Proposed hydrostratigraphical classification and nomenclature: application to the Palaeozoic in Saudi Arabia

Despite the need of stratigraphers and hydrogeologists for a hydrostratigraphical classification, such widely accepted classification is lacking 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 hydrostratigraphical classification scheme is introduced here which takes into consideration 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 classification is a hierarchical scheme composed of two types of hydrostratigraphical units: aquizones and aquitards, which differ significantly in their 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 divided into mini-aquitards, meso-aquitards and mega-aquitards differing in their thicknesses and lateral continuities. Hydrostratigraphical units are easier to recognise on geophysical logs than lithostratigraphical units; they have fewer boundaries and therefore, it is easier for hydrogeologists to identify them.To test the applicability of the proposed classification the Palaeozoic succession of Saudi Arabia has been chosen to illustrate such an application. The stratigraphical interval between the Precambrian Arabian Shield aquifuge below and the Lower Triassic Sudair mega-aquitard above is named here the Najd Aquasystem, whose boundaries largely coincide with those of the Palaeozoic Erathem. The Najd Aquasystem, in turn, is divided into two aquagroups called 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 Aquagroup is divided into two superaquifers named the Jalamid below and the Rafhah above. Each of these superaquifers is composed of two named aquifers separated from each other by aquitards of different ranks. the hydrogeological characteristics of each aquifer were briefly discussed.