Stratigraphic correlation of the Awahab and Tafelberg Formations, Etendeka Group, Namibia, and location of an eruptive site for flood basalt volcanism

Abstract

Detailed field and geochemical investigations in the vicinity of the type section of the Tafelberg Formation of the early Cretaceous Etendeka Group, NW Namibia, have revealed the existence of a large eruptive vent in the lower part of the regional volcanic sequence produced by Strombolian and Vulcanian eruptions. The vent is filled with the thick, differentiated, ponded Kudu-Run olivine-rich basaltic flow, which has a distinctive low Zr/Y geochemical signature as well as a Tafelberg-type tabular basalt and the Nil Desperandum latite. Field evidence indicates that the Kudu-Run basalt and the latite were erupted from fissures located within the vent. Associated with the vent is an extensive pyroclastic apron extending from the vent edge and which is interbedded with the regional stratigraphy. Blocks of Precambrain basement lithologies occur within this deposit and indicate that the vent was excavated to a depth of at least 350 m below the palaeosurface at that time. The original Tafelberg Formation type section described by Erlank et al. [Erlank, A.J., Marsh, J.S., Duncan, A.R., Miller, R.McG., Hawkesworth, C.H., Betton, P.J., Rex, D.C. 1984. Geochemistry and petrogenesis of the Etendeka volcanic rocks from SWA/Namibia, 195–247. In: Erlank, A.J. (Ed.), Petrogenesis of Volcanic Rocks of the Karoo province. Special Publication of the Geological Society of South Africa, vol. 13, 395 p.] the Tafelberg Gully section, crosses from the lower part of the regional sequence into the intra-vent sequence and returns to the regional sequence higher up. In doing so it includes some of the localized intra-vent flows and excludes a number of flows which are part of the regional sequence in its lower part, thus rendering it inappropriate as a type section. A revised type section for the Tafelberg Formation is described by combining the upper part of the Tafelberg Gully section with a new section of 14 flows at the base of the regional sequence in the Tafelberg North (TBN) section some 2 km N of the Tafeleberg Gully. Distinctive flows in the TBN section can be mapped southwards where their precise stratigraphic relationship to the northward-thinning Springbok and Goboboseb Quarts Latite members of the Awahab Formation can be demonstrated. These stratigraphic relationships are entirely consistent with palaeomagnetic reversal stratigraphy and demonstrate that the same N–R–N polarity sequence occurs in the type sections of the Awahab and Tafelberg formations. Thus, the Awahab and Tafelberg magma systems were contemporaneous but the Tafelberg system outlived that of the Awahab. The Awahab system was built from eruptive centres located S of the Huab River whereas the Tafelberg vents were located further north.

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.