Bull. nat. Hist. Miis. Land. (Geol.) 54(1): 1-32 



Issued 25 June 1998 



The Jurassic and Lower Cretaceous of Wadi 



Hajar, southern Yemen 



THE NATURAL 

 HISTORY MUSEUM 



MICHAEL K. HOWARTH & NOEL J. MORRIS 



Department of Palaeontology, The Natural History Museum. Cromwell Road. London SW? 



5B ) 



29 JUf4 1998 



PRESENTED 



GEIVffi R AL LIBRARY 



Synopsis. Jurassic and Lower Cretaceous rocks inWadi Hajar are about 600 m thick, and consist of the Kohlan (oldest), Shaqra, 

 Madbi, Naifa (Billum and Kilya Members), Hajar (Arus and Mintaq Members) and Qishn Formations, of which the Hajar 

 Formation and its two members and the two members of the Naifa Formation are newly proposed here. The arenaceous Kohlan 

 Formation overlies the Precambrian and contains no fossils inWadi Hajar, but is dated elsewhere as Lower and Middle Jurassic. The 

 calcareous Shuqra Formation is Callovian in age and contains many brachiopods, bivalves and gastropods. The argillaceous Madbi 

 Formation also contains many brachiopods and molluscs, and a few ammonites that date it as Oxfordian. The Naifa Formation, 

 Billum Member is calcareous with occasional Upper Oxfordian to Upper Kimmeridgian ammonites; the Kilya Member is more 

 marly and contains many Beckeri Zone (Upper Kimmeridgian) and Hybonotum Zone (Lower Tithonian) ammonites. After a 

 disconformity representing the remainder of the LowerTithonian, the Hajar Formation, Arus Member is calcareous and marly, with 

 gypsum veining and microbialite boulders in the lower part, and contains many UpperTithonian ammonites. The Mintaq Member 

 is highly calcareous, and contains many top Tithonian to mid-Berriasian ammonites. After another disconformity missing out the 

 Valanginian and Lower Hauterivian, the Qishn Formation consists of sandy limestones, from which an Upper Hauterivian 

 ammonite was obtained, followed by the Orbitolina Limestone of Upper Barremian to Aptian age. 



INTRODUCTION 



From its mouth on the Gulf of Aden, 430 km ENE of Aden and 80 km 

 SW of Mukalla, Wadi Hajar extends inland for approximately 120 

 km north-westwards (Fig. 1). In Jurassic times the area now occu- 

 pied by the south-eastern part of Wadi Hajar was part of a relatively 

 slowly subsiding shelf sea, compared to the more rapidly sinking 

 Hajar Basin to the north-west and the Balhaf Basin to the south- 

 west. In this area Jurassic and Lower Cretaceous rocks lie directly on 

 basement metamorphic and igneous rocks of Precambrian age, and 

 they are overlain by younger Cretaceous andTertiary rocks. Because 

 of its large areas of good outcrop and relative ease of access, Wadi 

 Hajar has been the scene of much exploratory collecting in recent 

 years by those seeking standard successions through the Jurassic and 

 suites of macro- and microfossils. for use in age determinations of 

 borehole successions in basinal areas that lack outcrops of Jurassic 

 rocks. In May 1991 two specimens of an undescribed Berriasian 

 species of Spiticeras from the Mintaq Salt Dome in the upper part of 

 Wadi Hajar were submitted to one of us (MKH) for determination. It 

 was immediately clear that these represented an ammonite fauna and 

 an age that had not been described before from Yemen. We were able 

 to visit Wadi Hajar in November 1991, when we obtained more than 

 100 specimens from this new and splendid Berriasian ammonite 

 fauna at Mintaq. We also collected about 30 ammonites from a 

 difficult-to-interpret section in eastern Jebel Billum, of Upper 

 Tithonian age, and several Upper Oxfordian ammonites from a 

 nearby locality just above the base of the Naifa Formation . The latter 

 ammonites were from one of two faunas that had been sampled by 

 previous investigators, and after determination by Drs L.F. Spath 

 and W.J. Arkell, ought to have resulted in an Upper Oxfordian to 

 Kimmeridgian age being given to the Naifa Formation. We located 

 and redetermined these previous collections, and we were then sent 

 for determination some beautifully preserved examples of the basal 

 Tithonian ammonileKatroliceras, as well as some top Kimmeridgian 

 ammonites. These led us to believe that there was a great deal more 

 variety in the Wadi Hajar Kimmeridgian and Tithonian ammonites 



than had been described before, and that although the Mintaq Salt 

 Dome and Jebel Billum/Naifa CliffAVadi Kilya rocks were of very 

 similar appearance, they contained entirely different ammonite fau- 

 nas of very different dates, ie. Berriasian, and Upper Oxfordian to 

 basal Tithonian respectively. 



A new tentative correlation was drawn up of these Upper Jurassic 

 rocks from Mintaq to Wadi Arus, Jebel Billum, Naifa Cliff and Wadi 

 Kilya, that fitted in with the collections that were then available to 

 us. With this as a working hypothesis, we made another visit to Wadi 

 Hajar in January 1994. This yielded splendid collections of ammo- 

 nites from both existing and entirely new localities and ages, and 

 showed that the working correlation was largely correct, requiring 

 modification only in detail. With the 580 ammonites collected on 

 this second visit, we now had over 800 ammonites from the Jurassic 

 and Lower Cretaceous of Wadi Hajar, all from known levels, and 

 representing 14 biostratigraphical horizons from the middle of the 

 Callovian to the middle of the Berriasian, the majority being from 

 the top zone of theTethyan Kimmeridgian to the Berriasian. Fossils 

 of many other groups were also collected, foremost amongst them 

 being Callovian and Oxfordian brachiopods that are abundant in the 

 Shuqra and Madbi Formations. Numerous bivalves and a few 

 echinoids were obtained from the same two formations, but in the 

 Naifa and Hajar Formations faunas other than ammonites are much 

 rarer or absent. 



The purpose of this paper is to describe the Jurassic and Lower 

 Cretaceous of Wadi Hajar between the south-easternmost locality at 

 AlMa'abirand the Mintaq Salt Dome, 65 km to the north-west (Fig. 

 1 ). As such it takes in the river section at Naifa Cliff and the nearby 

 section in Wadi Kilya, both at Al Ma'abir, which together make up 

 the type section of the Naifa Formation: the various sections in the 

 outcrop along the southern side of Jebel Billum, where all the 

 Jurassic formations are developed; the splendid exposures in the 

 cliffs on both east and west sides of Wadi Arus and the road gorge 

 climbing out of that wadi to the south-east, where there are expo- 

 sures from the top of the Billum Member upwards, including the 

 type sections of the Hajar Formation; and the exposures of the 

 Mintaq Member in the Mintaq Salt Dome, where the beds have been 



© The Natural History Museum. 1998 



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