12 



M.K. HOWARTH AND N.J. MORRIS 



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Quaternary 

 Tertiary (Palaeocene, Eocene) 

 Cretaceous, post-Qishn Formation 

 Qishn Formation, U. Haut.-Aptian 

 Hajar Formation, U. Tithonian 

 Naifa Formation, U. Oxf.-Kimm. 

 Madbi Formation, Oxfordian 

 Shuqra Formation, Callovian 

 Kohlan Formation, ?M. Jurassic 

 Kohlan Volcanics, ?L.-M. Jurassic 

 Pre-Cambrian 

 Unmetalled road 



Fig. 7 Geological map of Jebel Billum and Wadi Arus. EWA and WWA are the locations of the east and west cliffs in Wadi Arus of Figs 13, 15, 16; WJB 

 is the western road entrance to Jebel Billum; CJB is the main Jurassic cliff section of central Jebel Billum, photographed from the western entrance in 

 Fig. 8, and in detail in Fig. 10; PC (Perisphinctid Cliff) is the long cliff (shown in Fig. 1 1 ) with perisphinctids low in the Billum Member at its base; EJB 

 (Eastern Jebel Billum) is (he section on the north side of the road where it first enters eastern Jebel Billum, depicted in the upper half of Fig. 12; SM is 

 the exposure of the Shuqra and Madbi Formations in east Jebel Billum, 1 km SW of the Perisphinctid Cliff. 



cliff access can be gained to the Kilya/Arus contact, where it is seen 

 to be an angular unconformity, the irregular bottom of the Arus 

 Member cutting down into the eroded top of the Kilya Member. 

 Access can also be obtained here to the scattered boulders in this 

 bottom bed and to the main 2.4 m thick bed of boulders. A rich basal 

 Upper Tithonian ammonite fauna was obtained from these boulders 

 (Fig. 17), mainly from 2.4 m bed of large microbialite boulders, but 

 the same ammonites also occur in the smaller boulders that are 

 scattered through the bed below down to the base of the Arus 

 Member. The whole ammonite fauna is new to Yemen, and includes 

 the remarkable discovery of two large examples of the boreal 

 ammonite Riasanites rjasanensis (Lahusen), the only record of this 

 genus in the Tethyan Province except in Argentina, and at a consid- 

 erably older horizon than its topmost Tithonian to Berriasian age in 

 the Boreal Province. Beds have apparently slumped and slipped 

 along low angle planes at the top of this west cliff, and it is not clear 



how much (if any) of the Mintaq Member is present higher up before 

 Cretaceous rocks are emplaced (?by slumping). 



The succession in the eastern cliff is considerably different: the 

 two points of correlation are the bed of microbialite boulders, which 

 are clearly the same on both sides of the wadi, and the limestone that 

 is the lowest horizon exposed on the eastern side (Fig. 1 6). Above the 

 latter limestone there are 35 m of marls and limestones of the Kilya 

 Member up to the base of the Arus, which are not seen in the west 

 side cliff, having been cut out by sliding, slumping or erosion. Near 

 the middle of the Kilya Member as developed in this eastern cliff 

 there are marls containing the Breadloaf Concretions, from which a 

 rich Upper Kimmeridgian. Beckeri Zone, ammonite fauna was 

 collected. It is similar to the fauna from the lower part of the Kilya 

 Member in Wadi Kilya and at Naifa Cliff. In fact, 2.5 m above the 

 base of the Kilya Member in the eastern cliff there is an horizon of 

 widely scattered, large (1 m diameter x 0.5 m thick) grey limestone 



