Tephrostratigraphy and morphometry of Wum Maar Volcano (Oku Volcanic Group- Cameroon Volcanic line): Implications for complex monogenetic volcanoes

Abstract

Field investigations of tephra deposits and morphometric characterisation were used to constrain the eruptive history and the complexity of the Wum Maar Volcano (WMV) on the Northwestern foot of the Oku Volcanic Group (Cameroon Volcanic Line). The WMV is a compound volcanic edifice made up of an irregular horse-shoe-shaped maar-crater bordered by overlapping scoria cones and a small lava flow field. The bulk ejecta volume computed using ArcGIS.10.3 and field topographic measurements is ∼0.133 km3. The deposits were subdivided into older volcanics, a pre-maar member composed of four Strombolian scoria cones and a maar-member made up of interbedded layers of lapilli-tephra, lapilli-ash, and ash-breccia deposits. The bedding characteristics (structure, bedding dip, direction, attitude, and contact relations) coupled with the componentry analysis of maar deposits indicate phreatomagmatic-derived fallouts and pyroclastic density currents emplaced under both dry and wet conditions due to variations in magma-water ratio during the different phreatomagmatic explosions. The thick paleosol between older volcanics and the pre-maar and the maar deposits suggests that before the formation of the WMV, an eruptive activity took place and comprised an effusive outpouring of a basaltic magma batch that exploited the network of fractures within the basement rocks to reach the surface. Long after this stage, other batches of basanitic and trachybasaltic magmas richer in volatiles erupted explosively (Strombolian eruptive dynamism) around the northern and southern rims of the maar crater, leading to the emplacement of several overlapping scoria cones. The last stage was characterised by the formation of the Wum maar crater through a phreatomagmatic explosion when batches of basanitic magma came in contact with groundwater. The volcanics are sub-aphyric and host mantle xenoliths. Although evenly distributed across the different maar units, these mantle xenoliths are frequent and large enough to suggest a rapid magma ascent. The center of the maar crater compared to the location of the scoria cones suggest that the explosion locus migrated eastwards. The relative proportion of lithic granite within the pyroclastic succession increases upward (highest in MU4), and the depth of the maar varies from <10 m at the western border to about 124 m at the eastern one, suggesting a vertical and a lateral migration of the explosion locus. The intensity of the phreatomagmatic explosion reduced with time as indicated by the clast-size distribution, with a progressive reduction in water availability wherein the eruption became more internally controlled, changing in style from phreatomagmatic to Strombolian at the end. We conclude that even with very small magma volumes, monogenetic eruptions can have very complex histories, as shown by the WMV, because of the taping system, multiple vents, change in eruptive style, vertical and lateral migration of eruption foci, and multiple magma batches.