Abstract:
Eolian ergs, and associated environs are sensitive to short- and long-term climate
changes. In one of the oldest, erg deposits, the ~2.0 Ga Makgabeng Formation, facies
association changes reflect one of the earliest recordings 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 interdunes that transitions vertically to thicker damp to wet
interdunes. 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 to the appearance of fluvial, sheet flood, eolian cross-beds sculpted by mass
flows, and thin playa deposits.
The vertical facies association demonstrates 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 towards 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 2.0 Ga erg demonstrates the impact of climate
change on erg development, resulting from shifts in the monsoonal impingement through
time.
