Abstract

In recent years there has been increasing awareness of the variety of structures formed by the deformation of unlithified sediment and of the difficulty in distinguishing these from structures produced in rock. Confusion of the two types clearly has serious implications, for example, in orogenic strain sequences where it is necessary to separate pretectonic slump structures from the results of later tectonism. Unfortunately, the features of soft sediment tend to fall within the realm of sedimentology whilst those of deformed rocks are the concern of structural geology so that the region where depositional, compactional, diagenetic and tectonic processes merge tends to be viewed from di­vergent positions and perhaps receives relatively little attention.

The meeting held at Aberystwyth provided a forum for workers of differing background to assess recent progress in clarifying this difficult area. About a hundred participants heard a day of talks which made it clear that a wide diversity of phenomena on a variety of scales falls within the conference topic.

The first part of the meeting was concerned with the structures and processes which are largely confined to soft sediments, beginning with large scale features such as olistostromes and moving on to the smaller structures such as convolute laminations. The later part continued through the compaction-lithification sequence, and the effects of tectonic strains became increasingly emphasised.

Discussion following the talks returned repeatedly to two points. Firstly, there was the problem of defining terms and especially how to define the onset of lithification. The second recurrent topic was the apparent paucity of reliable criteria for distinguishing between penecontemporaneous and post-lithification structures.

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