X. The Eucla Basin in South Australia

At some time prior to the Ptychagnostus gibbus Zone of the Middle Cambrian the area of deposition of Upper Precambrian (or Lower Cambrian) well‐sorted sands, silts and dolomite was affected by tectonic movements producing uplift of the Tyennan Geanticline and change in the shape of the depositional basin (Spry, Chapter I). Continued tectonic activity and more rapid sinking of the sea floor resulted in a change in sedimentary association from well‐sorted sediments of the orthoquartzite‐limestone suite to poorly sorted sediments of the greywacke suite. Initially siltstone was the main deposit in the Dundas, Huskisson River, Ulverstone, Deloraine and Beaconsfield areas and this has been likened to the initial euxinic phase of geosynclinical development elsewhere (Campana, 1961b).

Silt seems to have been the predominant normal deposit during the Middle and early Upper Cambrian, but siliceous oozes and some limestone were also formed. Carbonaceous, pyritic and calcareous silts were deposited. Inter‐bedded with the silts are poorly‐sorted greywackes and greywacke conglomerates with a disrupted framework and graded bedding. Banks and Jennings interpret these as mostly turbidity current deposits. The proportion of greywacke and conglomerate varies through the successions in a cyclic manner (Carey and Banks, 1954; Banks, 1956) such that a conglomerate‐rich section is followed by a greywacke‐rich section and this by a predominantly lutaceous section. These cycles may be interpreted as due to tectonic instability and variation in height of the source area. Faulting of Upper Middle Cambrian and Lower Dresbachian age has been demonstrated near Ulverstone. Campana and King state: “The proportion of coarse material increases upwards in the Dundas and Huskisson successions at least.”

Turbidity currents brought fragments of grey, red, black and banded cherts, banded slate, quartzite, basalt and golden mica (this last presumably from breakdown of Precambrian mica schist) to the Dundas area. In view of the known distribution of chert in western Tasmania a westerly or north‐westerly source is likely. Turbidity currents deposited fragments of chert, claystone, quartzite, slate, greywacke, quartz mica schist, chloritised basic lava and spilite in the Deloraine area indicating a source area with Precambrian rocks and earlier Cambrian sediments and lavas. Near Rocky Boat Harbour the source area contained dolomite, ultrabasic rocks, granite, and Precambrian quartzites and schists.

A difference between the fauna in the silts and in the greywackes is evident in the Hodge Slate at Dundas and the Kateena Formation near Ulverstone at least. The “dendroids” in the Hodge Slate are in the siltstone and the fragmentary trilobites and cystoids in the greywacke. This suggests that the fossils in the greywackes are thanatocoenotic as might be expected and introduces the possibility of remanié fossils and of shallow water fauna intercalated with deeper water fauna. The bathymetric conditions suggested by Hills and Thomas (1954) for the Cambrian of Victoria may thus not be applicable to Tasmania.

Deposition was also interrupted from time to time by lava flows, some of them, at least, submarine. The Mt. Read Volcanics may be Lower Cambrian but acid and basic lavas and pyroclastic rocks are interbedded with or overlie Middle and Upper Cambrian sediments at Zeehan, Dundas, Ulverstone, Smithton and Beaconsfield. Acid volcanic rocks are commoner near the Tyennan Geanticline and basic rocks further away. Possibly during the Dresbachian ultrabasic rocks were intruded as sheets and dykes into Precambrian and earlier Cambrian rocks and by Franconian time were exposed to erosion at Adamsfield.

Deposition may have commenced later at Smithton (Upper Middle Cambrian), Beaconsfield (Lower Dresbachian) and Adamsfield (Lower Franconian) than at Dundas (Lower Middle Cambrian).

Campana and King express the thoughts of Bradley (1957, pp. 114–115) and the author when they state: “The Dundas Group reflects a eugeosynclinical cyclic sedimentation under unstable tectonic conditions. The group is no doubt a synorogenic suite comparable with the Flysch as it was deposited in the narrow subsiding Dundas Trough which developed along the Mt. Read Volcanic Arc, and which is similar to the present deeps of archipelago areas. Such a comparison is enhanced by the succeeding Ordovician conglomerates and sandstones, comparable in some respects with the molassic deposits which displaced the Flysch sedimentation in the Pre‐Alpine troughs (Fig. 12).”

The Cambrian rocks were folded or tilted at least along the western and northern margin of the Tyennan Geanticline and near New River Lagoon, the Tyennan Geanticline was rejuvenated, the Asbestos Range Geanticline raised and the highland areas near Ulverstone and Zeehan uplifted late in the Cambrian or very early in the Ordovician.