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Interglacial Carbonates of the Umberatana Group, Northern Flinders Ranges, South Australia

The Umberatana Group in the northern part of the Adelaide Geosyncline has been subjected to a detailed sedimentological and chronostratigraphic analysis. The results of this study have uncovered the depositional history that includes at least 20 km of a giant reef complex, with a relief of 1.1 km,  from the mid-Cryogenian interglacial that prograded rapidly. Situated in the Balcanoona Formation, the reef complex displays facies similar to reefs from the Phanerozoic. Included among these are a basal forereef (slope) facies, above which are reef-margin facies, of both stromatolitic and non-stromatolitic frameworks, and an upper backreef (platform) facies comprised of shallow-water peloidal and oolitic carbonates. As the reef thickens in a basinwards direction, and the distribution of key facies, is consistent with progradation into deep water of the platform. Deposition of the upper Tapley Hill Formation was contemporaneous with progradation and following a major margin failure event deposition largely ceased. Reef growth continued for a short time after this event but eventually became extinct. The authors1 suggest the extinction could possibly been the result of global cooling of the climate, and/or eustatic sea level fall.

Global scale extreme environmental change characterised the Late Neoproterozoic, about 850-542 Ma. At this time there was severe glaciation on a global scale, the composition of the ocean and atmosphere were changing, and in the oceans early life forms that were evolving towards the stage of the Cambrian Explosion were gradually increasing in complexity. All of these factors add to the difficulty of understanding the evolution of the Earth1 (Hoffman et al., 1998a, b; Knoll & Carroll, 1999). An adequate understanding of the depositional environments and stratigraphical relationships during the successions of the Neoproterozoic is important for gaining an understanding of state of the environment of that time. The authors1 suggest the physical and geochemical clues ultimately hold the clues to help in gaining an understanding of many of the questions raised by study of these distant times.

The record of the Cryogenian Period and the Ediacaran Era are relatively complete in the Adelaide Geosyncline, Flinders Ranges, South Australia, including 2 global glacial periods in the Mid- and late-Cryogenian. An unusual outcrop in the Umberatana Group, a thick interglacial succession, are thick deposits of carbonate platform, about 1 km thick, from the Balcanoona Formation. For a distance of about 20 km the platform outcrops are continuous along strike then thin and end abruptly near Oodnaminta Hut, at which point they are onlapped by carbonate deposits and siliciclastic sediments of the Yankaninna Formation, that are of deep-water origin. Previously this feature was interpreted as a sharp transition from a shelf to a basin (Preiss, 1987) that is characterised by a steep 'submarine escarpment.' (Coats & Blissett, 1971; Preiss, 1987), a specific origin for the transition from platform to basin has not been determined until now.

The authors1 propose that a giant prograding reef complex is represented by the Balcanoona Formation near Oodnaminta Hut, the enigmatic submarine escarpment being the eroded margin of the reef complex. They based their proposal on sedimentological and stratigraphic data from the Umberatana Group in the region to the east and west of the Paralana Fault of the northern Adelaide Geosyncline. Their detailed study covered the sediments, of post-glacial age, of the Tapley Hill Formation, the platform carbonates from the Balcanoona Formation and the onlapping basinal sediments of the Yankaninna Formation.

Mildly deformed Neoproterozoic to Lower Cambrian synrift, post-rift and sediments that are probably of passive margin origin, that a have a stratigraphic thickness of more than 10 km, make up the Adelaide Geosyncline (Adelaide Fold Belt)(Adelaide Rift Complex). The name Adelaide Geosyncline was used by the authors1 to refer to the basin and its accumulated sediments in a non-genetic sense (Williams et al., 2008). As Rodinia was fragmenting and the Palaeo-Pacific Ocean was opening about 830 Ma these basin sediments were deposited unconformably above a crystalline basement of Mesoproterozoic age (Coats & Blissett, 1971; Preiss, 1987, 2000).

To the west the complex is flanked by the Gawler Craton, with its platform cover that is relatively undeformed, the Stuart Shelf, and to the east by the Curnamona Craton, both of which formed the margins of the intracontinental rift basin that was subsiding (Preiss, 1993; McKirdy et al., 2001). During the Delamerian Orogeny of the Cambrian-Ordovician the basin was deformed (Coats & Blissett, 1971; Preiss, 1987, 2000). An almost complete record of sedimentation, through what the authors1 describe as one of the important eras in Earth history, is provided by its current exposure.

There are 4 groups that make up the Neoproterozoic succession, the Umberatana Group comprising both glacial successions from the Cryogenian, Sturtian and Marinoan, and is the focus of this study by the authors1. Not many reliable ages have been determined for the Umberatana Group though a U-Pb age of about 650 Ma has been obtained from a tuffaceous horizon in the uppermost part of the Sturtian diamictites (Fanning & Link, 2006). This age agrees approximately with the Re-Os age of 643 ± 2.4 Ma that was obtained from the basal Tindelpina Shale Member of the Tapley Hill Formation (Kendall et al., 2006). The age of the Marinoan glacial event, represented by the Elatina Formation, can be tentatively set at 635 Ma by correlating the Elatina Formation with the Ghaub Formation from Namibia and from southern China, the Nantuo Formation, assuming global glaciation occurred approximately synchronously. According to the authors1 an age of about 580 Ma is preferred for the Elatina Formation by Calver et al., 2004. A U-Pb zircon age of 635.5 ± 1.2 Ma was obtained from a tuff near the top of the Ghaub Formation (Hoffman et al., 2004). A U-Pb zircon age 635.2 ± 0.6 Ma has been obtained from a tuff bed in the cap carbonate overlying the Nantuo Formation (Condon et al., 2005). According to the authors1 the ages of the latter 2 formations provide an age of approximate ending of the Marinoan Glaciation in Namibia and China respectively. They say that by correlating these formations with the Elatina Formation provides constraints leaving a length of time of more than 15 My for the Umberatana (Cryogenian) interglacial sequence.

Mt Jacob Region

Oodnaminta Hut and Illinawortina regions

Balcanoona Formation

Yankaninna Formation

Amberoona Formation

Enorama Shale and Wundowie Limestone Member

 

Sources & Further reading

  1. Giddings, J.A., Wallace, M.W. and Woon, E.M.S., 2009, Interglacial carbonates of the Cryogenian Umberatana Group, Flinders Ranges, South Australia, Australian Journal of Earth Science (2009) 56 (907-925). 
Author: M. H. Monroe
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Last Updated 07/05/2012

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                                                                                           Author: M.H.Monroe  Email: admin@austhrutime.com     Sources & Further reading