Karoo Strata Research Articles

Structural geometry and evolution of the Rukwa Rift Basin, Tanzania: Implications for helium potential

AbstractThe Rukwa Rift Basin, Tanzania is regarded as a modern example of a cratonic rift zone despite complex polyphase extensional and episodic inversion structures. We interpret 2D seismic reflection data tied to wells to identify and describe structures controlling stratigraphic sequences (Late Carboniferous to Pleistocene) in two main segmented Rukwa Rift domains, A and B, which are controlled by the Chisi and Saza shear zones. Fault geometry and stratal patterns are illustrated in relation to their kinematic interaction with folds. Fold structures reflect both extensional and compressional deformation and were mapped with a particular interest for their helium potential. We illustrate fault bend folds, fault propagation folds and fault propagation monoclines that are related to extension events. Folds related to compression exhibit various structural styles reflecting at least two phases of episodic and widespread inversion. First, Early Jurassic inversion phase which involved multi‐faulted anticlines in the Karoo strata. Second, a mild and widespread inversion structures during the Pleistocene which are characterised by both symmetrical and asymmetrical anticlines styles. Taken together, the extensional and compressional fold structures, stratal juxtapositions and unconformities define stratigraphic packages that are widely distributed in the Rukwa Rift Basin, and form potential subsurface traps for helium‐nitrogen–rich gases, from which some seep to the surface, evidently documented in thermal springs across the region.

Depositional style changes during the Permo‐Carboniferous–Early Jurassic evolution of the Central Kalahari Karoo Sub‐basin, Botswana

The Permo‐Carboniferous–Lower Jurassic Karoo succession in the Central Kalahari Karoo Sub‐basin of Botswana is largely confined to the subsurface, and contains coal, coalbed methane and groundwater resources, similar to other Karoo strata throughout southern Africa. Characterization of the stratigraphic architecture (i.e., spatiotemporal stacking character of sedimentary facies) is essential for various aspects of subsurface resource prediction (e.g., resource parameters; reservoir architecture; mining feasibility). This study uses sedimentary facies analysis on borehole cores from central Botswana and sequence stratigraphic principles to resolve large‐scale depositional style changes during the evolution of the greater Kalahari Karoo Basin in southwestern Gondwana. The results show that changes in the Karoo stratigraphic architecture define two, large‐scale overall fining‐upward depositional sequences (i.e., Lower and Upper Karoo) that are marked by regional subaerial unconformities. These regionally mappable depositional sequences contain pairs of high‐ to low‐amalgamation units, which are typified by stacked, multistorey sandstone bodies, succeeded by finer grained units that are sometimes rich in coal (e.g., the Dukwi‐Makoro‐Kamotaka vs. the Morupule‐Serowe‐Tlhabala formations in the Lower Karoo; the lower versus upper Mosolotsane‐Ntane formations in the Upper Karoo). The large‐scale depositional style changes, reflecting base‐level fluctuations and variation in the balance between accommodation and sediment supply, resulted from climatic changes (e.g., Permo‐Carboniferous deglaciation), tectonic events (e.g., regional pre‐Mosolotsane uplift event), and marine incursions. The latter is documented, for the first time, via the marine‐influenced ichnofauna in the uppermost Ecca Group. Further refining this initial sequence stratigraphic framework for the Botswanan Karoo will require acquisition of radioisotopic dates (e.g., U–Pb zircon ages) and modern seismic reflection data.

The 3D geometry of regional-scale dolerite saucer complexes and their feeders in the Secunda Complex, Karoo Basin

Dolerites in the Karoo Basin of South Africa commonly represent kilometre-scale, interconnected saucer-shaped structures that consist of inner sills, bounded by inclined sheets connected to stratigraphically higher outer sills. Based on information from over 3000 boreholes and mining operations extending over an area of ca. 500km2 and covering a >3km vertical section from Karoo strata into underlying basement rocks, this paper presents the results of a 3D modelling exercise that describes the geometry and spatial relationships of a regional-scale saucer complex, locally referred to as the number 8 sill, from the Secunda (coal mine) Complex in the northern parts of the Karoo Basin.The composite number 8 sill complex consists of three main dolerite saucers (dolerites A to C). These dolerite saucers are hosted by the Karoo Supergroup and the connectivity and geometry of the saucers support a lateral, sill-feeding-sill relationship between dolerite saucers A, B and C. The saucers are underlain and fed by a shallowly-dipping sheet (dolerite D) in the basement rocks below the Karoo sequence. The 3D geometric strata model agrees well with experimental results of saucer formation from underlying feeders in sedimentary basins, but demonstrates a more intricate relationship where a single feeder can give rise to several split level saucers in one regionally extensive saucer complex.More localised dome- or ridge-shape protrusions are common in the flat lying sill parts of the regional-scale saucers. We suggest a mode of emplacement for these kilometre-scale dome- and ridge structures having formed as a result of lobate magma flow processes. Magma lobes, propagating in different directions ahead of the main magma sheet, undergo successive episodes of lobe arrest and inflation. The inflation of lobes initiates failure of the overlying strata and the formation of curved faults. Magma exploiting these faults transgresses the stratigraphy and coalesces to form a ring-like inclined sheet that subsequently feeds a central flat lying roof at a higher stratigraphic level.On a regional scale, the kilometre-size saucer geometries reflect the lateral migration and transport of mafic magmas close to or at the level of the Karoo Supergroup, fed by only isolated feeders in the basement. On a more local scale, the complex internal geometries within saucers mainly reflect the flow pattern of the magmas and wall-rock accommodation structures.

Modelling of distribution and geometry of lithological complexes of the Ecca Group (the Karoo Supergroup) in SW Botswana

The Ecca Group, a subdivision of the Karoo Supergroup (U. Carboniferous-L. Jurassic) in SW Botswana is a sequence deposited as marine deltaic bodies considered to have been supplied from a cratonic source elevated north of the basin. The Karoo strata in this region are covered unconformably by sands of the Kalahari Beds (Upper Cretaceous – Recent). Therefore the bedrock outcrops are extremely rare, limited in size and a low number of boreholes drilled in this vas area (ca. 340 x 540 km) provide the only insights into the succession of the Karoo Supergroup. Very long distances between individual boreholes make correlation, interpolation between localities and interpretations of geometry of lithological bodies that would provide clues supporting basin analysis by traditional means very problematic. Therefore, modelling of the Ecca Group lithofacies associations with the use of Petrel software was performed, which resulted in the first approximation of the space-time relations between lithological complexes of this unit. These relations in turn suggest evolutionary trends of the basin during deposition of the Ecca Group strata. The model suggests two main zones of supply indicated by two distinctly different patterns of deltaic lithofacies associations, and their evolution controlled by post-Dwyka palaeotopography and its subsequent modifications by local subsidence in the centre of the depository. Initially rapid southward progradation of relatively fine-grained delta body located in the west of the area was followed by subsidence-induced aggradation interrupted by stages of abandonment and marine transgression. Such variations, emphasised by the presence of sandy clinoforms of the delta lobes separated by basinal ‘fines’, imply significant interplay between rates of supply and subsidence. On the other hand, the delta formed in the east contains relatively high proportion of coarse-grained sandstone facies overlying prodelta fines as laterally extensive tabular body formed most probably by lateral migration of distributary channels and delta-front mouthbars, and devoid of abandonment stages. Proximal litofacies of the “western delta” fill the subsiding depocentre and grade distally into synchronously deposited prodelta fines towards the south. By contrast, distal fine-grained prodelta facies fill basin depocentre in the eastern area and are overlain by proximal facies of the “eastern delta”.

The internal structure and geotectonic setting of the Xade and Tsetseng complexes in the western most part of the Kaapvaal Craton.

The Xade Complex is an unexposed Y-shaped body, approximately 100 km long and 25 km wide, located close to the western margin of the Kaapvaal craton in Botswana. The complex is characterized by large coincident magnetic and gravity anomalies. It is completely covered by varying thicknesses of Kalahari sediments as well as by Karoo strata, which means that detailed analysis of high resolution airborne magnetic data, ground gravity data and limited seismic data are essential in interpreting the internal configuration of the complex. An earlier interpretation of the first airborne magnetic survey of Botswana (Reeves, 1978) coupled with subsequent drilling discovered the Xade Complex and showed that it is made up of mafic and ultramafic rocks. However, the limited amount of drilling did not provide sufficient information to either interpret in detail its internal geology or its regional geotectonic setting (Meixner and Peart, 1984). New 2D and 3D gravity and magnetic field modelling have constrained the geometry of the complex as a syncline defined by folded mafic lavas and high-level sub-volcanic mafic sheets. The Xade Complex lies within a graben that forms the north-south arm of a triple junction with the faulted western margin of the Kaapvaal Craton. The focal point of the triple junction coincides with an inflection of the cratonic margin and is the likely site of the feeder zone to the mafic lavas of the Xade Complex. The Tsetseng Complex is shown to be an internally layered, magnetite-bearing gabbro.