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THE EDIACARAN QUESTION: NORTH AND SOUTH

RICHARD. J. F. JENKINS and CHRISTOPHER NEDIN

>Department of Geology and Geophysics, University of Adelaide, South Australia 5005

Jenkins, R.J.F. & Nedin, C. 1998. The Ediacara question: north and south. Inaugural Sprigg Symposium, University of Adelaide. Geological Society of Australia, Abstracts, 51: 25.

ABSTRACT

Ideas concerning formal international chronostratigraphic divisions within the wider Proterozoic have an extended history (e.g. Harland and Herod 1975). The "Ediacarien" Stage of Termier and Termier (1960) emphasised the significance of the evolutionary appearance of megascopic soft-bodied metazoans near the close of the Proterozoic, and the same concept was linked to the specific setting of the Flinders Ranges with the nomination of an Ediacaran System and Period by Jenkins (1981) and parallel consideration of an Ediacarian System by Cloud and Glaessner (1982). The lower boundary of the System of Cloud and Glaessner reflects the popular idea that a closing glacigenic cycle within the Proterozoic (the Elatina 'glaciation') preceded major metazoan radiations.

Two well recognised glacigenic intervals are present in the local thick Neoproterozoic succession, the 'Sturtian' and 'Elatina' mixtites (or equivalents) of the Umberatana Group, identified herein as "refrigerations" I and II respectively. New evidence points to the Bunyeroo Formation representing a further possible refrigeration "III" within the Wilpena Group and dropstones low in the Billy Springs Formation indicate yet another refrigeration "IV".

Investigations by others in the Kimberley of northwestern Australia (e.g. Cockeran et al. 1996, Plumb 1996a, b) have led to the view that two 'Marinoan' (later Neoproterozoic) glacigenic intervals occur in sequence, the older represented by the Walsh and/or Landrigan Tillites being the equivalent of the Elatina ‘glaciation’ in the Flinders Ranges (Fig. 1). The younger Egan Formation shows parallels with the Bunyeroo Formation in its general homotaxial placement, including its situation above an erosion surface developed on a local succession resembling the Sandison Subgroup. Similarly, in the northern Amadeus Basin of central Australia, the discovery of several 'relict' successions predating the glacigenic Olympic Formation or its lateral passage into conglomerates suggests that this refrigeration corresponds to the Bunyeroo Formation and its cap-carbonate is the equivalent of the Wearing Dolomite. An erosive surface is present at the top of the Olympic Formation cap-carbonate, preceding the shales of the Pertatataka Formation. This level is the likely equivalent of the placement of the proposed GSSP at the Wearing Dolomite in the Flinders Ranges. The secular delta13CCARB isotopic record over these intervals is strikingly similar in both regions.

The correlative stratigraphies of the Elatina Formation through the later Neoproterozoic in the Flinders Ranges and central Australia are so similar to the Vestertana Group of the north Finnmark area of Norway as to suggest the possibility that Baltica and the Australian Shield were conjugate at the time. There is a simple geometric solution to Australia occupying a "North Atlantic" placement during the later Neoproterozoic. The Elatina Formation may be correlative with the Smalfjiord Formation of Finmark and the Bunyeroo and Olympic Formations are the homotaxial correlatives of the Mortensnes glacials.

Harland and Herod (1975) linked their Ediacaran to the close of the ‘Varangian’ glaciation, or by implication the top of the Mortensnes Formation, the possible homotaxial equivalent of the proposed GSSP herein.

Rb-Sr measurements for shales corresponding to the Bunyeroo Formation and older Wonoka Formation in the Officer Basin and on the Stuart Shelf may be interpreted as generating a 9 point model II errorchron of 646 Ī16Ma (MSWD 2.0). The alignment may be compared with a plausible ‘younger’ regression of 648 Ī26 Ma (7 point model 3 errorchron, MSWD 3.7) for the data correlative with or near the upper glacigenic level in the Kimberley. Thus possibly proposed GSSP is of the order of 640-650 Ma old, making the duration of the Ediacaran System c. 100 Ma long.

The delta13CCARB secular isotopic record synthesised from the Officer Basin (Dey Dey Mudstone through Munyarai Formation), not only correlates between the Flinders Ranges succession (Bunyeroo to upper Wonoka-Billy Springs Formations) on the one hand, but gives a strong correlation with the Mackenzie Mountains of northwestern Canada (Keele through to mid Sheepbed Formations). The Wonoka and Twitia Formations are isotopic correlatives and the presently proposed GSSP position on the Wearing Dolomite corresponds to the upper sharp boundary of the Rapitan Group (Shezal Formation) and with the base of the Twitia. The erosive surface below the Narana Formation corresponds to the basal Nama Group unconformity of Namibia, a correlation supported by new sequential fossil finds in the Rawnsley Quartzite. In the Mackenzie Mountains this equivalence is with the upper Sheepbed through Risky Formations.

REFERENCES

Cloud, P. and Glaessner, M.F., 1982. Science,, 783-792.
Cockeran, M., Grey, K., Li, Z.X. & Powell, C. McA., 1996. Geological Society of Australia, Abstracts, 41: 97.
Harland, W.B. and Herod, K.M., 1975. Geological Journal Special Issue, 6, 189-216.
Jenkins, R.J.F., 1981. Transactions of the Royal Society of South Australia, 105, 635-643.
Kaufmann, A.J., Knoll, A.H. & Narbonne, G.M., 1997. Proceedings of the National Academy of Science, USA, 94, 6600-6605.
Plumb, K.A. 1996a. Geological Society of Australia, Abstracts, 41: 344.
Plumb, K.A. 1996b. 30th International Geological Congress, Abstracts, 2, 51.
Termier, H. and Termier, G., 1960. Revue Générale des Sciences Pures et Appliquées, 67, 79-87.