THE BIVALVE ANADARA TRAPEZIA AND OTHER MACROFOSSILS IN TUFFS OF MOTUKOREA VOLCANO, AUCKLAND. V.F. Bryner1 and J.A. Grant-Mackie2 c/- MacRae's Mining, RD3, Palmerston, Otago 'Geology Department, University of Auckland, Private Bag 92019, Auckland ABSTRACT From drill hole data it has been determined that Motukorea Volcano (Brown's Island), a centre within the Auckland Volcanic Field, 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 Pleistocene 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 North Island, 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 Hauraki Gulf 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 Auckland Volcanic Field. 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 Rangitoto Island, 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, lava apron, a collection of scoria 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 cockle, 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) Pecten novaezelandiae Rve, 1853. (2) oyster indet. (3) (3) (2) (2) Purpurocardia purpurata (Desh., 1854) KD Dosina zelandica Gray, 1835 (ID (4) (1) (5) (1) (18) Bivalvia 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.