Internal stratigraphic relationships in the Etendeka Group in the Huab Basin, NW Namibia: understanding the onset of flood volcanism

The Etendeka Igneous Province in NW Namibia forms the eastern most extent o f the Parana-Etendeka Flood Basalt Province and, despite only covering abou t 5% of the Parana-Etendeka, has been the focus of much interest, due to it s extremely well exposed nature. The Huab Basin in NW Namibia forms the foc us of this study, and formed a connected basin with the Parana throughout K aroo times (late Palaeozoic) into the Lower Cretaceous. It contains a conde nsed section of the Karoo deposits, which indicate early periods of extensi on, and Lower Cretaceous aeolian and volcanic Etendeka deposits, which have their correlatives in the Parana. In the Huab Basin, the volcanic rocks of the Etendeka Group consists of the Awahab and Tafelberg Formations, which are separated by a disconformity. Detailed examination of the Awahab Format ion reveals an additional disconformity, which separates olivine-phyric bas alts (Tafelkop-type) from basalt/basaltic andesites (Tafelberg-type) mat-ki ng out a shield volcanic feature which is concentrated in an area to the SE of the Huab River near to the Doros igneous centre. Early volcanism consis ted of pahoehoe style flows of limited lateral extent, which spilled out on to aeolian sands of an active aeolian sand sea 133 million years ago. This sand sea is equivalent to the sands making up the Botucatu Formation in the Parana basin. The early expression of flood volcanism was that of laterall y discontinuous, limited volume, pahoehoe flows of Tafelkop-type geochemist ry, which interleaved with the aeolian sands forming the Tafelkop-Interdune Member basalts. These basalts are on-lapped by more voluminous, laterally extensive, basalt/basaltic andesite flows indicating a step-up in the volum e and rate of flood volcanism, leading to the preservation of the shield vo lcanic feature. These geochemically distinct basalts/basaltic andesites for m the Tsuhasis Member, which are interbeded with the Goboboseb and Sprinkbo k quartz latite flows higher in the section. The Tsuhasis Member basalts, w hich form the upper parts of the Awahab Formation, are of Tafelberg-type ge ochemistry, but are stratigraphically distinct from the Tafelberg lavas, wh ich are found in the Tafelberg Formation above. Thus, the internal stratigr aphy of the flood basalt province contains palaeo-volcanic features, such a s shield volcanoes, and other disconformities and is not that of a simple l ayer-cake model. This complex internal architecture indicates that flood vo lcanism started sporadically, with low volume pahoehoe flows of limited lat eral extent, before establishing the more common large volume flows typical of the main lava pile. (C) 1999 Elsevier Science Ltd. All rights reserved.