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The AAPG/Datapages Combined Publications Database

CSPG Special Publications


Permian Triassic Systems and Their Mutual Boundary — Memoir 2, 1973
Pages 425-432

The Geological Sequence and the Permian-Triassic Boundary in Australia and Eastern New Guinea

J. M. Dickins


Upper Permian marine sequences are limited in area in comparison with Lower Permian. Uppermost Permian marine rocks have not so far been recognized in Australia and lowermost marine Triassic (Otoceratan) is known only from the Perth Basin. Brackish water beds of the same age are found in the Canning and possibly the Bonaparte Gulf Basin.

In the western part of Australia, Permian and Triassic rocks are known in the Bonaparte Gulf, Canning, Carnarvon and Perth Basins. The rocks in all basins are apparently separated by hiatus but without marked structural discordance. In the Bonaparte Gulf and Canning Basins, lagoonal or brackish water Lower Triassic overlies marine Permian but uppermost Permian is apparently lacking. In the Carnarvon Basin, non-marine sandstone is regarded as Upper Permian, and Lower Triassic with overlying Middle to Upper Triassic has recently been identified in drilling. In the Perth Basin, Otoceratan and younger Triassic in places overlies Upper Permian non-marine rocks with an hiatus. The position of the Permian-Triassic boundary is based on marine invertebrates and spores.

In eastern Australia, marine Lower and Upper Permian passes up into nonmarine Upper Permian and Triassic, in the Bowen, Sydney and Tasmania Basins. The boundary has been placed at the highest occurrence of the Glossopteris - flora and its associated microflora but currently this interpretation is under critical review. Breaks are recorded but their exact relationship to the boundary is not clear. The Tasman Geosyncline in eastern Australia was subject to instability and considerable igneous activity during the Permian and possibly into the Triassic. Radiometric dating shows a period of widespread granitic intrusion close to the Permian-Triassic boundary.

In the central part of Australia, non-marine Permian and Triassic are generally separated by hiatus.

In eastern New Guinea, marine rocks considered to be Permian lie on the erosional surface of a granite which gives a radiometric age close to the Permian-Triassic boundary. Lower Triassic has not been recognized.

The Early Permian is a time of widespread marine transgression and an overall cool climate. Withdrawal of the sea commenced in the Late Permian, and regression continued until latest Permian when it apparently retreated entirely from the present land mass. The climate became milder, and by Triassic time warm or even tropical conditions existed. In eastern Australia mountains were formed which cut off the inland basins from the sea. From the evidence available the radical transformation of the fauna during Permian and Triassic time may well have been connected with the change in paleographical and climatic conditions.

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