Eolian Dune Degradation and Generation of Massive Sandstone Bodies in the Paleoproterozoic Makgabeng Formation, Waterberg Group, South Africa

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 rippedup fragments of wind-ripple strata. Planar-based massive sandstone bodies are lenticular and characteristically are interbedded with lowangle 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 planarbased 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.

Secular changes in sedimentation systems and sequence stratigraphy

“…These saline pan deposits preserve micro-evaporitic features, such as those reflecting solution collapse and that have direct analogues in modern saline pans, and are one of the oldest reported examples (Simpson et al, 2004a). Climate change terminated the youngest erg (Simpson et al, 2002) as evidenced by massive sandstones generated by extreme precipitation events that degraded dune lee faces before truncation by the overlying Mogalakwena Formation fluvial system (Simpson et al, 2002;Eriksson et al, 2008a …”

Sedimentation patterns during the Precambrian: A unique record?

“…60 Ga Hornby Bay Group and Thelon Formation (NW Canada) sandstones are interbedded with the lower portions of aeolian cross-bed sets and dune toesets (Fig. 7), indicating genesis through mass-flow processes related to significant precipitation events; hyperconcentrated flows are inferred to have flowed down dune lee faces and eroded into them, with lobate deposits resulting on the dune plinths (Simpson et al, 2002). Several metres above the uppermost interdune bed, Simpson et al (2004b) documented saline pan deposits, up to about 2 m in thickness, with facies stacking patterns suggesting flooding and desiccation cycles of variable lengths (estimated at months to hundreds of years).…”

“…Several metres above the uppermost interdune bed, Simpson et al (2004b) documented saline pan deposits, up to about 2 m in thickness, with facies stacking patterns suggesting flooding and desiccation cycles of variable lengths (estimated at months to hundreds of years). Although not large by current erg sizes, the Makgabeng counts as one of Earth's oldest such deposits, and exhibits most of the features to be observed in any Phanerozoic -Modern desert succession, with strong evidence for an interplay of temporally dominant aeolian and subordinate aqueous processes and products (Eriksson et al, 2000, Simpson et al, 2002, 2004a.…”

“…The massive sandstones are interpreted as the product of massive rainfall events resulting in hyperconcentrated flows which passed down dune lee faces, eroding them in the toesets (Simpson et al, 2002). Centimetric scale; location on Makgabeng Plateau, Limpopo Province.…”

“…These sedimentary beds were laid down within two basins on the Kaapvaal craton (Fig. 4) and are dominated by fluvial deposits, with lesser desert, fan and lake sediments Simpson et al, 2002Simpson et al, , 2004b.…”