At the International Geological Congress held in Sydney, Australia, in August 1976 a Symposium of papers and discussion on ‘The Precambrian–Cambrian Boundary Problem’ was organized by the author in his capacity as Project-Leader/Chairman of the international Project 29 Working Group on the Precambrian–Cambrian Boundary. The following seven papers arose from this symposium. Project 29 is part of the current research under the auspices of the International Geological Correlation Programme (I.G.C.P.). which is sponsored jointly by the United Nations Educational. Scientific and Cultural Organization (U.N.E.S.C.O.) and the International Union of Geological Sciences (I.U.G.S.).

In the Anti-Atlas, trilobites occur below the zone of Fallotaspis tazemmourtensis of Hupé (1953) and even somewhat below the first archaeocyathids of the region. Most of them are opisthoparions (Hupetina gen.nov is described), and the olenellid Eofallotaspis gen.nov. probably is ancestral to Fallotaspis. The age is concluded not be younger than the lower Atdabanian of the Siberian platform. In the High Atlas, in the area called Ounein, a complete succession of trilobites from the low Lower Cambrian through the Middle Cambrian is being studied. The oldest trilobites encountered here are referred to the new genus Lemdadella. Correlation with the Anti-Atlas or other regions is uncertain.

Stromatolites of the groups Linella, Tungussia and Tifounkeia in association with thrombolites occur in abundance throughout the whole Lie de Vin Series, the middle part of the Adoudounian Group which consists of Calcaire lnférieur, Lie de Vin Series and Calcaire Supérieur. A Late Precambrian age is suggested for these stromatolites.

The oldest archaeocyathid fauna from Tiout (Anti-Atlas, Morocco) contains 15 species belonging to 11 genera. The following species: Aldanocyathus cribratus, Neoloculicyathus magnus, Coscinocyathus primus, C. secundus, C. fasciola, Tumulifungia marocana, Gordonifungia cowieiare new. The biostratigraphy of this archaeocyathid fauna demonstrates that the lower boundary of the Amouslekian stage should be moved downwards to within the ‘Calcaires Supérieurs’ of the Adoudounian (Assadassian or equivalent Ouneinian). One or perhaps more archaeocyathid and trilobite zones may yet be found beneath this boundary at Tiout. The uppermost ‘Calcaires Superiéurs’ may be dated, on the basis of Archaeocyatha, as equivalent to the Atdabanian stage, but not to the lowest part of it. The underlying fossiliferous horizons should be equivalent to all or part of the lower Atdabanian. The Precambrian–Cambrian boundary in Morocco can be looked for lower in the succession and defined on the basis of other fossil groups.

The Late Precambrian and Early Cambrian strata in eastern California and Nevada can be divided into 3 facies – the eastern Craton, the White-Inyo, and the Death Valley. These 3 facies supply evidence of 3 trilobite zones – the Fallotaspis, the Nevadella and the Bonnia-Olenellus; the inter-relationships are outlined. Correlations with the Siberian Platform successions are discussed and the Precambrian–Cambrian boundary is tentatively placed within the beds of the upper Reed Dolomite in the White-lnyo Range.

Two regions are considered to be the best for the definition of the Cambrian lower boundary in Siberia – in Yakutia (the middle course of the Aldan River), in Western Prianabarye (at the Kotuikan and Kotui). The stratigraphical relationships in these 2 regions are described. Some general problems of stratigraphy are then raised (a) the need to use lithologic-facies, palaeogeography and biostratigraphy together, (b) the boundary stratotype method of delimiting chronostratigraphical subdivisions and (c) the earliest trilobite zones, the Ediacara fauna and geochronology.

The discovery of Ediacara type faunas and later the Tommotian fauna at levels below the supposed first appearance of trilobites has brought new life to the discussion of the formal position of the Precambrian–Cambrian boundary. The oldest shelly faunas are apparently restricted to a few favoured areas, whereas transition beds developed as quartz sandstones, mostly unfossiliferous apart from trace fossils, seem to have a much larger regional distribution. The sequences of quartz sandstones overlie a Precambrian crystalline basement of a much greater age or clastic sediments, in part tillites, and are in turn overlain by a variety of rocks containing olenellacean trilobites.

The lowering to the Cambrian boundary makes the correlation of the widespread marine sandstones below levesl with trilobites highly pertinent. The correlation of some such sandstones in Greenland and Scandinivia is discussed below. In this account the Tommotial Stage is regarded as the basal Cambrian stage.

The area to the N and NE of the Amadeus Basin in central Australia is currently being studied by the Bureau of Mineral Resources as part of a detailed investigation of the Georgina Basin. This has led to the recognition of numerous rock units within a previously undifferentiated latest Precambrian to early Cambrian sequence, and preliminary correlations with the Amadeus Basin are possible. The sequence starts with the lower of the 2 late Precambrian tillites, and includes stromatolitic carbonates and ichnofossiliferous sandstones. These outcrop on the Hay River, Tobermory and Huckitta 1:250000 sheet areas. No early Cambrian carbonates are yet known from the eastern region, but occur on Huckitta in the Mount Baldwin Formation and in the Amadeus Basin in the Todd River Dolomite. These contain early Cambrian shelly fossils. It is tentatively suggested that the late Precambrian–early Cambrian sediments of the SW Georgina Basin are the platform equivalents of the basinal sediments of the Amadeus Basin, with fully marine sediments of this age occurring in the Georgina Basin only in NW–SE trending grabens that cut across the platform.

A magnetostratigraphic study of Upper Proterozoic to early Cambrian sediments at 4 localities in Central Australia reveals the presence of a magnetic polarity pattern composed of 2 long quiet intervals, first of reversed then of normal polarity, which is then succeeded by a long mixed interval. Lithologic comparisons suggest that this pattern is a reflection of the behaviour of the geomagnetic field at that time, and probably is not a result of varying sedimentation rates. Two sampled sections in the Arumbera Sandstone separated by 80 km both contain the long normal interval followed by the interval of mixed polarity, the detailed comparison of which demonstrates that the Arumbera-Box Hole boundary is unconformable. Elements from the late Precambrian Mt Skinner fauna in the Arumbera Sandstone indicate that the long normal interval is entirely pre-Cambrian, and that the base of the Cambrian occurs within the interval of mixed polarity.

A comparison with an additional site sampled across a fossil occurrence in the Central Mount Stuart Formation at Central Mount Stuart indicates that at least 1 of the Upper Proterozoic fossils, Hallidaya brueri, has a considerable vertical distribution in time.

If used properly, the magnetic polarity pattern discussed herein offers the possibility of precise intercontinental correlations of stratigraphic sequences near the base of the Cambrian. It is suggested that a stratotype sequence for the base of the Cambrian be chosen in such a manner that the boundary position corresponds to a geomagnetic reversal.

Nineteen members of the Working Group attended the meeting and constituted a good representation in experience, variety of specialist knowledge of disciplines within geology, and first-hand knowledge of appropriate global stratigraphy. The following resolutions as a whole were accepted, as an account of the meeting by those present, with a large majority. The minority of four who did not accept Resolution I requested that the Minority View (see below) should be recorded but it was decided not to include it in the Resolutions.