A large-scale flood event in 1994 from the mid-Canterbury Plains, New Zealand, and implications for ancient fluvial deposits
G.H. Browne
Institute of Geological & Nuclear Sciences, P O Box 30 368, Lower
Hutt, New Zealand
Published: Special Publication of the International Association
of Sedimentologists . 32, p. 99-109 (2002).
Abstract
Major flooding occurred in mid-Canterbury rivers of the South Island of
New Zealand on 9 January 1994. Maximum flood discharges of 5594 cumecs
were recorded in the Rakaia River, the highest in this river in over 40
years of recordings. Flooding in the large braided rivers of mid-Canterbury
is related to heavy orographic rainfall in the west, in alpine catchments
of the Southern Alps. Over a three-day period immediately preceding and
subsequent to the flooding, areas on the West Coast of the South Island
received 80-85% of their average January rainfall. On one of those days
(8 January 1994, the day prior to the flooding event in the Canterbury
rivers), the daily precipitation exceeded 190 mm for one of these West
Coast sites, some 40% of the average total monthly rainfall for January.
At the height of the flood, a 400 m-wide flood channel was created at
the mouths of the Rakaia and Rangitata rivers, with flood discharge eroding
the gravel beach normally fronting the rivers to the Pacific Ocean. This
flood channel was subsequently modified and eventually was plugged by
littoral sediment transported northward by long-shore drift. Extensive
chipping, scratching, and pitting of large boulders in the rivers indicates
that mechanical abrasion of fluvial clasts is an important agent in downstream
clast-size reduction.
Analogue strata of last glacial to latest Pleistocene age exposed in coastal
cliffs adjacent to the Canterbury Plains show little evidence of fine-grained
(silt- and clay-size) sediment. Where present, fine-grained sediment is
confined to discrete permeability-controlled layers or clay-bands (such
as along foreset stratification). Based on observations of flood deposits
in the modern deposits, these ancient deposits were probably deposited
with considerable fine-grained sediment. It is inferred that fines are
removed from the fluvial deposit either by aeolian transport, or by interstitial
water movement, some being concentrated in the distinct clay-bands
Facies development and sequence architecture
of a Late Quaternary fluvial-marine transition, Canterbury Plains and
shelf, New Zealand:
Implications for forced regressive deposits
Greg H. Browne and Tim R. Naish
Institute of Geological & Nuclear Sciences, Ltd P O Box 30 368 Lower
Hutt, New Zealand
Published: Sedimentary Geology, 158, p. 57-86 (2003)
Abstract
The Canterbury Plains, South Island New Zealand, comprise a c. 7,500
km2 coarse-grained, braidplain that accumulated during Quaternary glacio-eustatic,
sea-level fluctuations. The adjacent Canterbury Bight shelf covering c.13,000
km2, comprises coeval shelf-slope deposits, that are punctuated by advances
of the braidplain onto the shelf during periods of sea-level fall. This
study examines the sedimentological and stratal characteristics of outcropping
last glacial braidplain deposits, and then traces oscillations in the
position of the fluvial-marine transition over several late Quaternary
sea-level cycles using high-resolution seismic reflection profiles of
the Canterbury shelf and slope. Outcropping last-glacial Burnham Formation
sediments display numerous, aggradationally-stacked massive and cross
stratified gravel deposits with minor intercalated sand and mud. The gravels
accumulated as longitudinal bars and channel fills within an extensive
braidplain succession, with some evidence of frozen ground conditions
during deposition based on sedimentological features.
High-frequency (3.5 kHz) seismic reflection data of the subsurface Canterbury
shelf identify up to 7 unconformity-bound, Milankovitch-duration depositional
sequences. These sequences are inferred to correlate with successive 100-kyr,
sea-level cycles spanning Oxygen Isotope Stages 16 to 1 (last c. 700ka).
Each sequence displays a distinctive stratigraphic motif comprising 4
recurring seismic units:
2-D forward stratigraphic modelling, constrained by outcrop and seismic
data, indicates that rivers of the Canterbury region did not incise during
eustatic sea-level fall. This may be the case elsewhere too, where a coastal
plain is flanked by a lower gradient shelf. On the Canterbury shelf, fluvial
incision did not occur during Quaternary forced regressions, but instead,
subaerial accommodation was created and filled in by thick, fluvio-deltaic
deposits, as contemporary rivers graded to the glacial maximum shoreline.
Incision was restricted to three zones: (1) The lowstand shelf break,
where canyons of limited extent formed by nickpoint retreat, (2) the transgressive
coastline where rivers incised due to coastal erosion, and (3) the inner
braidplain adjacent to the Southern Alps where degradation was caused
by tectonic uplift.
Contact
Greg Browne
July 23, 2003