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1 2006

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The Jurassic section of Ras Sharwayn (Southern Yemen): stratigraphic analysis

Fazzuoli M., Morelli M., Pavia G.

Volumina Jurassica

2006

Vol. 4, no. 1

85-85

The present study was performed at Ras Sharwayn, about 250 km east of Al-Mukalla along the Gulf of Aden coast. Four Jurassic stratigraphic units have been distinguished. The Kohlan Formation (60 m) overlies unconformably the crystalline basement. Its lower and middle part consist of fluviatile sandstone and conglomerate. The upper part is made up of transitional coarse- and fine-grained sandstones and siltstones. Shallow marine fine-grained sandstones close the sequence. Fossils are present in the bioclastic storm layers of the top unit. The Shuqra Formation (71 m) is divisible into two members. The basal Calcareous-marly Member (45 m) consists of grey calcilutites alternating with fine calcarenites and subordinate nodular marls (inner to mid ramp). At the base, a very rich fossil palaeocommunity of muddy outer shelf occurs, characterized by Anomalodesmata pelecypods, less common brachiopods of orders Rhynchonellida (Somalirhynchia africana) and Terebratulida (Kutchithyris sp.), early, possibly new forms of infaunal Atelostomata echinoids (Mepygurus sp., Bothriopneustes(?) sp.). Among pelecypods, common taxa are Modiolus cf. subangustissimus, Bucardiomya cf. protei, Ceratomya sp., Gresslya sp., Procardia cf. latissima. The age, by benthic foraminifers and the nautiloid Paracenoceras giganteum, is Late Oxfordian. The Carbonate Member (26 m) changes from basal reddish marly calcilutites (mid ramp) to thick beds of red-brown, coarsely crystalline limestones and dolomites (inner ramp). The top beds contain fossils of colonial organisms, essentially stromatoporoids. The age, by stratigraphic frame and microfossils, is Late Oxfordian and possibly earliest Kimmeridgian. The Madbi Formation, (30 m) consists of yellowish marls alternating with marly calcilutite and bioclastic calcarenite (coquinas) corresponding to storm layers (mid to outer ramp). The very basal bed contains an oligotypical palaeocommunity dominated by large-sized brachiopods (Somalirhynchia n. sp.) with scattered Exogyra sp. and Alectryonia cf. pulligera. The rich brachiopod assemblage indicates a muddy bottom where oyster shells offered a semi-rigid ground on which the rhynchonellid palaeocommunity was growing. As to the age, a specimen of ammonoid, Orthosphinctes sp., collected at some seven metres above the base, undoubtedly refers to as Early Kimmeridgian. The Madbi Formation ends with an unconformity surface.The informal Clastic Unit (56 m) consists, from the bottom, of: red-brown dolomite; grey calcarenite with quartz grains; massive, white conglomerate with well rounded clasts of limestones, quartz and bioclasts (e.g. colonial organisms). The calcarenite and the conglomerate are gravity flow deposits, accumulated at the base of a scarp of possibly tectonic origin. The onset of the clastic sedimentation is connected with the tectonic phase that strongly deformed the Jurassic sedimentary basins, and formed horst and graben topography approximately at the Jurassic/Cretaceous boundary. Main advances of the present research in regard to the previous studies (Beydoun 1964) are: a detailed lithological and sedimentological analysis, new biostratigraphical data and some variations on the litostratigraphical subdivision and nomenclature.