Lithological control on the elevation of shore platforms in a microtidal setting

Abstract

The shore platforms on Shag Point, southern New Zealand, are quasi‐horizontal surfaces and are developed between supratidal and low water spring levels. A range of morphologies occur, with more exposed platforms having a distinct low‐tide cliff, in contrast to low‐tide surfaces where the seaward edge is buried beneath rubble and macro‐algal growth. The platforms range in width from 20 to 80 m and are eroded into Late Cretaceous/Early Tertiary fine marine sandstones and mudstones. Shore platforms have formed in two principal lithological units: a homogeneous unit that is characterized by few discontinuities, and a fractured unit with joints spaced about 0·5 m apart. Rock hardness is low in both units (L‐type Schmidt hammer rebound values of 31 ± 4), and there is little systematic variation in values between the two units in which platforms have developed. Case‐hardened concretions within the sandstone are significantly harder than surrounding rock and cause local relief of metre scale as the spherical diagenetic features are eroded from the bedrock. They do not, however, appear to affect broad‐scale platform geometry. Joints within the bedrock are a primary control on platform elevation. Platforms formed in jointed rock occur at the lower portion of the intertidal zone, in contrast to platforms formed in unjointed bedrock, in which horizontal surfaces occur at or above mean high water spring tide level. Rock structure, therefore, appears to be the primary determinant factor of platform geometry at Shag Point. Copyright © 2006 John Wiley & Sons, Ltd.

The use of the Schmidt Hammer and Equotip for rock hardness assessment in geomorphology and heritage science: a comparative analysis

“…Stephenson and Kirk (2000) working in New Zealand, found by using the Schmidt Hammer that weathering had reduced rock strength on some platforms by up to 50% and so played a major role in their development. Thornton and Stephenson (2006) found a relationship between rock hardness and shore platform elevation in Australia, while Kennedy and Dickson (2006) used the Schmidt Hammer to assess the importance of case hardening on shore platforms at Shag Point in southern New Zealand and found it less important than structural controls, notably jointing. Similarly, the Equotip has been used for assessing the importance of case-hardening in the development of raised rims on intertidal shore platforms in Japan (Aoki and Matsukura, 2008b).…”

Patterns of surface downwearing on shore platforms in eastern Canada

“…Most of the investigations conducted during the fi rst half of the last century were qualitative and largely based on the interpretation of potentially ambiguous fi eld evidence. More recent work within the last few decades has been more quantitative, and it has frequently employed: fi eld survey to determine shore platform morphology and elevation in relation to tidal levels (Trenhaile, 1972;Takahashi, 1977;Stephenson and Kirk, 1998); Schmidt Rock Test Hammers to measure rock hardness for absolute or comparative purposes, and to identify variations in rock hardness as a result of weathering (Trenhaile et al, 1999;Stephenson and Kirk, 2000;Kennedy and Dickson, 2006); and micro-erosion meters, or their variant transverse micro-erosion meters (TMEM will be used in this paper to refer to both types of instrument), to measure slow rates of surface downwearing (surface lowering or erosion in the vertical plane) (Kirk, 1977;Robinson, 1977a,b;Gill and Lang, 1983;Stephenson and Kirk, 1998;Foote et al, 2006;Trenhaile et al, 2006;Stephenson and Finlayson, 2009;Stephenson et al, 2010). Downwearing has been ascribed to abrasion where there is suitable loose material at a site (Robinson, 1977a;Blanco-Chao et al, 2003), but where abrasives are absent, it has been attributed to weathering and removal of the fi negrained debris by waves (Robinson, 1977b;Mottershead, 1989;Stephenson and Kirk, 1998).…”

Where is the seaward edge? A review and definition of shore platform morphology

“…The platforms are N 500 m wide at low tide, concave upward in profile, with gradients of 2.5°towards the high tide mark and 1-1.5°in the lower sections (Porter et al, 2010). Similarly on microtidal Shag Point, New Zealand, Kennedy and Dickson (2006) used burial by gravellysize material to define the seaward terminus of platforms found at low-tide elevation.…”