The geology of the late-Cretaceous Salpeterkop volcano near Sutherland: A geomorphologic benchmark

Abstract

The Salpeterkop volcano is spatially part of the Sutherland Suite of alkaline rocks in the Northern Cape. It is one of the best preserved volcanoes in South Africa with part of the tuff ring still intact, remnants of the ejecta mantle outside the crater still preserved, and is host to epiclastic rocks, including ash and lapillistone and water lain sediments, inside the crater. New dates from apatites and phlogopites from Salpeterkop suggest an age close to 70 Ma. This Upper Cretaceous age is supported by the silicified wood found within the epiclastic sediments. Its relationship to the alkaline rocks in this cluster is not entirely clear but field evidence suggests that initial olivine melilitites and ultramafic bodies were followed by the main eruption that produced Salpeterkop. Clear igneous components in the associated breccias and pyroclastics are rare but indicate that this volcano was linked to an alkaline (trachytic) intrusion driven by phreatic magmatism. This is further highlighted by the presence of (nepheline?) syenite xenoliths in some carbonatite breccias and dykes that are part of the later carbonatites with its associated hydrothermal alteration products. Although the carbonatites are largely late-stage, there is evidence of earlier carbonatite activity from a precursor carbonatite dyke that has off-set an olivine melilitite dyke. Relatively unaltered pyroclastics, associated with two vents, northeast and northwest of the crater respectively, represent the final phase of this volcanic centre. Finally, the preservation of the Salpeterkop crater and its associated volcaniclastics highlights the change of intense landscape denudation, that ensued from Gondwana break-up to the end of the Cretaceous, to a period of drastically reduced erosion rates during the Cenozoic Era.