Circular, Cryogenic Structures from the Hirnantian Deglaciation Sequence (Anti-Atlas, Morocco)

• Published in: Journal of Sedimentary Research
• Main Authors: Nutz, A., Ghienne, J.-F., Štorch, P. (Petr)
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2013
• Subjects: Ordovician; Anti-Atlas (Morocco); cryogenic structure; Ice; permafrost
• Online Access: https://doi.org/10.2110/JSR.2013.11; http://hdl.handle.net/11104/0218189

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 glacial platform and characterize the Hirnantian deglaciation sequence. The circular to ovoidal structures, which comprise fluvial sandstone beds and are sealed by shallow-marine strata, occur in closely spaced clusters. The circular structures are interpreted as periglacial in origin, related to cryogenic processes involving the growth and then decay of bodies of segregation ice. The size and diameter are comparable to the latest Pleistocene and current lithalsa-like structures. A conceptual emplacement model is proposed. The Hirnantian lithalsa-like structures constitute firm evidence for a Late Ordovician discontinuous permafrost during the deglaciation stage in the area of present Morroccan Anti-Atlas.