Relationship between volcanic dynamics, climate change and the thickness of the water infiltration zone within a karst context: the Pleistocene maars and tuff rings of the Middle Atlas (northern Morocco)

Hisham EL Mesbahi, J. M. Dautria, Hérvé Jourde, Olivier Alard, Philippe Munch, Jean Louis Bodinier, O Houali

    Research output: Contribution to journalArticle

    Abstract

    The small (≈500 km2) Azrou-Timadite basaltic area (Middle Atlas, northern Morocco) lies on a Liasic karst plateau with an altitude between 1,750 and 1,950 m. This area includes approximately 20 small-sized Early Pleistocene (2.5-0.8 Ma) strombolian cones and 30 Middle Pleistocene (around 0.6-0.5 Ma) hydrovolcanic edifices. While the edification of the first ones occurred when the climate was dry, the second ones formed during a wet episode. Hydrovolcanic edifices are peculiarly interesting because they display exceptional diversity in terms of morphologies and ejecta deposit mechanisms, which show an evolution from phreatic to lacustrine activity via phreatovolcanic activity, sometimes even within a single edifice. This diversity could be explained both in terms of relative depth and/or degrees of water saturation of the deep and shallow karst compartments beneath the volcanoes. During a wet climatic episode, the deep karst is probably permanently saturated with water while the epikarst has varying degrees of saturation that depends on the topography. Some rare edifices are typical maars with craters that deeply cut their basement (up to 55 m): their formation can be attributed to a magma-water interaction mainly within the deep karst compartment, with the epikarst playing a minor role. Other rare edifices are lacustrine tuff rings with gently sloping shallow craters several metres deep that imply the presence of flooded depressions, on the surface at the time of eruption, fed by the drainage of water from the epikarst. Paradoxically, one of the lacustrine tuff rings is associated with the deepest crater (80 m) in the area: this can only be explained if there was already a collapsed karst cavity occupied by a lake at the crater location. A dozen edifices, the largest ones, are complex tuff rings where base surge and fallout deposits are interbedded with mud and debris flows. For these tuff rings, we suggest a scenario involving relative low volumes of magma rising up to the surface in pulses and alternatingly interacting with water stored in the deep karst and in the epikarst, with the temporary formation of a lake inside the crater during the periods of quiescence between two pulses. Finally, it appears that the different hydrovolcanic dynamics, which developed in the Middle Atlas between 600 and 500 ka, depended mostly on the thickness of the water infiltration zone. This thickness, which is variable according to the eruption sites, probably depended on the morphology of the limestone substratum under the volcanic cover.
    Original languageEnglish
    JournalJournal of African Earth Sciences
    Publication statusSubmitted - 10 Oct 2019

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