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The Geology of Area 3, GEOL2016 Field Mapping Exercise, Mazabeko, KwaZulu-Natal Province, South Africa

Khanyisile Tshabalala
1597540
ABSTRACT
The mapping area is located. The goal of this study was to produce a geological map of the area, establish the stratigraphic sequence and recognize structures. The oldest rock succession crops out in the south, bottom left corner of the map and the main lithologies found in the area were a conformable sequence of komatiites, basalt and diamictite. The next sequence of rocks unconformably overly this succession and is a series of quartzites, phyllites with some sheared intercalated basalts. The major shear zone occurs along the main river channel and a fault also occurs along the same channel towards the southern part of the map with two dolerite intrusions of relatively different ages. These structures complexify the geology of the area.

Contents Page number
Introduction 4
Stratigraphy and Lithology
Stratigraphic succession
Igneous intrusions
Metamorphism 5-10
Structures
Shear zone
Fault 10
Discussion 10-13
Geological History 13-14
Conclusion 14
Acknowledgements 14-15
References 16
Appendices 17-19
Enclosures
Introduction
The GEOL2016 students from the University of the Wiwatersrand went on a twelve day field mapping excursion, 02-14 July 2018, as part of their curriculum. The study area, Mazabeko is a small village located on the coordinates (-28.5442464, 30.5924), about 70 Km south of Dundee in KwaZuluNatal (Fig A).It is characterized by a very rugged terrain, drying up riverbeds as well as no tarred roads. The study was carried out using basic field mapping techniques to produce a geological map of area at a scale of 1:6 000. The objective of this report is to present detailed:
descriptions of the stratigraphy of Mazabeko
descriptions of the structuresgeological evolution of the area
Define the stratigraphy of Mazabeko

Figure A: Geographical loaction of Mazabeko (https://www.google.co.za/maps/place/Mazabeko/@-28.4942576,29.9724245,237637m/data=!3m1!1e3!4m5!3m4!1s0x1ef1a172f5795ecd:0x2a811bfa8817c97f!8m2!3d-28.5342928!4d30.5854674).

Stratigraphy and Lithology
Stratigraphic succession
The River Formation
The River Formation is comprised only of komatiites. These are characterized by the spinifex texture (fig.2). The rock also displays a tree bark like texture and is pinkish peach to gray on weathered surface and sea blue-green on fresh surfaces. The komatiites crop out on the southern most part of the map along a dried up river channel. These rocks are not structurally deformed but are intensely fractured.

The Hill Formation
The Hill Formation crops out on the south western part of the map and is only comprised of basalt. The basalt was intensely foliated and at some places closest to the c shearzone the basalt was mistakened for talc schist as it could easily be scratched by finger nails and also displayed a very soapy and powdery texture. The basalt also had huge, randomly oriented chert and quartz veins.
lefttopFigure : Spinifex texture in the komatiites
The Gorge Formation
The Gorge Formation is comprised of only diamictite which crop out in a gorge on the southwestren part of the map. The diacmictite is matrix supported with a very big clast size range; from granules to boulder sized grains. The clasts are also of very different compositions; granite, quartz and volcanic rocks. The diacmictite also has very small quartz veins that are 0.5cm thick and are sparsely distributed and randomly oriented.

The Ravine Formation
The Ravine Formation is only plane-bedded quartzite that is average oriented at 1240|420|NNE with fine to medium sand sized particles. An erosive contact between the quartzite and diacmictite from the gorge formation was observed.

The Plains Formation
The Plains Formation is simply a wide range of morphologically different basalts. It is the same very extensive basalt but towards the river channel on the south western part of the map, the basalt is intensely foliated with multiple and huge quartz veins. The quartz veins bulge in and out displaying a sausage like texture. The same basalt further away from the river channel is not as foliated and further away it has quartz amygdales (fig 3), the proportion of amygdales decreases futher away in the north-easterly direction such that on the most north-eastern part of the channel the basalt is very massive with 0% amygdales.
The White Ridge Formation
The White Ridge Formation is comprised of interlaminated quartzites and phyllites. The quartzite is pale yellow to white with a tint of brown turning to orange. It is plane-bedded, planar cross bedded and trough cross stratified with medium to coasre sand sized grains and ripple marks. The quartzite is oriented at 1240|420|NNE. The phyllite is greyish- green with very small ~1mm cubic (barely visible), reddish- orange crystals which in the field were identified as pyrite. The phyllite has very well developed cleavage planes oriented at 940|880|NNE.

Figure 3: Amygdaloidal basalt with 40% quartz amygdales
The Cliff Formation
The Cliff formation is only comprised of vessicular and amydaloidal basalts. The vessicular basalt is greyish black with some vessicles filled by some white crystals either than quartz which could not be identified in the field. It is also has quartz veins that ~5mm thick and are randomly oriented. The amydaloidal basalt has pipe amygdales and deformed pillow basalts but looks exactly the same as the basalts previously described (fig. 4). It is in this formation that pillow lavas were observed.

Figure 4: Pipe amygdales pointing NNE, direction of dip.

The Buffalo Formation
The Buffalo Formation is comprised of two unconformable units; the quartzite is plane-bedded with ripple marks and medium to coarse sand sized grains. It is oriented at 1100|250|NNE. The diactimictie has very wide clast size rane; granules to boulder sized. However, most of the grains are granule to pebble size with just one boulder sized gneiss clast (fig. 5a-b). It is oriented at 1520|130|WSW (angular unconformity).
lefttop Figure 5a: Gneiss clast in the diamictieFigure 5b: Angular unconformity
(Loonat, 2018)
The Road Formation
The Road Formation is comprised of diamictite and silstone, it crops out on the most nothern part of the map by the road. It is in this diamictite that a facetted pebble futher north, along strike was found (fig. 6). Overlying this diamictite is a very pale brown, finely laminated siltstone with pebbles right at the bottom of the bed. The orientation of the siltstone was not measured (Ndou, 2018).

The Mesa Formation
The Mesa Formation is comprised only of massive, vessicular basalt which crops out most north- eastern part of the map (right on the edge of the map). The basalt has a very irregular weathering pattern, such that at some places on the outcrop it looks intensely foliated (Ndou, 2018).

Figure 6: A facetted pebble found in the diamictite (Mashala, 2018)
Igneous intrusions
The study area experienced only two dolerite dyke intrusions, both of these dykes are oriented east- west. The first dyke best crops out on the coordinates (266170,6838148), the dyke has a very variable crystal size, it is coarsest in the middle and is composed of plagiclase and pyroxene which range from 1-3mm in size. The dyke is not metamorphosed. The second dyke crops out on the coordinates (), it also has a varying crystals size range; 1-4mm, the biggest crystals are some sort of amphibole which could not be identified in the field.

Metamorphism
The Mazabeko Supergroup lithologies have been subjected to a compelx series of alterations. The rocks have been intensely fractured, foliated and some even contain metamorphic minerals such as chlorite, talc, actinolite and other amphiboles that could not be identified in the field because of weathering.

Structures
The study area also has some structural complexities; a major shear zone along the main river channel and a fault south of the same main river channel. The mylonite, the extremely extensive quartz veins, intensely foliated basalt and very coarse-grained quartzite along the main river channel are indicative of a major shearing event in the area. The quartzite and basalt offset were a clear indication of a faulting event. Since, the rocks were only horizontally displaced along strike, the fault can then be said to be a strike- slip fault in which the south eastern rocks have been displaced horizontally in the south-southwest direction.

Discussion
The first succession of lithologies indicate a very big volcanic, perhaps a rifting event with a transition from ultramafic to mafic or ‘normal’ magma outpourings. This huge volcanic event seems to have caused landscape instabilities and hence triggered debris flows. Von Brunn and Gold (1993) also described a massive diamictite with a clast size range from granule to cobbles in a fine-grained matrix. Moreover, stated that the diamictite is indicative of a mudflow deposit that was emplaced in a marine shelf environment, the sediment was delivered in a subsiding basin by downslope mass movement from a fault-bounded, elevated margin where highland glaciers are likely to have contributed clastic detritus. However, does not explain what triggered the mudflows.
This succession of rocks is then overlain by a sequence of plane bedded quartzites and basalt, the quartzite represents a unidirectional fluid flow, since the grain size ranges from medium to coarse sand, it can then be said that the sandstone (protolith) was deposited in a low energy environment. The end of sedimentation of the ravine formation is marked by a series of volcanic eruptions represented by a whole series of morphologically different basalts. Furthermore, evidence to suggest multiple eruptions is the preserved lava flows, ~8 different lava flows. These are huge volumes of magma outpourings. Some of the magmas differentiated by the release of volatiles.
The next succession of rocks marks the beginning of the deposition of sediments. The interlayered quartzite and phyllites represent a tidal setting with seasonal tidal activity marked by distinct layers of quartzite and phyllites. In this case, the protolith could identified but it is most likely that it was a mudstone. This set of rocks not only represent a tidal depositional environment but also preserves evidence of no overturning of beds since the trough cross beds are still scoop shape up. Moreover, the transition from trough cross beds to planar cross bed represent deposition by unidirectional flow and most probably through the process of dune migration.

The next set of basalt marks the end of sedimentation, because the basalt is very vesicular and has pipe amygdales, these represent a very volatile enriched magma. The pipe amygdales were found in their original position, that is, there were not found upside, which served as more evidence to support that the way up is indeed NNE. The pillow lavas could not be used as way up indicators because of the intense deformation. However, they represent subaqueous volcanic eruptions to be specific, sea-floor volcanism.

The next succession of quartzite represents the beginning of sedimentation. The plane-bedded and ripple marked quartzites indicate deposition by unidirectional flow in the lower flow regime. The quartzite is tilted and thus, forms and angular unconformity with the diamictite. The suggested theory to explain the unconformity is that, the quartzite was initially deposited horizontally, due to tectonic forces, it was tilted. It is the tilting tectonism that triggered debris flows in the area and thus, the deposition of the diamictite closest to the river channel, which marks the lowest point in topography in the area. The diamictite could also have been deposited during shearing as it is not affected by shearing.

The last rock succession unconformably overlies the preceding strata. The diamictite is of glacial origin since a facetted pebble was found. This glacial diamictite or tillite represents a period of deglaciation which no field evidence can be presented as to what triggered the deglaciation. However, it is the youngest diamictite in the area since clasts of other diamictites are observed. The youngest rock unit which marks the end of sedimentation, is the basalt which caps the entire stratigraphy of the mapping area.

The area also has two dolerite dyke intrusions which trend east-west, because the dolerites coarsen in the north-south direction. The first dyke intrusion (d1) is older than the second dyke intrusion (d2) because it is metamorphosed whereas d2 is not metamorphosed.

The entire area has been metamorphosed to greenschist facies as indicator minerals such as actinolite, actinolite and serpentine are found in the rocks also most of the basalts encountered are greyish green. The intensely foliated rocks along the river channel represent a major shearing event. This also explains why the area does not have so many smaller tributaries, since rivers only flow along zones of weakness, in this area shearing only took place where there are river channels today.

Geological History
Rifting to form sea.

Subsidence below sea level to form sedimentary basin.

Deposition of the diamictite and sandstone.

Dolerite dyke intrusion, d1.

Shearing.

Volcanic eruptions.

Subsidence and deposition of interlaminated sandstone and mudstone.

Volcanic eruptions.

Subsidence, deposition of sandstone.

Volcanic eruptions.

Subsidence, deposition of sandstone and diamictite.

Faulting and shearing.

Dolerite dyke intrusion, d2.

Uplift, weathering and erosion
Deglaciation and deposition of tillite
Volcanic eruptions.

Uplift, weathering and erosion until present day surface.

Conclusion
The 12-day mapping exercise resulted in the production of the geological map of Mazabeko, two geological cross sections; 1. W-E plane, 2. SW-NE plane, stratigraphic column and the construction of the geological history of the area. This study was purely based on observations; hence the stratigraphic order of events was determined using the basic principles of stratigraphy. Lastly, the talc schist like basalt is a clear indication of numerous shearing events as this was the highest grade of metabasalt observed in the area also there was no overturning.

Acknowledgements
The twelve-day mapping exercise is the first biggest project I have had in my academic career thus far. This project would not have been possible without the contribution and collaboration of others. My sincere gratitude:
To the Elandskraal staff for the warm welcome and ensuring that the campsite facilities are kept in a user-friendly state.

To the bus driver for transporting us safely to and from the study area.

To the Geosciences staff for the support and guidance.

To the lecturers for guidance, support and sharing your knowledge.

To the tutors for the support, care and guidance especially Ms. H.K Maselela for the support you gave me when I was not medically fit at the field and going the extra mile to ensure the successful completion of this project.

To my classmates for making the field school a very memorable experience, especially my working group for sharing most of the journey with me.

To Ms. T. Makhateng for helping with the use of GIS software to produce the map and editing the report.

To Ms. N. Nkosi for helping with the editing of the report.

References
Burke, K., Kidd, W.S.F. and Kusky, T.M. 1984. The Pongola structure of the south-eastern Africa: The world’s oldest preserved rift? Journal of Geodynamics, 2, 35-49.

Cole, E.G. (2012) Lithostratigraphy and depositional environment of the Archean Nsuze group. M.Sc. (Geology). unpublished. University of Johannesburg. Retrieved from: https:// ujdigispace.uj.ac.za (Accessed: 09 September 2018).

Ndou, H. (2018), Road and Mesa Formation descriptions.

Siahi, M., Hofmann, A., Hegner, E. and Master, S. 2016. Sedimentology and facies analysis of Mesoarchean stromatolitic carbonate rocks of the Pongola supergroup, South Africa. Precambrian Research, 278, 244-264.

Von Brunn, V. and Hobday, D.K. 1976. Early Precambrian tidal sedimentation in the Pongola supergroup of South Africa. Journal of Sedimentary Petrology, 46 (3), 670-679.

Von Brunn, V. and Gold, D. J. C. 1993. Diamictite of the Archean Pongola supergroup sequence of southern Africa. Journal of African Earth Sciences, 16 (3), 367-374.

Appendices
Appendix A1
Table 1: The Stratigraphy of Mazabeko
MAZABEKO SUPERGOUP Groups Subgroups Formations Members
Road Mesa Basalt
Road Tillite/ Glacial diamictite
Hills Flowing Channels Buffalo Diamictite
Quartzite
Cliff Basalt
White Ridge Quartzite
Phyllite
Quartzite
Phyllite
Dried up river beds Plains Basalt
Ravine Quartzite
Houses Gorge Diamictite
Hill Basalt
River Komatiites

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