Eustatic Influence on Quaternary Braid-Plain Development in the Canterbury Region of New Zealand
G. P. Thrasher, J. M. Beggs, R. J. Smith
The Canterbury Basin, is located east of the Southern Alps, South Island, New
Zealand. Quaternary sediments are approximately 400 m thick and consist of an
alternating sequence of fluvial gravels, coarse-grained marine
deposits and
fine-grained
marine
deposits. This sequence extends 150 km, from the mountains
to the shelf break. The gravels consist of glacial outwash deposited by braided
rivers during eustatic low stands. The
marine
units consist of clastic material
reworked from the fluvial strata and deposited in a shelf environment during
eustatic high stands. Because of the simple subsidence history of the basin the
depositional patterns reflect the interplay of sediment supply and base level
changes. Aggradation of coarse deposits occurs during regressions and low-st
nds. Transgressions and high stands result in downcutting by rivers, coastal
retreat, and reworking of the fluvial sequence by nearshore processes.
The aquifer properties of the fluvial gravels and the aquiclude properties of
the fine-grained marine
sediments make this sequence an important source of
ground water. In the Canterbury region, the interaction between a periglacial
outwash braid plain and
marine
processes during eustatic cycles can be examined,
revealing important analogues between the Quaternary system and
hydrocarbon-bearing reservoir systems in subsurface situations.
Two distinct depositional environments may lead to sequences which could
possess reservoir characteristics: widespread sheet-like units of the braid
plain; and lenticular, better-sorted marine
units composed of near-shore
reworked braid-plain deposits. The interspersed fine-grained
marine
strata,
which represent aquicludes in the Canterbury region, could represent important
intraformational seals and flow barriers in a hydrocarbon-bearing sequence.
AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995