Growth, destruction and facies architecture of effusive and explosive volcanics in the Miocene Shama basin, southwest of Saudi Arabia: Subaqueous–subaerial volcanism in a lacustrine setting

The Harrat Shama Volcanic Basin (HSVB) is a part of a small, well-exposed intra-continental extensional basin that formed during the opening of the Red Sea, containing 5 km of Miocene bimodal volcanics and volcaniclastic rocks. The Shama basin accumulated a thick fluvio-lacustrine fill in which two distinct volcanic sequences and their deposits can overlap with each other. In addition, complete facies architectures of the Shama volcanics have been recognized providing a complex mixed siliciclastic and volcaniclastic basin infill in the respective basin where volcanism took place. The lower sequence is composed of hyaloclastites, zeolite-bearing bedded tuffs, and bedded accretionary-lapilli-tuffs and an upper sequence, is made up of pumiceous lapilli-tuffs and peperitic breccias capped by basaltic lava flows. The former is interpreted to have been dominated by discrete, phreatomagmatic fall deposits, which are attributed to an overall high eruption rate in a lacustrine setting, followed by a dominantly subaerial pumiceous lapilli tuff deposits and volcaniclastic sediments sited in the upper part of the basin with paleosols and/or fluvial deposits in between the two sequences. These deposits could be related to polygenetic volcanoes and tectonic structures, such as faults and rift-zones. These two sequences display a complex succession of effusive and explosive volcanisms and their reworked deposits, with abundant evidences of magma–water interaction such as peperites for non-explosive magma–water interaction with the lacustrine water-saturated sediment and standing water body in a lake environment. The difference eruption dynamics and fragmentation mechanisms between the two sequences reflect progressive environmental changes from subaqueous or watery to subaerial or dry. Fluvial erosion and deposition completed the evolution of the emergent marginal part of the Shama basin. ama basin then experienced volcano growth and degradation that formed the two sequences; NW-SE-trending basement faulting triggered multiple flank collapses and volcanic debris avalanches, and voluminous pumiceous lapilli-tuff eruptions produced a caldera (upper sequence). Lacustrine conditions persisted during the destruction and post-destruction stages of the volcanoʹs evolution, as evidenced by magma–water interactions. Shama basin is a small-volume volcano, similar to tuff rings; however, its magma compositions, complex eruption styles, and inter-eruptive breaks suggest, that it closely resembles a volcanic architecture commonly associated with large, composite volcanoes. The main cause of such complex eruptive behavior resides in the stratigraphic, structural, and hydrogeological characteristics of the substrate above which the volcanoes were emplaced, rather than on the compositional characteristics of the erupting magma, which do not show significant variation among the different deposits.