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