JURASSIC AND LOWER CRETACEOUS OF WADl HAJAR 



galar (Oppel)) and the Madbi Formation {Ataxioceras desmoides 

 Wegele, Idoceras ci'. farquluirsoni Spath, Perisphinctes momhassamis 

 Dacque, P. (Pachysphinctes) wbustiis Spath, P. (Divisosphinctes) cf. 

 inaeqitalis Spath). In fact, none of these Kimmeridgian ammonites 

 occur in the Shuqra and Madbi Formations, which are of entirely 

 older dates: the basal limestone inWadiArus is the topof the Billum 

 Member of the Naifa Formation, and the overlying marls and 

 nodules (the Breadloaf Concretions) belong to the Kilya Member of 

 the same formation and contain the same ammonites as at Naifa 

 Cliff. This part of the succession in Wadi Arus ends at the same 

 unconformity as found at Jebel Billum, that misses out the whole of 

 the Lower Tithonian except for part of the basal zone. It is overlain 

 by more marls (some with gypsum veining), concretions and 

 microbialites', containing a splendid ammonite fauna entirely new 

 to Yemen, of basal Upper Tithonian age, then by the massive 

 limestones and interbedded marls that are so obvious in the road 

 gorge leading down into Wadi Arus. Both belong to a new formation 

 (here named the Hajar Formation); the upper massive limestones are 

 largely poor in ammonites here, though a bed full of a new species of 

 Upper Tithonian or basal Berriasian Substeueroceras was found in 

 the upper half of the unit. 



Another section that is crucial to an understanding of Beydoun's 

 dating of the 'Naifa Formation' is in the Mintaq salt dome. As in 

 Wadi Arus, there are thick massive limestones and interbedded 

 marls in the Mintaq section that were identified as Naifa Formation 

 by Beydoun, though here they are underlain not by marls and 

 concretions, but by evaporites of the Sabatayn Formation. Many 

 microfossils (foraminifera and calpionelids) were obtained from 

 Beydoun's samples from the limestones of the Mintaq section, and 

 they were dated as Upper Tithonian. 



For the age of the rocks in Wadi Hajar that he referred to his Naifa 

 Formation, Beydoun now had two incompatible dates: top Oxfordian 

 to Kimmeridgian from the ammonites at Naifa Cliff and Jebel 

 Billum. or Upper Tithonian from the microfossils at Mintaq and the 

 meagre evidence in Wadi Arus. He chose the Upper Tithonian date 

 from the Mintaq evidence. This was unfortunate, because the lime- 

 stones at Mintaq and the upper limestones at Wadi Arus are not the 

 same as those at Naifa Cliff/AI Ma'abir and Jebel Billum, and Upper 

 Tithonian is a whole stage too high for the date of the Naifa 

 Formation at its type locality in Naifa Cliff. 



One final section to be considered is that at Jebel Madbi. Though 

 we have not seen it ourselves, from Beydoun's detailed section and 

 from subsequent photographs, it can be interpreted according to the 

 nomenclature used by Beydoun himself at Naifa Cliff and Jebel 

 Billum. The position of the base of the marly Madbi Formation is 

 clear in Jebel Madbi, and this is its type locality. After a thickness of 

 100 m of marls, there are 74 m of limestones, then 78 m of more 

 marly limestones, before a return to massive limestones, 

 conglomeratic at the base, which attain the very large thickness of 

 434 m in Jebel Madbi. Because of the prominence of the latter 

 limestones with conglomerates at their base, Beydoun identified 

 them as the Naifa Formation, which made his underlying Madbi 

 Formation 252 m thick, including the 74 m thick sequence of 

 limestones in the middle. In fact, the latter limestones on Jebel 

 Madbi are the lower half of the Naifa Formation (the Billum 

 Member), the next 78 m of more marly limestones are the upper half 

 the of same formation (the Kilya Member, exactly as at Naifa Cliff 

 and in Wadi Kilya), and the overlying 434 m of conglomerates and 

 massive limestones are the Hajar Formation, with the conglomerates 

 at the base marking the unconformity. 



'Calcareous bodies formed by algae, bacteria and cyanobacteria; they include 

 stromatolites, thrombolites and dendrolites. 



Table 1 Comparison of the lithological nomenclature for the Jurassic and 

 basal Cretaceous in Wadi Hajar as proposed by Beydoun in 1964-68 

 and as used subsequently. In the left hand coluinn N shows the range of 

 the limestones at the type locality of the Naifa Formation at Naifa Cliff 

 and close vicinity, while M shows the range of the limestones in the 

 Mintaq Salt Dome. 



BEYDOUN 

 1964-68 



SUBSEQUENT 

 INTERPRETATIONS 



THIS PAPER 



Formation 



Formation 



Member 



Formation 



Member 



M — ' 



NAIFA 



N — ; 



NAIFA 



HAJAR 



Mintaq 



Arus 



MADBI 



U. Madbi Shales 



NAIFA 



Kilya 



Madbi Porcellanites 



Billum 



MADBI 



L. Madbi Shales 



MADBI 



SHUQRA 



SHUQRA 



SHUQRA 



KOHLAN 



KOHLAN 



KOHLAN 



Beydoun's incorrect correlation between Naifa Cliff/Jebel Billum 

 and Wadi Arus/Mintaq/Jebel Madbi has had profound effects on all 

 later work. From the obvious lithological divisions in a section like 

 Jebel Madbi, the terms Lower Madbi Shales (for the whole of the 

 real Madbi Formation), Madbi Porcellanites (for the Billum Mem- 

 ber of the Naifa Formation), Upper Madbi Shales (for the Kilya 

 Member of the Naifa Formation), and 'Naifa Formation' (for the 

 Hajar Formation as proposed here) have become widely used in 

 recent years (see Table 1 ). Thus the term 'Naifa Formation' has been 

 transferred to rocks of Upper Tithonian and Berriasian age. This has 

 been done despite the fact that as a consequence there is no such 

 'Naifa Formation' at its type locality at Naifa Cliff. If lithostrati- 

 graphical nomenclature is to be used in a meaningful and practical 

 way, then notice has to be taken of priority of usage and the rocks that 

 occur at type sections, because to contend that a formation does not 

 occur at its type locality leads to unacceptable instability of nomen- 

 clature. So the Naifa Formation is used here as originally defined 

 from the rocks that occur at its type locality at Naifa Cliff and the 

 immediate vicinity. Also the base of the Madbi Formation is drawn 

 where it was first proposed at the base of the argillaceous formation 

 in Jebel Madbi; it is about 100 m thick and extends as far up as the 

 base of the overlying limestones, which are the same as the lime- 

 stones of the Naifa Formation as defined at Naifa Cliff. On Jebel 

 Madbi the Naifa Formation consists of a lower half of about 74 m of 

 limestones, and an upper half of about 78 m of more marly lime- 

 stones, up to the unconformity overlain by conglomerates and 

 limestones of the Hajar Formation. 



In his original definition in the Naifa Cliff/Al Ma'abir area (which 

 includes Wadi Kilya), Beydoun was quite clear that the Naifa 

 Formation consists of a lower limestone half, overlain by a more 

 marly upper half, which we now call the Billum and Kilya Members 

 respectively. The Kilya Member is terminated by an obvious 

 unconformity in Wadi Arus, where it is followed by the Hajar 

 Formation, with marls, limestones and concretions at the base (here 

 named the Arus Member), then by thick massive limestones above. 

 The latter limestones are prominent and thick at Mintaq, where they 

 are here named the Mintaq Member of the Hajar Formation. The 

 whole succession appears to attain its thickest, and perhaps most 

 complete, development on Jebel Madbi, which would repay careful 

 investigation and ammonite collecting. Beydoun's interpretation of 



