Outcrop-Subcrop Sequence and Diagenesis of Upper Jurassic Arab-Hith Formations, Central Saudi Arabia

Abstract

Outcrop-subcrop of the Upper Jurassic Arab-Hith Forma- f tions in central Saudi Arabia were studied in detail. The Arab Formation is t mostly poorly exposed owing to the extensive slumping, collapsing and i brecciation resulted from the dissolution of the interbedded anhydrite units. Except for an excellent exposure at Dahl Hit and limited exposurest o :I the east of AI-Hair and Al-Badi (Fig. 3),. the Hith outcro~ had been re- I moved totally by underground water solutIon of the anhydnte. 1 A composite stratigraphic sequence of the Arab and Hith was established I: fro~ the outcrop-subc~o~ relationship. Fo.ur carbonate u~its have been recogrnzed; the basal UnIt IS generally persIstent and undIsturbed, whereas .' three other units are mostly made of slumped, contorted and fractured 1, limestones as well as of brecciated limestone. While most of the coarse- I grained limestone of the Arab in the outcropping area underwent leaching t and recrystallization, the rocks in the subcrop were leached, recrystallized I anddolomitized. 1 Dissolution of the Arab- Hith anhydrite units has caused serious geotech- Ii nical problems such as slumping, brecciation, cavities, vugs and fractures of I the rocks in the outcrop-subcrop areas. Presence of vertical fractures and .J po~sible presence of permeability barriers to the east of the Sulaiy (Lower r 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.