6 - The interpretation of massive rain-out and debris-flow diamictites from the glacial marine environment

Summary

Abstract

Lithofacies and clast fabric analyses of four massive diamictites from the glacigene Dwyka Formation show two (Floriskraal and Douglas) to be primarily of rain-out origin and the other two (Elandsvlei and Kransgat River) to have formed by subaqueous sediment gravity flow. Ancient massive diamictites, which exhibit no bedding or other internal structures, thus may have a multiple origin as they can form directly by dense rain-out or indirectly by resedimentation of glacial material. Rain-out and resedimentation can occur simultaneously, and distinction between the two processes in ancient diamictites is problematical. For the interpretation of ancient massive diamictites a lithofacies analysis must therefore be a first priority, because results of fabric analyses and other criteria are often inconclusive. Diagenesis, compaction and dewatering can obliterate diagnostic features necessary for the successful interpretation of massive diamictites.

Introduction

Glacial sequences from Precambrian to Late Palaeozoic in age consist mostly of homogeneous diamictites lacking sedimentary structures as well as observable grain fabrics. Such sequences attain thicknesses of several hundreds of metres over large areas and their interpretation always poses a problem to glacial geologists.

A massive diamictite is here defined as being a homogeneous rock body without bedding or other internal structures on a macro- and mesoscale (Fig. 6.1). A diamictite sequence may, however, be thickly bedded although bedding planes are commonly absent in diamictites up to several tens of metres thick. According to Petti – john (1975) truly massive beds are probably very rare and radiography of such seemingly homogeneous beds has in many cases revealed internal bedding structures.