Coastal landforms and sediments of the Marlborough Sounds (1987)
AuthorsLauder, Glen A.show all
This thesis examines coastal form and sediments of the Marlborough Sounds, New Zealand. An important aspect of coastal behaviour in this landscape stems from linkages between catchment and coast. Focus is therefore placed on the manner in which sediment delivered from catchment sources is redistributed within the shore and offshore domains. Coastal response is shown to depend on two factors: the form of the receiving sites and the mobility of sediments within them. Investigation of coastal landforms at a range of scales identifies the framework within which sedimentation takes place. Consideration of landscape sediment redistribution at Quaternary, Holocene and human timescales establishes the locations in the coastal landscape in which change has taken place. A key factor in coastal response relates to the wide size range of sediments delivered. The fractionation of sediment within the coastal domains is used as an index by which to identify the controls on coastal sedimentation. A new conceptual model of coastal behaviour, the Ordered Response Model, is developed as a framework within which to investigate coastal response. The model is operationalised in three ways. This is done first with regard to coastal sediments and their grain-size interpretation, secondly in the context of shoreline form and sediment redistribution, and thirdly in relation to form and sediment trapping within coastal embayments. The patterns of sediment redistribution are seen to reflect trapping behaviour in the coastal landscape at a range of scales. Sediments are investigated from the viewpoint of the factors which determine their retention or accumulation in or rejection from a coastal site. Shore sites are distinguished on the basis of the extent to which they trap materials delivered to them from catchment sources. Governing factors are shoreline gradient and size grade of materials. A primary fractionation of sediments takes place at the shore and the finer fractions are by-passed to the nearshore. Sediment fractions that are relatively immobile under prevailing environmental conditions develop paved lag surfaces at a range of scales. Sediments that accumulate at the shore are distinctive in their mixed sand and xv gravel composition with a dominant mode in the granule and very coarse sand grades (-291 to 091). Sediment deposited on the intertidal surfaces is found to be redistributed by a distinctive mechanism. Migratory intertidal bedforms defined here as "clastic waves" are a means by which the low energy shores disperse sediment which is delivered to them. These waves are a distinctive form of the shoreline of the Marlborough Sounds, and have attributes different from other shoreline forms identified in the literature. Clastic waves are shore-parallel, crescentic or lunate forms with longshore crest dimensions of 0.5 to 30m, length dimension perpendicular to the crest of up to 20m, and crest heights of 0.05m to 0.5m. Rates of intermittent migration vary from 1m/day to 10m/year. Key factors in their development are identified as low wave energy, tidal range, intermediate to low intertidal gradients (<1:20) and a mixed sand and fine gravel grain-size. Bathymetric form is found to reflect the varying influence of sub-bottom morphology, sediment accumulation and hydraulic reworking. Analysis of sediment thickness identifies a mean thickness over sub-bottom of 7.33m in Pelorus Sound. Spatial variations in sediment thickness identifY marginal embayments as significant sediment traps. Mean sedimentation rates calculated over a 6,000 year timespan give Pelorus Sound a spatially averaged rate of 1.22mm/yr. Sub-bottom form is shown to have a stronger role in determining bathymetric form than previously reported. Due to the constraining effect of shallow sub-bottom form on sedimentary processes sediment thicknesses in the inner Pelorus Sound are not greater than those found in channels or embayments in the middle reaches of the Sound. A mean thickness of 5.75m from sub-bottom seismic profiles in the inner Pelorus equates to a sedimentation rate of 0.96mm/year over 6,000years, at about which time the river valleys of the Marlborough Sounds were drowned by postglacial rising sea-levels. Analysis of sub-bottom form reveals evidence of previously unreported drowned terrace remnants, which are correlated to subaerial terrace remnants. On the basis of both long profile patterns along these remnant surfaces and an analysis of bathymetric form of marginal bays and channels, an interpretation is developed of the origin of form in Pelorus Channel and Tory ChanneL Sediment trapping behaviour is identified as the most distinctive attribute of this coastal landscape, and shown to operate at a range of nested scales. As a XVI consequence of trapping behaviour, the operation of any part of this coastal landscape must be considered in relation to its operation as a whole.