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New evidence is presented from the Arroyo del Soldado Group (ASG, Uruguay) and the Nama Group (NG, Namibia), strongly suggesting the occurrence of non-global, Phanerozoic-type glacial events. Age of the NG (550-535 Ma) is well constrained by biostratigraphy and U-Pb ages of ash beds. The Polanco Formation of the ASG was likely deposited between 570 and 540 Ma, based on biostratigraphy, chemostratigraphy and U-Pb ages of detrital zircons and basement units (Gaucher et al., 2004, 2007).
In the carbonatic Polanco Formation of the ASG, evidence of glaciation occurs at two levels. Unit B is characterized by a moderate sea level drop, as reflected by sedimentary structures. A drop in δ13C values of carbonates from +5.5 to ?4.5 ‰ PDB is observed, negative values encompassing 200m of section. Corresponding 87Sr/86Sr ratios fall from 0.7085 to 0.7073. The same unit hosts Cloudina riemkeae, index fossil of the late Ediacaran. Correlation with the Shuram-Wonoka-upper Doushantuo anomaly (ca. 560 Ma) is probable. The transition between the uppermost Polanco and the overlying Barriga Negra Formation is characterized by: (1) remarkable sea-level drop leading to shelf exposure, karstification and reworking; (2) δ13C values of carbonates drop from +2.4 to ?2 ‰ PDB; (3) 87Sr/86Sr ratios fall again to 0.7073; and (4) termination of carbonate deposition. The karstic surface thus formed has been correlated across the Río de la Plata Craton (Barker surface).
The Vingerbreek Member and underlying carbonates of the NG, show the following characteristics suggesting the occurrence of a glacial event: (1) sea-level drop, exposure and reworking of the underlying limestones of the Kuibis Subgroup (Germs, 1995); (2) a negative δ13C excursion from +6 to ?2 ‰ PDB; (3) occurrence of striated pavements and diamictites. The Vingerbreek Member is constrained between 549 and 545 Ma, as shown by U-Pb ages of ash beds. An age around 547-548 Ma seems most probable, and a correlation with the Barker surface is suggested.
A trend from the global or near-global glaciations of the Cryogenian to limited glacial events in the Ediacaran, and finally to a largely ice-free world in the Cambrian is envisaged (Gaucher et al., 2007). This trend parallels the diversification of metazoans, climate amelioration being one of the major ecological factors that permitted their explosive radiation.
Gaucher, C., Sial, A., Blanco, G., Sprechmann, P., 2004. Chemostratigraphy of the lower Arroyo del Soldado Group (Vendian, Uruguay) and palaeoclimatic implications. Gondwana Res. 7: 715-730.
Gaucher, C.; Sial, A.; Ferreira, V., Pimentel, M., Chiglino, L., Sprechmann, P. 2007. Chemostratigraphy of the Cerro Victoria Formation (Lower Cambrian, Uruguay): evidence for progressive climate stabilization across the Precambrian-Cambrian boundary. Chem. Geol., 237: 28-46.
Germs, G. 1995. The Neoproterozoic of southwestern Africa, with emphasis on platform stratigraphy and paleontology. Precambrian Res. 73: 137-151.
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