Stratigraphic correlation of the Awahab and Tafelberg Formations, Etendeka Group, Namibia, and location of an eruptive site for flood basalt volcanism
Introduction
The Etendeka Igneous Province in NW Namibia is the smaller component of the early Cretaceous Paraná-Etendeka continental flood basalt province. Despite its relatively small volume in comparison with its South American counterpart, the excellent exposures in an arid dissected terrain has resulted in Etendeka-based volcanological, petrological and stratigraphic studies contributing significantly to the understanding of the giant Paraná-Etendeka province as a whole (e.g. Ewart et al., 2004a, Ewart et al., 2004b, Milner et al., 1995, Marsh et al., 2001 and many references therin). Understanding the origin and emplacement of large-volume continental flood basalt provinces, such as the Paraná-Etendeka province, remains a major problem for volcanologists and petrologists. Critical to addressing these problems is the establishment of a detailed stratigraphy within volcanic sequence and the location of the fissures from which lava flows were erupted. These problems are inter-related as detailed stratigraphy may help constrain the location of eruptive sites. Specific problems hampering conventional stratigraphic mapping is the compositional monotony of the basaltic sequences and the fact that lava flows are usually pahoehoe compound flows (in the terminology of Walker, 1971) that cannot be unambiguously traced laterally for any distance. The latter problem is particularly acute in older sequences which may be preserved only in scattered and eroded remnants. Over the past two decades geochemistry has become an essential tool in establishing the stratigraphy of flood basalt sequences (e.g. Swanson et al., 1979, Marsh et al., 1997). However, where sedimentary horizons are interbedded with the basaltic sequence (e.g. lower part of the Karoo – see Lock et al., 1974) or where distinctive widespread silicic sheets are interbedded with basalts (e.g. the Etendeka – see Milner et al., 1995) stratigraphic mapping and correlations may be done without recourse to geochemistry.
In the Etendeka volcanic sequence a number of quartz latite sheets within the basaltic sequence has facilitated the stratigraphic correlation of the flood volcanic sequence between the numerous eroded remnants that constitute the province (see Milner et al., 1994, Marsh et al., 2001). Nevertheless stratigraphic correlation between (and sometimes within) a number of remnants remains problematical. In the southern Etendeka region Milner et al. (1994) recognised the Awahab and Tafelberg Formations. The type section of the Awahab Formation is at Awahab (14° 09′ 23″E; 20° 38′ 13″S – see Fig. 1) in the volcanic remnant S of the Huab River, and this sequence can be correlated with the volcanic sequence in the Goboboseberge remnant between the Brandberg and Messum complexes. The Awahab Formation consists of mafic lavas and two prominent quartz latites (QLs) – the lower Goboboseb QL which is interbedded in the mafic lavas and the upper Springbok QL which caps the sequence. The type area for the Tafelberg Formation is at Tafelberg (14° 09′ 03″E; 20° 10′ 15″S) on the eastern edge of the main Etendeka remnant. This formation is dominated by mafic lavas with a lower interbedded Wereldsend QL and the capping Beacon and Grootberg QLs. Each of the quartz latite units is compositionally and petrographically distinct (Milner and Duncan, 1987). The Goboboseb and Springbok QLs of the Awahab Formation extend across the Huab River and occur over wide areas in the southern part of the main Etendeka remnant, but thin northwards and finally pinch out some 10 km S of Tafelberg. At several locations between Awahab and Tafelberg the Wereldsend QL of the Tafelberg Formation can be observed to lie on the eroded upper surface of the Springbok QL and it has been proposed that there is a disconformity between the Awahab and Tafelberg Formations (see Fig. 3 of Milner et al., 1994 and Fig. 9 of Jerram et al., 1999). However the exact nature of the disconformity and the precise stratigraphic relationship between the units in the lower parts of the two formations remains uncertain.
Additionally, it was discovered that the Nil Desperandum latite (see Marsh et al., 2001 – previously the Tafelberg latite of Erlank et al., 1984) which outcrops in the Tafelberg type section, has limited lateral distribution and appeared to be confined to a palaeovalley cut into the basaltic sequence in the lower part of the Tafelberg Formation. Subsequent detailed mapping of this feature and the surrounding volcanic sequence has revealed that the palaeovalley it is an eroded eruptive vent. It has also become apparent that the so-called Tafelberg type section (Erlank et al., 1984 Fig. 6) is not a correct representation of the regional volcanic sequence at Tafelberg. The mapping has also clarified the correlation between the lower parts of the Tafelberg and Awahab Formations. All these new data and their interpretation are the subject of this paper.
Section snippets
Nomenclature and analytical techniques
The Tafelberg Formation is dominated by aphyric mafic lavas with intergranular textures known as the Tafelberg type. They exhibit a continuous chemical compositional range from <50% to 58% SiO2 , and extend from the basalt into the basaltic andesite field in the TAS diagram (Marsh et al., 2001). Silica-poor and silica-rich members of this suite cannot be distinguished on the basis of field or petrographic character and only when a chemical analysis is available is it possible to identify a rock
The Etendeka Formation – Tafelberg type section
The eastern edge of the main Etendeka remnant is marked by a prominent dissected escarpment which, at Tafelberg (14° 09′ 03″E; 20° 10′ 15″S), exposes one of the thickest continuous sections (some 900 m) through the volcanic sequence. The Tafelberg area has featured in the earliest dating and palaeomagnetic studies of Etendeka volcanism (Siedner and Miller, 1968, Siedner and Mitchell, 1976, Gidskehaug et al., 1975) and this lead Erlank et al. (1984) to focus their geochemical studies on the
The Tafelberg vent complex
The Tafelberg vent (Fig. 2, Fig. 3) is about 3 km wide and up to 5 km long but its full extent cannot be determined as it is partly covered to the N by the upper part of the regional volcanic sequence which builds Tafelberg . The complex consists of breccias, flows of basalt and a latite, all confined by steep, inward-dipping walls of the vent. At present erosion levels these walls are built of the basaltic flows of the lower part of the Tafelberg sequence. Associated with the vent is an
Conclusions
Detailed mapping and additional geochemical and palaeomagnetic studies at the Tafelberg type section for the Tafelberg Formation of the Etendeka Group has revealed the presence of a deep eruptive vent at Tafelberg. The vent is associated with a pyroclastic blanket, up to 3 m thick, which occurs within the regional volcanic sequence and extends for more than 650 m from the vent margins. The nature of this pyroclastic deposit suggests that the vent was excavated by an initial phase of Strombolian
Acknowledgements
This paper has been a journey of serendipity over several years. Thanks to Roger Swart of NAMCOR who assisted with field observations in the preliminary part of the study in 1999. Work on the Tafelberg North section in 1996 involved Paul Renne and John Glen (Berkeley Geochronology Centre) who provided the palaeomagnetic results. Thanks to Shiloh Marsh for field assistance during the completion of mapping in the vent complex in 2004.
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