Placer Formation and Termination of the Orange Sand Hi

Page 1 of 81 Sedimentology 1 1 2 3 4 �� 5 6 �� 7 8 �� 9 10 11 EDUARDO GARZANTI1*, PEDRO DINIS2, PIETER VERMEESCH3, SERGIO ANDÒ1, 12 13 ANNETTE HAHN4, JOÃO HUVI5, MARA LIMONTA1, MARTA PADOAN1 ALBERTO 14 1 3 1 15 RESENTINI , MARTIN RITTNER , GIOVANNI VEZZOLI 16 17 18 19 �� 20 �� 21 �� 22 2� �� 23 �� 24 3 25 � �� 26 �� 27 4�� 28 5 29 �� 30 31 ��Corresponding author. Tel.: +39-02-64482088 32 33 34 E-mail: [email protected] (E. Garzanti), [email protected] (P. Dinis), [email protected] 35 (P.Vermeesch), [email protected] (S. Andò), [email protected] (A.Hahn), 36 [email protected] (J.Huvi), [email protected] (M.Limonta), [email protected] 37 38 (M.Padoan), [email protected] (A. Resentini), [email protected] (M.Rittner), 39 [email protected] (G. Vezzoli) 40 � 41 � 42 43 � 44 45 � 46 � 47 48 � 49 50 � 51 � 52 53 ��: Sedimentary petrology, heavy minerals, detrital-zircon geochronology, Raman counting 54 55 of deep-sea silt, grain density and geochemistry of placer deposits, garnet and pyroxene chemistry, 56 longshore sediment transport, Moçamedes Desert and Cunene River sands. 57 58 59 60 Sedimentology Page 2 of 81 2 1 2 3 4 5 �� 6 7 8 This study focuses on the causes, modalities and obstacles of sediment transfer in the longest cell of 9 10 littoral sand drift documented on Earth so far. Sand derived from the Orange River is dragged by 11 12 swell waves and persistent southerly winds to accumulate in four successive dunefields in coastal 13 14 Namibia to Angola. All four dunefields are terminated by river valleys, where eolian sand is flushed 15 16 back to the ocean. And yet sediment transport continues at sea, tracing a 1800 km-long submarine 17 18 19 sand highway. Sand drift would extend northward to beyond the Congo if the shelf did not become 20 21 progressively narrower in southern Angola, where drifting sand is funnelled towards oceanic depths 22 23 via canyon heads connected to river mouths. Garnet-magnetite placers are widespread along this 24 25 coastal stretch, indicating systematic loss of the low-density feldspatho-quartzose fraction to the 26 27 28 deep ocean. More than half of Moçamedes Desert sand is derived from the Orange River, and the 29 30 rest in similar proportions from the Cunene River and from the Swakop and other rivers draining 31 32 the Damara orogen in Namibia. The Orange fingerprint, characterized by basaltic rock fragments, 33 34 clinopyroxene grains, and bimodal zircon-age spectra with peaks at ∼0.5 and ∼1.0 Ga, is lost 35 36 37 abruptly at Namibe, and beach sands farther north have abundant feldspar, amphibole-epidote 38 39 suites, and unimodal zircon-age spectra with peak at ∼2.0 Ga, documenting local provenance from 40 41 Paleoproterozoic basement. Along this oblique-rifted continental margin, beach placers are 42 43 dominated by Fe-Ti-Cr oxides with more monazite than garnet, and thus have a geochemical 44 45 46 signature sharply different from beach placers found all along the Orange littoral cell. High- 47 48 resolution mineralogical studies allow us to trace sediment dispersal over distances of thousands of 49 50 kilometers, providing essential information for the correct reconstruction of source-to-sink 51 52 relationships in hydrocarbon exploration and to predict the long-term impact of man-made 53 54 infrastructures on coastal sediment budgets. 55 56 57 58 59 60 Page 3 of 81 Sedimentology 3 1 2 3 4 5 6 �� 7 8 �� 9 �� 10 11 �� 12 13 Talking Heads, Once in a lifetime, 1980. 14 � 15 16 � 17 �� 18 1 19 20 2 The long-distance transfer of huge detrital masses from one site to another on the Earth's surface is 21 22 23 3 of essence to understand sedimentological processes, enhance the resolution of source-to-sink 24 25 4 studies for hydrocarbon exploration, and improve the quality of environmental management 26 27 5 (Dickinson, 1988; Kaminsky ��, 2010; Scott ��, 2014). Hundreds of million tons of sediment 28 29 6 are carried each year for thousands of kilometers along the largest fluvial systems, such as the 30 31 7 Amazon, the Ganga-Brahmaputra or the Yellow River (Hay, 1998; Milliman & Farnsworth, 2011). 32 33 8 Equally established is the underwater transport of large sediment volumes over similar distances via 34 35 36 9 turbidity currents, as across the present Bengal and Indus Fans or their ancient analogues (Ingersoll 37 38 10 ��, 2003). Far less documented is the transport of large sediment volumes in the shallow sea, 39 40 11 which can cover distances of a thousand kilometers and more under the action of persistent 41 42 12 longshore currents. The major implications of littoral sediment transport for the paleogeographic 43 44 13 interpretation of ancient sedimentary deposits remain notably underexplored. 45 46 14 Only a few cases of modern ultralong-distance littoral sand transport have been documented in 47 48 49 15 detail so far. Swell-driven coastal-transport systems hundreds of kilometers in length exist in many 50 51 16 parts of the world (Silvester, 1962; Davies, 1972), as in southern Brasil (Calliari & Toldo, 2016) or 52 53 17 eastern Australia, where sand is dragged alongshore for ∼1500 km to north of Fraser Island, the 54 55 18 largest sand island on Earth (Boyd ��, 2008). Even longer is the littoral cell fed by the Orange 56 57 19 River, the object of the present study, rivaled in length only by the mud-dominated Amazon cell 58 59 60 Sedimentology Page 4 of 81 4 1 2 3 4 5 20 (Allison & Lee, 2004; dos Santos ��, 2016). Littoral dispersal of Nile sand under the action of 6 7 21 longshore currents fuelled by strong northwesterly winds, which has fed the beaches of Gaza and 8 9 22 Israel through most of the Quaternary, terminates at Haifa Bay and Akhziv submarine canyon ∼700 10 11 23 km from the Delta (Inman & Jenkins, 1984; Garzanti ��, 2015a). The littoral cell associated with 12 13 24 the Columbia River, the third largest in the United States by discharge, extends for 165 km only 14 15 16 25 (Ruggiero ��, 2005), although sand transport by turbidity currents continues for ∼1100 km along 17 18 26 a devious route reaching far into the Pacific Ocean (Zuffa ��, 2000). 19 20 27 The present article builds upon previous studies that monitored sediment dispersal along the 21 22 28 Atlantic coast of southern Africa – from the Orange River to the Namib Erg, and beyond to the 23 24 29 Skeleton Coast and Moçamedes Desert of southernmost Angola (Vermeesch ��, 2010; Garzanti 25 26 30 ��, 2012a, 2014, 2015b) – and focuses on physical processes of sediment mixing and unmixing 27 28 29 31 in the terminal tract of this ultralong submarine sand highway (Fig. 1). We will investigate 30 31 32 specifically provenance of sand accumulating in the Moçamedes Desert north of the Cunene River 32 33 33 mouth (Fig. 2) and along the Angolan coast farther north (Fig. 3), in order to understand where, 34 35 34 how, and why the compositional fingerprint of Orange-derived sand is eventually lost after ∼1800 36 37 35 km of littoral drift. We will examine whether and to what extent northward sand transport is 38 39 36 blocked on land by incised river valleys and/or at sea by submarine canyons, the influence of 40 41 42 37 atmospheric and oceanic circulation, and the role played by shelf width, representing both a control 43 44 38 and a consequence of longshore sediment drift. An accurate evaluation of such factors is essential to 45 46 39 understand the physical processes promoting such a large-scale transfer of sediment volumes, and 47 48 40 how they are reflected in the composition of continental-margin sediments. 49 50 41 � 51 52 42 �� 53 43 54 55 44 The coastal region of southern Angola is affected profoundly by atmospheric and oceanic 56 57 45 circulation in the southeastern Atlantic Ocean. Here the warm, southward flowing Angola Current 58 59 60 Page 5 of 81 Sedimentology 5 1 2 3 4 5 46 converges with the cold, northward flowing Benguela Current, forming the Angola-Benguela front 6 7 47 (Meeuwis & Lutjeharms, 1990; Shannon & Nelson, 1996; Lass ��, 2000). Reflecting this oceanic 8 9 48 circulation pattern and the influence of the subtropical high-pressure system, climate along the coast 10 11 49 is very arid between ∼30ºS and ∼15ºS. Annual rainfall increases progressively from as low as 15 12 13 50 mm at Foz do Cunene and Baia dos Tigres, to 20 mm at Tombua, 50 mm at Namibe, and 200 mm at 14 15 16 51 Lobito (Guilcher, 2010). Rainfall increases much more rapidly inland to

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