Reconstructing the Cratering process of the Monoun Maar Volcano (Cameroon Volcanic Line): Insights from Volcanic Facies and componentry analysis

The Monoun Maar Volcano (MMV), located on the Cameroon Volcanic Line (CVL), is a small-volume polygenetic volcano characterized by eruptive activity and interaction with CO ₂ -rich fluids. This study presents a revised tephrostratigraphy of the MMV, integrated with grain-size and componentry analyses, to reconstruct its eruption sequence, tephra emplacement mechanisms, and cratering processes. The MMV forms a complex, sigmoidal tephra ring surrounding a 0.62 km ² maar. Seven stratigraphic units record a multi-phase eruptive history. Activity began with emplacement of a pyroclastic surge (U1), followed by oxidized scoria-fall deposits (U2) and a lava flow (U3). After a repose interval, eruptive loci migrated and produced a surge-dominated sequence with siderite-coated clasts (U4), followed by a lithic-rich tuff breccia (U5), a fresh scoria-fall deposit (U6), and a final surge sequence (U7). Grain-size data from unconsolidated beds analysed from Units 4 to 7 indicate polymodal distributions between − 4.3ϕ and 3.6ϕ, poor to very poor sorting, and median diameters ranging from − 2ϕ to -1ϕ. Componentry data show average proportions of 19.4 vol.% lithic clasts, 21.4 vol.% fresh juvenile clasts, and 59.2 vol.% recycled juvenile clasts. Stratigraphic relationships, sedimentary structures, and componentry variations indicate a two-stage evolution of the MMV. The first stage corresponds to emplacement of Units 1–3 and construction of the western sector of the crater system. The second stage involved renewed activity in the central and eastern sectors, as reflected by a paleosol, angular discontinuities, and the emplacement of Units 4–7 in the south-eastern and south-western sectors. The irregular sigmoidal morphology of the maar, together with paleosols, angular discontinuities, and three eruptive basins of variable depth, indicates prolonged vertical and lateral migration of the eruption loci, producing an amalgamated crater complex. A conceptual model based on fluctuations in fresh juvenile content suggests that changes in explosion loci were the principal control on temporal variations in eruptive style during Group II activity. Within this sequence, Unit 6 records the most juvenile-rich and dominantly magmatic phase, whereas Unit 4 records the least juvenile-rich and most excavational phase.