Abstract

Massive sandstone bodies in the 1.85 Ga Makgabeng Formation, South Africa, are located within the middle and upper part of the formation and are associated with the oldest reported deposits of barchan and barchanoid-ridge dunes. Bases of massive sandstone bodies are channelized or planar. Channel-based bodies typically overlie low-angle or up to 26° inclined wind-ripple strata. Channel margins vary from low angle to vertical. Rarely, massive sandstones at channel margins display vague horizontal stratification and/or contain ripped-up fragments of wind-ripple strata. Planar-based massive sandstone bodies are lenticular and characteristically are interbedded with low-angle to horizontal dune toesets. These lobate massive sandstone bodies vary from 5 cm to 6 m in thickness and from 3 m to possibly over 50 m in width; some of the sandstone bodies onlap dune reactivation surfaces. Rare dewatering structures, parting lineations, and adhesion structures occur at tops of the massive sandstone bodies. Lower boundaries of massive sandstone bodies change from channelized to planar-based down the dune foresets.

Massive sandstones were generated by two different mechanisms, which are inferred to have been triggered by significant precipitation events. The presence of steep margins, channeling, rip-up fragments, and dewatering features, and a lack of tractional structures, indicate that most massive sandstones were deposited from hyperconcentrated flows down the dune lee face. Flows were initially turbulent, analogous to hyperconcentrated flows, resulting in scouring of wind-ripple strata. As hyperconcentrated flows migrated onto the dune plinth, rapid deposition produced lobate massive sandstone deposits with planar bases. This sedimentation is linked to development of a hydraulic jump at the slipface-plinth intersection, which increased flow depth and reduced flow velocity. Some flows maintained their turbulence onto the dune plinth, resulting in erosion of the plinth and scouring into the underlying preserved dune deposits. Massive sandstone bodies linked to reactivation surfaces are more likely a result of partial lee-face collapse, which generated translational slides.

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