup (Figure 2.2) was deposited unconformably on metasediments and granite, but on its northwestern margin, and on the Mt Babbage Inlier, the Umberatana Group (Figure 2.2) unconformably overlies the basement (Coates and Blisset 1971; Preiss, 1987). On the southern margin of the Curnamona Province, the Burra Group unconformably overlies the basement in a series of thrust slices (Berry, 1977; Paul et al., 1999). On its western margin, flat-lying Adelaide Fold Belt rocks unconformably overlie the Gawler Craton on the Stuart Shelf (Preiss, 1987). Further south in the Fleurieu Arc (Figure 2.1), basement and Adelaide Fold Belt rocks are interleaved in thrust blocks, as thick-skinned thrusts have transported thrust sheets of Adelaide Fold Belt rocks and underlying basement westward on to the basement (Flottmann and James, 1997).
Rutland et al. (1981) and Preiss (1987) identified eight structural elements within the Adelaide Fold Belt (Figure 2.1; Table 2.1). They are fundamental to the development of the Adelaide Fold Belt. The Northern Flinders Zone was an area of deep trough development, initially the Willouran Trough (see below) and then later, the Yudnamutana Trough, whereas the Central Flinders Zone may have been emergent during deposition of the Burra Group (Preiss, 1987). Most fundamental is the Torrens Hinge Zone. It was a platform area early in the basin development but began to subside during deposition of the Burra Group, continuing until deposition ceased in the Adelaide Fold Belt. It underwent mild basin inversion during the Delamerian Orogeny (Preiss, 1987).
Figure 2.1. locality map showing the extent of the adelaide Fold Belt, from Kangaroo island in the south to the peake and Denison inlier in the north, with the eight struc- tural elements of rutland (1981) and preiss, (1987: pirSa regional GiS data). 2.1.2 Stratigraphy and Geochronology of the adelaide Fold Belt.
2.1.2.1 Historical Development of the Stratigraphy
Mawson and Sprigg (1950) first developed a three fold stratigraphy of the Adelaide Fold Belt, comprising the Torrensian, Sturtian and Marinoan Series. Sprigg (1952) added the Willouran Series to this initial stratigraphy, placing it below the Torrensian. These series are chronostratigraphic and away from their type sections the boundaries cannot be defined precisely (Preiss, 1987). Therefore Thomson et al. (1964) first defined four new stratigraphic units; the Callanna Beds, and the Burra, Umberatana and Wilpena groups (Figure 2.2) which could be mapped and correlated across the Adelaide Fold Belt. The Callanna Beds was formalized as the Callanna Group by Forbes et al., (1981). Preiss (1987) described
Denison Inliers Peake &
Kangaroo Island
the Adelaide Fold Belt stratigraphy in detail, and made subsequent modifications to the Burra and Umberatana groups stratigraphy (Preiss, 1997; Preiss et al., 1998; Preiss and Cowley, 1999). Preiss (1982) suggested that the Precambrian and Cambrian deposits define three major successions, each with its own tectonics and palaeogeography, and separated by major regional unconformities. He proposed that the Callanna and Burra groups be combined in the Warrina Supergroup, the Umberatana and Wilpena groups combine as the Heysen Supergroup and the Cambrian sedimentary rocks that sit above the Adelaide Fold Belt be termed the Moralana Supergroup (Preiss, 1982). The Warrina Supergroup reflects the initial intracontinental rift development of the Adelaide Fold Belt and the Heysen Supergroup reflects the break-up of Rodinia and transition to a passive margin (Figure 2.2: Powell et al., 1994; Preiss, 2000). However some authors (e.g., Veevers, 1997; Direen and Crawford, 2003) suggest that the break-up should be placed much later, during the latest Neoproterozoic). The Moralana Supergroup reflects the change in tectonics to an active continental margin prior to the Delamarian Orogeny (Haines and Flottman, 1998).
The chronostratigraphic subdivision has been retained. It places all of the rocks within the Adelaide Fold Belt in the Adelaidean which Preiss (1987) restricted to the period of deposition of the Adelaide Fold Belt. It has four sub-divisions; the Willouran, Torrensian, Sturtian and Marinoan (Figure 2.2), which reflect the four-fold stratigraphic division, except that the base of the Sturtian is placed at the base of the Belair Subgroup (of the table 2.1. elements of the adelaide Fold Belt with their main stratigraphic, sedimen- tological and structural features (rutland et al., 1981; preiss, 1987).
element
Stratigraphy/
Sedimentology
Structure
northern Flinders Zone
Callanna to Wilpena.
Deep troughs and thick
packages
Arcuate folds, the trends of which swing from NE-SW in the NE to E-W in the south to NW-SE in the NW. Continues to Peak and Denison Inlier. Salt-related breccias
Central Flinders Zone
Callanna to Wilpena. Mainly shallow water.
Broad dome and basin folds. Diapiric breccias
nackara arc
Callanna to Wilpena.
Sediment wedge thickens to the southeast, deeper water sediments
Arcuate fold and thrust belt which trend N-S in the south, NE-SW in the NW and E-W in the east.
Southern Flinders Zone
Callanna to Wilpena.
Shallower water sediments A zone of arcuate upright folds.
houghton
anticlinal Zone Callanna to Wilpena.
Mainly lower Adelaidean rocks exposed
Fleurieu arc Burra to Wilpena Has the most intense deformation and highest grade metamorphism.
torrens hinge Zone
Burra to Wilpena.
Shallow water and sub-aerial sediments
Flexuring and faulting, marks the transition from undeformed thinner AFB and Cambrian packages to the west from strongly folded packages to the east
Stuart Shelf (+ Spencer Shelf)
Callanna(?), Umbertana & Wilpena
A platform area with flat-lying AFB
Burra Group) and the base of the Marinoan is placed at the base of the Yerelina Subgroup (Preiss, 1987).
2.1.2.2 The Callanna Group.
The Callanna Group comprises the Arkaroola and Curdimurka subgroups (Figure 2.2). At the type area of the Arkaroola Subgroup, in the Arkaroola area, the Curdimurka Subgroup Figure 2.2. Semi-figurative stratigraphic section of the Adelaide Fold Belt, showing
the age controls and chronostratigraphy on the left and and tectonic setting on the right.
Formation names for units in the Willouran Range area are shown in brackets under their Subgroup names. Colours represent the main lithology in each unit.
Shale Siltstone Sandstone Carbonate Glaciogenic Basalt Evaporites 777 ± 7 Ma ~723 Ma Radiometric date Correlation date 827 ± 6 Ma (Wingate et al., 1998) 777 ± 7 Ma (Preiss, 2000) ~ 621 Ma ~ 635 Ma ~723 Ma (Brasier et al., 2000) ~ 555 Ma (Zhang et al., 2005) (Kendall et al., 2006) 797 ± 5 Ma (Drexel, 2009) 802 ± 10 Ma (Fanning et al. 1986) 798 ± 5 Ma 799 ± 4 Ma (Fabris et al., 2005)
}
680 ± 23 Ma (Mahan et al., 2010)(
643 ±2.4 Ma 657 ± 5.4 Ma 659 ± 6 Ma (Fanning & Link, 2008)?
Wonoka Fm Bunyeroo Fm
ABC Range Qtzite
Nuccaleena Fm Brachina Fm Acraman Impact Callanna Group Burra Group Umberatana Group Wilpena Group Arkaroola Subgroup Curdimurka Subgroup Emeroo Subgroup Bungarider Subgroup
(Myrtle Springs Formation)
Pound Subgroup
Yudnamatana Subgroup Belair Subgroup Nepouie Subgroup
(Tapley Hill Formation)
Yerelina Subgroup Upalinna Subgroup (Amberoona Formation) Mundallio Subgroup (Skillogalee Dolomite) Boorloo Siltstone Cooranna Fm Hogans Dolomite Recovery Fm Dunns Mine Lst Rook Tuff Dome Sandstone Wooltana Volcanics Wywyana Fm Paralana Quartzite Intr a C o ntinent al Rift Break Up P a ssive M argin
Willour
an
Torrensian
Sturtian
Marinoan
W a r r in a S u p e r g r o u pM o r a l a n a S u p e r g r o u p (not included in the AFB)
H e y s e n S u p e r g r o u p 1 2 3 4 5
Rb - Sr Whole Rock Dates
1 1 2 3 4 5 6 588 ± 35 Ma (Webb, 1980) 601 ± 68 Ma (Webb et al., 1980)
676 ± 204 Ma (Webb & Coats, 1980)
690 ± 21 Ma (Jenkins & Cooper, 1998)
675 ± 54 Ma (Compston et al., 1987, using data from Webb et al., 1983))
750 ± 53 Ma (Webb & Coats, 1980)
Figure 2.3. outcrop of the Callanna Group (pirSa regional GiS data).
The Callanna Group outcrops in isolated fragments throughout the northern two thirds of the Ad- elaide Fold Belt, including the Peak and Denison Inliers.
does not crop-out, and in the type area of the Curdimurka Subgroup, in the Willouran Range (Figure 2.1), the Arkaroola Group occurs only as megaclasts within breccias. Forbes (1990) quoted a personal communication from Preiss, that the Dome Sandstone overlies the Noranda Volcanics (a correlative to the Wooltana Volcanics in the Willouran Range) in a megaclast in the central Willouran Range, otherwise no contact between the Arkaroola and Curdimurka subgroups has been found.
The Arkaroola Subgroup consists of the basal Paralana Quartzite, which lies unconformably on the Meso- to Palaeoproterozoic Mt Painter Inlier, and the successively overlying Wywyana Formation and Wooltana Volcanics (Figure 2.2). It crops out as an intact succession in the vicinity of the Mt Painter Inlier. Elsewhere it occurs as megaclasts within breccias scattered through the Adelaide Fold Belt (Figure 2.3). Around the Mt Painter Inlier, the Paralana Quartzite and Wooltana Volcanics show rapid variations in thickness, controlled by syn-
Denison Inliers Peake &
Gawler Craton
depositional faulting (Coats and Blisset, 1971) and they reach a maximum thickness of about 2,700 m (Priess, 1987).
The Wooltana Volcanics are interpreted to correlate with the Beda Volcanics, which inter- tongue with sandstone of the Backy Point Beds, the basal unit of the Adelaidean on the Stuart Shelf (Mason et al., 1978). Other probable correlatives include the Cadlareena Volcanics in the Peake and Denison Inlier and the Wilangee Basalt in the Barrier Ranges of western NSW (Preiss, 1987). Hilyard (1990) concluded from geochemical similarities that the Wooltana Volcanics and the Gairdner Dyke swarm, a group of NW – SE trending dolerite dykes that intrude the Gawler Craton, have the same source. Mafic volcanic mega-clasts are also common in diapiric breccias throughout the northern and central Adelaide Fold Belt and the Nackara Arc, (Dalgarno and Johnson, 1968; Preiss, 1987; Hilyard, 1990). Together, all these units have been included within the Willouran Basic Province and were derived from a mantle plume (Crawford and Hilyard, 1990). Baddeleyite from the Gairdner Dyke swarm gave an age of 827 ± 6 Ma (Wingate et al. 1998), which provides the minimum age for the initiation of the Adelaide Fold Belt. In the eastern Curnamona Province, the Little Gabbro, interpreted by Gibson et al. (1996, 1997) to be a magma chamber that fed dykes at a higher level has been dated by Wingate et al., (1998) at 827± 9 Ma. Hilyard (1990) estimated that the Willouran Basic Province covered about 210,000 square kilometres.
Zhao et al (1994) concluded from geochemical data that the Gairdner Dyke swarm and the coeval Amata Suite in the Musgrave Block formed from the decompressional melting of a large-scale mantle plume. In Australia, the diameter of the plume is in excess of 1,000 km, from the Adelaide Fold Belt to the central Australia. They suggested that the plume may have been responsible for large-scale crustal extension and thinning, leading to the formation of the Centralian Superbasin (Zhao et al. 1994). Von der Borch (1980), Zhao et al., (1994) and Williams and Gostin (2000) placed the plume head below a postulated triple point centred in the vicinity of Leigh Creek. Li et al., (1999) suggested that the break-up of Rodinia was initiated by a mantle plume beneath South China, which they placed adjacent to the Adelaide Fold Belt at this time. Mafic and ultramafic intrusives in South China have been dated at 828 ± 7 Ma, and they suggested that the Gairdner Dyke swarm forms part of a radial dyke swarm, the focus of which was to the southeast of the Adelaide Fold Belt (Li et al., 1999).
Coats and Blisset (1971) and Preiss (1987) concluded that deposition of the Paralana Quartzite was fault controlled within an intracontinental rift basin. In a later paper, based on the absence of conglomerates and sedimentary breccias, (Preiss, 2000) suggested that the Paralana Quartzite was deposited on a gradually subsiding peneplained stable craton and not within a rift zone. Where it crops out around the southern half of the Mt Painter Inlier, the thickness is fault-controlled, with Coats and Blisset (1971) showing that the faulting is syn-depositional.
The Curdimurka Subgroup crops out widely but in dismembered packages and as megaclasts in breccia, with the most complete section at its type section in the Willouran Range (Figure
2.3). Its initial depositional area is thought to be reflected in the distribution of its occurrences in breccias (Preiss, 1987) and so it was likely restricted to the Northern and Central Flinders Zones, the Nackara Arc and Houghton Anticlinal Zone. It has been divided into six units (Figure 2.2) and consists mainly of siltstone and shale with minor carbonate and sandstone (Murrell, 1977; Forbes, 1980; Preiss, 1987). Rowlands et al. (1980) interpreted it to have been deposited in a lacustrine environment in an intracontinental rift. There are two dates published for the Curdimurka Subgroup. In the Willouran Range, a SHRIMP U-Pb age of 802 ± 10 Ma from zircons taken from a lenticular porphyritic dacite from within the Rook Tuff has been reported (Fanning et al., 1986). The second is for a volcanic unit, the Oodla Wirra Volcanics, that occur as rafts within the Mt Grainger Diapir in the Nackara Arc. Two two separate samples of felsic volcanic rocks gave zircon U – Pb ages of 798 ± 5 Ma and 799 ± 4 Ma (Fabris et al., 2005); i.e. within error of the Rook Tuff. They are interpreted to have been deposited within evaporitic siltstone of the Callanna Group (Fabris et al., 2005). The Curdimurka Subgroup is described in more detail in section 2.3.5.
2.1.2.3 The Burra Group
The Burra Group is divided into four subgroups; the Emeroo Subgroup, the Mundallio Subgroup, the Bungarider Subgroup and the Belair Subgroup (Figure 2.2: Preiss, 1987; Preiss and Cowley, 1999). All are Torrensian in age except for the Belair Subgroup which is lowest Sturtian. The division of the Burra Group broadly reflects a gradual deepening of the basin; from the clastic Emeroo Subgroup through the carbonate-dominant Mundallio Subgroup, mainly represented by the Skillogalee Dolomite, to the shale and siltstone of the Bungarider Subgroup. The Belair Subgroup has a coarse-grained clastic unit at its base, overlain by a finer-grained clastic unit and records the final shallowing of the basin at this time. All subgroups are widespread except for the Belair Subgroup, which is limited to the southern Adelaide Fold Belt (Preiss, 1987; Preiss and Cowley, 1999). Preiss (1993) suggests that the limited occurrence of the Belair Subgroup may be due to its erosion prior to deposition of the Umberatana Group in the northern Flinders Ranges. The acid to basic Boucaut Volcanics have been dated as 777 ± 7 Ma (quoted as a personal communication from Preiss, 1998, Foden and Barovich, 2000). Fanning and Link (2008) suggested that this date is unreliable but no reason is given as to why. They crop out in two occurrences in the southern part of the Olary Region. Based on structural associations, Forbes (1978) suggested that they are older than the Burra Group but Preiss (1987) placed them near the base of the Emeroo Subgroup. Undated basalt flows interbedded with sandstone of the Emeroo Subgroup have been intersected in drill core in the Port Pirie region (Cowley and Parker, 1987; Parker et al., 1990).
A U-Pb age of 797 ± 5 Ma has been published recently (in a South Australian Geological Survey Report Book) for a porphyry interpreted to intrude the Skillogalee Dolomite at the Burra Copper mine (Drexel, 2009; Reid, 2009). The dated porphyry sample was from one of five pods that were mapped over 30 m, with the maximum thickness being two metres (Drexel, 2009). Previously the pods had been interpreted to have intruded the Skillogalee Dolomite after the Delamerian Orogeny. As this date contradicts much of the earlier dating,
it is addressed here. To do so requires some mention of conclusions from this study, and the chapters from where those conclusions are drawn are given.
As both the intrusion into the Skillogalee Dolomite and the Rook Tuff are based on intrusive rocks, in both cases, presumably shallow intrusives, it may be concluded that both ages are correct, but it does not follow that the Skillogalee Dolomite and the Rook Tuff are the same age; only that they were intruded by felsic volcanics at the same time. This means that the entire Curdimurka Subgroup and Emeroo Subgroup of the Burra group are somewhat older than 800 Ma. If the age correlation of the Wooltana Subgroup with the Gairdner Dyke Swarm is correct, then about 6 - 8,000 m of sediments were deposited in about 27 million years, at a rate of about 1,000 m per 4 – 4.5 million years. Not only that, two periods of extension are required to occur in that time. On the other hand, if the correlation is incorrect, then there is no way of knowing how old the Callanna Group is, except to say that it is older than about 800 Ma.
The second possibility is that they are indeed volcanic, with shallow intrusive intruding into newly deposited sediments. If this is the case, then the Rook Tuff and rocks of the Curdimurka Sub-group correlate with the upper Skillogalee Dolomite and the Myrtle Springs Formation. In the Willouran Range area, one of the distinguishing features of the Skillogalee Dolomite is the abundant magnesite layers (e.g., Belperio, 1990; Franks, 2004). Nowhere is magnesite seen in the Curdimurka Subgroup. The Emeroo Subgroup may then be equivalent to the Dome Sandstone. It is clear from detrital zircon spectra that these rocks have seen different sediment source areas and have received different proportions of material from the source areas (Chapter 4). Field observations show that the contact between the Curdimurka Subgroup and Burra Group are always faulted (Chapters 8 and 9) and so the juxtaposition could be purely structural. In the case of the Boorloo Siltstone this would require about 4,000 m of vertical movement on a reverse fault. Although this could explain the spatial relationship between the Boorloo Dolomite and Emeroo Subgroup, the timing of the movement would be during the deposition of the Umberatana Group. This would require north-directed thrusting, field evidence shows that the main movement in the Willouran Trough at this time was to the southeast, controlled by movement on a decollement at the base of the Dome Sandstone. There is late, north-directed movement on some faults, but overall this movement is of the order of ten’s to hundred of metres at most; not the 4,000 m required. On the southwestern margin of the Euchre Pack Domain, the Skillogalee Dolomite is in contact with the Boorloo Siltstone along what is interpreted to be a reverse fault; the Bungarider Fault. However, if the Boorloo Siltstone is younger than the Skillogalee Dolomite, this makes the Bungarider Fault a normal fault, which does not accord with the regional tectonics or structure.
Or finally, the rocks dated have not been interpreted correctly. The rocks were collected some 30 years ago, during which time the understanding of the Adelaide Fold Belt has increased greatly, particularly with regard to the origins and characteristics of the breccias. The porphyry pods are within about 30 m of the mapped edge of a diapir that contains acid volcanics clasts. Their pod-like geometry, sub-parallel to the diapir (Figure 18 of Drexel,