Pyroclastic acidic volcanic rocks are intercalated between sedimentary rocks of the Roraima Formation at the Tafelberg in Suriname. From these rocks, a 14-point whole-rock Rb-Sr isochron of 1,599 ± 18 m.y. was obtained with initial 87Sr/86Sr = 0.7075 ± 0.0032 (47.2 × 109 yrs 87Rb half-life; errors with 95 percent confidence level). K-Ar measurements on five samples give dates ranging from 1,573 to 1,681 m.y. Some 200 m.y. have thus elapsed between the consolidation of the Guiana Shield during the “Trans-Amazonian orogeny” 1,810 ± 40 m.y. ago and the development upon it of the Roraima basin (or basins) of deposition. Isotopic age measurements on dolerite dikes and sills cutting the Roraima sedimentary rocks (the Roraima Intrusive Suite) indicate that the basalt intrusions took place within a relatively short time interval (some 50 m.y. at most) after the deposition of the Roraima Formation.

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