Stratigraphy and provenance of Pliocene greywacke‐bearing conglomerate, Cardrona valley, Otago, New Zealand

Abstract

A synclinal sedimentary basin in the Cardrona valley, Otago, New Zealand, has formed along the Nevis‐Cardrona Fault Zone, a major fault zone associated with late Cenozoic Alpine Fault related deformation. The sedimentary basin records a conformable transition from Miocene low‐energy fluviolacustrine sedimentation to fluvial gravel deposits (>300 m thick) associated with late Miocene‐Pliocene initiation of mountain building in the region. The lower gravel unit, the Tuohys Breccia (new name), contains mainly subangular schist clasts, with subordinate quartz, derived from the uplifted schist basement adjacent to the sedimentary basin. This unit is conformably overlain by the Waiorau Conglomerate (new name), which contains up to 80% greywacke clasts. There is no greywacke basement in the Cardrona area, and these gravels must have been transported >60 km. The greywacke clasts have a high quartz (c. 40%) and feldspar content (c. 40%) and low lithic content (c. 20%), typical of Torlesse Terrane, rather than Caples Terrane, greywackes. The clasts have high SiO₂ (>70 wt%), high La/Sc ratios (typically >3), and high La/Y ratios (>1), and all these parameters are characteristic of Torlesse Terrane provenance. Torlesse Terrane greywacke basement is now at least 60 km to the northeast of the Cardrona area, implying transport of the clasts towards the southwest. Imbrication in the greywacke gravels supports this southwest transport direction, and rare lamprophyre clasts derived from the north also support southward transport. This southward to southwestward transport is opposite to the present Cardrona River flow direction, and major changes to the regional paleodrainage have occurred due to progressive compression. The present Manuherikia River, 50 km to the east, provides a modern analogue for long distance (c. 70 km) transport of greywacke gravels across schist basement without incorporation of a significant schist clast component.

Keywords:

• river capture
• paleogeography
• Kaikoura Orogeny
• Otago Schist
• tectonics
• geochemistry
• Tuohys Breccia
• Waiorau Conglomerate
• new stratigraphic names