| dc.description.abstract | Geological mapping is the process of a geologist physically going out into the field and recording geological information from the rocks that outcrop at the surface. Information that the geologist looks for will include: lithology, boundaries between rock types and structures present on rock surfaces. This data is partly analyzed from the field (in-situ) and in the laboratory with the help of computer software. The whole project involved collecting data from GPS location, visual observations at the outcrops or stations, measuring the extents of the available structures and other morphological features using tools like geological compass, and later plotting the data on geological maps, collected samples for laboratory analysis. The structural data was collected from the different stations. This helped in constructing stereograms and rose diagrams for the structures like joints, beds and folds encountered during the mapping exercise. The structural data and its resultant graphical plots helped in determining the history of deformation in the area. Both macroscopic and microscopic petrographic analysis was carried out on the collected samples whose results reveal the mineralogical composition of the rocks in the mapped area and the metamorphic status of the area was inferred. Petrographic studies confirmed presence of quartz and clay minerals (as the dominant minerals) together with some micas, alkali feldspars and iron oxides in the rocks of the mapped area. The type of metamorphism that affected these rocks is mainly regional and of low grade evidenced by foliation and some index minerals such as chlorite. Sandstones and shales were metamorphosed to quartzites and phyllites respectively. Deep, quiet and shallow water dynamic environments of deposition were proposed due to the presence of shale and quartzites and some structures such as beds and laminations in shales.
The area lies on a generally rugged topography with intervening valleys, steep gorges, flat topped ridges and rounded hills. The climate of the area is composed of two wet seasons with mean annual precipitation of about 750mm to 1250mm and mean annual minimum temperatures of 12.5-15°C to a maximum of 25-27.5C.
The rock types in Gayaza include shales, quartzites, conglomerates, phyllites, laterites, Aplites and mylonites (only found in area I). The rocks were deposited in the order of shales followed by sandstones (later metamorphosed to quartzites) which were later intruded by granites and as a result of erosion and weathering the granites formed arena structures which predominate much of the Karagwe-Ankolean system.
The major structures in the area mapped include; beddings, faults, folds and joints all exhibiting two major trends; the northwest-southeast (NW-SE) trend which is similar to that of the regional folds and the northeast-southwest (NE-SW) direction similar to that of the cross folds. The minor structures include; micro-folds, micro-faults, quartz veins, boudins, rods, foliation, lamination and mud cracks. | en_US |
