Abstract:

More than a hundred circular structures are observed at the topmost part of the Late Ordovician glaciogenic record in the central Anti-Atlas, southern Morocco. The structures were formed in the outer glaciated platform and characterize the Hirnantian deglaciation sequence. They are distributed within a 20 meter thick stratal interval and, at present, relicts of them still occupy several square kilometers in map view. The circular to ovoidal structures, which comprise fluvial sandstone beds and are sealed by shallow marine strata, occur in closely spaced clusters. They are 13–320 m in diameter, with half of them in the 40–90 m range. Stratal wedging and bending, as well as extensional and reverse centimeter- to meter-scale soft-sediment fractures, demonstrate that formation processes initially involved synsedimentary uplifts. Subsequent subsidence and collapse resulted in the formation of circular structures, which essentially correspond to localized depocenters in the form of small-scale sag basins. Regarding the overall deglaciation context, the circular structures are interpreted as periglacial in origin, with vertical movements ascribed to cryogenic processes involving the growth and then decay of bodies of segregation ice. The size and diameter distributions, which are comparable to the recent (Younger Dryas) and current lithalsa structures, suggest that Hirnantian circular features relate to similar genetic processes and can be referred to as lithalsa-like structures. A conceptual emplacement model is proposed, which considers that a long-term sedimentary aggradation over the fluvial plain prevented any significant landforms (either a bump or a depression) from being expressed at the depositional surface at the time of the circular structure formation.

The Hirnantian lithalsa-like structures constitute firm evidence for a Late Ordovician discontinuous permafrost during the deglaciation stage. Assuming that the late Quaternary meridional zonation is valid, it suggests the Anti-Atlas was at latitudes in the 55–65° interval. If combined with the rare pingo structures described in the literature, which attest that coeval continuous permafrost conditions occurred at higher paleo-latitudes, the Moroccan circular structures provide evidence for an alternate South Pole position during the Hirnantian (444 Ma).

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