Basotu Craters

About 30 explosion craters aligned north-northeast form two parallel lines. They are seldom more than 40 m high, are up to 100 m deep and a number contain crater lakes. They cut through a Precambrian basement of varied lithology and Neogene tuffs and limestones. The tuffs are up to 12 m thick, poorly bedded and contain crystals of biotite, aegirine, quartz and feldspars as well as lithic fragments of basement in a matrix which is dominantly carbonate. Overlying the tuffs is a limestone layer up to 0.5 m thick consisting of about 75% calcite, and containing crystals similar to those in the tuff; there are many drusy cavities up to 2.5 cm across. The limestone is considered to be a carbonatitic ash and the thickening of the underlying tuff eastwards suggests a source in that direction, possibly Hanang (No. 163-00-026). The craters are either simple, comprising a rim of agglomerate and tuff, or composite with a single cone surrounding more than one crater. They are generally circular or elliptical and seldom more than a kilometre in diameter. The agglomerate fragments are mostly about 1 cm in diameter, but may be up to 1 m, and consist of basement metamorphic rocks and dolerites with fragments of carbonate rock around two craters. Crystal fragments in the tuffs include aegirine, augite and nepheline and the matrix is predominantly of carbonate with a little fine-grained quartz and plagioclase. The carbonate forms 15-45% of the rock and is either a calcite/dolomite mixture or magnesian calcite.