Outcrop-subcrop sequence and diagenesis of upper jurassic arab-hith formations, Central Saudi Arabia

الملخص EN

Outcrop-subcrop of the Upper Jurassic Arab-Hith Formations in central Saudi Arabia were studied in detail.

The Arab Formation is mostly poorly exposed owing to the extensive slumping, collapsing and breeciation resulted from the dissolution of the interbedded anhydrite units.

Except for an excellent exposure at Dahl Hit and limited exposures to the east of Al-Hair and Al-Badi (Fig.

3).

the Hith outcrop had been removed totally by underground water solution of the anhydrite.

A composite stratigraphic sequence of the Arab and Hith was established from the outcrop-subcrop relationship.

Four carbonate units have been recognized; the basal unit is generally persistent and undisturbed, whereas three other units are mostly made of slumped, contorted and fractured limestones as well as of breeciated limestone.

While most of the coarsegrained limestone of the Arab in the outcropping area underwent leaching and reerystallization, the rocks in the subcrop were leached, recrystallizcd and dolomitized.

Dissolution of the Arab-Hith anhydrite units has caused seriousgeotech-nieal problems such as slumping, breeciation, cavities, vugs and fractures of the rocks in the outcrop-subcrop areas.

Presence of vertical fractures and possible presence of permeability barriers to the east of the Sulaiy (Lower Cretaceous) escarpment seem to have caused the rise of the underground water table in the Riyadh area, which may further introduce potential health hazards by vertical charging of the sewage water into the underlying Riyadh Aquifer.