Abstract
The area mapped (18kni2), crosses the boundary between the Brook Street terrane of Permian - Early Jurassic age and the Murihiku terrane of Triassic, north of Ohai in western Southland. This study focused on the lithology and lithostratigraphy of the Murihiku terrane, especially along newly cut forestry roads. The Murihiku rocks range from varieties of siltstone and silty arkosic sandstones to coarse grained tuff aceous lithic sandstones, from minor to more extensive lens-shaped conglomerates and breccias. Distinct lithologies have been recognised and new formations both formal and informal are proposed. Approximately 1800 m of Murihiku strata are exposed in this study area.
A distinctive unit mapped previously as the Ardross Member, is elevated to formation status in this study. The Ardross Formation is Etalian in age, and is situated stratigraphically above the siltstones of the Red Bog Formation in the North Etal Group. The Ardross Formation is an ammonite-bearing mudclast, tuffaceous conglomerate, 2-3 m thick and readily recognisable in the field. Ammonites are mainly Amphipopanoceras fraseri, with less common Leiophyllites marshalli. At least three different periods of deposition can be recognised in this formation: (1) the encasement of the ammonites within a tuffaceous mud, (2) the deposition elsewhere in the basin of feldspathic crystal tuff and tuffaceous siltstone. These are proposed to have formed the Ardross Formation as a result of (3) debris flow redeposition, widespread along the basin margin, during a period in the Etalian.
A fossiliferous conglomerate, 5-10 m thick and with a lateral extent of at least a kilometre, is Kaihikuan in age and contains minor granitoid and schist clasts. This conglomerate is proposed to be the marker bed for the base of the Taringatura Group. The presence of the granitoid and schist clasts is significant, as they indicate input of sediment from a continental interior during the Kaihikuan.
A distinctive suite of coarse lithic sandstones and conglomerates has been informally grouped as the Mad River Road Formation (MRR Formation). Alternating beds within the suite are interpreted to represent turbidite sequences, one of which is called the Flaggy Facies. A portion of this facies shows soft sediment slumping characteristic of sediment close to a slope margin. This facies and overlying conglomerates and sandstones are interpreted to represent deposits within a submarine fan system.
Conglomerates of the MRR Formation commonly contain conspicuous highly weathered andesitic clasts, provisionally identified as the Park Volcanics Group which outcrops elsewhere within the MRR Formation as a cluster of andesite volcanics, either extrusive or intrusive.
The range of rock types and facies associations of the Murihiku terrane is interpreted to represent various depositional settings within a volcanic arc-related basin, mostly recognised as submarine fan accumulations, which were fed material via turbidity and debris flows channellised in canyons. The Murihiku basin has been interpreted from macrofossils and paleomagnetic analysis to have been located at a low-latitude (66°S) during the Triassic. These sediments were shuffled along the margin of Gondwana to be in proximity with the Brook Street terrane, allowing for their docking by the Late
Cretaceous.