Fluvial‒marine interactions and ichnofacies in the Ordovician Peninsula Formation, Hermanus, South Africa

A moderately deformed succession of the Ordovician Peninsula Formation in the syntaxis zone of the Cape Fold Belt is well exposed at Hermanus, some 120 km east-southeast of Cape Town in South Africa. The oldest deposits recognised at Hermanus represent a regressive shallow-marine prodelta succession of storm-redeposited sandstone, silty bioturbated sandstone and siltstone. Some clean sandstone beds show abundant fugichnia and other trace fossils characteristic of the Skolithos Ichnofacies. The presence of hummocky cross-stratification and evidence of longshore current activity indicates water depths were probably just below storm wave-base. Between storms, flocculation of organic-rich mud from buoyant hypopycnal plumes associated with river floods suggests a prodelta environment in front of the prograding Peninsula Formation fluvial succession. A ravinement surface, with at least 4 m erosional relief, between the marine and fluvial successions may have been induced by a drop in relative sea-level. The overlying fluvial succession shows sedimentary features typical of a bed-load dominated fluvial system with planar and trough cross-stratified sandstone and scattered pebbles. These form upward-fining cosets (4‒8 m thick) of predominantly downstream-accreting macroforms that indicate deposition from high-energy channelised flows with up to 90° palaeocurrent variability between cosets. They represent a broad, braided to low-sinuosity, fluvial channel system that drained across a low slope towards the south-southeast. Based on correlation with the ‘Rooiels marker’ north of Hangklip, the shallow-marine and overlying fluvial successions at Hermanus may represent part of the lower Peninsula Formation, equivalent to part of the ‘Leeukop member’ in the Cape Peninsula. • Palaeo-environments in the Ordovician lower Peninsula Formation, Cape Fold Belt • Initial shallow marine deposition in a storm-dominated prodelta environment • Skolithos Ichnofacies • Ravinement surface between marine and overlying fluvial deposits • Braided to low sinuosity fluvial channels flowed to the south-southeast