Géométrie tridimensionnelle de la partie nord-ouest du complexe de Bushveld, Afrique du Sud
O.A. BAMISAIYE, P.G. ERIKSSON, J.L. VAN ROOY, H.M.BRYNARD, S. FOYA, A. BILLAY, V. NXUMALO, A.M. ADEOLA.
Abstract: The surbsurface geometry of the Rustenburg Layered Suite (RLS) in the northwestern part of the Bushveld Complex was investigated by analysis two hundred borehole log data. The objective of this study is to produce a regional-scale 3D model. A geostatistical method was employed in analysing the borehole logs lithostratigraphic units. Cross sections, fence diagrams, and isopach maps were constructed to provide insight into the emplacement and regional deformation of the complex. The results reveal the geometry at depth as well as the nature of the RLS to the roof and footwall rocks.
Keywords: 3D geometry; geostatistics; Northwestern Bushveld Complex; Rustenburg Layered Suite (RLS); emplacement.
Résumé: La géométrie du sous-sol de la Formation de Rustenburg (FR) dans la partie du Nord-ouest du complexe de Bushveld a été analysée à l’aide des données de deux cents puits de forage. L’objectif de cette étude est de produire un modèle en 3D d’échelle régionale. La méthode géostatistique a été exploitée lors de l’analyse des données litho-stratigraphiques obtenues des puits de forage. Des coupes transversales, des blocs diagrammes et des cartes isopaques ont alors été construits dans le but de préciser l’emplacement et la déformation régionale du complexe de Bushveld. Les résultats révèlent aussi bien la géométrie souterraine que la nature de la Formation de Rustenburg entre sa base et son sommet.
Mots clés: Géométrie en 3D, géostatistique, Nord-ouest du complexe de Bushveld, Formation de Rustenburg (F.R), emplacement.
Plan
Introduction
Regional geology of the area
Data and method
Results
Result of geo-spatial analysis of borehole data around northwestern bushveld
Amandelbult section
Union section
Gap areas between Amandelbult and Union section
Discussion and conclusions
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INTRODUCTION
The Western Bushveld with its conspicuous layering spans from the northeastern edge of the Crocodile river fault south of Thabazimbi through the Pilanesberg complex to east of Brits. The area of focus is the northern parts of the Pilanesberg complex to the northeastern end of the Crocodile River fault representing an area of about 70 km. The area is divided into Amandelbult section and Union section (Figure 1). Previous structural information about the study area were based on geophysical investigations and inferences drawn from scattered field observations (Chunnet and Rompel, 2004; Campbell, 2011). Some of the structural features that had been identified in the northwestern part of the Bushveld Complex area include the Roodedem graben (Greyvensteyn, 2001), the gap areas (McLaren and Villiers, 2004; Scoon and Teigler, 1994; Eales and Cawthorn 1996; Kruger, 2005) and potholes (Irvine et al 1983; Viljoen et al, 1986; Carr and Groves, 1994; Carr et al. 1999; Viljoen, 1999; Lomberg et al.,1999). However, detail about the nature and actual geometry of these features are not fully understood due to paucity of information about the subsurface (Turner,1989; Davis, 2002). This paper considers a regional aspect of the structural geology of the northwestern part of the BC with the aim of shedding more light on the subsurface configuration of the area.
3D models as well as borehole strip logs and hole to hole section of the logs were utilized to constrain the subsurface geometry and geological relationships of complex 3D structure around the Northwestern Bushveld with more focus on the Middellaagte/Roodedam Graben (at Amandelbuilt section, the gap areas and Union section. In general, this paper presents the results of the geospatial analyses of subsurface geological data obtained from compilation of borehole records, geophysical and field mapping records (obtained from the Council for Geosciences in Pretoria) for better understanding of the subsurface disposition of the Northwestern BC.
REGIONAL GEOLOGY OF THE AREA
The Bushveld Igneous rocks were emplaced just above the Transvaal sequence about 2060 Ma (Walraven et al., 1990). These igneous rocks occur as a sequence of layered rocks (which exhibit layering analogous of sedimentary rocks) and form the largest layered intrusion in the world; covers land area of about 67,375 square km and also holds the largest deposit of PGM, chromium, titanium, vanadium, iron and tin (Godel et. al.2006; 2007; Wilson et al.1998). Layers in the BC have remarkable lateral extent (Cawthorn and Molyneux1986; Maier and Eales 1997). Evidence for probable continuity at depth was given by Webb et. al. (2011), Cawthorn and Webb, (2001) and Kruger (2005) based on stratigraphy, textures, mineral composition, and geophysical studies.
The BC consists of mafic to ultra-mafic rocks which represent the RLS as well as acid rocks known as Lebowa Granites which intrude and overlay the RLS together with Rashoop Granophyre (Barnes and Maier, 2002: SACS, 1980; Von Gruenewaldt et al. 1986). The RLS outcrops prominently as western, eastern, northern, far northern, far western and lastly the southwestern part which is covered by recent deposits (Kinnaird et al., 2005). The RLS rocks at the northwestern part of the BC consist of repeated succession of Upper Zone, Main Zone, Critical Zone, Lower Zone and Marginal Zone. Repeated magma injection coupled with mineral precipitation and interaction with new magma addition is responsible for the repetition of mineral layer or cyclic unit in the RLS (Cawthorn, 1999, Cawthorn, 2006).
DATA AND METHOD
Borehole data obtained from the Council for Geoscience, were first sorted and entered into Excel sheet before importing into RockWorks15® software. Location maps were then generated to verify the coordinate of each borehole. Grouping the lithologies into stratigraphic units based on SACS, 1980 classification scheme was carefully carried out. The base of the Upper Zone in this study was placed above the Pyroxenite Marker horizon while the base of Giant Mottled Anorthosite marks the base of the Main Zone and directly overlay the Bastard Merensky layer or the Merensky Pyroxenite. The Upper Critical Zone is identified by the appearance of the Merensky Reef and chromitite layers. The Middle chromitite unit is marked by the presence of alternating norite/pyroxenite/anorthosite layers with chromitite layers and transforms to another cycle of pyroxenite and chromitite layers which define the Lower Critical Zone.
Structural contours connecting points with the same elevation at both top and base of each stratigraphic unit were subsequently generated using Regression Kriging spatial interpolation technique in order to create a continuous surface from the input data. This was followed by calculation of stratigraphic thicknesses by subtracting the lower contact elevation from the upper or top elevation (taking the orientation of each well and collar elevation into consideration) using the grid maths option in the software and modelling of the borehole data were subsequent carried out.
Verification of initial result was done by first ascertaining that there was no error in the input data, by verifying with raw data plot and plotting of strip logs and fence diagrams to view the lateral continuity as well as for detection of subtle structural features. Determination of appropriate interpolation method with varying degree of user determinant input was followed by subsequent overlaying the surface geology to establish geographic correlation and continuity and finally by checking with known geological information.
RESULTS
RESULT OF GEO-SPATIAL ANALYSIS OF BOREHOLE DATA AROUND NORTHWESTERN BUSHVELD
This area is divided into two sectors for the purpose of this discussion i.e. the extreme northeastern part or the Amandelbult area while the second area includes the union section and the northern part of Pilanesberg Complex. The structure contour interval at Amandelbult section reveals a similar pattern in each of the stratigraphical units from the Overburden unit to the top of the Main Zone (as shown in Figure 1).
Figure 1: Geologic map of Northwestern Bushveld Complex showing the main sections and farm names mentioned in text with Amandelbult section at the northeastern edge and Part of the Pilanesberg Complex in the south.
AMANDELBULT SECTION
From the 3D model, the Amandelbult area is occupied by rocks of RLS, these rocks are tabular at the surface. The entire 20 km stretch of this section is marked by continuous downward –dipping and step-like display of all the stratigraphic units as indicated in Figures 2, 3 and 4. The entire RLS layers in this area dip steeply to the south, and the complete sequence (cycle) of RLS rocks (from the Main Zone to Lower Zone) occurs at different depths less than 100 m in the north to over 2200 m southeast. This continuity in structure and thickness, pattern might suggest that the structure developed or was reactivated during the intrusion of the RLS. The Complete magma sequence at different depths and southwards thickening of the RLS rocks in this area might suggest syn-emplacement faulting or/ and reactivation of old structures. The Merensky reef exposure is shallow in the north (at a depth of 50 m) and deepens around Middellaagte in the north, to about 2400 m at Zondereinde farm in a step-like fashion. Mining operations occurs over the entire section in down dip direction, at different depths. Structural trend in the area is NW-SE except in the southwest, where the trend is NE-SW.
Figure 2: Strip log of some of the boreholes in Amandabult section of northwestern Bushveld showing the first level of the ‘stratigraphic cycle’. Note the central drop representing a graben structure.
Figure 3: Exploded 3D Model of one of the complete step-like sequence in the Amandelbult section (some of the stratigraphic units are exempted for clarity)
Figure 4: Stratigraphic fence diagram of Amandelbult section showing the step-like pattern of different ‘stratigraphic cycle’ (some levels are omitted for clarity). Note that the thickness of the Critical Zone increases down-dip.
Figure 2 and 4 show a graben shape at the central part of Amandelbult section. The structure is about 3 km wide in the northern part and the width increases downwards towards the south. The structure is confined between the bases of the Main Zone to the Lower Zone and does not extend to the surface. The western part of the structure shows a throw of about 700 m, while the eastern part is about 500 m down.
Profile drawn on the Main Zone structure contours and isopach map for part of the Amandelbult section show an inverse pattern of down- dip verging folds as indicated on Figure 5.
Figure 5 : Western n Bushveld Complex with Profile A-A1 (on Lower Zone interval structure contour map) of the northern part of the Amandelbult area showing connected down dipping fold structures that are probably related to pothole.
UNION SECTION
The Union section is located on the southwestern side of the Amandelbult section (see Figure 1) and the northern gap separates these two sections over an average distance of about 11 km. The Union Section is an extension of the of the RLS layers at the Amandelbult section with a thick succession of Main Zone gabbro-norites. The thickness of the feldspathic rocks in the UG2- Merensky reef interval in this section is about 30 m, while it is about 20 m at Amandelbult section. Thick succession of mottled anorthosite and magnetite is present in this area. The southern gap area between the Union section and the northern part of the Pilanesberg Complex is marked by a gradual decrease in elevation.
GAP AREAS BETWEEN AMANDELBULT AND UNION SECTION
The two gap areas identified in northwestern Bushveld Complex include the Northern gap and Southern gap. The Northern Gap separates the Amandelbult Section from the Union Section with a distance of about 11km. This area is marked by slopes that dip towards the Amandelbult Section. The Southern Gap occurs between the Union Section and north-eastern fringes of Pilanesberg Complex. .The northern and southern gap areas are characterized by the presence of Upper Zone rocks. The model reveals a gradual increase in elevation from the northwest, interrupted by the presence of domes and a small basin structure (see Figure 6).
Figure 6: Presence of slopes (faults) along the northern and southern gap area and isolated local dome structure on grid model of Northwestern Bushveld
DISCUSSION AND CONCLUSIONS
While strong abrupt thinning on both sides of the central section of Upper Zone isopach residual map coincides with the gap areas. The profile across the area reveals that the gap areas coincide with the downthrown sides of a centrally located positive thickness domain. The central part of the northwestern Bushveld on Upper Zone residual isopach maps show a centrally located NW thickening trend with abrupt thinning to the east and west. However, on the Main Zone a NW thickening trend in the north and SE thickening trend in the south marks residual isopach map the same central portion. Strong southeast thickening coincides with structural negative or structure low areas of Amandelbult section, the thickness is maximal in the axial portion of this structure. This area is marked by structure controlled down-dip accumulation of sequence of RLS rocks.
The Amandelbult and Northern sections outcrop with repeated magma cycles and continuous step- like layering pattern probably show evidence for multiple magma injection. The structural trend in this area is NNW as defined by the centrally located downthrown section bounded by opposite dipping faults to form the Roodedem/Middellaagte graben was probably produced during an extensional phase. The continuous step-like structure allowed a down-dip deposit of over 2500m based on available borehole records. However, Maier, (2013) has identified that the same structural pattern extend to over 10 km based on recent mining activity in the area. This indicates that the area might be close to magma feeder site.
The NE-SW outcrop trend cuts across the NNW structural trend, and probably defines the magma influx direction. This area is also marked by the presence of folding that probably relates to pothole structures. On the structural profile the length of the fold is mostly less than one kilometre and the height is not more than 500 m.
The Northern and Southern Gap area coincides with fault zone. The Upper Zone is better develops around the gap areas, and exhibits a central NNW trend with steep slopes along the gap area. There is an inverse relationship between structure and thickness of both regional and residual structures in the area, implying that the structures were already in place before the magma emplacement, although some of these structures were probably reactivated or modified during the RLS emplacement. The difference in the structural pattern between the top of the Upper Zone and the base of the Main Zone unit for most parts of the Western Bushveld might probably be due to slight tectonic movement. Another possibility is the crystallization of Upper Zone unit from a much latter magma influx which occured at the level of the Pyroxenite Marker horizon (Kruger et al., 1987; Cawthorn et al, 1991).
Formation of most of the structures such as normal faulting, slumping and layer parallel slip movement has been attributed to structural readjustment of the underlying Transvaal Supergroup rocks to the weight of additional magma load (Smith and Basson, 2006; Eriksson and Reczko, 1995; Carr et al., 1994).
Shape and distribution of potholes were described as circular, elongate, ovoid and irregular by Viljoen et al., (1986) while, Carr et al (1994) related the geometry to minor listric faulting at the floor of the BC. Carr et al. (1999) however described pothole pattern as small scale discontinuities of larger structural discontinuities. Syn-magmatic deformation had been suggested for the formation of pothole in the BC (Mitchell and Scoon, 2007; Smith and Basson, 2006). From these descriptions, the structure delineated above (Figure 5) around the Western Bushveld might be related to pothole structures.
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Acknowledgements
We are grateful to the management of Council for Geosciences, Pretoria, South Africa for granting access to borehole data and other information. Our profound gratitude also goes to all the staff of the Mineral Resources Department of CGS, Marietjie Schalekamp, Abera Tessema, and Thomas Abram of Central region for assistance in various ways during the data capturing and interpretation process. This research was supported by bursary from the University of Pretoria.
To cite this article
Electronic reference
O.A. Bamisaiye, P.G. Eriksson, J.L. Van rooy, H.M.Brynard, S. Foya, V. Nxumalo, A.M. Adeola, A. Billay, 2016. “Subsurface geometry of the northwestern Bushveld complex, South Africa” Canadian journal of tropical geography/Revue canadienne de géographie tropicale [Online]. Vol. (3) 1. Online in May 5, 2016, pp. 28-36. URL: http://laurentian.ca/cjtg
Authors
BAMISAIYE O. Adun
Department of Geology, University of Pretoria
Private bag X20 Hatfield, Pretoria 0028
South Africa.
E-mail: seyiadun@yahoo.com
ERIKSSON P.G. and VAN ROOY J.L.
Department of Geology, University of Pretoria
Private bag X20 Hatfield, Pretoria 0028
South Africa
BRYNARD H.M.; FOYA S. ; NXUMALO V. and BILLAY A.
Council for Geoscences, Pretoria
South Africa.
ADEOLA A.M.
Centre for Geoinformation Science
Department of Geography, Geoinformatics and Meteorology
University of Pretoria
Private bag X20 Hatfield, Pretoria 0028
South Africa.
