Abstract

In the Falkland Islands, diamictite of Carboniferous to Early Permian age unconformably overlies Devonian sandstone and quartzite and extends gradationally upward into black shale of Permian (?) age. The unit, previously termed Lafonian Tillite, consists dominantly of diamictite containing 5–50 per cent subrounded to subangular clasts up to boulder size set in a very poorly stratified, sandy mudstone matrix, now recrystallized. Maximum clast diameter is about 7 m, and clast types include those derived locally from both the Cape Meredith basement complex and the Devonian sequence as well as volcanic, carbonate, and mafic hypabyssal rocks of unknown source. No striated floor was recognized, and striated and clearly faceted clasts are exceedingly rare.

The stratigraphic section ranges in thickness from 350 to 850 m and includes isolated intercalations of stratified rocks up to 50 m thick. These range from linear channel-fillings and small fan-shaped bodies of well-sorted sandstone and minor conglomerate in the west to extensive, bedded intercalations of pebbly mudstone, graded graywacke, and shale with dispersed pebbles in the east. These intercalations define a western and an eastern facies within the Lafonian Diamictite; an intervening third facies consists of disrupted and contorted slabs derived from sandstone of the western facies. Transport from west to east is indicated by small-scale sedimentary structures, trends of the channels, eastward decrease in mean and maximum clast size in the diamictite, diamictite clast-fabric, and eastward thinning of the entire unit.

The linear sand bodies are interpreted as subglacial eskers debouching onto small submarine fans; the western facies is accordingly assigned to marginal glacial conditions. The eastern facies is interpreted as offshore marine (?) in origin, and the intermediate facies is interpreted as slope deposits resulting from submarine mass movement. It is considered likely that the diamictite was derived from shelf ice fringing southeastern South America during Late Paleozoic time.

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