Revised stratigraphy and first geochronology of the Miocene submarine volcanic succession at Kennaook/Cape Grim, northwestern Tasmania

Abstract

Kennaook/Cape Grim in far northwestern Tasmania, Australia, was a site of submarine intraplate basaltic volcanism during the Miocene. The succession is exceptionally well preserved and is dominated by pillow lava, massive lava and pillow fragment breccia. The total volume of volcanic products (1.1 km3) is relatively small on a global scale, but the preservation is world class. The oldest unit, the Woolnorth Tuff lies unconformably on the Neoproterozoic Rocky Cape Group. The first 40Ar/39Ar dating of the volcanic rocks reveals an early Miocene (24.5–23 Ma) age for the entire sequence. The Woolnorth Tuff is composed almost entirely of devitrified basaltic glass shards and olivine crystal fragments. The Woolnorth Tuff is overlain by the Slaughter Bluff Volcanic Breccia (23.12 ± 0.19 Ma) at Kennaook/Cape Grim. The Slaughter Bluff Volcanic Breccia is dominated by diffusely bedded pillow fragment breccia. North of Kennaook/Cape Grim, the Little Trefoil Basalt (ca 24 Ma) intrudes the Woolnorth Tuff. To the south of Kennaook/Cape Grim, the Woolnorth Tuff is overlain by the Studland Bay Basalts (23.73 ± 0.08 Ma) and intruded by a newly recognised unit, the Hippo Basalt (24.52 ± 0.12 Ma). The Studland Bay Basalts comprise mounds of basaltic pillow lavas followed by a succession of diffusely bedded, matrix-dominated, pillow fragment breccia and basaltic breccia. Detailed field mapping and sampling have revealed that the environment of deposition of all Kennaook/Cape Grim units was submarine and that they were emplaced in relatively rapid succession. KEY POINTS First 40Ar/39Ar geochronology for the Kennaook/Cape Grim volcanic succession. Little Trefoil Basalt has been re-interpreted from a subaerial extrusive unit to a submarine intrusive unit. A new intrusive unit, the Hippo Basalt, has been recognised. The stratigraphy of the Kennaook/Cape Grim volcanic succession has been revised.