Molluscan death assemblages from uplifted Holocene terraces, Mahia Peninsula, New Zealand, interpreted from present-day intertidal ecology

Abstract

Beu, A.G., August 2016. Molluscan death assemblages from uplifted Holocene terraces, Mahia Peninsula, New Zealand, interpreted from present-day intertidal ecology. Alcheringa 41, xxx–xxx. ISSN 0311-5518.

Macrofossils from cover beds of marine terraces at Table Cape, Mahia Peninsula, uplifted coseismically ca 250, 1400, 1850 and 3500 years ago are compared with the fauna living on the nearby 200-m-wide intertidal rock platform. The inner platform is dry at low tide, apart from shallow pools containing Diloma aethiops and abundant but unexpected specimens of Zeacumantus subcarinatus and Cominella glandiformis exposed to the sun; all are common fossils in the terrace cover beds. The outer platform is carpeted densely with the alga Hormosira and shelters a diverse fauna, again all common fossils in the terrace cover beds. Sand and gravel containing shell fragments and supporting Zostera sea-grass turf unexpectedly covers ca 5% of the platform in low areas along the east coast. The fossil fauna is listed from 47 samples from two trenches excavated east–west and north–south through the terrace cover beds. Death assemblages of 158 molluscs, including 24 new records as fossils, and 11 taxa in other phyla delimit sediment derived from four shell accumulation sites identified around the present cape: storm beach, high-tidal strand line, hollows in the rock platform and Zostera flats. Up to 32 of the 47 samples (68%) possibly were deposited in Zostera flats; the rest were sand beach accumulations deposited at and near the high-tidal strand line. All fossils common in the cover beds lived on the rock platform. Differences between the deposits in the two trenches result from two factors. Protection along the east coast by protruding tephra beds and an outer rampart armoured with the macroalga Durvillaea prevented erosion of Zostera-supporting sediment which, therefore, was retained after each uplift event. The exposed eastern position of Table Cape also caused sediment to be transported predominantly westwards, onshore on the east coast but away from Table Cape along the north coast.

A.G. Beu [[email protected]], Paleontology Department, GNS Science, PO Box 30368, Lower Hutt 5040, New Zealand.

Key words:

• coseismic uplift
• neotectonics
• ecology
• Holocene
• Mahia Peninsula
• Mollusca
• palaeoenvironments

Acknowledgements

Funded by Backbone Fund Contract CO5XO412 (National Paleontology Collection) from the New Zealand Foundation for Research, Science and Technology, and GNS Science Direct Crown Funding. Mrs Kathleen Mato (Nukutaurua, Mahia Peninsula) kindly allowed access to Table Cape in Dec 2015. K.R. Berryman (GNS) and S. Irwin (formerly of Geology Department, Victoria University of Wellington) helped collect samples and discussed their interpretation, and K.R. Berryman helped with the manuscript in many ways. B.A. Marshall (Museum of New Zealand Te Papa Tongarewa) helped with identifications and provided access to the Te Papa collection. I am grateful to B.W. Hayward (Geomarine Research, Auckland) for many improvements to the manuscript. J. L. Raven (Wellington) also suggested improvements. I am also grateful to two anonymous referees and to the editor, Steve McLoughlin, for helpful suggestions that improved the final manuscript. K. Clark (GNS) compiled Fig. 1 and commented on the manuscript, M. Terezow (GNS) carried out macrophotography and SEM operation, P. Carthew (GNS) rearranged Fig. 1 and prepared the other figures, and D. Houghton (GNS) prepared the tables.