Borehole LLE1 – an intra-caldera succession of the Goedgenoeg and Makwassie Formations, Ventersdorp Supergroup

Abstract

Abstract During the late 1980s a deep exploration borehole (LLE1) was collared on the Makwassie Formation outcrop at Makwassie Hills, near Leeudoringstad. The borehole intersected a 2420 m thick succession of rhyolitic quartz porphyries and interbedded andesitic mafic lavas of the Makwassie Formation, with a further 1301 m of dacitic feldspar porphyries and andesitic mafic lavas of the Goedgenoeg Formation, before it was stopped at 3721 m, still in the Goedgenoeg Formation. The mafic lavas in the Goedgenoeg Formation are low viscosity basaltic flows, as are the mafic lavas interbedded in the Makwassie Formation. The Goedgenoeg Formation feldspar porphyries and the Makwassie Formation quartz porphyries are of extrusive origin, emplaced as ignimbrites and ash-flow tuffs, ascertained by macroscopic and petrographic characteristics. Individual ignimbrite or ash-flow units are several tens of metres to hundreds of metres thick, regularly separated by volcanoclastic sandstone and calcareous shales. Geochemically the lower part of the Goedgenoeg Formation feldspar porphyries in borehole LLE1 correlates with the low-Zr dacite group of the Bothaville area, and the upper part with the high-Zr dacites. The lower part of the Makwassie Formation quartz porphyries correlates with the high-Zr/Cr rhyolite group of the Bothaville area, and the upper part with the low-Zr/Cr rhyolite group. The Bothaville area’s Makwassie Formation dacite unit is absent from the LLE1 profile. The interbedded andesitic mafic lavas are similar in composition to those in the Bothaville area. There is no structural or stratigraphic indication (through lithological or geochemical means) of duplication in this borehole’s stratigraphic profile. The borehole drilled into a caldera succession, part of a cluster of calderas associated with half-graben structures aligned from west-northwest to east-southeast, from the Makwassie Hills towards Bothaville. These and other Platberg caldera clusters are developed outside the Central Rand Basin, mainly to the west of it.