Diapirism and the Fold Zone controversy of the Ordovician glaciomarine Pakhuis Formation, South Africa

Abstract

Abstract The Fold Zone controversy centers around firstly, the relationship between the direction of ice-flow and the geometry of the fold structures, and secondly the relative importance of subglacial or proglacial buckling as a folding process compared to the role of load casts or ball-and-pillow structures. The more recent recognition of diapirs associated with the Fold Zone resulted in further field investigations. Understanding the advance-retreat sequence constrains the interpretation of the glaciotectonic processes. The Sneeukop Member, the unconformity and Steenbras Member are interpreted as a continuum ice-expansion sequence without a major retreat interlude. The palaeo-ice-flow data are reinterpreted to accommodate the regional variation of ice-flow patterns. It is concluded that the Pakhuis ice sheet expansion overriding the Peninsula basin took place by means of tributary troughs converging on the main Peninsula trough thus explaining the relationship between the direction of ice-flow and fold trend variations across the basin. The folds also vary regionally in style between upright and overturned. Diapirs were recognized both as blind mushroom structures below the unconformity and piercing upwards. Chaotic breccias and lit-par-lit intermixing of tillite and deformed, attenuated banded sandstone are interpreted as blowout material presumably formed by a piercement diapir venting at the surface. Diapirism in the Pakhuis glacial context is understood as the consequence of an ice sheet expansion over a water-logged Peninsula sand aquifer. The over-pressurized sand aquifer gave rise to diapirs and associated structures. The main features of the Fold Zone are compared to modern examples as described in the literature. Push moraine and crevasse-filling examples are considered as the best-fit models to explain the genesis of the Fold Zone.