A Report of Geologic Mapping project of Igayaza, Isingiro District.

Abstract

This geologic mapping exercise was carried out in Igayaza located in Isingiro district in south western part of Uganda from 30th January 2021 to 8th February 2021. The major aim of this field mapping was to train students to obtain hands-on experience in field geological mapping activities from field preparations to geological written report and practically learn field geological mapping and field data collection techniques.

The area mapped is within the Karagwe Ankolean system of rocks which stratigraphically overly the Basement complex rocks. The Karagwe-Ankolean system (1400-950Ma) in Uganda is the northern most extension of the Kibaran mobile belt. The sediments of this system occupy a continuous area in southern and central Kabale, southern Mbarara, Bushenyi, Isingiro, Rakai and south eastern Masaka districts. The Buhweju plateau and surrounding hills in Bushenyi district belong to this system as well.

Argillaceous and arenaceous rocks (shales, phyllites and quartzites) are characteristic of the area except for parts of granitic intrusions (arenas). Stratigraphically, rocks were deposited in the order of shales, sandstones that later metamorphosed to quartzites that occupy the top of ridges which were later intruded by granites. Quartzites overlie shales which overlie slates. Slates are underlined by phyllites and finally granites at the base.

The major structures in the area are beddings, faults, folds and joints all exhibiting two major trends; NW-SE trend which is similar to that of the regional folds and NE-SW direction similar to that of the cross folds whereas the minor ones include: micro-folds, micro-faults, quartz veins, foliation, laminations and mud cracks. Dip and strike structural measurements were made by taking geologic bearings for in situ rocks at the geological stations that were set up. The structural measurements were then used to construct rose diagrams and stereograms for structures such as joints and bedding. The results of which further substantiated the presence of the two tectonic regimes as stated in literature.

At the end of the mapping exercise, hotspot presentation was done where students would take lecturers to the most interesting areas and present their observations to the lecturer along with the students.

Petrographic analyses done on the thin sections of selected rock samples collected from the field revealed the mineralogical composition of rock samples from the study area. From these analyses, it was found out that the rocks of the Igayaza area are of a low metamorphic grade and generally assumed the entire geology of the Karagwe-Ankolean system. This is evidenced by foliation and some index minerals such as chlorite.