Characterisation and gradient constraint of mixed debris flow/braided river dominated fans by analysis of Skeleton Coast fan systems in NW Namibia
Final Report Abstract
Its location on a tectonically relatively stable passive margin and its degree of interaction with the sea make the Horingbaai fan-delta an exceptional record of coastal activity, providing insights into the response of ephemeral fluvial systems to changes in climate, sea-level and continent-scale uplift. Drainage pattern and facies architecture characterizes the 10 km x 13 km Horingbaai fan-delta as braided river dominated, developing a gradient of 0.01 and issuing seaward of the coastal escarpment of NW Namibia. In terms of its gradient the Horingbaai Fandelta is rather steep compared to other braided river dominated fans and classifies close to the modal range for debris flow dominated fans. The gradient of the Horingbaai fan-delta and of several other Skeleton Coast fan systems populate a "depositional slope gap" postulated by earlier authors within a gradient range of 0.009-0.026. Both catchment and coastal depositional settings classify as arid to hyper-arid. The fan comprises upper, middle and lower segments. Upper and middle fan are dominated by a braided river system, only in the upper fan are fluvial sediments interleaved with hyperconcentrated flow deposits. The lower fan-delta has been repeatedly inundated by the marine system, as is the case with the lower portions of the fan presently. Plio-Pleistocene sea-level highstands have left marine terraces on the fan surface which enable correlation with the offshore marine record and provide timelines to constrain fan growth. The bulk of Horingbaai Fan-delta progradation took place at ∼2.7-2.4 Ma, between the formation of a widespread erosional surface incising the middle Pliocene Karpfenkliff and Kamberg Formations, but prior to the emplacement of the warm-water faunabearing late Pliocene "Oyster Terrace" (∼2.4-2.2 Ma), an equivalent of the +30 mP (marine terrace package) in coastal southwestern Africa. Substantial, episodic supply of coarse debris during the late Pliocene is not restricted to the Horingbaai fan-delta, but also registered by an increase in offshore sedimentation rates and aggradation in other coastal drainage systems, issuing on the same post-Kamberg erosional surface. Major fan progradation is contemporaneous with widespread uplift (~12±5 m/Ma) and climate change occurring in southwestern Africa, the last associated with the intensification of the northern hemisphere glaciations. Younger fan growth phases constrained by <10 m asl marine terrace bodies yield mostly cold-water fauna and are substantially weaker and more episodic. This corresponds to the onset of strong glacial/interglacial climatic fluctuations on top of a general aridification trend and the introduction of colder sea-surface temperatures after 2.2 Ma.
Publications
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(2013): The braided river dominated Horingbaai Fan-delta, NW Namibia: Dating of Skeleton Coast fan progradation using marine terrace stratigraphy.- Abstract Joint Meeting GV, DMG, Sediment, Tübingen 2013
Stollhofen, H., Stanistreet, I.G., von Hagke, Chr., Nguno, A.
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(2014): Constraining aggradation and degradation phases of alluvial fans in the sedimentary record: a case study from the Namib Desert/NW Namibia.- Geophysical Research Abstracts 16, EGU2014-9099 (EGU General Assembly 2014)
von Hagke, Chr., Stollhofen, H., Malatesta, L.C., Stanistreet, I.G.
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Pliocene-Pleistocene climate change, sea level and uplift history recorded by the Horingbaai fan-delta, NW Namibia. Sedimentary Geology, Volume 309, 15 July 2014, Pages 15-32
Stollhofen, H., Stanistreet, I.G., von Hagke, Chr., Nguno, A.
