Depositional Environments and Stratigraphic Correlation of the Karoo Se- Quence in Northwestern Damaraland

Communs geol. Surv. Namibia, 6 (1990), 67-77 Depositional environments and stratigraphic correlation of the Karoo Se- quence in northwestern Damaraland E. Horsthemke, S. Ledendecker and H. Porada lnstitut für Geologie und Dynamik der Lithosphäre, Universität Göttingen, Goldschmidtstrasse 3, D-3400 Göttingen, West Germany The continental sediments of the Karoo Sequence in the Huab Basin, northwestern Damaraland, Namibia, are interpreted as marginal equivalents of a more than 2000 m thick succession of Gondwana beds that have been accumulated in the Parana Basin of South America. The sediments, which attain a maximum thickness of 250 m, reflect a change from cold and nival to warm and arid climates. In the Huab Basin, the succession begins with tillites and periglacial diamictites and rhythmites (depositional units 1 and 2). These are covered by fluvial sandstones and coal deposited in a cold tundra-like environment (depositional unit 3). The succeeding fluvial sandstones and calcareous soils (depositional unit 4) cover a wide area and indicate a moderate, dry climate. The following, probably lacustrine, succession includes sediments with autochthonous carbonates (stromatolites, oolites), a widespread Mesosaurus bone bed and various marginal environments indicating warm climatic conditions (depositional units 5 and 6). The aqueous sequence is terminated by red beds of the Gai-as Formation (depositional unit 7). After a phase of erosion and non-deposition, continental sedimentation revived with fluvial deposits (depositional unit 8) and terminated with aeolian sandstones in a desert environment (Etjo Sandstone Formation, depositional unit 9). While the Huab sediments are broadly correlated with those of the Parana and Karoo basins, age determinations from fossils suggest that units 1 and 2 are Permo-Carboniferous and that units 3 to 7 are Permian. The exact age of the Mesozoic units 8 and 9 is uncertain Introduction Sequence. A hilly to mountainous topography was in- cised into the basement rocks during the Permo-Car- Sedimentary deposits of the Karoo Sequence in boniferous glaciation (Martin, 1981) and it controlled, northwestern Damaraland are exposed in the Gobo- together with the subsidence of the Paraná Basin, the boseb Mountains south of the Ugab River, as well as deposition of the Karoo Sequence up to the Gai-as For- in an area approximately 100 by 50 km wide on both mation (unit 7). The top of the Karoo Sequence is made the northern and southern sides of the Huab River (Fig. up of more than 500 m of Mesozoic mafic-to-acid lavas 1). This latter area, referred to as the Huab Basin in the of the Etendeka Formation which partly interfinger with following discussion, has been investigated since 1987 the uppermost sediments. with the aim of establishing the stratigraphic sequence In this study the Karoo Sequence has been subdi- and depositional conditions. vided into nine depositional units which are described The basement to the Karoo Sequence is schist and and interpreted in respect of their sedimentary facies. granite of the Upper Proterozoic to Cambrian Damara A schematic east-west cross-section through the Huab 67 Horsthemke, Ledendecker and Porada area is illustrated in Fig. 2. Correlations with Karoo Se- metres. At one locality a thickness of 7 m was observed. quence sediments in South Africa and South America Exposures of the sediments are rare and show that the (Uruguay, Brazil) are further discussed. lateral extension is restricted to several tens of square metres. Previous work Lithofacies A Mesosaurus bone bed discovered by Reuning and The base of the Karoo Sequence comprises an unsort- von Huene (1925) on the northern flank of the Doros ed, unbedded, matrix-supported conglomerate contain- Crater (Fig. 1) was the first evidence of a Karoo age ing clasts ranging in diameter from a few centimetres to for the sedimentary succession of the Huab Basin. several tens of centimetres and set in a pelitic to psam- However, a detailed description and subdivision of the mitic matrix. Hodgson (1972) reported that the clasts sequence at Doros Crater was only presented 32 years consist mainly of quartz (80%) and additionally quartz- later by Reuning and Martin (1957), who correlated the ite (10%) and schist (10%). Frets (1969) described an lower part with the Dwyka Formation and the upper occurrence of striated pavement in the western Huab part with the Stormberg Series of South Africa. Dur- area which indicates that the ice movement was to- ing subsequent mapping of parts of the Huab Basin by wards the west. Frets (1969) and Hodgson (1972), and after discovery of a vertebrate fauna north of Doros Crater by Keyser Interpretation (1973), the present stratigraphic subdivision evolved: The sediment has been interpreted as a tillite by Re- SACS (1980) attributes the sedimentary succession to uning and Martin (1957) and Hodgson (1972). Martin the Dwyka Group, the Lower Ecca Group and the Drak- (1981) also described parts of the Huab Valley as a ensberg Group. deep glacially shaped valley. The restricted distribution of the tillite suggests a proximal position for the Huab Description of the depositional units area which was probably dominated by erosion during the glaciation and subsequent melting periods. Depositional unit 1 Depositional unit 2 These sediments overlie the basement discordantly and in general attain a thickness of a few tens of centi- The sediments of this unit, which attain a maximum 68 Depositional environments and stratigraphic correlation thickness of 15 m, either directly overlie the tillite de- lateral extent. In upward-fining successions, the platy posits or small restricted areas of Damaran basement. sandstone and siltstone contain abundant trace fossils of burrowing organisms. Sandstone intercalations inter- Lithofacies finger laterally with the carbonaceous shales, while at This unit is characterised by varying successions of the basal contact they often contain eroded particles of conglomerate, sandstone, claystone and, in some places, the underlying material. Unit 3 is capped by a calcare- marl or limestone. The sequence begins in many places ous-ferruginous palaeosol along the eastern margin of with conglomerate, up to 50 cm thick, which contains the study area, well exposed in the Twyfelfontein and pebbles up to 15 cm in diameter set in a matrix which Verbrande Berg region. is more psammitic than that of the tillite. Slump structures are frequent in places where the conglomerate Interpretation interfingers with the sandstone. The sandstone exhibits The association of mainly pelitic sediments, rootlet different sedimentary structures: planar bedding, lami- horizons and coal seams overlain by and partly inter- nar, bedding and several types of cross-stratification are fingering with sandstones indicates a swampy, fluvially common. controlled depository. Two sedimentary environments The fine-grained parts of the unit, i.e. claystone, marl can be distinguished in extended glacial valleys: a sta- and siltstone, are mostly developed as rhythmites. Indi- ble meandering system with clastic channel fills and vidual rhythmite layers are up to 5 cm thick and sepa- wide areas characterised by permanent moisture and rated by remarkably subtle horizontal contacts caused abundant vegetation (Cairncross and Cadle 1988). by different grain size and various contents of calcite. According to Retallack (1980), the Lower Permian All psammitic to pelitic layers of unit 2 frequently ex- Gangamopteris-Glossopteris floral assemblage may be hibit arthropod trace fossils of the Umfolosia type (Sav- readily compared with present-day boreal plant com- age, 1971). munities. Also, the boreal wetlands, bogs and shallow pools of Canada have been taken as an example to Interpretation explain the Lower Permian coal deposits of Australia The association of matrix -supported conglomerate (Martini and Glooschenko, 1985). Cairncross and Ca- and thinly interbedded shale-siltstone successions is dle (1988), in describing the sedimentary facies of the believed to represent a periglacial diamictite-rhythmite Witbank Coalfields (Vryheid Formation, South Africa), couple. Whereas frequently developed slump structures have distinguished phases of prevailing coal formation in the diamictite may be the result of solifluction-in- and of predominantly clastic sedimentation. Climatic duced earthflow, interbedding of coarse-grained sand- changes have been claimed to be the main reason for this stone.or gravel and unsorted conglomerate probably variation. Thus, in cold periods and under waterlogged indicate fluvial transport. The fine-grained rhythmites conditions, a widespread autochthonous vegetation are thought to be deposited in small protected basins could develop, whereas in warmer more moderate cli- under low-energy conditions as indicated by the uni- matic intervals, sheet -like fluvial deposits were spread form lamination, straight and continuously crested rip- over the area. Similarly, it is thought that deposition of ples and undisturbed trace fossils. unit 3 began under cold climatic conditions which gave rise to the formation of coal in the lower part of the Depositional unit 3 succession. With climatic melioration, a sandy fluvial facies (unit 4, described below) established itself and, Sediments of unit 3 are widespread in the Huab Basin towards the east, was accompanied by the formation of and form the first blanketing cover, up to 50 m thick, early calcareous-ferruginous soils. over the basement. They include shale and siltstone

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