Ancient subaqueous successions in the Hart-Carson LIP, north Western Australia

Thursday, 2 February 2017
Marina/Gretel (Hobart Function and Conference Centre)
Karin Orth1, Christopher Phillips2 and Julie Hollis2,3, (1)University of Tasmania, Hobart, Australia, (2)Geological Survey of Western Australia, East Perth, WA, Australia, (3)Ministry of Resources, Greenland, Nuuk, Greenland
Abstract:
The Carson Volcanics are the only volcanic unit in the Palaeoproterozoic Kimberley Basin. The unit covers around 160,000 km2 and is the remnant of the extrusive portion of the mafic Hart-Carson Large Igneous Province. Stacked basalt lavas form 80% of the succession and have the trap-like appearance typical of extensive subaerial flood basalts. However, intercalated sedimentary rocks with symmetrical ripples, desiccation cracks and stromatolites are evidence of localised shallow marine to emergent depositional environments.

Pillow basalt and pillow breccias also indicate periods of subaqueous deposition on the Yampi Peninsula and near Mt House Station. In the Mt House Station area, the pillow-basalt breccia may extend over 20 km along strike and is up to 30-40 m thick. Pillows are isolated and supported in a poorly sorted breccia matrix. Pillows can be up to 5 metres long and 1-2 m across. Many bifurcate and some display complex shapes. Hollow pillows are common and some pillows have abundant quartz amygdales. Pillow margins commonly display small (5-10 cm long) joints at right angles to the pillow rim with extensive, internal polyhedral jointing. Pillow rinds are thin (less than a few mm) with subtle ropey wrinkles and marks where pillow surfaces are exposed. The breccia contains abundant angular basalt clasts varying between sandsized and pebble-sized clasts. Coarse clasts are disaggregated basalt or pillow fragments. In some areas there is mudstone between fragments or quartz as secondary infill. The breccia appears to be predominantly hyaloclastite with some peperite. The succession is interpreted as a lava delta, which formed where lava entered a shallow marine environment.

On Yampi Peninsula, pillow basalt appears to burrow into siltstone and is capped by massive basalt. Unlike most other areas in the Carson Volcanics, the succession on the Yampi Peninsula is dominated by volcaniclastic breccia, sandstone and siltstone. Breccias are both monomictic and polymictic and have sedimentary or volcanic dominated matrix. Some breccia contains boulder-sized clasts. The Carson Volcanics are thickest (800 m) in this area. Contrary to the current interpretation of the Carson Volcanics on the Yampi Peninsula as a volcanic edifice, the succession may represent the remnant of a submarine depocentre.

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