Temperate mixed carbonate-siliciclastic sediments and authigenic minerals are the
current surficial deposits at shelf and slope depths (30-1015 m water depth) on the
north Kaipara continental margin (NKCM) in northern New Zealand. This is the
first detailed study of these NKCM deposits which are described and mapped
from the analysis of 54 surficial sediment samples collected along seven shorenormal
transects and from three short piston cores. Five surficial sediment facies
are defined from the textural and compositional characteristics of this sediment
involving relict, modern or mixed relict-modern components. Facies 1
(siliciclastic sand) forms a modern sand prism that extends out to outer shelf
depths and contains three subfacies. Subfacies 1a (quartzofeldspathic sand) is an
extensive North Island volcanic and basement rock derived sand deposit that
occurs at less than 100-200 m water depth across the entire NKCM. Subfacies 1b
(heavy mineral sand) occurs at less than 50 m water depth along only two
transects and consists of predominantly local basaltic to basaltic andesite derived
heavy mineral rich (gt30%) deposits. Subfacies 1c (mica rich sand) occurs at one
sample site at 300 m water depth and contains 20-30% mica grains, probably
sourced from South Island schists and granites. Facies 2 (glauconitic sand)
comprises medium to fine sand with over 30% and up to 95% authigenic
glauconite grains occurring in areas of low sedimentation on the outer shelf and
upper slope (150-400 m water depth) in central NKCM. Facies 3 (mixed
bryozoan-siliciclastic sand) consists of greater than 40% bryozoan skeletal
material and occurs only in the northern half of the NKCM. Facies 4 (pelletal
mud) occurs on the mid shelf (100-150 m water depth) in northern NKCM and
comprises muddy sediment dominated by greater than c. 30% mixed carbonatesiliciclastic
pellets. Facies 5 (foraminiferal mud and sand) contains at least 30%
foraminifera tests and comprises two subfacies. Subfacies 5a consists of at least
50% mud sized sediment and occurs at gt400 m water depth in southern NKCM
while subfacies 5b comprises gt70% sand sized sediment and occurs at mid to
outer shelf and slope depths in the northern NKCM.
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A number of environmental controls affect the composition and distribution of
NKCM sediments and these include: (1) variable sediment inputs to the NKCM
dominated by inshore bedload sources from the south; (2) northerly directed
nearshore littoral and combined storm-current sediment transport on the beach and
shelf, respectively; (3) offshore suspended sediment bypassing allowing
deposition of authigenic minerals and skeletal grains; (4) exchange between the
beach and shelf producing similar compositions and grain sizes at less than 150 m
water depth; and (5) the episodic rise of sea level since the Last Glaciation
maximum approximately 20 000 years ago which has resulted in much sediment
being left stranded at greater depths than would otherwise be anticipated.
Sedimentation models developed from other wave-dominated shelves generally
do not appear to apply to the NKCM sediments due to their overall relative
coarseness and their mosaic textural characteristics. In particular, the NKCM
sediments do not show the expected fining offshore trends of most wavedominated
shelf models. Consequently, sandy sediments (both siliciclastic and
authigenic) are most typical with mud becoming a dominant component in
southern NKCM sediments only at greater than 400 m water depth, over 350 m
deeper than most models suggest, a situation accentuated by the very low mud
sediment supply to the NKCM from the bordering Northland landmass.
