The stratigraphy, sedimentology and structure of the Cango group north of Oudtshoorn, C.P.

[摘要] ENGLISH ABSTRACT:This investigation aims at determining the paleogeographicand tectonic setting of a Precambrian suite of low grademetamorphic rocks north of Oudtshoorn, Cape Province, bymeans of a stratigraphical, sedimentological and structuralanalysis.The Cango Formation is raised to Group status, while formalnames are introduced in a new, more detailed stratigraphicsubdivision.The lowest unit, the Matjies River Formation, was depositedin an environment varying from that of an unstable shelf(Nooitgedagt Member) to an isolated platform (Cave Member).The main source area lay to the west. After submergencethe turbidites of the Groenefontein Formation were depositedin deeper water. Transport was in a westerly directionalong the trough axis, with the main supply from the flanksto the north and south. During subsequent uplift the basinwas locally eroded and the Vaartwell conglomerates (partlycannibalized from the Groenefontein Upper Member) weredeposited in alluvial fans(polymictic member) and onbeaches (monomictic member). Transport was roughly inan easterly direction. Westward transgression of the searesulted in upward grading of the Vaartwell to infralittoral(Uitvlugt Formation) and circalittoral (Gezwinds KraalFormation) deposits. The later grade laterally into thefluvio-deltaic sediments of the Schoongezigt Formation,derived from the east. An overall flysch- molasse - flyschcyclicity is in evidence.The regional setting is visualized as an embayment situated atthe southern edge of a continent and connected in the southeastto the open sea. The source areas were probably of anon-volcanic nature.The Cango rocks were subsequently deformed by horizontal,northward-directed compression as shown by the style offolding and thrust faulting. The regional strike is arcuate,convex to the north and conforms to the outcrop pattern of theoverlying Cape Fold Belt in this area. This is either a post-Cape phenomenon affecting both the Cango and Table Mountain Group, or it indicates that the pre-Cape tread controlled the deformation of the latter.Metamorphism of the Cango rocks did not exceed the lower greenschists facies.The Schoemans Poort Formation was deposited unconformablyin isolated depressions (mainly as fluvial sediments) alongthe margin of elevated highlands after uplift and erosionof the Cango Group. It represents a molasse deposit.A suite of basic rocks subsequently intruded both the CangoGroup and Schoemans Poort Formation.The distinctive paleogeography of the Cange Group and theSchoemans Poort Formation precludes any correlation with thepossibly time-equivalent Malmesbury Group or KlipheuwelFormations respectively.Seen in perspective the depositional and deformational historyof the Cango sediments in all important aspects resembles themarginal basin association of the miogeosynclinal zone. Assuch they may contribute to a better understanding of thefundamental processes responsible for diastrophic cycles.

[发布机构] Stellenbosch University