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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.

2

Indirect biostratigraphy in condensed successions: a case history from the Bajocian of Normandy (NW France)

Pavia G., Martire L.

Volumina Jurassica

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2009

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Vol. 7, no. 1

67-76

EN

The fossil assemblages of the Bajocian of Normandy are affected by taphonomic condensation, and the use of these assemblages for biostratigraphy must be carried out with great care because of the taphonomic reworking (reelaboration) of most of them. Nevertheless such beautiful fossils, ammonites in particular, retain their value at least as taxonomic references if their relative stratigraphic position is recognized. Such a goal has been achieved in the Bretteville section, where the "Oolithe Ferrugineuse de Bayeux" Formation (OFB), consisting of 14 beds contained within 170 cm of strata, is exposed. Each bed contains a condensed ammonite assemblage in which the overall chronologic interval represented by fossils is longer than the time of sedimentation of the bed, and no ammonites can be defined as contemporaneous with the enclosing matrix, except for the topmost two beds. The timing of biologic and sedimentary events in this Fe-oolitic succession must thus be established in an indirect way, through a stratigraphy of fossils based on their taphonomically delayed first occurrence. In particular, we can only define the maximum age of each layer, constrained by the youngest recorded fossil. On the basis of this approach, the lower part of the OFB is shown to fall within the middle to upper part of the Bajocian Stage (Humphriesianum to Parkinsoni chrons), whereas the uppermost OFB can be referred to as the uppermost Bajocian (latest Parkinsoni Chron). We conclude that, at Bretteville and in general for the Fe-oolites of Normandy, fossiliferous horizons with condensed assemblages have no value for definition of the biostratigraphic standard scale, and that indirect biostratigraphy is useful to assign minimal chronologic values to taxa when (1) their stratigraphical range has not yet been established in expanded successions, (2) they are new taxa, and (3) they are present only in a condensed succession that suffered from the same taphonomic constraints as the Bajocian of Normandy.

3

The tangled dimorphism within the ammonite Stephanoceratidae of the Late Bajocian

Pavia G.

Volumina Jurassica

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2006

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Vol. 4, no. 1

242-243

EN

Stephanoceratidae ammonoids achieved maximum diversification in the Upper Bajocian. At the topmost Lower Bajocian, two phyletic lineages produced important taxa which divided into a net of widespread supraspecific entities. The first one refers to the dimorphic Lokuticeras regarded by Galacz (1994) as the origin of the long-ranging subfamily Cadomitinae, from which the typically Late Bajocian Garantianinae derived at the beginning of the substage. The second lineage started from the dimorphic Phaulostephanus which was proposed by Pavia (2000) as the ancestor of an archipelago of minor taxa, such as Parastrenoceras, Subcollina, the polyphyletic superfamily Perisphinctaceae, the corresponding dimorphic genera Caumontisphinctes and Infraparkinsonia as possible stem of Parkinsoniidae. All these stephanoceratids are grouped in the literature into detailed dimorphic pairs, some of which do not need any particular comments, whereas others still lie in a undefined, not totally shared position. The following case-history could clarify the situation: * Cadomitinae: the coupling of Cadomites (M) and Polyplectites (m) is confirmed by all authors. * Garantianinae: Metz (1992) and Gauthier et al. (2002) clearly demonstrated that the inner whorls of Orthogarantiana (M) are homologous to the adult phragmocones of Strenoceras (m); consequently the previous combination Orthogarantiana-Torrensia (Pavia 1983) cannot be valid any longer. A distinct group of forms (cf. Pavia 1973, pl. 19) shows mature features which recall the microconchs of the genus Parkinsonia, in particular open umbilicus and very short apertural lappets: the possibility to transfer these forms to the parkinsonid family must be taken into account. Pseudogarantiana is accepted as the microconchiate counterpart of later garantianinids derived from Orthogarantiana, such as Hlawiceras and other supraspecific names. Moreover, an accurate taxonomic revision is necessary as the number of microconchiate taxa is too low when compared with that of the macroconchs. Benatinites, Parastrenoceras, Subcollina: they seem to be derived from Phaulostephanus by iterative evolution in different times. As to their dimorphism, (M) Benatinites matches the (m) Lugariceras (Schlögl et al. 2006). On the contrary the dimorphism of Parastrenoceras and Subcollina is far to be clarified; the microconchiate Torrensia is a good candidate, at least for Subcollina with which it shares the zigzag pattern of the ventral ribs.

4

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.

5

Relative biostratigraphy in condensed succession: a case history from the Bajocian of Normandy

Pavia G., Martire L.

Volumina Jurassica

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2006

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Vol. 4, no. 1

201-202

EN

The classic dm-thick sections of the Bajocian of Normandy, NW France, such as those of St. Honorinedes- Pertes (formerly d’Orbigny’s stratotype), Sully and St. Vigor in the surroundings of Bayeux are affected by taphonomic and stratigraphic condensations (Pavia 1994). The use of their fossils for biostratigraphic purpose to date the embedding sediments must be avoided because of the taphonomically reworked state of most of them. Such beautiful fossils, ammonites in particular, would become useful again as taxonomic reference if their relative position is recognized. This goal is what Gauthier et al. (1996) tried to achieve in the more than 2 m thick section of Feuguerolles, south of Caen; but the fossil preservation is poor and the Quaternary weathering altered the Fe-oolitic limestone so that beds are not clearly discernible and taphonomic analysis is difficult. A succession similar to that of Feuguerolles was exposed more than ten years ago during a highway construction at Bretteville, SW Caen (Martire & Torta 2000). The succession of the Oolite Ferrugineuse de Bayeux Formation consists of evenly bedded limestones which total the thickness of 140 cm. Even more important beds are lenticular so that, correlating two separated sections, 20 metres far one to the other, we numbered 13 interfingered layers each one with a distinct fossil assemblage. Beds are rich in fossils and a large collection of them is stored in the Museo di Scienze Naturali di Torino. Most ammonites, if not all, are reelaborated and the taphonomic reworking could have been repeated many times; consequently beds contain fossils belonging to different taphorecords, each one characterized by a definite state of fossil preservation discernible from that of other mixed fossils. The taphonomic condensation (sensu Gomez & Fernandez-Lopez 1994) does not allow any reliable biostratigraphy. Nevertheless, the detection of bed interfingering and the careful collection of fossils, bed by bed, led us to consider the 13 successive fossil assemblages as the product of spatially restricted sedimentary events whose age could be outlined by the biochronologic relationships of ammonites contained in superposed beds. Each bed gets in this way a minimal relative age. This provided a relative biostratigraphic succession even if all fossils are reelaborated, which helps in using fossils at least for taxonomic purposes. It is worth noting that such a practice does not constitute any reference either for standardizing biozonation nor for correlation purpose except at regional extent. An example from the Bretteville section could put into practice this procedure. Bed 9 delivered more than 35 identified ammonites; all specimens are reelaborated and biochronologically span the Humpriesianum and Niortense zones; no taxa of the above Garantiana Zone are present. The overlying bed 10 is equally rich in reelaborated ammonites with taxa again referable to the Humpriesianum and Niortense Zones, but also taxa such as Pseudogarantiana and Prorsisphinctes stompus which point to the Garantiana Zone. In conclusion, we are confident to assign the bed 9 to the passage between Niortense and Garantiana zones.

6

Nomenclatural suitability in ammonoid classification: generic versus subgeneric status of dimorphic pairs

Pavia G.

Volumina Jurassica

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2006

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Vol. 4, no. 1

241-242

EN

Dimorphism in ammonoids is fully accepted, since many decades, on the basis of both the fossil record and the biology of modern cephalopods. Anatomy, ecology, genetics support definition of coupling among living animals. Ammonoids offer only indirect evidences for dimorphic interpretation; actually the possibility of coupling macro- and microconch counterparts bases on parameters controlled by autecologic, environmental and palaeobiogeographic constraints which are in turn filtered by taphonomic factors, so that the institution or the delimitation of a dimorphic taxon never is an easy practice. Callomon (1981) summarized the conceptual and technical factors that could guide such analyses: comparison of individuals that show signs of maturity, study of assemblage variability, and consequently of the dimorphic size ratio, evaluation of the phylogenetically stepped development of dimorphism. Of course comparison of fossils of the same age is fundamental; it is worth noting that a careful taphonomic analysis is needed to avoid any mixing of heterochronous palaeobiologic entities. As often, of course, the evidence does not help in the dimorphic practice, the definition of a pair of corresponding morphs does not occur frequently in literature. Nevertheless, in Middle and Late Jurassic Ammonitida, dimorphic coupling constitutes nearly the rule. The presence of micro- and macroconchs in the same bed or set of beds let to reconstruct a dimorphic species which approaches to biospecies: cf. Westermann & Riccardi (1979) for Bajocian Otoitidae of Argentina and Pavia (1983) for Bajocian Stephanoceratidae of S France. But in most circumstances, evidences are not conclusive in favour of a single binomen, and the dimorphic counterparts are classified in different morphospecies which are arranged in separate supraspecific taxonomic levels, usually with subgeneric relationship. The subgeneric rank, though most popular, is not applied by all authors (e.g. Fernandez-Lopez 1985) who prefer to keep dimorphic counterparts distinct at generic level. Different reasons support this position. I would like to stress a single practical aspect which takes into account the nomenclatural tie of subgenera: the homonymy. In fact, if two homonym morphospecies are supposed to be dimorphs, and then have to be referred to the same genus, according to priority it is necessary to change the younger binomen. It is easy to imagine what confusion could derive, mainly if later such a homonymy becomes inconsistent. This possibility is real: within Bajocian Stephanoceratidae (Pavia 1983, p. 83) a single specific name is repeated two-times or even more in binomens which could be coupled: e.g. "Normannites" hoffmanni Westermann vs. Teloceras hoffmanni (Schmidtill & Krumbeck), "Itinsaites" helveticus Maubeuge vs. "Polystephanus" helveticus Maubeuge, and others in different ammonite families too. The separation of dimorphs at generic level would avoid this danger without decreasing the palaeobiologic sense.