Baharia Oasis

by John Ball; H. J. L. Beadnell

UPPER CRETACEOUS.

Cenomanian.

7.—Sandstones, Clays and Marls—These, the lowest and oldest beds, form the floor and parts of the walls of the depression. They are best developed and exposed in the north end, where their maximum thickness is about 170 metres; the base of the series is not seen, and thus their total thickness cannot be estimated. In the north part of the oasis they are capped, with apparent conformability, by Eocene limestones, although an intervening band of limestone-grit may occasionally occur. In the south, the series is followed by the higher Cretaceous divisions, while in the isolated hills within the depression these beds are capped either by Eocene limestones, by basalt or dolerite, or by Post-Eocene ferruginous sandstones and quartzites.

They consist of friable false-bedded variegated sands and sandstones, with harder dark-brown ferruginous bands, alternating with sandy shales and clays, passing through every gradation. Some of the sandstones are micaceous.

The clays are frequently saliferous, and bands of fibrous gypsum are occasionally seen.

Large well-formed groups of barytes crystals are occasionally found in the sandstones of the oasis-floor, and hard thin vein-like masses of a sandstone, in which the cementing material is barytes, are frequently met with in the eastern part. The weathering of these veins, which are evidently produced by the action of infiltrated solutions in cracks, gives rise to the peculiar spheroidal belted nodules which often strew the ground, and of which the existence has been already mentioned by Ascherson and Zittel.

Although these strata are, as a rule, unfossiliferous, organic remains are common in certain beds, and especially in the dark-brown ferruginous concretionary bands of the sandstones. The commonest form is Exogyra, individuals of which are found crowded together in some bands; in most cases they are pseudomorphs in ironstone.

In the hills between Mandisha and El Bawitti this series consists of soft false-bedded variegated sands and sandstones with ferruginous bands, alternating with sandy shales, the whole locally showing a slight westerly dip of between 3° and 7°. Some of the sandstones contain flakes of colourless mica. The whole series is capped and protected by a hard bed of dolerite, which has been intruded along a bedding plane into the series, and now forms the summit of these hills.

The following section was measured about 4 kilometres west of Mandisha—

Top.		Metres.
	More or less columnar dolerite, much broken up by weathering.	10
	Sandstones, sandy clays; unfossiliferous	65
	Ferruginous sandstone with casts of Exogyra; sandrock and sandstone, with ferruginous unfossiliferous bands	10

The section is much obscured by downwash and talus of angular fragments of dolerite, sand, shale, etc. This downwash of shale and clay is often covered by a hard coating of salt and sand cemented together, sometimes having the appearance of regular beds.

The following is a detailed section measured at the well-marked isolated conical hill near the western escarpment, 10 kilometres north-east of El Qasr (See [Plate I.]) The series is here capped by 6 metres of nodular and hard compact limestone with Nummulites, Operculina, Lucina, etc., of Lower Eocene (Libyan series) age.

Top.	(Hard compact limestone with Nummulites,etc., Eocene).	Metres.
	Shaly clays with ferruginous bands,occasionally containing Exogyra shells; alternations ofshales and shaly sandstones.		7·4
	Soft shaly and crumbly sandstones, withmany hard ferruginous bands and shaly clays. In the sandstone occursolid vertical rods of ironstone 5 centimetres in diameter, withconcentric structure		21·0
	Gray marls with plant-stems andleaves		1·5
	Yellow sandstone, well-marked thickcompact bed, false-bedded		11·5
	Hard ferruginous band withfish-remains		0·2
	Gray shales		1·5
	Soft beds of crumbly sand, hardersand-rock, with clayey layers and ferruginous bands		6·0
	Shaly clays with dark-red ferruginoussandy layers, with gasteropod and Exogyra shells. Greenshaly sandstone on top, and gray sandy clays below		16·0
	White and gray-bedded sand-rock		9·0
	Obscured beds, one containing smallExogyra and gasteropods.		15·0
	Shaly sandstone with hard reddish nodularferruginous bands containing gasteropods and Exogyra.		3·5
	Sandy beds, hidden by downwash		6·0
	Ferruginous bands containingExogyra casts	⎱ ⎰	20·0
	Sandy shaly clay, false-bedded, withwhite sand-rock. Plant-remains
	Bed of white sand-rock		1·0
	Thinly laminated gray shale		2·0
	Reddish-brown nodular ferruginousband		0·2
	Sandy or clayey beds, obscured bydownwash	⎱ ⎰	9·0
	Sandy clay
	Sandy beds with dark-brown ferruginousbands		13·0
	Sandstone with hard ferruginous bands,full of Exogyra		6·0
	Sandy bed, crowded with well-preservedExogyra shells		0·2
	Yellow sandy clay with numerousExogyra casts		3·0
	Dark carbonaceous shale with obscureplant-remains	⎱ ⎰	15·0
	Sandy beds with bone-fragments
Floor of Oasis.			168·0[41]

At the extreme north end of the depression the cliffs and the lower part of the conspicuous dark hill, Jebel Horabi, are composed of shales, clays and sandstones belonging to this series. The cliffs are capped by Eocene limestone-grits and limestone as in the section just described. The upper part of Jebel Horabi, however, consists of a mass of ironstone, often coarsely pisolitic, with some limonite, and red and yellow ochre. This iron-ore appears to replace the sandy beds more or less irregularly at the base, but the great mass of mineral above is probably a later and quite distinct deposit. (Vide under [Post-Eocene]). Traces of shells occur in places.

In the isolated, limestone-capped hill ([Plate I]), 6½ kilometres north-west of Ain el Haiss (the northern spring) these beds are more fossiliferous than usual.

Top.	(Limestone, the basal bed of the intermediateseries).
	Limestone, the basal bed of theintermediate series.
	Sandy beds with hard brown ferruginousconcretionary bands, containing numerous casts of Cyprimeria(Artemis?) Arca, Cucullæa, Odostomopsisabeitrensis, Baculites aff. syriacus, andNeolobites Vibrayeanus	⎫ ⎬ ⎭	50 m.
	Clays, often shaly
	Hard ferruginous sandstone with pocketsof clean white sand

Numerous other sections at different points were examined and measured. Such a series is naturally very variable, and individual beds do not maintain their characters over wide limits, so that correlation of different sections is not possible, except in the roughest manner.

In the southern part of the depression fossils are more frequent, otherwise the beds are very similar.

The series under consideration is by no means poor in organic remains, although the latter are more or less restricted to certain bands and localities. No doubt most of the fossil shells have been removed by solution, etc., as those now existing are generally preserved as pseudomorphs in limonite.

The following is a list of fossils which have been obtained from this series:—[42]

Cucullæa sp.	Odostomopsis abeitrensis.
Arca sp.	Natica sp.
Exogyraflabellata.	Baculites (syriacus?)
E. mermeti.	Neolobites vibrayeanus.
Venus(Artemis?) sp.	Fish-teeth and bones.
Silicified wood and plant-remains(including leaves of Dicotyledons).

As regards the age of these beds, it has already been mentioned that the discovery of Exogyra by Capt. Lyons some years ago, in the basal bands of the series, showed them at least to be Cretaceous, although their correlation with the Overwegi stage, on the supposition that the specimens collected were referable to E. Overwegi, is now shown to be inadmissible. The fairly extensive assemblage of organic remains obtained from this and the overlying series enables the age of the beds to be determined with accuracy, and Dr. Blanckenhorn, who had the first opportunity of making a careful examination of the collection, has pronounced them to be Cenomanian.[43]

The general lithological character of the beds and the prevalence of false-bedded sandstones containing in places silicified wood and plant-remains, including leaves of large trees, point to the conclusion that the beds of this series were deposited in fairly shallow water, perhaps in an estuary, at no great distance from the land.

6.—Limestones and Variegated Sandstones.—In the walls of the south end of the oasis, the series just described is always overlain by a bed of hard, and usually more or less crystalline, limestone. This limestone forms the lowest member of the series now under discussion. At the extreme southern end of the oasis, on the western side, this basal limestone forms a narrow platform, of only some 200 metres width, separating the respective escarpments of the two series 6 and 7. On the same side further north the upper escarpment is usually further back, and in some localities the beds do not form a single well-marked escarpment at all.

The series also occurs forming a line of hills within the depression, these hills owing their existence to a remarkable syncline, by which the beds in question have been folded down to the level of the floor of the depression.

The beds of the series form, as a rule, a well-marked group, individual beds often being traceable for great distances and correlable in different parts of the oasis.

The group as a whole consists of alternations of brown limestone[44] (with grey crystalline varieties) and variegated sandstones and clayey beds. A typical section, as exposed on the west scarp in N. lat. 27° 53′, is as follows:—

Top.	(Buff-coloured limestone and whitechalk of Division 5.)		Metres.
	1.	Soft brown sandstones and sandy beds,much obscured	7·6
	2.	Sandy beds, passing up into a very hardgrey concretionary sandstone, showing ripple-marks	4·6
	3.	Thin-bedded dark sandstones, mostly veryhard	1·5
	4.	Hard compact thick-bedded grey sandstone,very concretionary, false-bedded	2·1
	5.	Shaly sandy beds with ferruginousconcretions	1·5
	6.	Brown impure limestone	0·6
	7.	Soft variegated sandy beds andsandstone	9·0
	8.	Rather friable buff-coloured sandstone,more or less false-bedded, with peculiar iron-staining	6·0
	9.	Gray rather concretionary sandstone, withdark-brown ferruginous bands	3·0
	10.	Brown crystalline limestone with calcite,passing into harder grayish crystalline limestone, flinty attop	6·1
	11.	Gray marls and shaly clays	3·0
	12.	Brown crystalline limestone	0·6
			45·6
Base.	(Sandstones and clays of Division7.)

In Jebel Hefhuf, the long narrow ridge-shaped hill a few kilometres south-east of El Bawitti ([Plate VII]), the beds of this series are implicated in the fold, and are found tilted at angles of 30 and 40 degrees. They here consist of a considerable thickness of crystalline limestone, calcareous grits, shaly clays and sandstones; the softer beds always form a gully between the harder.

The following were noticed at a point about 7 kilometres from the western end of the ridge:—

Top.			Metres.
	Calcareous grit, usually siliceous, withbones (often a true bone-bed).	⎫ ⎪ ⎬ ⎪ ⎭	21
	Sandy clays, etc., with silicifiedwood[45]
	Thick-bedded brown sandstone, siliceousand ferruginous in part.
	Green and gray iron-stained sandyclays
	Thick-bedded brown crystallinelimestone
	Brown crystalline limestone with calcitecavities near base		24
Base.			45

Below come the clays and sandstones of the lowest series, No. 7.

In the section above, the limestones are usually too crystalline to show any traces of organic remains, although numerous casts of shells occur at certain points. The calcareous grit, a very well-marked bed, has been observed at points widely distant within the oasis; in places it is a true bone-bed,[46] often highly siliceous and passing into a hard quartzite. Its occurrence here, at the top of the Cenomanian series and below the White Chalk (to be described later) is of great interest, as its position shows it to be probably homotaxial with the bone-beds of Dakhla Oasis[47] and the Eastern Desert,[48] where they are of Campanian age. The bone-beds on this horizon must thus have been deposited over an enormous area.

If followed round from the south end of the oasis along the east scarp, the beds of this division are found to gradually become more and more calcareous, consisting often almost entirely of limestone,[49] and to dip more and more to the south-east, till in the neighbourhood of the large sandstone-hill near the scarp in latitude 28° 5′, where the tilting reaches its maximum, they dip at 45° and partly disappear under the sandy covering of the ground between the hill and the scarp. Further north the dip grows less and the beds re-appear, forming the top of the main scarp at the oasis-edge.

The organic remains in Division 6 include ammonites, echinids, with Ostrea, Exogyra and masses of Serpulæ; fossil wood and fragments of bone also occur. These fossils are in places abundant, some bands being almost entirely formed of the shells of Exogyra. The ammonites, although apparently specifically identical with those of the lower series, attain a much greater size. The assemblage of forms and the prevalence of bands of limestone point to deeper water conditions than the foregoing underlying series.

The following is a list of forms obtained as far as determined:—

Rhabdocidaris, probablynov. sp.
Diplopodiamarticensis.
Heterodiadema libycum.
Toxaster radula.
Hemiaster lusitanicus(roachensis?)
Serpula(Galeolaria) filiformis.
Pinna, probablynov. sp.	Corbula sp.
**Inoceramuscripsi.	**Natica sp.
Ostrea sp.	Tylostoma syriaca.
Exogyraflabellata.	Turritella nov. sp. aff.nodosa.
E.olisiponensis.	**Turritella sp.
E. mermeti.	Cerithium sp.
PlicatulaReynesi.	Pterodonta aff. inflata.
Spondylus (?)	Murex.
Modiola sp.	**Fusus sp.
Cardium sp.	**Drillia pleurotomoides.
Isocardia sp.
NautilusMunieri.
**Pachydiscusperamplus.
Neolobitesvibrayeanus.

[**]Indicative of a somewhat later age than Cenomanian.

The above fauna taken as a whole has a decided Cenomanian aspect, although some of the forms (marked **) are indicative of a somewhat later age, i.e., Turonian and Senonian. As the next bed above is the Danian White Chalk, it is probable that some of the upper beds of this series form a transitional stage between the Cenomanian and Danian. The presence of the bone-bed, as already mentioned probably of the same age as the Dakhla and Eastern Desert bone-beds, i.e., Campanian, supports this view. In any case, however, the great thickness of beds, of Senonian-Danian age, of the southern oases of Dakhla and Kharga, including the bone-beds, the Exogyra Overwegi series and the ash-gray shales, is here apparently only represented by a few metres of deposits.

DANIAN.

5. Thick-bedded White Chalk and grey crystalline limestone.—This series conformably overlies the upper member of the last. On the west side the chalk forms a well-marked, snow-white, tortuous escarpment, lying at a considerable distance (5 to 10 kilometres) from the oasis-wall. Towards the south, however, it approaches the depression, its escarpment following immediately above and behind that of the underlying series. South of the extreme end of the depression proper, the chalk escarpment forms a long narrow bay, the furthest point of which is 20 kilometres distant. Thence it runs northwards, so that to the south-east of the depression the beds of the series are seen at some little distance from the oasis-edge, but further north they appear to come to the edge and to overlie the brown limestone as part of the main scarp in the neighbourhood of the large hill already mentioned in latitude 28° 5′ N., where the strata dip so steeply into the plateau. There is some difficulty in correlating the beds of this group on the two sides of the oasis, for while on the west they are chalky and in places fossiliferous, being only in part altered to a crystalline limestone, on the east, doubtless owing to the folding which produced the strong dipping already referred to, they are almost entirely composed of hard crystalline and horny greyish-white limestone, apparently devoid of fossils. (Vide [p. 60]).

Within the depression it is probable that the White Chalk is represented in part by some of the hard gray crystalline limestone in the synclinal fold of Jebel Hefhuf.

On the west and south-west sides of the depression the White Chalk covers a considerable area, which is characterized by its rough surface and by numerous depressions, some of considerable size, eroded by wind-borne sand. To the south it extends to Farafra, being continuous with the chalk of that oasis.

Lithologically, on the west side of the oasis, the series is represented by a thick-bedded, snow-white, pure foraminiferal chalk, 30 to 45 metres thick, partly altered in the upper part into hard gray crystalline limestone. It was probably deposited in water of considerable depth.

On the west side of the depression the White Chalk yielded a fair assemblage of fossil-remains, the chief among which were great numbers of corals. In the upper part Nautili are occasionally met with, while Spirorbis, Pecten, Gryphæa, fragments of Inoceramus and other shells are fairly common. In addition echinids occur sparingly and sharks’ teeth are seen here and there. One of the best fossil localities is 24 kilometres north of Ain el Haiss.

The following is a list of those obtained:—

Cælosmilia.	Exogyra overwegi.
Linthia nov. sp.	Pecten farafrensis.
Spirorbis.	Spondylus nov. sp. aff. S. Dutempleanus.
Inoceramus.	Nautilus sp.
Gryphæa vesicularis	Corax pristodontus.

As regards the age of this White Chalk the fossil facies has a very young Cretaceous aspect. Zittel[50] has already shown that the White Chalk of the Libyan Desert, studied by him in Farafra, Dakhla and Kharga, is Danian, and the identity of the chalk of Baharia with that of those oases was satisfactorily proved on the traverse from the latter oasis to Farafra.

The White Chalk brings our description of the Cretaceous series of Baharia to a close. As will be shown, it is overlain unconformably by Eocene deposits. Throughout the deposition of the Cretaceous in this area it is clear from the character of the beds that the sea-floor had been continually sinking. After the deposition of the white chalk subsidence probably ceased and the area became one of elevation, the Cretaceous beds rising to form land. During this elevation much folding must have taken place, and subsequently gradual subsidence set in until in Lower Eocene times the area again became marine and the deposition of the rocks now to be described began to take place.

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