The Upper East Region is underlain by two main geologic formations: the Pre-Cambrian Basement Complex and the Paleozoic Consolidated Sedimentary Formations. The Pre-Cambrian Basement Complex rocks are made up of crystalline igneous and metamorphic rocks which cover about 92% of the region. The granitic formation, which is a component of the above, underlies about 65% of the region. The Birimian rock formation is the second largest and covers about 25% of the region. The Lower Birimian Series comprises phyllites, schists, shales, greywackes and siltstones whereas the Upper Birimian consists of thick beds of agglomerate, pyroclastic and hypabyssal intrusive rocks and the rock types are mostly grewackes and phyllites.
Characteristically, the pyroclastic and basic volcanics in this region have been altered and metamorphosed into hornblende schists and amphibolotes and the folds in these rock formations dips in the direction of 300 – 900 in the NE – NW directions.
The Tarkwaian Rock Formation underlies about 2% of the region and it is found mostly in the Bolgatanga Municipal and the Bongo District. This rock formation is considered to be of shallow water continental origin, derived from the Birimian granitic complexes and has been subjected to low-grade metamorphism. The rock types are mainly phyllites, quartzites, schists and conglomerates.
The Paleozoic Consolidated Sedimentary Rocks, locally referred to as the Voltaian Formation, underlies about 8% of the region. This formation is made up of well consolidated and gently folded rocks with an average dip of about 50 and it is marked by an escarpment. On the basis of lithology and field relationships, the Voltaian is sub-divided into three sub-groups: Upper, Middle (Oti and Obosum) and Lower Voltaian (Basal Sandstones). These rocks are found at the South Western and South Eastern parts of the region. The Oti and Obosum formation have nodular structures and it is intensely weathered. The rock types are sandstones, shales, arkose, mudstone beds, conglomerate and limestone. Basal sandstones underlie the South Central to South Eastern parts of the region. These rocks when weathered are characteristically purple. These rocks include quartzites-sandstones, sandstones and pebbly grits.