Paleofjords that drained glaciers 300M years ago preserved NW Namibia

The Kaokoland region of northwest Namibia is a desertic land: Vast barren plateaus are deeply dissected by several hundred-meter-deep U-shaped valleys in which ephemeral streams flow. The hard floors and walls of these valleys surprisingly display abundant glacial erosion features characteristic of flowing glaciers such as scratches, striae, grooves, and smooth, elongated hills called whalebacks.

Such features are usually found in Canada or Scandinavia that were recently (20,000 years) covered by huge ice sheets. Discovering such marks of ice flow in Namibia, which did not undergo any recent glaciation, is therefore thrilling and challenging.

These glacial erosion features are sealed in place by remnants of less-resistant sedimentary rocks-conglomerate, sandstones, and mudstones encompassing large, far-traveler lonestones-which indicate that these valleys were inundated by the sea, on which icebergs drifted, after the flowing glaciers retreated.

The dating of these sedimentary rocks indicates an age of 300 million years, corresponding to an icehouse period. Africa, and therefore Namibia, was at that time part of the Gondwana Supercontinent located close to the south pole.

Thus, as foreseen as early as the 1950s by the German geologist Henno Martin, famous for having shared his experience of his two years spent in the Namibian desert (Sheltering Desert), a French-American-Austrian joint team demonstrated yesterday in the journal Geology that 300-million-year-old glaciers drained through-and likely carved-these valleys, which, after glaciers vanished, were inundated by the sea and therefore turned into fjords.

These fjords were subsequently filled with sediments for 130 million years before being exhumed in more recent times. Owing to preferential erosion of the less-resistant sedimentary rocks filling up the valleys, these ancestral fjord morphologies are now shaping the desertic landscape of NW Namibia.

Although modern fjords are abundant on continental shelves at high latitude, these Namibian fjords are the unique example of preserved ancient fjord morphologies and therefore serve for tackling climate changes associated to the penultimate icehouse our planet experienced.

Research Report: "Fjords network in Namibia: A snapshot into the dynamics of the late Paleozoic Glaciation"

Related Links
Geological Society Of America
Beyond the Ice Age

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