Studland Bay Basalts, Cape Grim, Tasmania, Australia - A world class exposure of submarine basaltic lavas.

Abstract:

The Studland Bay Basalts (SBB) are part of a predominantly submarine volcanic succession at Cape Grim in far northwestern Tasmania, Australia. Volcanism at Cape Grim was part of a broader episode of Cenozoic intraplate basaltic volcanism in Tasmania. The volcanic succession is exceptionally well preserved and exposed in rock platforms and steep coastal cliffs.

The oldest unit in the Cape Grim succession, the Woolnorth Tuff (WT) is composed almost entirely of devitrified basaltic glass shards and olivine crystal fragments; sedimentary structures are consistent with deposition in a sub-aqueous environment. The WT is overlain by the Slaughter Bluff Volcanic Breccia (SBVB). The SBVB is dominated by a 15-25 m thick diffusely bedded pillow fragment breccia. North of Cape Grim, the Little Trefoil Basalts intrude as sills into the WT.

To the south of Cape Grim, the WT is overlain and intruded by the SBB. The SBB consists of pillow lavas, lava lobes and sills, pillow breccia and volcanic conglomerate. The sills form ellipsoidal mound shapes ~50 m long with unique joint patterns. Pillow lavas and lava lobes are exceptionally well exposed and can be viewed in 3 dimensions. The basalts are typically olivine-phyric with a groundmass that includes plagioclase and pyroxene. A detailed field mapping and sampling project has revealed that the environment of deposition of all SSB units was submarine and that they were emplaced in relatively rapid succession. ³⁹Ar/⁴⁰Ar dating reveals an Early Miocene age for the entire sequence; the SBB sills have an age of 24.52 ± 0.12 Ma and the pillow lavas have an age of 23.73 ± 0.08 Ma.

At Flat Top Bluff, the southern-most exposure of the SBB, lobate basalt lavas are overlain by 10-30-m-thick mounds of basaltic pillow lavas followed by 50-m-thick succession of diffusely bedded, matrix dominated pillow fragment breccia and basalt breccia. Bedding orientations generally dip to the south-southwest suggesting an inland northeastern source. This sequence represents an episode of submarine volcanic edifice construction and subaerial emergence.

The SBB provide an excellent example of a submarine basaltic volcanic edifice dominated by effusive eruptions from multiple separate sources over a geologically short period (hundreds of thousands of years).