Journal of the Geological Society, London, Vol. 156, 1999, pp. 771–777. Printed in Great Britain.
Climate, sediment supply and tectonics as controls on the deposition and preservation of the aeolian–fluvial Etjo Sandstone Formation, Namibia
NIGEL MOUNTNEY1,3, JOHN HOWELL1, STEPHEN FLINT1 & DOUGAL JERRAM2 1Stratigraphy Group, Department of Earth Sciences, University of Liverpool, Brownlow Street, Liverpool L69 3BX, UK 2Department of Geological Sciences, University of Durham, South Road, Durham DH1 3LE, UK 3Present address: Department of Earth Sciences, Keele University, Keele, Staffs ST5 5GG, UK (e-mail: [email protected])
Abstract: Deposition and subsequent preservation of the Jurassic–Cretaceous Etjo Sandstone Formation of Namibia represents a complex interplay between climatic and tectonic factors and related variations in extrabasinal sediment supply. The aeolian and fluvial deposits indicate semi-arid to arid climatic conditions throughout the deposition of four distinct sedimentary units. The succession records either an upward increase in aridity or an upward increase in aeolian sediment supply, represented by a transition from a fluvially dominated basal unit, through a marginal fluvial–aeolian unit to an exclusively aeolian unit. A combination of inherited palaeotopography and syndepositional extensional faulting provided the space necessary for the accumulation of much of the succession. A basinwide unconformity (super surface) divides the succession. This hiatus resulted partly from a lack of available preservation space and partly from a shutdown in aeolian activity related to a regional climatic reorganization. A subsequent shift in the palaeowind direction from northwesterly to southwesterly exploited sand reserves in the Parana´ Basin of South America and led to the resumption of aeolian sedimentation across the region. Variations in preserved bedform thickness were directly controlled by differential amounts of tectonic subsidence across the basin. A second major super surface towards the top of the succession resulted from the regional shutdown of large tracts of the aeolian system following the eruption of Etendeka flood basalts across the region.
Keywords: Namibia, aeolian environment, fluvial environment, climate, tectonics.
The response of arid and semi-arid continental depositional fluvial units and the significance of the surfaces that separate systems to regional changes in climate and tectonic setting is them are investigated at a variety of scales to distinguish complex and difficult to recognize within preserved succes- between accumulations resulting from autocyclic bedform sions. Aeolian–fluvial interactions result from both the local- climbing within the erg and its immediate margin, and ized, short-term interplay between competing depositional accumulations resulting from regional changes in climate, processes and from more regional, longer-term responses to extrabasinal sediment supply and/or rates of tectonic changes in allocyclic controls such as climate, extrabasinal subsidence. sediment supply and basin tectonics (Talbot 1985; Blakey 1988; Kocurek & Havholm 1993). Whilst the central parts of well established erg systems (sand seas) almost certainly do respond to changes in allocyclic controls, such responses may Geological setting often be subtle, being expressed, for example, as slight vari- The Jurassic–Cretaceous aeolian–fluvial Etjo Sandstone ations in the angle of climb of dune bedforms or in the Formation is exposed over more than 3000 km2 in the Huab preserved thickness of bedsets. In erg-margin areas, however, Basin of northwestern Namibia (Fig. 1). The basin forms an where competing aeolian and fluvial processes are often finely eastern extension to the much larger Parana´ Basin of South balanced, any changes in external controls will be reflected by America which developed through the Late Palaeozoic and predictable variations in depositional style, as characterized in Mesozoic as an intracratonic basin (Zalan et al. 1991). Subse- the preserved succession by an upward increase in aeolian quently, the basin underwent extension in Late Jurassic to dominance, an upward increase in fluvial dominance or a Early Cretaceous times, prior to the break-up of West period of non-deposition, depending on the nature of the Gondwana and the opening of the South Atlantic (Dingle change. Several previous studies of aeolian–fluvial interactions 1992; Light et al. 1993). Basement rocks in the Huab Basin have been applied to determine the nature and extent of comprise the Late Proterozoic Zerrissene turbidite system climatic and tectonic controlling mechanisms on semi-arid which forms part of the Damara Orogen (Miller 1983; Swart continental systems (e.g. Jurassic Kayenta–Navajo transition, 1992). These are overlain by 200 m of continental sedimentary Herries 1993; Jurassic Page Sandstone, Jones & Blakey 1997; rocks of the Karoo Supergroup (Horsthemke et al. 1990). The Permian Cedar Mesa Sandstone, Langford & Chan 1988; Etjo Sandstone Formation rests unconformably on these Silurian Tumblagooda Sandstone, Australia, Trewin 1993). Karoo sedimentary rocks, attaining a maximum thickness of In this study we document and interpret the facies architec- 200 m in the basin centre, but thinning to absent as it onlaps ture of the aeolian–fluvial Etjo Sandstone Formation of against the southern basin margin. The northern basin margin Namibia. The geometric relationships between aeolian and is not exposed, being covered by up to 500 m of Etendeka
771
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/156/4/771/4887220/gsjgs.156.4.0771.pdf by guest on 01 October 2021 772 N. MOUNTNEY ET AL.
Fig. 1. Simplified geological map of the Huab region of northwestern Namibia. Positions of logged sedimentary sections depicted by white circles. These act as control points for the fence diagram presented in Fig. 2, the position of which is signified on the map by the solid black lines. Numbered white squares show the positions of the two architectural panels presented in Fig. 3. Mapping based on the 1:50 000 geological maps of Horsthemke (1992) and Ledendecker (1992), supplemented by aerial photographic interpretation by the authors.
flood basalts which directly overlie the Etjo Sandstone For- termed locally the Gaias Formation (Horsthemke 1992; mation. Aeolian strata in the upper part of the Etjo Sandstone Ledendecker 1992). This basinwide unconformity exhibits a Formation interfingers both vertically and laterally with these pronounced palaeotopography which partly accounts for lava flows (Milner et al. 1994; Jerram et al. 1999). On the basis thickness variations in the overlying Etjo Sandstone For- of 40Ar/39Ar dating of the basalts (Renne et al. 1996), termin- mation (Fig. 2). The Etjo Sandstone Formation attains a ation of aeolian sedimentation is estimated to have occurred at maximum thickness of 200 m, but thins southwards as it 132&1 Ma. No age information for the base of the Etjo onlaps against the southern basin margin. The distribution of Sandstone Formation has been forthcoming and hence the fluvial and aeolian facies indicates complex spatial and tem- duration of aeolian–fluvial sedimentation remains unknown. poral variations in depositional style which may be related to four distinct phases of sedimentary evolution, each of which is summarized below. Data Data have been collected at three distinct scales. (1) Fifty-nine vertical Krone Member logged sections demonstrate the extent of the major aeolian and fluvial units, and spatial variations in facies distribution and sedimentary The basal 10–15 m comprises a cross-bedded, predominantly style (Fig. 2). (2) Eight architectural panels, totalling 10 km in length, clast-supported pebble and cobble conglomerate that cuts have been constructed at the ‘set scale’. These enable semi-regional down into the underlying Karoo Group. Clasts are predomi- lateral tracing of sedimentary units and bounding surfaces, and the nantly reworked from Damaran metasedimentary rocks reconstruction of individual bedforms within specific parts of the erg exposed at the basin margins. Palaeocurrent data (Fig. 3) (Fig. 3). (3) Key sedimentary units and their bounding surfaces have been investigated at the ‘laminae scale’. Certain sedimentary struc- indicate that the main axial drainage system within the basin at tures, such as wavy and contorted laminae, desiccation cracks, rooted this time flowed from NE to SW along the line of the and/or bioturbated horizons are important palaeoenvironmental indi- present-day Huab River. To the south, tributary streams cators (Kocurek 1981; Kocurek & Havholm 1993) and are crucial to flowed from SE to NW into the basin axis. The presence of the correct interpretation of regionally extensive events. numerous horizontally laminated sandstone and pebbly sand- stone horizons, together with rarely preserved dessication cracks interbedded with the conglomerates (Mountney et al. Sedimentary units 1998), is interpreted to represent deposition in a high energy The Etjo Sandstone Formation lies unconformably on flow environment dominated by flash floods and braided, Permian fluvio-lacustrine sediments of the Karoo Group, ephemeral stream flows (Miall 1978; Nichols 1987). The
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/156/4/771/4887220/gsjgs.156.4.0771.pdf by guest on 01 October 2021 CONTROLS ON AEOLIAN–FLUVIAL SYSTEMS 773
Fig. 2. Fence diagram depicting the three-dimensional geometry of the Huab Basin during deposition of the Etjo Sandstone Formation and the spatial distribution of the four major sedimentary units. Diagram constructed from 59 logged sections (vertical black columns) and 8 architectural panels of the type shown in Fig. 3. See Fig. 1 for section locations. Vertical exaggeration 50:1.
sandstone units signify waning flow conditions, culminating in evident, with the dunes migrating to the SSW and ESE. These emergence, drying out and the development of dessication migration directions are thought to have been controlled by features. palaeotopography whereby the wind was preferentially funnelled along NE–SW and NW–SE orientated palaeovalleys at various sites within the basin (Mountney et al. 1998). Lower Fluvial–Aeolian Unit (Mixed Unit) The fluvial conglomerates are overlain conformably by up to 30 m of mixed fluvial and aeolian sandstones, and rare pebbly Main Aeolian Unit (Main Unit) sandstones that represent a transition from fluvial to aeolian Above, there is an abrupt transition to large-scale cross- sandsheet deposition, with the development of isolated aeolian bedded sandstones that dominate the formation, attaining a dunes. Occasionally, stream deposits truncate horizontally maximum thickness of 150 m. These deposits consist exclu- laminated sandsheet and cross-bedded dune sand deposits. sively of fine- to medium-grained, yellow-white quartz sand- Facies indicate a semi-arid environment where conditions stones that are very mature, and contain grains that are well suitable for aeolian transport and bedform generation were rounded and well sorted. Individual bed-sets are typically 10 m frequently curtailed by the resumption of fluvial activity. Wavy thick, although in the basin centre single beds up to 52 m thick laminae and adhesion structures in some sandsheet deposits are observed at the base of the unit (Fig. 3a). Foresets consist indicate the close proximity of the water-table to the depo- of grainflow/grainfall cross-strata, with wind rippled sand sitional surface. Preserved aeolian dune bedforms within this deposits preserved in the basal parts of sets. These deposits are unit rarely exceed 2 m in thickness and are laterally tran- interpreted to represent large-scale aeolian dunes. Trough sitional into sandsheet facies or are cut out abruptly by sandy cross-beds, 200–500 m wide, are exposed in sections orientated fluvial facies. Grainflow/grainfall cross-strata within dune perpendicular to the palaeowind (Fig. 3a). In sections parallel deposits reveal dip directions which vary by over 60) within a to the wind these relate to gently climbing interdune or single bed, indicating curved slipfaces (Fig. 3). This, together first-order surfaces (Kocurek 1996; Brookfield 1977), and are with the lateral discontinuity of the beds suggests that these interpreted to represent large transverse dune bedforms with deposits represent isolated barchan dunes migrating over crescentic slipfaces. Some sets contain superimposition sandy fluvial and aeolian sandsheet plains. Despite the broad (second-order) surfaces (Fig. 3), indicative of the deposits spread in palaeocurrent data collected from the cross-stratified of slipfaceless dunes with smaller, superimposed dune bed- aeolian sandstones, two dominant migration directions are forms migrating over their surface—compound draa in the
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/156/4/771/4887220/gsjgs.156.4.0771.pdf by guest on 01 October 2021 on 01 October 2021 by guest Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/156/4/771/4887220/gsjgs.156.4.0771.pdf 7 .MOUNTNEY N. 774 TAL. ET
Fig. 3. Two-dimensional architectural panels depicting bed geometry and bounding surface relationships. (a) basin centre. (b) southerly part of the basin. A distinctive super surface extends across both sections at the base of the Main Unit. Note the 50 m thick set (with numerous superimposition surfaces) at the base of the Main Unit in panel 1. The equivalent set is only 5–10 m thick in panel 2. Also note the large-scale trough-cross stratified nature of the sets towards the left-hand end of panel 1 (perpendicular to the palaeowind). See Fig. 1 for locations. Palaeocurrent data presented in rose diagrams are a composite of data from across the basin. CONTROLS ON AEOLIAN–FLUVIAL SYSTEMS 775
terminology of McKee (1979). Reactivation (third-order) extensive super surfaces that represent substantial periods of surfaces relating to minor changes in wind direction are non-deposition (e.g. Jurassic Entrada Sandstone, Kocurek common throughout the succession. Palaeocurrent data reveal 1981; Jurassic Page Sandstone, Kocurek et al. 1991; Permian an average foreset dip of 24) towards 040) and indicate a Cedar Mesa Sandstone, Langford & Chan 1988; Jurassic unidirectional palaeowind (Hodgson 1972; Fig. 3). Wingate Sandstone, Clemmensen & Blakey 1989). Super sur- faces have been used to divide apparently continuous aeolian deposits into a series of separate sedimentary accumulations Upper Aeolian Unit (Upper Unit) that result from distinct periods of aeolian sedimentation. Overlying aeolian sandstones (0–60 m thick) are distinct from The surface that separates the Mixed Unit from the Main deposits of the Main Unit because they occur after the initial Unit represents a sharp and distinctive horizon that can be eruption of basaltic lava in the region. These initial lavas are traced for in excess of 30 km from the basin centre to the restricted in spatial extent and consequently aeolian deposits of southern margin of the basin. This surface separates deposition the Upper Unit may lie either directly on the lowermost basalts in the Mixed Unit from deposition in the overlying Main Unit or on deposits of the Main Unit. In the latter case, the two across much of the basin and is more extensive than those successions are separated by a laterally extensive bounding surfaces that cap accumulations of single bedforms (first-order surface. bounding surfaces). This surface contains additional evidence to support the notion of a hiatus. Vertical tubular burrows (Skolithos) that extend down from the surface are common Synsedimentary tectonic development and most likely represent burrowing activity into a partially stabilized substrate. Rootlets indicating surface stabilization The onset of seafloor spreading along the Namibian margin are also common. Extensional growth faults that displace was preceded by E–W-orientated continental extension. deposits of the Mixed Unit commonly terminate at the afore- Gladczenkoˇ et al. (1997) estimate that the South Atlantic rift mentioned surface and do not displace beds in the overlying system underwent extension for c. 25 Ma prior to break-up. A Main Unit. We interpret this surface to be a basinwide super series of N–S and NNE–SSW orientated extensional faults surface that divides two temporally distinct aeolian dominated traverse the Huab Basin (Fig. 1). Milner & Duncan (1987) and units. Clemson et al. (1997) document evidence for active extension The surface dividing the Main Unit from the Upper Unit is during, and after, the eruption of the Etendeka basalts also interpreted to be a super surface, albeit one that has (132 Ma). Extensional faults are common in the Mixed formed due to somewhat unusual circumstances, namely the Aeolian–Fluvial and Main Units. In outcrop, small-scale faults rapid and widespread emplacement of Etendeka flood basalts display bed offsets of up to 3 m and individual beds are that resulted in a major reorganization in erg dynamics. commonly thicker in immediate hangingwall, indicating syn- sedimentary tectonic development at this time. On a 5–10 km scale, deposits of the Krone Member and Mixed Unit clearly thicken into localized fault controlled depocentres (Fig. 2, Line Discussion: controls on the development and preservation 2), whilst these deposits are substantially thinner or absent of the Etjo Sandstone Formation from the adjacent footwall areas. Within deposits of the Main Here we assess the relative roles of variations in climate, Unit, preserved bedsets clearly thicken towards the basin extrabasinal sediment supply, tectonics and palaeotopography centre (Fig. 3). Pre-existing pronounced topography on the in controlling depositional style and preservation potential in Karoo surface, generated prior to the onset of deposition of the Etjo Sandstone Formation. The Krone Member represents the Etjo Sandstone Formation, would have played a significant a fluvial system that earlier in its history carved out the role in generating accommodation space into which the fluvial palaeotopography observed at the top of the Karoo Super- and aeolian sandstones were subsequently deposited. Thus, the group. This ephemeral fluvial system represents the result of accumulation and preservation of the Etjo Sandstone For- flash flooding in semi-arid wadi channels. The overlying mation was in part controlled by inherited palaeotopography transition from conglomeratic fluvial to sandy fluvial and ff and in part controlled by di erential synsedimentary tectonic aeolian facies most likely reflects an increase in aridity that subsidence. Active extension was underway during deposition resulted in a shutdown of the fluvial systems draining the basin of the Etjo Sandstone Formation, somewhat earlier than margins. Instead, lower energy fluvial systems reworked the previously documented and certainly prior to the eruption of sand fraction of the fluvial deposits already in the basin. the overlying Etendeka flood basalts. Aeolian sand grains in the Mixed Unit are less well sorted and less mature than those in the Main Unit. This suggests that aeolian activity resulted from local reworking of fluvial Super surfaces and sedimentary sequences in the Etjo deposits rather than from the onset of extrabasinal aeolian Bounding surfaces that extend laterally for several kilometres sediment input. Within the Mixed Unit, the predominance of in aeolian successions result either from migration of large- aeolian sandsheet facies over aeolian dune facies indicates that scale dune bedforms (first-order bounding surfaces) or from there was insufficient sand grade material to build substantial hiatuses in erg development (super surfaces) (Talbot 1985; dunes across the basin. Indeed, 20–30% of the aeolian sand- Kocurek 1988). A super surface bounds a temporally distinct sheet deposits consist of medium to coarse sand grains which period of erg accumulation. These regional bounding surfaces form bimodally sorted, wind rippled sandsheets but are too represent the termination of ergs or at least large portions of coarse to build dunes. ergs and, therefore, mark significant hiatuses in the evolution Preservation of aeolian deposits requires that the accumu- of basins filled by erg sequences (Kocurek 1988). Several lation is placed below the regional baseline of erosion studies of ancient aeolian systems have recognized that pack- (Kocurek & Havholm 1993). This may be achieved either by ages of aeolian strata are commonly bounded by laterally regional subsidence or by incorporation of the deposits into
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/156/4/771/4887220/gsjgs.156.4.0771.pdf by guest on 01 October 2021 776 N. MOUNTNEY ET AL.
the saturated zone (i.e. placement below the water-table). is only possible to crudely relate local climate within the Huab Subsidence may occur due to tectonism, sediment loading or Basin to broader scale regional climatic patterns. Certainly, the sediment compaction. Incorporation into the saturated zone entire region was characterized by semi-arid to arid climatic may be the result of either a relative rise in the water-table due conditions throughout the Late Jurassic and Early Cretaceous. to subsidence, or an absolute rise in water-table due to a The Botucatu erg at this time covered much of the Parana´ change in climate. An absolute water-table rise at the time of Basin and other arid climate deposits of Mesozoic age are deposition of the Mixed Unit appears unlikely because over reported from elsewhere in Namibia, Botswana and South 90% of the sandsheet deposits are composed of wind-rippled Africa (Johnson et al. 1996). sands indicative of deposition above the water-table, rather Preservation of the Main Unit within the Huab Basin was than wavy or adhesion laminated sands that are indicative of controlled by a combination of pre-existing topography on the deposition at or close to the water-table (Kocurek 1981). Karoo surface developed prior to the onset of deposition of Where wavy and adhesion laminae are observed they are only the Etjo Sandstone Formation and synsedimentary tectonic local in extent, possibly representing deposition in isolated subsidence. blow-out hollows. Tectonic subsidence is one obvious mech- Following the hiatus in deposition represented by the super anism through which the accumulation may have been pre- surface separating the Mixed Unit from the Main Unit, there served. Displacement across synsedimentary extensional faults was a switch from a northwesterly to a prevailing south- within the basin would have generated the accommodation westerly wind direction (Fig. 3). This tapped upwind sand space necessary for the preservation of the deposits. For reserves of the Botucatu erg in the Parana´ Basin, resulting in example, faulting towards the southern basin margin created the migration of large aeolian bedforms into the Huab Basin. minor depocentres in which remnants of the Mixed Unit have Available accommodation space within the basin was rapidly been preserved (Fig. 2). In contrast, immediately adjacent to filled. The basal set within the Main Unit thickens from only these minor depocentres, the Mixed Unit is often absent and 8–10 m at the southern basin margin to 52 m in the lowpoints the Main Unit then lies directly on sedimentary rocks of the toward the basin centre (Fig. 2). This variation in preserved set Karoo Supergroup. thickness is unlikely to have been controlled by original Generation of the super surface separating the Mixed Unit bedform amplitude (which must have exceeded 52 m) but from the Main Unit is most readily attributed to a combi- instead reflects variation in available preservation space across nation of climatic, topographic and tectonic factors. Firstly, the basin. Higher in the Main Unit succession preserved sets whilst the inherited palaeotopography and synsedimentary representing the deposits of subsequent aeolian bedforms tectonic subsidence encouraged preservation of the Mixed rarely exceed 10 m in thickness, indicating that sediment Unit in low-points around the basin, elsewhere preservation is supply by this time had largely filled the basin, reducing incomplete and an unconformity is present in the form of an available accommodation space. The upper part of the Main erosive super surface. The basinwide extent of this surface Unit consists of predominantly aeolian dune deposits indicates that former deposits of the upper part of the Mixed separated by minor interdune deposits that accumulated under Unit were reworked on a regional scale prior to deposition of conditions of positive angles of bedform climbing (Rubin & the Main Unit. Secondly, the widespread occurrence of trace Hunter 1982). fossils and rootlets on the super surface, which contrasts with The cessation of deposition of the Main Unit occurred due their absence elsewhere in the Etjo Sandstone Formation, to the widespread emplacement of the initial Etendeka flood indicates a period of stabilization where dune development and basalts over large tracts of the region (Jerram et al. 1999). migration was curtailed. This most likely resulted from a Although the initial lavas did not cover the entire erg system, climatic shift towards conditions that encouraged colonization. their eruption did result in a dramatic decrease in sediment Alternatively, a decrease in sediment supply may have termi- availability and led to a period of significant aeolian reworking nated dune migration enabling opportunistic species to of the surviving parts of the active dune system. This rework- become established (Thomas & Tsoar 1990). The development ing resulted in the generation of a second major super surface of a vegetative cover would have had the knock-on affect of across the region. Three successively smaller dune fields char- reducing sand mobility and further stabilizing the system, acterize the Upper Unit and represent the re-establishment and promoting super surface development. subsequent shutdown of aeolian systems under conditions There is a distinct change in depositional style across the of progressively decreasing sediment availability following super surface from conditions of restricted aeolian sediment repeated eruptions of basalt. supply in the Mixed Unit to an abundant aeolian sediment supply, capable of building a basinwide, continuous erg in the Main Unit. This, together with the better sorting and increased maturity of sand grains in sediments of the Main Unit, signals Conclusions the widespread and volumetrically significant introduction of Within the aeolian dominated Etjo Sandstone Formation, extrabasinally derived aeolian sand into the region. The likely climate and related extrabasinal sediment supply have exerted cause of this event was a switch from northwesterly to south- a primary control on the style of deposition. Conversely, westerly palaeowinds (Fig. 3) and suggests a major climatic inherited palaeotopography and synsedimentary extensional reorganization in the region. The widespread Botucatu faulting, expressed as a complex pattern of regional subsid- Sandstone Formation being deposited at this time in the ence, has controlled the preservation of this succession. Thus, Parana´ Basin of South America (Zalan et al. 1991) provided an the resulting record of sedimentation reflects the complex ample upwind source of aeolian grade material for the Main interplay between these major allocyclic controls. Unlike the Unit. The Etjo Sandstone Formation at this time effectively extensively studied Permian-Jurassic aeolian-fluvial succes- formed a downwind extension of the continental-scale sions exposed on the Colorado Plateau of the western United Botucatu erg (Dingle 1992). Since the precise age of the onset States (Blakey et al. 1988), which developed predominantly in of deposition of the Etjo Sandstone Formation is unknown, it a foreland setting, sediments of the Etjo Sandstone Formation
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/156/4/771/4887220/gsjgs.156.4.0771.pdf by guest on 01 October 2021 CONTROLS ON AEOLIAN–FLUVIAL SYSTEMS 777
accumulated in an active rift environment and display a ——&H, K.G. 1993. Eolian sequence stratigraphy—A conceptual distinctive basin-scale architecture that reflects their regional framework. In:W,P.&P,H.W.(eds)Siciliclastic Sequence Stratigraphy. American Association of Petroleum Geologists, tectono-stratigraphic setting. Memoirs, 58, 393–409. ——, K,J.&H, K. 1991. Outcrop and semi-regional three- BG PLC and Conoco UK Ltd are gratefully acknowledged for their dimensional architecture and reconstruction of a portion of the eolian Page joint sponsorship of this research. We would like to thank S. Milner, Sandstone (Jurassic). In:M,A.D.&T,N.(eds)The three- H. Stollhofen and I. Stanistreet for their helpful discussions and B. dimensional facies architecture of terrigenous clastic sediments and its Frommherz, J. Sorrensen and D. Pirrie for their constructive implications for hydrocarbon discovery and recovery. Society of Economic Paleontologists and Mineralogists Concepts in Sedimentology and reviews. The Geological Survey of Namibia is thanked for use of its Paleontology, 3, 25–43. facilities and the loan of a fieldwork vehicle. L,R.&C, M.A. 1988. Flood surfaces and deflation surfaces within the Cutler Formation and Cedar Mesa Sandstone (Permian), Southeastern Utah. Geological Society of America Bulletin, 100, 1541–1549. References L, S. 1992. Stratigraphie der Karoosedimente der Huabregion (NW Namibia) und deren Korrelation mit zeita¨quivalenten Sedimenten des Sedimentary Geology, B , R.C. 1988. Basin tectonics and erg response. 56, Parana´ Beckens (Su¨damerika) und des Gro§en Karoobeckens (Su¨dafrika) 127–151. unter besonderer Beru¨cksichtigung der u¨berregionalen geodynamischen ——, P,F.&K, G. 1988. Synthesis of Late Paleozoic and und klimatischen Entwicklung Westgondwanas. Go¨ttinger Arbeiten zur Mesozoic eolian deposits of the western interior of the United-States. Geologie und Pala¨ontologie, 54, 1–87. Sedimentary Geology, 56, 3–125. L, M.P.R., M, M.P., G,R.J.&B, N.L. 1993. B, M.E. 1977. The origin of bounding surfaces in ancient aeolian Seismic sequence stratigraphy and tectonics offshore Namibia. In: sandstones. Sedimentology, 24, 303–332. W,G.D.&D,A.(eds)Tectonics and Seismic Sequence C,L.B.&B, R.C. 1989. Erg deposits in the Lower Jurassic Stratigraphy. Geological Society, London, Special Publications, 71, Wingate Sandstone, Northeastern Arizona—Oblique dune sedimentation. 163–191. Sedimentology, 36, 449–470. MK, E.D. 1979. An introduction to the study of global sand seas. In:MK, C, J., C,J.&B, J. 1997. Structural segmentation and the E.D. (ed.) Global sand seas. US Geological Survey, Professional Paper, influence of basement structure on the Namibian passive margin. Journal of 1052, 1–19. the Geological Society, London, 154, 477–482. M, A.D. 1978. Lithofacies types and vertical profile models of braided river D, R.V. 1992. Structural and sedimentary development of the continental deposits, a summary. In:M,A.D.(ed.)Fluvial sedimentology. Memoir margin off Southwestern Africa. Communications of the Geological Survey of the Canadian Society of Petroleum Geology, 5, 597–604. of Namibia, 8, 35–43. M,R.MG. 1983. The Pan-African Damara Orogen of South West G, T.P., H, K., E, O., M, H., N,S.&S, Africa/Namibia. In:M,R.MG. (ed.) Evolution of the Damara J. 1997. South Atlantic volcanic margins. Journal of the Geological Society, Orogen. Geological Society, South Africa, Special Publications, 11, London, 154, 465–470. 431–515. H, R.D. 1993. Contrasting styles of fluvial-aeolian interaction at a M,S.C.&D, A.R. 1987. Geochemical characterisation of quartz downwind erg margin: Jurassic Kayenta-Navajo transition, Northeastern latite units in the Etendeka Formation. Communications of the Geological Arizona, USA. In:N,C.P.&P,J.D.(eds)Characterization Survey of S.W.A./Namibia, 3, 83–90. of Fluvial and Aeolian Reservoirs. Geological Society, London, Special ——, ——, E,A.&M, J.S. 1994. Promotion of the Etendeka Publication, 73, 199–218. Formation to Group status: A new integrated stratigraphy. H, F.D.I. 1972. The geology of the Brandberg–Aba Huab area, South Communications of the Geological Survey of Namibia, 9, 5–11. West Africa. PhD thesis, Orange Free State. M, N.P., H, J.A., F,S.S.&J, D.A. 1998. Aeolian and H, E. 1992. Fazies der Karoosedimente in der Huab-Region, alluvial deposition within the Mesozoic Etjo Sandstone Formation, NW Damaraland, NW-Namibia. Go¨ttinger Arbeiten zur Geologie und Namibia. Journal of African Earth Sciences, 27, 175–192. Pala¨ontologie, 55, 1–102. N, G.J. 1987. Syntectonic alluvial fan sedimentation, Southern Pyrenees. ——, L,S.&P, H. 1990. Depositional environments Geological Magazine, 124, 121–133. and stratigraphic correlations of the Karoo Sequence in North-western Damaraland. Communications of the Geological Survey of Namibia, 6, R, P.R., G, J.M., M,S.C.&D, A.R. 1996. Age of Etendeka 63–75. flood volcanism and associated intrusions in Southwestern Africa. Geology, 24, 659–662. J, D.A., M,N.P.&S, H. 1999. Facies architecture of the Etjo Sandstone Formation and its interaction with the basal Etendeka R,D.M.&H, R.E. 1982. Bedform climbing in theory and nature. flood basalts of NW Namibia: Implications for offshore analogues. In: Sedimentology, 29, 121–138. C, N., B,R.&C,V.(eds)Oil and Gas Habitats of the South S, R. 1992. The sedimentology of the Zerrissene turbidite system, Damara Atlantic Region. Geological Society, London, Special Publications, 153, Orogen, Namibia. Geological Survey of Namibia Memoir, 13, 1–54. 367–380. T, M.R. 1985. Major bounding surfaces in aeolian sandstones—A climatic J, M.R., V V, C.J., H, W.F., K,R.&S,U. model. Sedimentology, 32, 257–265. 1996. Stratigraphy of the Karoo Supergroup in southern Africa: An T, D.S.G. & T, H. 1990. The geomorphological role of vegetation in overview. Journal of African Earth Sciences, 23, 3–15. desert dune systems. In:T,J.B.(ed.)Vegetation and erosion. Wiley, J,L.S.&B, R.C. 1997. Eolian-fluvial interaction in the Page Chichester, 471–489. Sandstone (Middle Jurassic) in South-Central Utah, USA—A case study of T, N.H. 1993. Mixed aeolian sandsheet and fluvial deposits in the erg-margin processes. Sedimentary Geology, 109, 181–198. Tumblagooda Sandstone, Western Australia. In:N,C.P.&P, K, G. 1981. Significance of interdune deposits and bounding surfaces in D.J. (eds) Characterization of Fluvial and Aeolian Reservoirs. Geological aeolian dune sands. Sedimentology, 28, 753–780. Society, London, Special Publications, 73, 219–230. —— 1988. First-order and super bounding surfaces in eolian sequences – Z, P.V., W, S., A, M.A.M., V, I.S., C˜, J.C.J., Bounding surfaces revisited. Sedimentary Geology, 56, 193–206. A, V.T., N, E.V.S., C,J.R.&M, A. 1991. The —— 1996. Desert aeolian systems. In:R,H.G.(ed.)Sedimentary Parana´ Basin, Brazil. In:L, M.W., K, D.R., O, D.F.A. & Environments: Processes, Facies and Stratigraphy. Blackwell Science, E,J.J.(eds)Interior Cratonic Basins. American Association of Oxford, 125–153. Petroleum Geologists Memoirs, 51, 681–708.
Received 30 November 1997; revised typescript accepted 24 November 1998. Scientific editing by Lynne Frostick, Martyn Pedley and Duncan Pirrie.
Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/156/4/771/4887220/gsjgs.156.4.0771.pdf by guest on 01 October 2021