THE NUIZIAN ARTESIAN BASIN, ITS REGIONAL HYDROGEOLOGICAL ASPECTS AND PALAEOHYDROLOGICAL RECONSTRUCTION

Transcription

1 THE NUIZIAN ARTESIAN BASIN, ITS REGIONAL HYDROGEOLOGICAL ASPECTS AND PALAEOHYDROLOGICAL RECONSTRUCTION I. H. HIMIDA Water Resources Division, Desert Institute, Cairo, United Arab Republic. SUMMARY In the present study different geological, hydrogeological and hydrogeochemical data are used for interpretation of the regional hydrogeological aspects and evolutionary history of one of the most extensive artesian basins in the world, in North-East Africa Hydrogeochemically, the area of the artesian basin is subdivided into three main regions that contain different genetic types of ground water. Many hydrogeochemical zones are recognized and described within the areas of these hydrogeochemical regions. In the evolutionary history of the artesian basin three main stages are outlined, the most important of which extends from the middle of the Early Cretaceous up to the end of the Pliocene period. For the interval from the Late Eocene till the present time the hydrogeological aspects of the artesian basin are illustrated by a series of palaeohydrogeological schematic maps. Many of the important hydrogeological aspects of the area of study are explained on the basis of a palaeohydrogeological reconstruction of the artesian basin. RÉSUMÉ LE BASSIN ARTESTEN DE NUBIE-DESCRIPTION DE SES REGIONS HYDRO- CEOLOGIQUES ET RECONSTITUTION PALEOHYDROGEOLOGIQUE Dans la présente étude, l auteur a utilisé différentes données géologiques, hydrogéologiques et hydrogéochimiques pour donner une interprétation des aspects hydrogéologiques régionaux et ppur retracer l histoire de l évolution de l un des bassins artésiens les plus btendus du monde, au nord-est de l Afrique. Du point de vue hydrogéochimique, ce bassin artésien se subdivise en trois régions princpales où la genèse des eaux souterraines a été différente. A l intérieur de ces régíons, l auteur dist.ingue de nombreuses zones hydrogéochimiques dont il donne une description. Retraçant l évolution du bassin artésien, il donne un aperçu des trois stades principaux dont le p!us important va du milieu du crétacé inférieur à la fin du pliocènc. Pour la période allant de I éocène supérieur à l époque actuelle, les caractéristiques hydrogéologiques du bassin artésien sont illustrées par une série de cartes schématique paléohydrogblugíques. Nombre de caractéristiques hydrogéologíques importantes de la zone étudiée sont expliquées à partir d une reconstitution paléohydrogéologique du bassin artésien. The present study deals with the regional hydrogeological and palaeohydrogeological aspects of an extensive artesian basin that covers nearly the whole territory of the Libyan Desert as well as a part of the desert area extending to the east of the Nile Valley (the Eastern Desert). The Libyan Desert constitutes the north-eastern part of the African Continent. It is one of the most arid regions in the world, where on the greater part of the desert area the amounts of rainfall and, consequently, runoff are practically negligible. Previously, it was common to give the name Sahara to the whole desert area extending in North Africa. Since the beginning of the 20th century, it has been the custom to restrict the name Sahara to the western part of this desert area, while the eastern part has been given the name the Libyan Desert. The western boundaries of the Libyan Desert, which separate it from the Sahara, can be located along the series of highlands running north-west of El Fashir and 3 70

2 The Nubian Artesian Basin - regional hydrogeological aspects consisting of the Ennedi, Eridi, Tibesti, and Tummo Plateaus and the Hammadas desert area of Fezzan. The eastern side of the Libyan Desert is well defined by the Nile Valley. The southern regions of the desert are limited by the northern reaches of Darfur and Kordofan in Sudan, while the desert area extends northwards until it reaches the Mediterranean. The whole area described above constitutes one huge unbroken tract of true desert area which is characterized by extreme aridity. The only exception is the narrow littoral zone extending along the Mediterranean coastal area and characterized by scanty rainfall, seasonal irregular runoff and poor plant cover. However, moving southwards, the amount of rainfall rapidly diminishes and the plant cover becomes so scarce that within about km from the coast the desert area is totally devoid of any plant cover. The only islands of life within the interior of this huge desert area are the oases regions which are located in a series of depressions in the desert plateau and owe their existence exclusively to the ground-water resources. The most important of these oases are Kharga, Dakhla, Farafra, Bahariya and Siwa Oases in the Egyptian part of the Libyan Desert and Ghabub and Kufra in the Libyan part. Since 1957, much attention has been paid to developing the oases regions in the Western Desert of the U.A.R. Development programmes are included in what is called the New Valley Project. This is a great project of land reclamation in the oases regions, where water supplies are planned to be totally obtained from ground-water resources. Accordingly, pre-development studies dealt mainly with the problems of evaluating the ground-water potentialities in these regions. Two hundred and eight-four exploratory water wells, ranging in depth from 400 m to 1200 m, were drilled in the different oases regions during the last 10 years for the purpose of carrying out subsurface geological studies and conducting various hydrological observations. In addition to these wells a number of exploratory oil wells and water wells had previously been drilled by the oil companies in different regions of the Libyan Desert. These wells provide an important source of information about the subsurface geology and the hydrogeology of the northern part of the Libyan Desert, where some of the wells penetrated a sedimentary section more than 5000 m in thickness, Many types of survey were carried out in different parts of the Libyan Desert of the U.A.R., with concentration on the oases regions. These included topographical surveys of different scales, geological surveys, gravimetric surveys, and aeromagnetic surveys, in addition to electric well logging and conduction of electric profiles in some areas. Hydrological tests and observations included periodic measurement of ground-water levels and discharges, periodic analysis of ground-water samples, and the carrying out of pumping tests for some wells. The present study includes analysis and interpretation of the results of the above-mentioned research with the purpose of establishing the regional hydrogeological aspects of the ground-water basin extending below the surface of the Libyan Desert, and of reconstructing its hydrogeological conditions in the different geological periods throughout its formation. This palaeohydrogeological approach proved to be of great help in tackling the problems of origin and formation of the ground water of the Libyan Desert.. PHYSIMIZAPHY The Libyan Desert exhibits distinctive physiographic aspects which differ considerably from those of both the Sahara region extending on the western side and the Arabian 37 1

3 I. H. Himidu Desert lying to the east of the Nile Valley. It is generally characterized by the following main aspects: (i) extreme aridity, (ii) absence of drainage lines, (5) uniformity. The Libyan Desert, together with the Sahara and Arabian Deserts, is included in the great arid belt that extends from the Atlantic Ocean through North Africa and the Near and Middle East and ends in South-East Asia. However, the Libyan Desert is considered the most arid region within this belt. Both the Sahara region and the Arabian Desert are characterized by a more, mild climate than the Libyan Desert. An annual amount of rainfall ranging between 150 and 200 mm precipitates seasonally on the coastal zone of Libyan Desert. Other than in the coastal area, the amount of rainfall becomes less than 20 /year on the vast interior territory of the desert area. Appreciable amounts of rainfall can be recorded in the Central Sudan, where it increases considerably southwards, and also on the highlands bordering the Libyan Desert on its south-western and western sides. Besides the extreme aridity, other distinctive features of the Libyan Desert are its uniformity and the absence of drainage lines. The whole interior of the Libyan Desert is entirely flat and lacks any sign of drainage lines belonging to a comparatively recent age. Drainage lines are abundant only at the peripheries of the desert plateau, especially in the coastal area. Both the uniformity of the desert surface and the absence of drainage lines are features resulting from the extreme aridity of the area. Physiographic Regions of the Libyan Desert In spite of the apparent homogeneity and uniformity of the Libyan Desert, closer examination of the desert surface permits the recognition of six physiographic regions, each of which exhibits a particular landscape, They are briefly described as follows: 1) The Northern Mediterranean Coastal Plain: this is a relatively narrow plain running in an east-west direction parallel to the coast line. It is bounded at its southern side by a low escarpment rising about 100 m above sea level. The coastal plain is characterized by the occurrence of a series of elongated ridges oriented in the same direction as the plain and alternating with shallow depression areas. Such ridges are composed of oolitic limestones, and are described as consolidated ancient littoral dunes, 2) The Marmarican Homoclinal Plateau; forming the extreme northern area of the Libyan Desert plateau. It extends from the Nile Delta westward to Cyrenaica and from the North Mediterranean Coastal Plain southward to the Qattara Depression. This is an almost flat plateau sloping regionally both to the north and to the west and rising from 300 m to 400 m above sea level. It consists mainly of Middle Miocene intercalations of limestones and sandstones. 3) The Limestone Plateau; covering an extensive area to the west of the Nile Valley. It is a plateau consisting of compact limestones of Eocene age underlain by softer formations of Cretaceous age. It extends southward until it ends by an escarpment about 300 m high overlooking the Sandstone Plateau. 4) The Sandstone Plateau; sometimes referred to as the Sandstones Wide Plains (Shata, 1967). It covers more than half the area of the Libyan Desert and forms an extensive plateau extending from the Nile Valley in the east to the Tibesti and Ennedi highlands in the west and from Kordofan region on the south to the Limestone Plateau in the north. This plateau consists essentially of rocks of Nubian Series. 5) The Great Sand Sea: this is the largest sand area in the world. It covers an extensive area on the extreme western frontiers between Egypt and Libya. 6) The Libyan Desert Depressions: these are natural excavations in the desert 3 72

4 The Nubian Artesian Basin - regionul hydrogeological aspects plateau ranging in depth from 1 O0 m to 400 m. They are generally classified into three groups. The northern group of depressions includes the Qattara (the largest depression in the Libyan Desert), Moghara, Qarra, Siwa and Ghabub Depressions; the middle group includes the Bahariya and Faiyum Depressions while the southern group includes the Farafra, Kharga and Dakhla Depressions. The depressions of the Libyan Desert, according to most of the recent authors, owe their origin to tectonic factors at the first stages of their development. These tectonic factors, later on, were supplemented by weathering and erosion action along lines of weakness. Among the above-mentioned physiographic regions of the Libyan Desert, the depression areas are especially interesting as they include the oases regions and the areas liable to be reclaimed. GEOLOGY General Regional Geological Aspects The Libyan Desert, in respect of its regional geological aspects, represents a huge monoclinal structure where the older formations always outcrop at the southern localities, whereas, moving northwards, they generally increase in thickness and disappear under the younger formations. Along the coastal zone of the Red Sea, as well as in the most southern regions of the Libyan Desert, are outcrops of rocks belonging to the crystalline basement complex which are composed of granites, granodiorites, diorites, gneisses, schists, etc. The age of these rocks in most localities is assigned to the Precambrian. However, in many localities intrusive rocks of younger ages are recorded (see Fig. 1). Moving from the southern localities of the Libyan Desert northwards, the rocks of the basement complex are gradually overlain by a series of mostly unfossiliferous formations consisting of sands, sandstones, clays and shales which are commonly termed the Nubian Series. The Nubian Series varies in thickness from some few tens of metres in the northern regions of Sudan to about 250 m in the southern localities of Kharga Oasis, 900 m in the northern localities of the same oasis, and about 1800 m in Bahariya Oasis, while it attains a thickness of more than 3500 m in the northern localities of the Libyan Desert. Regionally, the Nubian Series changes gradually from mainly continental sandy facies in the southern regions of the Libyan Desert to intercalations of sands, sandstones and clays of alternating continental and shallow marine facies in the central regions of the desert area, to mainly marine facies in the northern regions where it consists of thick beds of clays intercalated by beds of limestones, dolomites and sandstones. The Nubian Series in most regions of its extension in the Libyan Desert lacks any guide fossil by which it can be stratigraphically subdivided. However, in the northern regions of the Libyan Desert, where marine facies dominate, a limited number of fossil species was recoroed in the Nubian Series penetrated by the deep exploratory oil wells. According to these findings, the section of the Nubiaa Series in these regions proved to comprise rock formations of Cambrian, Devonian, Carboniferous, Jurassic, and Lower and Upper Cretaceous ages. Generally, it may be concluded that the Nubian Series, by their stratigraphical position, range from the Cambrian to the Upper Cretaceous. To the north of the Kharga Oasis region, the Nubian Series are overlain by rock series of proper marine facies consisting of phosphate beds, chalky limestone, and 3 73

5 - I. H. Himidu I!I I Y Y Y Y Y LI-- 374

6 The Niibiaii Artesian Basiil - regioid hydrogeologicut nspecrs LEGEND TO FlG.1 Quaternary deposits. Miocene rocks, limestone. sandstone. and sandy clays. Palaeocene rocks (Palaeocene-Eocene). clays. shales and limestone. Senonian rc.:ks. chalk and limestone. Marstrichtian rocks. purplish clays in Kharga and Dakhla Oascs. Ccnomanian rocks, sandsstone, sometimes with clayey intercalations Lower CretacccÜs rocks, sandstone, with clay intercalations and limestone. Jurassic rocks. sandstcne, clays and shales. Paleozoic and Mesozoic undifferentiatcd rocks. sandstcae, with dolomite intercalations. Palaeozoic (Cambrian and Devonian?) rocks? sandstone. with siltstones. igneous and metamorphic rocks of basement complex. Zcne or mainly fresh water of infiltration corigin with mineralization lower than 1 g/l. Zone or brackish water of mixed origin within the Nubian in the region of Qattara Depression. Zone of highly saline ground water and brines of originally sedimentation origin. Zone of salty and saline grcund water in the Post-Nubian sediments in the northern part of the artesian basin. Fault. shales of Upper Cretaceous and Palaeocene ages. In Farafra Oasis, the Nubian Series are overlain by Upper Senonian chalk and crystalline limestones. Shales, limestones, chalky Iimestones and argillaceous limestones of Palaeogene age cover a relatively extensive territory of the Libyan Desert. In the southern regions of the Libyan Desert, Palaeogene formations (generally) conformably overlie the Upper Cretaceous formations, while in the northern regions (at least partially) the Upper Cretaceous formations are overlain by Palaeogene rocks. Continental, freshwater and marine deposits of Oligocene age consisting of coarse-grained sandstones, conglomerates and shales occupy only a very limited area of the northern region of the Libyan Desert. At the southern localities of the Siwa Oasis region, Palaeogene formations gradually disappear underneath the Miocene formations. Marly limestones, sandstones, and shales of proper marine facies overlie the Oligocene deposits in the northern part of the Libyan Desert. These Miocene rocks constitute the Marmarican Homoclinal Plateau. Pliocene deposits are recorded in some regions in the most northern part OC the Libyan Desert and the Nile Valley. They consist of both continental and marine deposits composed of sands, clays, gravels and conglomerates. They are largely developed in the form of elevated beaches in the northern part of the Libyan Desert. Pleistocene and Recent deposits are represented by fluvial deposits in the Nile Valley and Delta, gravel terraces bordering the Nile Valley, lacustrine deposits covering the ground surface in some of the depressions, wadi fillings at the edges of the desert plateau and in the Eastern Desert, consolidated dunes along the coastal zone of the 375

7 I. H. Himida Mediterranean, and the widespread free-moving sand deposits in the form of sand dunes and sand sheets. Regional Geological Structure From the structural point of view, the Libyan desert represents a part of the shelf areas that bound the Arabo-Nubian Massif (the Shield area). The rocks of the Arabo-Nubian Massif constitute the mountain range extending along the Red Sea. They also outcrop at some localities in the southern regions of the Libyan Desert. The shelf area of the Libyan Desert, structurally, can be subdivided into two major units: (i) the unstable shelf area (mobile area), (ii) the stable area. The unstable shelf area constitutes the northern part of the Libyan Desert. The boundary between this area and the stable shelf area is not well defined. However, it can be tentatively traced from the central regions of the Sinai Peninsula in a south-west direction, passing the Nile Valley at latitude 28' north, to the southern regions of Farafra Oasis and eventually in a western direction crossing the Libyan Desert. The mobile shelf area of the Libyan Desert is characterized by a relatively complete and thick section of sedimentary rocks belonging to the Palaeozoic, Mesozoic and Cainozoic eras; the rocks of the basement komplex are generally situated at great depths. This area has suffered more intensive tectonic movements throughout its geological history. It is characterized by the occurrence of a series of major anticlines and synclines oriented in a NE-SW direction; folds, overthrusts and diapirism are common here. The stable shelf area in the Libyan Desert extends from the regions where rocks of the basement complex outcrop in Central Sudan and north-eastern regions of Chad northwards up to the contact between the stable and unstable shelf areas. This area is characterized by a relatively thin section of sedimentary rocks and a shallow basement complex. Tectonic features in the area indicate that tectonic movements have been less intensive. A number of grabens and horsts cross this massif. Extensive but very gentle domes are recorded; on the other hand, true anticlines are absent. Faulting prevails in both tectonic units, but is more common in the stable shelf area. HYDROGEOLOGY Regional Hydrogeological Aspects Ground water has been exploited and used for a long time in the oases regions of the Libyan Desert by more numerous, and probably more prosperous, populations than at present. Remains of hundreds of old wells are recorded in these regions. Many hydrogeological aspects of some regio.ns of the Libyan Desert have been described in the works of various authors. The works of such authors as Beadnell (1908, 1909), Ball (1927), Sandford (1935), Hellstrom (1940), and Murray (1952) were mainly concerned with the problems of the source and directions of movement of ground water in the Libyan Desert. On the other hand, the works of more recent authors, such as those of Paver and Pretorius (1954), Pavlov and Burdon (1959), and Himida (1965, 1966, 1967), have dealt mainly with the hydrogeological conditions of the regions of Kharga and Dakhla Oases and quantitative studies of their ground-water resources. Later on, the work of Omara et al. (1970) is concerned with the hydrogeological conditions and origin of the ground water in the region of Farafra Oasis. 376

8 The Nubian Artesian Basin - regional lzydrogeological uspects Regional hydrogeological aspects of the Libyan Desert were firstly touched on by Ball (1927), who was the first to construct a water-level map for the Western Desert in Egypt, using water levels from different springs and wells discharging from different water-bearing formations. Later on, Sandford (1935), Hellstrom (1940) and Murray (1952) modified Ball s map and extended it to cover the whole of the Libyan Desert. Obviously, these authors considered that the different water-bearing formations in the Libyan Desert constitute one hydrogeological system. However, because of the lack of data at this period of time, they were not able to describe the configuration of the hydrogeological system in the Libyan Desert or its regional hydrogeological aspects. From an examination of recent information, with consideration of the regional geological structure of the Libyan Desert, the hydrogeological and hydrological aspects and hydrogeochemical zonation of the different water-bearing formations, it can be concluded that the ground-water horizons described in the different oases regions constitute only small scattered portions of an extensive multilayered artesian basin that covers nearly the whole territory of the Libyan Desert and a great part of the Arabian Desert to the east of the Nile Valley. From the structural geological point of view, this is a major artesian monocline of the platform type. Hydrogeologically, it is an artesian basin of the semi-closed type where the different water-bearing formations gradually disappear northwards under the younger formations and discharge only by upward leakage through the overlying beds. The eastern boundaries of the artesian basin can be strictly delineated by the series of mountain ranges consisting of basement rocks and extending parallel to the Red Sea coast. The southern boundaries can be defined by the outcrops of basement rocks in different regions of North Sudan and in the north-eastern regions of Chad. The western boundaries of the artesian basin cannot be sharply defined. However, they can be tentatively traced along the localities where the rocks of the basement complex either outcrop on the surface or are recorded at shallow depths (see Fig. 2). The artesian basin thus covers an area of about 2.5 million square kilometres, including nearly the whole territory of Egypt, the eastern part of Libya, the northern regions of Sudan and the north-eastern part of Chad. According to its areal extent, the artesian basin can be considered as one of the most extensive artesian basins in the world. Considering the fact that the water-bearing formations of the Nubian Series constitute the major and principal part of the structure of the artesian basin, the latter was called by the present author the Nubian Artesian Basin (Himida, 1965). Two main water-bearing complexes can be distinguished in the structure of the Nubian Artesian Basin. The regional hydrogeological aspects of each water-bearing complex are briefly described in the following: (i) The Nubian Series Water-Bearing Complex; this is the main water-bearing complex and is represented by the water-bearing formations and horizons contained in the Nubian Series overlying the basement complex. Water-bearing formations of the Nubian Series contain the main bulk of the ground-water resources within the Nubian Artesian Basin. Nubian formations, wherever tapped by wells, are found to contain ground water. In the regions of Kharga and Dakhla Oases the deep wells tapped more than 32 water-bearing horizons, kainly consisting of sandstone beds separated locally by confining beds of clays. Subsurface geological studies and hydrogeological investigations proved that both water-bearing horizons and water-confining beds have no regional lateral extension, with the exception of an upper bed metres thick of varigated clays which covers an extensive area in these regions. In the New Valley area, the ground water of the Nubian Series is exploited in the regions of Kharga, Dakhla, Mawhub West, Abu-Monqar, Farafra and Bahariya Oases 377

9 1. H. Himidu SCHEMATIC HYDROGEOLOGICAL MAP OF THE NUi3lAN ARTESIAN BASIN Amo moinly Cowred by proper Nubion Srrics formorioil, À<m sovrrrd by volconic rocks. Arco moinly covrrcd by rocks of the boscment camplci. Boondory line of the franc of highly SbfrOe ground wolcr ond brines of grncsis, io the r. Post-Nubion wotcr-beoring crrmplrx, Boundory lrne of fhe ironr of highly sdmc wotcr and brines io rhc deeper Nubion Serks wotcr-b#oring complex lin rhe ~olocozoic fwmotions~. ZO\--- Boundory line bctwcm tnc rcgions ofbmchshartcsmn Wore< 01 mixed odgh ond the fresh orferhn Water of moinly infiltrotion origin Suggested mom cntchmenf brms of the Nubion Artclion ~asm in rrcenr rimd. Mom orcos of notum1 discharge of Ihr Nobmn Artesion Sosin. r.m o~ artlfic,d cnpioit~tron of groundwaterin thc orresan barin. Worer-lcd contour liner of Ihr Nubion Serrer Wohl beormg complcx lin mcfres obwc seo Irurll Moi diriclions of rcchorge of?he ortesran basin. Suggested boundorics of the Nubion Irlesion BOSin. A/A OricntoNon of hydrogeological cross secrions 1 Fig. I J. FIGUKE

10 The Nubian Artesian Basin - regional hydrogeological usgects either by shallow native wells, or by deep wells, or by both. Shallow native wells, ranging in depth between 30 and 150 m, tap the uppermost water-bearing horizons of the Nubian Series, while the deep wells, ranging from 300 to 1200 m in depth, tap the middle and lower water-bearing horizons in the above-mentioned regions. Ground water of the Nubian Series in most regions of the Nubian Artesian Basin is characterized by its high hydrostatic pressure. In most localities of the oases of the Libyan Desert as well as in many localities along the Gulf of Suez there is a progressive increase of ground-water hydrostatic pressure from the uppermost water-bearing horizons to the lowermost ones. Previously, nearly all the shallow and deep wells tapping the Nubian Series in the oases regions were exploited under flowing regime. In present times a great number of the wells stopped flowing as a result of intensive exploitation of ground water and the consequent formation of regional cones of depression in some localities of the oases regions. Future exploitation of ground water in these regions should be carried out using adequate types of artesian pumps after investigating the possible exploitation regime. Another important aspect of the ground water of the Nubian Series in the oases regions is its low mineralization compared with the mineralization of the ground water contained in the Post-Nubian water-bearing horizons. It is very interesting to notice that the mineralization of the ground water of the Nubian Series in most of the oases regions decreases appreciably from the uppermost water-bearing horizons to the lowermost ones, resulting in what is called reverse zonation of ground-water salinity. Ground-water temperature of the Nubian Series ranges from 25OC to 4OoC, depending on the depth of the water-bearing horizon. (ii) The Post-Nubian Water Bearing Complex; this is represented by the water-bearing horizons contained in the fissured limestones, sandstones and marly limestones of the Upper Senonian, Eocene and Miocene formations extending to the north of the Kharga-Dakhla regions. Ground-water potentialities of the water-bearing formations making up the Post-Nubian water-bearing complex are much lower than for the Nubian Series water-bearing complex. Moreover, geological, hydrogeological and hydrogeochemical studies have proved that the water-bearing formations of the Post-Nubian complex are mainly recharged by upward leakage of the artesian water contained in the underlying formations of the Nubian Series, Ground water of the Post-Nubian water-bearing formations is largely exploited in the Farafra Oasis region (Upper Senonian water-bearing formations) and in the Siwa and Qarra Oases regions (Middle Miocene formations) using shallow dug wells and springs. Water temperature in the shallow wells exploiting the Post-Nubian formations in these regions ranges from 16OC to 28OC, depending on the depth of the discharged water-bearing formation. Ground water is generally exploited under flowing regime. The Nubian Artesian Basin, like all the other artesian basins, can be subdivided laterally into catchment areas, pressure areas and natural discharge areas. Although numerous regions on the different boundaries of the Nubian Artesian Basin have been acting as catchment areas during certain past geological periods, as wil be explained later on, the most probable catchment areas in recent times can be located on the highlands of north-eastern regions of Chad, central regions of Sudan and probably on the Tibesti Plateau. This can be judged from the water-level contour map (see Fig. 2). According to the water-level contour lines, ground-water movement takes place primarily from south-west to north-east. It is worth mentioning that the water-level contour lines of the Upper Senonian water-bearing formations in Farafra 379

11 I. H. Himida Oasis indicate that the movement of the ground water in these formations has nearly the same trend as that of the underlying Nubian Series water-bearing formations. Pressure areas of the Nubian Artesian Basin cover most of the basin area both in the Libyan Desert and in the desert area to the east of the Nile Valley. Throughout these areas, ground water in the water-bearing formations mostly occurs under hydrostatic pressure. The Nubian Artesian Basin being of a semi-closed type, the natural discharge of the ground water of the different formations takes place, at least partially, in a series of depressions extending in the northern regions; such as the Qattara Depression, Siwa Oasis, Ghabub Oasis and others. Regional Hydrogeochemical Zonation of the Ground- Water-Bearing Formations Considering the ground-water mineralization, its main chemical types and mode of formation within the Libyan Desert, the following hydrogeochemical regions and hydrogeochemical zones can be recognized: A. Region of Mainly Fresh Ground Water of Infiltration (Meteoric) Origin; this region covers a huge territory extending from tbe southernmost boundaries of the artesian basin up to a latitude north of Bahariya Oasis. This area is mainly covered by rocks of the Nubian Series, Ground water within this region is mainly fresh, and its mineralization rarely exceeds 1000 ppm and in most cases does not exceed 500 ppm. In some cases the mineralization of ground water is as low as ppm, as for example in the case of some shallow wells in Bahariya Oasis and some of the deep wells in Farafra Oasis, According to the ground water chemical types, this region within the area of the New Valley Project can be subdivided into the following hydrogeochemical zones (see Fig. 3): (i) Zone of chloride-sulphate-sodium-calcium water in the upper water-bearing horizons of the Nubian Series in Dakhla Oasis. (ii) Zone of bicarbonate-chloride-sodium-calcium water; predominating in the lower water-bearing horizons of the Nubian Series both in the El-Zaiyat region (midway between Kharga and Dakhla Oases) and in the northern part of Kharga Oasis. Ground water of this zone is characterized by the presence of appreciable amounts of NaHC03. (5) Zone of mixed water chemical types with predominance of bicarbonatechloride-sodium water in the lower water-bearing horizons and chloride-bicarbonatesodium water together with chloride-sulphate-sodium water in the upper water-bearing horizons of the Nubian Series in the Ginah area (Kharga Oasis). (iv) Zone of chloride-bicarbonate-sodium water in the southern localities of Kharga Oasis. (v) Zone of chloride-bicarbonate-magnesium-sodium water in the upper water-bearing horizons of the Nubian Series in Bahariya Oasis. Ground-water mineralization within these horizons is mostly very low. However, in some wells ground water of mineralization exceeding 1 O00 ppm is recorded. This is mostly due to continental salinization (secondary salinization). (vi) Zone of chloride-sulphate-magnesium-sodium and chloride-bicarbonatemagnesium-sodium water in the Upper Senonian water-bearing formations in Farafra Oasis. Mineralization of ground water within this zone generally ranges between 300 and 800 ppm. However, brackish and even saline water is recorded in some wells.this is mainly due to the effect of continental salinization. 380

12 The Nubian Artesian Basin - regional hydrogeological aspects JO c HYDROGEOCHEMICAL ZONATION MAP OF THE ARTESIAN WATER- BEARING HORIZONS IN THE WESTERN DESERT OF U. A. R. FIGURE

13 I. H. Himidu (vii) Zone of predominance of bicarbonate-chloride-calcium-magnesium (or magnesium-calcium) water and less frequently chloride-bicarbonate-magnesium-sodium water in the Nubian Series water-bearing horizons in Farafra Oasis. Mineralization of ground water is extraordinarily low and does not exceed 300 ppm. B. Region of Brackish and Salty Ground Water of Mixed Origin; this extends from the northern limits of the region where ground water of meteoric origin predominates up to a latitude north of the Qattara Depression (see Fig. 3). This region, vertically, comprises many water-bearing horizons in the Post-Nubian formations as well as in the upper part of the Nubian Series (see Fig. 1). Mineralization of ground water within this region mostly ranges from 2000 to 10,000 ppm. Shallow wells and springs in Siwa and Qarra Oases and also in the Qattara Depression, located within the above-mentioned region, exploit water-bearing horizons of the Middle Miocene formations. A zone of mainly chloride-sulphate-sodium water is recorded in both Siwa and Qarra Oases. Mineralization of ground water in these localities ranges from about 2000 to 7000 ppm. C. Region of Highly Saline Water and Brines of Sedimentation Origin; this extends to the north of the above-mentioned region. Within this region all the water-bearing formations under the surface ones are charged by highly saline water and brines. Mineralization of ground water may attain a figure of 300,000 ppm, and the ground-water chemical is predominantly sodium chloride. In vertical section, the upper limits of this zone gradually become deeper southwards until it underlies the region of brackish and salty ground water in the area of the Qattara Depression where it is represented by the lower water-bearing horizons of the Nubian Series (see Fig. 1). The distribution and aspects of the above-mentioned different hydrogeochemical regions and zones within the area of the Nubian Artesian Basin can be explained, to a large extent, on the basis of a palaeohydrogeological analysis of the artesian basin. PALAEOHYDROGEOLOGY Palaeohydrogeological studies, generally, are concerned with the reconstruction of the hydrogeological conditions of a region in the different geological periods during which it developed. Obviously, palaeohydrogeological analysis of a ground-water basin can be of primary importance in solving the problems of origin and history of the ground water contained in it. Besides the present geological structure and hydrogeological conditions, the palaeohydrogeological studies of the Nubian Artesian Basin were based on a great amount of diversified data and information concerning its geological history, palaeogeographical aspects, its palaeoclimates and the evolution of the landforms within its area. Reconstruction of the hydrogeological conditions of the Nubian Artesian Basin, from the Middle Eocene to the present time, is translated into a seriesofschematic maps (Fig. 4). For the construction of these schematic palaeohydrogeological maps of the Nubian Artesian Basin, the following items were considered: 1) The areas covered by sea water. 2) The areas occupied by both continental and marine deposits. 3) Distribution of the different water-bearing formations within the area of the artesian basin. 4) Regions of distribution of the main ground-water genetic types, 3 82

14 The Nrthiari Artesiatz Basin - regional hydrogeological aspects 383

15 I. H. Himida 5) Suggested main catchment areas and discharge areas of the artesian basin. 6) Regions of intensive tectonic movements affecting the regime, and directions of movement of the ground water. 7) The main directions of ground-water movements. The main changes in the hydrogeological conditions of an area, especially the changes in its hydrogeochemical aspects, and the trends in which these changes take place are chiefly originated by the changes in the palaeogeographical environments of this area. The most important changes in the palaeogeographical environments are the interaction between the sea environment and the land environment. The transgression of the sea on a land area may result in the.xommencement of a hydrogeological cycle, the first part of which (the sedimentation period) is characterized by the replacement of the already-formed fresh infiltration (meteoric) ground water by saline sea water and formation of sedimentation sea water (connate water) in the newly deposited sediments in the marine environments. The second part of the hydrogeological cycle generally begins with the regression of the sea. This period (infiltration period) is characterized by the infiltration of fresh meteoric water into the water-bearing formation, resulting in the gradual dilution of the already-formed sea water, washing of the water-bearing formation, displacement of the highly saline water seawards and, eventually, freshening of the ground water. Four major stages can be outlined in the geological history of the Nubian Artesian Basin: A, The first stage extends from the beginning of the Palaeozoic era up to the middle of the Early Cretaceous. During this period, geological data indicate that the northernmost part of the area now occupied by the Nubian Artesian Basin was repeatedly covered by sea water, resulting in the occurrence of many hydrogeological cycles. The history of the area during this stage can be summarized as follows: (i) The first hydrogeological cycle begins with the transgression of the Cambrian sea southwards in the north of Gondwanaland. The shore line probably reached the latitude of the present location of Bahariya Oasis (Amin, 1961). During this period, for the first time, was initiated the process of replacement of the meteoric infiltration ground water by the saline sea water in the already-existing thin mantle of the continental deposits. At the same time, during the sedimentation of the Cambrian deposits, sedimentation (connate) sea water deposits were formed simultaneously in them The geological history of the area during the Ordovician period is not well understood; however, many authors believe that the marine regime continued throughout this time (e.g., Amin, 1961; Said, 1962). Silurian deposits are not recorded in the area of the Libyan Desert. Accordingly, it is believed that by the end of Ordovician times an epierogenic uplift of the land occurred in the northern part of the African Continent. Consequently, by these times the sedimentation period ended and an infiltration period commenced which extended till the end of the Devonian period. During this time the area of the Nubian Artesian Basin developed in continental conditions and was subjected to continuous washing and freshening of the water-bearing formations, replacement of the sea water by fresh meteoric water, and displacement of the highly saline ground water seawards. (ii) The beginning of the second major hydrogeological cycle can be assigned to the end of the Devonian and beginning of the Carboniferous periods when a widespread transgression of the sea occurred owing to the sinking of the land surface in the northern part of the Libyan Desert (e.g. Ball, 1939; Amin, 1961; Said, 1962). Transgression of the sea attained its maximum extent in Middle Carboniferous times, when the shore line probably reached the latitude of Bahariya Oasis. During this 384

16 The Nubian Arfesiart Basin -regional Jtydrogeologicd aspects period and up till the end of Carboniferous times there was a dominance of processes of replacement of fresh already-infiltrated water in the pre-existing sediments by sea water, and deposition of saline connate sea water in the newly formed Carboniferous sediments. At the end of the Carboniferous period an extensive uplift of the land in the northern part of the Libyan Desert is recofded. Consequently, all the area now occupied by the Nubian Artesian Basin was again subjected to continental environments from the beginning of the Permian period up till the end of the Triassic period when the water-bearing formations were washed and freshened by the fresh infiltration water, and the saline and highly saline water was displaced to the north. Geological data indicate that at this period there still existed a marine gulf in the south-eastern localities of the Libyan Desert area. At the beginning of the Mesozoic era, sea transgression is recorded only ín the Sinai Peninsula and probably in the northernmost part of the Libyan Desert (Amin, 1961), while the whole of the remainder of the artesian basin developed in continental environments. (iii) The first major Mesozoic hydrogeological cycle can be assigned to the end of the Triassic and beginning of the Jurassic periods when the land surface in the northern regions of the Libyan Desert suffered downwarping, resulting in the occurrence of sea transgression from north to south in the form of two great gulfs (Amin, 1961). The sea transgression attained its maximum extent during the middle of the Jurassic period when the shore line reached latitude 29" north of Siwa Oasis. By the beginning of this transgression another period of sedimentation was initiated in this northern part of the Nubian Artesian Basin. However, by the end of Jurassic times and the beginning of the Early Cretaceous period the greater part of the Libyan Desert was again subjected to continental conditions. During this time a short infiltration period occurred, when the water-bearing formations of Jurassic and pre-jurassic ages were subjected to processes of washing, replacement of saline sea water by fresh infiltration water, and displacement of the highly saline water and brines to the north. On the other hand, palaeolithological studies indicate that a great part of the continental deposits of the Nubian Series was deposited particularly during this period (Amin, 1961; Said, 1962). B. The second and most important stage in the history of the Nubian Artesian Basin extends from the middle of the Early Cretaceous up to the end of the Pliocene period. During the middle of the Early Cretaceous is recorded the beginning of the most intensive and widespread transgression of the sea on the northern part of the African Continent, taking place from north to south as a result of an extensive progressive lowering of the land surface. By the end of the Lower Cretaceous period, all the regions of the North African Continent were submerged under sea. Transgression of the sea at the end of the Cretaceous period and the beginning of Palaeogene probably reached the southern regions of Egypt and the northern regions of Sudan (e.g., Ball, 1939). The advent of the sea on the continent during the above-mentioned period initiated the most extensive and prolonged hydrogeological cycle in the whole geological history of the region. Water-bearing horizons of the Nubian Series containing, in most localities, fresh infiltration water were subjected to contamination and replacement of their ground water by saline sea water. On the other hand, connate sea water was being formed in the nearly deposited Upper Cretaceous sediments and the younger ones. In the southernmost regions of the area now occupied by the Nubian Artesian Basin continental environments were retained, with infiltration of fresh meteoric water in 385

17 I. II. Himida the formation of the Nubian Series exposed in these regions.. At the end of Palaeocene times the sea had begun lo retreat northwards, and by that time an extensive and prolonged period of infiltration had started. At this time also, the Nubian Artesian Basin began to acquire its aspects and configuration. Steps in the development of the Nubian Artesian Basin from the beginning of the Eocene period up till the present time are illustrated in a series of schematic palaeohydrogeological maps (Fig. 4). Sed rekression during Early Eocene times reached the southern regions of Farafra Oasis. However, in the eastern and western regions of the artesian basin a marine regime continued because of the presence of two great gulfs (Fig. 4a). The boundary line between the zones of highly saline water and brines and that of brackish water most probably extended into the sea zone, not far from the shore line, as a result.of the relatively high hydraulic gradient of the ground water caused by the sinking of the northern part of the artesian basin. The main catchment areas of the Nubian Artesian Basin at this time were located on the south-western highlands as well as the southern and south-eastern regions of the artesian basin. At the same time, rainfall on the extensive area of the artesian basin probably played rhe main role in the processes of washing the water-bearing formations, freshening their ground water and displacing the highly saline water and brines to the north. At the end of Eocene times and the beginning of the' Oligocene period an intensive uplifting of the land in the northern regions of the Nubian Artesian Basin area was recorded. As a result, the sea receded and an extensive area of the artesian basin was.developed in continental environments. At that time the shore line probably reached the latitude of Fayum (Ball, 1939; Said, 1962). Consequently, during this period the infiltration processes continued, taking place also in the land areas previously covered by the Ebcene sea. A cycle of mountain formation took place at this time along the eastern boundaries of the Nubian Artesian Basin, with the resulting formation of high mountain ranges in the region now'.covered by the Red Sea. These mountain ranges most probably played an active role as catchment areas of the artesian basin besides the other areas mentioned above (Fig. 4b). At the beginning of the Miocene period, in the northern regions of the Libyan Desert, a more or less stable regime was recorded, with a tendency of slight subsidence and, limited sea,transgression southwards, On the greater part of the artesian basin area continental environments were still dominating, with the continuation of the infiltration processes (Fig. 4c). At the end of the Miocene period the general sinking of the land appears to have ceased and been followed by upward movement, with the regression of the sea to the north:.these movements were accompanied in the eastern part of the country by an intensive cycle of tectonism which resulted in the uplifting of the whole region of the Sinai Peninsula above sea level and the formation of different systems of faulting and folding, especially in the northern regions of the artesian basin area. The beginning of formation of the Nile Basin is assigned to this period (Ball, 1939; Said, 1962). At this time most of the territory now occupied by the Nubian Artesian Basin was subjected to continental conditions for a long period (Fig. 4d). Starting from the beginning of the Pliocene period, there commenced a great tendency of land to sink in the northern regions of the Nubian Artesian Basin area, resulting in the gradual raising of the sea level, which attained a level about 180 m higher than. its present level,. The shore line, at this time, retreated until it reached the latitude of Cairo and Wadi El-Natrun. However, in the Nile Basin, land subsidence 386

18 seems to be much more intensive. The intensive land subsidence in the Nile Valley resulted in the intrusion of the sea southwards in this last area in thc form of an elongated gulf which probably reached the southern regions of Egypt and the northern localities of Sudan when the transgression of the sea attained its maximum extent by the Middle Pliocene period. Ingression of the sea water in the Nile Valley, obviously, resulted in the contamination of the water-bearing horizons of the pre-pliocene sediments with saline sea water and, at the same time, in the formation of connate sea water in the newly deposited Pliocene sediments in the Nile Valley and in some of the northernmost localities of the artesian basin area. In the later part of the Pliocene period the land subsidence ceased and was followed by intensive upward movement accompanied by intense folding and faulting in the eastern part of the artesian basin. As a result of these tectonic movements the mountain ranges of Sinai and of the Eastern Desert were thrust to much higher levels than before. The presence of a large number of dry valleys filled with Pliocene and Pleistocene deposits in the Eastern Desert having a general orientation from east to west, in addition to the palaeoclimatic data, indicates that the artesian basin had by this time been receiving intensive recharge from the eastern regions (Fig. 4e). C. The third stage in the evolutionary history of the Nubian Artesian Basin begins from the end of the Pliocene period and extends up till the present time. At the end of Pliocene times all the territory of the Nubian Artesian Basin was developed in continental conditions not differing much from the present prevailing conditions. However, by these times, the amount of rainfall was probably much higher, especially in the areas of high altitudes. At this time, on the other hand, the depressions of Kharga and Dakhla Oases were already formed. It can be concluded that the other depressions of the Libyan Desert were also formed. Since the end of the Pliocene period, no transgressions of the sea are recorded on the area of the Nubian Artesian Basin. Considering the present prevailing hydrogeological aspects of the Nubian Artesian Basin and its evolutionary history, as described above, the following conclusions can be stated: 1) The occurrence of considerably thick water-bearing formations containing highly saline water and brines in the northern regions of the Nubian Artesian Basin can be explained by the fact that these formations were mainly deposited in marine environments and that they have been submerged under the sea numerous limes during the long period of their geological history, where they were subjected to sedimentation process e s. 2) The occurrence of extensive water-bearing formations containing fresh water in the central and southern regions of the artesian basin can be explained by the fact that these areas were mainly developed in continental environments where they have long been subjected to processes of infiltration of fresh meteoric water in the water-bearing formations, of washing and of displacement of any original sea water. On the other hand, these regions were developed in a marine environment for a relatively short period; the southernmost regions of the artesian basin have been developed entirely in continental conditions. 3) The noticeable lowering of ground-water mineralization in the water-bearing horizons of the Nubian Series relative to that of the Post-Nubian water-bearing horizons in the same locality can be attributed to the evolutionary history of both water-bearing complexes. Nubian water-bearing formations in most localities were 3 87