JURASSIC AND LOWER CRETACEOUS OF WADI HAJAR 



25 



Formation, it had certainly fallen from the shelly sandstone. 5 m 

 thick at the base of the Qishn Formation immediately above. It is 

 Upper Hauterivian in age, and is the first date obtained for this basal 

 part of the formation. The Orbitolina Limestone is generally recog- 

 nized to be Upper Barremian in age from the foraminifera 

 [Palorbitolina lenticularls (Blumenbach) and Choffatella deciplens 

 Schlumberger) that it contains (Beydoun, 1968: 92). Three ammo- 

 nites were recorded by Beydoun, 1968: 93) from the upper half of 

 the 498 m thick Qishn Formation in the Wadi Masila section referred 

 to above: Cheloniceras (C.) comiieliamis (d'Orbigny) (BMNH 

 C.86982: figured Howarth, 1998, pi. 21. fig. 3), Cheloniceras (C.) 

 sp. indet. (BMNH C. 86983, recorded as C. martini (d'Orbigny) by 

 Beydoun; figured Howarth, 1998, pi. 21, fig. 4) and ^Cheloniceras 

 (BMNH C.71676, recorded as Doiivilleiceras sp.), which all indi- 

 cate an Upper Aptian age at that level. However, the specimen of 

 Crioceratites from the base of the formation is a whole stage earlier 

 than previous dates obtained for the Qishn Formation. 



CORRELATION WITH EAST AFRICA 



The Jurassic and Cretaceous rocks of Yemen and Somalia were 

 deposited before the opening of the Gulf of Aden, which com- 

 menced with rifting in the Oligocene followed by sea-tloor spreading 

 from the late Miocene onwards (Abbate et al. 1988: 440-442). The 

 geology of the southern coastal areas of Yemen and northern Soma- 

 lia was described and compared by Beydoun (1970), from whose 

 work the diagram of Fig. 2 1 was drawn, using the best-fit of the pre- 

 rifted structures. The best described Jurassic sequence in northern 

 Somalia is near Bihendula, which can be seen from Fig. 21 to have 

 been only 225 km south-west of the Wadi Hajar area in the Jurassic, 



compared with the present-day post-rifting separation of 580 km. 

 The sequence near Bihendula was described by MacFadyen (1933) 

 and the ammonites in the succession were described by Spalh 

 (1935). A summary of the age assessments was given by Arkell 

 (1956: 308-312), and a more recent update of the lithostratigraphy, 

 including a vertical section of the succession at Bihendula. can be 

 found in Bosellini (1989: 412 &c, fig. 33). As expected, there is 

 much similarity between the Jurassic sequences of Wadi Hajar and 

 Bihendula, and the following comparison can be made: 



WADI HAJAR 



Hajar Formation 



Naifa Formation, Kilya Member 



Naifa Formation, Billum Member 



Madbi Formation 



Shuqra Formation 



Kohlan Formation 



BIHENDULA 



?Gawan Limestone 

 Daghani Shales 

 Wanderer Limestone 

 Gahodleh Shales 

 Bihen Limestone 

 Adigrat Sandstone 



The initial correlation is based on similar or near-identical lithol- 

 ogy in the two areas, suggesting that they were probably within the 

 same basin of deposition. However, there are differences in relative 

 thicknesses (eg. the Daghani Shales are much thicker (370 m) than 

 the Kilya Member (65 m) or the combined Kilya andArus Members 

 (113 m)), but the ammonites and other faunas described from the 

 two sequences lend considerable support to this correlation. The 

 most useful ammonites for age dating in Somalia were collected 

 from a section that extended through all the formations, at Daghani, 

 6 km east of Bihendula. which was described by MacFadyen (1933: 

 28) and Spath (1935: 206-208). The correlation given above is not 

 the same as the correlation suggested by Beydoun (1964: 45, table 

 4), which was not based on a comparison of the ammonite faunas in 

 the two areas. 



The Kohlan and the Adigrat Sandstone are similar arenaceous 



Fig. 21 The Jura.ssic outcrops in Wadi Hajar and northern Somalia, showing their relative position before the formation ot the Gull' of Aden in the 



Miocene. Outcrops of Jurassic are shown in solid black. Redrawn from Beydoun. 1970. figs 2-4: the tit between Yemen and Somalia is at the .SOO fathom 

 (approx. 1000 m) isobath, which leads to a minimum (and very small) amount of superposition of Basement. Me.sozoic and recent volcanic rocks in the 

 overlap triangle between Aden and Djibouti. 



