Abstract

Interbedded eolian dune, interdune and playa deposits, ~1.8 GA Makgabeng Formation, Waterberg Group, South Africa: Climatic Implications

Simpson, E. L., Kuklis, C.A, Department of Physical Sciences, Kutztown University, Kutztown, PA 19530, Eriksson, K.A., Department of Geological Sciences, Virginia Tech, Blacksburg, VA 24061, Eriksson, P.G, and Bumby, A. J., Department of Earth Sciences, University of Pretoria, Pretoria 0002, South Africa

Interbedded eolian dune, interdune and playa deposits of the 1.8 Ga Makgabeng Formation record one of the oldest examples of dunefield destruction by a shift from an arid or semi-arid to a more precipitation-dominated climate. Three facies are recognized: eolian dune, interdune, and playa. The eolian dune facies is composed of wind-ripple cross strata that varies from low- to high-angle dip. Associated with the high-angle strata are grainflows. Interdune deposits are composed of the following structures: wave, combined-flow, current and wind ripples, adhesion structures, mudcracks, algal rollups, and evaporite casts. Interdune strata interfinger with dune deposits. These sedimentary structures are arranged in drying-upward sequences with massive, horizontally stratified or current-rippled sandstones at bases of some interdune deposits. Vertically, wave- and combined flow-ripple deposits follow and give way to wind-ripple and adhesion structures with superimposed evaporite casts. The lateral extent of interdune deposits is limited to a maximum of 100 meters before pinching out or being eroded by the overlying dune. Playa deposits can be traced for over 5 km and consist of the following sedimentary structures: wave, combined-flow, current and wind ripples, planar-laminations, soft-sediment deformation structures, and evaporitic collapse features. Water-lain stratification is the predominate sedimentary structure. Commonly, graded beds of coarse- to fine-grained sandstone are present. The base of the graded bed is erosional, with flames and loads at the base, and is overlain by current followed by combined flow- or wave-ripple deposits. Horizontal wind-ripple strata caps some of the beds. These beds are interpreted to represent filling of the playa during a flood event; the scoured base and current ripples record surface flow as the playa fills. Combined flow- and wave-ripple deposits record the deepest water phase. The playa water dropped by either infiltration, evaporation or a combination of the two to expose the upper part of the bed to wind processes. Overall the playas record several phase of fill and desiccation. Interdune deposits with associated massive sandstones reflect increased frequency-severity of precipitation events producing wet interdune deposits leading to playa deposition.

Kutztown University of Pennsylvania

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D.J. Moore
Oct. 2002
Last modified Sept. 2004
moore@kutztown.edu