Ezz El-din Abdel Hakim Khalaf

The Mesozoic Natash volcanics are rift-related monogenetic volcanic complex that erupted in south Eastern Desert of Egypt. The architecture of their building has evolved through three phases namely: (1) tuff cone consisting of explosive -dominated pyroclastics that stand for an intensity eruption peak; (2) the effusive lava-conquered phase, characterized by prevalent basalt-trachyte-rhyolite fissure eruptions; and (3) late-stage subvolcanic plugs/domes. The first phase began with the emplacement of spatter-rich ignimbrites followed by grain-supported lapilli-tuffs and hyaloclastics in a shallow-water environment (phreatomagmatic phase) with a vent-opening phase. This initial phase is followed by deposition of clast-supported conglomerates of lahar facies that represent significant erosional period, separating the pyroclastics and effusive phases. The second phase comprises multiphases displaying compositional stratification (primitive beneath evolved) of pahoehoe Hawaiian and rubbly vulcanian lavas, overlying the pyroclastic deposits. The third phase is the youngest and comprises plugs of aphyric trachytes. The volcanics occasionally have vesicle sheets, cylinder-elongate vesicles, gas blister, and inflation clefts, reflecting degassing, rupturing, brecciation/quenching history, and pressurized flows. U-Pb zircon ages from two samples of trachyte volcanics gave concordant age of 92 ± 0.9 Ma. One trachyte plug yielded concordant age of 86.6 ± 1.3 Ma. These ages represent two events belonging to the major Senonian volcanic activities. The investigated volcanics have alkaline affinity and display a continuous composition ranging from basalt to trachyte and rhyolite. Their chemical compositions display linear, curvilinear, and scattered trends on the Harker diagrams matched with the petrographic data and reflect comagmatic genesis for the entire sequence. Crystal fractionation of olivine + pyroxene + plagioclase + Fe-Ti oxides + apatite plays a role in the volcanic evolution. The origin of the mafic rocks is derived from deep mantle source in the garnet facies via low degree of partial melting. Such magma source is affected by fluid-rich metasomatism generated by dehydration subducted crust on the EM I, EMII, and HIMU mantle reservoir during Neoproterozoic time. The basalt-trachyte-rhyolite suites and their pyroclastics from the Natash area, together with Mesozoic alkaline volcanics from the surrounding terrains in Africa, constitute a remnant of extensive magmatisms, having large extension, high eruption rate, and OIB-type chemical composition.

Bahariya monogenetic volcanic field is characterized by important geomorphological features (geomorphosites), namely, sub-circular maar-tuff ring, scoria cones, and domal-shaped tumuli. These geomorphosites constitute an asset for geoeducation, geotourism and miscellaneous social activities. They offer important knowledge into the paleoenvironmental and climatic factors that affected the style of volcanism at the occasion, and eventually shaped the diverse landforms found in the volcanic field. Bahariya Oasis is exclusive for its excellent locations where many volcanic heritages of high value give evidence of phreatomagmatic and effusive-controlled phases which formed volcanic landscapes under humid to dry climate. The geoheritage and archeological sites of early settlements are abundant in the Bahariya Oasis, accentuating the scientific magnitude of this region. There have been seven geosites recognized such as (1) the scoria cone, (2) the lava flows and their surface morphological features, (3) the pseudopillow fractures, (4) columnar joints, (5) peperites, (6) tumuli, and (7) rootless cones. These geosites coupled with other unique sites define the Oasis as global geopark. The latter will consider as an excellent logistical network to endorse volcanic geosciences and raise the economic growth in this part of Bahariya Oasis. The diverse geological characteristics at the Bahariya make this area a high volcanic geodiversity that can be used for geoeducational programs and geotourism. Excursions and research programs carried out by universities will contribute to enhanced geoconservation for local sustainable development. Currently, in the Bahariya region, tourism is not well developed, but it is recommended that, roads be improved to give better accessibility to the geomorphosites, and interpretative panels, informative brochures, multi-media presentations, seminars and workshops, scientific lectures, and postcards be produced to inform tourists about the geology of the region.

This study reports on lava–sediment interaction focusing on the Neogene volcano-sedimentary sequence in the
Abu Treifiya Basin, Cairo–Suez district, as a detailed example. The dynamic lava–sediment interactions as
peperites happen on a variety of scales from simple sediment interbeds with the extrusive and intrusive basaltic
rocks and hydrothermal products at a large scale down to complex breccia horizons and bulbous lava–sediment
contacts at small scales. They have been identified for the first time at the Abu Treifiya Basin and can only be used
as widespread paleoenvironmental indicators with limitations to demonstrate magma and surface water nonexplosive
interaction. The study of peperite is important in establishing broad contemporaneity of magmatism
and sedimentation along with explosive hydrovolcanic hazards and this finding is significant for the reconstruction
of evolution in the study area. The basaltic lava peperites and sedimentary rocks are up to 350 m thick and
form a continuous stratigraphic section that is distributed regionally in the study area over a distance of 100 km.
Five types of peperites are described and interpreted as resulting frombasaltic lava bulldozed intowet, unconsolidated
sediments at their basal contacts. Evidence that the sediments were unconsolidated or poorly consolidated
and wet when the lava flowed over them include vesiculated sediment, sediment in vesicles and fractures in lava
flow and in juvenile clasts in the peperite and soft sediment deformation. All peperites in this study could be described
as blocky or fluidal on the basis of juvenile clast, but other shapes occur and mixtures of different clast
shapes are also found regardless of the host sediment. Blocky and fluidal clasts in the peperite display progressive
disintegration, suggesting decreasing temperature and increasing viscosity during fragmentation. Abundance of
blocky clasts with respect to fluidal clasts in the peperites indicates that the fluidalemplacement and low-volume
sediment fluidization in the early stages were immediately followed by quench fragmentation due to the high
viscosity of the magma. Sediment fluidization, formation of vapor films, magma–sediment density contrasts,
and explosive fragmentation as well as magma properties such as composition, viscosity, and vesicularity are
the main mechanisms invoked to generate the peperites. Variously combining these contrasting features to varying
degrees may form diverse juvenile clast shapes in peperitic domains. During cooling, the larger fluidal shaped
clasts settled to the base of the sequence, through the saturated sediment, producing the vertical (stratigraphic)
grading now preserved. Grading occurred, essentially, in situ during peperite formation and cannot be attributed
to remobilization or mass flow.
Peperites occur in phreatomagmatic intra-crater/conduit or vent-filling deposits and along contacts between
sediment and intrusions, extrusion, and hot volcaniclastic deposits in two environments. Carbonate–lava interactions
occur in shallow marine which changes to subaerial fluvio-lacustrine environment through the mingling
between lava flows and siliciclastic sediments during the onset of basaltic volcanism. This work suggests that
the Abu Treifiya Basin may be an important local for the study of subvolcanic phreatomagmatic processes and
associated phenomena.

This paper presents a stratigraphic and sedimentary study of Neoproterozoic successions of the South
Sinai, at the northernmost segment of the Arabian–Nubian Shield (ANS), including the Kid complex. This
complex is composed predominantly of thick volcano-sedimentary successions representing different
depositional and tectonic environments, followed by four deformational phases including folding and
brittle faults (D1–D4). The whole Kid area is divisible from north to south into the lower, middle, and upper rock sequences. The higher metamorphic grade and extensive deformational styles of the lower sequence distinguishes them from the middle and upper sequences. Principal lithofacies in the lower sequence include thrust-imbricated tectonic slice of metasediments and metavolcanics, whereas the middle and upper sequences are made up of clastic sediments, intermediate-felsic lavas, volcaniclastics, and dike swarms. Two distinct Paleo- depositional environments are observed: deep-marine and alluvial fan regime. The former occurred mainly during the lower sequence, whereas the latter developed during the other two sequences. These alternations of depositional conditions in the volcano-sedimentary deposits suggest that the Kid area may have formed under a transitional climate regime fluctuating gradually from warm and dry to warm and humid conditions. Geochemical and petrographical data, in conjunction with field relationships, suggest that the investigated volcano-sedimentary rocks were built from detritus derived from a wide range of sources, ranging from Paleoproterozoic to Neoproterozoic continental crust. Deposition within the ancient Kid basin reflects a complete basin cycle from rifting and passive margin development, to intra-arc and foreland basin development and, finally, basin closure. The early phase of basin evolution is similar to various basins in the Taupo volcanics, whereas the later phases are similar to the Cordilleran-type foreland basin. The progressive change in lithofacies from marine intra-arc basin to continental molasses foreland basin and from compression to extension setting respectively, imply that the source area became peneplained, where the Kid basin became stabilized as sedimentation progressed following uplift. The scenario proposed of the study area supports the role of volcanic and tectonic events in architecting the facies and stratigraphic development.