Evolution of the western Namibian drainage systems since Eocene times - a multi-methodical approach
Final Report Abstract
This project was focussed on the western Namibian drainage evolution since Eocene times applying a multi-method approach. The extraordinarily good state of preservation and outstanding outcrops even of the pre-Pliocene sediments allowed to deploy the full range of routinely used state-of-the-art methods (dating via TCN, zircon morphometrics and isotopes), as well as cutting-edge SSI dating of calcareous matrices in order to determine the absolute ages of sediments and their provenance at an exceptionally high level of resolution. Palaeodrainage reconstruction was done over a long period of time and large areal distribution using sediments of the recent rivers and their precursors. One of the main objectives, the set-up of a reliable stratigraphy of Namibia’s fluvial terraces, was – with little exceptions – done as scheduled. The data suggests four main phases of fluvial sediment deposition since the Miocene: I) a Namibia-wide late Early to Mid Miocene phase, with terrace ages ranging between ca. 14 and ca. 19 Ma; II) a diachronous Late Miocene-Late Pliocene phase (7.8 to 2.7 Ma) that began in the north and ended in the south; III) a likely latest Pliocene-Mid Pleistocene phase that occurred throughout the country except for the Orange River; and IV) a latest Pleistocene-Holocene phase, whose sediments have been deposited at almost all rivers. As fluvial archives of Eocene and Oligocene age are scarce, information about the fluvial network of this time remains selective. Presuming that, succeeding or not, an accompanying feasibility study of advanced dating techniques including 53Mn in magnetite does not take the primary scientific objective of this project at risk, it is bearable that the yet obtained data can currently not be used. Changing provenances of the fluvial sediments through time were obtained in all studied catchments. The huge amount of newly available detrital zircon data (almost 10.000 single grain ages including Th/U ratios, and morphological features) made it possible to detect variations in the sediment’s provenance at high resolution. Combined with an extensive database of southern African zircon ages (n > 80.000), potential changes in catchment area sizes were recognised. The detrital zircon data were interpreted assuming that short-termed fluctuation frequencies rule out tectonic influences and that persistent trends throughout the sediments of all ages within one catchment area potentially represent long-term tectonic movements, like uplift or sagging. Thus, there is quite a tectonic influence on the catchment areas of the central Namibian rivers in Mid Miocene to Pleistocene times, which could not be found for the Kunene and Orange rivers. Particularly the Miocene terrace deposition of the latter seems to have significantly been affected by sea-level highstands, whereas e.g. the Ugab catchment was continuously separated from the Kamanjab inlier by uplift. The detrital zircon age distribution patterns indicating short-termed fluctuations show that variability in discharge, and thus likely in precipitation, increased in Mid Miocene times. During the Pliocene it seems to have become much drier in southern and central Namibia, while the northern regions may have been more humid than today. This trend continued throughout the Pleistocene, which is characterised by the least variable detrital zircon age patterns pointing to a minimum drainage of the catchments. Although the current climate is still arid, there have been some more humid or pluvial intervals, as recorded in the Holocene sediments of the Orange and Swakop rivers. An unexpected, but highly promising finding is the possibility to date Pliocene snail shells by the SSI method. Several species are endemic and limited to certain climate conditions. The potential of this method for reconstructing the palaeoclimate of southern Africa or comparable regions increases significantly, when adding further methods like stable isotope analysis or clumped isotopes. Thus, the data of this project are the base for such future work. Irrespectively, the results help to unravel the Cenozoic fluvial history and some aspects of the climatic evolution of western Namibia in detail. All these data can be used to estimate amplitudes and processing speeds of past events like incision rates, changing sizes of catchment areas or discharge, which is of particular interest for southern African climate and palaeogeographic modelling. The now concluded project thus clearly demonstrates the importance of high-resolution, integrated approaches including detailed absolute dating of fluvial terraces, stratigraphical, sedimentological and high resolution provenance analyses in order to fully understand complex palaeoenvironmental changes and correlation issues.
Publications
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2016. Evolution of the western Namibian drainage systems since Eocene times – a multi-methodical approach. 35th IGC, 27/08-04/09/2016, Cape Town, South Africa
Gärtner A, Linnemann U, Merchel S, Niedermann S, Gerdes A, Nguno A, Rugel G, Scharf A, Le Bras L, Hofmann M, Zieger J, Krause R, Harazim S, Stutzriemer M, Rothe J
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Mass spectrometry and the evolution of the western Namibian drainage systems. Frühjahrstagung der Deutschen Physikalischen Gesellschaft, 06/03-10/03/2017, Mainz, MS 9.8 (4.1)
Gärtner A, Linnemann U, Merchel S, Niedermann S, Gerdes A, Rugel G, Scharf A, Le Bras L, Hofmann M, Zieger J
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2019. Attempts to understand potential deficiencies in chemical procedures for AMS: Cleaning and dissolving quartz. Nuclear Instruments and Methods in Physics Research Section B
Merchel S, Gärtner A, Bookhagen B, Chabilan A, Gurlit S
