Volcano-sedimentary characteristics in the Abu Treifiya Basin, Cairo–Suez District, Egypt: Example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn-volcanic basaltic rocks

Khalaf, E. A., A. A. Motelib, M. S. Hammed, and E. A. H. Manawi, "Volcano-sedimentary characteristics in the Abu Treifiya Basin, Cairo–Suez District, Egypt: Example of dynamics and fluidization over sedimentary and volcaniclastic beds by emplacement of syn-volcanic basaltic rocks", Journal of Volcanology and Geothermal Research, vol. 292, pp. 1-28, 2015.


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.

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