The nature of jointing, Tasman Peninsula, southern Tasmania

Abstract

Existing literature describing the joints in high dolerite cliffs and at the Tessellated Pavement on the Tasman Peninsula offers conflicting views concerning their origin. Recent writers have preferred a tectonic origin. Although regional structures do affect both pavement and cliffrocks, these are not pervasive at outcrop scale. The rock pillars of the high cliffs are true, thermally produced columns, and their nature can be demonstrated by direct inspection of polygons. Polygon faces are not readily identified in collections of joint measurements, but some common orientations have been united in cliff exposure to yield ragged lineaments. This yields the illusion of more recent tectonic control, although many such orientations are clearly at least Jurassic in age. The apparently regular patterns of the pavements are regionally variable, and joint continuity and orientation are complex functions of lithology, bed thickness and pre-stressing, with new joints forming due to relaxation and weathering processes. Pavement joints are generally very recent although they too replicate much older structural trends controlled by east-west shears. Pavement ornamentation provides fine examples of physical and chemical weathering processes.