SEDIMENTATION-RATES, BASIN ANALYSIS AND REGIONAL CORRELATIONS OF 3 NEOARCHAEAN AND PALAEOPROTEROZOIC SUB-BASINS OF THE KAAPVAAL CRATON AS INFERRED FROM PRECISE U-PB ZIRCON AGES FROM VOLCANICLASTIC SEDIMENTS

Calculation of sedimentation rates of Neoarchaean and Palaeoproterozoi c siliciclastic and chemical sediments covering the Kaapvaal craton im ply sedimentation rates comparable to their modern facies equivalents, Zircons from tuff beds in carbonate facies of the Campbellrand Subgro up in the Ghaap Plateau region of the Griqualand West basin, Transvaal Supergroup, South Africa were dated using the Perth Consortium Sensit ive High Resolution Ion Microprobe II (SHRIMP II). Dates of 2588 +/- 6 Ma and 2549 +/- 7 Ma for the middle and the upper part of the Nauga F ormation indicate that the decompacted sedimentation rate for the peri tidal flat to subtidal below-wave-base Stratifera and elastic carbonat e facies, southwest of the Ghaap Plateau at Prieska, was of up to 10 m /Ma, when not corrected for times of erosion and non-deposition. Dates of 2516 +/- 4 Ma for the upper Gamohaan Formation and 2555 +/- 19 for the upper Monteville Formation, indicate that some 2000 m of carbonat e and subordinate shale sedimentation occurred during 16 Ma to 62 Ma o n the Ghaap Plateau, For these predominantly peritidal stromatolitic c arbonates, decompacted sedimentation rates were of 40 m/Ma to over 150 m/Ma (Bubnoff units). The mixed siliciclastic and carbonate shelf fac ies of the Schmidtsdrif Subgroup and Monteville Formation accumulated with decompacted sedimentation rates of around 20 B, For the Kuruman B anded Iron Formation a decompacted sedimentation rate of up to 60 B ca n be calculated, Thus, for the entire examined deep shelf to tidal fac ies range, Archaean and Phanerozoic chemical and elastic sedimentation rates are comparable. Four major transgressive phases over the Kaapva al craton, followed by shallowing-upward sedimentation, can be recogni zed in the Prieska and Ghaap Plateau sub-basins, in Griqualand West, a nd partly also in the Transvaal basin, and are attributed to second-or der cycles of crustal evolution. First-order cycles of duration longer than 50 Ma can also be identified. The calculated sedimentation rates reflect the rate of subsidence of a rift-related basin and can be asc ribed to tectonic and thermal subsidence. Comparison of the calculated sedimentation rates to published data from other Archaean and Protero zoic basins allows discussion of general Precambrian basin development . Siliciclastic and carbonate sedimentation rates of Archaean and Pala eoproterozoic basins equivalent to those of younger systems suggest th at similar mechanical, chemical and biological processes were active i n the Precambrian as found for the Phanerozoic. Particularly for strom atolitic carbonates, matching modem and Neoarchaean sedimentation rate s are interpreted as a strong hint of a similar evolutionary stage of stromatolite-building microbiota. The new data also allow for improved regional correlations across the Griqualand West basin and with the M almani Subgroup carbonates in the Transvaal basin. The Nauga Formation carbonates in the southwest of the Griqualand West basin are signific antly older than the Gamohaan Formation in the Ghaap Plateau region of this basin, but are in part, correlatives of the Oaktree Formation in the Transvaal and of parts of the Monteville Formation on the Ghaap P lateau. (C) 1998 Elsevier Science B.V. All rights reserved.