Paper No. 320-8
Presentation Time: 10:45 AM
AEOLIAN BEACH DEPOSITS IN THE 2.64 GA VRYBURG FORMATION, GRIQUALAND WEST, SOUTH AFRICA
The <10 to >100 m thick Vryburg Formation, Schmidtsdrift Subgroup, is the basal unit of the Transvaal Supergroup in Griqualand West, SW Kaapvaal craton, and rests unconformably on the 2.8 to 2.7 Ga Ventersdorp Supergroup. The 2642 Ma Vryburg conglomerates, quartz arenites, shales and carbonates are interpreted as transgressive, shallow, marginal and tidal near shore deposits. They are gradationally overlain by stromatolitic, oolitic and calc-arenitic carbonates of the Boomplaas Formation, above which the Lokammona Formation shales terminate the >300 m thick, overall deepening, transgressive cycle. Within the Vryburg Formation, conglomerates and quartz wackes with carbonate lenses are overlain by compositionally mature sandstones exhibiting typical characteristics of aeolian deposition. These medium grained quartz arenites, with mostly spherical and very well rounded grains, exhibit diagenetic and/or metamorphic syntaxial quartz overgrowth making them clean, white quartzites. Sedimentary structures such as ripple marks and cross-bedding appear at scales of a few cm to <1 m thick. Intercalated sets of planar cross-beds at shallow angles to each other, forming wedges up to 20 m long and over 2 m high, are typical of aeolian sand dunes. The large aeolian lee-beds are regularly rippled, whereas the troughs of large dune structures contain coarse grained planar sandstone beds with symmetrical and asymmetrical ripples (interdune deposits). Dune wedge bases are mainly erosional and traces of grain avalanching are evident on cross-bed planes. These aeolian facies within marginal marine to fluvial deposits make the Vryburg Formation a complete beach-near-shore Neoarchean section, one of the oldest such well-preserved sections in the world.