New Zealand Journal of Geology and Geophysics abstracts
Anatomy and origin of authochthonous late Pleistocene forced regression
deposits, east Coromandel inner shelf, New Zealand: implications for the development
and definition of the regressive systems tract
Barry E. Bradshaw
Geoscience Australia
GPO Box 378 Canberra,
ACT 2601, Australia
Campbell S. Nelson
Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, New Zealand
Abstract High-resolution seismic reflection data from the
east Coromandel coast, New Zealand, provide details of the sequence stratigraphy
beneath an autochthonous, wave dominated inner shelf margin during the late
Quaternary (0-140 ka). Since c. 1 Ma, the shelf has experienced limited subsidence
and fluvial sediment input, producing a depositional regime characterised by
extensive reworking of coastal and shelf sediments during glacio-eustatic sea-level
fluctuations. It appears that only one complete fifth-order (c. 100 000 yr)
depositional sequence is preserved beneath the inner shelf, the late Pleistocene
Waihi Sequence, suggesting any earlier Quaternary sequences were mainly cannibalised
into successively younger sequences. The predominantly Holocene-age Whangamata
Sequence is also evident in seismic data and modern coastal deposits, and represents
an incomplete depositional sequence in its early stages of formation. A prominent
aspect of the sequence stratigraphy off parts of the east Coromandel coast is
the presence of forced regressive deposits (FRDs) within the regressive systems
tract (RST) of the late Pleistocene Waihi Sequence. The FRDs are interpreted
to represent regressive barrier-shoreface sands that were sourced from erosion
and onshore reworking of underlying Pleistocene sediments during the period
of slow falling sea level from isotope stages 5 to 2 (c. 112-18 ka). The RST
is volumetrically the most significant depositional component of the Waihi Sequence;
the regressive deposits form a 15-20 m thick, sharp-based, tabular seismic unit
that downsteps and progrades continuously across the inner shelf. The sequence
boundary for the Waihi Sequence is placed at the most prominent, regionally
correlative, and chronostratigraphically significant surface, namely an erosional
unconformity characterised in many areas by large incised valleys that was generated
above the RST. This unconformity is interpreted as a surface of maximum subaerial
erosion generated during the last glacial lowstand (c. 18 ka). Although the
base of the RST is associated with a prominent regressive surface of erosion,
this is not used as the sequence boundary as it is highly diachronous and difficult
to identify and correlate where FRDs are not developed. The previous highstand
deposits are limited to subaerial barrier deposits preserved behind several
modern Holocene barriers along the coast, while the transgressive systems tract
is preserved locally as incised-valley fill deposits beneath the regressive
surface of erosion at the base of the RST. Many documented late Pleistocene
RSTs have been actively sourced from fluvial systems feeding the shelf and building
basinward-thickening, often stacked wedges of FRDs, for which the name allochthonous
FRDs is suggested. The Waihi Sequence RST is unusual in that it appears
to have been sourced predominantly from reworking of underlying shelf sediments,
and thus represents an autochthonous FRD. Autochthonous FRDs are also
present on the Forster-Tuncurry shelf in southeast Australia, and may be a common
feature in other shelf settings with low subsidence and low sediment supply
rates, provided shelf gradients are not too steep, and an underlying source
of unconsolidated shelf sediments is available to source FRDs. The preservation
potential of such autochthonous FRDs in ancient deposits is probably low given
that they are likely to be cannibalised during subsequent sea-level falls.
Keywords sequence stratigraphy; forced regressive deposits;
regressive systems tract; incised-valley fill; sea-level change; Coromandel
shelf; New Zealand; late Pleistocene; Forster-Tuncurry shelf; Australia
G02043; Online publication date 25 February 2004
Received 19 August 2002; accepted 10 September 2003
New Zealand Journal of Geology & Geophysics, 2004, Vol. 47: 81-99
0028-8306/04/4701-0081 © The Royal Society of New Zealand 2004
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