PROPOSED HYDROSTRATIGRAPHICAL CLASSIFICATION AND NOMENCLATURE - APPLICATION TO THE PALEOZOIC IN SAUDI-ARABIA

Despite the need of stratigraphers and hydrogeologists for a hydrostra tigraphical classification, such widely accepted classification is lac king and was ignored by the 1983 code of the North American Commission on Stratigraphical Nomenclature (NACSN). This study is an attempt to fill this' vacuum. A simple and universally applicable hydrostratigrap hical classification scheme is introduced here which takes into consid eration the physical properties of the rocks, especially porosity and permeability, in addition to other variables such as thickness of the unit and its areal extent. The proposed hydrostratigraphical classific ation is a hierarchical scheme composed of two types of hydrostratigra phical units: aquizones and aquitards, which differ significantly in t heir intrinsic permeabilities. Aquizones include five ranks which are named in ascending order: subaquifer, aquifer, superaquifer, aquagroup and aquasystem. The aquifer is the fundamental unit. Aquitards are di vided into mini-aquitards, meso-aquitards and mega-aquitards differing in their thicknesses and lateral continuities. Hydrostratigraphical u nits are easier to recognise on geophysical logs than lithostratigraph ical units; they have fewer boundaries and therefore, it is easier for hydrogeologists to identify them. To test the applicability of the pr oposed classification the Palaeozoic succession of Saudi Arabia has be en chosen to illustrate such an application. The stratigraphical inter val between the Precambrian Arabian Shield aquifuge below and the Lowe r Triassic Sudair mega-aquitard above is named here the Najd Aquasyste m, whose boundaries largely coincide with those of the Palaeozoic Erat hem. The Najd Aquasystem, in turn, is divided into two aquagroups call ed the Buraydah below and the Widyan above and separated by the 600 m- thick Qusaiba Mega-aquitard. The Buraydah Aquagroup is composed of two superaquifers: the Saq and the overlying Hail, whereas the Widyan Aqu agroup is divided into two superaquifers named the Jalamid below and t he Rafhah above. Each of these superaquifers is composed of two named aquifers separated from each other by aquitards of different ranks. th e hydrogeological characteristics of each aquifer were briefly discuss ed.