Re-Interpretation of Neoproterozoic Sediments of the Moroccan Anti-Atlas Mountains: Implication for the Snowball Earth Model

Neoproterozoic Snowball Earth hypothesis argues for some of the most controversial glacial events in Earth's history. Low-latitude diamictite/tillite and dropstones deposits combined with thick overlying marine cap-carbonates are used as evidence for repeated world-wide glacial events during the Cryogenian. The Anti-Atlas conglomerate of the Kerdous Inlier, near Tafraoute, have been suggested as part of these glacial deposits. However, no detailed field evidence yet existed in Kerdous Inlier documenting these diagnostic glacial characteristics in the Neoproterozoic conglomerate PII-III and PIII.

New field data of the conglomerate PII-III in the Kerdous Inlier have identified localities with up to meter-sized granitoid and schist clasts. This conglomerate is matrix-supported of pelitic composition with weakly developed schistosity, where the clasts are polymictic, angular to sub-rounded and poorly sorted. Overlying this unit is the conglomerate PIII which contrast in composition and level of deformation from the underlying conglomerate PII-III. This polymict para-conglomerate PIII is matrix-supported with no schistosity, composed of coarse to granular sand, where the addition of quartzite clasts progressively dominate the conglomerate sequences up section. In addition, this conglomerate unit is recognized by well-preserved depositional features with fine to medium sandstone with channel beds geometry, unidirectional current ripples, cross-bedding, and graded bedding. We interpret this conglomerate unit to be deposited in alluvial wedge, alluvial fan or braided river setting. Absence of bioturbation including Ediacranfauna indicate that conglomerate PIII represents continental setting.

The provenance of PII-III is locally derived from the Paleoproterozoic Tassrirt granite and Kerdous schist and PIII is both Paleoproterozoic Kerdous Schist and Mesoproterozoic Lkest Quartzite. The carbonate caps and evaporite deposits that typically overlays glacial deposits in the Snowball Earth model are missing here probably due to erosion or no deposition. We interpret the conglomerate PII-III to be associated with the late phase of Pan-African collision forming the Gondwana supercontinent and conglomerate PIII associated with post Pan-African extension and opening of Iapetus Ocean.