Stratigraphy and depositional setting of the Witpoort Formation (Witteberg Group, Late Devonian) in the Eastern Cape, South Africa

Abstract

The Witpoort Formation, a Late Devonian (Famennian) coastal siliciclastic sedimentary succession, has not received a detailed sedimentary facies analysis in the past three decades, despite significant palaeontological discoveries from its strata. Here, sedimentary facies, sediment dispersal patterns and ichnology are, for the first time, applied in combination to form a sedimentological background to the only high-latitude palaeoecological dataset from its time stage. The results of this thesis indicate a non-marine, sheet-braided fluvial deposit in which transgression of the coast led to the development of marginal-marine, estuarine facies, favouring the preservation of body and trace fossils. The deposits accumulated in erosively based, valley-fill fluvio-estuarine cyclothems along an approximately WNW–ESE trending shoreline bordering the Agulhas Sea. Towards the top of the succession (Perdepoort Member) fluvial progradation, and the lack of marginal-marine facies, indicate maximum regression, consistent with a globally recognised sea-level fall during the late Famennian. The end-Devonian Hangenberg Event is contemporary with the uppermost strata of the Witpoort Formation, in concord with prior assessments. Evidence for glaciation associated with this event in South Africa remains tenuous, though it is predicted that careful study of the Skitterykloof Member in the uppermost Witpoort Formation will find that periglacial processes were operating during its deposition, and perhaps also lower in the stratigraphic column. The Kellwasser Event, defining the Frasnian–Famennian boundary, is not recognised in the studied stratigraphic column, though it is predicted that the event will have occurred during deposition of strata that underlie the Witpoort Formation. Analysis of eustatic patterns from the global Famennian stratigraphic record and comparison with the stratigraphy described herein indicates that eustasy formed an important control on sedimentation in the Witpoort Formation. This presents the strongest evidence for constraining the chronology of its strata. There is an urgent need for further multi-proxy analyses, especially geochemical and palynological, as have been conducted in Euramerican strata, to increase the chronostratigraphic resolution and recognition of Late Devonian extinction events in the Witteberg Group. Fossil biodiversity of the Witpoort Formation is principally known from the Waterloo Farm locality, which is here shown to be lower in the stratigraphic column, and therefore older, than previously thought. Waterloo Farm is here positioned in the upper Rooirand Member. This deposit appears to have formed in a fresh- to brackish-water inner estuary, possibly tens of kilometres from the shoreline. Analysis of a new fossil locality, Rabbit Ridge, from lower in the stratigraphy, presents a comparable estuarine depositional environment to Waterloo Farm. Higher degrees of marine influence at Rabbit Ridge are indicated by the invertebrate and trace fossil signatures. The ecology of invertebrate communities and trace fossils indicate variable salinity levels and depositional rates from a spectrum of estuarine sub-environments that occurred at intervals throughout the Rooirand Member.