Stratigraphy and lithology of Miocene silicic volcanics in the Shama Area, Saudi Arabia

الملخص EN

Volcanic rocks of Jabal Shama and Harrat Shama are considered to be remnants of Early Miocene volcanism related to the opening of the Red Sea rift.

This type of volcanism is characterised by the local occurrence of felsic lava flows and pyroclastics.

The upper part of the succession is composed of slightly silicified flow-banded rhvolite; brec- ciated and devitrified lavas are also common.

Beneath the rhyolite is the Green Tuft unit, comprising two main lithologic types : 1.

At the base of the sequence, massive to brecciated felsic lava How.

strongly hydrother- malised.

overlies locally exposed basalt.

Perlite lenses and layers, corresponding to hydrated vitrophyric lavas and showing various textures, are commonly intercalated with hydrotnor- malised felsic lava flows.

2.

Extensive ground surge deposits and pumice flows, the representatives of ignimbritie explosive volcanism at distant centres or vents, from the upper part of this unit.

The Green l uff unit is.

in general, poorly exposed elsewhere, but has been preserved in the Shama area by the overlying felsic lava flows.

An erosional unconformity at the top of the formation and the lack of intense hydrothermal alteration in the upper volcanics indicate a pause in volcanic ac- tivity between the deposition of these two main volcanic units.