SIMPSON, E. L.¹, K. A. ERIKSSON², P. G. ERIKSSON³, and A. BRUMBY^3
1Dept. Physical Sciences, Kutztown Univ., Kutztown, PA
²Dept. Geol. Sciences, Virginia Tech, Blacksburg, VA
³Dept. Geology, University of Pretoria, Pretoria, South Africa

Abstract: Genesis of Massive Sandstones Interbedded with Eolianites: the 1.8 Ga Makabeng Formation, South Africa

The 1.8 Ga Makgabeng Formation presents a unique opportunity to address the genesis of massive sandstones interbedded within eolian sequences because of 3D exposure that permits characterization of the geometry of sandstone bodies and their location in relation to dune morphology. The Makabeng Formation is bounded above and below by fluvial sediments. Massive sandstones are located within the upper part of the formation and are associated with deposits of barchan and barchanoid dunes. Bases of massive sandstones are channelized or flat. Channelized bodies typically overlie low-angle or up to 26° inclined wind-ripple strata. Channel margins vary from low-angle to vertical and overhanging. Rarely massive sandstones at channel margins display vague horizontal stratification and/or contain ripped-up fragments of wind-ripple strata. Flat-based, lenticular massive sandstones are commonly interbedded with low-angle to horizontal dune toesets and onlap dune reactivation surfaces. Poorly preserved dewatering structures are present at tops of some massive beds. Lower boundaries of massive sandstones change from channelized to sharp-based down the dune foresets. Steep margins, channelization, lack of tractional structures, rip-up fragments, and the presence of dewatering features indicate that the massive sandstones were emplaced as water-saturated flows down the dune lee face. Linkage to the reactivation surface suggests that precipitation events were the triggering mechanism for initiation of partial lee-face collapse. Flows were intially turbulent resulting in scouring of wind-ripple strata. As flows migrated onto the dune plinth, some flows eroded into the underlying preserved dune whereas others lost momentum and their deposits are preserved as sharp-based, lenticular sandstones interbedded with plinth deposits.

AAPG Search and Discovery Article #90928©1999 AAPG Annual Convention, San Antonio, Texas