Evidence for climate shifts in the ~ 2.0 Ga upper Makgabeng Formation erg, South Africa

Abstract

Eolian ergs and associated environs are sensitive to short- and long-term climatic changes. In one of the oldest erg deposits, the ~2.0Ga Makgabeng Formation, vertical changes in facies associations reflect one of the earliest records of short-term climatic shifts in a continental setting.The Makgabeng Formation is separated into lower and upper erg deposits by a playa or saline pan deposit. The lower erg deposit consists of dune sets with thin lenses of dry and deflationary interdune strata that transition vertically to thicker damp to wet interdune strata. A laterally persistent playa deposit in the middle of the section consists of mudstone with deep and shallow penetrating mud cracks and subordinate siltstone and sandstone interbeds. Above this lower mudstone interval, the playa strata are sandy. Overlying the playa deposit, the upper erg deposit consists of thick eolian sets with thin lenses of dry interdune deposits. Grain size change near the top of the upper erg deposit corresponds with the appearance of fluvial, sheet flood, eolian cross-beds sculpted by mass flows, and thin playa deposits.Vertical stacking of facies associations demonstrates temporal shifts in precipitation and fluctuating water tables. Fluvial and playa deposits record high-water tables whereas low-water tables are reflected in the core erg. The transition from a lower to higher water table is recorded by wet interdune interspersed within the dune strata toward the top of this interval. Rapid climatic amelioration occurred near the termination of the Makgabeng erg resulting in impingement of ephemeral river systems, development of playas, and generation of massive sand flows. This 2.0Ga erg demonstrates the impact of climate change on erg development, resulting from shifts in monsoonal impingement through time.