THE BIVALVE TRAPEZIA AND OTHER MACROFOSSILS IN TUFFS OF , .

V.F. Bryner1 and J.A. Grant-Mackie2 c/- MacRae's Mining, RD3, Palmerston, Otago 'Geology Department, , Private Bag 92019, Auckland

ABSTRACT

From drill hole data it has been determined that Motukorea Volcano (Brown's Island), a centre within the Auckland , erupted probably some 8,000 yr before present (BP) through a ridge of Tertiary Waitemata Group strata flanked by channels containing a regressive to transgressive sequence of Late sediments. Early phreatomagmatic eruptions formed a tuff ring. Xenoliths in these tuffs include rare whole fossil shells and more common fragments down to millimetric size. These were, with one exception, part of a near-shore, fully marine assemblage, believed to have been derived from channel-filling Pleistocene sediments approximately 25m below present sea-level. The exception is Anadara trapezia, the most abundant fossil present in the pyroclastics. It is a living Australian brackish water species, found also in Quaternary near-shore and estuarine deposits around the , including in sediments about 32, 000 to 34, 000 yr and perhaps 37,600 yr old near Rangitoto with which the source deposit at Motukorea is tentatively correlated. If we accept that these dates are correct and that the Anadara shells were derived from the level postulated, then either the region has suffered some 15m of uplift in the subsequent period or the currently accepted Late Pleistocene sea-level curve is in error by that amount; the former alternative is the favoured one.

INTRODUCTION

Explosive volcanic activity often results in the inclusion of country rock fragments in the erupted tuffs as they accumulate. Occasionally, when eruption occurs through fossiliferous strata, the fossils themselves become xenolithic inclusions in the tuffs and provide a valuable guide to the age and depositional environment of the strata below. This paper reports such an occurrence for the Motukorea Volcano in Auckland's inner and considers the implications of the faunal association, its age and its probable present depth of burial.

123 Tane, VoL 34, 1993 REGIONAL SETTING AND GEOLOGY

In the Auckland area basement rocks are Permian-Jurassic meta-greywackes and meta-argillites overlain by Lower Miocene sandstone, mudstone and tuffaceous beds of the Waitemata Group. Waitemata strata are in turn overlain by Quaternary sediments or volcanics, the latter erupted from one of the 60 or more centres of the . During the Pleistocene, glacial low-stands of sea-level have alternated with interglacial highs, with trangressive and regressive sequences and associated cut or built terraces. As a consequence of the last glacial low-stand (approximately 20,000 yr before present - BP) and subsequent sea-level rise, the Waitemata Harbour forms an extensive drowned river valley system.

GEOLOGY OF MOTUKOREA

Motukorea (Brown's Island) is a volcano located in the Waitemata Harbour, south of , one of the centres within the Late Quaternary basaltic Auckland Volcanic Field (Fig. 1). The island is composed of the remnant of an early-formed tuff ring, an extensive, mostly submarine, apron, a collection of cones and mounds, spatter walls and debris flow deposits (Bryner 1991). From geomorphic evidence Searle (1964) thought Motukorea to be less than 20,000 years old. Bryner (1991) concluded from sea-level at the time of eruption that it is about 8,000 years old.

THE PRE-ERUPTION ENVIRONMENT OF MOTUKOREA

Lithologies and landforms around and beneath Motukorea were determined by Bryner (1991) utilising drill hole and pricking logs which accompanied a report by Mead and Firth (1952) on the geology of Motukorea, shown in part on Fig. 1. Prior to eruption a low ridge of Waitemata strata formed a northwards extension of Musick Pt, with a high-point located under the eastern rim of the tuff ring rising to at least lm above present sealevel and outcropping in the eastern corner of Crater Bay. Approximately 300m north, along Line 3, the upper surface of the Waitemata sediments is about 6m below present sealevel (BPSL). About one kilometre west of Line 3, the slope of the upper surface of the lava flow suggests that the Waitemata Group surface drops to 25m BPSL. This ridge was flanked to the west and southwest by the Tamaki Stream (a tributary of the ancestral Waitemata River - Searle 1964), which cut down to

124 Tane, Vol. 34, 1993 Fig. 1. Geological map of Motukorea showing location of relevant drill lines; borehole and pricking sites omitted (simplified from Bryner 1991). f 173 to fl 79 are fossil localities within 1:50,000 topo sheet Rll.

36m BPSL into the Waitemata Group surface. Further west, on the other side of a rise to 12m BPSL at the edge of the lava flow, is a younger, narrower channel, cut 5-10m into the Waitemata Group surface by the Tamaki Stream after Motukorea erupted (Fig. 2).

PLEISTOCENE SEDIMENT COVER

The Waitemata strata are partially covered by Pleistocene sediments which

125 differ h? character to the west and east of the island (Fig. 2). To the west, the older of the two Tamaki Stream courses (nearer the island) has been infilled by a regressive sequence of shell-bearing silts and sands passing up to peat- and wood-rich silts. In borehole 66 (BH 66) (not shown but lying between BH65 and BH74 to the south of the Diamond Line) the regressive sequence is overlain by a 10m transgressive sequence. To the east the undulose surface on the Waitemata rocks is overlain by up to 5m of dark grey silt on the undulating eroded surface of which the distal volcaniclastics were deposited.

Fig. 2. 3-D projection of cross-sections along the Diamond and Triangle lines to the west of Motukorea. Numbers at the top of sections are borehole numbers. Lengths of vertical lines indicate depths of boreholes.

126 FOSSILS OF THE MOTUKOREA TUFFS

Motukorea erupted through a ridge of Waitemata strata parts of which were covered by Pleistocene sediments, into a subaerial or very shallow marine environment. Initial eruptions were phreatomagmatic and formed a tuff ring. Part of this is exposed in cliffs 10-24m high on the eastern side of the island. The tuff sequence can be separated into three units: lower and transitional units of predominantly surge-deposited beds and an upper scoriaceous unit deposited by airfall (Bryner 1991). Sedimentary clasts included amongst Motukorea ejecta correlate well with lithologies known to occur in the vicinity. The only group of lithologies identified from nearby drilling not erupted as lithics are the woody peats and silts, such as found in the old Tamaki Stream course to the west of Motukorea. In addition rare, well-preserved fossil shells, large shell fragments, and abundant millimetric fragments occur at six stratigraphic levels (seven localities) within the tuffs, lapilli-tuffs and lapillistones of the lower and transitional units. Shells and fragments are free of adhering lithified sediments. The presence of discrete fossil-rich horizons in the tuffs is most likely the result of either an eruption focus moving with time (resulting in the bed(s) from which the fossils were derived being disrupted by separate eruptions), or the spalling of the tuff ring and country rock as widening of the eruption crater took place, or both. It can be seen from the macrofossils identified (New Zealand Fossil Record File Numbers Rll/fl73-fl79 - Table 1) that Anadara trapezia fragments are in greatest abundance. The modern Sydney mud , Anadara trapezia, does not live in the region today but has been recently reported (Grant-Mackie and Cook 1990) in Late Pleistocene shelly silts nearby on the western side of Rangitoto and in the adjacent channel. It is reasonable to assume that all three records of the species in this area are from the one deposit, the age of which is of some significance in determining the history of sea-level and environmental change in the area.

REASSESSMENT OF THE AGE OF ANADARA IN AUCKLAND

Grant-Mackie and Cook (1990) reported radiocarbon ages of 25,430 ±990 years before present (yrs BP) and 37,600 ±1,800 yrs BP for Anadara from Rangitoto Island. Recent information from Dr J.W.A. McKee (pers. comm. Oct 1991), Institute of Geological and Nuclear Sciences Limited, Lower Hutt, to J.A. Grant-Mackie indicates that the two dates were in fact from the same valve, and the first-mentioned should have been reported as >25,000 ±1000 yrs BP (since

127 Rll/f 173 174 175 176 177 178 179

Maoricolpus roseus (Q. & G., 1834) 1

Zeacolpus vittatus (Hutt., 1873) (4) KD Turritellidae indet. (2)

Cominella (C.) adspersa (Brag. 1789) (4) 1 (?1)

Cominella sp. 1

Penion sulcatus (Lam., 1816) 1 1

Gastropoda indet. (2)

Anadara trapezia (Desh., 1839) (16) (15) (1) (22) 1(6) (15) (1)

Tucetona laticostata (Q. & G., 1835) (1) Rve, 1853. (2) indet. (3) (3) (2) (2)

Purpurocardia purpurata (Desh., 1854) KD

Dosina zelandica Gray, 1835 (ID (4) (1) (5) (1) (18) indet. (4) barnacle indet. (1)

Table 1. Abundance and diversity of fauna within the Motukorea tuffs. Fossil locality numbers within Sheet Rll head columns in which numbers of individuals or identifiable fragments (in parentheses) are given. The oyster appears to be the common mud oyster Tiostrea chilensis lutaria(Hutton , 1873); the barnacle is a single compartment of a balanomorph; and the unnamed Cominella in Rll/fl77 is a form related to, but apparently different from C. adspersa, perhaps more like the extinct C. fascinervosa (Bartrum and Powell, 1928).

the background limit of the accelerator mass spectrometry - AMS - system at that time was c.25,000 yrs BP), and thus should have been disregarded with the receipt of the second determination. Dr McKee commented further that this second result, although acceptable, is still close to the background (of 39,400 ±2,000 yrs BP) that existed for the AMS dating system at the time, making it prudent to attempt to confirm it by further determinations once the procedure had reached a higher level of precision. In August 1991 two further dates were obtained on shells from the western Rangitoto island deposit (fossil locality Rl 1/f 148) by AMS as follows:

128 No. R16051/1, NZA 1964; on one valve of Anadara trapezia: 31,860 ±500 yrs BP

No. R16051/2, NZA 1965; on one valve of spissa: 33,950 ±560 yrs BP.

The 2000 year difference in these dates may be due in part to differences in the accumulation of 14C in shells of the two species during their life-times, differences that are known to exist because, e.g., of differences in diet and metabolism. The dates indicate a slightly younger age than the earlier date and perhaps accumulation over at least a 2,000 year period, or perhaps accumulation of the lm-thick shellbed over the full 6,000 years spanned by the three dates, if the oldest is not rejected. As stated above, it is reasonable therefore to assume that the Motukorea shells, and the shellbed(s) from which they were derived into the tuffs, are also of ages around 32, 000 - 37, 000 yrs BP.

DISCUSSION AND CONCLUSIONS

The absence of adhering lithified sediments suggests that all fossils almost certainly came from underlying unconsolidated Pleistocene sediments. All species found are Pliocene to Recent in age (Beu and Maxwell 1990). The assemblage, with the exception of Anadara, suggests a near shore, fully marine environment (Morton and Miller 1968) for the sediments from which they were derived. The different environments favoured by the fossils suggests eruption through two stratigraphically separate shell beds, with most fossils except Anadara trapezia being derived from the grey shelly silts. Anadara trapezia could have been derived from peaty silts such as those found filling the old Tamaki Stream valley to the west of Motukorea. Alternatively, Anadara valves may have been transported from their life environment (at Motukorea Anadara found are all subadults) and deposited with the other shells. The presence of Anadara trapezia, as the only locally extinct species in the fauna, is of special significance. The species today is "characteristic of shallow-water, sea-grass communities in typically hyposaline estuarine environments from central to southern . The fossil record of the species (in ) indicates that its modern, disjunct distribution is but a remnant of a much more extensive, maximal range during the Late Pleistocene, clearly a time when environments favourable to the species existed through the entire subtropical and temperate coastline of Australia" (Kendrick 1990:45).

129 Kendrick (1990) goes on to suggest that the presence of the species in the fossil record indicates conditions of combined marine-freshwater influences with sustained river discharge (ie. not subject to seasonal aridity and thus, fluctuating salinity). These observations make less problematic the occurrence of Anadara trapezia in the Auckland area in sediments of Last Glacial age. The Rangitoto Anadara were reported as all juveniles (Grant-Mackie and Cook 1990); in fact none has a hinge-line longer than 25mm, and most are less than 15mm long. They were interpreted as probably having been transported. Valve fragments in the Motukorea tuffs include individuals with a hinge-line at least 35mm long and the number of thick-shelled heavily ribbed Anadara fragments indicates these and larger valves were common in the deposit(s). Fully grown adults reach at least 50mm in hinge length so the Motukorea shells may also have been mostly subadult and perhaps not in situ, but certainly they are likely to have been transported a shorter distance than the Rangitoto shells. It is easy to imagine a contemporary Tamaki estuary offshore from the present mouth having the environmental conditions suitable for Anadara trapezia. If the above is so, then the Anadara-bearing shellbed(s) through which the Motukorea volcanics were erupted are likely to have been accumulated essentially at contemporary sealevel. Borehole logs give us clear indications of the likely depths. Fig. 2 shows sections based on interpretation of these logs, unfortunately unsupported by cores or cuttings which have not been preserved. The shelly sand at the top of the sequence overlies the lava apron and laps onto the edge of the youngest tuffs; it is thus younger than Motukorea and can not have been the source of the fossils in the tuffs. A unit of shelly silt is recorded between 34 and 25m below modern sealevel, overlain to c. 15m by silt, peat and wood. The latter could be a mixed swamp/estuarine deposit accumulated partly in an environment suitable to Anadara trapezia. However, no shells are recorded from this unit and had they been present they would have been likely to be large thick-shelled adults sufficiently abundant and prominent to have been noticed and logged. It is more likely therefore that Anadara came from the underlying "shelly silt" along with other molluscs, with which they would have become associated by post-mortem seawards transport of subadult valves from their estuarine life environment. Accepting this conclusion, sealevel at the time Anadara lived in the earlier Tamaki estuary would have been some 30m to 20m lower than the present. This is to be compared with Anadara-bearing shell beds in the Rangitoto Channel 15-18m below the present sealevel and on the western margin of Rangitoto Island at present highwater mark (Grant-Mackie & Cook 1990).

130 Sea-level at 32-37,000 yrs BP (during oxygen isotope event 3.1) was some 65-85m (Shackleton & Opdyke 1973) or c. 50m (Chappell & Shackleton 1986) below the present, in either case significantly lower than any of the above three measurements suggests. This, together with the variations among the three, strongly indicate faulting effects, perhaps associated with , as has already been argued by Grant-Mackie & Cook (1990). Alternatively, more than one period of Anadara colonisation is recorded by the three areas. Multiple migration of Anadara to the North Island has certainly occurred (Beu et al. 1990), but faulting is at this stage the favoured explanation, although the evidence is not strong. Pocknall et al. (1989) also preferred tectonic effects to explain discrepant Holocene sea-level data in the Firth of Thames.

ACKNOWLEDGEMENTS

We particularly wish to thank Dr JWA McKee, Institute of Geological and Nuclear Sciences Ltd, Lower Hutt, for his valuable comments and assistance on the radiocarbon dates. The manuscript has also benefitted from critical examination by Drs M R Gregory and IEM Smith. Draughting was done by Ms L Cotterall and the manuscript typed by Ms R Bunker, whose contribution we acknowledge with thanks.

REFERENCES

Beu, A.G. & Maxwell, P.A. 1990: Cenozoic of New Zealand. New Zealand Geological Survey Paleontological Bulletin 58: 518p Bryner, V.F. 1991: "Motukorea: the eruption of a centre in the Auckland Volcanic Field." Unpublished thesis, University of Auckland Library: 126p. Chappell, J. & Shackleton, N.J. 1986: Oxygen isotopes and sea level. Nature 324 (6093): 137-140. Gilmour, R. 1953: Submarine investigations and borings: West Tamaki Strait, Motukorea area. New Zealand Engineering: 201-204 Grant-Mackie, J.A. & Cook, S. de C. 1990: A Late Quaternary Anadara-bearingdeposi t disturbed by Rangitoto lava. New Zealand Natural Science 17: 73-79. Kendrick, G.W. 1980: A Pleistocene molluscan fauna with Anadara trapezia (Deshayes) (Bivalvia:Arcoida) from the Dampier Limestone of Shark Bay, . Western Australian Museum Records, Report of the France-Australia Bicentennary Expedition Committee: 33-48. Kendrick, G.W., Wyrwoll, K-H. & Szabo, B.J. 1991: Pliocene-Pleistocene coastal events and history along the western margin of Australia. Quaternary Science Reviews 10: 419-439. Mead, A.D. & Firth, CW. 1952: "The geology of Motukorea." Report dated 4 September 1952, Auckland Metropolitan Drainage Board. Pocknall, D.T., Gregory, M.R., & Greig, D.A. 1989: Palynology of core 80/20 and its implications for understanding Holocene sea-level changes in the Firth of Thames, New Zealand. Journal of the Royal Society of New Zealand 19: 171-179. Searle, E.J. 1964: "City of volcanoes: a geology of Auckland." Paul's, Auckland: 112p. Shackleton, N.J. & Opdyke, N.D. 1973: Oxygen isotope and paleomagnetic stratigraphy of equatorial Pacific core V28-238: oxygen isotope temperatures and ice volumes on a 105 year and 106 year scale. Quaternary Research 3: 39-55.

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