Saline pan deposits from the ∼1.8 Ga Makgabeng Formation, Waterberg Group, South Africa

Abstract

Interbedded medium- to fine-grained sandstones and mudstones, which occur within an 800-m-thick areally extensive aeolian dune and interdune deposit of the 1.8 Ga Makgabeng Formation, Waterberg Group, South Africa, are interpreted as saline pan deposits. Few examples of saline pan deposits have been documented from the Precambrian rock record; most of those described have been assigned to alkaline-saline and playa lake settings. Seven saline pan facies are recognized in the Makgabeng Formation: (A) strongly asymmetrical ripples, (B) inversely graded laminations, (C) slightly asymmetrical ripples, (D) symmetrical ripples, (E) massive sandstone, (F) horizontal lamination, and (G) massive-graded mudstone. These facies are interpreted respectively as current-ripple, wind-ripple, combined-flow ripple, wave-ripple, suspension (E and G) and upper plane-bed deposits (F). XRD analysis of saline pan mudstones from drill core south of the study area indicates the presence of trace amounts of anhydrite and gypsum. Additional evidence for evaporite deposition includes evaporite pseudomorphs in the underlying dune deposits and distinctive structures linked to efflorescent salt crust development. Stacking of facies reflects fluctuations (10's to 100's of years) in the hydrologic regime of the saline pan, whereas the efflorescent crusts are consistent with shorter period (months or years) flooding and desiccation cycles. Maximum saline pan depths and geographic extent are recorded by thick, graded mudstone facies (G) near the base of the deposit, indicating a time of possible climatic amelioration and saline pan expansion, followed by semiarid to arid conditions reflected in the overlying dune deposits.