JURASSIC AND LOWER CRETACEOUS OF WADI HAJAR 



and many brachiopods, bivalves and gastropods were obtained from 

 the Shuqra and Madbi Formations (Fig. 6). 



Jebel Billum 



The long arcuate outcrop of sedimentary Jurassic rocks at Jebel 

 Billum is about 10 km long. It overlies Precambrian metamorphic 

 and igneous rocks (and basalts that are probably Lower/Middle 

 Jurassic in age) to the south-west, and is followed by the Qishn 

 Formation, then higher Cretaceous rocks and massive Tertiary lime- 

 stones to the north-east (Fig. 7). The peak of Jebel Billum itself is 

 formed by Palaeocene limestones of the Umm-er-Radhuma Forma- 

 tion. In the north-western part of the outcrop, a cliff in the upper part 

 of the Jurassic (Fig. 8) lies south-west of that peak. Here the Naifa 

 and Hajar Formations are overlain by the Qishn Formation, the 

 contact with the latter being readily recognizable by the sharp colour 

 change from grey to reddish-brown near the top of the cliff. The top 

 of the Madbi Formation occurs at the base of the cliff, and the 

 overlying Billum Member is 60 m thick, extending up to a prominent 

 white limestone (Fig. 10). The Kilya Member has a middle lime- 

 stone section between more marly beds above and below, as at Wadi 

 Kilya, but the exact position of the disconformable contact with the 

 Arus Member is uncertain owing to lack of access. It is at or some 

 distance below the prominent beds of massive grey limestone near 

 the top of the cliff which are overlain by the more sandy reddish- 

 brown beds of the Qishn Formation. The lower Billum Member part 

 of this cliff extends westwards to the road-cutting at the western 

 entrance to the Jebel Billum inlier, where it forms an overhanging 

 cliff, which was the vantage point for the photograph of Fig. 8 (also 

 Beydoun's (1964: pi. 9, lower) photograph; note that on his photo- 

 graph he placed the 'NaifaVQishn 'unconformity' at the position of 

 the Billum/Kilya Member contact). The Glochiceras and 

 Orthosphinctes shown in the bottom half of the Billum Member in 

 Fig. 12 were collected from material excavated from this western 

 road-cutting entrance to Jebel Billum. 



From the front of Jebel Billum, the cliff of Naifa and Hajar 

 Formations extends south-eastwards for several kilometres forming 

 long cliffs, near the top of which the Qishn contact is readily seen 

 from the sharp colour change to reddish-brown. The unconformity 

 between Naifa and Hajar Formations (ie. at the contact between the 

 Kilya Member and the Arus Member) is also well seen, especially in 

 the long easternmost cliff. At the base of this cliff near its eastern end 

 (Fig. 1 1; at locality PC on Fig. 7) many perisphinctids occur 7-8 m 

 above the base of the Billum Member. Many are fragments or 

 moulds of large specimens on limestone blocks and are neither well- 

 preserved nor collectable. Several photographs and some specimens 

 were obtained (list on Fig. 12), and they form the main evidence for 

 the dating of the bottom of the Billum Member as Upper Oxfordian. 



A succession that is more difficult to interpret is in a steep bank 

 and cliff on the north side of the road just before it leaves the eastern 

 end of the Jebel Billum outcrop (locality EJB on Fig. 7). The 

 Orbitolina Limestone of the Qishn Formation forms a small cliff at 

 the top of the bank, and parts of the underlying Arus Member are 

 exposed in the bank below, including the contact with the Kilya 

 Member, which can be clearly seen to be an unconformity here. The 

 bulk of the Kilya Member is obscured, but at the western end of the 

 outcrop the whole thickness of the Billum Member is well seen. The 

 main interest, however, is in the Arus Member, which is 61 .5 m thick 

 here, and in the 1 3 . 8 m of the Mintaq Member up to the disconformity 

 at the base of the Qishn Formation (Fig. 12). The Mintaq Member 

 has two horizons with ammonites: an upper bed of marls only 5 m 

 below the Qishn with crushed Substeueroceras, and a lower bed of 

 porcellanous limestones with many ammonites including Virgato- 



sphinctes. Choicensisphinctes. Siihsteueraceras and Blanfordiceras . 

 Both faunas are from the upper half of the Upper Tithonian, and the 

 upper one is important in providing evidence for the age of the beds 

 immediately below the Qishn Formation in Jebel Billum. The under- 

 lying shales and marls of the Arus Member contain similar ammonites 

 in a bed of rubbly limestone just below the middle of the member 

 (Fig. 12), and the age is somewhat lower in the Upper Tithonian. A 

 single specimen of Choceratites from the basal bed of the Qishn 

 Formation here is the first evidence for an Upper Hauterivian date 

 for the bottom of that formation. No ammonites were found in the 

 Kilya Member at this locality, nor have they been found anywhere 

 else in the Jebel Billum inlier, though there are large areas of outcrop 

 where they might be expected, all of which are difficult to reach (eg. 

 the flanks of the main Jurassic cliff in Figs 8, 10). 



The Kohlan, Shuqra and Madbi Formations are well exposed in 

 the valley (part of Wadi Ghiadhat) below the main Jurassic cliff in 

 Jebel Billum, and there are places where the contact with the 

 metamorphosed basement igneous rocks is seen. The upper half of 

 the Shuqra and the Madbi Formations were measured here, and large 

 collections were made of the immensely rich brachiopod faunas. 

 Bivalves and gastropods are also abundant at some horizons, and 

 there is a bed containing large specimens of the nautiloid Paraceno- 

 ceras near the top of the Shuqra Formation (Fig. 6). 



Wadi Arus 



4 km west of the western road entrance to the Jebel Billum inlier the 

 road descends into the upper end of Wadi Arus through a gorge that 

 exposes the full thickness of the Mintaq Member of the Hajar 

 Formation (Fig. 7). At the bottom the road turns northwards into the 

 wider part of Wadi Arus and after 1 km passes a prominent cliff on 

 the west exposing a bed of large microbialite boulders near the top 

 (Figs 13, 15). There are cliffs on both sides of the wadi here, and 

 correlation between them poses difficulties, though it appears that 

 the lowest limestone in the eastern cliff can be traced continuously 

 across the wadi and the unmetalled road to the middle of the western 

 cliff (Fig. 16). A short distance to the north a complex low-angle 

 fault system brings the Cretaceous down to the bottom of the wadi 

 and the Jurassic is not seen again farther north. Little visited this 

 locality and published vertical sections of the western cliffs (Little, 

 1925: pi. 25) showing the fault and beds up the wadi to the south, 

 though it is not possible to identify the cliff with the microbialite 

 boulders on his sections. He also gave a section (1925: pi. 26) of the 

 eastern cliffs, marking a bed of 'oil shales', which he said correlated 

 with a similar bed at Naifa Cliff. This is near the bottom of the lowest 

 exposure in the eastern cliffs shown on Fig. 16, and being near the 

 base of the Kilya Member, Little's correlation is nearly correct. 

 Beydoun also visited these exposures, and collected ammonites 

 from the Breadloaf Concretions on the east side (which he said 

 belonged to the Madbi Formation) and also a few from the top of the 

 Billum Member (which he said was part of the Shuqra Formation) 

 (Beydoun, 1 964: 35, 36), but he made no reference to the bed of large 

 microbialite boulders in the western cliff. 



The upper part of the Billum Member is seen in a small cliff just 

 south of that west cliff (Fig. 1 3, lower left), and its top bed containing 

 many crushed impressions of Strehlites and perisphinctids forms the 

 pavement at the bottom of the main cliff (Fig. 15). In the west cliff 

 there is a thickness 12.4 m of pale grey marls and limestones of the 

 Kilya Member up to a clear disconformity at the base of the Arus 

 Member of the Hajar Formation. The bottom bed of the Arus 

 Member consists of darker yellow-brown marls and limestones 

 containing many strings and cross bedded veins of gypsum, as well 

 as scattered microbialite boulders. On the south side of the western 



