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Volume 80, 1952
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Australasian Lower Devonian Shoreline

It is at this point that the paucity of our knowledge becomes most apparent. The following is a summary of available evidence as interpreted by the writer:

(1)

The more or less meridionally aligned deposits of Lower Devonian age in Eastern Australia are part of the enormous Tasman Geosyncline (Schuchert, 1916; Bryan, 1933, 1944; Andrews, 1938; Browne, 1947). This is a structure of world significance—a first order geosyncline. The sediments incorporated in this structure must have extended laterally something like 500 miles before being folded against the Australian shield, thus forming many of the mountain areas of Eastern Australia, including Tasmania.

The geosyncline extends from Queensland to Tasmania, but it apparently did not reach Antarctica. However, the geosyncline is still a major structure in the south of Tasmania. The large submarine bank south of Tasmania (vide fig. 2) is still probably a block of similar structure to Tasmania which has been let down under the sea in the general foundering of the crust between Tasmania and Antarctica.

The classification of the Tasman Geosyncline (vide Glaessner and Teichert, 1947) depends on whether the Tasman Sea area was an expanse of sea or land during the time concerned. If it were sea, then the Tasman Geosyncline may be regarded as an orthogeosyncline. The interpretation accepted in the accompanying palaeogeographic maps regards the Tasman Geosyncline as a parageosyncline developed between massifs in the big Austzealandic* continent. This interpretation pictures two more or less parallel geosynclines of large size—the Tasman Geosyncline and the New Zealand Geosyncline (so named by Schuchert in 1916). These geosynclines were preceded by earlier ones represented by the metamorphosed sediments of pre-Cambrian to early Cambrian age in Australia, New Zealand, and Antarctica.
(2)

The Lower Devonian beds of the Tasman Geosyncline carry faunas of both in-shore (Rhenish) and off-shore (Bohemian) facies. They provide evidence of a Lower Devonian shoreline running roughly meridionally down the east side of Australia. Lower Devonian faunas have been recognized in East Borneo (Rutten, 1940), Queensland, New South Wales, Victoria, and Tasmania. The presence of lavas and tuffs interbedded with Lower Devonian sediments in Eastern Australia indicates a cycle of vulcanism, which no doubt was associated with a crustal instability to be associated with the edge of a large geosyncline. The presence of radiolarian cherts of this age in East Borneo suggests deeper waters than prevailed at that time in the Tasman Geosyncline, where sandstones, quartzites, shales, and limestones predominate. The Queensland radiolarian cherts belong to a later period, viz. Middle Devonian (Bryan and Jones, p. 30).
(3)

The Lower Devonian beds in New Zealand form a part of another large geosyncline, enclosing strata with a long time range.
(4)

The more they are studied, the more alike one another the New Zealand and East Australian Lower Devonian faunas are found to be. Further *

[Footnote] * The name “Austzealandia” is here proposed for the palaeogeographic continent because the name “Australasia” stands for Australia and New Zealand as separate entities, and so does not lend itself to the description of a structure involving the space between the two areas in addition to the areas themselves,

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evidence of this similarity is provided in this paper (fig. 1). Of special significance is the description from Victoria and Tasmania of three species of the brachiopod Maoristrophia, a genus previously known only from New Zealand (Allan, 1947; Gill, 1948c).
(5)

A shoreline connected the beds in New Zealand with those in Australia, and the most likely location for this, in the writer's opinion, was to the south, because:
(a)

There are no Silurian or Lower Devonian rocks in Antarctica, as far as is known. This suggests that the area was a land surface during the Lower Devonian. Some Upper Devonian fish plates have been found at Granite Harbour in the Ross Dependency at the base of the Beacon Sandstone, which is mostly Gondwana in age (Woodward, 1921; Seward, 1914; David and Priestley, 1914, p. 243). Moreover, the Beacon Sandstone appears to belong to a different tectonic cycle from that involving the Lower Devonian beds further to the north.
(b)

The close similarity between the Lower Devonian faunas along the Tethys seaway (cf., for example, those from Indo-China) and those in Australia indicates a ready migration route from waters to the north of Australia into Australian waters. The string of Lower Devonian deposits from Northern Queensland (Chillagoe limestone) right down to Tasmania likewise indicates an uninterrupted coastline.
(c)

The close similarity between the south-east Australian and New Zealand Lower Devonian faunas indicates a ready migration route along a coast between those two areas. (b) and (c) together thus indicate a ready migration route—and therefore coastline—all the way from the Tethys seaway to New Zealand. To draw a palaeogeographic map connecting the northern part of Australia with New Zealand in a continuous land mass is not in keeping with the presence of the Tasman Geosyncline, nor with the palaeontological evidence put forward in this paper.
(d)

The series of coral bioherms of both Upper Silurian and Lower Devonian age extending down the east Australian states into Tasmania, suggests freedom from colder polar currents.
(e)

The isolation in Lower Devonian time of the Australia-New Zealand region (part of the Boreal marine palaeobiological province) from South America and South Africa (the Austral province, recently renamed the Malvinocaffric province—vide Rud. and E. Richter, 1942) demands an arrangement of land and sea which biologically isolates Australia and New Zealand from South America and South Africa. A land bridge connecting Australia and New Zealand with Antarctica would materially assist such isolation.

Allan (1947) has recently shown that the Malvinocaffric element previously (1935, 1942, 1945) believed present in the brachiopod fauna of the Reefton Beds does not really exist. There was also an alleged Malvinocaffric element in the trilobite fauna. Allan described Homalonotus (Burmeisteria) huttoni, noting that the subgenus was based on a species from beds in South Africa and the Falkland Islands. However, in a paper on Homalonotid trilobites, the writer (1949b) commented that “The presence of tubercles on Allan's species suggests its association with Burmeisteria, but it cannot be so placed because the facial sutures cut the genal angles. It is probably

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Text-figure 1

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better accommodated in Trimerus.” Allan also re-described Homalonotus (Digonus) expansus Hector, but Digonus like Trimerus is a Boreal form.

It would thus appear that the trilobites, like the brachiopods, show no Malvinocaffric affinities. The Australia-New Zealand region was biologically isolated from the South American-South Africa region as far as marine faunas are concerned. This is an important fact to be taken into account when drawing palaeogeographic maps. It does not encourage the theory of a Samfrau geosyncline.

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Text-figure 2

(6)

As already noted, the Tasman Geosyncline was of great lateral extent. Keeping in mind the principles of marine sedimentation, it is to be expected that sediments were provided from both sides of the geosyncline in order to give so wide a lateral spread of sediments all belonging to waters of moderate depth. A “Tasmantis” borderland on the east side of the Tasman

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  • Geosyncline has already been hypothecated (Bryan, 1933, fig. 1; Andrews, 1938, pl. 1; Jensen, 1926, p. 151), and this may well have extended far enough eastwards to provide a western borderland for the New Zealand Geosyncline. This is in keeping with the fact that islands like New Caledonia have no Silurian and Devonian rocks in them. The pre-Cambrian rocks there may well have been part of a land mass in Lower Devonian times. In view of the probable permanence of the Pacific Ocean (Marshall, 1948), the tectonic importance of the Marshall Line (Bryan, 1944; de Jersey, 1946), and the sedimentational likelihood that the big New Zealand Geosyncline possessed a borderland to the east, such has been inserted in the palaeogeographic maps (figs. 2, 3). This interpretation is supported by the fact that the Chatham Islands and Fiji Group have a lacuna in their stratigraphical column similar to that found in New Caledonia. So the Chatham Islands and Fiji areas were probably land surfaces in Lower Devonian times.

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