Late Miocene volcanic stratigraphy of central Banks Peninsula, Canterbury, New Zealand

Abstract

Banks Peninsula consists of Late Miocene alkalic to transitional volcanic rocks erupted from two major composite volcanoes (Lyttelton and Akaroa) and a host of minor monogenetic vents. Four volcanic groups are defined in this paper: Lyttelton (11-9.7 Ma), Mt Herbert (9.7-8.0 Ma), Akaroa (9.0-8.0 Ma), and Diamond Harbour (7.0-5.8 Ma). Three further formations, which are petrographically distinct and volumetrically minor, were erupted between 8.1 and 7.3 Ma, marking the transition from Akaroa to Diamond Harbour volcanism.

Eruption of 350 km³ of mildly alkalic to transitional hawaiite to trachyte lavas of the Lyttelton Volcanic Group constructed a major composite volcano which forms the northwestern half of the peninsula. Late in the history of activity, the Lyttelton crater was breached in the southeast.

The Mt Herbert Volcanic Group, comprising 100 km³ of mildly alkalic lavas and pyroclastic rocks, was then erupted from two main vents on the southeastern segment of Lyttelton Volcano. It consists of four formations (Kaituna Valley Hawaiite (oldest); Orton Bradley Formation; Port Levy Formation; Herbert Peak Hawaiite) and marks the migration corresponding to the onset of Akaroa volcanism which formed a 1200 km³ composite volcano of dominantly alkalic lavas and shallow intrusives.

Three formations of “Church-type” lavas (Darra Basanitoid; Chateau Intrusives; Church Basalt) totalling 5 km³ were then erupted in the central part of the peninsula after which 20 km³ of weakly alkalic olivine basalt and hawaiite lavas from monogenetic vents on the eroded Lyttelton Volcano formed the Diamond Harbour Volcanic Group (Stoddart Basalt and Kaioruru Hawaiite).

Keywords:

• Banks Peninsula
• Late Miocene
• stratigraphy
• volcanology
• new stratigraphic names
• Lyttelton Volcanic Group
• Mt Herbert Volcanic Group
• Akaroa Volcanic Group
• Diamond Harbour Volcanic Group
• Kaituna Valley Hawaiite
• Orton Bradley Formation
• Mt Bradley Volcaniclastic Member
• Port Levy Formation
• Herbert Peak Hawaiite
• Darra Basanitoid
• Chateau Intrusives
• Church Basalt
• Stoddart Basalt
• Kaioruru Hawaiite