Early Jurassic Rift-Valley-Related Alkaline Lake Deposits Interbedded with Karoo Flood Basalts, Southern Namibia

• Autores: Harald Stollhofen, Ian G. Stanistreet, Stephan Gerschütz
• Localización: Lake basins through space and time / coord. por Elizabeth H. Gierlowski-Kordesch, K. Kelts, 2000, ISBN 0891810528, págs. 167-179
• Idioma: inglés
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• Resumen

  • The lake sediments are preserved as two separate layers interleaved with Early Jurassic Kalkrand flood basalts (Duncan et al., 1984; Cerschutz, 1996) and are best exposed around Hardap reservoir, 15 km north of Mariental town in southwestern Namibia (Figure1). The 55-300 m thick volcano-sedimentary sequenceun conformably oversteps underlying Triassic(Stormberg) and latest Carboniferous-Early Permian(Dwyka and Ecca) sediments in the east onto latest Proterozoic-Early Cambrian (Nama) basement toward the west (Heath, 1972; Schalk and Germs,1980). The top of the sequence is defined by the erosive Cretaceous land surface with continental sediments of the Cenozoic Kalahari thermal sag basin draped thinly over the top. The 183.0 0.6 Ma to 186.0 0.8 Ma dated olivine-tholeiitic basalts (Duncanet al., 1997) of the Kalkrand Formation were extruded during extensional rifting between South America and Africa (Miller, 1992; Dingle, 1993). This was one of a series of early rifting episodes from the Permian onward (Figure 2) prior to ultimate extensional rift phases during the Early Cretaceous. The latter caused extrusion of the Etendeka-Parana flood basalts and onset of South Atlantic oceanic opening(Hawkesworth et al., 1992; Renne et al., 1996; Gladczenkoet al., 1997).

    Extensional tectonism was experienced in the study area along a northerly trending set of extensional167faults dipping toward the east. This is inferred (Stollhofenet al., 1998) to be conjugate to a westerly dipping break-away detachment system that connected with the more major detachment toward the west,along which oceanic opening was eventually effected(Maslanyj et al., 1992; Light et al., 1992, 1993).The Hardap area is therefore characterized by