Origin of the Qattara Depression, Egypt
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Origin of the Qattara Depression, Egypt CLAUDE C ALBRITTON JR * I JAMES E BROOKS ' J Institute for lhe °fEarth and Man al Southern Methodist University, Dallas, Texas 75275-0274 BAHAY ISSAWI 16 Misaha Street, Dokki, Gta, Cairo, Egypt AHMED SWEDAN Egyptian Geological Survey and Mining Authority, Salah Salem Street, Abbassiya, Cairo, Egypt ABSTRACT part of that river's descending course into the mately quadrangular in plan. By contrast, the Mediterranean Basin, apparently became in- segment to the east has an axial valley which As a working hypothesis, we propose that active after its waters were diverted into un- follows a broadly arcuate course extending the Qattara Depression originated as a stream derground courses through sinks and cav- through Moghra Lake and thence curves west- valley that was subsequently dismembered by erns. With the refilling of the Mediterranean southwestward along a line approximately par- karstic processes during the late Miocene and the accompanying diminution of subsur- allel to the base of the bordering cliffs to the epoch and afterward was deepened by defla- face aqueous activities and with the advent of north. tion and otherwise modified by mass wasting more arid climatic regimes, surface processes The Qattara Depression is excavated in sedi- and fluviatile processes. associated with deserts have completed the mentary rocks of Cenozoic age. Over much of The Depression is excavated in sedimen- shaping of the Depression to give it its pres- the floor, this bedrock is variously concealed by tary rocks of Tertiary age that have a regional ent configuration and character. younger deposits of wind-blown sand, saline dip of a few degrees toward the Mediterra- crusts and salt marshes {sabakha), alluvium, or nean Sea. The precipitous northern wall is the INTRODUCTION pediment gravel. scarp slope of a cuesta capped by carbonate The Tertiary strata dip northward toward the rocks of middle Miocene age. Its cliffy slopes The Qattara is the largest and deepest of the Mediterranean Sea at angles of a few degrees. descend to pediments graded southward to undrained natural depressions in the Sahara Superposed on this homoclinal structure are an axial valley. That valley, now extensively Desert (Fig. 1). Cliffy slopes mark its northern broad anticlinal and synclinal warps with covered by saline crusts, sabakha deposits, and western boundaries. Near its south bound- general westerly trends. and salt marshes, slopes westward into the ary, the floor rises and merges imperceptibly Seven sets of fractures classified according to deeper quadrangular part of the basin where with the surface of the Libyan Plateau, often direction of strike have been identified by analy- elevations as much as 134 m below sea level called the "Western Desert of Egypt." In most sis of LANDSAT imagery (El Shazly and oth- have been measured. Nearby sinkholes and reports, the over-all boundary has been taken at ers, 1976, p. 15-17). The most widely publi- cavernous subsurficial limestone attest to past the sea-level contour, and by that definition, es- cized and at the same time most controversial of karstic activity. Presumably karstification timates of the Depression's area have ranged be- these has been called the "Qattara fault zone," a reached a peak during the recession of the tween 18,100 and 19,500 km2. For comparison, set of east-trending fractures alleged to run tan- Mediterranean Sea late in the Miocene epoch the larger of these figures is equal to almost half gent to the extreme southern edge of the Depres- when the north-dipping aquifers discharged the area of Switzerland. Its maximum length is sion. This has been called a "major crustal into the then almost empty basin. ~300 km, and its maximum breadth is 145 km. shear" by Moody (1966), who would extend it As the Mediterranean Sea receded, what is The lowest point, 134 m below sea level, is near northward to connect with the Palmyra Fracture now the Western Desert of Egypt and its off- the western end about 35 km southeast of Qara, Zone of Syria. Neev (1975) identified the fault shore continuations were subjected to exten- the only permanently occupied oasis in the area. zone as transcurrent in nature and curved its sive erosion by streams. A major stream According to El Ramly (1967), average an- trace to the northeast to connect with the Eratos- issuing from the Gilf Kebir Highlands flowed nual precipitation ranges from 25 to 50 mm thenes Fracture Zone off Cyprus. Most recently, northward to near the present site of Siwa along the northern rim, decreasing to less than Neev and others (1982) proposed that the Oasis, its alluvium now mostly covered by 25 mm in the deeper western part. Bassler Qattara-Eratosthenes megashear is one in a aeolian deposits of the Great Sand Sea. If it (1968) reported that, despite these low averages, swarm of ancient left-lateral faults belonging to reached the Mediterranean Basin, then it may anomalous torrential rains during the winter a system extending from Anatolia across Africa have done so through the Sahabi Channel months have impeded field operations, "trans- from the Nile Delta to the delta of the Niger and System of Libya, or by a much shorter route forming access roads into expanses of mud and possibly across the Atlantic into the Amazon through the Qattara area to an exit near the lakes." Basin. On the Egyptian Geological Survey's head of the Ras Alam er Rum submarine can- Geographically, the Depression is divisible Geologic Map of Egypt (1981), faulting along yon offshore near Alexandria. In either case, into two parts by a line running west-northwest the southern end of the Depression is not the stream responsible for the Qattara cuesta, from along the southwestern escarpment of Cec- indicated. whether a tributary of the "Gilf River" or a ily Hill (Fig. 1) across the floor to a point 35 km Recent deep drilling and geophysical investi- northeast of Qara. The segment west of this line gations in and near the Depression have re- *Deceased November 1, 1988. shows no clearly defined axis and is approxi- vealed that the simple homoclinal structure Geological Society of America Bulletin, v. 102, p. 952-960, 10 figs., July 1990. 952 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/7/952/3381016/i0016-7606-102-7-952.pdf by guest on 30 September 2021 28° 29" Figure 1. The Qattara Depression: location and topography. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/102/7/952/3381016/i0016-7606-102-7-952.pdf by guest on 30 September 2021 954 ALBRITTON AND OTHERS M'amura Area NE depression Kalakh Fm. meters Marmarica Fm. M'amura Fm. / Moghra Fm. Qatrani Fm.I Qasr el-Sagha Fm.. KEY Qaret el Ghazalet SE part of el Diffa plateau M'amura STRATIGRAPHIC COLUMN / Diffa / plateau Plio-Pleistocene - Kalakh Fm. ( Qaret NE Miocene I el Ghazaiat.0 '}" depression Middle _ Marmarica Lower M'amura Firn, j Moghra Fm. \ Q ATt ARA 1 , DEPRESSION Oligocene, Lower - Qatrani Fm. s ! - Eocene, Upper -Qasr el-Sagha Fm. >>i a n >> — I a» •l I Figure 2. Cenozoic stratigraphie relationships, northwest Egypt (including the Qattara Depression). apparent in the surficial strata of Tertiary age STRATIGRAPHY invertebrate fossils of late Eocene age (Said, conceals a complex array of strongly deformed 1962). Fossil vertebrates collected by B. Slaugh- rocks ranging in age from Eocene to Precam- The Qattara Depression formed in sedimen- ter and D. Gillette (1982, personal commun.) brian (Said, 1962, p. 197-201; Salem, 1976). tary rocks, ranging in age from Eocene to bear out this age. The formation overlies the The Depression appears to overlie a horst trend- Pliocene-Pleistocene. Stratigraphic relations middle Eocene "Nummulites gizehensis beds," ing east-northeast bordered by the Northern and among these units are complex, displaying both believed to be correlative with the late Eocene Matruh basins to the north and by the Abu Gha- lateral and vertical changes (Fig. 2). Younger Maadi Formation of the Cairo area. radig Basin on the south. Estimated depths to sedimentary deposits of Quaternary age mantle Oligocene. Qatrani Formation. The Qatrani crystalline rocks constituting the Precambrian the eroded floor of the Depression in many Formation at its type section at Gebel Qatrani basement vary between 3.3 and 4.5 km beneath places. north of the Fayum Depression was described the Depression, as compared with 5.1 to 6.2 km by Beadnell (1905) as being 250 m thick and in the basins to the north (Meshref and others, Tertiary composed of variegated sandstone, shale, and 1980). marl. In the upper part, some beds contain a Of the several sets of surficial fractures rec- Eocene. Qasr el-Sagha Formation. The type fluvio-marine fauna. Other layers contain fossil ognized by El Shazly and associates, only three section of this unit was established in the north- wood; still others contain abundant silicified are represented on the tectonic map for the ern Fayum Depression, southeast of the eastern bones of land vertebrates. From this evidence, basement surface in the Meshref report. The Qattara Depression, by Beadnell (1905). There Said (1962) concluded that the Qatrani was most numerous trend north-northwest-south- it consists of 180 m of shale and limestone with deposited in a near-shore marine to estuarine or southeast. Fractures trending east-northeast- sandstone in the upper part (Said, 1962). The bay to deltaic environment. Bown (1982), how- west-southwest and northeast-southwest are unit contains both marine invertebrate fossils ever, suggested that the ichnofossils and rhizo- also common to the surface and subsurface re- and the fossils of terrestrial vertebrates. liths of the Oligocene Qatrani Formation in the constructions.