Fig. 1: Generalised geology of Shoe Island, the Slipper Island Group and part of the mainland. Bathymetric contours are shown together with inferred

Transcription

1 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

2 TANE 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 7km 2 ;(b) interbedded flows and scoriaceous breccia conformably overlain by (c) subaerially 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 Rhyolite 5, possible Beesons Island Volcanics 3 and Miocene andesite 2. 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

3 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

4 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 \Beacon Point 0 Recent dune sands pebbly sandstone (Holocene ) SLIPPER FORMATION 21 bedded pyroclastics Intrusion, with minor breccia m Fig. 4: Geological map of Slipper Island. Contours in metres. 45

5 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

6 coarsely spherulltlc 8 Q M!? <1!"3 x <^ flow layering (<(> vertical ) z'" anticline (^plunging) similar fold, axial plane horizontal (^/^dlpping ) sample number 29 m Fig. 6: Geological map and cliff-sections of Shoe Island. Contours in metres. 47

7 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 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 ( mm), predominantly rounded quartz (0.1-3mm), minor altered hypersthene (0.2-1 mm) and euhedral magnetite ( mm) 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

8 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 7km 2 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. 8, 9). The lower of these two members is a 100m plus sequence of interbedded flows and scoriaceous breccia that is conformably overlain by 70m plus of subaerially bedded pyroclastics of the upper member. These two members appear to be the western remnants of an andesite cone, with the pyroclastics overlying the intrusion at the south end of Double Bay. The age of these rocks is unknown but their fresh appearance and the physiographic expression of the cone indicates that they are young and of Pliocene or more probably Pleistocene age. The temporal relationship between Slipper Formation and the Minden Rhyolites is also unknown. The comparative recentness and unaltered nature of these andesites suggest that they are not part of the Beesons Island Volcanics (Coromandel Group of Skinner 4 (in press)). After more detailed mapping in the southern Coromandels however they may be linked with the Omahia Andesites 3 or form part of a new group. Whatever the 49

9 outcome Slipper Formation contains some of the youngest andesites of the Coromandel volcanic sequence. (a) Intrusion Penguin, Rabbit and most of Slipper Island consist of flow-layered and well-jointed andesite of a shallow intrusion (probably a large sill) (Figs. 4, 5). Inclusions are common and lenses of breccia are seen at the south end of Slipper Is. What appear to be multiple dykes occur on a promontory at the south end of the island. The intrusion forms a sharp contact with the interbedded flows and breccia at the north end of Slipper Is. (Fig. 4). The contact is marked by a 30m wide zone of monolithological breccia containing very angular blocks up to 2m across. The fine matrix constitutes less than 5%. The actual contact at Double Bay was not observed, but is thought to be abrupt. Further south, two thin (<3m) breccia zones strike at 010, are near vertical and are probably associated with faults. Jointing is the most prominent feature in the intrusion and three major sets can be recognised (Fig. 10). Cross or tension joints (T) are the dominant set and in the southern part of Slipper Is. these strike north-east and dip between 70 and 90. Longitudinal joints (L) are less prominent and were rarely able to be measured. Columnar jointing only occurs in some parts. The orientation of tension joints and flow-layering in the southern part of Slipper Is. suggest a possible NE-trending anticlinal fold between South Bay and Fortress Bay. It is interesting to note that this north-east trend is parallel to a Fig. 8: Photo showing airfall bedding of Slipper Formation pyroclastics in cliff-section, east Double Bay. A small fault (right side downthrow lm) cuts across the centre of the section. (Height of cliffs 60m). 50

10 positive gravity anomaly trend (particularly the isostatic anomaly) to the south-west of the Slipper Island Group 6. (b) Flows and breccias A sequence of interbedded flows and scoriaceous breccias, dipping approximately south-west and underlying pyroclastics, forms the basal part of cliffs on north-eastern Slipper Is. The flows, ranging from 5-20m thick, are a medium grey becoming reddish in the upper parts where they contain numerous inclusions. There appears to be little difference in mineralogy between the flows and intrusion (see petrography). C DOUBLE BAY B Fig. 9: Sketched cliff-section from North Trig to Double Bay showing structural relationships between Slipper Formation flows, scoriaceous breccias and the overlying pyroclastics. 51

11 (cj Pyroclastics Beds of grey, red-brown and cream coloured lapilli breccia and lapilli tuff conformably overlie the interbedded flows and breccias (Figs. 8, 9). Some beds are crudely graded but sorting is generally poor. Clasts arc angular to subrounded and mostly from 2-5cm, though some larger blocks reach 3m diameter. The dominant lithology is a vesicular black to red or red-brown porphyritic lava, with minor glassy or flow-banded andesite. These pyroclastics generally have dips of to the south-west, that appear to steepen eastwards, indicating that the eruptive centre was situated to the north-east (Fig. 1). (ii) Petrography In hand specimen, the intrusion and flow rocks are medium to medium-dark and generally coarsely porphyritic. In thin section there appears to be little difference in mineralogy between the flows (22634) and intrusion, though the flows have a finer grained, more glassy groundmass with fewer plagioclase microphenocrysts. Generally the rocks contain abundant phenocrysts of twinned, zoned and often partially resorbed plagioclase (0.3-6mm) that may comprise up to a quarter of the total rock (Table 2). Also present are pleochroic orthopyroxene and stumpy clinopyroxene phenocrysts ( mm).Cli common in the flows. Rare, altered phenocrysts of subhedral olivine (0.3-2mm), and opaques and chlorite pseudomorphs after hornblende (0.3-1 mm) are present in some sections. The groundmass may be intersertal, intergranular or pilotaxitic and consists of plagioclase microlites, opaques, granular pyroxenes and brown glass. Glomeroporphyritic clots (0.4-9mm) of euhedral intergrown crystals (0.5-6mm) of plagioclase with or without clinopyroxene and more rarely containing orthopyroxene and opaques sometimes occur. In two sections (22638, 22640) are found subrounded nodules (6-18mm) containing crystals of finely subophitic to coarsely intersertal plagioclase, pyroxene, opaques and quartz. Chemical analyses of two samples are presented in Appendix 1. Fig. 10: Types of jointing in an intrusion (after Hills 1 ). T = tension joint, L = longitudinal joint, \ = parallel joint, F = flow-layering. 52

12 3. Holocene A 30cm thick black, pebbly sand layer, containing Modiolus shell fragments (N49/f524) resting directly on weathered andesite, is exposed just above high tide level at the south end of South Bay (Fig. 4). This appears to be a fossil beach from a time of higher sea level and is probably of Flandrian age. GEOLOGICAL HISTORY During the Pliocene or Pleistocene rhyolite domes were extruded in the vicinity of Shoe Island and Watchman Rock, as well as in nearby areas on the mainland (Fig. 1). Sometime during the same period the Slipper Formation andesites were also intruded and erupted. Firstly a large intrusive body was intruded at an inferred shallow depth in an area south-east of the rhyolite domes. The overlying rocks were partially or fully removed by erosion from the intrusion prior to the formation of a small andesite cone off the north-east tip of Slipper Island. Early phases of this eruption spilled andesite flows from the vent, followed later by lapilli and ash showers that mantled the upper slopes of the cone. Subsidence into the vent during these eruptions was possibly responsible for the steep brecciated contact between the flows and intrusion. Terrestrial and marine erosion during times of low and high sea-levels respectively, since the intrusion and eruption of these rocks, has largely been responsible for shaping the present-day physiography. ACKNOWLEDGEMENT The authors would like to thank Dr R.F. Heming for his helpful suggestions and criticism of the manuscript. REFERENCES 1 Hills, E.S. 1965: "Outlines of structural geology." (3rd Ed) Methuen, London. 182pp. N.Z. Geological Survey. 1972: North Island (1st Ed) "Geological Map of New Zealand, 1:1,000,000." D.S.I.R., Wellington. 3 Schofield, J.C. 1967: Sheet 3, Auckland (1st Ed) "Geological Map of New Zealand, 1:250,000." D.S.I.R., Wellington. 4 Skinner, D.N.B. (in press). "Geological Map of New Zealand, 1:63,360." Part Sheets N35, N36 and N39, and sheet N40, Northern Coromandel. 5 Thompson, B.N. 1966: Geology of the Coromandel Region. N.Z.G.S. Report Woodward, D.J. 1971: Sheet 3, Auckland (1st Ed) "Gravity Map of New Zealand, 1:250,000." D.S.I.R., Wellington. S3

13 Table 1: Modal analyses of Minden Rhyolite lavas. Shoe Island Watchman Rock PHENOCRYSTS (>0.3mm) Total Plagioclase Quartz Orthopyroxene Hornblende GROUNDMASS (<0.3mm) Total Plagioclase Quartz Opaques Glass Plagioclase:Quartz ratio *1000 points per count. Table 2: Modal analyses of andesites from the Slipper Formation.* Slipper Island Rabbit Island 22634f PHENOCRYSTS f>3mm) % % % % % % Total Plagioclase Orthopyroxene Clinopyroxene Olivine P P - - Quartz P P 1.3 GROUNDMASS (<3mm) Total Plagioclase Orthopyroxene Clinopyroxene Opaques Glassy and indet p=pre»ent; *1000 points per count; f from flow, remainder from intrusion. 54

14 APPENDIX - CHEMICAL ANALYSES OF SLIPPER FORMATION ANDESITES Analyst: T.H. Wilson Si0 2 Ti A Total 22634: Slipper Formation flow : Slipper Formation intrusion