Late Bajocian Research Articles

Mirdita Zone ophiolites and associated sediments in Albania reveal Neotethys Ocean origin

The Mirdita Ophiolite Zone in Albania is associated with widespread melanges containing components of up to nappe-size. We dated matrix and components of the melange by radiolarians, conodonts, and other taxa. The components consist of radiolarites, pelagic limestones and shallow-water limestones, all of Triassic age, as well as ophiolites. Triassic radiolarite as a primary cover of ophiolite material proves Middle Triassic onset of Mirdita ocean-floor formation. The melange contains a turbiditic radiolarite-rich matrix (“radiolaritic flysch”), dated as Late Bajocian to Early Oxfordian. It formed as a synorogenic sediment during west-directed thrusting of ophiolite and sediment-cover nappes representing ocean floor and underplated fragments of the western continental margin. The tectonic structures formed during these orogenic events (“Younger Kimmeridian or Eohellenic Orogeny”) are sealed by Late Jurassic platform carbonates. The geological history conforms with that of the Inner Dinarides and adjoining areas; we therefore correlate the Mirdita-Pindos Ophiolite Zone with the Vardar Zone and explain its present position by far-distance west-directed thrusting.

Stratigraphy and Palaeobiogeography of Mesozoic Benthic Foraminifera - Part 3

The Adriatic Carbonate Platform (AdCP), was a separate shallowmarine depositional system characterized by a lack of terrigenous input and was connected to Gondwana towards the South via Gavrovo–Tripolitza or Apulia. It existed for approximately 120 MY, from the Early Jurassic Pliensbachian/Toarcian) to the end of the Cretaceous, resulting in a 4000–6500 m thick succession of almost pure carbonates. However, this is part of a thicker (>8000 m) sequence of predominantly carbonate rocks which forms the Karst Dinarides, and was deposited during more than 270 MY – at least from the Carboniferous (Moscovian) to the Late Eocene. Among many different groups of fossil organisms, benthic foraminifera are especially abundant and well preserved, so they, along with calcareous algae (Dasycladales), are the most important fossils used for age determination and stratigraphic subdivision of shallow-marine carbonate deposits. Within the 257 determined taxa belonging to different foraminiferal families which lived through the Mesozoic, numerous different index fossils occur in assemblages indicating various ages: Early Triassic, Anisian, Carnian, Norian–Rhaetian, Late Sinemurian, Early and Late Pliensbachian (Carixian and Domerian), Early and Late Aalenian, Early and Late Bajocian, Early and Late Bathonian, Callovian, Early and Late Oxfordian, Kimmeridgian, Tithonian, Berriasian, Valanginian, Late Hauterivian, Late Barremian, Early and Late Aptian (Bedulian and Gargasian), Early and Late Albian, Early, Middle and Late Cenomanian, Turonian, Coniacian, Santonian, Early and Late Campanian and Early and Late Maastrichtian. A total of 64 biostratigraphic units – biozones of different categories, from subzone to superzone, were defined within the stratigraphic interval from the Carnian to the Late Maastrichtian. This enabled very detailed biostratigraphic subdivision of the carbonate deposits within the Karst Dinarides. This is one of the most precise sequences, not only in this area, but also among former shallow marine deposits of the entire Neotethyan realm in the present Mediterranean region. The palaeobiogeographic characteristics of biotopes and the composition of foraminiferal assemblages during the Mesozoic were controlled by the position of the study area within the Neotethyan bioprovinces. Until the Albian, this area represented part of the Southern Neotethyan bioprovince, while from the Cenomanian to its final disintegration at the end of the Cretaceous it belonged to a separate, Central Mediterranean Neotethyan bioprovince.

Stratigraphy and Palaeobiogeography of Mesozoic Benthic Foraminifera - Part 2

The Adriatic Carbonate Platform (AdCP), was a separate shallowmarine depositional system characterized by a lack of terrigenous input and was connected to Gondwana towards the South via Gavrovo–Tripolitza or Apulia. It existed for approximately 120 MY, from the Early Jurassic Pliensbachian/Toarcian) to the end of the Cretaceous, resulting in a 4000–6500 m thick succession of almost pure carbonates. However, this is part of a thicker (>8000 m) sequence of predominantly carbonate rocks which forms the Karst Dinarides, and was deposited during more than 270 MY – at least from the Carboniferous (Moscovian) to the Late Eocene. Among many different groups of fossil organisms, benthic foraminifera are especially abundant and well preserved, so they, along with calcareous algae (Dasycladales), are the most important fossils used for age determination and stratigraphic subdivision of shallow-marine carbonate deposits. Within the 257 determined taxa belonging to different foraminiferal families which lived through the Mesozoic, numerous different index fossils occur in assemblages indicating various ages: Early Triassic, Anisian, Carnian, Norian–Rhaetian, Late Sinemurian, Early and Late Pliensbachian (Carixian and Domerian), Early and Late Aalenian, Early and Late Bajocian, Early and Late Bathonian, Callovian, Early and Late Oxfordian, Kimmeridgian, Tithonian, Berriasian, Valanginian, Late Hauterivian, Late Barremian, Early and Late Aptian (Bedulian and Gargasian), Early and Late Albian, Early, Middle and Late Cenomanian, Turonian, Coniacian, Santonian, Early and Late Campanian and Early and Late Maastrichtian. A total of 64 biostratigraphic units – biozones of different categories, from subzone to superzone, were defined within the stratigraphic interval from the Carnian to the Late Maastrichtian. This enabled very detailed biostratigraphic subdivision of the carbonate deposits within the Karst Dinarides. This is one of the most precise sequences, not only in this area, but also among former shallow marine deposits of the entire Neotethyan realm in the present Mediterranean region. The palaeobiogeographic characteristics of biotopes and the composition of foraminiferal assemblages during the Mesozoic were controlled by the position of the study area within the Neotethyan bioprovinces. Until the Albian, this area represented part of the Southern Neotethyan bioprovince, while from the Cenomanian to its final disintegration at the end of the Cretaceous it belonged to a separate, Central Mediterranean Neotethyan bioprovince.

Stratigraphy and Palaeobiogeography of Mesozoic Benthic Foraminifera - Part 1

The Adriatic Carbonate Platform (AdCP), was a separate shallowmarine depositional system characterized by a lack of terrigenous input and was connected to Gondwana towards the South via Gavrovo–Tripolitza or Apulia. It existed for approximately 120 MY, from the Early Jurassic Pliensbachian/Toarcian) to the end of the Cretaceous, resulting in a 4000–6500 m thick succession of almost pure carbonates. However, this is part of a thicker (>8000 m) sequence of predominantly carbonate rocks which forms the Karst Dinarides, and was deposited during more than 270 MY – at least from the Carboniferous (Moscovian) to the Late Eocene. Among many different groups of fossil organisms, benthic foraminifera are especially abundant and well preserved, so they, along with calcareous algae (Dasycladales), are the most important fossils used for age determination and stratigraphic subdivision of shallow-marine carbonate deposits. Within the 257 determined taxa belonging to different foraminiferal families which lived through the Mesozoic, numerous different index fossils occur in assemblages indicating various ages: Early Triassic, Anisian, Carnian, Norian–Rhaetian, Late Sinemurian, Early and Late Pliensbachian (Carixian and Domerian), Early and Late Aalenian, Early and Late Bajocian, Early and Late Bathonian, Callovian, Early and Late Oxfordian, Kimmeridgian, Tithonian, Berriasian, Valanginian, Late Hauterivian, Late Barremian, Early and Late Aptian (Bedulian and Gargasian), Early and Late Albian, Early, Middle and Late Cenomanian, Turonian, Coniacian, Santonian, Early and Late Campanian and Early and Late Maastrichtian. A total of 64 biostratigraphic units – biozones of different categories, from subzone to superzone, were defined within the stratigraphic interval from the Carnian to the Late Maastrichtian. This enabled very detailed biostratigraphic subdivision of the carbonate deposits within the Karst Dinarides. This is one of the most precise sequences, not only in this area, but also among former shallow marine deposits of the entire Neotethyan realm in the present Mediterranean region. The palaeobiogeographic characteristics of biotopes and the composition of foraminiferal assemblages during the Mesozoic were controlled by the position of the study area within the Neotethyan bioprovinces. Until the Albian, this area represented part of the Southern Neotethyan bioprovince, while from the Cenomanian to its final disintegration at the end of the Cretaceous it belonged to a separate, Central Mediterranean Neotethyan bioprovince.

The dogger reef horizons of the Moroccan Central High Atlas: New data on their development

In the Central High Atlas, N of Errachidia, three major reef horizons in the Bajocian carbonate series of the Atlasic Basin were studied recently. The results of the study are based on fieldwork with an emphasis on stratigraphy, morphology and sedimentary patterns of detailed field sections. The lower reef horizon is composed of smaller build-ups which developed during the Aalenian-Lower Bajocian interval. At that time the depositional environment favored the production of both basinal and platform facies. This reef horizon is located only on the southern platform rim. Its development within the trough was controlled by synsedimentary block tilting, triggered by rifting, which was related to an ancient fracture zones inherited from the Triassic. Formed during the later stages of a rising sea-level, this horizon reflects a high carbonate production on the southern platform area. In the basin area, the second reef horizon presents relatively larger bodies (“ patch reefs”). It consists of two major reef levels, separated by an interval of marls interspersed with thin-bedded limestones, developed most probably during Early? to Late Bajocian. For the duration of this time, within two 3rd order sequences, progradational shelfal units extended the platform facies northward to the basin center, covering a large area. Within a long-term transgressive/regressive cycle, throughout the Middle Aalenian-Bajocian, these reef horizons changed their structural growth to aggrading stacking patterns. The third reef horizon belongs to the lower part of the upper member of the Tazigzaout Formation, and shows the same sedimentary evolution as the second reef horizon underneath. It fits into a 2nd order geodynamic regressive cycle (upper part of Upper Bajocian-Bathonian pp.). It appears, that the development of the second and third reef horizon’s forced the platform to prograde towards the N, NW and E during a late highstand, when accommodation was decreasing. However, depending on the complexity of the sedimentary systems, and on the interplay between tectonics and eustatic sea level changes, the interpretations of the depositional history of these reef horizons remain complicated.

A forced regressive shelf-margin wedge formed by transition-slope progradation: lowermost Cretaceous Rauk Plateau Member, Jameson Land, East Greenland

The Middle Jurassic – lowermost Cretaceous succession of Jameson Land, East Greenland records a marine, overall regressive–transgressive–regressive cycle with regressive maxima in the Late Bajocian and Late Volgian separated by a transgressive maximum in the Kimmeridgian. Smaller-scale regressive interludes took place in the Late Callovian and Mid Oxfordian. A shelf-slope-basin physiography started to develop in the Late Callovian due to increasing rifting and a relief of several hundred metres was attained during maximum end-Jurassic regression and deposition of the Volgian Raukelv Formation. The formation consists of a forestepping stack of laterally extensive shelf-edge wedges, each several tens of metres thick, composed of coarse-grained sandstone, showing highangle clinoform bedding and containing marine body and trace fossils. These clinoform beds are superimposed on the large-scale clinoforms of the shelf–slope–basin. The wedges onlap older shelf deposits in a landward direction and are overlain by thin transgressive sandstones or mudstones, or directly by the next coarse-grained wedge. The top wedge, comprising the Rauk Plateau Member, is of Late Volgian (i.e. earliest Cretaceous) age and is characterized by steep clinoforms truncated by internal erosional downlap surfaces. The clinoforms are simple avalanche beds, a few tens of centimetres thick, or they may be several metres thick and contain large-scale cross-bedded intrasets of probable tidal origin. The erosional events were associated with downshift of the succeeding clinoforms, recording minor sea-level fall and forced regression. The top surface of the Rauk Plateau wedge is incised by a system of minor channels leading to a large canyon-like valley. The wedge was deposited by transition-slope progradation below wave base during a period of sea-level stillstand punctuated by minor, stepwise falls. It provides an excellently exposed example of a laterally derived, coarse-grained shelf-margin wedge, showing high-angle clinoform bedding and representing an ancient counterpart to Holocene and Late Pleistocene prograding infralittoral wedges seen on seismic profiles across Mediterranean shelf edges.

Specialization and iterative evolution of some Western Tethyan Bathonian ammonites [ Benatinites (B.) nov., B. (Lugariceras) nov. and Hemigarantia

Late Bajocian to Early Bathonian ammonites, now attributed to Benatinites and Lugariceras, and Late Bathonian Hemigarantia, previously considered as cryptogenic taxa, share very similar sutural and ornamentation characters, suggesting their derivation from the stephanoceratid stock, most probably from the Cadomites–Polyplectites line. The palaeogeographical distribution of these taxa is limited to the North-Western African and European margins of Western Tethys. Palaeontologically, they represent the result of the iterative evolution rather than a poorly known lineage.

The upper Shemshak Formation (Toarcian–Aalenian) of the Eastern Alborz (Iran): Biota and palaeoenvironments during a transgressive–regressive cycle

The siliciclastic, up to 4,000 m thick Upper Triassic–Bajocian Shemshak Formation is widespread across the Iran Plate, especially in the Alborz Mountains of northern Iran. In contrast to its lower, generally non-marine part, the upper part is marine. Based on the Tazareh section of the eastern Alborz, an integrated analysis of this marine interval is presented. The 1,700 m thick marine sedimentary succession records a gradual deepening from inner to mid and outer shelf environments from the Middle Toarcian to early Late Aalenian. During the Late Aalenian–Early Bajocian, the trend was reversed and infilling of the basin by a large delta system occurred. This general facies development reflects a nearly symmetrical transgressive–regressive (T–R) megacycle, terminated by the inter-regional mid-Cimmerian tectonic event. A renewed transgression in the early Late Bajocian initiated a subsequent sedimentary megacycle. The bioturbated mid and outer shelf sediments contain a low to moderately diverse benthic fauna dominated by deep burrowing bivalves, often preserved in the growth position. A hierarchy of four orders of sedimentary cycles can be recognized (parasequences, parasequence sets, unconformity-bounded third-order depositional sequences, and the 13 ma long second-order T–R megacycle). A regional correlation with the Jajarm area (200 km to the east) shows a very similar temporal facies pattern of the upper Shemshak Formation. The eastern Alborz T–R cycle is completely out-of-phase with other (eustatic) sea-level curves, suggesting regional tectonic control. Rough estimates of subsidence rates give an average value of 126 m/ma. However, much higher values for the Aalenian (230 m/ma), particularly the Late Aalenian (700 m/ma), indicate a distinct increase in subsidence rate towards the Early Bajocian mid-Cimmerian tectonic event. These high subsidence rates suggest that the sediments of the Shemshak Formation of the eastern Alborz formed in a (young) rift basin.

Downhole magnetic measurements of ODP Hole 801C: Implications for Pacific oceanic crust and magnetic field behavior in the Middle Jurassic

Downhole horizontal and vertical magnetic field measurements within the 474 m thick Jurassic crustal section drilled at ODP Hole 801C in the western Pacific show anomalies that are “in phase,” indicating that this site formed in the southern hemisphere and moved across the paleoequator to its present location at 18.6°N. The inclination computed from the horizontal and vertical anomaly logging data varies significantly downhole and can be explained by progressive rotation of the lava sequence as it is buried during accretion. This is compatible with the observed dips in the Formation Micro Scanner (FMS) data, which show up to 42° of rotation toward the fossil spreading axis. We restore the magnetic inclination to prerotated values and obtain an estimated pretilt inclination of 39.9° ± 6.6°, corresponding to a paleolatitude of 22.7° ± 5°S. The magnetic logging data document six polarity units within the drilled volcanic extrusive crust. The upper 132 m thick logged section of lavas contains four polarity units, which apparently formed 7 Myr after the formation of the underlying basement. The lower 212 m logged section of the hole is reversely magnetized with an intervening normal polarity zone. More than one reversal downhole would indicate a rapid reversal rate given the time it takes to form the extrusive section of oceanic crust: ∼45,000 years at a fast spreading rate of 66 km/Myr. Previous results from a contemporaneous section in Spain suggest rapid reversals throughout the late Bajocian–early Bathonian Stage (170–165 Ma). Nevertheless, the complicated formation history prevents quantification of the geomagnetic reversal rate for the 801C crustal section.

World distribution of middle Jurassic ammonites (Upper Aalenian to Middle Bathonian): relationships between biodiversity and palaeogeography

Abstract The break up of the Pangea takes place in the Jurassic; the palaeoceanographic consequences are the opening of seaways, particularly at the place of the future Atlantic and Indian oceanic areas. During the Toarcian, and from the late Aalenian to the middle Bathonian, the so-called “Hispanic corridor” (or “Atlantic seaway”) exists between the “western Tethys” and the “American Pacific border”, through the “Caribbean Tethys”. Two additional seaways which play as by-passes of the Pangea are proposed, one along the northern border of Laurasia (Boreal sea), and a second along the southern border of Gondwana (South Pacific Sea); however, if these two last could be effectively used for faunal exchanges and migrations, they are poorly documented. Near the end of the Aalenian, the Ammonitina sub-order undergoes a strong faunal turnover. The last Hammatocerataceae, a well known and diversified Liassic superfamily, gives birth to three distinct superfamilies which will dominate among others till the end of the Jurassic: Stephanocerataceae, Perisphinctaceae and Haplocerataceae. The analysis of the worldwide corresponding radiation of these three major taxa puts in light differences and similarities between the biogeographic provinces usually recognised. The Tethyan, Pacific and Boreal domains, and their associated epicratonic platforms are divided into sixteen “palaeobiogeographical provinces”; the listed ammonite taxa concern twenty-three subfamilies which range in fifteen biozones, from the late Aalenian to the middle Bathonian. The counting of the species of each subfamily within the several palaeobiogeographic provinces emphasises faunal similarities in terms of total biodiversity between the several provinces. To perform, a “hierarchical ascendant classification” is used: the results are illustrated in the shape of a relational tree where the nearest branches correspond to the provinces that show the most similar biodiversity. The northwestern Tethyan provinces (European platforms) share strong similarities between them and with the provinces of the southwest Tethyan margin (Maghreb). The Circum-Pacific provinces gather themselves, underlining their great faunal similarities. Finally, provinces that show a strong endemism are isolated; these are the Boreal areas and the southeast Tethyan margin. The total time variation of the diversity is expressed by the counting of species in each biozone from the late Aalenian to the middle Bathonian. The primary and global signal of the diversity evolution obtained is then independently related to each palaeobiogeographic province. The comparison between the two signals shows the differences of the time evolution of the diversity in the several provinces. The maxima of diversity are often diachronous in the several listed provinces: early Bajocian in North America; late Bajocian on the northwest European platforms. Finally, the palaeogeographic distribution of each ammonite subfamily is used to emphasise the evidence for the several seaways that would exist between the several provinces. The maps which have been constructed for the considered ammonite taxa show that peculiar seaways like the “Hispanic corridor” (Caribbean Tethys), the “South Gondwana” (South Pacific Sea) and the “North Laurasia” (Boreal Sea) bypasses could have been used by ammonites to invade several provinces. Nevertheless, some groups have a geographic distribution of which the causes still remain problematic.

THE PRE-OROGENIC VOLCANO-SEDIMENTARY COVERS OF THE WESTERN TETHYS OCEANIC BASIN: A REVIEW

The records of the Jurassic Western Tethys Ocean are the ophiolitic rocks now scattered in the Tertiary orogenic belts of the Alps, Apennines and Betic Cordillera. These ophiolites, involved in a convergent margin environment, are affected I) by HP/LT metamorphism derived from a subduction process or II) by very low grade overprint, corresponding to the tectonic prism at the margin of the overriding plate. On the whole, they share common characteristics: a- The MORB geochemical signature. b- The ophiolitic successions often “reduced” and thin. c- The volcano-sedimentary covers often directly overlying the serpentinised peridotites. d- The widespread occurrence of cherts as Jurassic pelagic sediment. In the thickest “complete” ophiolitic successions, basalt flows, generally thin, are preceded and followed by ophiolitic breccias. Only the basal portion of the breccias on top of the serpentinites (Levanto Breccias) has a tectonic origin, all other levels have a sedimentary origin. These breccia-basalt assemblages are overlain by thick sequences of Mt. Alpe Cherts and Calpionella Limestones, followed by Palombini Shales. In the reduced (or incomplete) successions, thin breccias and cherts were directly deposited, above the Levanto Breccias (ophicalcites pro parte), and followed by Palombini Shales. This stratigraphic pattern seems to be widespread in the whole Western Tethys ocean. In some sequences, transitional mid-ocean ridge (T-MOR) basalts are present and the ophiolitic rocks are associated with Variscan continental slices and debris, as in the Err-Platta succession (Central Alps) and in some exotic blocks in the flysch of the External Ligurides (Northern Apennines). In the Balagne (Corsica) T-MOR basalts are associated with quartzarenites. These occurrences show that an unroofed mantle and sections of oceanic crust evolved very near to a continental margin. The different radiolarian ages of the cherts deposited before, within, or on top of the MOR basalts allow to infer a minimum time interval for the Western Tethys oceanisation. This interval can be considered between 16 and 21 Ma (from Late Bajocian to Kimmeridgian/Tithonian). If we assume 1cm/yr spreading rate during this time, the basin would have reached about 150-200 km width. The same ages suggest that the ocean opening was diachronous along the Western Tethys basin. Mainly on the basis of the Northern Apennines and Corsica data, it is possible to reconstruct the following evolutive geodynamic, paleogeographic and sedimentary evolution of the Western Tethys ocean basin: 1- Bajocian/Bathonian stage: opening of the Ligurian Northern Apennines oceanic segment and, perhaps, also of the Ligurian, Western and Central Alps ones. 2- Bathonian/Callovian stage: opening of all the segments of the Western Tethys ocean basin. The volcano-sedimentary covers formed during these two stages are constituted by breccias, basalts and siliceous pelagites (cherts). 3- Tithonian/Berriasian: end of the ocean spreading (Tithonian) and beginning of the quiescent stage in the whole basin, marked by the lack of any tectonic activity and by the sedimentation of the Calpionella Limestones and, locally, of mixed siliceous-calcareous deposits (Nisportino-Murlo Fm.). 4- Hauterivian/Santhonian: this is the longest quiescent stage of the basin, dominated by the sedimentation of the Palombini Shales and Limestones. Some siliciclastic deposits are shed from both passive continental margin sides. During the Early Cretaceous, there is also evidence of a rare intraplate magmatism in Southern Tuscany. The Western Tethys ophiolitic successions are similar to those of present day, slow spreading oceans, in particular to those of the Atlantic Ocean (Mutter and Karson, 1992; Tucholke and Linn, 1994). The Galician North Atlantic margin provides a model for the process of mantle denudation. For the oceanic evolution, the model of Tucholke and Linn (1994) is particularly taken in consideration. According to this model, tectonic extension was one major process in the Western Tethys oceanic development.

Chariocrinus (Crinoidea: Articulata) from the Latady Formation, Behrendt and Hauberg Mountains, Ellsworth Land, Antarctica

Specimens of a late Bajocian to early Callovian isocrinid assigned to Pentacrinites cf. P. californicus (Clark), and specimens collected by Hikuroa in 1999–2000, are described as a new species of Chariocrinus. All specimens were collected from rock exposures of the Latady Formation at eight localities in the Behrendt and Hauberg Mountains, Ellsworth Land, Antarctica. The fauna, preserved in situ at all localities, is essentially complete, but due to tectonism, localities may have been moved many kilometres from their original position. Chariocrinus latadiensis n. sp. is compared with congeneric species and those of Hispidocrinus. It is the first record of the genus in the Southern Hemisphere, which has European Tethyan affinities. Columnals are pentastellate to pentalobate, and the symplectial articula appear diagnostic of Hispidocrinus but are included in Chariocrinus on the basis of spineless axillaries and fused adjacent crenulae; those of Hispidocrinus are spined and separated by a furrow or smooth band, respectively. A single pluricolumnal (?Apiocrinus) is the only associated crinoid found at any of the localities. Chariocrinus latadiensis lived a semi‐sessile existence on a volcarenite substrate in association with vagrant ammonites, epifaunal and infaunal bivalves, and epifaunal brachiopods. The benthic taxa lived in shallow, low energy marine environments in the Middle Jurassic, part of a submarine back‐arc basin near the shore off southeastern Gondwana.

High‐resolution 40Ar/39Ar dating of the oldest oceanic basement basalts in the western Pacific basin

We report new 40Ar/39Ar ages for the oldest Pacific oceanic floor at Ocean Drilling Program Site 801C in the Pigafetta basin and Site 1149D close to the Izu‐Bonin subduction zone in the Nadezhda basin. These ages were determined by applying high‐resolution incremental heating experiments (including 15–30 heating steps) to better resolve the primary argon signal from interfering alteration signatures in these low‐potassium ocean crust basalts. Combined with previous results from Pringle [1992] for Site 801B and 801C, we arrive at a multistage history for the formation of the Pigafetta ocean crust. The oldest part of the Pacific plate was formed at the spreading ridges at 167.4 ± 1.4/3.4 Ma (n = 2, 2σ internal/absolute error), offering an important calibration point on the Geological Reversal Timescale (GRTS) since it represents the old end of the Mesozoic magnetic anomalies. This mid‐ocean ridge basalt sequence, however, is overlain by more tholeiites and alkali basalts that were formed 7.3 ± 1.5 Myr later around 160.1 ± 0.6 Ma (n = 7, 2σ internal error). The older age group is confirmed independently by radiolarian ages ranging from Late Bajocian to Middle Bathonian (167–173 Ma [Bartolini and Larson, 2001]) and by profound differences in the structural characteristics of this basement section [Pockalny and Larson, 2003]. Thin layers comprising hydrothermal deposits separate these sequences, which in addition to the difference in isotopic age show distinct major and trace element compositions. This indicates that key volcanic and hydrothermal activity took place 400–600 km away from the spreading ridges, on the basis of a Jurassic ∼66 km/Myr half spreading rate in the Pacific. It remains unclear if these processes were active continuously after the initial formation of the Pacific oceanic crust, but all our observations seem to point to an episodic history. Site 1149D gives another important calibration point on the GRTS of 127.0 ± 1.5/3.6 Ma (n = 1, 2σ internal/absolute error) for anomaly M12 that is slightly younger when compared to current timescale compilations (134.2 ± 2.1 Ma [Gradstein et al., 1995]). This might suggest that the dated basalt from Site 1149D does not represent the age of the ocean crust formed at its ridge axis; it may also be part of the Early Cretaceous intraplate events that have produced dolerite sills in the Pacific crust at Sites 800 and 802 around 114–126 Ma.

Shallow marine syn-rift sedimentation: Middle Jurassic Pelion Formation, Jameson Land, East Greenland

The Middle Jurassic Pelion Formation – Fossilbjerget Formation couplet of Jameson Land, East Greenland, is a well-exposed example of the Middle Jurassic inshore–offshore successions characteristic of the rifted seaways in the Northwest European – North Atlantic region. Early Jurassic deposition took place under relatively quiet tectonic conditions following Late Permian – earliest Triassic and Early Triassic rift phases and the Lower Jurassic stratal package shows an overall layer-cake geometry. A long-term extensional phase was initiated in Middle Jurassic (Late Bajocian) time, culminated in the Late Jurassic (Kimmeridgian–Volgian), and petered out in the earliest Cretaceous (Valanginian). The Upper Bajocian – Middle Callovian early-rift succession comprises shallow marine sandstones of the Pelion Formation and correlative offshore siltstones of the Fossilbjerget Formation. Deposition was initiated by southwards progradation of shallow marine sands of the Pelion Formation in the Late Bajocian followed by major backstepping in Bathonian–Callovian times and drowning of the sandy depositional system in the Middle–Late Callovian. Six facies associations are recognised in the Pelion–Fossilbjerget couplet, representing estuarine, shoreface, offshore transition zone and offshore environments. The north–southtrending axis of the Jameson Land Basin had a low inclination, and deposition was sensitive to even small changes in relative sea level which caused the shorelines to advance or retreat over tens to several hundreds of kilometres. Eight composite sequences, termed P1–P8, are recognised and are subdivided into a total of 28 depositional sequences. The duration of the two orders of sequences was about 1–2 Ma and 360,000 years, respectively. The Upper Bajocian P1–2 sequences include the most basinally positioned shallow marine sandstones, deposited during major sealevel lowstands. The lowstands were terminated by significant marine flooding events, during which sandstone deposition was restricted to northern, more proximal parts of the basin. The Upper Bajocian – Middle Bathonian P3–4 sequences show an overall progradational stacking pattern. The sequence boundary at the top of P4 marks a significant shift in stacking pattern, and the Upper Bathonian – Middle Callovian P5–8 sequences show large-scale backstepping, terminating in a widespread condensed succession at the distal, southern end of the basin. The largescale backstepping was governed by combined tectonically-induced subsidence, reflecting increased rates of extension, and eustatic sea-level rise. The depositional trends of the Pelion Formation – Fossilbjerget Formation couplet provide a well-exposed analogue to contemporaneous subsurface deposits which form major hydrocarbon reservoirs on the west Norway shelf, and in the Northern North Sea.

The Jurassic of East Greenland: a sedimentary record of thermal subsidence, onset and culmination of rifting

The Late Palaeozoic – Mesozoic extensional basin complex of East Greenland contains a record of deposition during a period of Rhaetian – Early Bajocian thermal subsidence, the onset of rifting in the Late Bajocian, its growth during the Bathonian–Kimmeridgian, culmination of rifting in the Volgian – Early Ryazanian, and waning in the Late Ryazanian – Hauterivian. The area was centred over a palaeolatitude of about 45°N in the Rhaetian and drifted northwards to about 50°N in the Hauterivian. A major climate change from arid to humid subtropical conditions took place at the Norian–Rhaetian transition. Deposition was in addition governed by a long-term sea-level rise with highstands in the Toarcian–Aalenian, latest Callovian and Kimmeridgian, and lowstands in the latest Bajocian – earliest Bathonian, Middle Oxfordian and Volgian. The Rhaetian – Lower Bajocian succession is considered the upper part of a megasequence, termed J1, with its base in the upper Lower Triassic, whereas the Upper Bajocian – Hauterivian succession forms a complete, syn-rift megasequence, termed J2. The southern part of the basin complex in Jameson Land contains a relatively complete Rhaetian–Ryazanian succession and underwent only minor tilting during Middle Jurassic – earliest Cretaceous rifting. Rhaetian – Lower Jurassic deposits are absent north of Jameson Land and this region was fragmented into strongly tilted fault blocks during the protracted rift event. The syn-rift successions of the two areas accordingly show different long-term trends in sedimentary facies. In the southern area, the J2 syn-rift megasequence forms a symmetrical regressive–transgressive–regressive cycle, whereas the J2 megasequence in the northern area shows an asymmetrical, stepwise deepening trend. A total of eight tectonostratigraphic sequences are recognised in the Rhaetian–Hauterivian interval. They reflect major changes in basin configuration, drainage systems, sediment transport and distribution patterns, and in facies and depositional environments. The sequences are bounded by regional unconformities or flooding surfaces and have average durations in the order of 10 Ma. They are subdivided into conventional unconformity-bounded depositional sequences with durations ranging from tens of thousands of years, in the Milankovitch frequency band, up to several million years. Deposition was alluvial and lacustrine in the Rhaetian–Sinemurian, but almost exclusively marine during the Pliensbachian–Hauterivian time interval when a marine strait, up to 500 km wide and more than 2000 km long, developed between Greenland and Norway, connecting the Arctic Sea and the North Sea. Coal-bearing fluvial and paralic deposits occur, however, at the base of the onlapping Middle Jurassic succession in the central and northern part of the basin complex. The sedimentary development is similar to that in the Northern North Sea and on the Norwegian shelf, and East Greenland offers important onshore analogues for virtually all of the types of deeply buried Jurassic depositional systems of these areas and especially their hydrocarbon reservoirs.

Transgressive–regressive cycles and Jurassic palaeogeography of northeast Iberia

A correlation and sequence stratigraphy study of Jurassic successions has been carried out in the main sedimentary basins of northeast Iberia, i.e., Asturias, Basque-Cantabrian, and Iberian basins, based on the identification of transgressive–regressive cycles. The development and palaeogeographic evolution of the epicontinental carbonate platforms of northeast Iberia were largely controlled by major tectonic activity at three main intervals, at the beginning of the Jurassic, in the Lower–Middle Jurassic transition, and during the uppermost Jurassic, respectively. During Early Jurassic times, northeast Iberia was the site of a single and large carbonate ramp opened to the north. This carbonate ramp suffered a progressive drowning, evolving from an inner to hemipelagic ramp systems, with the local development of suboxic environments in the deepest areas located to the north. During the Middle Jurassic, different open carbonate platforms were formed including the development of swells in intermediate areas. During the Upper Jurassic, the outer ramp areas were progressively moving to the east Iberian Basin, and the ammonite faunas showing a markedly Tethyan affinity thereafter. Three first order T–R cycles bounded by major discontinuities associated with significant time gaps are identified. They extend respectively from the latest Rhaetian to the Early Aalenian, from the Middle Aalenian to the Early Oxfordian and from the Early Oxfordian to the Late Berriasian. Major transgressive peaks occurred at the Middle Toarcian (Bifrons Zone), at the Late Bajocian (upper Niortense and Garantiana zones) and at the mid-Kimmeridgian (Divisum Zone). Each of the first order cycles includes four second-order T–R cycles. Cycles 1.1–1.4 are identified in the northern basins. Cycles 2.1–2.4 display some differences in age of transgressive peaks from one basin to another. Cycles 3.1–3.4 are mainly identified in the Iberian Basin. The correlation with other separated Boreal and Thethysian basins demonstrates that the number and age of T–R cycles varies from one basin to another and are mainly controlled by the local or regional tectonic development. The transgressive peaks may reflect episodes of eustatic rise during the Jurassic. However, their different age from one basin to another is explained by different subsidence evolution.

THE MIDDLE JURASSIC SILICEOUS SEDIMENTARY COVER AT THE TOP OF THE VOURINOS OPHIOLITE (GREECE)

The age of the Vourinos ophiolitic massif, pertaining to the Jurassic Tethys Ocean, is not yet exactly known. Four sections in the cherts at the top of the basalts yielded well preserved radiolarian assemblages according to which the age of the end of the “ophiolitic activity” (= the end of ocean spreading) was immediately before or within the latest Bajocian interval. A radiometric (Ar/Ar) age of the metamorphic sole at the base of the ophiolite gave 171±4 Ma (late Bajocian), this is, an age slightly older than that of the cherts: this can mean that the metamorphic soles pertain to an intraoceanic hot thrust, doubling the oceanic crust/lithosphere.

Stable isotope records (O, C) of Jurassic aragonitic shells from England and NW Poland: palaeoecologic and environmental implications

Shells of fully marine Middle to Upper Jurassic molluscs from England and north-western Poland were analysed with respect to their stable isotope (δ 18O, δ 13C) compositions, and palaeoecological and environmental life conditions of these molluscs were inferred from them. Light microscopical and SEM inspection and an analysis of the minor element content (Fe, Mn, Mg, Sr) suggest rather unaltered isotope signals. The δ 18O and δ 13C values show a characteristic distribution among three groups of co-occurring organisms. Benthic (adult) bivalves generally preserved higher δ 18O and δ 13C values than ammonites, whereas planktic bivalve larvae tend to possess the lowest δ 18O but higher δ 13C than adult bivalves. As this distribution pattern is found in numerous horizons and sections of Bathonian to Kimmeridgian age in NW Poland and England, it is thought to reflect real palaeoenvironmental parameters. All observations can be incorporated in a single model that assumes (i) seasonally induced temperature stratification of the water column, (ii) a correlation between phytoplankton blooms and reproduction season of planktic–planktotrophic bivalves, and (iii) insignificant vital effects with respect to the δ 13C in bivalves, but strong biological control in ammonites. In addition, the δ 18O evolution suggests that the Late Bajocian to Middle/Late Bathonian and Early Oxfordian to Late Kimmeridgian were considerably warmer than the latest Bathonian to Late Callovian time interval. The oxygen isotopic records from other European regions indicate a similar pattern of long-term palaeotemperature evolution. The comparatively high water temperatures during the Callovian to Oxfordian of the Isle of Skye (NW Scotland) are enigmatic, however. In the Early Oxfordian, sea surface and bottom temperatures began to rise in continental Europe and England. These changes coincide with a south-westward drift of the West European crustal plate, but a causal relationship remains to be demonstrated.

Evolutionary Rates of Jurassic Ammonites in Relation to Sea-level Fluctuations

Other| August 01, 2001 Evolutionary Rates of Jurassic Ammonites in Relation to Sea-level Fluctuations JOSÉ SANDOVAL; JOSÉ SANDOVAL 1Departamento de Estratigrafía y Paleontología, Universidad de Granada, 18071- Granada, Spain Search for other works by this author on: GSW Google Scholar LUIS O'DOGHERTY; LUIS O'DOGHERTY 2Facultad de Ciencias del Mar, Universidad de Cádiz, 11510- Puerto Real (Cádiz), Spain Search for other works by this author on: GSW Google Scholar JEAN GUEX JEAN GUEX 3Institut de Géologie et Paléontologie, BFSH-2, Université de Lausanne, CH-1015 Switzerland Search for other works by this author on: GSW Google Scholar PALAIOS (2001) 16 (4): 311–335. https://doi.org/10.1669/0883-1351(2001)016<0311:EROJAI>2.0.CO;2 Article history accepted: 16 Jan 2001 first online: 03 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation JOSÉ SANDOVAL, LUIS O'DOGHERTY, JEAN GUEX; Evolutionary Rates of Jurassic Ammonites in Relation to Sea-level Fluctuations. PALAIOS 2001;; 16 (4): 311–335. doi: https://doi.org/10.1669/0883-1351(2001)016<0311:EROJAI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search nav search search input Search input auto suggest search filter All ContentBy SocietyPALAIOS Search Advanced Search Abstract An analysis is presented of the diversity and faunal turnover of Jurassic ammonites related to transgressive/regressive events. The data set contained 400 genera and 1548 species belonging to 67 ammonite zones covering the entire Jurassic System. These data were used in the construction of faunal turnover curves and ammonite diversities, that correlate with sea-level fluctuation curves. Twenty-four events of ammonite faunal turnover are analyzed throughout the Jurassic. The most important took place at the Sinemurian-Carixian boundary, latest Carixian-Middle Domerian, Domerian-Toarcian boundary, latest Middle Toarcian-Late Toarcian, Toarcian-Aalenian boundary, latest Aalenian-earliest Bajocian, latest Early Bajocian-earliest Late Bajocian, Early Bathonian-Middle Bathonian boundary, latest Middle Bathonian-earliest Late Bathonian, latest Bathonian-Early Callovian, earliest Early Oxfordian-Middle Oxfordian, earliest Late Oxfordian-latest Oxfordian, latest Early Kimmeridgian, Late Kimmeridgian, middle Early Tithonian and Early Tithonian-Late Tithonian boundary. More than 75 percent of these turnovers correlate with regressive-transgressive cycles in the Exxon, and/or Hallam′s sea-level curves. In most cases, the extinction events coincide with regressive intervals, whereas origination and radiation events are related to transgressive cycles. The turnovers frequently coincide with major or minor discontinuities in the Subbetic basin (Betic Cordillera). You do not currently have access to this article.

A thyreophoran dinosaur from the Early Bajocian (Middle Jurassic) of the Isle of Skye, Scotland

Synopsis The proximal parts of the right ulna and radius of a thyreophoran dinosaur from Bearreraig Bay, Isle of Skye, represent the first occurrence of this type of dinosaur from the Middle Jurassic rocks of Scotland. The incompleteness of the bones and the lack of more diagnostic features does not allow a precise identification although the ulna has more features in common with that of a eurypodan (stegosaurs and ankylosaurs) than a basal thyreophoran. The earliest identifiable stegosaur from Europe that has similar proximal expansion of the ulna is the middle Callovian (Middle Jurassic) Lexovisaurus of England and France. The ulna of the ankylosaur Mymoorapelta from the Upper Jurassic of Western Colorado, also shows close morphological similarity to the Scottish bone. Other ankylosaur fragments have been recorded from as early as the late Bajocian (Middle Jurassic) of England. The stratigraphic horizon from which the new bones come from is the Bearreraig Sandstone Formation (Early Bajocian, Middle Jurassic).