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The Jurassic succession of Ras Sharwayn, South-eastern Yemen

Fazzuoli M., Morelli M., Pavia G., Al-Thour K., Chiocchini M., Reale V., Taddei E.

Volumina Jurassica

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2009

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Vol. 7, no. 1

135-145

EN

Four Jurassic stratigraphic units have been recognised at Ras Sharwayn, about 300 km east of Al-Mukalla along coast of the Gulf of Aden. The Kohlan Formation (60 m) unconformably overlies 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. The sequence ends with shallow marine fine-grained sandstones. The Shuqra Formation (71 m) can be divided into two members. The lower Calcareous-marly Member (45 m) consists of grey bioclastic limestone and subordinate nodular marl (inner to mid ramp). Its age is Late Oxfordian. The upper Carbonate Member (26 m) changes from basal reddish marly limestones (mid ramp) to thick beds of red-brown, coarsely crystalline limestones and dolomites (inner ramp). The topmost beds contain fossils of colonial organisms, essentially stromatoporoids. Its age is Late Oxfordian, and possibly earliest Kimmeridgian. The Madbi Formation (>30 m) consists of yellowish marl alternating with marly limestone and bioclastic limestone (coquinas) corresponding to storm layers (mid to outer ramp). With regard to its age, a specimen of Orthosphinctes sp., collected a few metres from the base, possibly refers to the Early Kimmeridgian. The Madbi Formation ends with an unconformity surface. The informal elastic unit (56 m) (including the Naifa Formation) consists, from bottom upwards, of: red-brown dolomite; grey detrital limestone with quartz grains; massive, white conglomerate with well rounded limestone clasts, quartz and bioclasts (e.g. colonial organisms). The last lithotypes are gravity flow deposits, accumulated at the base of a scarp possibly tectonic in origin, approximately at the Jurassic-Cretaceous boundary. This detailed lithological, sedimentological and biostratigraphical study has provided revised litostratigraphical subdivision and nomenclature and improved the stratigraphic control.

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

Fazzuoli M., Morelli M., Pavia G.

Volumina Jurassica

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2006

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Vol. 4, no. 1

85-85

EN

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.

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The birth of Jurassic: from the sea or from the sky? Evidences from the Northern Apennines (Italy)

Fazzuoli M., Orti L.

Volumina Jurassica

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2006

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Vol. 4, no. 1

279-279

EN

At the Triassic/Jurassic (T/J) boundary one of the most important mass extinctions occurred. The inferred causes for this extinction are numerous: eustatic variations; climatic changes; extraterrestrial impact events; strong volcanic episodes etc. One of the diagnostic evidences of a bolide impact in the stratigraphic records is the occurrence of PDF structures in quartz grains ("shocked quartz"). According to Bice et al. (1992), shocked metamorphosed quartz grains has been found in the Il Fiume Gorge, near Corfino (Tuscany) within three closely spaced shaly beds, at the boundary between the Rhaetic (Upper Norian) Calcare a Rhaetavicula and the inferred Lower Jurassic (Hettangian) Calcare Massiccio, suggesting "that multiple impacts occurred in latest Triassic, one of which coincided with a locally, and perhaps globally, significant extinction at the T/J boundary". According to our observations at the same stratigraphic section cited by Bice et al. (1992) the following results can be summarized: - The upper part of the Calcare a Rhaetavicula consists of dark grey bioclastic wackestone beds alternating with bioclastic packstone beds (storm layers) and with marly shales. - The top interval of this formation, 2 m thick, consists of 8-30 cm thick beds of dark grey oolitic grainstone, interlayered with three cm thick levels of marls; another 30 cm thick marly level tops this transition interval. In these marly levels the shocked quartz grains were found. - Above a sharp boundary, the overlying lithologies belong to the Calcare Massiccio (massive limestone). The lower interval, 90 cm thick, is slightly bedded and it is represented by barren mudstone; upwards the rock is massive, cliff forming, and also consists of barren mudstone, more or less dolomitized, up to about 30 m, where bioclastic and intraclastic grainstone/packstones occur. In this level abundant Rhaetic benthic foraminifers are present, such as Gandinella apenninica and Aulotortus sinuosus. - At the regional scale, a Rhaetic fossil assemblage (with Triasina hantkeni etc.) has been found also in other stratigraphic sections of the Calcare Massiccio in the same area (Fazzuoli et al. 1988). In conclusion, in the upper part of the Calcare a Rhaetavicula a lowering of the sea level occurred and the depositional environment evolved from a mid ramp to a high energy inner ramp and then to a lagoon, typical of the Calcare Massiccio. During this lowstand, according to the occurrence of shocked quartz, one or more impact events took place. However, considering the occurrence of Triassic fossils well above the levels with the shocked quartz, it is possible to affirm that the acclaimed extra-terrestrial event occurred within the Rhaetic, not at the Triassic/Jurassic boundary.