Grooves in the heart of glacial reconstructions: how to distinguish ice stream beds and iceberg keel ploughmarks in Earth's ancient glacial record

We investigate the origin of an exceptionally well-preserved Late Palaeozoic soft-sediment glacial pavement at Oorlogskloof, South Africa and make a palaeoenvironmental reconstruction of deglaciation at this time. Elongate bedforms exposed at the base of the Dwyka Group have previously been interpreted as subglacial flutes and used as evidence of a fast-flowing soft-bedded ice stream. However, analysis of near-identical bedforms of a similar age in Brazil have revealed formation through iceberg ploughing in a shallow glaciomarine environment, warranting re-examination at Oorlogskloof. Fine-resolution orthomosaics and digital terrain models generated from aerial and ground-based structure-from-motion, coupled with field observations, were used to undertake detailed geomorphological mapping of landforms at the site. Elongate bedforms (oriented ESE-WNW) include V-shaped grooves and striations, tapered flutes, and sharp crested asymmetrical ridges with centimetre-scale sediment gravity flow lobes on their slopes. These are accompanied by arcuate push ridges with elongate limbs which parallel the adjacent bedforms. We propose the elongate features were streamlined by a grounded iceberg keel in a shallow marine environment, where the convex core of the arcuate push ridges indicates scouring towards the WNW. Asymmetrical ridges are interpreted as lateral berms since the preservation of sediment gravity flow lobes on their gently dipping slopes necessitates an unconfined substrate at the margins of the grounded iceberg keels. These features further testify to the soft sediment condition of the pavement at the time of bedform formation. Our analysis reveals a pavement formed by iceberg ploughing near a dynamic, retreating ice sheet margin and contests the evidence for an LPIA ice stream positioned over this site, with key implications for locating the position of the palaeo-ice margin over this region of Gondwana. We contend that the site also serves as a valuable sedimentary analogue for contemporary and Pleistocene deglacial landsystems that remain largely inaccessible on today's seafloor.