Tanqua Karoo Research Articles

Contribution of research borehole data to modelling fine-grained turbidite reservoir analogues, Permian Tanqua–Karoo basin-floor fans (South Africa)

Outcrop analogue studies can be augmented and constrained by drilling research wells through the same stratigraphic interval. Close-to-outcrop wells help to validate outcrop observations with well log and core data and thus improve the use of such data in actual field developments. Research wells located further away from the outcrops increase the spatial data coverage and can give important insight into regional facies distributions and net:gross changes. In the Tanqua–Karoo Basin (South Africa), seven wells were drilled into fine-grained sand-rich basin-floor fans and interfan mudstones to supplement outcrop data. Three close-to-outcrop wells proved useful in establishing characteristic log responses of the main architectural elements identified from the nearby outcrops. Lithofacies were correctly identified in more than 80% of cases using an artificial neural network. Borehole images provided detailed information on sedimentary structures, including a wealth of palaeocurrent data from climbing ripples that significantly enhanced the interpretations based only on outcrops. Wells sited away from the outcrops supplied information on lateral thickness and facies trends and intrafan stacking patterns, which helped to define the stratigraphic evolution of the fans. The combined data indicate that deposition was controlled in part by subtle basin-floor topography, and that intrafan lobe switching took place, leading to internal subdivisions that potentially caused effective compartmentalization of the basin-floor fan.

Vegetation of quartz fields in the Little Karoo, Tanqua Karoo and eastern Overberg (Western Cape Province, South Africa)

Vegetation of quartz fields in the Little Karoo, Tanqua Karoo and eastern Overberg (Western Cape Province, South Africa)

Provenance evolution and chemostratigraphy of a Palaeozoic submarine fan-complex: Tanqua Karoo Basin, South Africa

Geochemical studies of mudstones from boreholes in the deep water Permian Skoorsteenberg Formation in the Karoo Basin of South Africa were undertaken to assess stratigraphic and provenance evolution and the feasibility of chemostratigraphic correlation in this sedimentary setting. Data for 38 major and trace elements including rare earth elements were obtained from four boreholes from 304 samples covering proximal to distal parts of the turbidite complex. There are geochemical signals displaying systematic stratigraphic trends across the basin that imply a continuous evolution of one pre-eminent source terrain that supplied the sediment. Changes in ratios of TiO 2/Al 2O 3, La/(La+Lu) and Th/(Th+Y) suggest an increasingly mafic contribution to the depositional system with time. Perversely, however, increasing Zr and Hf abundance up the stratigraphic succession suggest that there was also an increase in the felsic content with time. These apparently conflicting interpretations may be explained by a single source terrain that became increasingly igneous (e.g. less sedimentary rocks being weathered and supplying new sediment) with time although the progressively denuded rocks must have been predominantly mafic. The chemical index of alteration decreased with time possibly suggesting that a less intense weathering regime in the hinterland developed with time. The geochemical data set from the Skoorsteenberg Formation in the Karoo Basin have demonstrated that chemostratigraphy is not an ideal approach to correlation since the geochemical signals have substantial statistical noise that is not easily related to lithology.

Erratum to Three-dimensional geological models from outcrop data using digital data collection techniques: an example from the Tanqua Karoo depocentre, South Africa

Erratum to Three-dimensional geological models from outcrop data using digital data collection techniques: an example from the Tanqua Karoo depocentre, South Africa

Three-dimensional geological models from outcrop data using digital data collection techniques: an example from the Tanqua Karoo depocentre, South Africa

Abstract Recent technological advances have made the collection of digital geological data from outcrops a realistic and efficient proposition. The world-class exposures of Permian basin-floor turbidite fans of the Tanqua depocentre, Karoo Basin, South Africa have been the focus of one such study. These outcrops are faulted at a subseismic scale (displacements of up to 40 m), with continuous exposures of up to 40 km in depositional dip and 20 km strike directions. Digital data collection has been undertaken using a variety of methods: differential global-positioning systems (DGPS) mapping, surveying using laser total station and laser rangefinders, ground- and helicopter-based digital photography and photogrammetry, and digital sedimentary outcrop logging as well as geophysical data from boreholes. These data have then been integrated into several 3-D geological models of the study area, built using a subsurface reservoir-modelling system. The integrated dataset provides insights into the stratigraphic evolution of a deep-water fan complex by allowing true 3-D analysis and interpretation of data collected in the field. The improved understanding of these deep-water fan systems will improve existing models of offshore analogues by enhancing understanding of geometries and trends not resolvable from existing offshore data and by identifying potential problematic areas for fluid flow. Initial results from the application of this approach have been successfully applied to the conditioning of stochastic geological models of a subsurface deep-water reservoir from the North Sea.

Deep-sea fan pinch-out geometries and their relationship to fan architecture, Tanqua Karoo basin (South Africa)

Exceptional exposures of Permian basin floor fans (fans 3, 4) and a slope fan (fan 5) in the Tanqua Karoo foreland basin of South Africa have enabled an investigation of the relation between the pinch-out geometries and fan architecture. The pinch-out geometry of fan 3 is characterized by the down dip transition from thin to medium bedded sheet deposits to pinch-out fingers, which are overlain by younger prograding sheet deposits. This geometry reflects the progradational stacking pattern of the fan. In contrast, the fan 4 pinch-out fingers consist of stacked channel fills in the same conduit. This pinch-out configuration relates to the dominant aggradational style observed on the mid and distal parts of fan 4. Fan 5 represents a slope fan comprising an axial channel conduit, which branches down slope into three distributary channels. The distal fan is characterized by larger channel fills, which may represent bypass channels to other basin floor fans. The very thick-bedded nature of the youngest channel fill unit suggests early bypass followed by retrogradation as indicated by the presence of thinner bedded heterolithic channel fill deposits along the axial conduit. Although some of the massive pinch-out channels exhibit basal scour, their depositional morphology suggests that they mainly originated due to the infill of subtle topographic depressions by low concentration turbidity currents. Instead of describing these features as channel fills, the use of the term pinch-out fingers is preferred.

High resolution stratigraphic analysis of a turbidite system, Tanqua Karoo Basin, South Africa

The Tanqua submarine fan complex is located in the southwestern Karoo Basin of South Africa, and comprises four basin floor fans (Fans 1–4) and one slope fan (Fan 5). Fan 3 exhibits an extensively exposed turbidite system, which enables a detailed investigation of its architecture and growth history. A fan wide stratigraphic correlation has enabled the construction of a deterministic 3D fan model, illustrated by correlation panels and isopach maps. The stratigraphy of Fan 3 comprises 10 depositional units, interpreted as six sandy fan growth phases separated by four intervals of reduced deposition. The clinoformal geometry of the units defines Fan 3 as a prograding fan characterized by three depositional phases. The initial phase of turbidite sedimentation is represented by basal scours and deposition of thin-bedded channel-fill and sheet deposits. The second phase is characterized by widespread channeling, moderate overbank deposition and significant mid- and outer-fan construction. The final phase is marked by bypass of the proximal axial channel system, overbank aggradation and reduced progradation on the mid- and distal-fan.

ABSTRACT: Methods and Tools for Digital Data Collection and Construction of 3-D Geological Computer Models in the Field; Examples from the Tanqua Karoo Turbidites, South Africa

ABSTRACT: Methods and Tools for Digital Data Collection and Construction of 3-D Geological Computer Models in the Field; Examples from the Tanqua Karoo Turbidites, South Africa

Anatomy, geometry and sequence stratigraphy of basin floor to slope turbidite systems, Tanqua Karoo, South Africa

The Tanqua area of the Karoo basin, South Africa, contains five Permian deep‐water turbidite fan systems, almost completely exposed over some 640 km2. Reconstruction of the basin‐fill and fan distributions indicates a progradational trend in the 450 m+ thick succession, from distal basin floor (fan 1) through basin‐floor subenvironments (fans 2, 3 and 4) to a slope setting (fan 5). Fans are up to 65 m thick with gradational to sharp bases and tops. Facies associations include basin plain claystone and distal turbidite siltstone/claystone and a range of fine‐grained sandstone associations, including low‐ and high‐density turbidite current deposits and proportionally minor debris/slurry flows. Architectural elements include sheets of amalgamated and layered styles and channels of five types. Each fan is interpreted as a low‐frequency lowstand systems tract with the shaly interfan intervals representing transgressive and highstand systems tracts. All fans show complex internal facies distributions but exhibit a high‐frequency internal stratigraphy based on fan‐wide zones of relative sediment starvation. These zones are interpreted as transgressive and highstand systems tracts of higher order sequences. Sandy packages between these fine‐grained intervals are interpreted as high‐frequency lowstand systems tracts and exhibit dominantly progradational stacking patterns, resulting in subtle downdip clinoform geometries. Bases of fans and intrafan packages are interpreted as low‐ and high‐frequency sequence boundaries respectively. Facies juxtapositions across these sequence boundaries are variable and may be gradational, sharp or erosive. In all cases, criteria for a basinward shift of facies are met, but there is no standard ‘motif’ for sequence boundaries in this system. High‐frequency sequences represent the dominant mechanism of active fan growth in the Tanqua deep‐water system.

ABSTRACT: Reservoir Architecture of Turbidite Elements, Permian Tanqua Karoo, South Africa

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Abstract: High-resolution 3-D architecture and pinchout geometries of fasin floor fans, Tanqua Karoo, South Africa

Abstract: High-resolution 3-D architecture and pinchout geometries of fasin floor fans, Tanqua Karoo, South Africa

Abstract: Reservoir Characterization of Fine-Grained Outer Sheet Sands, Tanqua Karoo, South Africa

Abstract: Reservoir Characterization of Fine-Grained Outer Sheet Sands, Tanqua Karoo, South Africa

Abstract: Architectural and Reservoir Characteristics of Fine-Grained Depositional Lobes, Tanqua Karoo, South Africa

Abstract: Architectural and Reservoir Characteristics of Fine-Grained Depositional Lobes, Tanqua Karoo, South Africa

Reserve selection in the Succulent Karoo, South Africa: coping with high compositional turnover

The Succulent Karoo biome is home to the world's richest succulent flora. It has approximately 1954 endemic plant species, and is the only semi-arid region to qualify as a hotspot of global significance. Despite its importance, only 2% of the biome is currently protected. Based on its flora, the biome can be divided into 12 bioregions, reflecting its high compositional turnover in relation to environmental and geographical gradients. Only three of these bioregions (the Gariep Centre, the Namaqualand Rocky Hills and the Tanqua Karoo) contain National Parks, and three contain large (over 10 000 ha) provincial reserves (the Gariep Centre, the Namaqualand Rocky Hills and the Little Karoo). The current reserve system does little to conserve biodiversity, with only one reserve significantly conserving Red Data Book (RDB) plant diversity. Using a RDB plant species database of 3874 records at a quarter degree scale (QDS = 15′×15′), we used hotspot analyses and iterative reserve selection algorithms to identify possible locations for future reserves. The hotspot analysis and iterative analyses yielded similar results for the top 11 QDS, mainly due to very high local endemism. Also because of the local endemism and the high species turnover within the biome, the real-world iterative algorithm (starting with the seven already reserved QDS) selected a very large total number of QDS (59% of the total in the biome) to conserve all RDB species. As a possible alternative to conservation planning based on QDS, we also assessed priorities at the scale of bioregions, but showed that representation at this geographic level misses important areas defined at a finer scale. We suggest that if the objective is to maximise the retention of RDB species in the landscape (to pre-empt extinction by scheduling the allocation of limited conservation resources), at least the top 5% of QDS (n=11) selected by the iterative procedure, and identified as the core conservation sequence by analysis of endemicity and threat, should be given priority for reservation. Less extensive and, in some cases, less formal conservation action can be applied to QDS later in the sequence, based on species-specific monitoring and action plans. Of the 11 core areas, four fall in a node centred on the Vanrhynsdorp Centre, two fall in a node centred on the Kamiesberg, and the remaining five are isolated. With existing reserves, the core areas capture 50% of the RDB flora in 8% of the biome.

The physical environment and major plant communities of the Tankwa- Karoo National Park

Apart from Acocks (1988) there are no published descriptions of the vegetation of the greater Tanqua and Doring River drainage basin (Bayer et al. 1993). A botanical and physical description of the Tankwa- Karoo National Park (TKRNP) which occurs in Veldtype 31b (Acocks 1988) is provided. The three dominant geological formations, older glacigenic deposits of the Dwyka Group, followed by the succession of siliciclas-
 tic sediments of the Permian Ecca Group, with flat dolerite sills and dykes, underlie eight distinct plant communities. The plant communities can be divided into large open plains dominated by Galenia africana and Tripteris sp. in the erosion rills, Malephora luteola and Augea capensis common in the low lying areas and Zygophyllum microcarpum, Brownanthus ciliatus and Galenia crystallina common on the more shaly concave plains and low shale hills. Slightly elevated rocky areas are dominated by Ruschia cf. robusta, Ruschia spinosa communities, while crusts of stemless mesembs such as Rhinephyllum macra denium, Hereroa fimbriata and Cheiridopsis acuminata are found on the desert paved areas. Annual Asteraceae covers all the denuded and sparsely vegetated areas after good winter rains while annual mesembs colonise on the more sodic sites. A total of 259 plant species were collected sporadically over a period of eight years, this includes 65 succulents and seven species endemic to the Tanqua Karoo and immediate adjacent area of the Roggeveld Mountain Range and Sutherland. Four Tanqua Karoo endemic species were found in the park.

Abstract: Characterization of Channel Depositional Settings in a Fine-Grained Submarine Fan, Tanqua Karoo, South Africa

Abstract: Characterization of Channel Depositional Settings in a Fine-Grained Submarine Fan, Tanqua Karoo, South Africa

Two new species of <i>Zygophyllum</i> (Zygophyllaceae) from the Western Cape, South Africa

Two new species of Zygophyllum L. from the Western Cape are described, namely, Z. fuscatum Van Zyl from the coastline between Betty’s Bay and De Hoop Nature Reserve and Z botulifolium Van Zyl from the Tanqua Karoo.

Abstract :Architectural Changes within Channel Depositional Settings of a Fine-Grained Turbidite, Tanqua Karoo, South Africa

Abstract :Architectural Changes within Channel Depositional Settings of a Fine-Grained Turbidite, Tanqua Karoo, South Africa

Architectural Characteristics of Fine-Grained Submarine Fans: A Model Applicable to the Gulf of Mexico: ABSTRACT

ABSTRACT Submarine fan deposits in the Gulf of Mexico and in many areas of the Atlantic Ocean fall into the category of fine-grained, low overall sand/shale ratio basin-floor fans. Models published over the years that have been applied to either exploration or development are primarily based on sand-rich fans, most of which were deposited in active margin settings and are not applicable to the areas mentioned above. Observations from the excellent outcrops of the Permian Tanqua Karoo in the southwestern part of the Karoo Basin, South Africa, the Jackfork Formation (Pennsylvanian) in Arkansas, and the Deep Sea Drilling Project Leg 96 on the Mississippi Fan make it possible to construct a macro-scale model that addresses the architecture of both the macro- and meso-scale depositional features of fine-grained submarine fans deposited in generally unconfined settings. At the entrance to the basin floor in the base-of-slope area, the large conduit that was initially carved out across the basin slope, may start to widen. Most of the high- and low-density flows moving from the submarine canyon in the upslope area through the conduit (fan valley or upper fan channel), will be too small to occupy the entire width of the lower part of the conduit. The result is a complex build-up of channel fills incised into each other, forming amalgamated sandstone bodies interbedded with remnants of shale-rich levee-overbank deposits. Once in the basin proper the degree of erosion of flows will be reduced and channel-levee complexes result. The channel fills are primarily amalgamated sandstones whereas the levee-overbank deposits are often thin-bedded, low-contrast, low-resistivity sandstones with high porosity and permeability. The interbedded shales reduce vertical conductivity of fluids but proper production techniques make these deposits potentially very productive. The channels gradually become smaller and loose their influence to govern the direction of the head of density flows. A gradual widening of the deposits results, known as sheet sands or depositional lobes. They have high sand/shale ratios and the overall sheet sand deposition suggests lateral switching resulting from seafloor compensation rather than frequent channel progradation.

Permian Passive Margin Submarine Fan Complex, Karoo Basin, South Africa: Possible Model to Gulf of Mexico

ABSTRACT The tectonic development of the southwestern part of the intracratonic Karoo Basin was related to a subduction zone and an orogenic fold belt. However, the entire Lower Permian Ecca Group basin-fill succession exhibits the depositional characteristics of a passive-margin setting. The subbasins that developed during subsidence accumulated large submarine fan complexes that were succeeded by a river-dominated deltaic system. The subbasin fill in the west-southwestern area (Tanqua Karoo) is approximately 250 m thick and is comprised of six arenaceous submarine fan systems separated by basinal shales. The uppermost fan system is overlain by prodelta shales and siltstones, followed by delta front and delta plain sediments. The fan complex displays middle and outer fan deposits over an area of about 650 km2. The individual fans vary in maximum thickness from 24 to over 60 m. The cyclicity of the fan deposition is believed to have been controlled by fluctuations in relative sea-level with an average periodicity of about 100,000 years. Paleocurrent directions vary between the six units, reflecting changes in deltaic point source locations between SSE and WSW. The sixth fan system is located to the south, downlaps onto the last of the five sequences, and may represent a slope fan with major slumps and channels. The strata are not affected by folding or vegetation, permitting three-dimensional viewing of channel fills and levee overbank relationships, lateral migration of channels, and the transition from channelized to non-channelized lobe and sheet deposits. The sediments of the different subenvironments seem to have their own typical sedimentary structures which makes it possible to start defining the placing of thin-bedded turbidites and their potential relationships to channel-fill deposits. The gentle tectonic development of the southwestern Karoo Basin, as is reflected in the distribution of the lower shale formations, low sand/shale ratio, and overall fine grain size throughout the entire submarine fan/deltaic mega-sequence reflects a passive margin setting. These well-exposed outcrops contain many sedimentary characteristics that can be related to those of the Mississippi Delta and Fan. Therefore, this area can provide important elements that will be useful for the development of models of deep-water sands in the northern margin of the Gulf of Mexico.