NEW INSIGHTS ON THE LATE PERMIAN KAROO BASIN EVOLUTION FROM SHRIMP-RG U-PB ZIRCON DATING

摘要: New U-Pb ages were determined using the Sensitive High Resolution Ion Microprobe Reverse Geometry (SHRIMP-RG) for 180 zircon-grains recovered from six ash beds in the southwestern Karoo basin of South Africa. Samples were collected from two distinct loci of deepwater basin floor and submarine slope deposition within the Permian-aged Ecca Group in the Tanqua and Laingsburg depocenters. Previous work constrained the depositional ages of the> 1500 m thick deepwater to fluvial succession to between 270-255 Ma. This poor chronostratigraphic resolution precluded the creation of a robust stratigraphic correlation between the depocenters and impeded interpretation of basin evolution. The new data suggest that deepwater deposition initiated at~ 275 Ma (Collingham Fm.), while the youngest ages (in submarine slope deposits) are late Permian. The Tanqua and Laingsburg depocenters were filled at different times. Sandstone deposition in the Laingsburg depocenter initiated sometime after 275 Ma and deposition of the first thick turbidite succession (Fan A) is bracketed into a period of~ 15 my, while the Tanqua depocenter was sand starved. The topmost sample from the Laingsburg depocenter and the lowermost from the Tanqua depocenter yield similar maximum depositional ages and are interpreted as indicative of synchronous submarine deposition in the two loci at this time (~ 255 my). These data indicate that submarine slope deposition was still active at a time when biostratigraphic data (non-marine reptile assemblages) suggest fluvial floodplain conditions existed elsewhere in the Karoo basin. All rhyo-dacitic ash samples …