JURASSIC AND LOWER CRETACEOUS OF WADI HAJAR 



13 



doggers, which are so similar to the large doggers 7.5 m above the 

 base of the Kilya Member in Naifa Cliff (Fig. 3), that a direct 

 lithological correlation between them might be postulated. The 

 microbialite boulders in the eastern cliff contain the same ammo- 

 nites as in the boulders of the west cliff, and above them are 40.9 m 

 of marls and limestones before much more massive limestones are 

 reached that mark the base of the Mintaq Member. These can be 

 traced continuously along the cliff to the south, then up the road 

 gorge out of the wadi to the south-east up to the base of the Qishn 

 Formation, making a total thickness of 85 m of Mintaq Member 

 (Fig. 18). There are huge slumped blocks of these Mintaq Member 

 limestones on the south side of the road gorge, especially at its lower 

 end where it turns into the wadi, which are seen in a photograph 

 published by Beydoun (1964: pi. 1 1 ). Most of the limestones contain 

 no ammonites (or other fossils), but about 31 m below the top there 

 is a bed containing abundant crushed Substeuewcems and a few 

 Protacanthodiscus, which give an UpperTithonian or basal Berriasian 

 age to this part of the Mintaq Formation. Still higher, about 16 m 

 below the Qishn Formation, a very large (0.5 m d.\<imt\.Qr)Aspidocenis 

 was seen and photographed, though it could not be collected. 



The Mintaq Salt Dome 



About 50 km farther up Wadi Hajar to the north-west is an area 

 where Jurassic rocks outcrop in three small salt domes. The first of 

 these is the Mintaq Salt Dome, 8 km beyond Sidara, in which the 

 Mintaq Member has been pushed into an orientation with vertical 

 bedding around the periphery of the dome, by rising gypsum and 

 halite deposits in the underlying Sabatayn Formation. There are two 

 main outcrops of the Mintaq Member here, the south-western and 

 north-eastern outcrops, separated by the core of the dome in which 

 there are two plugs of Recent basalts (Fig. 19). The south-western 

 outcrop gives a continuous measurable succession from its contact 

 with the Sabatayn evaporites, through 215.7 m of limestones with 

 subsidiary marls, up to its contact with the overlying Qishn Forma- 

 tion, where a disconformity misses out part of the Beniasian, the 

 Valanginian, and Lower Hauterivian stages (Fig. 20). The upper 

 two-thirds of the Mintaq Member and its contact with the Qishn 

 Formation are well exposed in a gulley (Fig. 14; at locality X on Fig. 

 19), near the bottom end of which a rich ammonite fauna was 

 collected from beds 27-77. A few ammonites occur sporadically in 

 higher and lower beds. All are new to Yemen and are of mid- 

 Berriasian age. Of particular interest is the abundance of theTethyan 

 ammonite Spiticeras and the South American Berriasian genus 

 Argentiniceras. There are no ammonites in the top 64 m of the 

 Mintaq Member, the age of which, though probably still Berriasian, 

 is not known accurately. The ammonite-bearing beds were not 

 located on the hillside east of the main gulley (Fig. 14, background 

 at top right), nor were they located during a search of the extensive 

 north-eastern outcrop of the Mintaq Member in the salt dome. The 

 latter outcrop appears to expose the same beds as in the south- 

 western outcrop, and there are large areas of exposure of vertical or 

 slightly overturned beds on a high 1 km long NW-SE trending ridge, 

 up to a contact with a small area of Qishn Formation. 



LITHOSTRATIGRAPHY AND 

 BIOSTRATIGRAPHY 



The Kohlan Formation 



LiTHOLOGY AND TYPE SECTION. The term Kohlan Formation was 

 first used by Beydoun (1964: 31), being derived from the 'Kohlan 



Series' of Lamare (1930: 52), which was proposed for the arena- 

 ceous beds at the base of the Jurassic on the high plateau at Kohlan, 

 75 km NW of Sana'a. At that type locality the formation consists of 

 about 250 m of conglomerates, sandstones and sandy marls, with 

 plant impressions in the top 30 m. Beydoun (1968: 64) took the 

 section about 1 km SE of the river crossing at Al Ma'abir as the 

 reference section and location for the Kohlan Formation in Wadi 

 Hajar, where it is about 55 m thick and lies directly on eroded and 

 peneplaned basement rocks. At the top it is overlain conformably by 

 the Shuqra Formation, the base of which is placed at the bottom of 

 the lowest bed of calcified sandstone that shows evidence of marine 

 condidons, bioturbation and contains some shelly layers . The Kohlan 

 Formation is also well exposed in the central part of the Jebel Billum 

 inlier, and a photograph of its contact with the Precambrian base- 

 ment was given by Beydoun (1964; 44, pi. 8, upper figure). According 

 to the German Geological Map (explanation of sheet D-39-61/73) 

 the basal sedimentary Jurassic rocks in the Jebel Billum inlier are 

 underlain by tuffs and basalt and trachyte lavas, that are themselves 

 of Jurassic age. The evidence for the Jurassic (rather than 

 Precambrian) age of these igneous rocks is not known to us. 



At both Al Ma'abir and Jebel Billum the Kohlan Formation 

 consists of coarse-grained sandstones, pebble beds and conglomer- 

 ates, with a few horizons of finer silts and silty marls in the upper 

 part. The conglomerates contain many boulders and pebbles of the 

 basement beds, cross-bedding is frequent, some beds are cut by 

 quartz seams, and there are haemerite cemented patches in the upper 

 beds. The formation is probably fluviatile or estuarine throughout. 



BIOSTRATIGRAPHY. There is no fossil evidence for the date of the 

 Kohlan Formation in Wadi Hajar or most other areas ofYemen. In the 

 Dhufar region of Oman, immediately east ofYemen, the occurrence 

 of a fossil plant low in the formation suggests that it might be of 

 Lower Jurassic age at that level (Beydoun, 1964:46; 1968: 65). The 

 date of the top of the formation is delimited by the age of the base of 

 the overlying Shuqra Formation, and the only new evidence we have 

 to add is the occurrence of three species of echinoids near the base of 

 the Shuqra These and the abundant faunas of Daghanirhynchia 

 immediately above, are Bathonian or Lower Callovian in age, which 

 suggests that the top of the Kohlan Formation might lie within the 

 Bathonian. 



The Shuqra Formation 



LiTHOLOGY AND TYPE SECTION. After the earlier use of the term 

 'Shuqra Limestone', the name Shuqra Formation was first used by 

 Beydoun (1964: 31-35; 1968: 105-07) for a calcareous formation 

 above the arenaceous Kohlan Formation and below the argillaceous 

 Madbi Formation. The type locality is at Jebel Urays, 15 km north of 

 Shuqra, and 80 km north-east of Aden, where it follows the Kohlan 

 Formation unconformably, and consists of 98 m of limestones, 

 truncated at the top by Tertiary basalts. 



The Shuqra Formation is well exposed in the wadi below the main 

 Jurassic cliff in central Jebel Billum, where it is conformable with 

 both the Kohlan Formation below and the Madbi Formation above, 

 and is about 100 m thick. At Al Ma'abir the Shuqra Formation occurs 

 1 km south-east of the river crossing, and is about 70 m thick, where 

 it follows the Kohlan Formation conformably, but the upper part is 

 only partly exposed and the contact with the Madbi is faulted. 



The Shuqra Formation is calcareous throughout, though the lime- 

 stones are nodular, rubbly or marly at some horizons, and it is sandy 

 in the lower part, where the basal beds are transitional from the 

 Kohlan Formation. The middle part is more heavily calcified and has 

 more massive, fine-grained limestones. At the top it ends with a bed 

 with ripple marks then a ferruginous surface at Al Ma'abir and two 



