Footsteps Through Time

Footsteps Through Time
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Wednesday, 22 March 2017

Ancient Origins -Evolution of the Zambezi

Reproduced from the Zambezi Society March 2017 Newsletter


Lovers of the Zambezi River who are interested in learning more about its very ancient history, will be delighted to read a fascinating article by Fenton "Woody" Cotterill, Andy Moore and Roger Key which was reproduced in the The African Fisherman magazine last year.  

The article is a non-technical summary of the original paper published in the South African Journal of Geology under the title:  ‘The Zambezi River: an archive of tectonic events linked to Gondwana amalgamation and disruption, and subsequent evolution of the African Plate’. 
What is particularly fascinating to the layman about this latest research is a) geological study now establishes the ancient Proto-Zambezi as the Earth's oldest-known river (minimum 280 million years old) and b) that the ancient Proto-Zambezi ran in exactly the opposite direction (west across Africa into the Karoo) to what it does today (east into the Indian Ocean). 

We reproduce an extract below: (you can read the full article via the link given at the end): "Africa's Zambezi is now established as Earth's oldest known river – the direct descendent of an ancient Proto‑Zambezi river. The origins of the Zambezi have been deciphered from geological evidence across the vast wilderness of the Chicoa, Gwembe, Luangwa, Mana and Sebungwe valleys of south‑central Africa; here, rock formations and landforms preserve the central repository of the river’s history back into the Palaeozoic Era before 280 Ma (million years ago).The Proto-Zambezi river originated in the drainage system that was established as vast Dwyka ice-sheets were receding across the super-continent of Gondwana. At this time the immense highlands of the Trans-Gondwana mountains directed the westerly flow of glacial meltwaters into the interior of Gondwana. With its catchment entirely contained within Gondwana, this massive river sustained a vast, inland sea centred on modern‑day Botswana.... 

..An unbroken chain of evidence places a minimum age of 280 Ma on the Zambezi's origins, when Gondwana’s ice-sheets began to melt as the super-continent drifted northwards. Central Africa’s rift valleys preserve the rock ledgers (historic record) that tell us the Proto‑Zambezi existed long before Africa became a continent. Importantly, this geological record of the ancestral Proto‑Zambezi is contiguous with the modern river. In particular, the evidence reveals that precursors of the modern Luangwa and Middle Zambezi already existed in the early Permian. This geological record in the Luangwa and Middle Zambezi valleys (the central African rifts) has survived despite the complete reversal of the Zambezi’s flow caused by Gondwana’s breakup, which forged the African continent.

The west-flowing Proto-Zambezi drainage system maintained a vast inland Karoo sea (K).  The original extent of the Karoo sediments was likely larger before the breakup and erosion associated with the daughter continents of Gondwana.  This map details the centres of Karoo deposition (shallow lakes, seas and deltas) depicted in darker shading, as presented in surviving rock formations in southern and central Africa. The superimposed dashed line approximates the position of the modern, east-flowing Zambezi river.

The greatest change in over 280 million years of the Zambezi’s evolution entailed the complete reversal of its flow.  The process of break up was protracted, as over millions of years, river by river, flow in the Proto‑Zambezi’s catchment was redirected into the Indian Ocean, to reduce the significance of the Proto‑Zambezi’s inland terminus (in central Botswana).... 

....Geologically recent captures of the Chobe, Kafue and Upper Zambezi testify to the relative youth of the (modern) river’s topology.  The Great Equatorial Divide on Africa’s high plateaux comprises the Zambezi’s modern watershed, and its present position testifies to repeated re-shuffles of links between long lived rivers. To summarize, we now estimate the modern Zambezi basin to be barely 250 thousand years old.......

...Its status as Earth’s oldest, big river – yet known – underwrites the conservation status of the Luangwa‑Zambezi wilderness. The geological continuity preserved across these landscapes reveals how the ecosystems of the Zambezi have endured in an unbroken chain, ever since the first ecosystems of Dwyka times formed along glacial streams. The criterion of Ancient Evolutionary Heritage positions the Zambezi as a unique outlier among Earth’s most spectacular landscapes, including the Grand Canyon. One is challenged to name World Heritage Sites that match these iconic values, in preserving comparable geological and paleoecological records."

This summary article is reproduced in full by The African Fisherman in print (issue 172 - April/May 2016) or online at this link.  

The full scientific article is re-produced as a downloadable PDF on the Zambezi Society website HERE.

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