Paleofjords that drained glaciers 300 million years ago still preserved
in the desertic landscape of NW Namibia
Boulder, Colo., USA: 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.
FEATURED ARTICLE
Fjords network in Namibia: A snapshot into the dynamics of the late
Paleozoic Glaciation
Pierre Dietrich and colleagues
Contact: pierre.dietrich@univ-rennes1.fr, Université de Rennes 1–CNRS,
Géosciences Rennes UMR 6118, Rennes, France
URL:
https://pubs.geoscienceworld.org/gsa/geology/article/doi/10.1130/G49067.1/607263/Fjord-network-in-Namibia-A-snapshot-into-the
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