Deshayesi Zone Research Articles

The heteromorph ammonite genus Ammonitoceras Dumas, 1876 (Ancyloceratidae) in the lower Aptian (Lower Cretaceous) of the Les Ferres Aptian Basin (southeastern France)

Ammonitoceras Dumas, 1876 is a genus of heteromorph ammonites characterized by the presence of a peculiar ontogenetic stage in its inner whorls: the Ammonitoceras stage. But in spite of its wide paleogeographic and biostratigraphic extension throughout the Aptian (Lower Cretaceous), this genus remains poorly known. In the present work we study specimens of Ammonitoceras from the lower Aptian Deshayesites multicostatus (Deshayesites deshayesi Zone) to Dufrenoyia furcata (Dufrenoyia furcata Zone) subzones of the Les Ferres Aptian Basin (southeastern France). The results are as follows: (1) representatives of Ammonitoceras from this area are regarded dimorphic with criocone macroconchs and ancylocone microconchs, (2) their ontogenetic sequence is described, (3) their intraspecific variability is significant and concerns the adult size and the duration of the ontogenetic stages, especially the Ammonitoceras stage, (4) two species are recognized: the earlier Ammonitoceras ucetiae Dumas, 1876, characterized by a brief Ammonitoceras stage on average, and the latter Ammonitoceras lahuseni (Sinzow, 1906), characterized by a longer Ammonitoceras stage on average. The sample of the Deshayesites grandis Subzone (Deshayesites deshayesi Zone) is composed of specimens too fragmentary to be identified at species level.

Revising the timing and causes of the Urgonian rudistid-platform demise in the Mediterranean Tethys

Abstract The widespread demise of the Urgonian rudistid-platform ecosystem in the Mediterranean Tethys was so far considered as a progressive phenomenon starting at the Barremian–Aptian transition and culminating in the mid-early Aptian in concert with the Oceanic Anoxic Event (OAE) 1a. The re-assignment of the final demise of the Urgonian-type peri-Vocontian rudistid platforms to the latest Barremian by means of ammonite biostratigraphy forces us to rethink the chronology of events and underlying causes. Re-examination of the best-documented platforms of the Mediterranean Tethys supports a large scale demise of the Urgonian rudist-dominated platforms in the latest Barremian, as demonstrated directly by ammonite occurrences and indirectly by correlations. In most cases, the last rudistid sequences, dominated by caprinid rudists of peri-Vocontian affinity, are topped by an exposure surface. Regional investigation reveals that the duration represented by the exposure surface depends on the regional geodynamics. While the hiatus straddles the middle part of the M. sarasini Zone (latest Barremian) in subsiding platforms, it is of greater duration in quiescent tectonic context spanning the upper M. sarasini Zone up to the D. forbesi–D. deshayesi zones interval (late early Aptian). This exposure event is estimated to have lasted at least 1 Myr and may have resulted from a high-amplitude relative sea-level fall. It is here suggested that a short-lasted, global cooling phase and associated glacio-eustatic sea-level fall may have interrupted rudistid-platform ecosystem in the latest Barremian and subsequently led to hiatus development.

Revision of the lower Aptian (Lower Cretaceous) ammonite species Cheloniceras cornuelianum (d’Orbigny, 1841)

The aim of the present work is to determine the morphological characteristics of the lower Aptian (lower Cretaceous) ammonite species Cheloniceras cornuelianum (d’Orbigny, 1841) (Douvilleiceratinae, Douvilleiceratidae) on the basis of 32 specimens from the Paris Basin (France), including the lectotype of the species and 23 topotype specimens. It is generally admitted that this species occurs in the Deshayesites forbesi-Dufrenoyia furcata zones interval (lower Aptian). Our study has lead to several unexpected results. First, the study sample of C. cornuelianum is very probably younger and has more primitive traits than expected: it is probably dated around the limit between the Deshayesites forbesi and Deshayesites deshayesi zones and is morphologically close to the earlier genus Procheloniceras Spath, 1923. Secondly, the lectotype is an extreme specimen with much derived traits compared to its original population. It constitutes an example of type-specimen not morphologically representative of its original population, responsible of a risk of misinterpretation of the species. Finally, the intraspecific variability and the proportion of asymmetric specimens of the study sample of C. cornuelianum are very important but there is no evidence of sexual dimorphism.

The stable O- and C-isotope record of fossils from the upper Barremian–lower Albian of the Caucasus — palaeoenvironmental implications

Late Barremian to early Albian palaeotemperatures have been determined through oxygen isotope analyses of well-preserved brachiopod, bivalve, gastropod and ammonite shells, and belemnite rostra from the Caucasus. Results from calcitic invertebrate skeletal elements from the upper Barremian Sartousiana, Securiformis and Waagenoides zones indicate palaeotemperatures of 19.2, 12.0–17.1 and 11.3–14.3 °C, respectively. In contrast, early Aptian fossils of the Weissi and Deshayesi zones reveal significantly higher palaeotemperatures (20.5–22.3 and 17.1 °C, respectively), than those obtained from brachiopod shells of the Securiformis Zone. Isotopic palaeotemperature data are not available for the Furcata Zone in the Caucasus. The warmest conditions in our study interval developed during the late Aptian Melchioris-Abichi Chrone according to data on bivalves, gastropods and the ammonite Parahoplites (20.8–27.1 °C) and the Subnodosocostatum Chrone (?25.3 °C), on the basis of a shell of the oyster Amphidonte. However, the potential influence of minor freshwater incursions into the upper epipelagic zone in which Amphidonte lived, as well as the predominance of ammonites that preferred cool conditions in the Subnodosocostatum Zone, leads to an alternative interpretation in which cooling initiated at the beginning of the late Aptian. There remains considerable uncertainty in the interpretation of δ18O values in fossils from the Subnodosocostatum Zone but it is possible that one or two cooling events interrupted the warmest conditions during the late Aptian. The highest δ13C values derived from fossils found in the Subnodosocostatun Zone (up to 8.3 and 3.9‰ for shallow-water bivalves and deeper-dwelling belemnite taxa, respectively) have been linked to an episode of late Aptian volcanic activity. On the basis of our stable oxygen isotope record, two ethological groups can be recognised among Aptian ammonites from the Caucasus: cool and thermophilic dwellers. All Early Cretaceous belemnites from this area seem to have had a preference for cool-water conditions.

New insights on the age of the post-Urgonian marly cover of the Apt region (Vaucluse, SE France) and its implications on the demise of the North Provence carbonate platform

Our contribution presents an integrated litho-, bio- and chemostratigraphic study of the best-preserved outcrops of the post-Urgonian marly cover (A1 unit) of the North Provence carbonate platform. The study of the rich ammonite fauna from the Apt–Gargas area (SE Vaucluse) confirms that the A1 unit is older than previously assumed and dated to the upper (but non-uppermost) part of the lower Aptian Deshayesites forbesi Zone of the Mediterranean standard zonation. This interval is assigned to the lower part of the Roloboceras hambrovi Subzone as defined in the present contribution which is time equivalent to the spreading of the OAE 1a. Microfossil occurrences previously documented in the A1 unit are consistent with our ammonite-age calibration. Unfortunately, the curves of the carbon and oxygen stable isotopes exhibit a correlative signal probably caused by an early diagenetic overprint which questions the previous use of the C-isotope signal for precise chronostratigraphic correlation with respect to the OAE 1a excursion. The top of the A1 unit is marked by a burrowed firmground locally infilled and capped by glauconite-rich sandy marls which grade into the blue–grey muddy marls of the overlying A2 unit. The hiatus associated to this discontinuity is equivalent to the culmination of the OAE 1a, including the uppermost D. forbesi Zone (=upper R. hambrovi Subzone as herein defined) and the lower D. deshayesi Zone. The revised calibration of the post-Urgonian marl implies that the ammonite-age calibration of the North Provence carbonate platform should be revised and questions the timing and driving mechanisms of its stepwise demise.

Two ammonite species under the same name: Revision of Deshayesites deshayesi (d’Orbigny, 1841) based on topotype material (Lower Aptian, Lower Cretaceous, Northeast of France)

After a review of the current state of knowledge about the classical Lower Aptian (Lower Cretaceous) ammonite index-species Deshayesites deshayesi (d’Orbigny, 1841) and the problems concerning the acceptation of this species, a topotype sample of 159 Deshayesites from the Argiles à Plicatules Formation (Paris Basin, northeastern France), including the lectotype of the latter species, is studied statistically. According to the Deshayesitidae evolutionary pattern highlighted by Bersac and Bert (2012), the present study shows that the whole studied population from the Argiles à Plicatules Formation belongs actually to a single species with a large intraspecific variability under the Dipolar Variation. Nothing supports the hypothesis of polyzonality of the level that provides the Deshayesites within the Argiles à Plicatules. The characteristics of the Deshayesites studied relate the latter to Deshayesites annelidus Casey, 1963 rather than to what is usually considered to be D. deshayesi. Consequently, D. deshayesi is revised, D. annelidus is treated as a junior synonym of D. deshayesi, and the species previously usually considered as D. deshayesi has to be renamed in D. multicostatus Swinnerton, 1935. Accordingly, it is proposed to move the limit of the D. deshayesi Zone downward.

Multi-proxy orbital chronology in the aftermath of the Aptian Oceanic Anoxic Event 1a: Palaeoceanographic implications (Serre Chaitieu section, Vocontian Basin, SE France)

In the early Aptian, the Oceanic Anoxic Event (OAE) 1a is well defined by a negative ?13C excursion followed by a positive δ13C excursion, spanning the Deshayesites deshayesi and Dufrenoya furcata ammonite biozones. A cyclostratigraphic approach is performed in the Vocontian Basin, France, to estimate the time required for the carbon cycle recovery following the major disturbance associated to OAE1a and to provide durations of ammonite and foraminifer biozones. The Serre Chaitieu section, which consists of hemipelagic blue-grey marls with occasional marker limestone horizons and encompassing the Deshayesites deshayesi Zone to the end of the Epicheloniceras martini Zone, was used as a reference section in the Vocontian Basin. Using field Spectral Gamma Ray (SGR), 450 measurements were performed throughout the section, and a sample of each measured sediment was collected to further perform calcimetry, clay mineralogy, and magnetic susceptibility (MS) measurements. Detrital clay mineral assemblages consist of illite, illite/smectite mixed-layers (I-S), kaolinite and chlorite. Fluctuations of clay minerals are mainly driven by climate change, progradation/drowning of peri-vocontian platforms and sea-level changes. The proportions of illite and kaolinite covary and fluctuate in opposition with I-S. Cyclic fluctuations of relative proportions of clay minerals are particularly well recorded by the kaolinite/chlorite ratio (K/C). Spectral analyses, using the multi-taper and the amplitude spectrogram methods, were performed on SGR, MS, CaCO3 and K/C signals to detect sedimentary cycles related to an orbital forcing throughout the series. The geochronometer 405-kyr eccentricity cycle well expressed and significant (up to 99% confidence level) is used to provide a robust temporal framework. More than five 405-kyr eccentricity cycles are recognised, providing a total duration of at least 2.49 myr for the whole sedimentary succession. The minimum duration of the D. furcata Zone is assessed at 0.42 myr, and the duration of the E. martini Zone at 1.52 myr. Amplitude spectrograms show a strengthened signal of obliquity during the D. furcata Zone, which is coherent with the global cooling that has been depicted for this interval, and which could have favored the development of lowextension polar ice and thus the lowering of the sea level. Durations of C-isotope zones, worldwide correlated, are also calculated. From these results, the duration of the return to equilibrium in the carbon cycle in the aftermath of OAE1a could be calculated at 1.35 myr.

Stratigraphic updating and correlation of Late Barremian–Early Aptian Urgonian successions and their marly cover, in their type region (Orgon-Apt, SE France)

Comparing the stratigraphy of the rudist bearing limestones of Orgon (the type locality for the Urgonian stage of d'Orbigny) with that of the Urgonian limestones of the Monts de Vaucluse-Apt region, where the stratotype of the Aptian sensu d'Orbigny is located, shows that the corresponding platform carbonates have distinctive ages, late Barremian (Gehrartia sartousiana zone), and Bedoulian (Deshayesites oglanlensis to the middle part of the Deshayesites deshayesi zone) respectively. Nevertheless the Upper Barremian Orgon limestones are still present in the western part of the Monts de Vaucluse where they are capped by a Palorbitolina lenticularis–Heteraster oblongus (Pa1) guide level, ascribed to the Imerites giraudi zone. The overlying rudist beds are characterized by the presence of Caprinidae (Offneria-Pachytraga-Praecaprina assemblage), and spread over the Monts de Vaucluse-Apt region; they also extend to the Mont Ventoux, and to the eastern Languedoc too. These latter beds, assigned to the early Bedoulian, i.e. the D. oglanlensis–D. weissi zones, are interrupted by a drowning discontinuity capped by Palorbitolina beds (Pa2) overlain by bioclastic or coral limestones of early late Bedoulian age (lower part of the D. deshayesi zone). The succeeding marly cover, including marly limestones and the lowermost part of the “Gargas marls”, which marks the regional demise of shallow carbonates, is ascribed to the late Bedoulian (Deshayesites grandis subzone and Dufrenoyia furcata zone) and represents the lowermost part of the “Aptian”sensu d'Orbigny.

Sedimentology, biostratigraphy and event stratigraphy of the Early Aptian Oceanic Anoxic Event (OAE1A) in the Apulia Carbonate Platform Margin – Ionian Basin System (Gargano Promontory, southern Italy)

The stratigraphic and biotic signatures of severe environmental changes across the late Early Aptian Oceanic Anoxic Event (OAE1a) in central Tethyan settings have been investigated in the almost undeformed Apulia Carbonate Platform Margin-Ionian Basin system (ACPM-IB) exposed in the Gargano Promontory (southeastern Italy). The nature of the observed Lower Aptian biofacies shifts within the investigated carbonate system are analyzed using an integrated biostratigraphic approach, based on ammonite-calibrated orbitolinids and caprinids coupled with the published δ13C and δ18O record of coeval pelagic sections of the Ionian basin (Coppitella, eastern Gargano, and Paliambela, northwestern Greece). Detailed field analysis of facies and tracing out of key biosedimentary and chemostratigraphic markers of the Lower Aptian have elucidated the evolution of the platform margin and allowed integrated correlations across the Gargano margin to basin transition.It is shown here that the global-scale environmental changes leading to the OAE1a event (i.e., fluctuations of surface-ocean temperature, available trophic resources and, above all, pCO2) are matched consistently by significant shifts of the biotic associations inhabiting the ACPM. The onset of greenhouse, mesotrophic conditions in the surface ocean undergoing an increasing acidification by CO2 excess favored the rapid spread of calcite shelled, filter feeding, eurytopic opportunist organisms (chondrodontids, ostreids, chaetetids sponges, along with minor bryozoans) as well as echinoids and orbitolinids (foramol productivity mode); this change is recorded by the deposition of a 4–6 m Crisis Interval (CI) that sharply overlies the “Urgonian”-type, rudist-rich platform margin complex (Montagna degli Angeli Limestones) formed from predominantly aragonite shelled, stenotopic organisms (mostly caprinid rudists, with minor corals and Dasycladales and Bryopsidales green algae) (chlorozoan productivity mode). The CI heralded the incipient drowning of the ACPM which occurred immediately after the deposition of a hothouse “out of balance” brachiopod (Orbirhynchia nadiae)-cyanobacteria association (microbial productivity mode). The biostratigraphically constrained CI and the related drowning of the ACPM have been physically correlated with specific chemostratigraphic segments of the δ13Ccarb curve that is available for the facing, proximal Ionian Basin. The early (earliest ?) Late Bedoulian, greenhouse CI (early D. deshayesi – early P. cormyi zones) was deposited during the late C2 interval, whereas the early Late Bedoulian, hothouse “out-of-balance” guild and the ACPM drowning (early deshayesi zone) accompanied the subsequent negative excursion and culmination, respectively, at the very base of the Chemostratigraphic Selli Level (CSL) (latest C2 and C3 interval). These correlations permit regional to global interpretations of historical patterns and explanatory paleoeanographic and paleoecologic hypotheses; furthermore, they suggest that the Apulia shallow-water ecosystem reacted to the environmental disruptions linked with the OAE1a sooner than that of the facing open ocean. The onset of the drowning event was synchronous with the hothouse-induced bloom of the “out of balance” brachiopod-cyanobacteria association that occurred simultaneously with the pronounced negative excursion of the δ13Ccarb curve. This suggests a cause-and-effect relationship with the sudden environmental perturbations linked with the injection into the atmosphere-hydrosphere system of 13C-depleted CO2. The numerical age model of the major biotic and stratigraphic events at the Lower Aptian ACPM, along with their inferred genetic processes call for different timings and causal mechanisms associated with platform demise in northern and central Tethyan settings.

Impact of Early Aptian carbon cycle perturbations on evolution of a marine shelf system in the Basque-Cantabrian Basin (Aralar, N Spain)

The Aptian Oceanic Anoxic Event 1a (OAE1a, ca.120 Ma ago) is one of the most prominent of a series of geologically brief intervals in the Cretaceous reflecting a major perturbation in the global carbon cycle. This carbon cycle perturbation is recorded in the C-isotope stratigraphy. In this study we present a new carbonate and organic carbon isotope record across OAE1a from expanded shallow marine sections (Igaratza and Iribas) in the Basque-Cantabrian Basin (N Spain). The sediments studied were accumulated on a mixed carbonate–siliciclastic and carbonate shelf. The δ 13C carb and δ 13C org curves obtained show a pronounced negative spike in the Deshayesites weissi Zone and a subsequent positive excursion within the newly described D. deshayesi – D. furcata transition Zone, approximately equivalent to the upper D. deshayesi Zone of other sections. Specific changes in facies patterns and a reduction in neritic carbonate production predate the negative C-isotope perturbation. These changes coincide with global carbonate crises in neritic and pelagic environments. Corresponding with the beginning of OAE1a (Selli level) a condensed interval rich in ammonites is observed. This horizon is linked to a major transgression with high biodiversity. The “ammonite bloom” suggests enhanced primary productivity in the surface waters of the Basque-Cantabrian Basin at improving environmental conditions for selected biocalcifiers. The ultimate recovery of shelf carbonate deposition occurred after OAE1a and it coincided in time with the most positive values of the carbon-isotope excursion. Based on carbon-isotope stratigraphy it has been deduced that black shales corresponding to Selli Level are missing in Aralar, so that anoxia is not recorded in the studied sections of the marine shelf environment. An organic-rich interval (Aparein level) postdates the Early Aptian major positive excursion. The Aparein event should be considered in the future research of OAEs.

Biostratigraphic characterization by means of ammonoids of the lower Aptian Oceanic Anoxic Event (OAE 1a) in the eastern Iberian Chain (Maestrat Basin, eastern Spain)

It is now generally accepted that the Oceanic Anoxic Event 1a [OAE 1a] correlates with the lower part of the Leupoldina cabri planktonic foraminiferal Zone. Its calibration against the standard ammonite scale, however, seems to be more problematic. This is due, in part, to the fact that ammonites are scarce and/or of little diagnostic value from a biochronological viewpoint in the lower Aptian pelagic successions where the black shale horizons are better developed. We have been able to characterize OAE 1a geochemically in the relatively shallow water deposits of the eastern Iberian Chain (Maestrat Basin, eastern Spain), where ammonite faunas are rich. The interval corresponding to this event is dominated by the genera Roloboceras and Megatyloceras, accompanied by Deshayesites forbesi and Deshayesites gr. euglyphus/spathi. This assemblage is characteristic of the middle/upper part of the Deshayesites weissi Zone. The first occurrence of the species Deshayesites deshayesi (d'Orbigny), which marks the base of the overlying zone, takes place in our sections some metres above the OAE 1a interval. In the historical stratotype region of Cassis-La Bédoule (southern Provence Basin, southeastern France), the OAE 1a interval is also characterized by the presence of Roloboceras and Megatyloceras. Nevertheless, it has usually been correlated with the D. deshayesi Zone. In our opinion, this discrepancy is due to divergences in the taxonomic assignments of the deshayesitids present in these beds. In fact, the specimens attributed by French authors [Ropolo, P., Conte, G., Gonnet, R., Masse, J.P., Moullade, M., 2000. Les faunes d'Ammonites du Barrémien supérieur/Aptien inférieur (Bédoulien) dans la région stratotypique de Cassis-La Bédoule (SE France): état des connaissances et propositions pour une zonation par Ammonites du Bédoulien-type. Géologie Méditerranéenne 25, 167–175; Ropolo, P., Moullade, M., Gonnet, R., Conte, G., Tronchetti, G., 2006. The Deshayesitidae Stoyanov, 1949 (Ammonoidea) of the Aptian historical stratotype region at Cassis-La Bédoule (SE France), Carnets de Géologie / Notebooks on Geology Memoir 2006/01, 1–46.] to D. deshayesi and D. dechyi can be reinterpreted as belonging to D. forbesi. Following this reinterpretation, the Roloboceras beds (equivalent of OAE 1a) of Cassis-La Bédoule would also correspond to the D. weissi Zone. This age is additionally corroborated by data from southern England [Casey, R., 1961a. The stratigraphical palaeontology of the Lower Greensand. Palaeontology 3, 487–621; Casey, R., 1961b. A Monograph of the Ammonoidea of the Lower Greensand, part III. Palaeontographical Society, London, pp. 119–216], and by our recent observations in Le Teil (Ardèche Platform, southeastern France), where the Roloboceras faunas are also associated with Deshayesites consobrinus and Deshayesites gr. euglyphus, taxa that are characteristic of the D. weissi Zone.

Demise of the northern Tethyan Urgonian carbonate platform and subsequent transition towards pelagic conditions: The sedimentary record of the Col de la Plaine Morte area, central Switzerland

The sedimentary succession of the Col de la Plaine Morte area (Helvetic Alps, central Switzerland) documents the disappearance of the northern Tethyan Urgonian platform in unprecedented detail and suggests stepwise platform demise, with each drowning phase documented by erosion and phosphogenesis. The first identified drowning phase terminated Urgonian carbonate production in a predominantly photozoan mode. Using a correlation of the whole-rock δ 13C record with the well-dated record from SE France, its age is inferred to as Middle Early Aptian (near the boundary between the weissi and deshayesi zones). A subsequent drowning phase is dated by ammonites and by a correlation of the whole-rock δ 13C record as Late Early Aptian (late deshayesi to early furcata zone). A third drowning phase provides an ammonite-based age of Early Late Aptian ( subnodosocostatum and melchioris zones) and is part of a widely recognized phase of sediment condensation and phosphogenesis, which is dated as latest Early to Middle Late Aptian (late furcata zone to near the boundary of the melchioris and nolani zones). The fourth and final drowning phase started in the latest Aptian ( jacobi zone) as is also indicated by ammonite findings at the Col de la Plaine Morte. The phases of renewed platform-carbonate production intervening between the drowning phases were all in a heterozoan mode. During the ultimate drowning phase, phosphogenesis continued until the Early Middle Albian, whereas condensation processes lasted until the Middle Turonian. Coverage of the external margin of the drowned Urgonian platform by a drape of pelagic carbonates started only in the Late Turonian. During the Santonian, the external part of the drowned platform underwent normal faulting and saw the re-exposure of already lithified Urgonian carbonates at the seafloor. Based on the here-inferred ages, the first drowning phase just precedes oceanic anoxic episode 1a (OAE 1a or “selli event”) in time, and the second drowning phase partly overlaps with OAE 1a. The onset of the third drowning event slightly predates two further periods of increased organic-matter accumulation in the Vocontian Basin (Noir and Fallot levels), and the onset of the fourth and final drowning phase may coincide with two further periods of increased organic-matter accumulation in the Vocontian Basin (Jacob and Kilian levels, part of OAE 1b). These correlations indicate a relationship between the so-called anoxic episodes and the stepwise demise of the Urgonian platform, even if the onset of environmental change is registered earlier on the platform than in basinal sediments.

Lithostratigraphy and biostratigraphy of some Lower Cretaceous outcrops from Northern Sinai, Egypt

Lower Cretaceous successions of the Risan Aneiza, El Tourkumanyia and Manzour areas (Northern Sinai, Egypt) were studied for biostratigraphic purposes. These successions were subdivided into three rock units, from base upwards: the Malha, the Risan Aneiza and the Halal formations. Each formation included enriched cephalopod horizons. Fifty cephalopod species were identified. The age and stratigraphic distribution of these cephalopods allowed the recognition of eight ammonite zones ranging in age from Barremian to late Albian. These zones are, from base to top: Subpulchellia oehlerti Zone (late Barremian), Barremites difficilis Zone (late Barremian), Deshayesites deshayesi Zone (early Aptian), Aconeceras nisus Zone (early middle Aptian), Epicheloniceras tschernyschewi Zone (late middle Aptian), Acanthohoplites nolani Zone (late Aptian), Knemiceras gracile Zone (early Albian) and Mortoniceras inflatum Zone (late Albian). The identified ammonite zones were correlated with national and standard international ammonite zones as proposed in previous studies of the same time interval.

Les surfaces a Rhizocorallium de l'Aptien inferieur sur la bordure meridionale du bassin vocontien (France Sud-Est), marqueurs de dynamiques locales; leur relation avec un evenement anoxique global

Abstract On the North-Provence margin, an omission surface with Rhizocorallium often characterizes the sub-Aptian discontinuity between Eocretaceous carbonates and Aptian-Albian marls. This discontinuity is recorded in various successions described along a palaeoslope joining the Vocontian Basin and the Provence platform, between the Jabron valley in the west and the Var valley in the east. The successions including Aptian marls always exhibit an omission surface with Rhizocorallium. Through this area, the Aptian discontinuity equates to at least part of the Goguel Level. The latter is represented by black shales occurring in the Vocontian Basin at the top of Deshayesi zone and is a regional expression of the global anoxic event OAE1a. Thus, the surface with Rhizocorallium can be regarded, on the North-Provence margin, as a marker coincident with the onset of the anoxic event which, in the Vocontian basin, succeeded to a nannoconid crisis. Representing a short event, the surface is also a marker of various processes: (1) current activity, deduced from a dominant orientation of Rhizocorallium. The Barremian-early Aptian carbonate succession is strongly eroded by the formerly more active currents along the Peipin Channel, south-east of Sisteron; these currents also led to the omission of the Bedoulian (= carbonate Lower Aptian) to upper Valanginian series from the top of the hemipelagic palaeoreliefs in the eastern Castellane arc; (2) tectonic activity in the Lower Aptian, marked by N080 degrees to N100 degrees extension faults and slumping of a semi-lithified carbonate sediment. Slumped material and fault scarps are burrowed with Rhizocorallium; (3) differential lithification of the Barremian limestones which has led to distinctive types of current erosion.

Lithological and faunal stratigraphy of the Aptian and Albian (Lower Cretaceous) of the type Speeton Clay, Speeton, north-east England

SUMMARY Study of the Aptian and Albian parts of the Speeton Clay Formation at Speeton, North Yorkshire, has produced a detailed stratigraphy and a large collection of stratigraphically constrained fossils, despite the general poor state of exposure. This has allowed the development of a readily applicable bed numbering scheme and detailed faunal (macrofossil, foraminifera and ostracod) range charts. The Barremian-Aptian boundary is marked by the appearance of abundant examples of the ammonite Prodeshayesites and a marked lithological change from black shale to sandy mudstone. Higher parts of the Aptian have yielded abundant faunas, including ammonites of the fissicostatus, forbesi and deshayesi zones; the base of the forbesi Zone marked by the incoming of Ewaldi Marl facies. Poorly fossiliferous silty shales of the Albian tardefurcata Zone are succeeded by a bed with glauconite and phosphate pebbles, interpreted as marking the base of the mammillatum Superzone. This is overlain by fossiliferous clays of the Lower Albian and part of the Middle Albian. Range charts for macrofossils and microfossils from the Upper Barremian to Middle Albian of Speeton are constructed and biozones indicated. Aptian and Lower Albian successions inland of Speeton are highly variable, probably due to intra mid-Cretaceous fault movements. The same general stratigraphy as at Speeton, however, may be seen across much of the southern North Sea.

Un exemple de spéciation lié à l'eustatisme: l'apparition précoce de Schackoina cabri (foraminifère planctonique mésogéen)

Investigation of planktic foraminifers from the Lower Aptian of the Vocontian Basin (SE France) reveals that Schackoina cabri makes a very discrete first appearance in the Weissi ammonite zone and not in the Deshayesi zone, i.e. before the global anoxic event OAE 1a and not after as was accepted in the literature. This discovery seems to support the recent model whereby speciation of planktic foraminifers occurred in marine lowstands and their real expansion during the ensuing sea-level rise.

The stratigraphy and palaeontology of the Lower Greensand of the Hoes Farm Borehole, near Petworth, Sussex

The Hoes Farm Borehole provides a cored sequence through the Atherfield Clay, Hythe Beds and the lowest part (Fittleworth Beds) of the Sandgate Beds. Macrofaunas and ostracods have been collected from many levels and, for the first time, the Ostracoda from the English Aptian have been examined over an extended sequence with good macropalaeontological control. A 0.7 m seam of fuller's earth, recorded near the base of the Hythe Beds, yielded an ostracod fauna of bowerbanki Zone age. A new stratal unit, the Hoes Farm Member, mainly of deshayesi Zone age, has been defined at the base of the Hythe Beds.