Guillermo Meléndez

The Middle-Upper Jurassic ooliticironstone level in the Iberian range (Spain). Eustatic implications

Abstract The oolitic ironstone level occurring at the Callovian-Oxfordian boundary across wide areas in the central IberianRange (Eastern Spain) is interpreted here as formed on an extended, very shallow to temporarity emerged, uniform carbonate platform. Main evidence to support this interpretation, comes from both sedimentological analysis of facies and taphonomic analysis of the ammonites. The sedimentological analysis gives support to the idea of iron oolites being formed on, or in the surroundings of, emerged areas. On the other hand, the taphonomic analysis shows that some inner moulds of ammonites from this level display evidence of taphonomic reworking, such as the presence of ellipsoidal abrasion facets on the final part of the last preserved whorl, or annular abrasion furrows carved on the external region. These features would have developed by the action of directional currents under extremely shallow conditions. Bathymetric implications are relevant for the interpretation of the sea level fluctuations at this stratigraphic interval: A relative lowstand of sea level is proposed for the Upper Callovian-Lower Oxfordian interval in the studied area.

FOSSILIZATION OF AMMONITES AND SEDIMENTARY EVENTS IN DEEP ENVIRONMENTS OF CARBONATE PLATFORM (HIGHEST MIDDLE TO LOWEST UPPER OXFORDIAN, IBERIAN RANGE, SPAIN)

In the Ricla area (Zaragoza, Aragonese Branch of the Iberian Range), at the top of the Yatova Formation, grey-reddish wackestone limestones grade into yellow-green siliciclastic limestones of the Aldealpozo Formation. These changes of facies between the two successive formations are associated with syndepositional palaeoreliefs developed during the Late Oxfordian. The uppermost deposits of the Yatova Formation represent an Oxfordian condensed section, from the upper Bifurcatus Zone (Middle Oxfordian) and Hypselum Zone (Upper Oxfordian). These deposits are interpreted as developed in an open marine, moderately deep carbonate platform, showing uniform low-energy conditions with extremely reduced carbonate and terrigenous background sedimentation, and very low sedimentation rates. The low diversity of the benthic fauna, scarce development of sponge bioherms and ammonite populations inhabiting the platform are palaeobiological criteria which corroborate these palaeoenvironmental conditions. Ammonite assemblages are composed of Sub-Mediterranean taxa. Over 900 ammonite specimens have been collected from the upper Bifurcatus and Hypselum zones. Oppeliidae (45,2%) and Perisphinctidae (37,9 %) are dominant. Aspidoceratidae (14,3%) are common. Haploceratidae (2,2%) are scarce. Two phylloceratids and a lytoceratid have been found. Ammonoids are commonly preserved as concretionary calcareous internal moulds of reelaborated elements. Resedimented shells are scarce. The degree of packing of ammonite remains and the stratigraphical persistence display high values. Taphonomic features indicative of sedimentary starving in deep carbonate platform environments are: 1) high concentrations of reelaborated ammonites, 2) taphonic population of type two, 3) phragmocones completely filled with sediment, and 4) homogeneous concretionary internal moulds, bearing no signs of abrasion, bioerosion or dense encrusting by organisms (such as serpulids, bryozoans or oysters). In conclusion, the occurrence of these ammonite associations confirms the development of an advanced deepening phase, within a 3rd order deepening/shallowing cycle, in the Aragonese platform, during the late Bifurcatus to Hypselum zones.

Current trends in systematics of Jurassic Ammonoidea: The case of Oxfordian-Kimmeridgian Perisphinctids from Southern Europe

Abstract Recent progress in the characterization of morphological variability of ammonites species has helped outline a biostratigraphic-evolutionary classification of many groups of ammonites on the basis of the concept of biospecies, or successive horizontal, homogeneous assemblages. In the case of Middle Oxfordian Perisphinctinae and Upper Oxfordian-Lower Kimmeridgian Ataxioceratinae a similar pattern can be drawn after recognizing a sharp sexual dimorphism within each group and evaluating the large horizontal continuous variability in each successive assemblage. Some prominent cases of discontinuous variability, interpreted as possible cases of intraspecific, non sexual, polymorphism, such as in Larcheria-Otosphinctes-Dichotomosphinctes, Subnebrodites-Orthosphinctes, and the forms with and without parabolic nodes of Orthosphinctes (Ardescia) and Ataxioceras (Parataxioceras), are discussed and analyzed. Taxonomic and nomenclatural problems derived from this approach are evaluated and the need for a nomenclatural revision is considered. The typological taxonomy, widely used during the last century, characterized by an abundance of morpho-taxa is however regarded as a useful tool for recognizing and characterizing morphologies and to give valid names to both sexual dimorphs until a deeper knowledge of successive dimorphic pairs is achieved.

Stratigraphy and tectonics of a time-transgressive ophiolite obduction onto the eastern margin of the Pelagonian platform from Late Bathonian until Valanginian time, exemplified in northern Evvoia, Greece

Abstract The obduction of an ophiolite sheet onto the eastern Pelagonian carbonate platform complex of the Hellenides began during the Late Bathonian and ended with the final emplacement of the ophiolite during Valanginian time. The early stages of obduction caused subaerial exposure of the platform, recorded by an unconformity of Callovian age, which is marked by laterites overlying folded and faulted, karstic substrates. The laterites have distinct ophiolitic geochemical signatures, indicating that emergent ophiolite had been undergoing lateritic weathering. This unconformity coincides with widespread western Tethyan, Callovian gaps, indicating that the obduction in the Hellenides was probably related to far-reaching plate tectonic processes. Resumed gravitational pull and rollback of the subducted, oceanic leading edge of the temporarily exposed ophiolite. Platform drowning continued into Tithonian–Valanginian time, documented initially by reefal carbonates and then by below-CCD, carbonate-free radiolarian cherts and shales. Subsequently, siliciclastic turbidites, which apparently originated from uplifted Variscan basement, were deposited together with and over the radiolarite as the ophiolite nappe sheet advanced. The nappe substrate underwent tectonic deformations of varying intensity, while polymictic mélange and syntectonic sedimentary debris accreted beneath the ophiolite and at the nappe front. The provenience of the ophiolite nappe complexes of northern Evvoia most probably has to be looked for in the Vardar ocean.

Callovian and the Callovian - Oxfordian transition sedimentary record in NE Iberian Chain: Taphonomic analysis and palaeogeography

A comparative study is made on Callovian to middle Oxfordian sequences along the Northeastern Iberian Chain (E. Spain). In the NW areas, from Veruela-Ricla to Tosos (SW-S of Zaragoza), Callovian deposits are represented by expanded carbonate sequences (mudstone and marls with a variable content in clastics) ranging from early Bullatus to Athleta Biozone. Farther East in the near area of Moneva-Arino (Sierra de Arcos) and in the area of Calanda (SE of Zaragoza), this stratigraphic interval is represented by a strongly condensed, 2-4 m thick, highly incomplete sequence. Bullatus to early Anceps biozones are partly represented under bioclastic or oolitic packstone facies. The Callovian-Oxfordian transition interval deposits are represented by a decimeter-thick ironooid fossiliferous limestone condensed sequence (low values of sedimentation rate) formed by expanded sediments (high values of instant rate of sediment accumulation). Ammonite recorded associations frequently show traces of reelaboration and clear evidence of taphonomic condensation. The palaeogeographic evolution of the platform is reconstructed on the basis of extensive sedimentologic studies and on the analysis of taphonomic gradients shown by ammonite associations. Such taphonomic gradients clearly show a shallowing trend of the platform during the Callovian, which would take place earlier in the SE areas (Sierra de Arcos-Calanda) from Gracilis Biozone onwards, and later, from Coronatum Biozone onwards, in the NW areas (Veruela-Ricla to Tosos), the SE area acting as a shallow to temporarily emerged palaeogeographic threshold. The shallowing process would lead to the widespread emersion of the platform from latest Callovian (Lamberti Biozone) to earliest Oxfordian (Mariae and Cordatum biozone, p.p.)

Oxfordian perisphinctid ammonites from Chacay Melehué, Argentina

A significant assemblage of Oxfordian perisphinctids was collected at Chacay Melehué, Neuquén, Argentina. It is composed of well preserved specimens belonging to the family Perisphinctidae:Perisphinctes aff.promiscuusBukowski (Perisphinctinae),Tenuisphinctes herreroduclouxi (Leanza) (Perisphinctinae), andSubvinalesphinctes pseudokranaus n. sp. andS. prophetae (Gygi & Hillebrandt) (Vinalesphinctinae). The family Ataxioceratidae is represented byLithacosphinctes aff. janus (Choffat) (Ataxioceratinae). The subfamily VinalesphinctinaeMeléndez & Myczynski, 1987 is discussed and organized into three genera:SubvinalesphinctesWierzbowski,VinalesphinctesSpath andCubasphinctesJudoley & Furrazola, which seems to cover the whole of the subfamily, ranging, at least, from the lowerplicatilis to the lowerbifurcatus zones, Middle Oxfordian in Cuba, part of Mexico, Chile and Argentina. The root of the subfamily is most likelyS. pseudokranaus n. sp. The paleobiogeographic affinites are mainly Tethyan, and Tethyan-Caribbean during the Middle Oxfordian.ZusammenfassungEine bemerkenswerte Assoziation von Perisphinctiden aus dem Oxfordium wurde bei Chacay Melehué (Neuquén, Argentinien) aufgesammelt. Sie beinhaltet gut erhaltene Stücke der Familie Perisphinctidae mit den ArtenPerisphinctes aff.promiscuusBukowski (Perisphinctinae),Tenuisphinctes herreroduclouxi (Leanza) (Perisphinctinae) sowieSubvinalesphinctes pseudokranaus n. sp. undS. prophetae (Gygi & Hillebrandt) (Vinalesphinctinae). Die Familie Ataxioceratidae ist mitLithacosphinctes aff. janus (Choffat) (Ataxioceratinae) vertreten. Die Unterfamilie VinalesphinctinaeMeléndez & Myczynski, 1987 wird diskutiert und in drei Gattungen aufgegliedert:SubvinalesphinctesWierzbowski,VinalesphinctesSpath undCubasphinctesJudoley & Furrazola, welche die ganze Unterfamilie abzudecken scheinen, die von derplicatilis bisbifurcatus Zone, Mittel Oxfordium in Kuba, Teilen Mexikos, Chile und Argentinien verbreitet ist. Der Ursprung dieser Unterfamilie liegt höchstwahrscheinlich beiS. pseudokranaus n. sp. Die paläobiogeographischen Bezüge sind vorwiegend zur Tethys bzw. tethyalkaribisch während des Mittel-Oxfordiums.

Taphonomic gradients inMiddle Jurassic ammonites of the Iberian Range (Spain)

Abstract Three categories of ammonites fossils have been distinguished in Middle Jurassic of the Iberian Range, according to their state of preservation: reworked or reelaborated elements, resedimented elements and accumulated elements. The relative frequency of reelaborated vs. resedimented ammonites reflects the trends of the hydrodynamic energy in the sedimentary palaeoenvironments, as well as the polarity of the sequences and their discontinuities. Four categories of abrasion surfaces developed on concretionary internal moulds of ammonites are of interest as paleobathymetric indicators: annular furrows, ellipsoidal facets, roll facets, and truncational facets. The transition from associations constituted by accumulated or resedimented ammonites, as well as from associations of resedimented or reelaborated ammonites with truncational or roll facets, to associations of reelaborated ammonites with ellipsoidal abrasion facets or annular abrasion furrows, is interpreted as a taphonomic cline indicative of shallowing gradients from subtidal to inter- or even supratidal environments.

Oxfordian ammonites from Rostam Kola, northern East Alborz, North Iran

The study of a set of ammonite samples from outcrops of the Lar Formation in northermost Iran (northwestern East Alborz) indicates a latest middle to early late Oxfordian age for this fauna. All the studied ammonites belong to the subfamilies Passendorferiinae (Passendorferia uptonioides and P. gygii) and Ataxioceratinae (Orthosphinctes ariniensis, Orthosphinctes sp. A and O. sp. B), and show dominant Submediterranean affinities, being largely comparable to ammonite successions recorded in South European epicontinental platforms, such as Iberian Range, French and Swiss Jura, Southern Germany and Polish Jura. Affinities with true Mediterranean (Northwest Tethyan) Province are also close. However, differences are marked by the virtual absence of representatives of suborders Phylloceratina and Lytoceratina in the recorded sequence, one of the dominant faunal components there. Nevertheless, the material available is scarce for supporting strong conclusions.

A correlation of subsurface andsurface data of the Middle-Upper Jurassic between the Ebro Basin and the central Iberian chain (Eastern Spain)

Abstract Evidence coming from subsurface studies of Middle and Upper Jurassic in the Ebro Basin (easten Iberia) shows ageneral homogeneity and a good correlation with Iberian Chain on what concerns the distribution of facies and the main litho-stratigraphic units: (1) the thick, carbonate Chelva Formation (Middle Jurassic); (2) the sponge limestone, Yatova Formation (middle-upper Oxfordian), (3) The Sot de Chera formation (upper Oxfordian — lower Kimmeridginan) formed by a uniform marly interval; (4) the Loriguilla Formation (Kimmeridgian), formed by a homogeneous interbedding of shaley marls and limestones. The uppermost Jurassic unit, the oncolitic limestone Higueruelas Formation (Tithonian to Berriasian) has not been evidenced so far in the Ebro Basin due, most probably, to erosion. Correlation appears much less evident with other adjacent areas, such as the Catalan Coastal Range, farther East, or the Pyrenees, at the North, where lithological successions are distinct and facies are predominantly dolimitic. On the other hand, the Middle-Upper Jurassic successions appear much more complete at the southern part of the Ebro Basin than in the north, where they are progressively affected by pre-Cretaceous and pre-Tertiary erosion.

THE STRATIGRAPHY OF THE UPPER BATHONIAN TO MIDDLE OXFORDIAN SUCESSION OF THE ARAGONESE BRANCH OF THE CORDILLERA IBÉRICA (SPAIN) AND ITS EUROPEAN CONTEXT

The Upper Bathonian-Middle Oxfordian succession of the Aragonese branch of the Cordillera Iberica is one of the most completely developed in Europe and includes localities of international importance for Jurassic bio- and chronostratigraphy. Of particular importance are a potential stratotype for the Upper Bathonian of North West Europe and reference sections for a number of Submediterranean Province Middle Oxfordian biostratigraphic units. The intervening Callovian sequence, albeit locally strongly condensed, also includes faunas of key stratigraphical importance. The sequence of stratigraphically important ammonite faunas for this interval is here reviewed and placed in its European context.