Late Proterozoic periglacial aeolian deposits on the West African Platform, Taoudeni Basin, western Mali

The Late Proterozoic Bakoye 3 Formation is a predominantly aeolian unit deposited in the glacially influenced cratonic Taoudeni Basin of western Africa. The Bakoye 3 can be divided into five distal units, two proximal units, and a local upper massive sandstone. The basal Unit 1 shows a complex interfingering of aeolian and subaqueous structures, and is interpreted as the precursor of the overlying erg sequences. Unit 2 consists of compound, trough cosets of aeolian cross-strata dominated by grain-flow strata. The unit is interpreted to represent draas with superimposed, small, crescentic dunes. A super bounding surface marks the termination and planation of the erg. Unit 3 is distinguished from the underlying Unit 2 by its larger, overall simple sets of trough cross-strata, interpreted to represent simple, large, crescentic dunes. Unit 4 occurs only locally in laterally discontinuous, large troughs. In one case the trough is filled by small sets of tabular cross-strata dominated by grain-flow deposits. At another section, wedges of coarse-grained wind-ripple strata fill the trough. Unit 4 may represent remnants of ergs or, more likely, local deposition in depressions. The depressions, in the latter scenario, formed with the development of a second super surface that truncates Unit 3. Unit 5 consists of very large sets of wind-ripple cross-strata with less common sets of grain-flow deposits. These deposits are believed to represent enormous dunes with large plinths and subordinate slip face development. A third super surface separates Unit 5 from overlying marine deposits. Together, Units 1–5 represent the core of the ergs in a distal position relative to adjacent upland source areas. Proximally, aeolian deposits are simple, smaller, trough sets interpreted as moderate sized crescentic dunes. Coarse-grained braided stream deposits are prominent. Locally, the top of the Bakoye 3 is marked by channelized mass-flow deposits containing aeolian blocks, and is believed to have resulted from iceberg grounding. An overall environment for the Bakoye 3 is one of uplands marked by ice sheets, with outwash plains extending distally to aeolian ergs. Super surfaces, all marked by polygonal fractures and coarsegrained sediment, represent periods of erg termination that may be linked to glacial-fluvial-aeolian cycles.