Tidal deposits from the Archaean Moodies Group, Barberton Mountain Land, South Africa

Abstract

Tidal deposits are recognised from the Archaean Moodies Group which is older than 3300 m.y. Lithologies, sedimentary structures and vertical sequences are used to characterize four tidally influenced facies: sandstone—shale, medium- to fine-grained quartzose sandstone, conglomerate—sandstone and medium- to coarse-grained orthoquartzitic sandstone. The sandstone—shale facies with abundant flaser, lenticular and wavy bedding accumulated on mid-tidal flats. Two subfacies are recognized within the medium- to fine-grained sandstones. The first consists predominantly of small-scale bipolar cross-bedding and is considered to have formed on lower intertidal sand flats. The second, which is characterized by large-scale cross-beds associated with plane bedding, probably formed in a complex of flood tidal delta environments. The conglomerate—sandstone facies consists of upward-fining units enclosed within rhythmically interlayered argillaceous sands and clays. These sediments are interpreted as tidal channels which meandered across estuarine tidal flats. Ebb-oriented trough cross-beds passing upwards into plane-bedded sands with clay partings, coupled with the geometry of the sand body, suggest that the medium- to coarse-grained orthoquartzites represent subtidal and intertidal sand shoals. A palaeoenvironmental synthesis of the facies relationships indicates that back-barrier tidal flats occurred lateral to a non-barred estuary.