Facies development and sequence architecture of a late Quaternary fluvial-marine transition, Canterbury Plains and shelf, New Zealand: implications for forced regressive deposits

Abstract

The Canterbury Plains, South Island, New Zealand, comprise a c. 7500 km 2 coarse-grained, braidplain that accumulated during Quaternary glacio-eustatic, sea-level fluctuations. The adjacent Canterbury Bight shelf covering c.13,000 km 2, 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 seven unconformity-bound, Milankovitch-duration depositional sequences. These sequences are inferred to correlate with successive 100-ka, sea-level cycles spanning Oxygen Isotope Stages 16 to 1 (last c. 700 ka). Each sequence displays a distinctive stratigraphic motif comprising four recurring seismic units: 1. Basinward of the glacial maximum shoreline, wedge-shaped units displaying steeply dipping clinoforms that onlap the continental slope are interpreted as “perched lowstand deltas” belonging to the lowstand prograding wedge systems tract (LST). 2. Irregular hummocky units up to 10 m thick, containing high-amplitude discontinuous reflectors, are interpreted as representing stranded coastal deposits of the transgressive systems tract (TST). 3. Low-amplitude seismic units which offlap and downlap onto the TST, infilling local paleotopography, and interpreted as comprising fine-grained marine sediments of the highstand systems tract (HST). 4. Basinward thickening units (up to 40 m thick), containing a strongly progradational series of offlapping, inclined (0.5–1.0°), high-amplitude reflectors, that downstep towards the basin are interpreted as coarse-grained, fluvio-deltaic sediments, similar to the last glacial Burnham Formation, deposited during glacio-eustatic sea-level fall, or forced regression. We assign this unit to the regressive systems tract (RST), which displays a gradational lower boundary overlain by a sharp planar regionally extensive sequence boundary or ravinement surface. 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.