Lithostratigraphy of the Mesoproterozoic Bethesda Formation

Abstract

The metamorphic Bethesda Formation forms part of the Areachap Group, a volcano-sedimentary unit affected by the 1.2 to 1.0 Ga tectonothermal event in the Namaqua Sector of the Mesoproterozoic Namaqua-Natal Metamorphic Province. It comprises mainly pale brown biotite- and muscovite-rich metapelitic and quartzitic schist and gneiss with subordinate amphibolite, quartzite and calc-silicate rocks. It is commonly migmatitic with pegmatitic leucosomes and veins, deeply weathered and generally poorly exposed. The extrusion of volcanic rocks in the Areachap Group is constrained between 1.30 and 1.24 Ga, whereas the Bethesda Formation is younger than 1224 ± 20 Ma based on detrital zircon dating. The regional foliation of the Areachap Group generally strikes northwesterly, reflecting isoclinal F 2 folding which obscures contact relationships. Upper amphibolite facies metamorphism in the Bethesda Formation at about 18 km depth is dated at 1205 ± 16 Ma. The close association of the Bethesda Formation with the calc-alkaline metavolcanic Jannelsepan Formation, with which it is in contact, suggests that they both formed in a volcanic arc setting shortly before the onset of the Namaqua orogeny. The Areachap Group as earlier defined included the Sprigg, Van Wykspan, Rateldraai, Bethesda and Jannelsepan Formations in the Upington area. Lithological correspondence between the Bethesda Formation and the quartzo-feldspathic Van Wykspan and contact-metamorphic Rateldraai Formations suggests that these formations should be included in the Bethesda Formation, as implemented here. The conglomeratic Sprigg Formation might be younger than the other formations of the Areachap Group, but more data on detrital zircons is needed to evaluate this. For practical reasons the original name Bethesda Formation for this mappable unit is retained, although there is a complete lack of primary characteristics, way-up and base or top criteria.