Baharia Oasis

by John Ball; H. J. L. Beadnell

Another point of disturbance is on the western plateau in N. lat. 28° 26′, E. long. 28° 49′, some 11½ kilometres north-west of El Qasr. Here (Fig. 2) along a N.E. and S.W. line, crossing an otherwise level plateau, the Eocene beds, and probably also the Cretaceous below, are thrown up in a sharp anticlinal fold, which however, is not continuous but consists of a number of isolated bulges, with dips up to 50°. The plateau at this point consists of Eocene Operculina and Nummulitic limestone, and the effect of the anticlinal fold is to expose the beds of sandstone below, which, as already mentioned, are probably Cretaceous.

Fig. 2.—Sketch showing probable relations of Eocene and Cretaceous in anticline on Western Desert plateau, 11½ kilometres N.W. of El Qasr.

Eocene.—1. Grey crystalline limestone. 2. Hard white gritty chalky limestone. 3. Grey calcareous sandstone. 4. Limestone with Nummulites and Operculina.

Upper Cretaceous.—5. Calcareous-grit. 6. White sandstone. 7. Crystalline limestone. 8. Cenomanian clays and sandstones with Exogyra flabellata, E. Mermeti and E. olisiponensis, etc.

With regard to the age of the tectonic movements which produced the synclinal and anticlinal folds of Baharia, it must be admitted that it is not easy to speak with absolute certainty, owing to want of evidence at one or two points. It appears certain, however, that they belong to two great movements, separated by a vast interval in time. The first disturbance of the beds would appear to have taken place at the end of the Cretaceous period, when it seems probable that the area in question underwent upheaval into dry land. It seems tenable that it was during this movement that the large flat anticline, so well seen in the south part of Baharia and in the north of Farafra, was produced, as the Eocene beds forming the plateau immediately to the east of the Cretaceous beds do not show any sign of disturbance.[60] The subsiding Cretaceous land, on which these Eocene deposits were laid down, must therefore have had the form of a long, flat, irregular ridge of anticlinal structure, probably extending from Dakhla through the oases of Farafra and Baharia to Abu Roash.

The other folds of Baharia, being parallel to the anticline just dealt with, would at first sight appear to date from the same movement. With regard to the syncline, however, two observations appear which force us to consider its age as very much later; these are, first, the occurrence of nummulites and other Eocene fossils within the fold at a point 16 kilometres north of Ain el Haiss, and secondly, the fact that at the northern extremity of the fold, where it meets the eastern scarp, the Eocene beds are affected. With regard to the first of these, the beds themselves, from which the fossils had been derived, were not observed to be actually affected by the fold, and therefore the evidence here, although suggestive, is not conclusive. The evidence at the north end of the fold, however, if substantiated, proves that the age of the fold must be post-Eocene. With regard to the anticlinal fold on the north-western plateau there is no question, as this clearly affects the Eocene beds; it also is parallel to the other folds. These folds affecting Eocene beds, lead us to the conclusion that there was another period of possibly ever more important earth-movements, of the exact date of which we cannot be certain, although it is not impossible that it was closely connected with those great earth-movements of Pliocene times, which gave rise to the chief topographic fixtures of N.E. Africa and S.W. Asia.

Summary.Summary of the Geological History of the Oasis.—The oldest[61] sedimentary deposit in Egypt is the Nubian Sandstone of the Cretaceous. From the general absence of marine shells in this rock we may premise that the deposit was laid down in an inland sea or lake, which must have covered an enormous extent of country. In the northern part of Egypt the upper part of the Nubian Sandstone becomes fossiliferous, the fauna having an undoubted Cenomanian aspect; this is the case in Wadi Araba, at Abu Roash and in Baharia. Further to the south, however, as in Dakhla, the Nubian Sandstone first becomes fossiliferous in Senonian times. We may explain this difference in the two localities on the supposition that a gradual subsidence was taking place with a corresponding gradual encroachment of the sea from the north, which covered the northern part of the country (including Wadi Araba, Abu Roash and Baharia) in Cenomanian times but did not reach the latitude of Dakhla till considerably later, i.e., in Senonian times.

From the Cenomanian onwards throughout the Cretaceous, the Baharia area was one of continued subsidence, the lithological and palæontological characters of the beds showing evidence of a gradual increase in conditions of depth, until the maximum was reached in the deposition of the White Chalk in Danian times.

Between the deposition of the uppermost Cretaceous rocks and the Eocene it is probable that a considerable interval elapsed, during which the Cretaceous was elevated into land, with much folding and fracturing of the rocks and subsequent denudation.[62] It was probably during this upheaval that the Cretaceous of Baharia assumed its anticlinal structure.