Vent-filled Breccia and Adjacent volcanic flows at Port Chalmers.

Allen, JM

INTRODUCTION Port Chalmers lies on the north side of Otago Harbour, close to the geographic centre of the Dunedin Volcanic Complex of Upper Miocene to Lower Pliocene age (Coombs, 1965). Geologic mapping there has disclosed a westerly dipping succession of volcanic flows, erupted from a vent somewhere east of Port Chalmers Peninsula, in the sequence basalt, trachyte, basalt, phonolite. At a time at least as late as the eruption of the lowest basaltic flows, a new vent formed centred on Port Chalmers itself, but is now choked by the Port Chalmers Breccia owing to collapse of the vent walls. Trachyte is the least abundant rock type, confined to a narrow strip between Careys Bay and Deborah Bay (see Map 1), and is herein referred to as the Deborah trachyte. It is underlain by at least 350 - 1050 feet of basaltic flows. This is quite different from the distribution of trachyte at Port Chalmers mapped by Professor W.N. Benson, as part of a less detailed and broader project encompassing the whole of the Dunedin area. Benson's unpublished map of the Dunedin district (see Fig. 1) shows trachyte forming much of the lower part of the Mihiwaka - Port Chalmers ridge. He considered (Benson, 1942) this trachyte as forming part of the Initial Eruptive Phase of trachyte flows and tuffs, and regarded the basalt shown on his map (see Fig. 1) low in Sawyers Bay, in the southern part of Port Chalmers Peninsula1 and between Careys Bay and Rocky Point, as remnants of later flows which abut the trachyte, rather than earlier flows which underlie it. I 2. The following quote (Benson, 1942, p.90) is probably a fair summary of his views:- "The earliest eruptive materials •••• are anorthoclase trachytes . •• erupted in very large amountin the central originally-depressed Port Chalmers - Hoopers Inlet region, where trachytic agglomerates and tuffs associated with small trachytic flows and cut by very many trachyte dykes built up a low cone. ••••• In the centre of this mass, near Portobello, the trachyte agglomerate contains a few fragments of feldspar basalt and trachyandesite (? ) the only indication of pre-trachytic effusions. Before the trachyte magma was exhausted basaltic magma rose ••••• Its expulsion commenced with the formation of massive agglomerate followed by finer grained tuffs and more widespread flows of basalt. The earlier members of this series of flows were invaded by the latest trachytic dykes •••• Some crust-movement was in progress and the thinning out of the basaltic agglomerate and tuffs (in so far as it may not be due to its moulding against the trachyte cone) suggest the rise of two low anticlinal ridges running in an E-N-E and N-N-W direction respectively and intersecting a little south of Portobello." Benson appeared to regard the division between initial and first eruptive phas e.~ as sharp, and recognised only one possible occurrence of a trachyte flow in the F:Lrst Main Eruptive Phase:- "Northeast of Port Chalmers on the western side of Deborah Bay the earlier members of this group of (First Main Eruptive Phase) basalts are interstratified with a thin flow or sill of anorthoclase trachyte of extent too small to indicate on the map and doubtless derived from one of the latest trachytic dykes which ascend into earlier basaltic flows of the first main eruptive phase." (Benson MS , p.100). The Deborah trachyte is undoubtedly equivalent to that mentioned above by Benson, and cannot be referred to the Initial Eruptive Phase as conceived by him (Benson, 1942; 1959). The Port Chalmers Breccia is a largely unsorted and well cemented rock with a rich variety of volcanic fragments, mostly ranging from / I 3. trachyandesite to phonolite, with less common schist and syenite fragments. Last century and early this century it was quarried as a building stone and used extensively for constructing the lower parts of buildings and for street kerbing. The Supreme Court building in Dunedin is built entirely of Port Chalmers Breccia. The stone is relatively soft with a high water absorption, and on weathering the matrix tends to weather preferentially and form a flaky skin (Marshall, 1929). It is unlikely to regain favour as a building stone. Boult (1905) considered an explosive origin likely for the breccia because of the presence of schist and syenite fragments, while Marshal! (1906) believed the breccia to represent the "detritus thrown out from a crater in which violent spasmodic steam eruptions occurred". Bens on (1942) linked widespread flood plain agglomerates occurring between the first and second eruptive phase lavas with explosive eruptions from the Port Chalmers and other breccia-filled vents.

52 leaves; Maps 1 folded; 27 cm.

1968Allen

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