The nature of the Late Paleozoic glaciation in Gondwana as determined from an analysis of garnets and other heavy minerals: Discussion

Gravenor (1979) has used chattermark trails found upon the surfaces of detrital garnet grains as palaeoenvironmental indicators of both the degree and multiplicity of glaciation in an area. Such a practice of using this microtexture analysis to authenticate ancient glacial deposits (developed from Folk (1975); Gravenor et al. (1978)) may prove erroneous or, at the least, questionable in the light of scanning electron microscope theory (Margolis and Kennett 1971; Whalley and Krinsley 1974). In addition, some problems recently identified regarding the origin of these chattermark trails (Bull 1977; Rocha-Campos and Krauspenhar 1978) need to be considered. A major aim of Gravenor (1979, p. 1138) was ". . . to examine the evidence provided by an analysis of heavy minerals, particularly garnets . . . to provide an explanation for certain of the controversial aspects of this (the Gondwana) glaciation." (our parentheses). The evidence cited depended heavily upon the acceptance that chattermark trails are not only mechanically formed but that they are glacially formed. We wish to question the hypothesis of formation of glacial grinding as the sole mechanism of chattermark genesis and in support of these views we shall draw upon recent results that we have obtained from analysis of garnet and quartz grains taken from "nonglacial" deposits (Table 1). More detailed presentation of these data and their implications to chattermark formation is presently in preparation (Bull et al. , in preparation). The deposits were taken from beaches on Freetown Peninsula, Sierra Leone, and from Cape Comorin beach and dune complex, Tamilnadu State, southern India. In both instances, fieldwork and literature searches have shown that the deposits sampled are not likely to have been affected either by ancient or more modern glacial modification. Those samples from the Freetown Peninsula have derived from Archaean granitic rocks either from the north-central Sierra Leone or possibly from the south along the West African coastline. Those samples taken from Cape Comorin have also derived from more ancient basement rocks (gneiss, granulite, and charnockite) of the Ghats Mountains to the west. The implications of our findings (Table 1) are that chattermark trails can seemingly be found in the same or higher percentages upon quartz and garnet grains from nonglacial sediments as those identified from glacial sediments by Gravenor. Furthermore, these results are compatible with those of RochaCampos and Krauspenhar (1978) who considered the features to be formed in a subaqueous environment with later chemical etching. The samples taken from both India and Sierra Leone have, at one time or another, been subjected to modification in the beach environment. Whether the textures are mechanically formed in subaqueous environments later to be chemically etched or indeed solely the result of chemical action requires further consideration (Bull et al., in preparation). Nevertheless, it can be shown that chattermark trails are not exclusively formed by glacial modification (whatever "glacial" modification is: see Whalley and Krinsley (1974)). Most certainly, glaciers do impart chattermark trails upon grains, however, great caution must be exercised in using these textures for environmental reconstruction. By considering that chattermark trails were at

Field of study: Environmental science