Harald Stollhofen

The geochronology and significance of ash-fall tuffs in the glaciogenic Carboniferous-Permian Dwyka Group of Namibia and South Africa

Abstract Centimetre thick, laterally extensive tuff horizons occur within dark, marine mudstones of the Carboniferous-Permian Dwyka Group (Karoo Supergroup) in southern Namibia and South Africa. These pyroclastic deposits preserve the earliest evidence of volcanism in Karoo-equivalent strata of southern Africa. Four deglaciation sequences (DS I–IV) recorded in the Dwyka Group of Namibia and South Africa are capped by mudstone units such as the 45 m thick marine fossil-bearing Ganigobis Shale Member in Namibia in which 24 thin ash-fall horizons are preserved. Ion microprobe analyses (SHRIMP) of juvenile, magmatic zircons from the tuff horizons were used to determine their age. They permit a new radiometric age calibration of the top of deglaciation sequence II and of the Dwyka/Ecca Group boundary in southern Africa. Juvenile zircons of two tuff horizons near Ganigobis (southern Namibia) give 206∗ Pb 238 U ages of 302.0 ± 3.0 Ma and 299.2 ± 3.2 Ma (latest Kasimovian) for the top of DS II. Juvenile zircons from two tuff horizons of the basal Prince Albert Formation, sampled north of Klaarstroom and south of Laingsburg in the Western Cape (South Africa), were dated at 288.0 ± 3.0 and 289.6 ± 3.8 Ma (earliest Asselian). According to these age determinations, the deposition of Dwyka Group sediments in southern Africa started by the latest at about 302 Ma and ended at about the Carboniferous/Permian boundary, 290 Ma before present.

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

Abstract The Etendeka Igneous Province in NW Namibia forms the eastern most extent of the Parana–Etendeka Flood Basalt Province and, despite only covering about 5% of the Parana–Etendeka, has been the focus of much interest, due to its extremely well exposed nature. The Huab Basin in NW Namibia forms the focus of this study, and formed a connected basin with the Parana throughout Karoo times (late Palaeozoic) into the Lower Cretaceous. It contains a condensed section of the Karoo deposits, which indicate early periods of extension, 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 Formation reveals an additional disconformity, which separates olivine-phyric basalts (Tafelkop-type) from basalt/basaltic andesites (Tafelberg-type) marking 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 consisted of pahoehoe style flows of limited lateral extent, which spilled out onto 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 laterally discontinuous, limited volume, pahoehoe flows of Tafelkop-type geochemistry, 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 volume and rate of flood volcanism, leading to the preservation of the shield volcanic feature. These geochemically distinct basalts/basaltic andesites form the Tsuhasis Member, which are interbeded with the Goboboseb and Sprinkbok quartz latite flows higher in the section. The Tsuhasis Member basalts, which form the upper parts of the Awahab Formation, are of Tafelberg-type geochemistry, but are stratigraphically distinct from the Tafelberg lavas, which are found in the Tafelberg Formation above. Thus, the internal stratigraphy of the flood basalt province contains palaeo-volcanic features, such as shield volcanoes, and other disconformities and is not that of a simple layer-cake model. This complex internal architecture indicates that flood volcanism started sporadically, with low volume pahoehoe flows of limited lateral extent, before establishing the more common large volume flows typical of the main lava pile.

Mass and hyperconcentrated flow deposits record dune damming and catastrophic breakthrough of ephemeral rivers, Skeleton Coast Erg, Namibia

Channel-shaped deposits of well sorted sand with only 2–4% fine-grained material, being either massive and structureless or upward fining with basal lag, are found interbedded with aeolian sand in the Skeleton Coast Erg, Namibia. Detailed analyses of the channel-shaped deposits suggest that they are formed as hyperconcentrated flows within the erg. Grain-size analysis and whole rock geochemical modeling revealed that some of the fluvial sediments contain up to 70% aeolian sand, interpreted as a result of dune collapse into the fluvial system. In certain cases, this instantaneous supply of sand resulted in generation of sandy mass flows with laminar flow behaviour. The presence of smectite as dominant clay mineral proved to be of crucial importance in formation of mass flow deposits. These mass flows had no erosional capacity, and drape the palaeotopography. They are comparable to those generated by catastrophic collapse of dunes, described in the literature. This paper suggests that all these deposits should be termed intra-erg mass flows, as several of them carry little, if any debris. Based on their origin, intra-erg mass flows can be divided into two groups:

Lava-sediment interaction in desert settings; are all peperite-like textures the result of magma-water interaction?

Abstract This study reports on lava–sediment interaction in dry depositional settings focusing on the Early Cretaceous volcano-sedimentary sequence in the Huab Basin, NW Namibia, as a detailed example. Here an active aeolian sand sea (erg) system was progressively engulfed by lavas of the Etendeka Flood Basalt Province ∼133 Ma BP. This volcanic flooding resulted in: (1) the unprecedented preservation of large parts of the active dune system; (2) the development of a variety of sediment interlayers; and (3) a record of excellent examples of the dynamic interaction between lava and aeolian sand. The lava–sediment interaction happens on a variety of scales from simple sediment interbeds within the lavas at a large scale down to complex breccia horizons and bulbous lava–sediment contacts at small scales. ‘Peperite’ like textures are found where hot lava has dynamically interacted with unconsolidated aeolian sands. Such interaction could involve lava cascading down the front of dune faces, or active ‘aa’ style flow fronts ‘bulldozing’ into and oversteepening dune faces causing rapid influx of sand into the active flow front. Resulting breccia horizons have been found up to 5 m thick. Locally, the aeolian sand is baked to a quartzite by the hot lava. In contrast, on other surfaces, the lava leaves striations in the sand as it flows across it indicating local flow directions and less pronounced interaction. In places, wind ripples and topset beds are preserved on the stoss side of the dune indicating the passive nature of emplacement for some of the lava flows. Peperites are often thought to form due to the presence of water within the unconsolidated sediments that the juvenile magma comes into contact with. However, examples from arid environments demonstrate that peperite-forming processes are diverse and may not always require water. Therefore, it is suggested that the term peperite should be used to describe deposits consisting of sediments mixed with juvenile magmatic components where it is clear that there has been a dynamic interaction between the two. This leaves the presence, or lack, of water as an issue for interpreting the processes by which the peperite formed.

Death of a sand sea: an active aeolian erg systematically buried by the Etendeka flood basalts of NW Namibia

Here we report on a ‘fossilized’ sand sea that was progressively engulfed by the basal Etendeka flood basalts in NW Namibia. Preserved relict aeolian landforms include transverse barchanoid dunes and isolated barchan dunes. Present‐day preferential erosion of the lava flows exhumes relict aeolian bedforms preserved in the position in which they were migrating at the time of burial (c. 133 Ma). A passive eruption style of inflated pahoehoe flows has preserved the bedforms without significant deformation. The sediment interlayers record a decrease in sand supply and a change in palaeowind direction, which may have been driven by the ongoing break‐up of west Gondwana, or may be a direct result of the widespread volcanism itself.

Landscape distribution of Oldowan stone artifact assemblages across the fault compartments of the eastern Olduvai Lake Basin during early lowermost Bed II times.

The density and composition of Oldowan stone artifact assemblages deposited during the first ca. 20,000 years of lowermost Bed II times show a recurrent pattern of variation across recognized synsedimentary faults that compartmentalized landscapes of the eastern Olduvai Lake Basin. When active, the faults created minor topographic relief. The upthrown fault footwalls accumulated assemblages with relatively high densities of artifacts, including types retaining potential usefulness, particularly volcanic flaked pieces, manuports, pounded pieces, and split cobbles. Values for these assemblage characteristics decline toward the lower-lying hangingwall of the fault compartments, accompanied by an increase in the proportionate weight of artifact assemblages comprising quartzite, particularly flaking shatter and potentially useful detached pieces. Values reverse once again at faults, either on the downthrown, hangingwall side or on the upthrown side. The patterns are stronger for the volcanic components of the artifact assemblages than for the quartzite components, reflecting the additional influence of distance from the local source on quartzite assemblage characteristics reported previously. The landscape distributions of artifact assemblages are consistent with a landscape-fault model in which minor fault-induced topographic relief at times created a mosaic of vegetation environments repeated within each of the three fault compartments of the lake margin and distal alluvial fan. The fault-compartmentalized landscape model is currently supported only by sediment thickness and facies changes across synsedimentary faults, but it provides predictions for spatial variation in the cover abundance of trees, freshwater reservoirs and associated distributions of resources and hazards associated with stone artifact use and discard that can be tested if sample sizes of key paleoenvironmental indicators are increased.

Contrasting styles of ephemeral river systems and their interaction with dunes of the Skeleton Coast erg (Namibia)

Abstract The Skeleton Coast erg forms a prominent NNW trending dune belt, 6– 22 km wide, comprising dunes up to 50 m high, sub-parallel to the South Atlantic margin of Namibia. To a variable degree along its strike, the dune belt dams west-southwestward flowing ephemeral river systems on their route towards the Atlantic Ocean. The southern rivers, Koigab, Uniab and Hunkab, affecting the erg, are characterized by short, infrequent ephemeral flows from restricted catchments, whereas northern rivers, Hoanib and Hoarusib, are fed by large catchment areas favouring higher discharge and more frequent flows as the intertropical convergence zone and greater likelihood of monsoonal rainfall influence is approached. The potential for river damming increases in the same downwind direction through the erg because of increasing dune belt width and height and the change from low barchanoid dune forms in the south to large composite transverse dunes in the north. Whereas cross-sections of the southern part of the erg show considerable asymmetry with low barchans and sinuous crested dune forms eastwards but a high dune wall at the western edge, cross-sections over the northern erg are more uniformly symmetrical. As a consequence, the northern rivers are effectively dammed at the eastern erg margin, causing flood reservoir basins there. In contrast, southern rivers are dammed within the erg as the most pronounced and commonly encountered barrier is provided by the dunewall at its western border.

Lithostratigraphy and depositional environments in the Waterberg-Erongo area, central Namibia, and correlation with the main Karoo Basin, South Africa

Abstract The dissected landscape of the Waterberg-Erongo area, central Namibia, exposes Karoo-equivalent strata deposited in basins that occur throughout sub-Saharan Africa. Although many are of economic interest, including coal-bearing strata, their depositional history is not well understood. This study of the Waterberg-Erongo area provides detailed lithostratigraphical data, which suggest sedimentation from the late Early Triassic to the Early Jurassic in a fault-bounded depository. Subsidence and sediment supply were controlled predominantly by the northeast-southwest trending Waterberg-Omaruru Fault Zone, which defines the northwestern margin of the depository. Facies development and thickness distribution of the Karoo strata in the Waterberg-Erongo area, perhaps the most continuous of any of the Karoo basins, indicate a northeastwardly-migrating depocentre alongside that fault, in response to major extensional movements in the early pre-South Atlantic rift zone. Periodic fault movements repeatedly caused basinward progradation of the alluvial facies, which are reflected by stacked fining-upward cycles in the lithological record. On a broader scale, the results of this study suggest that the northward propagation of the rift zone between Southern Africa and South America, was partially accommodated by transfer lineaments. Local depocentres developed along these lineaments, such as those in the Waterberg-Erongo area, with localised enhanced subsidence greater than that revealed in other Namibian onshore exposures, dominated by the rifting itself.

Facies architecture variations and seismogenic structures in the Carboniferous-Permian Saar-Nahe Basin (SW Germany) : evidence for extension-related transfer fault activity

Abstract Sedimentary and volcanic strata in the late-orogenic intermontane Saar–Nahe Basin preserve a record of sedimentary responses to motion on contemporaneously active faults. Studies of such deposits can constrain both concepts for lateral and vertical sedimentary facies development and geometric and kinematic models of intrabasinal extension. During Carboniferous–Permian syn-rift development, the Saar–Nahe Basin underwent a marked intrabasinal segmentation generated by the offsets of longitudinal normal faults and dextral (oblique) strike-slip faults which were developed perpendicular to the SW–NE basin elongation. The latter are interpreted as transtensional transfer faults which compartmentalized the basin into segments of differing structural style, bed thickness and facies development. The fault zones themselves bound an assemblage of variously sized uplifted and downthrown blocks showing variable magnitudes and senses of displacement. Their activities are recorded by the contrasting depositional histories of tephrostratigraphically constrained sedimentary units occupying footwall and hanging wall block positions during basin filling. The influence of faults on the depositional system is seen in their effect as topographic barriers, thus restricting free dispersal and mixing of sediment. Particular sedimentary facies, such as coals and biogenic limestones, are confined to the immediate vicinity of the fault zones, indicating the isolation of sediment-starved areas near downthrown hanging walls. Other fault-generated depressions acted as depositional traps favouring considerable accumulations of basaltic lavas, volcaniclastic mass flows and fluvial channel deposits. In contrast, uplifted footwalls are characterized by the formation of unconformities and pedogenic features. The abundance of syn-tectonic deformation structures such as sediment-filled fissures, intraformational breccias, series of micro-faults, clastic dykes and other water-escape features within syn-rift strata and their distinct spatial linkage to fault zones implies considerable seismically triggered processes associated with faulting. The combined occurrence of abrupt variations in facies architecture across faults and seismotectonic structures suggests that the basin subsidence history involved a series of events rather than gradual change.

Onshore equivalents of the main Kudu gas reservoir in Namibia

Abstract Radiometric dates of Jurassic and Cretaceous flood basalts of the southern South Atlantic maritimes have thrown the stratigraphic contexts of Karoo and post-Karoo sequences into considerable question. Central to the problem is the status of volcano-sedimentary units, characterized by aeolian-related sedimentary interlayers, containing evaporites and other lacustrine elements. The main Kudu gas reservoir (‘Lower Gas Sand’) offshore Namibia, interleaved with undated flood basalts, is a prime example. A previously proposed onshore correlate is the fluvio-aeolian Etjo Sandstone, with a central Namibian type area, and comparisons have been made, particularly with units traditionally classified as Etjo Sandstone, near the Etendeka Plateau base in northwest Namibia, now recognized to be of younger age. We time-correlate the main Kudu Sandstone reservoir, however, with evaporitic fluvio-lacustrine and aeolian interlayers beneath and interleaved with Early Jurassic Kalkrand flood basalts of southern Namibia, situated onshore almost directly opposite the Kudu Field. The main Kudu Sandstone reservoir is transitional in palaeoenvironmental setting between Early Jurassic onshore analogues. Main Kudu reservoir geometries and associated lithologies can be better constrained with knowledge derived from Kalkrand sedimentary interlayers and their tectonic and volcanic controls. Kalkrand interlayer thicknesses are controlled primarily by down-to-the-east extensional fault system half-graben geometries, antithetic to an inferred down-to-the-west master detachment. Within resulting sub-basinal accommodation space, lava topography controlled thickness variation. When Kalkrand insights are taken into account with radiometric dating and biostratigraphy, a Namibian Mesozoic framework fixes Kudu reservoirs and analogues into a southern Gondwanan context. At least in its type area in central Namibia, the Etjo Formation is Early Jurassic, discounting a previously proposed widespread <100 Ma hiatus along a sub-Etjo disconformity. Instead, sequence boundary hiatuses of Permian-Triassic, Early Jurassic and Middle Jurassic-Early Cretaceous ages represent time-stratigraphic gaps, relating to major faulting episodes, recording extensional events associated with continental margin evolution. Understanding onshore analogues and refining the Namibian Mesozoic framework should help constrain seismic analysis of the Namibian margin and hydrocarbon reservoir geometries. Consequently, exploration focus should widen to include Karoo sequences beneath the basal Early Cretaceous transgression.