Tertiary monogenetic volcanism in the Gabal Marssous, Bahariya Depression, Western Desert, Egypt: implication for multi-phases, mafic scoria cone suite related to Red Sea rift in the Afro-Arabian realm

Abstract

An integrated stratigraphic, sedimentological, volcanological, and geochemical studies were conducted for the first time on the eruptive products of the Marssous volcano, Bahariya Depression, Western Desert, Egypt. The rarity of complex volcanological studies in the west Red Sea rift makes these investigated volcanoes important as they offer a clue to the style of volcanism, eruptive environment and magma genesis during magmatism complimentary to those areas extensively studied in the Arabian Peninsula toward Syria and Eastern Turkey. The Marssous volcano is small monogenetic scoria cone that shows a polyphase feeding system, consisting of unconformable superimposed characteristic effusive-to-explosive eruptive units, suggesting a wide spectrum of diverse eruptive styles through a complex feeder network. On the basis of the sedimentological characteristics, field relationships, lava flow textures and granulometric indicators, five volcanic units have been identified from the base to top as (1) coherent porphyritic massive basalts (Bpm), (2) stratified tuff beds (Unit 1), (3) crude-bedded lapilli tuff beds with bombs (Unit 2), (4) massive agglutinate beds (composed of spatter and fluidal bombs) formed by lava fountains (Unit 3), (5) porphyritic vesicular basalts (Bpv), and (6) subvolcanic feeder intrusions. The degree of vesicularity and the size of the clasts increase from thin Unit 1 (ash to lapilli) through more thick Unit 2 (lapilli-bomb) to Unit 3 (breadcrusted scoriaceous bomb) as an indicative of an increased magma flux and the high eruptive energy together with an enlarged of degassing fragmentation. There is consequently a progressive evolution from an initial Phreatomagmatic explosive stage followed an initial effusive event to dry magmatic explosive (Strombolian & Hawaiian) and effusive eruptive styles in the later. With eruption progression, the external water to fuel Phreatomagmatism was diminished relatively early in the eruptions giving way to accumulate a pyorclastic fall deposition of Strombolian to Hawaiian lava-fountain episodes together with effusive eruptions, all together forming the majority of the pyroclastic and effusive successions of Gabal Marssous. These eruptive phases have happened during a continuous deposition without any time pauses in a short period of time as a result of a single cone-forming eruptions. The Marssous volcano shares resemblances in terms of inferred eruption style and structures with other scoria cones elsewhere in the broad regional context such as North Africa, Mediterranean province, and Arabian Peninsula, and thus provides an outstanding field laboratory to explore scoria cones architecture and growth from a global perspective. This volcano event is a key tectono-stratigraphic marker for an early manifestation, coinciding with the initiation of Red Sea rifting opening.