Canterbury basin : a geophysical and stratigraphic investigation

Wilson, David, 1967-

A series of high resolution single channel seismic (SCS) lines from the Canterbury Bight are presented, that extend from the Rangitata River mouth to beyond the shelf break, penetrate up to 150m, and sample 7 (possibly 8) sea-level cycles. Sediments on the outer shelf comprise a series of prograding foresets deposited during periods of late sea-level regression and lowstand. When the sea-level was lowest the river appears to have deposited lobate sands onto the most recent foreset, building out a shore-face barrier. Rapid sea-level rise submerges and preserves these features. Periods of stillstand are observed as either elongate, locally prograding sands that overlie the transgressive erosion surface, or wave cut erosion features. During periods of sea-level highstand marine and fluvial sediments interfinger in a complex manner. Deposition in the Canterbury Bight, during the Late Quaternary, appears to be bimodal; during sea-level regression and lowstand, sediments are actively deposited within 20km of the present-day coastline and at the paleo-shelf edge. A gap of non-deposition developed and widened with sea-level fall - this encouraged the development of the regressive sequence boundary. During sea-level transgression, this gap narrowed and a marine ravinement surface developed that reoccupied and may have eroded the resgressive sequence boundary. During periods of sea-level highstand, deposition occurs at the coast where wave-action and longshore currents redistribute the sediment. I use the term Depositional Shoreline Break Point (DSBP) to represent the position where sea-level reached its minimum. DSBP features are recognised on all seismic dip-lines, and are used to determine the age of sediments and associated unconformities. A fuzzy-logic based numerical model is used to simulate sediment deposition in the Canterbury Bight during the most recent sea-level cycle. The results are consistent with the idea that the sediments are predominantly deposited during periods of relatively slow sea-level regression. I propose the transgressive surface as a seismically recognisable horizon that separates successive sediment sequences. Power spectra of topography and Bouguer gravity data from the central South Island exhibit the effects of deformation associated with oblique convergence between the Australian and Pacific plates. For cross-sections perpendicular to the length of the South Island, at wavelengths (λ > 60 ± 20km), topography correlates well with Bouguer Gravity suggesting that long wavelength topographic features are associated with, or may even cause, lithospheric density variations. Bouguer gravity anomalies in the Canterbury Basin are likely to be the result of shallow sediment density variations. Short wavelength (λ < 60km) periodic loads (topography) are only partially compensated by a relatively rigid (Tᵉ > 5km) lithosphere. Bouguer gravity observations suggest that the central South Island topography is compensated (and in places over-compensated) at depth, implying that either the elastic plate rigidity has been overestimated, or other mechanisms for isostatic compensation are occurring. A 2D numerical model allows for the consideration of distributed loads and restoring forces, thereby applying more realistic input parameters. The model that best fits seismic and gravity contraints corresponds to a broken elastic plate (Tᵉ = 30-40km) loaded with topography and a subsurface load (equivalent to between 50 and 80km subducted cold mantle-lithosphere).

The accommodation of sediments in the Canterbury Basin is discussed in terms of subsidence, compaction, bathymetric effects, and elastic flexure. Modelling of thermal subsidence indicates that ≈ 93% of the total thermal subsidence occurred by the middle Oligocene (30Ma), with only ≈ 193m since then. For regions where the dominant mode of sediment deposition is progradation, petroleum well data are likely to overestimate subsidence rates. The load of the Neogene sediment body resulted in ≈395m compaction of sediments at the Clipper 1 petroleum well (4507 total compacted thickness). Elastic flexure of the Canterbury Basin, by emplacement of the Neogene sediment load, is determined considering two conceptual models; the first considers a single load, the second, a stacked sequence of prograding clinoforms. The flexural model that best fits gravity and seismic control corresponds with a broken plate (Tᵉ = 30km) loaded with topography, subsurface load (equivalent to 50km subducted cold mantle-lithosphere) and the Neogene sediment package. The present-day Moho relief is determined assuming that, by the early Miocene (20Ma), the central South Island and Canterbury Basin lithosphere had reached a state of isostatic equilibrium, and that the Moho relief would reflect bathymetry at that time. Present-day Moho relief is the combination of paleo-Moho relief and best-fit elastic flexure. The results indicate a ≈ 3.4° westward dipping Moho.

157 leaves : ill., maps ; 30 cm.

1998Wilson

Open Access