Generalised Geology of Shoe Island, the Slipper Island Group and Part of the Mainland
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Fig. 1: Generalised geology of Shoe Island, the Slipper Island Group and part of the mainland. Bathymetric contours are shown together with inferred stream courses during lower sea-levels of the Last Glaciation. The inferred position of the Paku, Shoe and Watchman Rock rhyolite domes and North Trig andesite cone are shown. 42 TANE 20 1974 GEOLOGY OF SHOE ISLAND AND THE SLIPPER ISLAND GROUP by B.W. Hayward*, P.R. Moore*t, D.A.B. MacFarlan* SUMMARY Shoe Island and Watchman Rock are the eroded remnants of hypersthene rhyolite domes (Minden Rhyolite Subgroup; Pliocene-Pleistocene age). Slipper Formation is proposed for the two-pyroxene andesites of Slipper, Penguin and Rabbit Islands. This Formation (Pliocene-Pleistocene age) consists of three members: (a) a shallow intrusion having a minimum outcrop area of 7km2 ;(b) interbedded flows and scoriaceous breccia conformably overlain by (c) sub- aerially bedded pyroclastics. These last two members are the western remnants of a former andesite cone. INTRODUCTION Shoe Island and the Slipper Island Group lie off the east coast of the Coromandel Peninsula, 3-8km from Tairua (Fig. 1). The Slipper Island Group has previously been mapped as Minden Rhyolite5, possible Beesons Island Volcanics3 and Miocene andesite2. Shoe Island is recorded as rhyolite on all these maps. The islands were mapped by the authors during the A.U.F.C. scientific camp in August 1973 — Slipper Island by D.A.B.M. and P.R.M., Penguin and Rabbit Islands by B.W.H., and Shoe Island by P.R.M. Rock specimen numbers refer to the petrology collection of the University of Auckland Geology Department and the fossil record number is that of the N.Z. Fossil Record File. PHYSIOGRAPHY Shoe Island and the Slipper Island Group are generally westward-sloping islands. They are fully exposed to wave attack from the Pacific Ocean on the northern and eastern coasts, which have high cliffs of unweathered rock. The mainland shelters the western coasts which have low cliffs and sandy beaches. Slipper Island has a slipper-like profile when viewed from the north-west (Fig. 2). Its northern and eastern cliffs are up to 140m high, with narrow boulder beaches beneath them (Fig. 3). There are three sandy beaches on the west coast. A large east-west valley drains the northern two-thirds of the island with a swamp in its lower reaches where the outlet to the sea is restricted by a •Department of Geology, University of Auckland. Present Address: N.Z. Geological Survey, Lower Hutt. 43 boulder spit that is encroaching southwards into Home Bay (Fig. 4). Farming activities, including the partial draining of the swamp for an airstrip, have modified Home Bay. South Bay is nearer to its natural state, with a broad sandy beach backed by low, mobile sand dunes covered by sparse vegetation. Streams draining the south of the island flow into a swamp behind these dunes. The highest point on Slipper Is. is North Trig (142m), a narrow promontory atop high cliffs, that is the western remnant of an andesite cone. Penguin and Rabbit Islands have steep vegetated sides and flat tops. Rock exposures are confined to shore platforms and low cliffs except for a 30m high bluff at the south-east end of Rabbit Is. The shoreline of both islands consists of either large angular blocks (up to 10m across) or well-rounded gravel beaches. A low (2m ASL), possibly Flandrian, boulder terrace occurs at the south-west end of Penguin Is. (Fig. 5). Shoe Island has high (80-100m) near-vertical cliffs on the east coast, Fig. 2: Profile of Slipper Island from the north-west. Cliffs below North Trig (extreme left) are of Slipper Formation flows and pyroclastics whereas the remaining cliffs are of weathered andesite of the intrusion. The sandy beach of Home Bay can be seen. Fig. 3: North-east view of Slipper Island with Penguin and Rabbit Islands in the upper left and Watchman Rock in the middle right. Opoutere Beach on the Coromandel Peninsula is in the background. Rocks of the Slipper Formation intrusion are exposed in the cliffs of Beacon Point (lower centre) and the flows overlain by pyroclastics are exposed in the cliffs of Double Bay and North Trig (lower right). Photo: Whites Aviation. 44 showing large-scale flow structures in the flow-banded rhyolite. The cliffs on its west coast are only 10-12m high. Slipper, Penguin and Rabbit Islands are almost joined at low tide by a series of reefs. Bathymetric contours (Fig. 1) show these islands to sit on the edge of a shallow submarine peninsula extending from the mainland south of Tairua. A group of submerged low rocky pinnacles between Watchman Rock and 22632) beach boulder spit 0 \Beacon Point Recent dune sands pebbly sandstone (Holocene ) SLIPPER FORMATION 21 bedded pyroclastics Intrusion , with minor breccia 22640 500 m Fig. 4: Geological map of Slipper Island. Contours in metres. 45 a point 4km to the north may be the remnants of an eroded rhyolite dome. The presumed original dome shope of Shoe Is. is not reflected in the undersea topography, though two large rhyolite rocks break the surface 1km to the east. When sea level was about 100m lower during the last glaciation (which ended 15,000 years ago), this entire area would have been dry land, with the coastline out beyond the Aldermen Islands. Possible former courses of the Tairua River and other streams during this period of low sea level are indicated on Fig. 1. STRATIGRAPHY AND STRUCTURE Two major volcanic units occur on the islands mapped; the Minden Rhyolite Subgroup and what is here termed the Slipper Formation (andesitic). The age of these units is unknown, but both are considered to have been erupted within the Pliocene or Pleistocene. Fig. 5: Geological map of Penguin and Rabbit Islands. 46 coarsely spherulltlc 8 Q M x !? <1 !"3 <^ flow layering (<(> vertical ) z'" anticline (^plunging) similar fold, axial plane horizontal (^/^dlpping ) 22628 sample number 29° m Fig. 6: Geological map and cliff-sections of Shoe Island. Contours in metres. 47 1. Minden Rhyolite (Whitianga Group) Shoe Is. is part of an eroded dome of flow-banded, spherulitic rhyolite (Figs, 1, 6). Spherulitic rhyolite also forms Watchman Rock but its relationship to any eruptive centre has yet to be demonstrated. However it is possibly part of a postulated dome centred on rocks to the north. (i) Structure Flow-layering of the rhyolite lava on Shoe Is. dips at 20-60° to the east or south-east, being steepest in the north (Fig. 6). In the north-east a clear intrusive contact is exposed where the flow-layered lava overlies a lens of breccia. Large fold structures can be seen in the high eastern cliffs of the island, and smaller features on the western side. Two types of folds can be recognised (Fig. 6): (a) with fold axis parallel to the strike of the flow-layering. These are either similar or simple folds with the fold axis roughly horizontal e.g. north of Shag Bay. Dip of the axial plane ranges from horizontal to 30°. (b) with fold axis roughly perpendicular to the strike of the flow-layering. These are simple folds with a plunging axis. The plunge is from 5-25° to the east. The eastward dip of the flow-layering and the deduced movement direction from the folds, together with the intrusive contact in the north-east suggest that the centre of an extrusive rhyolite dome lay to the east of Shoe Is. No glassy chilled margins were observed. (iij Petrography The rocks are hypocrystalline, massive to coarsely or finely flow-layered rhyolites in varying shades of grey and pink, and having a vitrophyric to spheralophyric texture (Fig. 7). The phenocryst content (Table 1) ranges from 19-38% and the plagioclase/quartz ratio from 1 to 2. All samples have anhedral to euhedral plagioclase (0.1 -2.5mm), predominantly rounded quartz (0.1-3mm), minor altered hypersthene (0.2-1 mm) and euhedral magnetite (0.05-0.3mm) phenocrysts sitting in a brown spherulitic and glassy groundmass. Small accessory crystals of zircon, biotite and apatite sometimes occur in the groundmass whereas partially resorbed anhedral crystals of hornblende (0.5-2mm) only occur in the rhyolite of Watchman Rock; no potash feldspar or clinopyroxene phenocrysts were recognised. (iii) Breccia A 12-15m thick lens of tuffaceous breccia occurs in the north-eastern cliffs of Shoe Is. between lower massive spherulitic rhyolite and overlying flow-layered rhyolite (Fig. 6). The breccia is crudely graded and poorly bedded containing angular to sub-rounded clasts from 5-30cm with some up to 3m. Lithologies are mainly flow-layered or massive porphyritic rhyolite, with minor perlitic rhyolite and welded fine breccia. Near the base, clasts appear to have been derived from the underlying lava. Small lenses of breccia also occur in the south-western cliffs, and either represent rafts carried along by the lava or autobrecciation of the lava by release of gases. 48 Fig. 7: Photomicrograph of Shoe Island rhyolite showing glassy spherulites forming the groundmass (T/S 22628). Subrounded quartz (upper right) and plagioclase (lower left) phenocrysts are shown (X 50). 2. Slipper Formation (i) Description Slipper Formation is proposed for the andesite volcanics and an intrusion which outcrop on Slipper (Whakahau), Penguin and Rabbit Islands. The formation consists of three members. A large intrusion outcrops over a minimum area of 7km2 extending from Rabbit Is. to North-west Bay on Slipper Is. The two younger members only outcrop on the north end of Slipper Is. where the cliffs of Crater and Double Bays are nominated as a reference section (Figs.