اثار وسط المملكة العربية السعودية دراسة الادوات والمنشات الحجرية-غيرملون

Chapter 1
Geology, Geomorphology and Palaeoclimate
1.1 Introduction
This chapter looks into the physical and environmental aspects of central Saudi Arabia, which contain the study area, in particular and the Arabian Peninsula in general. A summary of known information on these aspects is given here in a non-technical manner. Quaternary palaeoclimatic conditions are also investigated in the Arabian Peninsula and surrounding regions either through observed phenomenon, i.e. deep erosion channels, or by the study of deep-sea cores in the Arabian Sea and the Red Sea or lake levels in Africa and Arabia.
From a geological viewpoint, the Arabian Peninsula and north Africa represent the remnant of an ancient continental landmass in the southern hemisphere, know as Gondwanaland (Beaumont et al 1976).
On the basis of the sedimentary record, the initial stage of the formation of the Red Sea is thought to date before the middle Oligocene (32 million years BP), when the Red Sea was a shallow sea extending southward from the "Tethyan Ocean" on to the Nubian- Arabian plate (Gass 1977). At the beginning of the Miocene period,
about 26 million years BP, the rift of the Red Sea, which is a proto-ocean in an early stage of development (about 20 million years BP), has undergone a drastic change thought to have been triggered during the onset of a major evaporite formation period; on the basis of the presence of evaporates 4-5 km thick (op. cit.; Kinsman 1975; Veevers 1981).
During the Miocene period, approximately 25 to 12 million years ago, the Arabian plate broke off from the African shield, and as a result, the Red Sea was formed.
Geologically, The Arabian Penin-sula itself is a thick crustal plate composed of ancient sedimentary and volcanic rocks. It can be divided into two major geological provinces (Figure 1.1):
First. The Precambrian Basement Com - plex (the Arabian Shield) which includes the western and central parts of Arabia, occupies an area of about 77,000 Km2. It is composed of metamorphosed, hig-hly deformed sediments intruded by granites (Abd el Rahman 1986; Fisher 1978; Powers et al 1966). Its slightly-arched surface is a peneplain which declines grad-ually to the north, northeast and east and underlies a thin sedimen-tary cover in the eastern part of Arabia (Chapman 1978).
Second. The Eastern Sedimentary Basin (the Arabian Shelf) occupies the eastern part of Arabia and constitutes around two thirds of the total area of the peninsula. The sedimentary formations of the Arabian Shelf have an average thickness of about 5,000 m. They start with the Cambrian and include all the geological forma-tions that follow including recent deposits (Abd el Rahman 1986). On the Arabian Shelf, a sequence of continental and shallow- water marine sedimentary rocks dip gently away from the Arabian Shield and into a number of deep sedimentary basins (Chapman 1978).
1