The origin and significance of load-induced deformation structures in soft-sediment and lava at the base of the Archaean Ventersdorp Supergroup, South Africa

Abstract

Many sedimentary deposits throughout the geologic record display structures that indicate the post-depositional disruption of their primary sedimentary features. Characteristically, the deformation, leading to irregular, broken or shifted stratification, takes place soon after deposition. The process of soft-sediment deformation, to which it is more commonly referred, can be attributed to four main mechanisms: reverse density gradation, liquefaction, slumping (slope deposit) and shear stress exerted by flow. Two or more of these mechanisms may act together and are notably described in purely sedimentary environments. As such, soft-sediment deformation can be considered an intra-formational response to the accumulation of a sediment pile, being largely confined to a single (sedimentary) bed, within undeformed (sedimentary) beds. However, deformation structures may also form where two compositionally and genetically contrasting substrates occur in intimate juxtaposition, as in the case of a volcano–sedimentary setting. Recognizable `soft-igneous' structures, analogous to those observed in sedimentary deposits, may form where lava flows have extruded over water-saturated sediments. In this instance, the lava `bed' substitutes for a sedimentary bed by inducing loading of the underlying sediments, resulting in the formation of deformation structures within both substrates, at their common interface. Such a situation commonly arises at the base of the Klipriviersberg Group, a±2000 m volcanic pile comprising the lower formations of the Ventersdorp Supergroup. Deformation structures in what was once soft-sediment and lava, commonly occur where lava flows comprising the basal Formation(s) of the Klipriviersberg Group overlie a pervasive, multi-lateral, multi-storey sand and gravel placer, the Ventersdorp Contact Reef (VCR). The VCR by definition forms the basal, (auriferous) conglomeratic portion of the Venterspost Conglomerate Formation (VCF) and is developed to varying degrees, predominantly around the western margin of the preserved Witwatersrand Basin. It should be noted that the terms VCR and VCF are often used synonymously on the gold mines where the basal conglomerate is economically exploited, and any reference to the one term generally implies its association with the other. Three main types of deformational structures are recognized in the VCF: small-scale sedimentary injection (perturbation) structures or “flames”, comprising entrained VCR sediment which penetrate the overlying, (then freshly deposited), lava base; hemispherical “ball-and-pillow” structures formed within the overlying lava, and soft-sediment structures exaggerated by the presence of pillow lavas. These structures are interpreted to be primarily of a hydro-dynamic origin, and collectively, form in response to a process referred to as rheologic settling. It is postulated that this type of syn-sedimentary deformation formed during the de-watering of the VCR, in response to differential loading of the water-saturated, unconsolidated, underlying sediments over which the lavas flowed. These circumstances induced a reverse density gradient at the sediment–lava interface, a condition formally termed Rayleigh–Taylor instability.