A field mapping project report for area A, Igayaza Subcounty, Isingiro District
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In February 2021, a class of Geology and petroleum students from Makerere University camped in Igayaza area for 11 days to carry out Geological mapping in the same area. Igayaza is located in Isingiro district about 30km south of Mbarara. The project was aimed at acquainting students with necessary practical skills in field data acquisition, data analysis, geological data reporting and usage of geological equipment’s. Different materials and methods were used in field mapping and these include; Global positioning systems for taking coordinates of lithologic boundaries, geological hammer for collecting samples, geological compass for taking structural measurements. The mapped area A is composed of rugged topography with ridges, rounded hills and V- shaped valleys. There is a broad stadium like low land in the Northern part of the area termed Arena which was formed by weathering and erosion of the granites that intruded the Karagwe Ankolean system. There are two drainage patterns in area A; Major parallel to sub parallel pattern which follows V- shaped valleys most of which lie in fault zones and a dendritic pattern in areas which were eroded uniformly like in the arena. The mapped area is composed of different rock types and these are; phyllitic shales near the base of the hills and ridges, grey shales on steep slopes, ferruginous shales on flat topped hills and valleys, quartzites and conglomerates at the top of the ridges. The stratigraphic interpretation of area A is based on the right way up of the strata and it shows that there are intercalations of argillaceous rocks and arenaceous horizons. This could have been caused by seasonal changes in the conditions at the time of deposition which varied the volume of water in the basins and this determined the sediments that were deposited at the time. Different geological structures were encountered in the mapped rocks in area A and these are; Joints, beds, folds, faults, quartz veins. The joints are mostly open but in some areas, they are filled with quartz veins or lateritic material. The joints have two major trends; the dominant set trends in the NW-SE and the other set trends in the NE-SW. Jointing in area A is related to the folding episodes and so the dominant set in NW-SE is related to the regional fold and the set in NE-SW is related to the cross fold. Beds are relatively thin and have two sets; the dominant set trends in NE-SW and is related to the cross fold. The minor set trends in NW-SE and is related to the regional fold. The folds in area A are relatively open and mostly trend in NE-SW direction and so are cross folds. The cross folds are plunging at relatively high angles due to refolding by the regional fold. Faults are common and mostly trend in the NW-SE direction. Few faults trend in the NE-SW direction. Faults are recognized by linear vegetation and fault breccia along the fault zone. Petrographic analysis was done on thin sections made from collected samples (ST4, ST1, FP, ST4 Breccia, ST5) and major minerals are quartz and hematite in samples; ST1, ST5 and ST4 Breccia, quartz, hematite and chlorite in samples; ST4 and FP. Area A was mostly affected by low grade regional metamorphism due to burial as evidenced by the presence of low grade metamorphic minerals like chlorite in shales. Relict bedding in some quartzites is also an indication of low grade metamorphism. Contact metamorphism could have also occurred in the area because there is a progression in metamorphism towards the granite in the arena. Thin section analysis shows wavy extinction in quartz which could be an indication of cataclastic metamorphism. From the analysis of the all the data obtained from the mapped area, a regional synthesis was constructed to bring out the whole geology of Karagwe Ankolean system in terms of rock types, stratigraphy, tectonic events and stress fields that could have affected the rocks.