Mid-Tertiary volcanism in the Northeastern Desert of Egypt was associated with the
African-Arabian plate rifting and opening of the Red Sea, a period of 30 Ma (Late
Oligocene). The resulted lava flows, predominately basaltic sheets, have mingled with
the underlying sediments, forming peperite along the contact between them. Field geology,
petrography, and geochemistry methods have been used to identify the behavior of the
lava-sediment interaction which produced peperite. Two types of peperite have been
recognized, fluidal at Naqb Ghul and El-Yahmum areas, and blocky at El-Qattamiya area.
Sometimes, the two types are represented together at the El-Qattamiya area. Fluidal
peperite is characterized by globular and equant juvenile clasts and with irregular
or ragged margins suggesting low viscosity, ductile fragmentation, and sediment fluidization.
Blocky peperites are characterized by angular, polyhedral, platy juvenile clasts and
a jigsaw-crack texture, resulted from quenching of magma in a brittle regime. The
coexistence of fluidal and blocky clasts together reflects progressive disintegration
and suggests decreasing temperature and increasing viscosity during fragmentation.
The presence of sediments in vesicles and in fractures in the peperite zone indicates
a non-explosive phase of interaction. Geochemical data suggests that different composition
across the magma/sediment boundary is a result of silicification and sediment fluidization.