Eruptive history of the Barombi Mbo Maar, Cameroon Volcanic Line, Central Africa: Constraints from volcanic facies analysis

Abstract

his study presents the first and detail field investigations of exposed deposits at proximal sections of the Barombi Mbo Maar (BMM), NE Mt Cameroon, with the aim of documenting its past activity, providing insight on the stratigraphic distribution, depositional process, and evolution of the eruptive sequences during its formation. Field evidence reveals that the BMM deposit is about 126m thick, of which about 20m is buried lowermost under the lake level and covered by vegetation. Based on variation in pyroclastic facies within the deposit, it can be divided into three main stratigraphic units: U 1 , U 2 and U 3 . Interpretation of these features indicates that U 1 consists of alternating lapilli-ash-lapilli beds series, in which fallout derived individual lapilli-rich beds are demarcated by surges deposits made up of thin, fine-grained and consolidated ash-beds that are well-defined, well-sorted and laterally continuous in outcrop scale. U 2 , a pyroclastic fall-derived unit, shows crudely lenticular stratified scoriaceous layers, in which many fluidal and spindle bombs-rich lapilli-beds are separated by very thin, coarse-vesiculatedash-beds, overlain by a mantle xenolith-and accidental lithic-rich explosive breccia, and massive lapilli tuff and lapillistone. U 3 displays a series of surges and pyroclastic fall layers. Emplacement processes were largely controlled by fallout deposition and turbulent diluted pyroclastic density currents under "dry" and "wet" conditions. The eruptive activity evolved in a series of initial phreatic eruptions, which gradually became phreatomagmatic, followed by a phreato-Strombolian and a violent phreatomagmatic fragmentation. A relatively long-time break, demonstrated by a paleosol between U 2 and U 3 , would have permitted the feeding of the root zone or the prominent crater by the water that sustained the next eruptive episode, dominated by subsequent phreatomagmatic eruptions. These preliminary results require complementary studies, such as geochemistry, for a better understanding of the changes in the eruptive styles, and to develop more constraints on the maar's polygenetic origin.

Temporal Evolution of the Barombi Mbo Maar, a Polygenetic Maar-Diatreme Volcano of the Cameroon Volcanic Line

“…A well-distinct 10 cm-thick paleosoil bed, which is indicative of relatively long quiescent period, separates the precedent units from the upper unit U 3 . From those observations, it was suggested that the BMM might be a polygenetic maar [7]. However, no systematic geochronological evidence existed to comfort that observation.…”

“…Recently, from a tephrostratigraphic examination, the about Figure 1. Location map of the study area along the Cameroon volcanic line (A) and the geological map (B), modified after [7]. (C) shows the sampling points (stars) around the BMM and the adjacent maar, and the position of the limburgitic lava flow (black dot).…”

“…Two other ages of 0.20 and 0.24 Ma were obtained on two lherzolite bearing trachy-basalts collected in the BMM pyroclastic sequence [9]. Although those young (relative to the 1 Ma) ages clearly suggest that volcanic activities at the BMM were more recent, they do not say anything about either the first or the last eruptive phase on the BMM or even on its potential polygenetic nature [7]. Moreover, during a recent field investigation, another likely poorly developed soil-bed was identified in the upper part of the BMM's pyroclastic deposits sequence [Chako Tchamabé, personal communication], implying that the eruptive history of the BMM could be more complex than previously thought [7].…”

“…The second unit (U 2 ), which is about 12 m-thick, consists of a scoria-rich layer containing various centimeter-to decimeter-sized spatter bombs, vesiculated scoria, few mantle xenoliths and country rocks; a lithic and mantle xenolith rich-explosive breccia; and two compact and consolidated layers of lapillituff and lapillistone. The upper unit U 3 comprises fallout and pyroclastic density current derived-beds piled up repetitively for about 90 m. Units U 1 and U 2 were thought to have been deposited during a volcanic episode that initiated the BMM formation [7]. A well-distinct 10 cm-thick paleosoil bed, which is indicative of relatively long quiescent period, separates the precedent units from the upper unit U 3 .…”

“…126 m-thick tephra pile preserved at its eastern flank was subdivided into three stratigraphic units U 1 , U 2 and U 3 [7]. The first unit (U 1 ) consists of about 26 m-thick series of alternating unconsolidated and consolidated sets of thinly and well-stratified lapilli-and ash-beds.…”