2004 Denver Annual Meeting (November 7–10, 2004)

Paper No. 5
Presentation Time: 2:45 PM


BASSETT, Kari N., BRADSHAW, John D. and WEAVER, Steve D., Dept. of Geological Sciences, Univ. of Canterbury, Prvt. Bag 4800, Christchurch, 8004, New Zealand, kari.bassett@canterbury.ac.nz

We envision the accretion of the Bowers Terrane island arc to the Gondwana margin to be similar to the accretion of the Finisterre arc in Papua New Guinea, producing an uplifted source shedding sediment into lateral basins. The arc accreted to the Gondwana margin, dated by both granitoid clast SHRIMP U/Pb zircon dating and trilobite fossils, in the Middle to Late Cambrian forming part of the Ross Orogeny. It was subsequently offset by orogen parallel strike-slip faults.

The majority of the Bowers Terrane is made up of the Sledgers Group, a mafic volcanic arc assemblage with intra-oceanic geochemistry interbedded with marine mudstones, sandstones, and rare limestone blocks. Mariner Group deposits, unconformably overlain by quartz arenites of the Leap Year Group in a regressive sequence of increasing continental affinity, occur in fault slivers on the western side of the central Sledgers arc axis. Basal black marine mudstones contain channels with large angular limestone blocks, interbedded with turbidites of quartz-rich sandstones. These coarsen up into submarine fan channel conglomerates containing clasts of mafic volcanics from the accreted Sledgers island arc and granitoids from the unroofing of the Ross continental arc. A deep water turbidite sequence contains 0.5 to 3 km long blocks of shallow marine mudstones with tidal indicators such as paired mud-drapes and ripples and blocks of deformed limestone with shattered edges and trilobite fossils and tracks. These record megacollapse of the continental margin. Younger fluvial conglomerates also contain both Ross continental arc granitoid clasts of I- and S-type affinity and Sledgers mafic island arc volcanic clasts but without the limestones. All SHRIMP U/Pb zircon crystallization ages from granitoid clasts are typical of the Ross Orogen, with inheritance ages characteristic of the Gondwana margin.

The combination of source types in the same channels suggests that the basin formed lateral to the uplift from the accretion of the Sledgers arc. The most likely setting is a basin lateral to the orogen similar to the Huon Gulf in the Solomon Sea fed by the Markham River from the uplifted mountains in Papua New Guinea.