The two geological horizons explored in this webpage bracket the Devonian Period. Both the Baviaanskloof Formation and the Witpoort Formation (shown on the geological column below) contain interfingering terrestial and shallow marine sedimentary deposits, with lenses of black shale that formed under lagoons and estuaries. This facies contains uniquely well preserved plant and animal fossils, explaining their selection for this report.
These localities are found within the Cape Fold Belt, a band of fold mountains that stretches along the southern coastline of South Africa. This geological province contains a nearly continuous sedimentary package deposited from the Cambrian Period until the Carboniferous Period (maybe 200 million years of cumulative deposition) called the Cape Supergroup. Folding of these strata dates back to the Palaeozoic, when compression of the Earth’s crust formed an immense mountain chain along the south of Gondwana, the Gondwanides, of which the Cape Fold Belt is only a fragment.
The Baviaanskloof Formation is the uppermost unit of the Table Mountain Group. It was originally termed 'the passage beds' because it contains the transition between the quartzite dominated Table Mountain Group strata and the overlying shale rich Bokkeveld Group. This transition has been linked to global sea level rise at the beginning of the Devonian Period, however, the Silurian-Devonian boundary is not clearly recognised in the Cape Supergroup. Plant fossils provide some constraint on the age of the deposits, indicating an Early Devonian age (see Impofu Dam).
The Bokkeveld Group is a marine sequence preserving several upward coarsening cycles of shoreline progradation (outbuilding) in which invertebrate fossils are, in places, abundant. However, it is not until the Late Devonian that a good record of southerly Gondwanan terrestrial ecosystems is preserved.
The Witpoort Formation represents the final stage of the Devonian Period, the Famennian. It is a clean quartzite band about 300 metres thick that is thought to have been deposited in shallow marine and beach-barrier environments. Dating of this formation is attained by correlation with a global regression (sea level fall) that took place at this time, and this is supported by vertebrate diversity, such as, at Waterloo Farm, the last appearance of placoderm fishes in the Cape Supergroup (3).
These localities are found within the Cape Fold Belt, a band of fold mountains that stretches along the southern coastline of South Africa. This geological province contains a nearly continuous sedimentary package deposited from the Cambrian Period until the Carboniferous Period (maybe 200 million years of cumulative deposition) called the Cape Supergroup. Folding of these strata dates back to the Palaeozoic, when compression of the Earth’s crust formed an immense mountain chain along the south of Gondwana, the Gondwanides, of which the Cape Fold Belt is only a fragment.
The Baviaanskloof Formation is the uppermost unit of the Table Mountain Group. It was originally termed 'the passage beds' because it contains the transition between the quartzite dominated Table Mountain Group strata and the overlying shale rich Bokkeveld Group. This transition has been linked to global sea level rise at the beginning of the Devonian Period, however, the Silurian-Devonian boundary is not clearly recognised in the Cape Supergroup. Plant fossils provide some constraint on the age of the deposits, indicating an Early Devonian age (see Impofu Dam).
The Bokkeveld Group is a marine sequence preserving several upward coarsening cycles of shoreline progradation (outbuilding) in which invertebrate fossils are, in places, abundant. However, it is not until the Late Devonian that a good record of southerly Gondwanan terrestrial ecosystems is preserved.
The Witpoort Formation represents the final stage of the Devonian Period, the Famennian. It is a clean quartzite band about 300 metres thick that is thought to have been deposited in shallow marine and beach-barrier environments. Dating of this formation is attained by correlation with a global regression (sea level fall) that took place at this time, and this is supported by vertebrate diversity, such as, at Waterloo Farm, the last appearance of placoderm fishes in the Cape Supergroup (3).
The Devonian Period, popularly known as 'the age of fishes', gave rise to the important preliminary steps in the development of life on land. Plants had attained vascular tissues already by the beginning of the Devonian, but then radiated, forming the precursors to all the main groups of plants that exist today. At this early stage, plants would have been small and delicate, confined to moist environments, but by the Middle Devonian, with the developments of roots and woody tissues may have spread into seasonally dry environments. Rooting plants consolidated the land surface, altering the water cycle by increasing infiltration, consolidating river banks, and increasing the rate and depth of soil formation. Vegetation fundamentally changed the nature of the landscape; indeed a change in the style of fluvial deposits is regarded from pre-Devonian sedimentary sequences (1). By the Late Devonian forests had spread globally, and we find the first evidence of trees in high-latitude deposits exemplified by the Witpoort Formation. Forestation, in turn, altered the carbon cycle, leading to a major decrease in atmospheric CO2 at this time (2). This explains contemporary global cooling and latest Devonian to Carboniferous glaciations, the first continental ice seen since the Himantian (end Ordovician Period) some 80 million years previously. Moreover, feedbacks related to the carbon and hydrological cycles, and consequent destabilization of the oceans and atmosphere (2, 3) allow that forestation is one of the plausible triggers for the Late Devonian biotic crisis that caused major overturn in vertebrate and invertebrate biodiversity.
References
1) Davies, N. S., & Gibling, M. R. (2010). Cambrian to Devonian evolution of alluvial systems: the sedimentological impact of the earliest land plants. Earth-Science Reviews, 98(3-4), 171-200
2) Algeo, T. J., & Scheckler, S. E. (1998). Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 353(1365), 113-130.
3) Gess, R. W. (2016). Vertebrate Biostratigraphy of the Witteberg Group and the Devonian-Carboniferous Boundary in South Africa. In Origin and evolution of the Cape Mountains and Karoo Basin (pp. 131-140). Springer, Cham.
References
1) Davies, N. S., & Gibling, M. R. (2010). Cambrian to Devonian evolution of alluvial systems: the sedimentological impact of the earliest land plants. Earth-Science Reviews, 98(3-4), 171-200
2) Algeo, T. J., & Scheckler, S. E. (1998). Terrestrial-marine teleconnections in the Devonian: links between the evolution of land plants, weathering processes, and marine anoxic events. Philosophical Transactions of the Royal Society of London. Series B: Biological Sciences, 353(1365), 113-130.
3) Gess, R. W. (2016). Vertebrate Biostratigraphy of the Witteberg Group and the Devonian-Carboniferous Boundary in South Africa. In Origin and evolution of the Cape Mountains and Karoo Basin (pp. 131-140). Springer, Cham.