Geology of the Buchanan quadrangle, Liberia

Abstract

Crystalline rocks of Precambrian age underlie the Buchanan quadrangle and are capped by Cretaceous and younger sedimentary rocks in the coastal area. The crystalline rocks, largely of metamorphic origin, include mainly gneisses of quartz diorite to granodiorite composition. Less common rock types are amphibolite, schist, quartzite, charnockite, and iron-formation. Most of the metamorphic rocks are of the amphibolite facies, but some near the coast belong to the granulite facies. Igneous rocks are predominantly diorite of the Cestos and Sehnkwehn batholiths. Two belts of Jurassic diabase dikes form prominent ridges that trend northwest across the quadrangle. Sedimentary rocks are mainly quartz sandstones, unconsolidated beach sands, and lagoonal deposits. The structure is dominated by northeast-and northwest-trending features. The northwest-trending Cestos shear zone is marked by a wide zone of mylonite that separates contrasting rock types. It is offset by northwesttrending faults that transect the quadrangle and are, at least locally, marked by narrow zones of shearing. The broad northwest-trending Todi shear zone near the coast separates granulite-facies metamorphic rock from amphibolite-facies rock to the northeast. Prominent folds in the quadrangle are broad and open; their wavelengths are measured in kilometers.