Print this pageLate Quaternary deposits on the Canterbury Shelf, New Zealand: High Resolution Seismic Expression of Lowstand and Transgressive Members of a Classic Braidplain System
THRASHER, G. P.; G. H. BROWNE, J. M. BEGGS, L. F. KRYSTlNIK, J. G. McPHERSON, and C. JONES
The broad, low-gradient continental shelf of the Canterbury Bight east of the South Island, New Zealand is characterised by a high-energy wave climate which transfers sediment northward by longshore drift. A high resolution seismic reflection survey was undertaken to characterise the stratal patterns and provide a basis for understanding sequence stratigraphic controls in this depositional system, which provides a potential analog for cyclical, gravel reservoir-bearing sequences in North America and elsewhere.
Four to six cycles can be recognised over the shelf, with the most recent lowstand shoreline recognisable at about -135 m. Each lowstand interval culminates in an offlapping unit with a width of at least 40 km. The top of the offlapping unit is highly erosional, and infilled with a low-amplitude seismic unit up to 10 m thick. Both of these units are erosively truncated and capped by downlapping toes of the succeeding cycle.
On the middle shelf, there appear to be broad wedges of low seismic amplitude facies corresponding to transgressive stands of sea level. On the inner shelf, the position of the last maximum transgressive shoreline is provisionally recognised about 15 km east of the present shoreline, and exhibits about 10 m of valley incision, similar in magnitude to erosion at the present shoreline. The hypothesis presented is consistent with the stratigraphic stacking patterns, but requires testing by drilling to verify the ages and lithology of seismic units.