Palynology of a peat layer interbedded with rhyolitic tephra Palynology of a peat layer.pdf  palynology

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P A L Y N O L O G Y OF A P E A T L A Y E R I N T E R B E D D E D W I T H R H Y O L I T I C T E P H R A L A Y E R S A T B U C K L A N D S B E A C H , A U C K L A N D :

A P R E L I M I N A R Y I N V E S T I G A T I O N

by R . M . Newnham 1 and J .A . Grant-Mackie 2

'Department of Geography, University of Otago, P.O. Box 56, Dunedin 'Department of Geology, University of Auckland, Private Bag 92019, Auckland

S U M M A R Y

Pollens from a peat layer sandwiched between two >1 m thick rhyolitic tephra layers at Bucklands Beach, Auckland, include the extinct palynomorph Proteacidites franktonensis Couper and indicate a probable Hautawan age (c. 2 Ma). This determination has important implications for the age and source of widespread rhyolitic tephra layers in the Auckland region, suggesting that a Coromandel Volcanic Zone source should be seriously considered. The palynofloras also indicate that the peat, which occurs at sea-level today, accumulated at a level close to the contemporaneous coastline. Significant vegetation and climate changes occurred within the time encompassed by peat accumulation, with the uppermost peat accumulating more slowly and under a slightly cooler and possibly drier climate than the lowermost peat.

I N T R O D U C T I O N

The presence around Auckland of peats and associated whitish muddy to sandy pumiceous deposits has long been noted. Hochstetter (1864) correctly identified the latter and suggested they had originated from eruption in the Taupo region. Most recently, Moore (1989) documented a number of sections around Auckland, showing from glass chemistry that the tephra layers could have come from centres in the Taupo Volcanic Zone, which has been active over the last 2 M a (Cole 1986). Moore undertook no work with the peats nor has she or any other worker obtained any numerical ages from the rhyolitic tephras within the region, nor any precise geochemical correlations with Taupo or Coromandel rhyolitic centres.

In April 1992, M r Geoff Fairfield of Bucklands Beach, Auckland, drew the attention of one of us (JAG-M) to a local section of interbedded peat and tephra layers. With him, Grant-Mackie visited the property of M r A.R. Croucher at 67 The Parade, Bucklands Beach, and sampled the peat temporarily exposed in a 2m high 4-5m wide site being prepared for garage construction. We report here the

133 Tane VoL 34, 1993

results of a preliminary palynological investigation of the peat unit and discuss the implications for the age and depositional environment of the peat/tephra sequence. We include recommendations for future work.

P R E V I O U S W O R K

There has been little detailed stratigraphic work undertaken on the interbedded peat and rhyolitic tephra layers mantling the Miocene Waitemata surface in the Auckland region (see Moore, 1989). Nevertheless, there is considerable potential for a combination of palynology of peats, geochemical fingerprinting of tephra layers, their depositional nature, numerical dating (e.g., 14C, K/Ar , fission track), and sea-level comparisons to provide data for a much more detailed knowledge of the Quaternary history of the region.

The vegetation history of the Auckland Isthmus is sketchy, with most known about the late Quaternary. Palynological profiles from Little Barrier Island (Harris, 1961) and from Pollen Island, in the west of Waitemata Harbour (Wright, 1951), dated as >20,000 years old (Chapman & Ronaldson, 1958), show few changes from present indigenous vegetation remnants. Lateglacial and postglacial vegetation changes on the Isthmus are described by Pocknall et al. (1989), Newnham (1990), and Newnham & Lowe (1991).

S T R A T I G R A P H Y

The excavation at 67 The Parade (Fig. 1) exposed a sequence a couple of metres thick, consisting of 1.2 m of very fine-grained peat (perhaps more correctly termed gyttja, mud with a very high content of fine organic detritus), with its base 2 m above local high tide mark, sandwiched between two whitish tephra layers of fine sand grainsize. The top of the upper tephra layer and bottom of the

Fig. 1. Location of Bucklands Beach site in east l 0 W e f w e r e

Auckland at 67 The Parade (arrow tip).

134 Tane, VoL 34, 1993

unexposed, but both layers were at least 1 m thick. Two samples were collected for pollen analysis, one (Rl l / f l83) from the base of the peat, the other (Rl l / f l82) from the top. These numbers, fl82 and f 183 denote localities entered in the archival New Zealand Fossil Record File of the Geological Society of New Zealand in 1:50,000 metre sheet district R l l . The sequence below the lower tephra layer is unexposed, but on the adjacent shore, c. 50 m to the north, mere is exposed at high tide level and below c. 0.5 m of pumiceous sand underlain by coarse, woody peat. It is assumed from other sections in the vicinity (see, e.g., those from Point England, Pakuranga and Highland Park in Moore, 1989) that these two sequences are in normal stratigraphic continuity.

P O L L E N R E C O V E R Y A N D P R E S E R V A T I O N

Standard extraction methods were employed (Faegri & Iversen, 1989) giving reasonable pollen yield and preservation quality for both samples. Two slides per sample were prepared and are lodged with the fossil collection of the Department of Geology, University of Auckland. It may be possible to improve pollen yield using more exhaustive methods. Pollen and spore counts and percentages based on a sum of dryland pollen are listed in Table 1. The estimated pollen concentrations are based on the method of adding exotic marker spores of known concentration to the samples.

DISCUSSION

Paleoenvironments

Both samples are dominated by pollen and spores indicative of restiad peat swamps: R l l / f l 8 3 contains abundant Empodisma (wire rush), Gleichenia (umbrella fern) and Cyperaceae (sedge) while R l l / f l 8 2 is dominated by Leptocarpus and to a lesser extent Empodisma and Cyperaceae (Table 1). These changes in local wetland vegetation may indicate that the site was comparatively closer to the coastline when the uppermost peats were deposited, as Leptocarpus is today characteristic of coastal and estuarine swamps. The dryland vegetation inferred from R l l / f l 8 3 , near the base of the peat unit, also indicates a coastal location, but perhaps further above sea level.

An unidentified species of Caryophyllaceae occurred quite close to the site during the time of R l 1/f 183, either in the restiad swamp or at its margins, where the swamp vegetation passed into shrub communities dominated by Leptospermum Kunzea (manuka/kanuka), Dodonaea (akeake), Pseudopanax and Pteridium (bracken). The inferred dryland vegetation of the Auckland region at

135

R l l / fl82 fl83

TALL GYMNOSPERM TREES: Agathis Dacrycarpus Dacrydium Halocarpus Libocedrus Phyllocladus podocarpoid Podocarpus Prumnopitys ferruginea Prumnopitys taxifolia Prumnopitys undiff.

(0) (0)

(5.7) (5.7) (0.8) (5.7) (3.3) (1.6) (2.5) (2.5)

(0)

(3.8) (5.7) (3.1) (1.3) (0.6)

15 (9.4) 8 (5.0)

15 (9.4) 11 (6.9) 4 (2.5) 2 (1.3)

TALL ANGIOSPERM TREES: Metrosideros Nestegis Nothofagus fusca type Weinmannia

1 (0.8) 1 (0.8)

18 (14.8) 1 (0.8)

24 (15.1) 1 (0.6) 4 (2.5) 4 (2.5)

SMALL ANGIOSPERM TREES & Aristotelia Ascarina Asteraceae Coprosma Dodonaea Griselinia Leptospermum type Leucopogon fasciculatus cf. Melicope Myrsine Pittosporum Pseudopanax Quintinia Scrophulariaceae

DRYLAND HERBS: Poaceae Haloragis

WETLAND PLANTS: Caryophyllaceae Cyperaceae

SHRUBS: 0 (0) 0 (0) 7 (5.7)

40 (32.8)

0 22

(0) (1.6) (2.5) (0.8) (0.8) d-6) (1.6) (3.3) (0.8)

(0)

1 3 0 1

11 1

(0.6) (1.9)

(0) (0.6) (6.9) (0.6)

18 (11.3) (0) (0)

(0.6) (0.6) (4.4)

(0) (0.6)

37 229

136

Empodisma 95 525 Gleichenia 0 238 cf. Leptocarpus 234 1

TREE FERNS: Cyathea dealbata type 1 17 Cyathea smithii type 1 6 Dicksonia squarrosa 0 6

FERNS: Hymenophyllum 1 0 monolete fern spores 2 0 Phymatosorus 0 1 Pteridium 0 6

EXTINCT: Proteacidites franktonensis 0 5

Pollen & Spore Concentration 272,963 136,321 (grains/cm3)

Table 1. The Parade pollen counts. Numbers in brackets are percentages for dryland taxa.

this time is mixed podocarp-kauri-angiosperm forest, resembling modern indigenous forest remnants in the area. Several species of tree fern were common but Nothofagus (beech) trees were probably only a minor component of the vegetation (Fig. 2). The presence of Ascarina (hutu) and Dodonaea and the overall inferred dryland vegetation composition suggests a moist, mild climate similar to the present. Dodonaea is today indicative of coastal forest.

The dryland vegetation inferred for R l 1/f 182 is rather different. Coprosma species, and to a lesser extent those of the Asteraceae (daisies), were probably common in the restiad mire and may also have featured in the dryland vegetation. The dominant tree pollen taxon, Nothofagus fusca type, indicates a change to beech (probably hard beech) forest, with some podocarps present, although generally at lower levels. Agathis (kauri) pollen was not recorded in this sample, nor were Ascarina or Dodonaea pollen. Overall, the inferred vegetation suggests a slightly cooler and possibly drier climate than for the previous sample. The higher estimated pollen concentrations (grains/cm3 of sediment; see Table 1) for R l 1/f 182 are interpreted as indicating a slower rate of sedimentation, rather man greater pollen deposition, as the vegetation communities inferred for R l 1/f 183 would pro