An Oligocene submarine rockfall/avalanche breccia, Fiordland, New Zealand

Abstract

Fan and canyon sediments of the Bal-leny Group (Cretaceous-Oligocene) of southern Fiordland, New Zealand, were deposited in small, fault-controlled, extensional basins and contain numerous examples of coarse mass-flow deposits. At Green Islets Peninsula, a 180 m thick breccia is well exposed in coastal outcrops. The massive, poorly sorted breccia is composed of pebble- to boulder-sized, angular blocks of the local basement and overlies, and locally interdigitates with, pelagic carbonate of the Chalky Island Formation.

The breccia apparently formed as a rock avalanche^) which cascaded off a basement high into a basin margin setting. The total volume of the breccia is c. 108 m3, and the most distal portion exposed appears to have travelled about 1000 m. Field relations do not unequivocally indicate whether the main breccia unit represents a single event consisting of an amalgam of several surges which forcibly interdigitated with the marl, or multiple events representing basin margin, debris-cone lithofacies that stratigraphically correlate to a thick sequence of marl. Enclosed in the Chalky Island Formation, at least 50 m below the base of the main breccia unit, are thin breccia beds, breccia packets, or single gneissic boulders that do indicate precursor debris slides and suggest a multiple event as, the most likely explanation.

Keywords:

• marginal basins
• Balleny Group
• breccia
• Chalky Island Formation
• Green Islets
• Kakapo Granite
• mass movements
• Puysegur Formation
• rockfalls
• sedimentation
• rates