Abstract

Improved constraints on the timing of Permian clastic sedimentation in the Karoo basin of South Africa have been revealed by new U-Pb single-grain zircon ages determined using the Sensitive High Resolution Ion Microprobe Reverse Geometry (SHRIMP-RG). Analyses were performed on 180 zircon grains recovered from six ash beds exposed in the southwestern part of the Karoo basin. Samples were collected from two distinct loci of deep-water basin-floor and submarine-slope deposition within the Permian Ecca Group: the Tanqua and Laingsburg depocenters. Previous work constrained depositional ages of the 1800-m-thick deep-water to fluvial succession to 270–255 Ma. This poor chronostratigraphic resolution precluded robust stratigraphic correlation between the depocenters and impeded interpretation of basin evolution. The new data suggest that deep-water siliciclastic deposition began ~ 275 Ma (the lower Permian Collingham Formation), while the youngest ages are latest Permian. The Tanqua and Laingsburg depocenters began sand deposition at different times. Sandstone deposition in the Laingsburg depocenter started sometime after 275 Ma, and deposition of the first thick turbidite-dominated succession (Fan A) is bracketed into a period of less than 20 My. The Tanqua depocenter was sand starved during this interval. Both the uppermost sample from the Laingsburg depocenter and the lowermost from the Tanqua depocenter yield similar maximum depositional ages. This indicates, within the resolution of the age data, synchronous submarine sandstone deposition in the two loci at this time (~ 255 Ma). These data indicate that submarine deposition was still ongoing at a time when fluvial floodplain conditions are interpreted elsewhere in the Karoo basin.

All the ash samples contain late Paleozoic zircon-grain age populations, while three samples contain small numbers of early Paleozoic, Proterozoic, and Archean zircon grains. Paleozoic zircon-grain age populations are interpreted as a record of arc activity linked to subduction. Increasing numbers of Precambrian zircons stratigraphically upward provide insight into changing source terranes during deposition into the Karoo basin.

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