Jerram Et Al Twyfelfontein Sandstone

Communs geol. Surv. Namibia, 12 (2000), 303-313 The Fossilised Desert: recent developments in our understanding of the Lower Cretaceous deposits in the Huab Basin, NW Namibia Dougal A. Jerram1, Nigel Mountney2, John Howell3 and Harald Stollhofen4 1Dept. of Geological Sciences, University of Durham, South Rd, Durham, DH1 3LE, UK. (email [email protected]). 2School of Earth Sciences and Geography, Keele University, Staffordshire, ST5 5BG, UK. 3Department of Earth Sciences, University of Liverpool, Liverpool, L69 3BX, UK. 4Institut für Geologie, Universität Würzburg, 97070 Würzburg, Germany. The Lower Cretaceous deposits in the Huab Basin, NW Namibia, comprise fluvial and aeolian sandstones, lava flows and associated intrusions of the Etendeka Group. The sandstones formed part of a major aeolian sand sea (erg) system that was active across large tracts of the Paraná-Huab Basin during Lower Cretaceous times (133-132 Ma). This erg system was progressively engulfed and subsequently preserved beneath and between lava flows of the Paraná-Etendeka Flood Basalt Province. Burial of this erg by flood basalts has resulted in the preservation of a variety of intact aeolian bed forms. Preserved bed forms vary in type and scale from 1 km wavelength compound transverse draa to isolated barchan dunes with downwind wavelengths of < 100 m. Due to the present-day preferential erosion of the lava flows, preserved aeolian dunes are now exposed in 3-D in the position in which they were migrating ~133 Ma ago. A relatively non-de- structive eruption style of inflated pahoehoe flows preserved the bed form geomorphology. These first pahoehoe flow fields, comprising olivine-phyric Tafelkop lavas, define a shallow shield-like volcanic feature. This volcanic feature centres around the Doros igneous centre marking this as the likely source for the lavas. Early lava flows followed low topography between dune build-ups, ponding in the inter- dune areas. Striations left on the sand surface by the lavas indicate the localised flow directions of the lava. Numerous sediment interlayers, preserved between the lava flows, record a change in palaeowind direction during volcanism. This change in climate may have been driven by the ongoing break-up of the West-Gondwana supercontinent or may be a direct result of the widespread volcanism. Introduction Korn and Martin (1954) and Reuning and Martin (1957) provided some of the earliest geological refer- Large parts of Central and Northern Namibia have ences to the Karoo and Etendeka stratigraphy and the been occupied by aeolian sand-seas (ergs) since Jurassic sediments and intrusions in NW Namibia. This contri- times. Major sand seas of this region include the Jurassic bution to the Henno Martin Commemorative Volume Etjo Formation in the Waterberg region (Holzförster et outlines and summarises the recent developments in our al., 1999), the Cretaceous Etjo Formation in the Huab understanding of the Lower Cretaceous deposits in the Basin (Mountney et al., 1998; Jerram et al., 1999a) also Huab Basin, NW Namibia. It reports on an ancient sand termed the Twyfelfontein Formation (Stanistreet and sea which was engulfed by, and interacted with, the ba- Stollhofen, 1999), the Tsondab Sandstone Formation sal Etendeka flood basalts in NW Namibia 133-132 Ma (Ward, 1988), and the Quaternary (recent) sediments ago, in a process that preserved much of the original and active erg system of the Namib Desert (Lancaster, dune morphology. 1995). During the Jurassic-Cretaceous, the African con- tinent rifted with, and then separated from, the South Geological setting and lithostratigraphic develop- American content. This rifting process and the concom- ment of the Etendeka Group in the Huab Basin itant impact of the Tristan Hot-Spot resulted in the mas- sive outpouring of flood basalts of the Paraná -Etendeka The Etendeka Igneous Province forms the most east- Flood Basalt Province immediately prior to and during ern extent of the much larger Paraná-Etendeka Flood the early stages of continental separation. The Paraná Basalt Province (Fig. 1A). The main outcrops and sub- -Etendeka Flood Basalt Province is one of the largest crops of Karoo sediments and Mesozoic flood basalts in of the continental igneous provinces with a present-day Namibia are shown in Fig. 1B, with the location of the preserved volume in excess of 1 x 106 km3 (Cordani and Huab Basin highlighted. In the Huab Basin, good 3-D Vandoros, 1967), with most of the volcanic activity oc- exposure of the Etendeka Group units can be found in curring between 135-130 Ma (Renne et al., 1996; Turn- the Huab Outliers region and immediately to the north er et al., 1994; Milner et al., 1995a). Often overlooked of the Huab River (Fig. 2A). The term ‘Huab Basin’ is the fact that flood basalts erupt over large areas of was introduced by Horsthemke et al. (1990) who iden- land where a variety of active continental environments tified a variety of stratigraphic units (that are now as- occur. Due to their high effusion rates, such lava flows signed to the Karoo Supergroup and Etendeka Group) have the potential to ‘fossilise’ and preserve important within a basinal feature centred around the present-day information concerning these continental environments Huab River valley (Fig. 2A). The stratigraphy within by encasing sediment and vegetation that would oth- the Huab Basin records the evolution of this region erwise have a low long-term preservation potential. In from Damaran times (500-600 Ma) through to the Early the case of the Paraná-Etendeka Flood Basalts, eruption Cretaceous break-up of Gondwana (130-133 Ma). The has preserved arid climate continental aeolian deposits succession can be divided into three major stratigraphic that reveal important information concerning the envi- units (Fig 2A): 1) Damara basement - deformed meta- ronmental conditions that prevailed in this part of West sediments and granites, 2) Karoo Supergroup sediments Gondwana during Early Cretaceous times. - continental fluvio-lacustrine sediments with marine 303 Jerram, Mountney, Howell and Stollhofen intercalations, and 3) Etendeka Group - fluvial-aeolian roo surface make up the basal 10-15 m of the Etendeka sediments and volcanics. Group deposits. This unit, termed the Krone Member The stratigraphic nomenclature for the Lower Cre- (Horsthemke et al. (1990), is characterised by predomi- taceous deposits in the Huab Basin has changed sig- nantly cross-bedded, mainly clast-supported, pebble nificantly since the early works of Reuning and Martin and cobble fluvial conglomerates that were deposited (1957) and Hodgson (1970). For clarity, Figure 3 sum- into river valleys cut down into the underlying Karoo marises the development of the nomenclature and out- sediments and through exposed highs of Damaran base- lines its stratigraphic context. ment. The clasts within the conglomerate are largely The Karoo stratigraphy in the Huab Basin records derived from Damaran meta-sediments exposed at the episodes of Palaeozoic extension that resulted in dep- basin margins. The main drainage direction for these osition of early intra-continental rift-fill successions fluvial systems is interpreted to have been from NE (Stollhofen et al., 2000; Jerram et al., 1999a). The accu- to SW (Mountney et al., 1998), similar to that of the mulation of the Karoo Supergroup sedimentary succes- present-day Huab River. sion was followed by a considerable time gap, possibly up to 120 Ma, prior to the onset of Etendeka Group dep- Mixed Aeolian Unit osition (Jerram et al., 1999a; Stollhofen, 1999). Dur- ing this period, a combination of pre-Etendeka rifting, The Krone Member is overlain by up to 30 m of mixed restricted extrabasinal sediment supply, and erosion of fluvial and aeolian sandstones that represent a transi- existing sediments, generated a large amount of accom- tion from a fluvial-dominated to an aeolian-dominated modation space in and around the area of the present- sedimentary environment. Ephemeral stream deposits day Huab River (Mountney et al., 1998; Mountney et truncate horizontally laminated sandsheet and cross- al., 1999a). This depositional hiatus is marked by a ba- bedded dune sand deposits. These are indicative of a sin-wide unconformity that separates Karoo strata from semi-arid environment where conditions suitable for the overlying deposits of the Etendeka Group. aeolian bed form generation were frequently interrupt- Below we outline the sedimentary and volcanic de- ed by episodes of fluvial activity, possibly on a seasonal posits that characterise the basal units of the Etendeka basis (Mountney et al., 1998). The extent of the mixed Group (summarised in figures 2 and 3). The reader is aeolian unit is incomplete throughout the basin and its referred to Mountney et al. (1998) for detailed descrip- upper limits are truncated by a bounding, basin-wide tions of the fluvial/aeolian deposits, and Jerram et al. super-surface (Mountney et al., 1999a). Palaeocurrent (1999a) for detailed descriptions of the volcanic depos- analysis of the well sorted, cross stratified sands within its in the Huab Basin. the Mixed Aeolian Unit indicate transport directions towards the south-west in the vicinity of the present-day Krone Member Huab River and towards the south-east in the southerly part of the basin (Mountney et al., 1998). The lowermost deposits that overlie the eroded Ka- 304 The Fossilised Desert: Lower Cretaceous deposits in the Huab Basin, NW Namibia 305 Jerram, Mountney, Howell and Stollhofen Main Aeolian Unit In the latter case, the two successions are separated by a laterally extensive bounding surface. Overlying sedi- Above the Mixed Aeolian Unit, there is an abrupt ment interlayers progressively decrease in thickness transition to large-scale cross-bedded sandstones that and lateral extent upwards. The last of the aeolian sedi- attain a maximum thickness of 150 m. These deposits ment units is recorded below the Goboboseb quartz lat- consist exclusively of fine- to medium-grained, red-yel- ite flow, a widespread marker horizon within the Etend- low to white aeolian sandstone units that are arranged eka igneous succession.

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