65-30 Mya
Formation of the Ancient Duricrusts of the African Land Surfaces

Introduction:

The geology history of South Africa and the development of its topography, drainage, land surfaces, soils and duricrust deposits are closely linked over the last 66 million years (the Cenozoic Era). Duricrusts are hard layers on or near the soil surface ranging in thickness from few centimetres to several meters. These formed through long periods of repetitive erosion and weathering of the Earth’s surface, due to the actions of wind, water, leaching and chemical activity, during alternating wet and dry periods of Earth’s history.

Silcrete and ferricrete duricrusts form caps of low lying hillocks in the Rûens near Wydgeleë and Napier.

Geology

Duricrust is the general term for a zone of chemical precipitation and hardening forming at or near the surface of sediment formations through soil forming (pedogenic) processes. Duricrusts are formed by the accumulation of soluble minerals deposited by mineral-bearing waters that move upward, downward or laterally by capillary action. Silcrete is a soil duricrust at the surface or near-surface deposit of soil, saprolite, or sediment that has been cemented by secondary silica, to form an indurated mass. Similar to the formation process of silcrete is that of ferricrete formed by iron oxide and calcrete formed by calcium carbonate.

Ferricrete duricrust at Skurwekoppie, Napier and sample from Hemel en Aarde Valley, Hermanus.

African Erosional Surfaces

At various localities around the Southern Cape, and further afield we see well preserved remnants of ancient land surfaces at varying elevations. In the 1950’s Lester Charles King was the first to recognise, date and coin certain high surfaces in southern Africa. The main three surfaces are the African erosional Surface (85 to 40 million years in age), the Post-Africa I Surface (20 to 15 million years in age) and the Post-African II Surface (8 to 3 million years in age). A variety of duricrust deposits formed during these periods, namely ferricrete called by farmers koffieklip (iron-rich), silcrete (silica-rich) and calcrete (calcium-rich) layers. In the Rûens of the Overberg ferricrete and silcrete capped hilltops are regarded as the remnants of the polycyclic ancient post-Gondwanan (African) erosion surface, heavily dissected subsequent to late Cenozoic uplift. This surface slope seawards resulting that the duricrust cappings occur at progressively lower elevations towards the south.

Grahamstown Formation

The Grahamstown Formation is a geology unit that includes the silcrete and ferricrete outcrops found in the coastal areas of the eastern, southern, and western Cape, at elevations ranging from 40 to 650 meters. It is named after Grahamstown, in the Eastern Cape, around which silcrete forms extensive sheets and outcrops, so abundant that it has been given the status of a formation by the South African Committee for Stratigraphy. SACS describe the Grahamstown Formation of Late Cretaceous to Paleogene in age (85 to 40 million years ago) as silcrete and ferricrete associated with deeply weathered saprolite (soft, earthy, typical clay-rich thoroughly decomposed rock). At Fort Grey in the Eastern Cape fossil wood was reworked from Permo-Triassic Beaufort strata and incorporated in early Cenozoic fluvial debris-flow deposits subsequently silcretised in the Grahamstown Formation.

Silcrete and ferricrete outcrops of the Grahamstown Formation in the southern Cape Province.

Examples of Duricrust exposures

West of Hermanus the ferricrete outcrops and duricrusts in the Hemel en Aarde Valley caps the rooting soils for the vines and can add to preserve moister in the soil. Farmers use earth moving machinery to break-up the hard “koffieklip” before planting the vines in mineral rich soils advantages to the production of the world class wines of the area. Fast areas of the Rûens had to be cleared of ferricrete duricrust before planting wheat, canola and barley.

The cap rock of Skurwekoppie south of Napier (34°27'26.17"S 19°54'17.22"E) is ferruginous cemented gravel, bolder layers and conglomerate of the Grahamstown Formation very resistant to weathering and commonly capping the hills of the Rûens. In the road cut we see brown, red to pinkish weathered soft shales of the Gydo Formation of the Devonian Bokkeveld Group forming the basement rocks upon which gravel and bolder beds of the Palaeocene-Eocene Grahamstown Formation were deposited. The conglomerate is cemented by the iron-rich minerals hematite, limonite and goethite. The well to poorly rounded quartzitic boulders and gravels were originally part of large alluvial fans and wash-out deposits on pedi-plains formed by braided stream along the foothills of the Bredasdorp Mountains associated with the African erosional and plaining cycle.

Skurwekoppie near Napier and vineyards of the the Hemel en Aarde Valley.

Silcrete Stone Tools

Of great interest to archaeologists are the silcrete stones that were widely used by Stone Age people for stone tool manufacture. The dominant raw material used for the Still Bay point production unearthed in the Blombos Cave 300km east of Cape Town is silcrete most possibly sourced from high level terraces near Riversdale and Albertinia some 30 km away. At Pinnacle Point near Mossel Bay researchers have determined that two types of silcrete tools were made and hardened using a heat treatment technique. There is evidence to suggest the technique may have been known as early as 165 000 years ago.

Bifacial silcrete points from Blombos and silcrete tools from Pinnacle Point.

65-30 Mya Formation of the Ancient Duricrusts of the African Land Surfaces

65-30 Mya
Formation of the Ancient Duricrusts of the African Land Surfaces