New Zealand Journal of Geology and Geophysics abstracts
Foraminiferal and molluscan evidence for the Holocene marine history
of two breached maar lakes, Auckland, New Zealand
Bruce W. Hayward
Hugh R. Grenfell
Anna Sandiford
Phil R. Shane
Department of Geology
University of Auckland
Private Bag 92 019
Auckland, New Zealand
Margaret S. Morley
Auckland War Memorial Museum
Private Bag 92 018
Auckland, New Zealand
Brent V. Alloway
Wairakei Research Centre
Institute of Geological & Nuclear Sciences Ltd
Private Bag 2000
Taupo, New Zealand
Abstract Drillhole records of fossil Foraminifera and Mollusca,
together with sparse tephra age control, document similar Holocene marine histories
of two of Auckland’s breached maars-Pukaki Lagoon, Manukau Harbour, and
Onepoto Lagoon, Waitemata Harbour. Following eruption, both maars slowly accumulated
carbonaceous mud in freshwater lakes, until they were breached by rising sea
level in the early Holocene (c. 8100 cal. yr at Onepoto, c. 7600 cal. yr at
Pukaki). Following breaching, both became saltwater tidal lagoons with silled,
subtidal basins rapidly accumulating marine mud as the underlying sediment compacted.
Onepoto Lagoon may have had deeper water than Pukaki, because it was colonised
by a foraminiferal fauna (Bolivina, Bulimina, Buliminella,
Spiroloxostoma) that prefers quiet, dysoxic bottom conditions. Both fossil
groups identify where the lagoons shallowed from subtidal to low tidal depths.
This occurs c. 15 m downhole (6900 cal. yr) in Pukaki and c. 9.5 m downhole
in Onepoto, after sea-level rise had levelled off at about its present height
(7000 cal. yr). Marine mud sedimentation slowed in the intertidal, accumulating
largely in response to 12 m and 5 m compaction of the maar fill, respectively.
Subtidal and low tidal fringe foraminiferal faunas of both lagoons are characterised
by Ammonia-Haynesina associations, whereas intertidal faunas above mean
low water are dominated (>90%) by Ammonia. Pukaki Lagoon foraminiferal
faunas differ from Onepoto by their higher subtidal diversity of benthic foraminiferal
tests and the presence of planktic tests in the subtidal section. These differences
are inferred to relate to the significantly more exposed conditions outside
the entrance to Manukau Harbour, where juvenile benthic tests were lifted into
suspension and, together with the planktics, carried by the strong tidal currents
up the harbour channels into Pukaki Lagoon. These introduced tests settled out
of suspension in the quiet subtidal waters and accumulated in the sediment.
Once Pukaki Lagoon had been filled with mud to intertidal depths, most introduced
tests were apparently flushed away by the outgoing tides and did not accumulate.
The presence in the Onepoto sequence (9.8-8.7 m) of the gastropods Micrelenchus
huttonii and Notoacmea helmsi f. scapha indicate that Zostera
seagrass once grew in the lagoon at around spring low tide level.
Keywords benthic foraminifera; Mollusca; New Zealand;
Auckland; Pukaki Lagoon; Onepoto Lagoon; Holocene; tidal elevation; compaction
G01037 Received 1 November 2001; accepted 8 August 2002 ; published 6
December 2002
New Zealand Journal of Geology & Geophysics, 2002, Vol. 45: 467-479
0028-8306/02/4504-0467 $7.00/0 © The Royal Society of New Zealand
2002
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