Abstract

In this paper a stratigraphic scheme is proposed for the Neoproterozoic Rosh Pinah Formation in southwestern Namibia. The formation forms part of the Port Nolloth Zone in the Pan-African Gariep Belt and has become famous for hosting two major economic base metal deposits at Rosh Pinah and Skorpion. It is characterised by the presence of volcanic rocks in an otherwise clastic-dominated sediment succession. Genetically, it is related to progressive opening of a failed intracratonic rift graben in the east (Rosh Pinah Graben) that was separated by a basement horst from a half graben to the west, which developed into the Adamastor Ocean. During early rifting, sedimentation started along the western flank of an actively rising fault scarp as alluvial fans, alluvial plains and fan deltas. The main rock types are arkose, dolomitised limestone, calcarenite, siltstone, mudstone and a variety of volcanic and pyroclastic rocks related to bimodal, predominantly felsic ~750 Ma magmatism. Sedimentation of the Rosh Pinah Formation began during a glacial period that is correlated with the global 720 to 750 Ma Sturtian glaciation.

The Rosh Pinah Formation displays repetitive sedimentary cycles that reflect rapid deposition with intervening quiescent periods related to reactivation on basin bounding faults with subsequent thermal and mechanical subsidence. Sedimentary facies play an important role in the localization of sedimentary-exhalative and hydrothermal replacement mineral deposits as fine-grained anoxic sediments. While deposition of the Rosh Pinah Formation volcano-sedimentary successions was sustained in the more active parts of the basin, background sedimentation to the west is reflected by more distal deposits, ranging from planar laminated allodapic limestone to mudstone, followed by platform carbonates. In proximal positions, tilting of the rift shoulders led to partial erosion of the Rosh Pinah Formation and thus to intraformational breccias and olistostromes.

The Rosh Pinah Formation is overlain by a regressive, siliciclastic succession, which has been interpreted as reflecting eustatic seawater fall in preparation of a major, probably syn-Marinoan (~580 Ma) glaciation. Finally the original geometry of the Rosh Pinah Formation depository was largely overprinted by intense ductile deformation during the Gariepian orogeny, which started ~580 Ma and reached its peak at ~545 Ma.

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