Quantification and causes of the terrigeneous sediment budget at the scale of a continental margin: A new method applied to the Namibia-South Africa margin

Basin Research

Volumn 24, Issue 1, 2012, Pages 3-30

  Guillocheau, F ^a,b   Rouby, D ^a,b   Robin, C ^a,b   Helm, C ^a,b   Rolland, N ^a,b,d   Le Carlier de Veslud, C ^a,b   Braun, J ^c  

^a UNIVERSITÉ DE RENNES 1   (France)

^b CNRS   (France)

^c OBSERVATOIRE DE GRENOBLE   (France)

^d CV Associés Le Robinson   (France)

Author keywords

[No Author keywords available]

Indexed keywords

ACCUMULATION RATE; CENOZOIC; CLIMATE CONDITIONS; CONTINENTAL MARGIN; CRETACEOUS; GIS; PASSIVE MARGIN; POROSITY; SEDIMENT BUDGET; SEISMIC VELOCITY; STRATIGRAPHY; TERRIGENOUS DEPOSIT;

ATLANTIC OCEAN; NAMIBIAN MARGIN;

EID: 84855525338     PISSN: 0950091X     EISSN: 13652117     Source Type: Journal    
DOI: 10.1111/j.1365-2117.2011.00511.x     Document Type: Article

References (98)

1. Aizawa, M., Bluck, B., Cartwright, J.A., Milner, R., Swart, R. & Ward, J. (2000) Constraints on the geomorphological evolution of Namibia from the Offshore Stratigrahic Record. Commun. Geol. Surv. Namibia, 12, 337-346.
2. Allen, P.A. & Allen, J.R. (1990) Basin Analysis. Principles and Applications. Blackwell Scientific Publications, Oxford.
3. Artemieva, I.M. & Mooney, W.D. (2001) Thermal thickness and evolution of precambrian lithosphere: a global study. J. Geophys. Res., 106, 16387-16414.
4. Bauer, K., Neben, S., Schreckenberger, B., Emmermann, R., Hinz, K., Fechner, N., Gohl, K., Schulze, A., Trumbull, R.B. & Weber, K. (2000) Deep structure of the Namibia continental margin as derived from integrated geophysical studies. J. Geophys. Res., 105, 25829-25853.
5. Berger, W.H., Lange, C.B. & Wefer, G. (2002) Upwelling history of the Benguela-Namibia System: a synthesis of leg 175 results. In: Proceedings of the Ocean Drilling Program Leg 175 Scientific Results, 175Benguela Current, Sites 1075-1087 (Ed. by W.H. Berger, G. Wefer & C. Richter), pp. 1-103. National Science Foundation, College Station, TX.
6. Bolli, H.M., Ryan, W.B.F., Foresman, J.B., Hottmann, W.E., Kagemi, H., Longoria, J.F., Mcknight, B.K., Melguen, M., Natland, J., Proto-decima, F. & Siesser, W.G. (1978) Cape basin continental rise - sites 360 and 361. In: Initial Reports of the Deep Sea Drilling Project, Vol. 40 (Ed. by H.M. Bolli & W.B.F. Ryan, et al.) pp. 29-182. U.S. Government Printing Office, Washington.
7. Bonnet, S. & Crave, A. (2003) Landscape response to climate change: insights from experimental modeling and implications for tectonic versus climatic uplift of topography. Geology, 31, 123-126.
8. Bray, R., Lawrence, S.R. & Swart, R. (1998) Source rock, maturity data indicate potential off Namibia. Oil Gas J., 96, 608-614.
9. Brown, L.F., Benson, J.M., Brink, G.J., Doherty, S., Jollands, A., Jungslager, E.H.A., Keenan, J.H.G., Muntingh, A. & Van Wyk, N.J.S., eds. (1995) Sequence Stratigraphy in Offshore South African Divergent Basins, Vol. 41. AAPG Studies in Geology, American Association Petroleum Geologists, Tulsa, 184pp.
10. Brown, R., Summerfield, M.A. & Gleadow, A.J.W. (2002) Denudational history along a transect across the Drakensberg escarpment of Southern Africa derived from apatite fission track thermochronology. J. Geophys. Res., 107, 2350. doi: 10.1028/2001JB000745.
11. Burke, K. (1996) The African plate. S. Afric. J. Geol., 99, 341-409.
12. Burke, K. & Gunnell, Y., eds. (2008) The African Erosion Surface: A Continental-Scale Synthesis of Geomorphology, Tectonics, and Environmental Change over the Past 180 Million Years (Geol. Soc. Am. Memoir, 201, 66pp.
13. Chumakov, N.M., Zharkov, M.A., Herman, A.B., Doludenko, M.P., Kalandadze, N.N., Lebedev, E.L., Ponomarenko, A.G. & Rautian, A.S. (1995) Climatic belts of the Mid-Cretaceous time. Stratigr. Geol. Corr., 3, 241-260.
14. Clemson, J., Cartwright, J.A. & Booth, J.R. (1997) Structural segmentation and the influence of basement structure on the Namibian passive margin. J. Geol. Soc., 154, 477-482.
15. Clemson, J., Cartwright, J.A. & Swart, R. (1999) The Namib rift: a rift system of possible Karoo age, Offshore Namibia. In: The Oil and Gas Habitats of South Atlantic (Ed. by N.R. Cameron, R.H. Bate & V.S. Clure), Geol. Soc. Spec. Publ. London, 153, 381-402.
16. Clift, P., Gaedick, C., Edwards, R., Lee, J.I., Hildebrand, P., Amjad, S., White, R.S. & Schlüter, H.U. (2002a) The stratigraphic evolution of the Indus fan and the history of sedimentation in the Arabian Sea. Mar. Geophys. Res., 23, 223-245.
17. Clift, P., Lee, J.I., Clark, M.K. & Blusztajn, J. (2002b) Erosional response of South China to Arc rifting and Monsoonal strengthening; a record from the South China Sea. Mari. Geol., 184, 207-226.
18. Clift, P.D. (2006) Controls on the erosion of Cenozoic Asia and the flux of clastic sediment to the Ocean. Earth Planet. Sci. Lett., 241, 571-580.
19. Corner, B., Cartwright, J.A. & Swart, R. (2002) Volcanic passive margin of Namibia: a potential fields perspective. In: Volcanic Rifted Margins (Ed. by M. Menzies, S.L. Klemperer, C.J. Ebinger & J. Baker), Geol. Soc. Am. Spec. Paper, 362, 203-220.
20. Cox, K.G. (1989) The role of mantle plumes in the development of continental drainage patterns. Nature, 342, 873-877.
21. Deschamps, F., Dauteuil, O., Bourgeois, O., Mocquet, A. & Guillocheau, F. (in press) Post-breakup evolution and palaeotopography of the North Namibian margin during the Meso-Cenozoic. Tectonophysics.
22. de Wit, M. (2007) The Kalahari epeirogeny and climate change: differentiating cause and effect from core to space. S. Afric. J. Geol., 110, 367-392.
23. de Wit, M.J., Marshall, T.R. & Partridge, T.C. (2000) Fluvial deposits and drainage evolution. In: The Cenozoic of Southern Africa (Ed. by T.C. Partridge & R.R. Maud), Oxford Monogr. Geol. Geophys. 40, 55-72.
24. Dingle, R.V. & Hendey, Q.B. (1984) Late Mesozoic and tertiary sediment supply to the Eastern Cape Basin (Se Atlantic) and Palaeo-drainage systems in Southwestern Africa. Mar. Geol., 56, 13-26.
25. Dingle, R.V., Siesser, W.G. & Newton, A.R. (1983) Mesozoic and Tertiary Geology of Southern Africa. Balkema, Rotterdam, 375pp.
26. Duff, P.M.D. (1993) Holmes' Principles of Physical Geology. Chapman & Hall, London, 791pp.
27. Emery, K.O. & Uchupi, E. (1984) The Geology of the Atlantic Ocean. Springer Verlag, Berlin.
28. Emery, K.O., Uchupi, E., Bowin, C., Phillips, J. & Simpson, E.S.W. (1975) Continental margin off Western Africa: Cape St. Francis (South Africa) to Walvis (South-West Africa). Am. Assoc. Petrol. Geol. Bull., 59, 3-59.
29. Exxon World Mapping Project. (1985) Tectonic Map of the World. Exxon Production Research Company, Houston, USA.
30. Fernandez, M., Afonso, J.C. & Ranalli, G. (2010) The deep lithospheric structure of the Namibian volcanic margin. Tectonophysics, 481, 68-81.
31. Gallagher, K. & Brown, R. (1999a) The Mesozoic Denudation history of the Atlantic margins of Southern Africa and Southeast Brazil and the relationship to offshore sedimentation. In: The Oil and Gas Habitats of South Atlantic (Ed. by N.R. Cameron, R.H. Bate & V.S. Clure), Geological Society Spec Publ. London, 153, 41-53.
32. Gallagher, K. & Brown, R. (1999b) Denudation and uplift at passive margins: the record of the Atlantic margin of Southern Africa. Philos. Trans. Roy. Soc. London, 357, 835-859.
33. Galloway, W.E. (2001) Cenozoic evolution of sediment accumulation in deltaic and Shore-Zone Depositional Systems, Northern Gulf of Mexico Basin. Mar. Petrol. Geol., 18, 1031-1040.
34. Gerrard, I. & Smith, G.C. (1982) Post paleozoic succession and structure of the Southwestern African continental margin. In: Studies in Continental Margin Geology (Ed. by J.S. Watkins & C.L. Drake), Am. Assoc. Petrol. Geol. Mem., 34, 49-74.
35. Gladczenko, T.P., Hinz, K., Eldhom, O., Meyer, H., Neben, S. & Skogseid, J. (1997) South Atlantic volcanic margins. J. Geol. Soc., 154, 465-470.
36. Gladczenko, T.P., Skogseid, J. & Eldhom, O. (1998) Namibia volcanic margin. Mar. Geophys. Res., 20, 313-341.
37. Goudie, A.S. (2005) The drainage of Africa since the Cretaceous. Geomorphology, 67, 437-456.
38. Gurnis, M., Mitrovica, J.X., Ritsema, J. & van Heijst, H. (2000) Constraining mantle density structure using geological evidence of surface uplift rates: the case of the African superplume. Geochem. Geophys. Geosyst., 1, 1588GC000035.
39. Hirsch, K.K., Scheck-Wenderoth, M., van Wees, J.-D., Kuhlmann, G. & Paton, D.A. (2010) Tectonic subsidence history and thermal evolution of the orange basin. Mar. Petrol. Geol., 27, 565-584.
40. Holtar, E. & Forsberg, A.W. (2000) Postrift development of the Walvis Basin, Namibia: results for the exploration campaign in quadrant 1911. In: Petroleum Systems of South Atlantic Margins (Ed. by M.R. Mello & B.J. Katz), Am. Assoc. Petrol. Geol. Mem., 73, 429-446.
41. ICS. (2004) International Stratigraphic Chart. International Commission of Stratigraphy. Available at:.
42. Jackson, M.P., Cramez, C. & Fonck, J.P. (2000) Role of subaerial volcanic rocks and mantle plume in creation of South Atlantic margins: implications for salt tectonics and source rocks. Mar. Petrol. Geol., 17, 477-498.
43. Jones, S.M., White, N.J., Clarke, B.J., Rowley, E. & Gallagher, K. (2002) Present and past influence of the iceland plume on sedimentation. In: Exhumation of the North Atlantic Margin (Ed. by A.G. Dore, J.A. Cartwright, M.S. Stocker, J.P. Turner & N.J. White), Geol. Soc. Spec. Publ. London, 196, 13-25.
44. Kohn, B.P., Gleadow, A.J.W., Brown, R.W., Gallagher, K., Lorencak, M. & Noble, W.P. (2005) Visualizing thermotectonic and denudation histories using apatite fission track thermochronology. Rev. Mineral. Geochem., 58, 527-565.
45. Lavier, L.L., Steckler, M.S. & Brigaud, F. (2001) Climatic and tectonic control on the cenozoic evolution of the West African margin. Mar. Geol., 178, 63-80.
46. Leturmy, P., Lucazeau, F. & Brigaud, F. (2003) Dynamic interactions between the Gulf of Guinea passive margin and the Congo River drainage basin: 1. Morphology and mass balance. J. Geophys. Res., 108, 2383-2396.
47. Light, M.P.R., Maslanyj, M.P., Greenwood, R.J. & Banks, N.L. (1993) Seismic sequence stratigraphy and tectonics offshore Namibia. In: Tectonics and Seismic Sequence Stratigraphy (Ed. by G.D. Williams & A. Dobb), Geol. Soc. Spec. Publ. 71, 67-85.
48. Liu, X. & Galloway, W.E. (1993) Sediment accumulation rate: problems and new approach. In: Rates of Geologic Processes, Tectonics, Sedimentation, Eustasy and Climate; Implications for Hydrocarbon Exploration (Ed. by J.M. Armentrout, R. Bloch, H.C. Olson & B.F. Perkins), GCS-SEPM Foundation 14th Annual Research Conference, pp. 101-107.
49. Maslanyj, M.P., Light, M.P.R., Greenwood, R.J. & Banks, N.L. (1992) Extension tectonics offshore Namibia and evidence for passive rifting in the South Atlantic. Mar. Petrol. Geol., 9, 590-601.
50. Mcmillan, I.K. (1990) Foraminiferal Biostratigraphy of the Barremian to Miocene Rocks of the Kudu 9a-1, 9a-2 and 9a-3 Boreholes. Commun. Geol. Survey Namibia, 6, 23-29.
51. Mcmillan, I.K. (2003) Foraminiferally defined biostratigraphic episodes and sedimentation pattern of the cretaceous drift succession (Early Barremian to Late Maastrichian) in seven basins of the South African and Southern Namibian Continental margin. S. Afric. J. Sci., 99, 537-576.
52. Melguen, M. (1978) Facies evolution, carbonate dissolution cycles in Sediments from the Eastern South Atlantic (Dsdp Leg 40) since the early cretaceous. In: Initial Reports of the Deep Sea Drilling Project, Vol.40 (Ed. by H.M. Bolli & W.B.F. Ryan, et al.) pp. 981-1024. U.S. Government Printing Office, Washington.
53. Métivier, F. & Gaudemer, Y. (1997) Mass transfer between Eastern Tien Shan and adjacent basins (Central Asia): constraints on regional tectonics and topography (P 1-17). Geophys. J. Int., 128, 1-253.
54. Métivier, F. & Gaudemer, Y. (1999) Stability of output fluxes of large rivers in South and East Asia during the last 2 million years: implications on floodplain processes. Basin Res., 11, 293-303.
55. Métivier, F., Gaudemer, Y., Tapponier, P. & Meyer, B. (1998) Northeastward growth of the Tibet plateau deduced from balanced reconstruction of two depositional areas: the Qaidam and Hexi Corridor basins, China. Tectonics, 17, 823-842.
56. Métivier, F., Gaudemer, Y., Tapponnier, P. & Klein, M. (1999) Mass accumulation rates in Asia during the cenozoic. Geophys. J. Int., 137, 280-318.
57. Mohriak, W., Rosendahl, B.R., Turner, J.P. & Valente, S.C. (2002) Crustal architecture of South Atlantic volcanic margins. In: Volcanic Rifted Margins (Ed. by M. Menzies, S.L. Klemperer, C.J. Ebinger & J. Baker), Geol. Soc. Am. Spec. Paper, 362, 159-202.
58. Moore, A.E. (1999) A reappraisal of epeirogenic flexure axes in Southern Africa. S. Afric. J. Geol., 102, 363-376.
59. Moore, T.C., Rabinowitz, P., Boersma, A., Borella, P.E., Chave, A.D., Duée, D., Fütterer, D.K., Jiang, M.J., Kleinert, K., Lever, A., Manivit, H., S., O.C., Richardson, S.H. & Shackleton, N.J. (1984) Proceedings of the Deep Sea Drilling Program, Initial Reports, Washington, 74, 1080pp.
60. Ni, S., Ding, X., Helmberger, D.V. & Gurnis, M. (1999) Low-velocity structure beneath Africa from forward modeling. Earth Planet. Sci. Lett., 170, 497-507.
61. Ni, S., Tan, E., Gurnis, M. & Helmberger, D. (2002) Sharp sides to the African superplume. Science, 296, 1850-1852.
62. Nyblade, A.A., Owens, T.J., Gurrola, H., Ritsema, J. & Langston, C.A. (2000) Seismic evidence for a deep upper mantle thermal anomaly beneath East Africa. Geology, 28, 599-602.
63. Nyblade, A.A. & Robinson, S.W. (1994) The African superswell. Geophys. Res. Lett., 21, 765-768.
64. Nyblade, A.A. & Sleep, N.H. (2003) Long lasting epeirogenic uplift mantle plumes and the origin of the Southern African plateau. Geochem. Geophys. Geosyst., 4, 1105, doi DOI:11 10.1029/2003GC0000573.
65. Partridge, T.C. & Maud, R.R. (1987) Geomorphic evolution of Southern Africa since the mesozoic. S. Afric. J. Geol., 90, 179-208.
66. Paton, D.A., di Primio, R., Kuhlmann, G., van der Spuy, D. & Horsfield, B. (2007) Insights into the petroleum system evolution of the Southern Orange Basin, South Africa. S. Afric. J. Geol., 110, 261-274.
67. Paton, D., van der Spuy, D., di Primio, R. & Horsfield, B. (2008) Tectonically induced adjustment of passive margin accommodation space; influence on the hydrocarbon potential of the Orange Basin, South Africa. Am. Assoc. Petrol. Geol. Bull., 92, 589-609.
68. Pickford, M., Senut, B., Mein, P., Gommery, D., Morales, J., Soria, D., Nieto, M. & Ward, J. (1996) Preliminary results of new excavations at Arrisdrift, Middle Miocene of Southern Namibia. C. R. Acad. Sci. Paris, 322, 991-996.
69. Poag, C.W. & Sevon, W.D. (1989) A record of Appalachian denudation in Post-Rift Mesozoic and Cenozoic sedimentary deposits of the U.S. Middle Atlantic Continental margin. Geomorphology, 2, 119-157.
70. Pysklywec, R.N. & Mitrovica, J.X. (1999) The role of subduction induced subsidence in the evolution of the Karoo basin. J. Geol., 107, 155-164.
71. Raab, M.J., Brown, R.W., Gallagher, K., Carter, A. & Weber, K. (2002) Late Cretaceous reactivation of major crustal shear zones in Northern Namibia: constraints from Apatite Fission Track Analysis. Tectonophysics, 349, 75-92.
72. Raab, M.J., Brown, R.W., Gallagher, K., Weber, K. & Gleadow, A.J.W. (2005) Denudational and thermal history of the early cretaceous Brandberg and Okenyenya igneous complexes on Namibia's Atlantic passive margin. Tectonics, 24, TC3006, doi:DOI: 3010.1029/2004TC001688.
73. Rayner, R.J., Waters, S.B., Mckay, I.J., Dobbs, P.N. & Shaw, A.L. (1991) The mid-cretaceous palaeoenvironment of Central Southern Africa (Orapa, Botswana). Palaeogeogr. Palaeoclimatol. Palaeoecol., 88, 147-156.
74. Ritsema, J. & van Heijst, H. (2000) New seismic model of the upper mantle beneath Africa. Geology, 28, 63-66.
75. Ritsema, J., van Heijst, H. & woodhouse, J. (1999) Complex shear wave velocity structure imaged beneath Africa and Iceland. Science, 286, 1925-1928.
76. Rouby, D., Bonnet, S., Guillocheau, F., Gallagher, K., Robin, C., Biancotto, F., Dauteuil, O. & Braun, J. (2009) Sediment supply to the orange sedimentary system over the last 150My: an evaluation from sedimentation/denudation balance. Mar. Petrol. Geol., 26, 782-794.
77. Rust, D.J. & Summerfield, M.A. (1990) Isopach and borehole data as indicators of rifted margin evolution in Southwestern Africa. Mar. Petrol. Geol., 7, 277-287.
78. Scholtz, A. (1985) The palynology of the upper lacustrine sediments of the Arnot Pipe, Banke, Namaqualand. Ann. S. Afr. Museum, 95, 1-109.
79. Sclater, J.G. & Christie, P.A.F. (1980) Continental stretching: an explanation of the mid-cretaceous subsidence of the Central North Sea basin. J. Geophys. Res., 85, 3711-3739.
80. Ségalen, L., Renard, M., Pickford, M., Senut, B., Cojan, I., Le Callonnec, L. & Rognon, P. (2002) Environmental and climatic evolution of the Namib Desert since the Middle Miocene: the contribution of carbon isotope ratios in Ratite Eggshells. C. R. Geosci., 334, 917-924.
81. Ségalen, L., Rognon, P., Pickford, M., Senut, B., Emmanuel, L., Renard, M. & Ward, J. (2004) Reconstitution of dune morphologies and palaeowind regimes in the Proto-Namib since the Miocene. Bull. Soc. Géol. France, 175, 537-546.
82. Selby, M.J. (1985) Earth's Changing Surface: An Introduction to Geomorphology. Clarendon Press, Oxford, 602pp.
83. Senut, B., Pickford, M. & Ségalen, L. (2009) Neogene desertification of Africa. C. R. Geosci., 341, 591-602.
84. Siesser, W.G. (1980) Late miocene origin of the Benguela upwelling system off Northern Namibia. Science, 208, 283-285.
85. Smith, R.M.H. (1986) Sedimentation and palaeoenvironments of late cretaceous crater-lake deposits in Bushmanland, South Africa. Sedimentology, 33, 369-386.
86. Smith, W.H.F. & Sandwell, D.T. (1997) Global seafloor topography from satellite altimetry and ship depth soundings. Science, 277, 1957-1962.
87. Stevenson, I.R. & Mcmillan, I.K. (2004) Incised valley fill stratigraphy of the upper cretaceous succession, proximal orange basin, Atlantic margin of Southern Africa. J. Geol. Soc., 161, 185-208.
88. Tinker, J., de Wit, M. & Brown, R. (2008a) Mesozoic exhumation of the Southern Cape, South Africa, quantified using apatite fission track thermochronology. Tectonophysics, 455, 77-93.
89. Tinker, J., de Wit, M. & Brown, R. (2008b) Linking source and sink: evaluating the balance between onshore erosion and offshore sediment accumulation since Gondwana Break-up, South Africa. Tectonophysics, 455, 94-103.
90. Trumbull, R.B., Sobolev, S.V. & Bauer, K. (2002) Petrophysical modeling of high seimic velocity crust at the Namibian Volcanic margin. In: Volcanic Rifted Margins (Ed. by M. Menzies, S.L. Klemperer, C.J. Ebinger & J. Baker), Geol. Soc. Am. Spec. Paper, 362, 221-230.
91. Tucholke, B.E. & Embley, R.W. (1984) Cenozoic regional erosion of the Abyssal sea floor off South Africa. In: Interregional Unconformities and Hydrocabon Accumulation (Ed. by J.S. Schlee), Am. Assoc. Petrol. Geol. Mem., 36, 145-164.
92. Uenzelmann-Neben, G., Schluter, P. & Weigelt, E. (2007) Cenozoic oceanic circulation within the South African gateway: indications from seismic stratigraphy. S. Afric. J. Geol., 110, 275-294.
93. UNESCO, Choubert, G. & Faure-Muret, A. (1990) International Geological Map of Africa, CGWM & UNESCO.
94. van der Beek, P., Summerfield, M.A., Braun, J., Brown, R.W. & Fleming, A. (2002) Modeling postbreakup landscape development and denudational history of the Southeast African (Drakensberg Escarpment) margin. J. Geophys. Res., 107, 2351.
95. van der Spuy, D. (2003) Aptian source rocks in some South African Cretaceous basins. In: Petroleum Geology of Africa: New Themes and Developing Technologie (Ed. by M.A. Arthur, D.S. MacGregor & N.R. Cameron), Geol. Soc. Spec. Publ. London, 207, 185-202.
96. Walford, H.L., White, N.J. & Sydow, J.C. (2005) Solid sediment load history of the Zambezi delta. Earth Planet. Sci. Lett., 238, 49-63.
97. Wefer, G., Berger, W.H., Richter, C., Adams, D.D., Anderson, L.D., Andreasen, D.J., Brüchert, V., Cambray, H., Christensen, B.A., Frost, G.M., Giraudeau, J., Gorgas, T.J., Hermelin, J.O.R., Jansen, J.H.F., Lange, C.B., Laser, B., Lin, H.L., Maslin, M.A., Meyers, P.A., Motoyama, I., Murray, R.W., Perez, M.E., Pufahl, P.K., Spiess, V., Vidal, L., Wigley, R., & Yamazaki, T. (1998) Proceedings of the Ocean Drilling Program, Initial Reports, 175, 577pp.
98. Wickens, H.D.V. & Mclachlan, I.R. (1990) The stratigraphy and sedimentology of the reservoir interval of the Kudu 9a-2 and 9a-3 boreholes. Commun. Geol. Surv. Namibia, 6, 9-22.