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Investigation of the crust in the southern Karoo using the seismic reflection techniqueLoots, Letticia 07 July 2014 (has links)
Several seismic reflection surveys were conducted in the late 1980s and early 1990s
under the auspices of the SA National Geophysics Programme. These surveys targeted the Bushveld Complex, Limpopo Mobile Belt (Limpopo Province), Witwatersrand Basin,
Vredefort Dome and the Beattie magnetic anomaly (BMA) in the Southern Karoo. The ~100 km seismic reflection profile described in this study (SAGS-03-92) covers the BMA, the Southern Cape Conductive Belt (SCCB) and the Karoo/Cape Fold Belt boundary. The profile runs from approximately Droëkloof in the south to Beaufort West in the north along the N12 national road. The profile was acquired in 1992, but the complete profile was not interpreted or published prior to this study. The purpose of this study is to successfully reprocess the data and to do a structural and stratigraphic interpretation in order to try and understand the geological history and processes that led up to the formation of the rocks in that area.
SAGS-03-92 reveals a clear image of the crust in the southern Karoo. The crust is
interpreted to be around 37 km thick in the area of investigation and can be classed into three parts: upper crust, middle crust and lower crust. The upper crust consists of the Karoo and Cape Supergroup rocks that dip slightly to the south. This interpretation has been confirmed by two deep boreholes (BH No. 3 and KW 1/67). The seismic fabric shows quite a strong character in the upper crust and the interpreted boundaries between the different lithologies (The Table Mountain, Bokkeveld and Witteberg Groups of the Cape Supergroup and the Dwyka, Ecca and Beaufort Groups of the Karoo Supergroup) are for the most part quite easy to identify. Within the Cape Fold Belt (CFB), however, the seismic character becomes distorted in such a way that it is very difficult to make out any features. This is possibly due to the severe faulting and folding that occurred when the CFB formed. An unconformity that can continually be followed throughout the profile (although it disappears in the south of the profile possibly due to deformation when the CFB formed) separates the upper crust from the middle crust and the unconformity is clearly indicated by a strong series of reflectors on the seismic profile.
The middle crust is interpreted to consist of granitic-gneisses belonging to the
Bushmanland Terrane (part of the Namaqua-Natal Belt (NNB)). The seismic profile suggests that the NNB gneisses continue beneath the Cape Fold Belt. The seismic fabric dips steeply to the north. The middle crust also hosts the source of the Beattie Magnetic Anomaly (BMA).
There is an area of high reflectivity under the BMA on the seismic profile that differs
significantly from the surrounding seismic character. This area is characterised by a beanshaped cluster of strong reflections dipping north and south. It is ~10 km wide, with a
thickness of ~8 km and occurs at a depth of ~6 km to ~10 km.
The lower crust is interpreted to consist of either granites belonging to the Areachap
Terrane, Richtersveld or Kheis Province (NNB) or rocks belonging to the Kheis Province.
The seismic fabric of the lower crust dips moderately to the south. The Moho is recognised at ~37 km depth at ~68 km from the south of the profile, but for the rest of the profile, it is unclear where the Moho is encountered.
The research done for this study correlates well with work done under the auspices of
Inkaba yeAfrica, especially the work done by Ansa Lindeque
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Electrical conductivity experiments on carbon-rich Karoo shales and forward modelling of aeromagnetic data across the Beattie AnomalyBranch, Thomas Cameron January 2014 (has links)
The Beattie Magnetic Anomaly is the world’s longest terrestrial magnetic anomaly with a strike length of over 1000 km and a wavelength in excess of 100 km. Collinear with this is a large belt of elevated crustal conductivities called the Southern Cape Conductive Belt. Historical crustal interpretations proposed a common source of serpentinized ophiolite as an explanation for both the anomalous crustal magnetic susceptibility and electrical conductivities. Spreading between the Western and Eastern Cape of South Africa the mid- to lower crust that hosts these anomalies is obscured by the overlying Cape and Karoo Supergroups. Between 2003 and 2006, three high resolution geophysical experiments were completed across the surface maximum of the Beattie Magnetic Anomaly (BMA) and the Southern Cape Conductive Belt (SCCB). These included a magnetotelluric (MT) survey and near vertical reflection and wide angle refraction seismic profiles. Within the MT inversion model the SCCB appeared as a composite anomaly, which included a mid-crustal conductor which is spatially associated with the BMA and a laterally continuous upper crustal conductor which is located at depths equivalent to the lower Karoo Supergroup. Subsequently; the upper crustal conductor was identified in northern and eastern extensions of the magnetotelluric profile; a distance in excess of 400 km. Historical magnetometer and Schlumberger Sounding experiments have previously identified elevated conductivities in the Karoo sequences which were attributed to the Whitehill and Prince Albert formations. These carboniferous, transgressive sediments are known to be conductive from borehole conductivity surveys and direct measurements at surface. In order to constrain the conductive properties of these sediments, impedance spectroscopy (IS) experiments were completed on core samples collected from a historical borehole drilled near to the MT profile. Part One of this thesis presents the results of these experiments, which support the proposition that the Whitehill and Prince Albert Formations are responsible for the laterally continuous, sub-horizontal, upper crustal conductor visible in the MT inversion model. Vitrinite reflectance studies were performed on the same samples by the Montanuniversität, in Leoben, these results corroborate the proposition that elevated organic carbon, of meta-anthracite rank, is the primary conductive phase for the Whitehill and Prince Albert formations. Part two of this thesis completed forward modelling exercises using historical aeromagnetic data previously collected across the Beattie Magnetic Anomaly. Preliminary models were unable to fit the geometry of any single magnetic model with conductors present in the MT inversion model discounting the proposition that the SCCB and BMA arise from a single crustal unit. Two constrained models were arrived at through an iterative process that sought a best fit between the measured data and the NVR crustal interpretations. The first model, proposes a largely resistive unit which incorporates portions of elevated crustal conductivity; these conductors are spatially correlated to crustal portions also characterised by high seismic reflectivity. The size of this modelled body suggest the likely host of the BMA is an intermediate plutonic terrane, analogous with the Natal sector of the Namaqua Natal Mobile Belt as well as the Heimefrontfjella in Dronning Maud Land, Antarctica, with magnetite hosted within shear zones. This is in agreement with previous studies. The second model proposes a lower crustal sliver imaged in the NVR data at depths proximal to the Curie Isotherm for magnetite and hematite as the source of the BMA. At these depths geomagnetic properties such as burial magnetisation or thermo-viscous remanent magnetism (TVRM) can potentially be linked to regional scale tectonic processes and can theoretically elevate a body’s net magnetic susceptibility. TVRM has been proposed for long wavelength crustal anomalies elsewhere.
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Integrated geophysical investigation of the Karoo Basin, South AfricaScheiber-Enslin, Stephanie E 10 May 2016 (has links)
A thesis submitted to the Faculty of Science, University of the Witwatersrand,
Johannesburg, in fulfilment of the requirements for the degree of Doctor of
Philosophy
Johannesburg, August 2015
School of Geosciences, University of the Witwatersrand / The possibility of extensive shale gas resources in the main Karoo Basin has
resulted in a renewed focus on the basin, and particularly the Whitehill Formation.
The main Karoo Basin has been the subject of geological studies since before the
1920s, but geophysical data provides an opportunity to shed new light on the
basin architecture and formation. In this thesis, I use regional gravity, magnetic
and borehole data over the basin, as well as vintage seismic data in the southern
part of the basin. Modern computational capacity allows for more information to
be extracted from these seismic data, and for these data to be better integrated
with potential field data. The integration of datasets in a three-dimensional model
(3D) has allowed for a better understanding of the shape of the basin and its
internal structure, in turn shedding light on basin formation.
A new depth map of the basin constructed using this extensive database
confirms that the basin deepens from on- to off-craton. The basin is deepest along
the northern boundary of the Cape Fold Belt (CFB), with a depth of ~4000 m in
the southwestern Karoo and ~5000 m in the southeastern part of the basin.
Sediment thickness ranges from ~5500 to 6000 m. The Whitehill Formation along
this boundary reaches a depth of ~ 3000 m in the southwest and ~4000 m in the
southeast. Despite limited boreholes in this region, the basin appears to broadly
deepen to the southeast. These seismic and borehole data also allow for mapping
of the Cape Supergroup pinch-out below the Karoo basin (32.6°S for the
Bokkeveld and 32.4°S for the Table Mountain Group), with the basin reaching a
thickness of around 4 km just north of the CFB. The gravity effect of these
sediments in the south is not sufficient to account for the low of the Cape Isostatic
Anomaly near Willowmore and Steytlerville. This ~45 mGal Bouguer gravity low
dominates the central region of the southern Karoo at the northern border of the
CFB. The seismic data for the first time show uplift of lower-density shales of the
Ecca Group (1800 – 2650 kg/m3) in this region, and structural and seismic data
suggest that these lower density sediments continue to depth of 11 to 12 km along
normal and thrust faults in this region. Two-dimensional density models show that
these shallow crustal features, as well as deeper lower crust compared to
surrounding regions, account for the anomaly.
These seismic and borehole data also allow for constraints to be placed on
the distribution and geometry of the dolerite intrusions that intruded the basin after
its formation, and in some cases impacted on the shale layer, to be constrained. The
highest concentrations of dolerites are found in the northwest and east of the basin,
pointing towards two magma sources. The region of lowest concentration is in the
south-central part of the basin. Here the intrusions are confined to the Beaufort
Group, ~1000 m shallower than the shale reservoir, suggesting it should be the
focus of exploration efforts. These dolerite sills are shown to be between 5 and 30
km wide and are saucer-shaped with ~ 800 m vertical extent, and dips of between
2° and 8° on the edges. The sheets in the south of the basin extend for over 150
km, dipping at between 3° and 13°, and are imaged down to ~ 5 km. This change
in dip of the sheets is linked to deformation within the Cape Fold Belt, with
greater dips closer to the belt, although these sheets do not appear to intrude strata
dipping at more than 15 to 20°.
In order to understand the shape of the Karoo basin and construct a 3D model
of the basin, an understanding is needed of the underlying basement rocks. The
Beattie Magnetic Anomaly (BMA) that stretches across the entire southern part of
the basin forms part of the basement Namaqua-Natal Belt. Filtered magnetic data
confirm that the Namaqua and Natal Belts are two separate regions with different
magnetic characteristics, which is taken into account during modelling. The BMA
is shown to be part of a group of linear magnetic anomalies making up the Natal
Belt. The anomaly itself will therefore not have an individual effect on basin
formation, and the effect of the Natal Belt as a whole will have to be investigated.
An in-depth study of outcrops associated with one of these linear magnetic
anomalies on the east coast of South Africa suggest the BMA can be attributed to
regions of highly magnetic (10 to 100 x 10-3 SI) supracrustal rocks in Proterozoic
shear zones. Along two-dimensional magnetic models in the southwestern Karoo
constrained by seismic data, these magnetic zones are modelled as dipping slabs
with horizontal extents of ~20-60 km and vertical extents of ~10-15 km. Body
densities range from 2800- 2940 kg/m3 and magnetic susceptibilities from 10 to
100 x 10-3 SI.
These, as well as other geophysical and geological constraints, are used to
construct a 3D model of the basin down to 300 km. Relatively well-constrained
crustal structure allows for inversion modelling of lithospheric mantle densities
using GOCE satellite gravity data, with results in-line with xenolith data. These
results confirm the existence of lower density mantle below the craton (~3270
kg/m3) that could contribute to the buoyancy of the craton, and an almost 50
kg/m3 density increase in the lithospheric mantle below the surrounding
Proterozoic belts. It is this change in lithospheric density along with changes in
Moho depths that isostatically compensate a large portion of South Africa’s high
topography (<1200 m). The topography higher than 1200 m along the edge of the
plateau, along the Great Escarpment, are shown to be accommodated by an
asthenospheric buoyancy anomaly with a density contrast of around 40 kg/m3,
while still mimicking the Bouguer gravity field. These findings are in line with
recent tomographic studies below Africa suggesting an “African Superplume” or
“Large Low Velocity Seismic Province” in the deep mantle.
The basin sediment thickness maps were further used to investigate the
formation of the main Karoo Basin. This was accomplished by studying the past
flexure of the Whitehill Formation using north-south two-dimensional (2D)
profiles. Deepening of the formation from ~3000 m in the southwest to ~4000 m
in the southeast is explained using the concept of isostasy, i.e., an infinite elastic
beam that is subjected to an increasing load size across the Cape Fold Belt. Load
height values increase from 4 km in the southwest to 8 km in the southeast. This
larger load is attributed here to “locking” along a subduction zone further to the
south. The effective elastic thickness (Te) of the beam also increases from around
50 km over the Namaqua and Natal Belts in the southwest to 80 km over the
Kaapvaal Craton and Natal Belt in the southeast. The changes in Te values do not
correlate with changes in terrane, i.e., a north to south change, as previously
though. The large extent and shape of the Karoo basin can therefore, in general,
be explained as a flexural basin, with the strength of the basement increasing
towards the southeast. Therefore, while factors such as mantle flow could have
contributed towards basin formation, reducing the load size needed, it is no longer
necessary in order to account for the large extent of the basin. This flexure model
breaks down further to the southeast, most likely due to a very high Te value. This
could be the reason for later plate break in this region during Gondwana breakup.
It is inferred that this increase in Te is linked to the buoyancy anomaly in the
asthenospheric mantle.
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Facies architecture and reservoir quality of Unit B, Permian Laingsburg Formation, southwestern Karoo Basin, South AfricaLombard, Donovan Joseph 03 1900 (has links)
Thesis (MSc)--Stellenbosch University, 2013. / This study presents a facies outcrop characterization and petrographical analysis
of Unit B of the Permian Laingsburg Formation. Unit B is interpreted as a base-ofslope
system, which represents a strikingly sand-rich succession. The base-of-slope
system is defined by a channel-levee complex. The study provides systematically a
clear understanding and description on reservoir heterogeneities, in terms of facies
distribution, physical processes and architectural elements. The dataset included
detailed sedimentary logs, photomosaic interpretations, supplemented by a
petrographical study to determine the textural and compositional attributes of the
studied sandstones.
Seven lithofacies was recognised within Unit B, based on detail observation and
description on grain size and sedimentary structures. They mainly consist of 1) thick
to massive bedded ‘structureless’ sandstone, 2) horizontal and ripple cross-laminated
thin-medium bedded sandstone, 3) silty sandstone, 4) structureless siltstone, 5)
hemipelagic mudstone, 6) muddy slump, and 7) sandy slump. Palaeocurrent analysis
indicates that the mean sediment transport direction of Unit B was to the E and NE.
Lithofacies 1 comprises thickly to massive bedded, frequently amalgamated,
mostly very-fined grained sand, mixed grading, irregular to sharp upper contacts,
structured upper bedding planes, large floating mudstone clasts and granules, rare
groove and flute casts. Also, scour and fill features have been documented.
Lithofacies 1 has been interpreted to result from channelized sandy debris flow
currents. Lithofacies 2 composes of thin-medium bedded, very fine-grained sand,
ungraded, sharp upper contacts, discrete units with traction bed forms, horizontal and
cross-lamination, mud-draped ripples, internal erosional surfaces and preserved crests. Lithofacies 2 shows diagnostic sedimentary features for a deep-water bottom
reworking current. Lithofacies 5 composes of very fine–grained mud, ‘structureless’
to finely horizontally laminated, fissile mudstone. Deposition resulted from
suspension settling of mud fractions out of a low-energy buoyant plume. Lithofacies
6 composes of contorted and convoluted bedding, steeply dipping layers and irregular
upper contacts. Deposition occurred via slumping on an unstable slope. Lithofacies 7
composes of fine–grained ‘structureless’ sandstone, amalgamated units, with dark
floating mudstone granules. Lithofacies 7 has been interpreted to form from
channelized flows evolving into slump deposition on an unstable slope.
The petrographic data reveals that the reservoir quality of the sandstones is
strongly controlled by depositional processes and diagenetic products. The sediments
of the Karoo Basin appear to be diagenetically controlled as a function of burial
depth. The major diagenetic products controlling the reservoir quality of the
sandstones, includes compaction (mechanical and chemical), and authigenic porefilling
constituents (quartz cement, feldspar dissolution and partial to complete
replacement, calcite cement, chlorite and illite). Compaction played a major role in
the evolution of the sediment, as compared to the effect of quartz cementation, and is
considered here to have caused irreversible destruction of depositional porosity and
permeability. The sediment has undergone intense mechanical compaction during
early-stage diagenesis, low temperature and shallow depth of burial (probably the
first 2 km). The high burial palaeotemperature (250 ± 500C) or more specifically the
high geothermal gradient of the Karoo Basin consequently increased the number of
diagenetic reactions. The high burial temperatures may have increased pressure
dissolution and quartz cementation. With compaction been limited, quartz
cementation and the authigenesis of chlorite and illite at deeper depths may have had
a profound effect on the permeability distribution of the studied sandstones. After the
completion of diagenesis, the pore systems of these sandstones were completely
destroyed by low-grade regional burial metamorphism.
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