New Zealand Journal of Geology and Geophysics abstracts
Foraminifera-based estimates of paleobathymetry using Modern
Analogue Technique, and the subsidence history of the early Miocene
Waitemata Basin
Bruce W. Hayward
Geomarine Research
49 Swainston Rd
St Johns, Auckland, New Zealand
email: b.hayward@geomarine.org.nz
Abstract Cluster and Canonical Correspondence
Analyses were used to group 52 early Miocene (Waitemata Group) benthic
foraminiferal faunas into eight associations and relate them to proxies
for paleobathymetry and bottom-water energy. Modern Analogue Technique
(MAT) was used to estimate the paleodepth of each fossil fauna by
comparing their generic composition with 371 modern New Zealand faunas.
MAT estimates are mostly consistent with, but no more precise than,
those inferred by conventional subjective means. MAT estimates could be
improved by broadening the modern database. The less precise paleodepth
estimates for outer shelf and bathyal faunas were improved using the
known upper depth limits of several key bathyal taxa. The combined use
of MAT and conventional methods is advocated to provide the most robust
paleoenvironmental interpretations.
The paleobathymetry estimates are consistent with previously inferred
regional subsidence during formation of the Waitemata Basin. The faunas
document the submergence of a land with up to 100 m of rolling relief,
with initial creation of both sheltered bays (Elphidium and Melonis
associations) and exposed coasts (Amphistegina and Cribrorotalia,
Gaudryina, Cibicides associations). Dysoxic faunas (Nonionella
dominated) provide evidence for the accumulation of fine sediment on
the quiet floor of several sheltered, deeper water inlets (20-40 m). As
subsidence continued, the former ridge crests became islands and
finally disappeared, coincident with cessation of coarse terrigenous
sediment supply. Paleobathymetry estimates imply that several coarse,
shallow-water gravel or shell hash units slid down the submarine slopes
of these small islands to be interbedded with mid-outer shelf
sediment (with Bolivina and Cibicides dominated
faunas). A period of sediment starvation (hiatus or thin mudstone)
ensued: in the south, it spanned an interval during which the basin
subsided from c. 150 m down to >400-500 m; in the north, it lasted
until the basin floor reached lower bathyal depths (c. >1700 m, Oridorsalis-Nodosaria
association), before sand turbidites flowed in from the northwest.
Keywords benthic foraminifera; New Zealand;
Waitemata Basin; Canonical Correspondence Analysis; Modern Analogue
Technique; early Miocene
G03065; Received 25 July 2003; accepted 22 November 2003; Online
publication date 1 December 2004
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47:
749-767
0028-8306/04/4704-0749© The Royal Society of New Zealand 2004
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