Sole Marks and Combined-flow Storm Event Beds in the Brixton Formation of the Siliciclastic Archean Witwatersrand Supergroup, South Africa

Abstract

ABSTRACT Sole marks and associated storm event beds are described for the first time from the middle Archean Witwatersrand Supergroup. The sole marks are excellently preserved and include a variety of marks formed by combined flow: turnabout, curvilinear, and low-angle intersecting groove marks; bipolar prod marks; recurved flute marks; and shallow scour hollows. The sole marks are intimately associated with other sedimentary structures formed by combined flow or oscillatory flow, such as wave ripple marks, undulatory parallel lamination, hummocky cross-stratification, and hummock-and-swale structures. The storm event beds with sole marks were deposited in a transitional shelf setting and grade upwards into swaly and trough cross-bedded orthoquartzite deposited on the inner shelf. Interbedded storm event beds display a variety of sole mark assemblages, reflecting widely varying flow conditions in the storm-generated bottom boundary layer. Oscillatory currents were mainly oriented perpendicular to paleoshore and were combined in some cases with offshore-directed storm-surge relaxation currents. Some sole mark assemblages, specifically those with turnabout and low-angle intersecting groove marks, contain evidence for the presence of shore-oblique or shore-parallel geostrophic currents. The storm event deposits in the Witwatersrand Supergroup are thus compared with models derived from odern storm deposition.