Phonolitic lava domes and other features of the Dunedin Volcano, East Otago

Abstract

Outward dips of flows on the periphery of the Dunedin Miocene volcanic massif indicate the flanks of a complex, eroded shield volcano. Pillow-form lavas low in the section on the southeast flank suggest that the volcano continued to impinge on the sea for some time during its growth. Mapping of flow planes and contact relations show that the more felsic lavas, notably benmoreites, nepheline benmo:eites, and phonolites, were erupted as steep-sided lava domes of internal growth, as short coulees, and as wider-ranging flows extending in some cases at least 7 km from their vents. Strongly developed flow “schistosity” sometimes linea ted by microcorrugations, may indicate the site of lava spines forced upwards through viscous lava domes. Units such as the Logan Point and Waitati Phonolite as mapped by Benson were each erupted from several quite separate vents and cannot validly be used as datum planes for the study of post-effusive crustal deformation, nor can they be used with confidence as stratigraphic markers. An important fault zone, here termed the Leith Valley Fault Zone, cuts the volcanic massif on its northwest flank.