Remnants Blocks of Pyroclastic Surge Deposits inBambili, Cameroon Volcanic Line: New Insights intothe Lithostratigraphy of Mount Bamenda

摘要

Field studies in Bambili locality (NE of Bamenda city, West-Cameroon), situated in the central part of Cameroon Volcanic Line, have recently permitted us to identifier above the welded massive lapilli tuff (mlT), remnant blocks (up to 6.5 x 11 m) of pyroclastic surge deposits. The latter are characterized by well-sorted and distinctly stratified layers with thicknesses ranging from 8 to 35 cm and showing graded bedding. The layers are matrix-supported and heterolithic, with the lithic fragments consisting of devitrified fiammes, vitrophyres, trachytic and rhyolitic cognates, granites and ignimbrites. The mineralogy of these surge deposits is quasi identical to that of the welded mlT which consists of alkali feldspar (sanidine), quartz, plagioclase, clinopyroxene, biotite and Fe-Ti oxides. The presence of these remnants blocks of pyroclastic surge deposits in the Bambili locality, emitted probably from Mt Oku vent, permit to reconsider the chronostratigraphy of the Bamenda Highlands. In fact, after a trachytic lava flow, a pyroclastic flow deposits allowed the formation of the welded ignimbrites represented by dark grey and whitish units; pulsating hydrostatic and magma pressures have subsequently produced alternating phreatomagmatic pyroclastic surges, which have afterward covered the massif, followed by basaltic flows.