3.2 Ga eolian deposits from the Moodies Group, Barberton Greenstone Belt, South Africa: Implications for the origin of first-cycle quartz sandstones

Abstract

The 3.2Ga Moodies Group in Barberton Greenstone Belt, South Africa, contains the oldest recognized evidence for eolian processes on Earth. These eolian facies indicate that significant wind regimes existed in the Archean atmosphere. Eolian deposits are developed in the lower Moodies Group in the central part of the belt and consist of stacked inversely graded strata ranging in thickness from 2 to 10mm.Individual strata consist of fine- to medium-grained quartz sandstone at the base that passes vertically into very coarse- to coarse-grained quartz sandstone. Strata make up cross-bed sets up to 3–5m thick. Sets of wind ripple strata are either tabular tangential or in the form of broad troughs. This type of inversely graded stratification is the product of wind-ripple migration. The coarse grain size of the sediment combined with the low-angle of inclination of the strata is best interpreted as the product of sand sheet or dune plinth preservation. Based on the geometry of eolian set boundaries, dunes were probably simple barchan form.The tens-of-meters thick eolian deposits are underlain by braided stream deposits composed of subfeldspathic, pebbly sandstones. The vertical change from braided-stream subfeldspathic sandstones to eolian quartz arenites, highlights the underappreciated capacity of eolian processes to generate first-cycle quartz sandstones on landscapes pre-dating the advent of land vegetation.