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

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 (c) 2006 John Wiley & Sons, Ltd.