Sandstone Sheets and Related Intrusions from Coffee Bay, Transkei, South Africa

Abstract

ABSTRACT At Coffee Bay fine-grained greywackes, silts and shales are attributed to the Southern facies of the Lower Ecca of the Karroo System. Within this sequence a structureless shale is riddled with intrusive sandstones; sheets, often branching, dominate the complex, but flat cross-cutting sheets and offshoots, and steeper folded dykes, are also developed. Irregularities in the structures reflect anisotropies in the host material, or post-injection folding and (rarely) boudinage. Certain joints were in existence at the time of injection, but nevertheless appreciable compaction took place thereafter. Lack of any age difference where intrusives with different attitudes are in contact indicates the rapid coeval injection of all the sandstones; this took place through a thickness of 40 m, to which a compaction factor must be added. The origin of these intrusions is speculative, although clearly they are derived from turbidite greywackes. Their random orientation may reflect an isotropic state in both the stress field and the host rock. Or, more speculatively the sheets may result from compression in an adjacent source area, with, in contrast, the more discordant features reflecting differing conditions in the area of study.