Late Oxfordian Research Articles

Timing of sea level, tectonics and climate events during the uppermost Oxfordian (Planula zone) on the Iberian ramp (northeast Spain)

The middle Oxfordian warming climate and sea-level rise initiated the development of vast carbonate platforms in some western European basins. At the same time, however, siliciclastics and siliceous sponges dominated certain marginal areas of the Iberian ramp. There, siliciclastic input was particularly prominent during the latest Oxfordian and may have been related to a global sea-level fall, synsedimentary tectonic activity, or humid climatic conditions in the hinterland. Field analyses and computer modelling have been previously used to determine the factors that controlled sedimentation. However, it is still unclear if the specific conditions that prevailed during the latest Oxfordian were due to eustasy, tectonics or climate, and when precisely they occurred. Here, we document major changes in sedimentological, micropalaeontological, and mineralogical records on the Iberian ramp during this interval. Detailed sedimentary facies and palynofacies analyses combined with sequence-stratigraphic and cyclostratigraphic analyses of the Ricla Barranco section enable the establishment of a high-resolution time frame. Based on the quartz and mica percentage fluctuations, one large- and seven small-scale sequences are defined. The large-scale sequence boundaries correlate with third-order sequence boundaries Ox 8 and Kim 1 defined by Hardenbol et al. (1998). The large-scale maximum-flooding surface corresponds to the base of the most calcareous interval and to the maximum abundance of marine phytoplankton and opaque, equidimensional phytoclasts. The small-scale sequences correspond to the 100-kyr orbital eccentricity cycle. Calcareous nannofossils and clay minerals were used as palaeoclimatic proxies. Nannofossil abundances and fluxes are lower in the upper part than in the lower part of the interval studied, suggesting a decrease in sea-surface trophic conditions, also shown by an increase in the relative abundance of oligotrophic taxa. This upper part is also characterised by an increase in smectite, which coincides with the base of the large-scale highstand deposit, and is interpreted as reflecting the establishment of dry conditions. A first increase in smectite occurs in the lower part of the succession, and coincides with high percentages of quartz and mica. This latter mineralogical assemblage is interpreted as recording the onset of the Late Jurassic to Early Cretaceous rifting stage, which occurred just before the Planula–Galar ammonite subzone transition. The present study points out a return toward optimum conditions for carbonate sedimentation only 300kyr after the prominent increase in siliciclastic input due to tectonic activity. The recovery of carbonate production was accompanied by a global sea-level rise and by decreasing rainfall on nearby land.

Biostratigraphy and ammonites of the Middle Oxfordian to lowermost Upper Kimmeridgian in northern Central Siberia

The Middle Oxfordian to lowermost Upper Kimmeridgian ammonite faunas from northern Central Siberia (Nordvik, Chernokhrebetnaya, and Levaya Boyarka sections) are discussed, giving the basis for distinguishing the ammonite zones based on cardioceratid ammonites of the genus Amoeboceras (Boreal zonation), and, within the Kimmeridgian Stage, faunas–for distinguishing zones based on the aulacostephanid ammonites (Subboreal zonation). The succession of Boreal ammonites is essentially the same as in other areas of the Arctic and NW Europe, but the Subboreal ammonites differ somewhat from those known from NW Europe and Greenland. The Siberian aulacostephanid zones—the Involuta Zone and the Evoluta Zone—are correlated with the Baylei Zone (without its lowermost portion), and the Cymodoce Zone/lowermost part of the Mutabilis Zone (the Askepta Subzone) from NW Europe. The uniform character of the Boreal ammonite faunas in the Arctic makes possible a discussion on their phylogeny during the Late Oxfordian and Kimmeridgian: the succession of particular groups of Amoeboceras species referred to successive subgenera is revealed by the occurrence of well differentiated assemblages of typical normal-sized macro and microconchs, intermittently marked by the occurrence of assemblages of paedomorphic “small-sized microconchs” appearing at some levels preceeding marked evolutionary modifications. Some comments on the paleontology of separate groups of ammonites are also given. These include a discussion on the occurrence of Middle Oxfordian ammonites of the genus Cardioceras in the Nordvik section in relation to the critical review of the paper of Rogov and Wierzbowski (2009) by Nikitenko et al. (2011). The discussion shows that the oldest deposits in the section belong to the Middle Oxfordian, which results in the necessity for some changes in the foraminiferal zonal scheme of Nikitenko et al. (2011). The ammonites of the Pictonia involuta group are distinguished as the new subgenus Mesezhnikovia Wierzbowski and Rogov.

Balanocrinus and other crinoids from Late Jurassic mudstones of France and Switzerland

Late Jurassic mudstones of France and Switzerland are rich in remains of the isocrinid Balanocrinus. From the early Late Oxfordian of Savigna (Departement du Jura, France) Balanocrinus brachiospina is described as new; the material includes all growth stages and skeletal elements, allowing the reconstruction of the animal and description of ontogenetic changes. The remains of Balanocrinus from Savigna are compared with Balanocrinus subteres (Munster in Goldfuss) from the Middle Oxfordian sponge facies of Holderbank (northern Switzerland) and with Balanocrinus pentagonalis (Goldfuss) from the Early Oxfordian of Les Rouges-Terres (Canton of Jura, Switzerland. The present material enables improved diagnoses of the two common Oxfordian species, B. subteres and B. pentagonalis. The Savigna locality includes two rare new comatulids, Palaeocomaster musculosus n. sp. and Andymetra donovani n. sp. The late Early Oxfordian locality of Andelot-Morval near Saint-Amour (Departement du Jura, France) furnished B. brachiospina n. sp., and the comatulids are represented by Palaeocomaster musculosus n. sp., Archaeometra scrobiculata (Goldfuss) and Semiometra petitclerci (Caillet). The paracomatulid Paracomatula morator n. sp. from Andelot is the last representative of the group. Numerous remains of the cyrtocrinid Tetracrinus galei n. sp. supplement the Andelot fauna.

Middle–Upper Jurassic (Upper Callovian–Lower Kimmeridgian) stable isotope and elemental records of the Russian Platform: Indices of oceanographic and climatic changes

New isotope (δ18O, δ13C) and elemental (Mg/Ca, Sr/Ca) data of well-preserved belemnite rostra, ammonite and gastropod shells from the Middle Oxfordian–Lower Kimmerdgian (Densiplicatum–Kitchini zones) of the Russian Platform are presented. This record is supplemented with published data from the Upper Callovian–Lower Kimmeridgian interval (Athleta–Kitchini zones). Significant differences in average temperatures calculated from δ18O values of particular fossil groups (5–15°C) show the thermal gradient and the presence of cold bottom waters in the Middle Russian Sea during the Late Callovian–Middle Oxfordian. An Upper Oxfordian–lowermost Kimmeridgian decrease in δ18O values and an increase in Sr/Ca ratios of cylindroteuthid belemnite rostra likely reflect a warming of the bottom waters of ca. 3.5°C. The gradual Late Oxfordian–earliest Kimmeridgian warming is followed by an abrupt temperature rise of 3–6°C that occurred at the transition of the Early Kimmeridgian Bauhini and Kitchini chrons.The occurrences of cold bottom waters and of (Sub)Mediterranean ammonites and belemnites in the Middle Russian Sea at the Middle–Late Jurassic transition are regarded as a result of the opening of seaways during a sea level highstand. The bottom waters are considered to have been formed in the cool Boreal Sea. The subsequent retreats of the cold bottom waters and of the (Sub)Mediterranean cephalopods from the Middle Russian Sea in the Late Oxfordian are explained by the restriction of water circulation during a sea-level fall. The Early Kimmeridgian rise of bottom temperatures of the sea is linked to a global climate warming. The data presented do not support a major cooling of the Arctic and a consequent glaciation in this region at the Middle–Late Jurassic transition. Since occurrences of cold water masses are diachronous in different European basins, the observed variations in sea water temperatures are interpreted as a result of changes in marine currents and water circulation.δ13C values of belemnite rostra from the Russian Platform are scattered but show the long-term Upper Callovian–Middle Oxfordian positive excursion consistent with the previously published isotope records of the Boreal Realm and terrestrial organic matter.

Strontium and carbon isotope stratigraphy of the Late Jurassic shallow marine limestone in western Palaeo-Pacific, northwest Borneo

Strontium and carbon isotope stratigraphy was applied to a 202m-thick shallow marine carbonate section within the Late Jurassic Bau Limestone at the SSF quarry in northwest Borneo, Malaysia, which was deposited in the western Palaeo-Pacific. Strontium isotopic ratios of rudist specimens suggest that the SSF section was formed between the latest Oxfordian (155.95Ma) and the Late Kimmeridgian (152.70Ma), which is consistent with previous biostratigraphy. The δ13Ccarb values of bulk carbonate range from −0.10 to +2.28‰ and generally show an increasing upward trend in the lower part of the section and a decreasing upward trend in the upper part of the section. A comparable pattern is preserved in the δ13Corg isotope record. Limestone samples of the SSF section mainly preserve the initial δ13Ccarb values, except for the interval 84–92m, where an apparent negative anomaly likely developed as a result of meteoric diagenesis. Comparing with the Tethyan δ13Ccarb profile, a negative anomaly in the lower SSF section can be correlated with the lowered δ13C values around the Oxfordian/Kimmeridgian boundary. In addition, δ13Ccarb values of the Bau Limestone are generally ∼1‰ lower than the Tethyan values, but comparable with the values reported from Scotland and Russia, located in Boreal realm during the Late Jurassic. This suggests that either the Tethyan record or the other records have been affected by the δ13C values of regionally variable dissolved inorganic carbon (DIC). The Late Jurassic δ13CDIC values are thought to have been regionally variable as a result of their palaeoceanographic settings. This study shows that δ13C chemostratigraphy of the Palaeo-Pacific region contributes to an improved understanding of global carbon cycling and oceanography during this time period.

Dinosaur track record on a shallow carbonate-dominated ramp (Loulle section, Late Jurassic, French Jura)

In 2006, sauropod and theropod trackways were discovered at the locality of Loulle in the French Jura Mountains. During the Late Oxfordian (sensu gallico), Loulle was located at the southeastern margin of the carbonate-dominated French Jura platform. The upper part of the section contains a 94-cm-thick dinosaur track-bearing interval. Dinosaur tracks were recorded in a tidal-flat setting punctuated by frequent flooding and emersion phases leading to desiccation, microbial mat development, and new sediment inputs. Four types of tracks were identified relative to their morphologies and the rheology of the tracked sediment. Tracks were recorded on sediment with different degrees of water saturation, covered with a thick, cohesive, and elastic microbial mat. This mat cover prevented tracks from erosion agents but also prevented the record of trackmaker anatomical details. Early lithification, probably driven by enhanced dolomitization, allowed the rapid fossilization of the tracks, protecting them from reworking during the subsequent dinosaur tracking phases. Sequential analysis reveals that the record of dinosaur tracks was made possible by a gain of accommodation at the scale of elementary (infra-Milankovitch) and small-scale (100 kyr) sequences. The loss of accommodation observed at the scale of medium-scale (400 kyr) and very small scale (20 kyr) sequences enhanced the ramp progradation at the Bimammatum-Planula boundary, allowing dinosaurs to walk on the Loulle tidal-flat, along a narrow ENE/WSW isthmus.

Reinterpretation of the Spanish Late Jurassic “Hispaniachelys prebetica” as an indeterminate plesiochelyid turtle (Testudines, Pancryptodira)

A partial postcranial skeleton (carapace, plastron, and other poorly preserved elements) of a turtle, from the late Oxfordian of the Betic Range of Spain, has recently been assigned to a new taxon, Hispaniachelys prebetica. This is one of the few European turtle taxa reported from pre-Kimmeridgian levels, and the oldest turtle so far known from southern Europe. The character combination identified in that taxon (including the presence of cleithra, and single cervical scale) did not allow its assignment to Plesiochelyidae, a group of turtles very abundant and diverse in the Late Jurassic of Europe. The revision of the single specimen assigned to this taxon led to the reinterpretation of some of its elements, being reassigned to Plesiochelyidae. This study confirms the presence of Plesiochelyidae in the Oxfordian. However, because the Spanish taxon does not present a unique combination of characters, it is proposed as a nomen dubium.

The shifting biogeography of reef corals during the Oxfordian (Late Jurassic). A climatic control?

Until recently, the Jurassic was thought to have been a period characterised by a predominantly warm and equable climate. During the Oxfordian (a time span of six million years in the Late Jurassic) the distribution of tropical coral reefs was limited to about 35°N and near to 25°S. However, in Middle Oxfordian time, coral reefs were abundant only at higher latitudes and almost entirely missing near the equator. During that time the area of maximum reef development had shifted poleward to a belt lying between 20°N and 35°N, leaving hardly any coral formations at the lower inner-tropical latitudes. After demise towards the end of Middle Oxfordian time, the low-latitude reefs recovered during the Late Oxfordian, accompanied by a southward migration of reef corals in the northern hemisphere. As suggested by stable isotope and palynological data, the faunal migration can be correlated with a significant rise in seawater temperature during the Middle Oxfordian.

The Oxfordian stable isotope record (δ18O, δ13C) of belemnites, brachiopods, and oysters from the Kachchh Basin (western India) and its potential for palaeoecologic, palaeoclimatic, and palaeogeographic reconstructions

The study combines the results of stable isotope (δ13C, δ18O) analyses of 187 belemnites, brachiopods, and oysters from the Middle to Upper Jurassic (Upper Callovian to Kimmeridgian) of the Kachchh Basin in western India. Generally, belemnites show lower δ13C and higher δ18O ratios than the benthic fauna. It is proposed that this discrepancy is caused by different life habitats of both groups with belemnites spending major parts of their life in deeper water outside the Kachchh Basin and entering shallow areas only occasionally (e.g., during seasonal migrations to spawning grounds). A mode of life as fast-swimming, nektonic predators similar to many modern squid species seems plausible. The study emphasises that results of stable isotope analyses should be evaluated separately for each fossil group. In addition to the influence of diagenetic alteration, weathering, salinity changes, and sea-level fluctuations, the ecology of the analysed fossil taxa has to be considered. Temperatures calculated from δ18O ratios point to relatively warm conditions around the Middle to Late Jurassic transition with no evidence for a glacial phase which had been proposed for this boundary. A distinct fall in temperatures is observed during the Middle Oxfordian reaching a minimum in the early Late Oxfordian. It is tempting to connect this event with the break-up of Gondwana causing stronger upwelling in the northwestern Tethys or alternatively the influx of polar water by the opening of the Transgondwanan Seaway. However, such connections cannot be verified without further data, preferably from localities in the southern Malagasy Gulf (e.g., Madagascar) and from younger horizons (Kimmeridgian–Tithonian).

Rate and synchronicity of environmental changes on a shallow carbonate platform (Late Oxfordian, Swiss Jura Mountains)

AbstractRecent global change occurs within decades and leaves a significant imprint on shallow carbonate platforms. To what extent can rate and synchronicity of environmental changes in the past be evaluated in order to allow comparisons with the processes and products of today? Sections of a carbonate‐dominated platform of Late Oxfordian age have been logged in detail in the Swiss Jura Mountains. The time frame, based on ammonite biostratigraphy and the hierarchical stacking of the sedimentary sequences, suggests that these formed by low‐amplitude, high‐frequency sea‐level changes in tune with the orbital cycles. The smallest unit, the elementary sequence, can thus be attributed to the 20 kyr precession cycle. By identifying sequence‐stratigraphic elements such as maximum‐flooding surfaces within an elementary sequence, an even higher time resolution can be obtained and reconstructions of changes in water depth and sea‐level are attempted. Sedimentation rates were highly irregular and discontinuity surfaces are common. Correlation of such surfaces from one section to the other is tentative because they could also have formed locally and are not necessarily synchronous. On the scale of elementary sequences, the vertical facies evolution permits interpretation of changes in water depth, which were produced by eustatic sea‐level changes (allocyclic) and/or by autocyclic processes. The distribution of clay minerals indicates changes of sea‐level, as well as of rainfall in the hinterland. Biotic changes were controlled by water depth and water quality, and also by nutrient and clay input. Carbon and oxygen isotopes do not show significant shifts within the elementary sequences. The original ecological, mineralogical and geochemical signals have partly been homogenized because of time‐averaging through bioturbation and through reworking by storms, and because of diagenetic modifications. Consequently, there is potential to interpret environmental changes on the studied carbonate platform but the time resolution is not better than a few thousand years. The rates and the synchronicity of the processes leading to the observed environmental changes can thus be estimated only within this time frame.

Microbial carbonates and corals on the marginal French Jura platform (Late Oxfordian, Molinges section)

Molinges was located on an Upper Jurassic ramp system of low-energy regime that developed at the southern margin of the French Jura platform. The sedimentary succession is characterized by the transition from a mixed siliciclastic-carbonate to a carbonate depositional setting that occurred during a long-term shallowing-upward trend. The disappearance of siliciclastics is explained by a climatic change, from humid and cold to drier and warmer conditions, previously identified in Late Oxfordian adjacent basins. The base of the section shows marl-limestone alternations of outer ramp. In its middle part, the section displays oncolitic marls, coral-microbialite beds and oncolitic limestones that deposited in a mid ramp position. Finally, the upper section part is made of oolitic limestones of inner ramp. In outer- to mid-ramp settings submitted to terrigenous inputs, the stacking pattern of deposits and facies evolution allow the identification of elementary, small-, medium-, and large-scale sequences. Small amplitudes of sea-level variations probably controlled rapid shifts of facies belts and reef window occurrences. In small-scale sequences, the coral beds developed during periods of sea-level rise. The decreasing rate of sea-level rise is marked by the downramp shift of the oncolitic limestone belt that led to the demise of coral-microbialite beds. These bioconstructions are mainly represented by thin biostromes in which corals never reach great sizes. The coral assemblages mainly include the genera Enallhelia, Dimorpharaea, Thamnasteria, and some solitary forms (Montlivaltia and Epistreptophyllum). They suggest relatively low-mesotrophic conditions in marine waters during the edification of the primary framework. Relatively cold water temperatures and periods of more elevated nutrient contents are probably responsible of the reduced coral development and the formation of a large amount of microbialites.

Magneto-, and isotope stratigraphy around the Jurassic/Cretaceous boundary in the Vysoká Unit (Malé Karpaty Mountains, Slovakia): correlations and tectonic implications

Magneto-, and isotope stratigraphy around the Jurassic/Cretaceous boundary in the Vysoká Unit (Malé Karpaty Mountains, Slovakia): correlations and tectonic implications Magneto- and isotope stratigraphic studies in the Vysoká Nappe (Hlboča section, Fatric Unit, Malé Karpaty Mts, Slovakia) were performed. A generally decreasing δ13C isotope curve is interpreted as a primary trend from the Late Oxfordian (3.3 ‰ V-PDB) to the Late Tithonian (1.8-1.4 ‰ V-PDB). Data from the Tithonian part of the Tegernsee Formation probably reflect "local" basin processes connected with the breccia formation in the latest Tithonian/earliest Berriasian and/or with possible diagenetic overprint. The C-isotope record of the Berriasian Padlá Voda Formation is more homogeneous (1.4-1.8 ‰ V-PDB) and assumed to be primary. Magnetostratigraphic investigations were focused on the Jurassic/Cretaceous (J/K) boundary strata. Upper Tithonian nodular limestones of the Tegernsee Formation differ substantially from Lower Berriasian calpionellid limestones of the Padlá Voda Formation in rock magnetic properties. Hematite is present in the Tegernsee Formation, while magnetite is the only magnetic mineral of the Padlá Voda Formation. Additionally, the latter formation contains superparamagnetic magnetite, which significantly influences its magnetic susceptibility. Correlation of normal and reversed magnetic intervals with the Late Tithonian global polarity time scale was supported by microfossil stratigraphy. M21n to M20n magnetozones were distinguished, including the short reversed Kysuca (M20n1r) Subzone within M20n. Interpretation of Lower Berriasian magnetostratigraphy was more complex due to presence of breccia horizons and a stratigraphic gap at the J/K boundary in the lower part of the Padlá Voda Formation embracing M19r and most of M19n magnetozones. This formation was also partially affected by remagnetization. Detailed correlation between the isotope- and magnetic stratigraphy of the Tithonian-Berriasian interval between Hlboča and Brodno sections is also complex due to J/K stratigraphical gap within the Hlboča section. The primary B component accounts for counter-clockwise rotation of the Vysoká Unit with a magnitude of ca. 50°. Since the paleodeclination of Paleogene and Karpatian-Eggenburgian rocks in the area is similar, the rotation must have taken place after Early Miocene. The paleoinclinations of several Upper Tithonian-Berriasian sections of the Central Western Carpathians and western part of the Pieniny Klippen Belt are consistent and indicate paleolatitude of 27-30°N.

The Aulacostephanidae (Ammonoidea) of the Oxfordian/Kimmeridgian boundary beds (Upper Jurassic) of southern England

Abstract: Four areas in southern England centred on Swindon and Westbury (Wiltshire), Bourton (north Dorset) and the Dorset coast near Weymouth (south Dorset) have yielded well-preserved late Oxfordian and early Kimmeridgian (Upper Jurassic) ammonites in abundance. These ammonites belong principally to the aulacostephanid genera Ringsteadia and Pictonia, and their microconch equivalents Microbiplices and Prorasenia. Systematic descriptions of these genera are included herein. Within the zonal and subzonal sequence of the English Oxfordian/Kimmeridgian Stage boundary beds, the established subdivision of the Late Oxfordian Pseudocordata Zone into Pseudoyo, Pseudocordata and Evoluta Subzones is confirmed. In the Early Kimmeridgian Baylei Zone, however, the evidence is that the sequence throughout much of southern England is incomplete compared with more complete sequences such as that at Staffin in the Isle of Skye, with the presence of only one faunal biohorizon, the densicostata horizon.

Tectonic and eustatic control on a mixed siliciclastic–carbonate platform during the Late Oxfordian–Kimmeridgian (La Rochelle platform, western France)

Boreal and Tethyan realms of Western Europe present significant sedimentological, paleontological, and stratigraphic differences. The purpose of this study is to constrain regional versus global controls on the dynamics of a sedimentary system located at the interface of these two realms in order to better understand the origin of their differences. Detailed sedimentological, palynofacies and calcareous nannofossil analyses were performed on two sections from the La Rochelle platform (western France). The Pas section includes part of the Late Oxfordian and Early Kimmeridgian, and the Rocher d'Yves section is assigned to the Late Kimmeridgian. They correspond to monotonous marl–argillaceous limestone alternations. Limestones are essentially mudstones with echinoderms, bivalves and foraminifera that suggest low-energy, open-marine conditions. Highly bioclastic and/or peloidal deposits occur commonly, and show wackestones to wacke-pack-grainstones textures. These deposits indicate frequent high-energy events, and are interpreted as storm deposits. Marls dominate in the most proximal depositional environments, while calcareous deposits are more important in more distal environments. The Rocher d'Yves section is globally more marly than the Pas section, suggesting a more proximal setting. Palynofacies are dominated by woody particles, suggesting shallow-water, proximal depositional environments. Calcareous nannofossils are ascidian spicules, coccoliths, and schizospheres. Watznaueria britannica dominate calcareous nannofossil assemblages in the Pas section. The Rocher d'Yves assemblages are quasi-exclusively composed of Cyclagelosphaera margerelii, and indicate more proximal paleoenvironments than those of the Pas section. Different orders of depositional sequences are defined, with sequence boundaries corresponding to the most rapid relative sea-level falls. They are hierarchically stacked, and correlate, on the basis of ammonite zones, with the sequences of contemporaneous sections from Tethyan and boreal realms. The stacking pattern of these sequences suggests an orbital control on sedimentation. Small-, medium- and large-scale sequences correspond to precession (20 ky) cycles and to 100 ky and 400 ky eccentricity cycles, respectively. The elementary sequences have durations shorter than 20 ky. The Kimmeridgian was a period of global sea-level rise that ended in the Late Kimmeridgian. More proximal depositional environments in the Rocher d'Yves section (Late Kimmeridgian) than in the Pas section (Early Kimmeridgian) imply a progradation of the La Rochelle platform during the Kimmeridgian. This progradation resulted from a slowdown of the subsidence in the Aquitaine Basin during the Kimmeridgian, corresponding to the first steps of Atlantic Ocean opening. High-frequency cycles on the La Rochelle platform formed in sync with Milankovitch orbital cycles, while tectonics controlled the formation of the low-frequency cycles.

High-frequency palaeoenvironmental changes on a shallow carbonate platform during a marine transgression (Late Oxfordian, Swiss Jura Mountains)

Based on a well-established bio- and sequence-stratigraphic framework, a narrow time window in the Bimammatum ammonite zone (Late Oxfordian) is investigated in six Swiss Jura sections representing a shallow-water carbonate platform. From the detailed facies and microfacies analysis of oncoid-rich (Hauptmumienbank Member) and ooid-rich (Steinebach Member) limestones, a microfacies classification is established, depositional environments are interpreted, and a depositional model for the Swiss Jura platform is proposed. The sequence- and cyclostratigraphic interpretation has been performed for the transgressive part of the medium-scale sequence Ox6+, independently for each section, with a very high time resolution at the scale of elementary depositional sequences. The good correlation of the elementary and small-scale sequences between the six studied sections and the similar number of elementary sequences in all sections strongly suggest that allocyclic processes were involved in their formation. The hierarchically stacked depositional sequences (small-scale and elementary sequences) result from orbitally controlled sea-level changes with periodicities of 100 and 20 kyr, respectively. Thickness variations in the correlated small-scale and elementary sequences imply variable sedimentation rates, probably resulting from differential subsidence due to the activity of tectonic blocks. The tectonically controlled platform morphology contributed significantly to the general pattern of depositional environments and, combined with high-frequency sea-level fluctuations, created a complex facies distribution in time and space on the Swiss Jura carbonate platform.

Les Gregoryceras (Ammonitina) de l’Oxfordien moyen terminal et supérieur téthysien : révision systématique, biostratigraphie et évolution

In prolongation of the previous studies, the Gregoryceras Spath, 1924 (Ammonitina, Peltoceratina) species from the uppermost Middle Oxfordian (Rotoides sub-zone) to the Early Late Oxfordian (Bifurcatus zone), are revised. Sections providing most of the studied specimens in this work (Spain, Algeria, Tunisia) are described and dated based on faunal comparisons between Tethyan and Subtethyan domains. It appears that the best tool for correlations is the presence of the genus Gregoryceras, the succession of its species being similar for the two considered domains. It allows us to complete the biostratigraphic scale based on the Gregoryceras species succession, and parallel to the standard zonation. During this period the revised species of Gregoryceras are G. fouquei (Kilian, 1889) and G. pervinquieri (Spath, 1913). G. fouquei is only present in the Rotoides sub-zone (uppermost Middle Oxfordian), and not in the Stenocycloides sub-zone (early Late Oxfordian), where G. pervinquieri (Spath, 1913). A new species, G. benosmanae nov. sp. is described as the most recent species of the genus (Grossouvrei sub-zone). The evolution of the youngest species of Gregoryceras continues the peramorphocline described for older forms. Concerning a potential dimorphism, new data (size differences, umbilicus enlargement during growth) are particularly provided by G. pervinquieri.

A NEW OXFORDIAN PLIOSAURID (PLESIOSAURIA, PLIOSAURIDAE) IN THE CARIBBEAN SEAWAY

Abstract: A new pliosaurid, Gallardosaurus iturraldei gen nov. et sp. nov., was found in the Viñales area, western Cuba, in sediments of the Jagua Formation, middle–late Oxfordian. This new taxon is characterized by: wide participation of the premaxilla in the outer margin of the external naris; frontal not participating in the orbital margin; postorbital in contact with the jugal and squamosal; presence of anterior pterygoid vacuity; cultriform process of parasphenoid convex and exposed in palatal view; pterygoid flanges high; jaw articulation low relative to tooth row; trihedral teeth in cross‐section and with smooth ridges at least in the labial face. A phylogenetic analysis suggests that Gallardosaurus forms a clade with Peloneustes, the most common pliosaurid genus occurring in the Oxford Clay. The Caribbean Seaway was, at least since the Oxfordian, a corridor that permitted the interchange for marine biota between Western Tethys and Eastern Pacific realms. Among vertebrates, bony fish and long‐necked plesiosaurs prevailed. However, marine pleurodiran turtles, metriorhynchid crocodilians, ophthalmosaurian ichthyosaurs, and pliosaurids (G. iturraldei gen. nov. et sp. nov.) have also been found, as well as at least two species of pterosaurs, and one camarasaurian dinosaur. Among these reptiles there were off‐shore pelagic forms such as the ichthyosaurs and metriorhynchids, together with the pliosaurid G. iturraldei gen. nov. et sp. nov.; other taxa were presumably less pelagic, such as the pleurodiran turtles and the cryptoclidid plesiosauroids. Gallardosaurus iturraldei gen. nov. et sp. nov. would have played the role of an active predator taking advantage of nectonic fish recorded in the area.

Spatial and temporal development of siliceous basin and shallow-water carbonate sedimentation in Oxfordian Northern Calcareous Alps

The Late Jurassic succession of Mount Rettenstein (central Northern Calcareous Alps, Austria) is unique in comparison to all other sections known in the Northern Calcareous Alps because it provides the oldest coexistence of radiolarite basin sedimentation with contemporaneous shallow-water carbonate intercalations. An up to 3.5-m-thick debris flow made up of shallow-water carbonate detritus with a radiolaritic matrix is overlain by thin (calcareous) radiolarite, followed by several hundreds of meters of shallow-water carbonates of the Plassen Formation. Benthic foraminifers (Labyrinthia mirabilis Weynschenk and Alveosepta aff. jaccardi) and the radiolarian associations indicate a depositional age of both the debris flow and the basal Plassen Formation around the boundary of Middle/Late Oxfordian resp. in the Late Oxfordian. This is as yet the first unambiguous evidence of Oxfordian shallow-water sedimentation in the Northern Calcareous Alps. This early neritic stage with the settlement of ooid bars and coral-stromatoporoid-reefs, evidenced by the debris flow resediments in siliceous basin sedimentation, is followed by the definite, rapid progradation of the actual Late Oxfordian/Kimmeridgian–Berriasian Plassen Carbonate Platform with its steep slope configuration. Assumably, this evolution was steered by a mixture of both global environmental and regional tectonic constraints.

Co-occurrence of the foraminifer <i>Mohlerina basiliensis</i> with <i>Bacinella–Lithocodium</i> oncoids: palaeoenvironmental and palaeoecological implications (Late Oxfordian, Swiss Jura)

Abstract. The stratigraphical and spatial distribution of the foraminifer Mohlerina basiliensis and Bacinella–Lithocodium oncoids was studied in the lagoonal deposits of the oncoid-rich Hauptmumienbank Member from the Swiss Jura Mountains (Late Oxfordian, NW Switzerland). Mohlerina basiliensis has a double-layered wall structure (thick outer hyaline-radial calcite layer with a lamellar structure and thin inner microgranular calcite layer) and is preferentially found in low-energy facies and associated with normal-marine bioclasts and Bacinella–Lithocodium oncoids. The distribution of Mohlerina basiliensis is relatively patchy. The ‘distal’ platform facies present a higher abundance and a more extensive stratigraphic occurrence of this foraminifer. The timing of the ecological evolution is given by the sequence- and cyclostratigraphic framework and illustrates the stepwise flooding of the platform. Bacinella–Lithocodium oncoids have diameters of a few millimetres to several centimetres and are composed of an alternation of Bacinella–Lithocodium meshwork and thin micritic laminae. They are found preferentially in low-energy facies (bioclastic wackestones) of open lagoons. The distribution of Bacinella–Lithocodium oncoids through time and space is also patchy; however, they appear more abundantly in ‘distal’ platform facies. This study reveals the co-occurrence of Mohlerina basiliensis and Bacinella–Lithocodium oncoids and thus suggests similar environmental conditions.

Late Oxfordian micropalaeontology, nannopalaeontology and palaeoenvironments of Saudi Arabia

ABSTRACT The Hanifa Formation in Saudi Arabia consists of a succession of carbonates, over 100 m thick, that were deposited during the Late Jurassic in an equatorial position on the west flank of the Neo-Tethys Ocean. It consists of the Hawtah and overlying Ulayyah members, each of which is considered as a third-order depositional sequence. The Hawtah Member is assigned an ?Early to Mid-Oxfordian age, based on brachiopod, nautiloid and coccolith evidence; ammonite, nautiloid, coccolith and foraminiferal evidence indicate a Late Oxfordian age for the Ulayyah Member. A detailed study of the microbiofacies and lithology of the late highstand succession of the Ulayyah member sequence was conducted in 41 cored wells distributed across Saudi Arabia. The aim of the study was to determine the most likely locations for porous and permeable grainstone lithofacies that host the Hanifa Reservoir in the region. A range of palaeoenvironments has been determined which include shallow-lagoon packstones and foraminiferal-dominated grainstones and deep-lagoon wackestones and packstones with Clypeina/Pseudoclypeina dasyclad algae. In addition, a series of basin-margin, shoal-associated biofacies are present that include stromatoporoid back-bank packstones and grainstones with the branched stromatoporoid Cladocoropsis mirabilis, bank-crest grainstones with encrusting and domed stromatoporoids. A few wells also proved the presence of intra-shelf, basin-flank mudstones and wackestones containing tetraxon sponge spicules, deep-marine foraminifera and coccoliths. The Hanifa Formation demonstrates the high environmental sensitivity of the Oxfordian biocomponents in Saudi Arabia. The study has exploited this feature to interpret the regional Late Oxfordian palaeoenvironmental variations, together with inferred hydrocarbon implications, with a moderately high degree of certainty. This essentially micropalaeontologically based study has revealed the approximate limit of an intra-shelf basin, with an irregular margin, located in the east-central part of the Saudi Arabian portion of the Late Oxfordian Arabian Plate carbonate platform. The basin is flanked by a belt of stromatoporoid banks that pass laterally into a back-bank facies before developing into a lagoonal facies. There is no evidence for shoreline of this basin, although the presence of rare charophytes, wood fragments and quartz grains in the northwest testifies to possible proximity of fluviatile input. The grainstone-dominated basin-margin facies presents good hydrocarbon reservoir facies and its juxtaposition to intra-shelf, potential source-rock basinal sediments provides important new exploration prospects in areas hitherto uninvestigated for hydrocarbon reservoirs, for which the overlying Jubaila Formation provides an efficient regional seal. The study provides a template for low-cost, high-value guidance for the selection of seismic survey sites in remote, under-explored areas where only a few wildcat well samples are available. The study could also be performed using cuttings samples where cores are not available. The varied biofacies within the Hanifa Formation could be applied for biosteering applications should this tool become necessary in coiled-tube, underbalanced horizontal development wells.