Interglacial carbonates of the Cryogenian Umberatana Group, northern Flinders Ranges, South Australia

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1 Australian Journal of Earth Sciences ISSN: (Print) (Online) Journal homepage: Interglacial carbonates of the Cryogenian Umberatana Group, northern Flinders Ranges, South Australia J. A. Giddings, M. W. Wallace & E. M. S. Woon To cite this article: J. A. Giddings, M. W. Wallace & E. M. S. Woon (2009) Interglacial carbonates of the Cryogenian Umberatana Group, northern Flinders Ranges, South Australia, Australian Journal of Earth Sciences, 56:7, , DOI: / To link to this article: Published online: 28 Sep Submit your article to this journal Article views: 552 Citing articles: 19 View citing articles Full Terms & Conditions of access and use can be found at

2 Australian Journal of Earth Sciences (2009) 56, ( ) Interglacial carbonates of the Cryogenian Umberatana Group, northern Flinders Ranges, South Australia J. A. GIDDINGS*, M. W. WALLACE AND E. M. S. WOON School of Earth Sciences, University of Melbourne, Vic. 3010, Australia. A detailed sedimentological and chronostratigraphic analysis of the Umberatana Group in the northern Adelaide Geosyncline has uncovered a depositional history involving the rapid progradation (at least 20 km) of a giant reef complex (up to 1.1 km relief) during mid-cryogenian interglacial times. The reef complex, which occurs in the Balcanoona Formation, displays facies similar to Phanerozoic reefs. These include a basal forereef (slope) facies, overlain by a reef-margin facies (consisting of both stromatolitic and non-stromatolitic frameworks), and an upper backreef (platform) facies consisting of shallow-water peloidal and oolitic carbonate. The thickening of the reef complex in a basinward direction, and the distribution of the key facies are consistent with the progradation of the platform into deep water. Progradation was contemporaneous with deposition of the upper Tapley Hill Formation and had largely ceased after a major margin failure event. Following this event, reef growth continued for a short time before becoming extinct, possibly as a result of global climatic cooling and/or eustatic sea-level fall. KEY WORDS: carbonates, Cryogenian, Flinders Ranges, microbial, progradation, reef, stratigraphy, Umberatana Group. INTRODUCTION Late Neoproterozoic time (* Ma) is characterised by extreme environmental change on a global scale. Severe global glaciations, changing oceanic and atmospheric compositions, and the gradual increase in biological complexity that occurred during the lead up to the Cambrian explosion, all represent significant challenges to our understanding of Earth evolution (Hoffman et al. 1998a, b; Knoll & Carroll 1999). Paramount to solving these environmental mysteries is an adequate understanding of the depositional environments and stratigraphic relationships of Neoproterozoic sedimentary successions. Ultimately, these successions hold the physical and geochemical clues to help answer many of the questions that have arisen in recent times The Adelaide Geosyncline in South Australia provides a relatively complete record of the Cryogenian and Ediacaran periods, including both mid- and late Cryogenian global glaciations. Present in the thick interglacial succession (Umberatana Group) is an unusual outcrop of thick platform carbonate deposits (*1000 m) belonging to the Balcanoona Formation. The platform outcrops continuously for some 20 km along strike before thinning and terminating abruptly near Oodnaminta Hut, where they are onlapped by deeper-water carbonate and siliciclastic sediments of the Yankaninna Formation (Figure 1). This feature has previously been interpreted as a sharp shelf-basin transition (Preiss 1987) characterised by the presence of a steep submarine escarpment (Coats & Blissett 1971; Preiss 1987). Until now however, a specific origin for this abrupt platform-basin transition has not been determined. In this paper we present new sedimentological and stratigraphic data from the Umberatana Group in the regions east and west of the Paralana Fault in the northern Adelaide Geosyncline (Figure 1). We have studied in detail the post-glacial sediments of the Tapley Hill Formation, platform carbonates of the Balcanoona Formation, and onlapping basinal sediments of the Yankaninna Formation. Our results show that the Balcanoona Formation near Oodnaminta Hut represents a giant prograding reef complex, the eroded margin of which provides an explanation for the previously described and enigmatic submarine escarpment. GEOLOGICAL SETTING AND AGE CONSTRAINTS The Adelaide Geosyncline is composed of mildly deformed Neoproterozoic to Lower Cambrian synrift, post-rift and probable passive-margin sediments, with a total stratigraphic thickness of 410 km (Figure 1). The name Adelaide Geosyncline (otherwise known as Adelaide Fold Belt or Adelaide Rift Complex ) is used here in a non-genetic sense to refer to the basin and its accumulated sediments (Williams et al. 2008). These were deposited unconformably over a Mesoproterozoic crystalline basement during breakup of Rodinia and opening of the Paleo-Pacific Ocean beginning at *830 Ma (Coats & Blissett 1971; Preiss 1987, 2000). The complex is flanked by the Gawler Craton and its relatively undeformed platform cover, the Stuart Shelf, *Corresponding author: jagiddings@gmail.com ISSN print/issn online Ó 2009 Geological Society of Australia DOI: /

3 908 J. A. Giddings et al. Figure 1 Location of the Adelaide Geosyncline and generalised geological map of the region south of Arkaroola, northern Flinders Ranges (modified from Coats 1973). Section localities are also shown: MJ, Mt Jacob; OHA C, Oodnaminta Hut; IL, Illinawortina. to the west, and the Curnamona Craton to the east, both of which once formed the margins of the subsiding intracontinental rift basin (Preiss 1993; McKirdy et al. 2001). The basin was subsequently deformed during the Cambrian Ordovician Delamerian Orogeny (Coats & Blissett 1971; Preiss 1987, 2000), and its current exposure provides an almost complete record of sedimentation through one of the most important eras in Earth history. The Neoproterozoic succession is divided into four groups (Figures 1, 2), of which the Umberatana Group encompasses both Cryogenian glacial successions (of Sturtian and Marinoan age) and forms the focus of this study. Reliable ages determined from within the Umberatana Group are few. However, Fanning & Link (2006) reported a U Pb zircon age of *658 Ma from a tuffaceous horizon within the uppermost Sturtian diamictites. This age is in approximate agreement with a Re Os age of Ma from the basal Tindelpina Shale Member of the Tapley Hill Formation (Kendall et al. 2006). The timing of the Marinoan glacial event, represented by the Elatina Formation, can be tentatively placed at 635 Ma by correlation with the Ghaub Formation in Namibia and the Nantuo Formation in South China (assuming approximate synchronicity of global glaciation: but see Calver et al. 2004, who prefer a *580 Ma age for the Elatina Formation). Hoffmann et al. (2004) obtained a U Pb zircon age of Ma for a tuff bed near the top of the Ghaub Formation and Condon et al. (2005) obtained a U Pb zircon age of Ma from a tuff bed in the cap carbonate overlying the Nantuo Formation, providing an approximate age for termination of Marinoan glaciation in Namibia and China, respectively. By correlation with the Elatina Formation, these constraints leave an interval of 415 Ma for deposition of the Umberatana (Cryogenian) interglacial sequence. FIELDWORK AND METHODOLOGY A detailed stratigraphic and sedimentological analysis of the succession of interest was undertaken in order to construct a chronostratigraphic and paleoenvironmental framework for the sediments. Stratigraphic sections were measured from a number of localities east (Mt Jacob) and west (Oodnaminta Hut and Illinawortina) of the Paralana Fault (Figure 2), using a 1.5 m Jacob Staff. Petrological analysis was carried out on 25 thinsections. All mineralogical information was obtained

4 Cryogenian interglacial carbonates 909 Figure 2 Geological map of the Oodnaminta Hut and Illinawortina regions, west of the Paralana Fault, showing the locations of each stratigraphic section. GHR, Grindel Hut Road locality. The map was produced from a combination of field mapping and high-resolution aerial-photograph interpretation.

5 910 J. A. Giddings et al. by scanning electron microscopy (SEM) and X-ray diffraction (XRD) at the University of Melbourne. A sediment-decompaction exercise was undertaken using the program OSXBackStrip Version 2.3 to determine the depositional thickness and water depths of individual units in the Oodnaminta Hut sections. The decompacted thickness of each unit was determined using the following input parameters: present stratigraphic thickness, the relative ages of upper and lower boundaries of each unit (estimated), grain density (rc), porosity coefficient (c) and surface porosity (jo). All formulas and algorithms used in OSXBackStrip Version 2.3 are from Allen & Allen (1990), and all lithological parameters were taken or estimated from those given for different lithologies in Allen & Allen (1990). CHRONOSTRATIGRAPHIC FRAMEWORK FOR THE UMBERATANA GROUP The Umberatana Group has previously been subdivided into four separate subgroups bound by basin-wide unconformities (Preiss et al. 1998) (Figure 2). The basal Yudnamutana Subgroup refers to all Sturtian glacial deposits, the overlying Nepouie and Upalinna Subgroups refer to the interglacial succession, and the uppermost subgroup, the Yerelina Subgroup, encompasses all glacial deposits of Marinoan age. Although individual formations within each subgroup vary between regions, the subgroups and the unconformities that define their upper and lower boundaries are recognisable across the Adelaide Geosyncline. The upper and lower limits of both the Yudnamutana and Yerelina Subgroups are defined by erosion during glaciation, while the boundary between the interglacial Nepouie and Upalinna Subgroups is defined by erosion resulting from regression and exposure at the top of the Balcanoona Formation and its lateral equivalents in other parts of the basin. This surface is interpreted to define the Sturtian Marinoan boundary (Preiss et al. 1998). In this study, parts of the interglacial succession (Nepouie and Upalinna Subgroups) have been subdivided into informal lithostratigraphic units to supplement the pre-existing stratigraphic nomenclature (Preiss et al. 1998), and to be used as chronostratigraphic markers for correlation between all sections. Detailed stratigraphic and sedimentological observations for each region (Mt Jacob and Oodnaminta Hut Illinawortina) are given below and all stratigraphic sections and chronostratigraphic correlations are displayed in Figures 3 and 4. Mt Jacob region POST-GLACIAL SUCCESSION: TAPLEY HILL AND BALCANOONA FORMATIONS Deposition immediately following Sturtian glaciation in the Adelaide Geosyncline is recorded in the Tapley Hill Formation (Thomson et al. 1964). This basin-wide unit lies conformably and disconformably above Sturtianaged glacial diamictites of the Merinjina Tillite in the northern Adelaide Geosyncline (Preiss 1987). East of the Paralana Fault it reaches a maximum thickness of 550 m near Mt Jacob, and five distinct stratigraphic units are identifiable (Figure 3). These are referred to here as the Tindelpina Shale Member (Thomson et al. 1964), and informally as the siltstone unit, cyclic unit, quartz sandstone unit and transitional shale unit. The laterally persistent Tindelpina Shale Member defines the base of the Tapley Hill Formation and has an average thickness of *50 m in our sections (Figure 3, 4). At the base of the Tindelpina Shale Member is a thin cap carbonate (510 m) consisting mostly of peloidal dolomite and lying with sharp, conformable contact above the glaciogenic Merinjina Tillite. The cap carbonate grades into the upper part of the Tindelpina Shale Member, which is characterised by dark grey, pyritic, dolomitic shale with regular *5 cm-thick dolomite interbeds. Towards the top of the Tindelpina Shale Member, these dolomite interbeds disappear and the Figure 3 Stratigraphic section through the post-glacial succession in the Mt Jacob region showing the informal stratigraphic units identified in the Tapley Hill Formation in addition to the Tindelpina Shale Member. Also shown are relative XRD peak height data for quartz (peak 101), calcite (peak 104) and dolomite (peak 104). THF, Tapley Hill Formation; BF, Balcanoona Formation.

6 Cryogenian interglacial carbonates 911 Figure 4 Chronostratigraphic diagram showing the stratigraphic sections measured near Oodnaminta Hut and Illinawortina. Dashed lines represent detailed correlations between each section. THF, Tapley Hill Formation; BF, Balcanoona Formation; YF, Yankaninna Formation; AF, Amberoona Formation; ES, Enorama Shale.

7 912 J. A. Giddings et al. unit grades into the overlying siltstone unit. The siltstone unit consists of calcareous siltstone that often forms a prominent ridgeline around m above the base of the Tapley Hill Formation (Figure 5a). In the Mt Jacob section the siltstone unit coincides with a change in the dominant carbonate mineralogy (from dolomite to calcite) and an increase in detrital quartz abundance (Figure 3). Overlying the siltstone unit is the cyclic unit, which is characterised by finely laminated calcareous shale with very regularly spaced cm-thick carbonaterich (30 60% carbonate) siltstone interbeds (Figure 5a, b). The average spacing of these carbonate-rich horizons is consistently *4 m in the stratigraphic section. Overlying the cyclic unit is a stratigraphic interval in the middle of the Tapley Hill Formation characterised by Figure 5 Field photographs of the Tapley Hill and Balcanoona Formations in the Mt Jacob region. (a) View looking south along strike showing the Tindelpina Shale Member (TSM), siltstone unit (su) and cyclic unit (cu) S, E. (b) Regularly spaced carbonate-rich horizons in the cyclic unit S, E. (c) Fine sandstone from the quartz sandstone unit displaying prominent cross-lamination (possible Bouma C division). Coin is 2.5 cm in diameter S, E. (d) Turbidite from the quartz sandstone unit. Note the cross-lamination near the base (x) and the presence of graded sand layers with erosive bases (s) S, E. (e) Limestone edgewise conglomerate from the upper part of the transitional shale unit. Note the curved, tabular clasts S, E. (f) Domical stromatolite from the lower Balcanoona Formation at Nepouie Gorge, south of the Mt Jacob section. Hammer for scale S, E.

8 Cryogenian interglacial carbonates 913 calcareous shale interbedded with siltstone, fine sandstone and occasionally pebbly sandstone (quartz sandstone unit). Partial Bouma sequences (commonly AB and BCD divisions) are a prominent feature through this interval (Figure 5c, d). Minor limestone edgewiseconglomerate interbeds (2 5 cm thick) are also present near the base of the quartz sandstone unit (Figure 5e). Their limestone intraclasts are tabular, thin (typically 53 mm thick), commonly curved, and closely resemble stromatolitic limestones found at the base of the overlying Balcanoona Formation. This indicates that deposition and erosion of equivalent shallow-water Balcanoona Formation carbonates was under way prior to deposition of the quartz sandstone unit. Lying above the quartz sandstone unit with abrupt contact is the upper regressive part of the Tapley Hill Formation, informally referred to here as the transitional shale unit. The unit reaches *330 m thickness near Mt Jacob and is dominated by finely laminated grey, silty shale with thin (up to 30 cm) interbeds of carbonaterich siltstone and limestone edgewise conglomerate (identical to those found near the base of the quartz sandstone unit). Edgewise-conglomerate beds increase in number and thickness (up to 1 m) through the conformable and gradational contact with the overlying Balcanoona Formation. The Balcanoona Formation (first defined by Coats & Blissett 1971 and later re-defined by Preiss et al. 1998) consists of stromatolitic limestone at the base (lower *20 m), with edgewise-conglomerate interbeds (5 cm to 3 m thick) and minor calcareous shale. These are overlain by purely stromatolitic dolomite. The stromatolites are dominantly broad domes ranging from *30 cm to *2 m in width (Figure 5f). Oodnaminta Hut and Illinawortina regions TAPLEY HILL FORMATION The stratigraphy of the Tapley Hill Formation in the Oodnaminta Hut region (Figure 4) is similar to that near Mt Jacob, with all five informal stratigraphic units recognisable. However, not all units are present in all sections and there are significant differences between the successions on either side of the Paralana Fault that reflect the different depositional histories of the two regions. The cap carbonate in the Oodnaminta Hut Illinawortina region differs markedly from that near Mt Jacob. In the Oodnaminta Hut sections (A C) it consists of finely laminated micritic limestone with thin (*1 mm) quartz silt interlaminations, interbedded with calcareous shale. It is *5 10 m thick and generally more shale-rich than at Mt Jacob. At Illinawortina the cap carbonate is highly lenticular and commonly only calcareous shale is present. The overall thicknesses of the Tindelpina Shale Member, siltstone unit, cyclic unit and quartz sandstone unit in the Oodnaminta Hut sections are comparable to those at Mt Jacob. However, the siltstone unit, quartz sandstone unit and transitional shale unit are not present in the Illinawortina section. The transitional shale unit is also missing from Oodnaminta Hut section C and is of variable thickness in the region of Oodnaminta Hut sections A and B ( m) (Figure 6a, b). The upper contact with the Balcanoona Formation is also sharp and erosional. Where present the transitional shale unit is also distinguished from that in the Mt Jacob section by the presence of slump-folded, laminated limestone, carbonate-intraclastic breccia, megabreccia and large isolated allochthonous blocks (Figure 6c, d, e). The intraclasts and large allochthonous blocks consist of dolomite, limestone or partially dolomitised limestone and range up to 850 m in length (Figure 6f). The allochthonous blocks display a variety of depositional textures, including finely laminated, stromatolitic and non-stromatolitic examples. Non-stromatolitic blocks are commonly characterised by cryptic microbial textures. BALCANOONA FORMATION The lower section of the Balcanoona Formation in the Oodnaminta Hut sections is characterised by an interval of dolomite megabreccia typically *100 m thick (Figure 4). The megabreccia facies consists of chaotic allochthonous dolomite with little or no matrix that forms the base of the Balcanoona Formation along the entire length of its outcrop to its lateral termination near Oodnaminta Hut (the submarine escarpment ) (Figure 2, 4). In places the megabreccia interfingers with, or is onlapped by, shale from the upper transitional shale unit of the Tapley Hill Formation. Lying conformably above the megabreccia facies is a massive dolomite unit characterised by the presence of cryptic, non-stromatolitic microbial textures (non-stromatolitic framework unit). This unit is thickest at the termination of the Balcanoona Formation (*675 m, Oodnaminta Hut section B) and thins gradually southward. It is missing in Oodnaminta Hut section A and reappears south of this locality with appreciable thickness (*200 m). Although difficult to distinguish in the field due to complete dolomitisation, the non-stromatolitic textures show a variety of morphologies that are similar to those found in allochthonous blocks within the underlying Tapley Hill Formation (Figure 7a, b). The most distinctive of these fabrics is characterised by lobate cavities (1 10 mm diameter) that have thin (51 mm), dark dolomicritic walls and are filled by fibrous-marine and coarse-equant cements (Figure 7c). These fabrics somewhat resemble calcified microbes (e.g. Renalcis or Wetheredella) described from younger (Cambrian, Devonian) sedimentary successions around the world (James & Kobluk 1978; Stephens & Sumner 2002). However, the size of the structures is not typical of microbes and their affinities are still unknown. For this reason, they are herein referred to as non-stromatolitic frameworks. Other non-stromatolitic (microbial-like) fossils appear to have dendritic and shrub-like growth patterns. Although the fabrics are best observed in partly dolomitised allochthonous blocks, dolomitisation appears to have been largely fabric retentive and some of the in situ non-stromatolitic frameworks within the Balcanoona Formation display north to northeast growth directions (Figure 7d). A stromatolitic dolomite unit with an average thickness of *360 m overlies the non-stromatolitic framework unit. The contact between these two units is transitional,

9 914 J. A. Giddings et al. Figure 6 Field photographs of the Tapley Hill Formation in the Oodnaminta Hut region. (a) Oblique aerial photograph looking west showing all informal stratigraphic units of the Tapley Hill Formation overlying the Merinjina Tillite (MT) and underlying the Balcanoona Formation (BF) with an irregular contact S, E (centre of image). (b) Oblique aerial photograph looking northwest at Oodnaminta Hut section C. Basinal megabreccias (bmb) of the Yankannina Formation can be seen lying unconformably above the cyclic unit of the Tapley Hill Formation, with all other units missing. The overlying shales and allochthonous blocks separate the basinal megabreccia from the lower laminated limestone (L1), recording the period in which final reef extinction occurred S, E (centre of image). (c) Slump folds in interbedded limestone and siltstone from the upper part of the transitional shale unit. These sediments underlie and intertongue with lower megabreccias of the Balcanoona Formation. Hammer for scale S, E. (d) Intraclastic breccia from the transitional shale unit showing angular dolomite clasts and limestone matrix. Hammer for scale S, E. (e) View looking south along strike between Oodnaminta Hut sections A and B at large dolomite megablocks within the transitional shale unit S, E. (f) Aerial photograph of the sequence between Oodnaminta Hut sections A and B showing the largest dolomite megablock lying in the upper Tapley Hill Formation S, E (centre of image). TSM, Tindelpina Shale Member; su, siltstone unit; cu, cyclic unit; qsu, quartz sandstone unit; tsu, transitional shale unit. where some non-stromatolitic frameworks coexist with the stromatolites. In general, the lower part of the stromatolitic unit is characterised by laminar and domical morphologies, while in the upper part columnar stromatolites are dominant. This is particularly the case towards the northern extent of the Balcanoona Formation near Oodnaminta Hut (Oodnaminta Hut section B). However, at other localities all morphologies may be

10 Cryogenian interglacial carbonates 915 Figure 7 Non-stromatolitic fabrics from the Balcanoona Formation (Oodnaminta Hut sections). (a) In situ cryptic microbial texture. The cavities are defined by protruding dolosparite crystals. Coin is 2 cm in diameter S, E. (b) In situ non-stromatolitic framework with a branching growth pattern. Coin is 2 cm in diameter S, E. (c) Photomicrograph of the fabrics displayed in (a), showing large, cement-filled, lobate cavities with thin micritic walls, reminiscent of calcified microbes (although much larger). University of Melbourne sample no. OH-06-4a. (d) Nonstromatolitic frameworks growing in a north to northeast direction at a high angle to bedding. Hammer for scale S, E. coexisting and this simple upper/lower pattern is not observed. The laminar stromatolites are typically flat to gently convex and are occasionally interbedded with thin beds of small, pseudocolumnar stromatolites. These pseudocolumnar stromatolites commonly form interconnected columns (*10 cm in height and 2 4 cm in width) (Figure 8a). The columnar stromatolites typically consist of tightly packed vertical columns that appear to form massive frameworks (Oodnaminta Hut section B) (Figure 8b). The columns are typically *4 cm wide and up to 15 cm in height, and expand upward before diverging into two parallel branches. Laminae are steeply convex to rectangular. Other minor morphologies observed include thin (51 cm) columnar stromatolites, branching and coalescing forms, and tuberous ribbed forms. All stromatolites have thin (51 mm) laminae consisting of peloidal dolomicrite, and microfenestral or clotted dolomicrite (grumeleuse structure), with subhedral dolomicrospar interlaminae (Figure 8c). The north to northeast growth direction of the stromatolites is the same as the underlying non-stromatolitic frameworks. In Oodnaminta Hut section B, the stromatolitic unit continues to the upper boundary of the Balcanoona Formation. However, in Oodnaminta Hut section A the upper part of the formation is distinguished by a bedded dolomite unit that reaches a maximum thickness of *370 m and thins northwards before pinching out above the underlying stromatolites *1.5 km south of Oodnaminta Hut section B. The dolomite unit consists predominantly of well-bedded, fenestral ooid and peloid grainstone (Figure 9a). Laminoid, bedding-parallel sheet cracks or cavities are common and partially filled by isopachous fibrous cement (Figure 9a, c). These fibrous cements are inclusion-rich, have a fascicular optic fabric, and generally resemble marine calcite cements (although lithologically are now dolomite) (Kendall 1985). The sheet crack facies also often contains small tepee structures (Figure 9d). The upper contact of this unit with the overlying Amberoona Formation is sharp and unconformable. The contact is often irregular and typified by large (1 10 m diameter) cavities. These cavities are filled with sandstone and dolomite breccias, indicating the probable subaerial exposure and karstification of the upper Balcanoona Formation consistent with that observed by Preiss et al. (1998). YANKANINNA FORMATION The Yankaninna Formation (Thomson et al. 1964, redefined by Preiss et al. 1998) occurs north of and onlaps

11 916 J. A. Giddings et al. Figure 8 Stromatolites from the Balcanoona Formation in Oodnaminta Hut section B. (a) Laminar and pseudocolumnar stromatolites from the lower part of the stromatolitic unit. Hammer for scale S, E. (b) Columnar stromatolites from the upper part of the stromatolitic unit S, E. (c) Photomicrograph showing the stromatolitic microtexture of interlaminated dolomicrite and dolomicrospar. University of Melbourne sample no. OH the Balcanoona Formation at its northernmost extent (Figure 2). It consists of a combination of pure carbonate and mixed carbonate siliciclastic sediments. We have placed the lower boundary of the formation at the base of a thick dolomite megabreccia unit (basinal megabreccia unit: Figures 4, 6b) that post-dates and onlaps the Balcanoona Formation. This megabreccia is *60 m thick in Oodnaminta Hut section C and extends along strike in a discontinuous (lenticular) fashion as far as Illinawortina, *14 km from Oodnaminta Hut. Here the unit is present as isolated dolomite blocks (510 m diameter) within calcareous shale. Between the two sections, very large allochthonous dolomite blocks (up to *450 m in diameter) occur within the megabreccia and lie with erosional contact above sediments in the cyclic unit of the Tapley Hill Formation (Figure 10a, b). Block composition mimics that of the Balcanoona Formation, consisting of stromatolitic and non-stromatolitic microbial dolomite. In Oodnaminta Hut section C, the basinal megabreccia unit is overlain by a *160 m-thick calcareous shale succession that also contains allochthonous limestone and partially dolomitised limestone blocks within discrete, laterally continuous horizons (Figure 10c). These limestone blocks display spectacular stromatolitic and non-stromatolitic fabrics identical to those found in the Balcanoona Formation. The non-stromatolitic (microbial-like) textures are again best observed in undolomitised or partly dolomitised limestone blocks and appear to form a variety of rigid, organised frameworks (Figure 10d, e, f). Overlying the basal shale interval of the Yankaninna Formation are three flaggy laminated limestone units (L1, L2, L3: Figure 4). These are between 20 m and 60 m thick and are separated by intervals of wellbedded and often slump-folded calcareous/dolomitic shale of comparable thickness (Figure 11a, b). Horizons containing allochthonous material (dolomitised laminated limestone, laminated limestone and shale blocks) are abundant throughout this interval. However, there is a distinct absence of any blocks containing stromatolitic or non-stromatolitic fabrics. Above this limestone shale interval in Oodnaminta Hut section C, green-grey calcareous shale and siltstone make up the remainder of the Yankaninna Formation (Figure 4). In the Illinawortina section, only two thick limestone units are present in the equivalent succession. In place of the lower limestone unit (L1) is a 150 m interval of slump-folded calcareous shale and siltstone with thin (up to 2 m) laminated-limestone interbeds displaying metre-scale erosion surfaces (Figure 11c, d). Above the upper limestone unit (L3), the Yankaninna Formation coarsens into siltstone and fine sandstone with abundant Bouma sequences and minor slump folding continuing through the upper 400 m of the unit (Figure 4). AMBEROONA FORMATION Overlying the Yankaninna Formation in Oodnaminta Hut section C and the Illinawortina section is calcareous shale and siltstone belonging to the Amberoona Formation (Thomson et al. 1964). In Oodnaminta Hut sections A and B, the Amberoona Formation directly overlies the Balcanoona Formation. Defining the base of the formation is a series of sandy and stromatolitic limestone beds (up to 1 m thick) interbedded with siltstone exhibiting prominent wave ripples (Figure 12a). The stromatolites are dominantly small, columnar bodies (*4 cm in width, up to 10 cm in height) (Figure 12b). The remainder of the Amberoona Formation is mostly grey calcareous shale and siltstone

12 Cryogenian interglacial carbonates 917 Figure 9 Well-bedded platform carbonate deposits from the upper Balcanoona Formation near section Oodnaminta Hut section A. (a) Peloidal and oolitic grainstone cemented by dolomicrospar. University of Melbourne sample no. OH (b) Horizontal, parallel sheet cracks in outcrop. Coin is 2 cm in diameter S, E. (c) Polished slab showing sheet cavity filled with fibrous isopachous marine dolomite cement (a, b) and coarse dolomite diagenetic cement (c). University of Melbourne sample no. AR-SC-1. (d) Tepee structure showing disruption of bedding-parallel sheet cracks S, E. that grades into green-grey sediments of the Enorama Shale. ENORAMA SHALE AND WUNDOWIE LIMESTONE MEMBER Lying near the base of the Enorama Shale (Thomson et al. 1964) are three prominent and basin-wide stromatolite horizons, together known as the Wundowie Limestone Member (first defined as a member of the Angepena Formation by Coats & Blissett 1971; currently defined as a member of the Enorama Shale by Preiss et al. 1998). These horizons (S1, S2, S3: Figure 4) vary between 0.5 m and 2 m in thickness and typically consist of white (buff-weathering) to pale-grey stromatolitic, sometimes sandy, limestone (Figure 12c). Above the Wundowie Limestone Member the Enorama Shale consists of green-grey, slightly calcareous shale (typically 510% carbonate). Lying unconformably over the Enorama Shale are the glaciogenic sands and diamictites of the Elatina Formation (i.e. Marinoan glacials). DISCUSSION Paleoenvironmental interpretation Close stratigraphic similarities between the successions east and west of the Paralana Fault in the lower Tapley Hill Formation reflect an overall similar depositional history in the post-glacial environment. A large transgression followed the Sturtian glaciation and the Tindelpina Shale Member (including the basal cap carbonate) lacks any shallow-water sedimentary structures, suggesting a deep-water environment (at least below fair-weather and storm wave-base). The siltstone unit at the top of the Tindelpina Shale Member, which displays a higher detrital-quartz silt fraction at Mt Jacob (Figure 3), is interpreted to represent a minor regression. Its absence at Illinawortina may suggest that this unit is a shallower-water proximal facies that thins out in a basinward direction. However, the siltstone unit does not contain any shallow-water features and was itself presumably deposited below storm wave-base. The depositional history of the middle part of the Tapley Hill Formation is almost identical in the Mt Jacob and Oodnaminta Hut regions, with the cyclic and quartz sandstone units being lithologically similar and of approximately comparable thickness. We interpret this interval to represent a deeper water environment than that of the siltstone unit. The regularly spaced carbonate-rich horizons within shales of the cyclic unit may represent condensed intervals with lower sedimentation rates, allowing a higher proportion of carbonate to accumulate. Alternatively, the increase in carbonate may signify a shallower depositional environment. In

13 918 J. A. Giddings et al. Figure 10 (a) View looking south from Oodnaminta Hut section C ( S, E) and (b) looking southeast towards Oodnaminta Hut section C ( S, E), showing the basal erosion surface of the basinal megabreccia unit (Yankaninna Formation) cutting down into and truncating beds within the cyclic unit of the Tapley Hill Formation. Note the regular spacing of the beds within the cyclic unit. (c) Isolated allochthonous limestone blocks from the shale interval above the basal megabreccia of the Yankaninna Formation. Person (arrowed) for scale S, E. (d) Mixed stromatolitic and non-stromatolitic framework textures in an allochthonous limestone block from the same interval S, E. (e) Non-stromatolitic framework consisting of lobate, spar-filled cavities that resemble calcified microbes (same as Figure 7a, c) S, E. (f) Photomicrograph of (e) showing the lobate spar-filled cavities with thin (51 mm) micritic walls (w). cu, cyclic unit; bmb, basinal megabreccia; L1, lower laminated-limestone unit; es, erosion surface. University of Melbourne sample no. YFblockTS2. either case, the regular spacing of the beds probably reflect Milankovitch orbital cyclicity. In both regions, the overlying quartz sandstone unit displays regular Bouma sequences suggestive of a turbiditic origin. The presence of turbidites in the Tapley Hill Formation has been previously recognised at other localities, and attributed to diapiric movements near Blinman (McKirdy et al., 2001). However, the large distance (*100 km) between the Mt Jacob Oodnaminta Hut regions and the Blinman diapir, and the absence of diapirs in this immediate region, suggests that the turbidites observed here may be unrelated to those in previously described sections. It may be that the turbidites reflect early growth of the Balcanoona platform and development of a slope during middle-tapley Hill times. They may indicate base-of-slope conditions,

14 Cryogenian interglacial carbonates 919 Figure 11 (a) View looking southwest at a laminated-limestone unit (L1) sharply overlying shales in the lower Yankaninna Formation S, E. (b) View looking northeast at the contact (white dashed line) between the Balcanoona Formation (BF) and onlapping sediments of the Yankaninna Formation (YF) (distance between BF and L2 symbols is *360 m). L2, L3, laminated limestone units S, E. (c) Slump folding (hammer for scale) ( S, E) and (d) an erosion surface (es) in laminated-limestone beds near the Illinawortina section ( S, E). suggesting that this interval represents the deepest water deposits in the post-glacial succession. In the upper part of the transitional shale unit of the Tapley Hill Formation, marked differences between the two regions become apparent. At Mt Jacob, the regressive nature of the unit and gradational contact with the overlying Balcanoona Formation reflects a conformable shallowing-upward succession. In the Oodnaminta Hut sections, however, the upper transitional shale is characterised by coarse megabreccia, syndepositional deformation structures (slumping folding), and an erosive contact with the overlying megabreccia facies of the lower Balcanoona Formation. The allochthonous blocks were probably derived from platform carbonates of the Balcanoona Formation elsewhere and are interpreted as evidence of large-scale mass flows caused by slope or platform margin instability. The abundance and size of allochthonous blocks (up to 850 m diameter south of Oodnaminta Hut section A) suggests the platform margin must have been steep (at least 458). The overlying Balcanoona Formation consists, from base to top, of a dolomite-megabreccia facies, overlain in sequence by a generally thick non-stromatolitic framework unit, a thick stromatolitic unit, and a bedded dolomite unit that thins and pinches out south of Oodnaminta Hut (Figure 4). The basal megabreccia facies is interpreted as talus accumulated on the lower slope as a result of erosion of the platform margin and subsequent mass flow. The non-stromatolitic framework unit that lies above the megabreccia facies consists of a variety of rigid, cryptic microbial-like frameworks that have an organisation similar to some Phanerozoic reef frameworks (e.g. calcified microbes: Grotzinger & Knoll 1995). We therefore interpret these as reef-building frameworks that must have grown in deep-water environments, immediately above the lower-slope breccias, in a dominantly basinward direction (north to northeast). Stromatolites from the overlying stromatolitic unit also form frameworks that have a growth direction approximately perpendicular to the slope, but probably grew in much shallower environments. The upper, bedded dolomite unit contains features indicative of high-energy deposition in shallow subtidal to intertidal environments, including an abundance of peloids, ooids, tepee structures and marine cements. This unit is here interpreted as a shallow-water platform facies. The internal geometry of the Balcanoona Formation (slope megabreccia overlain by reef-frameworks and platformal deposits) is consistent with that expected in a large prograding reef complex, with distinctive slope (forereef), reef-margin, and platform (backreef) facies (Figure 13a). The massive megabreccia at the base of the Balcanoona Formation, in addition to those in the upper

15 920 J. A. Giddings et al. to the progradational history of the reef complex. As the reef-margin and forereef facies advanced basinward, so too has the backreef facies (Figure 13a). The interpretation of a giant prograding reef complex is supported by the observation that the majority of the Balcanoona Formation in this region consists of rigid biological frameworks that grew and thicken in a basinward direction. This thickening of the reef-margin facies reflects the progradation of the platform into an area with greater accommodation space (deeper water). In addition, the sloping surfaces of older margins that are recognisable from aerial photographs (prograde surfaces: Figure 13b) also suggest progradational geometry for the Oodnaminta platform. The prograde surfaces show that the margin maintained a steep slope of 4*458 and high relief at least during the later stages of reef-growth. At times the slope must have been steep enough to create significant margin instability, leading to entire margin failure, and the transportation of large blocks of the margin into basinal environments. This almost certainly was the case during the ultimate margin-failure event, which resulted in deposition of the basinal megabreccia unit (lower Yankaninna Formation) and transportation of margin-derived material as far as Illinawortina (*14 km from the platform margin). Each of these periods of mass flow caused significant disruption and reworking of lower-slope and basinal sediments (upper Tapley Hill Formation). In the case of the basinal megabreccia unit, its erosional contact with the cyclic unit of the Tapley Hill Formation (Figure 10a, b) suggests that the entire upper transitional shale and quartz sandstone units (or equivalent sediments) were completely eroded away during a catastrophic massflow event (up to *150 m of section removed). Given the northward thinning of the transitional shale unit beneath the Balcanoona Formation it is possible that significant amounts of sediment may be missing from the southern Oodnaminta Hut sections as well, resulting from earlier mass-flow events. Figure 12 (a) Wave ripples in siltstone ( S, E) and (b) branching-columnar stromatolites in limestone from the lower Amberoona Formation in Oodnaminta Hut section C. Hammer for scale S, E. (c) Branching-columnar stromatolites from the Wundowie Limestone Member of the Enorama Shale in Oodnaminta Hut section C S, E. Tapley Hill and lower Yankaninna Formations (Oodnaminta Hut sections) represent the forereef and upperslope facies respectively. Laterally equivalent basinal sediments occur in the Illinawortina section. The nonstromatolitic framework unit and upper stromatolitic unit have both been assigned to the reef-margin facies. The presence of these biological frameworks above the basal megabreccia facies reflects the progradation of the reef margin over the top of the lower-slope forereef sediments. The occurrence and thinning out of the platform deposits (backreef facies) above the stromatolitic unit south of the terminal margin, is also testimony Estimation of margin relief and water depths The uppermost surface of the prograding reef complex can be used as a proxy for sea-level, since backreef lithologies must have been deposited in shallow water, close to sea-level. From this, it is possible to estimate the relief of the reef margin and depth of deposition of onlapping basinal sediments at any given point in the basin filling history. In order to do this accurately, it is necessary to decompact both the reef (Oodnaminta Hut section B) and off-reef (Oodnaminta Hut section C) sediments to obtain the thickness of each section at certain times throughout their depositional history (Figures 14, 15). Oodnaminta Hut sections B and C were separated into discrete units based on lithology and 5 km of overburden was added to represent Upper Neoproterozoic and Lower Cambrian strata. Sea-level is assumed to have been approximately equivalent to the top of the reef margin and remained reasonably constant throughout the basin-filling history, and the datum for the base of the sections (base of Tapley Hill Formation) to have

16 Cryogenian interglacial carbonates 921 Figure 13 (a) Schematic diagram of the Oodnaminta reef complex showing the distribution of the major facies and their key sedimentary features. (b) Aerial photograph of the Oodnaminta Hut region. White dashed lines highlight the prograde surfaces. The Yankaninna Formation (YF) can be seen onlapping onto the Balcanoona Formation (BF) on the right side of the image. MT, Merinjina Tillite; THF, Tapley Hill Formation; ab, allochthonous block. Figure 14 Burial history diagrams for Oodnaminta Hut sections B and C. The plots show the difference in thickness (depth) between the sections at particular times during the burial history (time 1, T1; time 2, T2) based on the chronostratigraphic correlations. At T1 and T2, this difference is taken to represent the depth of water at the base of the slope (assuming that the top of the reef complex is at approximate sealevel). This gives an indication of margin relief following its final failure. See text for details of the decompaction process. bmb, basinal megabreccia. been at the same level for each section. Based on the present thickness of each unit, and its lithological characteristics (grain density, porosity coefficient and surface porosity), the decompaction program (OSXBack- Strip V2.3) determines an initial thickness for each unit at the time of deposition. It also gives a total thickness

17 922 J. A. Giddings et al. Figure 15 Graphic representation of the decompaction estimates at times 1 to 3 (T1 T3). T1, beginning of basin filling (top bmb); T2, basin half-full (top YF6 or L3); T3, beginning of reef cover sequence. Oodnaminta Hut section B remains the same thickness from T1 to T3 and the thickness of section C gives an indication of water depth and margin relief at these times. THF, Tapley Hill Formation; BF, Balcanoona Formation; YF, Yankaninna Formation; bmb, basinal megabreccia. for the whole succession as the underlying units undergo compaction with the deposition of each new unit. Therefore, following progradation of the reef (deposition of the Balcanoona Formation) the difference in thickness between Oodnaminta Hut section B and C represents the depth of water to base of the slope. Following the terminal margin-collapse event, margin relief (elevation from the top of the basinal megabreccia unit to the top of the Balcanoona Formation) was *1100 m (after backstripping) (Figure 15). Assuming that the top of the reef margin represents approximate sea-level at the time, this value also represents the depth of water at the top of the megabreccia unit, at the beginning of the basin-filling phase. Assuming that sealevel remained approximately constant throughout the period of basin filling, water depths can be estimated for each unit in Oodnaminta Hut section B. Following deposition of the third laminated-limestone unit (unit YF6 in the decompaction exercise: Figure 15) the seafloor was at *426 m depth. However, if sea-level was lower than the top of the platform (as indicated by karstification) this depth might be an overestimate. Importantly, if the margin had relief of *1100 m, the majority of the organisms responsible for reef-building must have been growing in excess of 500 m water depth and were non-photosynthetic. After the remainder of the Yankaninna Formation was deposited, the basin had filled and shallow-water environments existed throughout the region (Figure 15). Deposition of wave-rippled siltstone and stromatolites in the lower part of the overlying Amberoona Formation would then have occurred at depths above fair-weather wave-base. A return to finer-grained sediments (shale and fine siltstone) above this interval signifies transgression and deeper water deposition for the remaining Amberoona Formation. Timing of reef progradation and extinction The stratigraphic and sedimentological data presented places constraints on the timing of reef progradation and extinction. Shallow-water carbonate development is likely to have occurred during deposition of the quartz sandstone unit of the Tapley Hill Formation. In both regions studied, edgewise-conglomerate beds containing thin, tabular limestone clasts occur below the quartz sandstone unit and before the appearance of turbidites, indicating the initiation of platform development. The presence of large platform-derived allochthonous blocks in the middle part of the overlying transitional shale unit suggests that a steep slope and significant margin relief had developed by this

18 Cryogenian interglacial carbonates 923 stage. The appearance of the first large block essentially represents the oldest recorded reef margin in the vicinity. Progradation of the Oodnaminta Reef (*20 km as exposed in continuous outcrop) is therefore likely to have occurred during later Tapley Hill times, while the upper part of the transitional shale unit was being deposited. The progressive thinning of the transitional shale unit northward is the result of rapid progradation, increasing slope angle, and increased erosion at the base of the slope caused by downslope mass flows. This supports approximate correlation of the Oodnaminta Hut reef complex with the Balcanoona Formation at Mt Jacob (Figure 16). At Illinawortina, where the basinal megabreccia unit consists of smaller, dispersed allochthonous blocks, there is no evidence for significant submarine erosion, yet the Tapley Hill Formation is considerably thinner than in the other sections. This may be due to its deeper water setting, where sedimentation rates were significantly lower than in more proximal settings. If the Illinawortina section does record a condensed sequence, then the entire period of reef progradation may be constrained to the upper *40 m of the Tapley Hill Formation in this section, immediately below the basinal megabreccia horizon (Figure 4). Figure 16 The timing of early platform development and progradation of the Oodnaminta Reef indicated on the Mt Jacob section. Early platform development probably began before deposition of the quartz sandstone unit of the Tapley Hill Formation and reef progradation was approximately contemporaneous with deposition of the Balcanoona Formation in the Mt Jacob region. The timing of the transition from syn-reef to postreef sediments (reef extinction) can also be established from the off-reef section (Oodnaminta Hut section C). Deposition of the basinal megabreccia unit (base of Yankaninna Formation) represents the final margin collapse event and is coincident with the termination of progradation (Figure 17). The cause of this event may be related to reef extinction (or at least a reduction of productivity) followed by the initiation of erosion, and/or subaerial exposure of the platform during regional late-sturtian regression. Low sea-level is evident in the Brighton Limestone and Balcanoona Formation across the Adelaide Geosyncline, which generally display shallowing-upward trends and commonly have identifiable erosional disconformities in their upper parts (Coats & Blissett 1971; Preiss 1987; Preiss et al. 1998). Exposure of the Oodnaminta platform during this time is consistent with the presence of sandstone/breccia-filled cavities at the top of the backreef facies, which we interpret as dissolution cavities caused by subaerial weathering. Although the age of this karst surface is difficult to determine in relation to the terminal mass-wasting event, it is possible that the two are synchronous. This exposure surface may be equivalent to the regional sequence boundary proposed by Preiss et al. (1998) to define the Sturtian Marinoan boundary. The continuance of this boundary along the submarine erosion surface at the base of the Yankaninna Formation, as proposed by Preiss et al. (1998), would be appropriate if the timing of the margin collapse was indeed coincident with the lowstand. Despite major margin failure at this time, the presence of small (510 m), undolomitised allochthonous reef blocks (containing non-stromatolitic framework fabrics) in the shale interval above the basinal megabreccia indicates that some reef growth continued after the margin-collapse event. The occurrence of these smaller limestone blocks in discrete horizons may indicate a final phase of intermittent and sparse reef growth. Above this interval (in the limestone units of the Yankaninna Formation), all allochthonous blocks lack reef-framework fabrics (both stromatolitic and non-stromatolitic), suggesting that reef growth had terminated by this stage. The presence of these laminated limestone units may be an indication of further sea-level fall, which reduced carbonate production on the platform and shifted the locus of deposition from the neritic zone to deeper-basin environments (Ridgwell et al. 2003) (Figure 17). This is consistent with the interpretation that the Balcanoona Formation remained emergent during the earliest Marinoan time, during deposition of the Yankaninna Formation (Preiss et al. 1998). Furthermore, the occurrence of a large regression (enough to subaerially expose much of the platform) may be an indication of global climatic cooling. The cessation of reef growth at the same point suggests the onset of conditions unsuitable for the survival of reef-building organisms, and it may be that low seawater temperature resulting from this global cooling was ultimately responsible for reef extinction. It seems unlikely that margin collapse or sea-level fall alone would have caused reef extinction.

19 924 J. A. Giddings et al. given that the majority of the reef-building organisms grew in several hundred metres of water even a large eustatic sea-level fall could not have resulted in the complete cessation of reef growth. Following the shutdown of platform progradation and reef extinction, sediments of the upper Yankaninna Formation filled the basin to the north, and carbonate and siliciclastic sediments of the Amberoona Formation subsequently buried the reef. The succession then shallows upward again and at the time of deposition of the Wundowie Limestone Member of the Enorama Shale, shallow-water environments probably prevailed across much of the basin. Following this phase of widespread shallow-water deposition, subsidence and transgression then led to deposition of the Enorama Shale. This was then followed by another regression and erosion heralding the onset of Marinoan glaciation. CONCLUSIONS Figure 17 Diagrammatic representation of the geological history of the Oodnaminta Reef. (a) Reef progradation and oversteepening of the margin. (b) Margin failure resulting from partial reef extinction and the initiation of erosional processes, leading to slope instability. (c) Global cooling and regression followed by reef extinction and basin filling. Limestone units in the lower Yankaninna Formation may be the result of platform exposure and the shifting of carbonate deposition to deeper-basin environments. (d) Basin filling and burial of the reef and platform. Water depths are taken from the decompaction exercise (Figure 15). Reef progradation continued after previous margin failure events (evident in large dolomite blocks in the upper Tapley Hill Formation beneath the reef), and (1) The stratigraphic framework described for the Umberatana Group has constrained the origin of the enigmatic submarine escarpment near Oodnaminta Hut (northern Adelaide Geosyncline). Here the Balcanoona Formation displays distinctive backreef (platform), reef-margin and forereef (slope) facies, with a geometry that reflects the progradation of a large reef complex. (2) Our results confirm that deposition of the Tapley Hill Formation following the Sturtian glaciation predominantly occurred in deep-water environments, consistent with a large transgression following global deglaciation. The development of accommodation space during deposition of the middle Tapley Hill Formation subsequently allowed the reef complex to advance at least *20 km into deep water (in a north to northeast direction), obtaining a maximum height of *1100 m. The presence of non-stromatolitic (microbial-like) frameworks deposited in several hundreds of metres of water indicates that the dominant reef-building organisms were non-photosynthetic. (3) Bed-correlation between the Oodnaminta Hut and Illinawortina sections demonstrates that reefgrowth occurred in a period equivalent to *40 m of deposition in the basinal environment (Illinawortina section), implying rapid progradation. Low sea-level and/or a shutdown of biological productivity probably led to massive margin failure and the erosion of the margin into its present form. Evidence for regional regression (karstification at the top of the Balcanoona Formation) may reflect a global climatic cooling event that, together with platform exposure, eventually led to reef extinction. (4) The internal and regional geometry of the Oodnaminta Reef, and the organisation of the biological frameworks resemble some Phanerozoic reefs. The occurrence of such reefs during the Cryogenian is of importance because it may suggest the presence of tropical conditions and environments favourable for the development of large, rigid biological frameworks during this period of extreme climatic fluctuation.