JURASSIC AND LOWER CRETACEOUS OF WADI HAJAR 



17 



ferruginous surfaces ("Iron Pans") at central Jebel Billum, This 

 middle part is conveniently divided off as the Middle Shuqra, 

 leaving the more marly limestones of the Upper and Lower Shuqra 

 above and below. The ripple-marks and the "Iron Pans" marking the 

 top surface were formed during a period of emergence above sea- 

 level. The Middle Shuqra, as so defined, is about 15 m thick at both 

 Al Ma'abir and central Jebel Billum. 



BlOSTRATlGRAPHY. The discovery of three specimens of the am- 

 monite Erymnoceras ( Pachyeiymnocems ) jarryi (Douville) is 

 important for dating the Shuqra Formation. Two were found 17 m 

 below the top of the Upper Shuqra in the sections at central Jebel 

 Billum and south-east of Al Ma'abir, and the third was in the top part 

 of the Middle Shuqra at the latter locality (Fig. 6). They date the 

 middle and upper parts of the Shuqra as Middle Callovian or low 

 Upper Callovian. No other ammonites have been recorded from the 

 Shuqra. Also of interest is a 0.5 m thick bed of limestone 23 m below 

 the top of the formation in central Jebel Billum, which contains 

 many large, solid, well-preserved specimens of the nautiloid 

 Paracenocems mehdionale Tintant; P. calloviense (Oppel) occurs at 

 a similar horizon at Al Ma'abir. Both are Callovian in age. 



Brachiopods are abundant in the Shuqra Formation. In the Lower 

 Shuqra they are dominated by two or three species of 

 Doghaniryhnchia. which are characteristic of this horizon. 

 Bihenithyris and Cererithyris occur in smaller numbers and there are 

 a few Coiiarosia and Arabicella. Somalirhynchia appears in the 

 Middle Shuqra. where it is the most abundant brachiopod, and is 

 accompanied by smaller numbers of Bihenithyris and Cererithyris. 

 In the Upper Shuqra, Somalirhynchia remains the most abundant 

 brachiopod, now occurring together with substantial numbers of 

 Conarosia, Arabicella. Arapsopleurum and Cererithyris, and occa- 

 sional Ara/?io//;vm and Ptyctothyris. Though generally Callovian in 

 age, these brachiopods are not well-dated independently elsewhere, 

 except that Somalirhynchia is not thought to occur below the Middle 

 Callovian, which is the probable date of the Middle Shuqra. 



Several specimens of Acrosalenia wyvillei Currie, Hemicidaris 

 gregoryi Currie andMetacrosaleniapseudocidaroides (Currie) were 

 found near the base of the Shuqra Formation at Al Ma'abir, and 

 although they are Bathonian or Callovian in age elsewhere, these 

 echinoids cannot be dated more precisely. 



Many bivalves and gastropods are found in the Shuqra Formation, 

 as seen in the long lists in the appendix. They include the genus 

 Eligmiis, of which E. aualites (Stefanini) is especially common in an 

 horizon 0-2 m above the Nautilus Bed in central Jebel Billum (Fig. 

 6). Eligmus ranges up to the top few metres of the Shuqra, but it does 

 not pass up into the Madbi Formation. The stratigraphical range of 

 Eligmus is not thought to extend higher than the top of the Callovian, 

 so, together with the presence of Erymnoceras, it is good evidence 

 that the top of the Shuqra Formation is near to the Callovian/ 

 Oxfordian boundary. 



The Madbi Formation 



LiTHOLOGY AND TYPE SECTION. The name was proposed by 

 Beydoun (1964: 31-36; see also 1968: 68) from its development on 

 Jebel Madbi, where it was described as an essentially argillaceous 

 formation of marls and shales, between the calcareous Shuqra 

 Formation below and calcareous Naifa Formation above. Beydoun 

 recorded a thickness of 252 m on Jebel Madbi. However, he did not 

 recognize that the limestones that come in approximately 100 m 

 above the base are the limestones of the Naifa Formation (Billum 

 Member). So approximately the upper 152 m of his 252 m thickness 

 on Jebel Madbi is the Naifa Formation, closely similar to its develop- 

 ment at Naifa Cliff and Wadi Kilya, and the argillaceous beds of the 



restricted Madbi Formation on Jebel Madbi are about 100 m thick. 

 The Madbi Formation is well-developed in Jebel Billum. where it 

 is 1 1 1.5 m thick, and also east of the road .south-east of the Al 

 Ma'abir river crossing, where it is about 70 m thick. It is argillaceous 

 throughout, and though marls are more common than shales, only 

 occasionally are they sufficiently calcareous to form harder beds of 

 rubbly or marly limestone. A few horizons are silty, gypsiferous or 

 bituminous. The formation is highly fossiliferous, with abundant 

 'shelly' faunas of brachiopods, bivalves and gastropods. There are 

 also many belemnites, but ammonites are rare and generally frag- 

 mentary and poorly preserved. A feature of the Madbi Formation at 

 both Jebel Billum and Al Ma'abir is the "storm beds" : the lowest one, 

 1-2 m thick, forms the bottom bed of the formation in central Jebel 

 Billum, but is cut out by faulting at Al Ma"abir, while there is another 

 one up to 4 m thick and 16 m higher at Al Ma"abir, and a top one 2 

 m thick and 43 m higher at central Jebel Billum. They are dark 

 brown, ferruginous, rubbly marls, containing immense numbers of 

 poorly sorted brachiopods, bivalves, oysters, gastropods and 

 belemnites, all heaped up in an unsorted mass as if thrown together 

 by storms (?or by earthquake induced slumping). 



BlOSTRATlGRAPHY. The Madbi Formation is Lower and Middle 

 Oxfordian in age: the age of the top is delimited by the Bimammatum 

 Zone, Upper Oxfordian, ammonites in the bottom few metres of the 

 overlying Naifa Formation, while the age of the base cannot be older 

 than the Middle or Upper Callovian date given by the ammonites in 

 the upper part of the Shuqra Formation. Age evidence from the 

 ammonites in the Madbi Formation itself consists of aPeltoceratoides 

 in the Middle Storm Bed and Perisphinctes in the Upper Storm Bed, 

 both at Al Ma"abir, which suggest a Lower and Middle Oxfordian 

 age respectively. The Shuqra/Madbi Formation junction is probably 

 close to the Callovian/Oxfordian boundary. 



Very large numbers of brachiopods occur in the Madbi Formation, 

 especially in the Storm Beds, but they are not well-dated independ- 

 ently elsewhere, and being potentially facies dependent, they do not 

 give a better age assessment of the formation. As in the Middle and 

 Upper Shuqra, Somalirhynchia is abundant, and there are now 

 representative species ofConarosia, Avonithyris. Bihenithyris, Cere- 

 rithyris, Dissoria, Eurysites, Mycerosia andPtyctothyris. Microthyri- 

 dina appears in considerable numbers in the Upper Storm Bed at 

 about the middle of the formation, and a few Somalithyris appear in 

 the upper part of the formation. 



Bivalves and gastropods are also abundant in the Madbi Forma- 

 tion. Many genera continue from the Shuqra Formation, except 

 Eligmus which is now absent, and are joined by Indogrammatodon. 

 Neocrassina and other genera. 



The Naifa Formation 



LiTHOLOGY AND TYPE SECTIONS. After its description in an un- 

 published Iraq Petroleum Company report by Pike & Wofford in 

 1939, the first published accounts of the Naifa Formation were by 

 Beydoun (1964: 31^6; 1968: 80). He designated the section at 

 Naifa Cliff on the south (right) bank of the river, 2 km upstream from 

 the road/river crossing at Al Ma"abir as the type locality. The middle 

 two-thirds of the formation are seen here (Figs 3, 9): the top was 

 removed by erosion before deposition of Quaternary rocks, while 

 the base is seen in nearby sections at the Al Ma'abir road/river 

 crossing and at Wadi Kilya. 



The Naifa Formation is essentially calcareous: thick, hard lime- 

 stones in the lower half, are followed by marls and limestones in the 

 upper half. This slight difference in lithology is the basis of its 

 division into two members, the lower Billum Member and the upper 

 Kilya Member, which are newly proposed here. 



