Late Aalenian Research Articles

Lower Jurassic deformation in the eastern Huincul High, Argentina

Subsurface characterization of the structure and sedimentary sequences in the Agua del Cajón area was performed to understand the Lower Jurassic deformation across the eastern Huincul High. Integration of seismic, well data, and fossil fauna allowed establishing the Late Triassic rifting and Jurassic compressional evolution underwent for this area. Four Late Triassic - Early Jurassic extensional depocenters were recognized, being the most significant the Agua del Cajón and El Salitral depocenters. In general, all of them present an E-W to WNW-ESE orientation and are flanked by NNE to NE-trend transfer zones. The contractional structural style of the Agua del Cajón area consists of south-vergence reverse faults and folds with two primary trends: (i) E-W to WNW-ESE and (ii) NE-SW to NNE-SSW. According to the slip and uplift amount, the first-order structural feature in the study area corresponds to the S-verging, E-W Huincul fault. Geometries and patterns in the seismic reflectors evidence two compressional episodes from late early Toarcian to Callovian time. The first one is associated with the inversion of the previous master normal fault of the El Salitral and Agua del Cajón depocenters during the late early Toarcian. The inversion of the NE-SW synthetic normal fault associated with the El Salitral depocenter also occurred during the first deformation pulses favored by its perpendicular orientation to NW-SE Jurassic σ1. NE-SW fault associated with the Agua del Cajon inversion shows no evidence of inversion-related deformation mechanisms. Instead, the formation of this structure is related to the previous NE-SW transfer zone. After a tectonic quiescence period in late Toarcian – late Aalenian, the compressional activity continued during the late Aalenian and Bajocian evidenced by progressive unconformities into Jurassic sequences. This activity would have continued until Callovian times. These results are in agreement with previously reported evolution in the eastern-central Huincul High region. However, they indicate that the contractional intraplate deformation in the southern Neuquén basin would have begun during the late early Toarcian; this is older than previously proposed.

New results of stable isotope and petrographic studies of Jurassic glendonites from Siberia

ABSTRACTWe present the results of an optical microscopy, cathodoluminoscopy and isotopic study on nine glendonite concretions (calcite pseudomorphs replacing metastable hexahydrate ikaite) from Lower-Middle Jurassic sediments of Northeast Russia (Anabar Bay and Lena River region). Glendonite concretions are mainly found within Late Pliensbachian, Toarcian, Aalenian, Bajocian and Lower Bathonian clastic sediments, correlating to episodes of global climatic cooling as determined by independent paleoclimate proxy data. Stable carbon and oxygen isotopic values of glendonite concretions suggest that the primary source of carbon was derived from diagenetically altered organic matter, and the source of oxygen was from seawater. The secondary diagenetic cement is characterized by a significantly lighter δ18O and significantly heavier δ13C signature than the isotopic characteristics of the bulk rock glendonite concretion. This secondary diagenetic cement is thought to have precipitated rapidly during burial diagenesis and since it occupies a significant volume of the glendonite concretion, it has the potential to significantly influence the isotopic composition of bulk rock glendonites.

The epiclastic barrier-island system of the Early‒Middle Jurassic in eastern Spain

During the Early-Middle Jurassic, several mainly volcaniclastic submarine eruptions, which followed well marked lineaments, took place in the Iberian Ranges of eastern Spain. Some of these volcanoes were occasionally emerged during sea-level lowstand periods. In this work we aim to present the signs indicating that a huge volcanic pile, located in the Sierra de Javalambre (Teruel, Spain), could have been subjected to emersion, reworking and colonization by flora. We also intend to date the volcanism and to delimit the most likely period of time when emersion, reworking and colonization were produced. The research work was supported by geological mapping, as well as by preparing stratigraphic sections and their palaeontological sampling. Biostratigraphical dating was achieved though taxonimical determination of ammonites, brachiopods, and data from pre-existing palynological studies. As a result of the shallow marine and subaerial partial erosion of the volcanic edifice, epiclastic bodies, some of which show large scale cross-bedding and bar geometry, were generated. These bedforms are interpreted as sandwaves constituting a barrier-island system protecting a lagoon on which low-energy sediments, rich in plant remains, together with washover fan facies were deposited. An Early Toarcian, Serpentinum Chronozone age for the volcanism is indicated by the brachiopod and ammonite content of infra- and inter-volcanic sediments. However, burial of the volcanic mound by the carbonates of the surrounding platforms ended in the Late Aalenian Bradfordensis Chronozone, about 10 million years after the eruption. This delay was partly due to a significant regional sedimentary gap marked by the hiatus of the Late Toarcian (from the Late Variabilis Chronozone) to the Late Aalenian (Bradfordensis-Concavum chronozones). The emersion of the volcanic pile, the colonization of the emerged islands by continental vegetation and the instauration of the epiclastic barrier-island system occurred between part of the Aalenian Murchisonae and part of the Bradfordensis chronozones. The concurrence of the porous epiclastic lithofacies together with the organic-rich lagoonal sediments could represent an analogue model of an untypical hydrocarbon system located in neritic position, which could probably be applied in other areas of extensive emerged submarine volcanoes present in the geological record.

Early Jurassic carbon and oxygen isotope records and seawater temperature variations: Insights from marine carbonate and belemnite rostra (Pieniny Klippen Belt, Carpathians)

New carbon and oxygen isotope records and discussion of the main variations in seawater temperature through the Sinemurian–Aalenian of the Pieniny Klippen Basin (northern Tethys Ocean) are presented herein. Comparison of the recorded changes in stable-isotope compositions of bulk carbonate and belemnite rostra from an open-marine environment with previously documented, predominantly restricted epicontinental data enables determination of major climatic events that were most likely of worldwide extent. A slight positive δ13C shift is recorded in the lowermost Upper Pliensbachian. A significant positive excursion in carbonate carbon isotope values is documented in the Lower Toarcian Serpentinum Zone. Furthermore, the δ13C values display a falling trend in the Lower–Upper Toarcian and relatively constant values in the Aalenian. Temperatures inferred from the δ18O values of well-preserved belemnite rostra suggest rather cool seawater conditions (10–13 °C) in the Pieniny Klippen Basin during the Late Sinemurian, warming by 4 °C in the Early Pliensbachian and then cooling by 8 °C in the Late Pliensbachian. The seawater temperature rose once more in the Early Toarcian and began to fall again during the Middle Toarcian. In the Middle–?Late Aalenian, seawater temperatures oscillated between 10 and 13 °C.

EARLY AND MIDDLE JURASSIC (PLIENSBACHIAN TO BAJOCIAN) RADIOLARIA FROM CHERTS OF KISELEVKA-MANOMA ACCRETIONARY COMPLEX (AMUR RIVER, EASTERN RUSSIA)

The lower part of chert sequence of Kiselevka-Manoma accretionary complex, Sikhote-Alin system (Eastern Russia) is characterized by the following radiolarian assemblages (in ascending order of age): 1. Early Pliensbachian to early Toarcian with Parahsuum izeenze , Bagotum maudense and Katroma clara. 2. Early Toarcian with Bistarkum rigidium and Citriduma hexaptera. 3. Middle-late Toarcian to early-middle Aalenian with Paronaella grahamensis, Praeparvicingula gigantocornis and Hsuum exiguum. 4. Early-middle Aalenian with Parahsuum ? grande, Parahsuum cruciferum and Hsuum altile. 5. Late Aalenian with Parahsuum ? hiconocosta, Transhsuum hisuikyoense and Nassellaria gen. et sp. indet. 1. 6. Early-middle Bajocian with Sella beniderkoulensis and Emiluvia splendida. New species Olanda inflata Bragin, n. sp. is described from this assemblage. Studied radiolarian assemblages are Tethyan in affinity, which allows to utilize the low-latitude zonal radiolarian scales for the Jurassic (Baumgartner et al., 1995b; Carter et al., 2010). The total stratigraphic range of chert sequence of the Kiselevka-Manoma accretionary complex can be estimated as Early Jurassic (Pliensbachian-early Toarcian)-Early Cretaceous (Barremian).

Middle Jurassic volcanism in a magmatic-rich passive margin linked to the Caudiel Fault Zone (Iberian Range, East of Spain): biostratigraphical dating

During the Early and Middle Jurassic, the Iberian Basin (East of Spain) recorded the transition from a generally magma-poor passive margin, which operated during the latest Triassic and most of Early Jurassic, to a magma-rich passive margin, developed from Pliensbachian to early Middle Jurassic. The mainly submarine pyroclastic volcanism, with minor lava flows, followed active NW and NE trending extensional fault zones. One of these zones of weakness, from which the volcanic deposits were expelled to the sea bottom, was the Caudiel Fault Zone. Two volcanic phases in relation to this weakness line over the Middle Jurassic times have been identified. The main phase occurred during the deposition of the El Pedregal Formation, and the other, less significant, is included between the Casinos and the El Pedregal formations. So far, the accurate age of these volcanisms, critical in the knowledge of the palaeogeographical and geodynamical evolution of the Iberian Basin, has remained uncertain. The ammonite content of the carbonate successions, with which these rocks are interbedded, allows precise biostratigraphical dating at the chronozone scale. For this purpose, 9 sections have been measured and 360 specimens of ammonites were collected and determined. The obtained data indicate that the Middle Jurassic volcanic deposits that are linked to the Caudiel Fault Zone and included into the El Pedregal Formation show a slight diachrony. They were deposited around the boundary between the Late Aalenian Concavum and the Early Bajocian Discites chronozones. However, in very restricted areas of the Caudiel Fault Zone, an older intra-Murchisonae volcanism was recorded.

Resolving the Middle Jurassic dinoflagellate radiation: The palynology of the Bajocian of Swabia, southwest Germany

Dinoflagellates underwent a major radiation during the Bajocian (Middle Jurassic, ~170–168Ma). The group originated in the Middle Triassic and dinoflagellate cysts were relatively low in diversity until the Bajocian, when over 100 species appeared. The Gonyaulacaceae expanded during this interval to become the dominant family of cyst-forming dinoflagellates, and has remained the principal dinoflagellate family throughout the Mesozoic and Cenozoic. However, Bajocian dinoflagellate cysts have received relatively little study. In order to unravel the pattern of the Bajocian dinoflagellate radiation, we have generated a high resolution, quantitative, palynological record through an expanded Middle Jurassic succession in Swabia, southwest Germany. Previous research has indicated a rapid, stepwise order of first appearances through the Bajocian. By contrast, we clearly demonstrate that there was a more gradual, continuous increase in diversity from the Late Aalenian to the Early Bathonian, although the number of first appearances was highest during the latest Early Bajocian to Late Bajocian. Major experimentation in excystment mode in the gonyaulacaceans occurred during the Late Aalenian and Early Bajocian, when multi-plate precingular, single-plate precingular and epicystal archaeopyle types first appeared. Through the integration of our data with published information from northwest Europe, we demonstrate that the appearance of dinoflagellate cyst taxa through the Late Aalenian–Early Bathonian appears to have been controlled by a major second-order transgression. Although the cause of this radiation is yet to be constrained, given the contemporaneous diversifications of other plankton groups, as well as ammonites, bivalves and fishes, it is likely that it formed part of the wider Mesozoic Marine Revolution. There is a prominent acme of Dissiliodinium giganteum in the Lower Bajocian which is a useful stratigraphical marker. Acanthaulax crispa is confirmed as a key index species for the Bajocian. One new species, Korystocysta aldridgeii sp. nov., is described.

Calcareous nannofossil zonation and sequence stratigraphy of the Jurassic System, onshore Kuwait

ABSTRACTThis paper presents the calcareous nannofossil zonation of the Middle and Upper Jurassic of onshore Kuwait and formalizes current stratigraphic nomenclature. It also interprets the positions of the Jurassic Arabian Plate maximum flooding surfaces (MFS J10 to J110 of Sharland et al., 2001) and sequence boundaries in Kuwait, and correlates them to those in central Saudi Arabia outcrops. This study integrates data from about 400 core samples from 11 wells representing a nearly complete Middle to Upper Jurassic stratigraphic succession. Forty-two nannofossil species were identified using optical microscope techniques. The assemblage contains Tethyan nannofossil markers, which allow application of the Jurassic Tethyan nannofossil biozones. Six zones and five subzones, ranging in age from Middle Aalenian to Kimmeridgian, are established using first and last occurrence events of diagnostic calcareous nannofossil species. A chronostratigraphy of the studied formations is presented, using the revised formal stratigraphic nomenclature.The Marrat Formation is barren of nannofossils. Based on previous studies it is dated as Late Sinemurian–Early Aalenian and contains Middle Toarcian MFS J10. The overlying Dhruma Formation is Middle or Late Aalenian (Zone NJT 8c) or older, to Late Bajocian (Subzone NJT 10a), and contains Lower Bajocian MFS J20. The overlying Sargelu Formation consists of the Late Bajocian (Subzone NJT 10b) Sargelu-Dhruma Transition, and mostly barren Sargelu Limestone in which we place Lower Bathonian MFS J30 near its base. The lower part of the overlying Najmah Formation consists of the Najmah Shale, which is subdivided into three subunits: (1) barren Najmah-Sargelu Transition, (2) Late Bathonian to Middle Callovian (lower Zone NJT 12) Lower Najmah Shale, and (3) Middle Callovian to Middle Oxfordian (upper Zone NJT 12 to NJT 13b) Upper Najmah Shale. Middle Callovian MFS J40 and Middle Oxfordian MFS J50 are positioned near the base and top of the Upper Najmah Shale. The upper part of the Najmah Formation is represented by the Late Oxfordian (Subzone NJT 13b) Najmah Limestone, and is overlain by the Kimmeridgian (Zone NJT 14) Jubaila Formation. Early Kimmeridgian MFS J60 and Late Kimmeridgian MFS J70 are positioned near the base and top of the Jubaila Formation. The positions of Late Jurassic MFS J80, J90 and J100 are not constrained by our biostratigraphic data and are positioned in the Gotnia Formation. The Upper Tithonian MFS J110 and the Jurassic/Cretaceous boundary are positioned in the Makhul Formation.

Aalenian–Early Bathonian (Middle Jurassic) radiolarian assemblages in the Tavas nappe within Lycian nappes in the western Taurides (SW Turkey): The first dating of carbonate platform drowning

Stratigraphic sections (Kizilca and Kizilca North) in the Tavas nappe, western Taurides, Turkey records the event of carbonate platform drowning in the western Tethyan realm. Our detailed biostratigraphic study of radiolarians and other microfossils places the important temporal constraints on this event and indicates that drowning occured in the geochronological interval from Early Jurassic (Hettangian–Sinemurian) through to Middle Jurassic (early Bathonian). The basal part of the Kizilca section is represented by platform carbonates with abundant benthic foraminifera and algae of Hettangian–Sinemurian to Pliensbachian age. Toward the upper part of the section, the occurrence of brecciated limestones with some ammonites and belemnites reveals that the drowning event began in the Pliensbachian. The first radiolarian assemblages of early–middle to late Aalenian age were obtained from the overlying units characterized by pelagic limestones and chert alternation. Just after the subsidence of the platform, ribbon chert sedimentation began in the late Aalenian, as indicated by the radiolarians. Successively, early–late Bajocian and latest Bajocian–early Bathonian radiolarian assemblages were obtained from overlying chert with mudstone–silicified mudstones. Similar to that observed in the Kizilca section, the Kizilca North section include successively pelagic limestones, pelagic limestone–chert alternation and chert–mudstone alternation of late Aalenian to early–middle Bajocian age according to radiolarian biostratigraphy. All of these observations indicate that a continuous stratigraphy recorded the drowning event without a large gap from Hettangian–Sinemurian to early Bathonian in the Tavas nappe. This drowning event has close similarities with coeval events in other Tethyan realms in terms of mechanism, facies characteristics and fossil assemblages. Therefore, this study can aid correlation studies of the Tavas nappe with the Jurassic paleogeography worldwide.

The Late Aalenian Polyplocussandstein Formation in SE Lower Saxony, Germany: Meandering distributary and crevasse splay sedimentation on a lower deltaic plain

The Late Aalenian Polyplocussandstein Formation in SE Lower Saxony, Germany: Meandering distributary and crevasse splay sedimentation on a lower deltaic plain

A new record of the enigmatic lobster genus Stenodactylina Beurlen, 1928 (Crustacea: Decapoda: Erymidae) from the Middle Jurassic of south-western Germany

A second species of the previously monotypic lobster genus Stenodactylina Beurlen, 1928, Stenodactylina rogerfurzei n. sp. is recorded from the upper Aalenian of the Wutach area (Baden-Wurttemberg, SW Germany). Like the type species, Stenodactylina liasina Beurlen, 1928, the new species is based only on a single chela. Originally the type horizon of S. liasina was erroneously reported as being late Toarcian in age, but it in fact came from a bituminous limestone bed within the lower Toarcian Posidonia Shale. Hence, the known stratigraphical range of Stenodactylina is expanded from the early Toarcian to the late Aalenian.

Discorhabdus as a key coccolith genus for paleoenvironmental reconstructions (Middle Jurassic, Lusitanian Basin): Biometry and taxonomic status

Abstract Biometric analyses of Discorhabdus coccoliths were performed on a set of 29 samples taken from the marl/limestone couplets of the reference section of Cabo Mondego (Lusitanian Basin, west Portugal), Late Aalenian to Early Bajocian in age. Mixture analysis performed on the whole dataset revealed a bimodal length size distribution for the Discorhabus pool composed of D. striatus and D. ignotus. The size limit at 5 μm proposed here can be reliably used to distinguish both populations, but optical characteristics, namely birefringence, form an additional criterion essential to discriminate the two species. An increase in Discorhabdus size occurred from the Late Aalenian to the Early Bajocian, due to a raise in size of the whole Discorhabdus pool and in the abundance of D. striatus. We propose that an increase in Discorhabdus size upward in the section is associated with a positive trend in paleoproductivity as supported by the increase of nannofossil accumulation rates as well as the δ13C values of bulk carbonates sampled in the Cabo Mondego section. However, the Discorhabdus size pattern may also represent a response to sea-surface temperature, salinity or an evolutionary trend. A relative supra-regional warming of sea-surface water masses is suggested by δ18O values of biogenic carbonates and apatites that were sampled in several Mediterranean Tethyan settings. Rise of sea-surface temperature may have triggered an increase in the abundances of D. striatus as well as of the size of the Discorhabdus pool. A hypothetical influence of changing sea-surface salinity on Discorhabdus size or species-specific Discorhabdus abundances is not supported by our micropaleontological data or by available geochemical data. Our results also reveal that the increase in Discorhabdus size matches the history of calcareous nannofossil turnover (radiation, diversity and increasing abundances) during the Middle Jurassic.

Les grottes d'Azé (Saône-et-Loire, France) : de la roche altérée aux sédiments

The Azé cave (south Burgundy), from alteration to ghost-rock. The goal of this study was to gain a better understanding of the formation of the Azé Caves and the relationships which exist at this site between the surrounding rock and the sediments. A detailed study of the limestones from the Late Aalenian to the Early Bathonian which make up the surrounding rock shows that they contain considerable quantities of quartz and hundreds of ppm of manganese. Certain levels contain numerous siliceous cherts. From the Jurassic, the region underwent several phases of immersion. This period was also marked by a tectonic phase of weak amplitude. The faulting which occurred at this time allowed the circulation of meteoric water through the limestone leading to the dissolving of calcium carbonates and to phases of dolomitisation and dédolomitisation. As a result, "ghost rock" was created in the different stratigraphie units. However the hydrodynamism of these flows was weak and the alteration of the rock was isovolumic without evacuation of the alterite. During the Oligocene, the Méconnais region underwent a more severe tectonic phase. The structure of leaning blocks that we see today was put in place and the "ghost rock" was fractured by larger faults. As a result a hydrodynamic potential was created between the different sides of the massifs and the alterite was removed by water thus creating the caves. From that time on bacterial activity was able to develop on the walls of the cavities and in the residual alterite. The bacteria oxidised the manganese and iron contained in the limestone leading to the accumulation of manganese and iron oxide deposits in the limestone and on the walls, often in the form of structured encrustings. These bacteria were still active during certain periods of the Quaternary as shown by the discovery of deposits on bear bones. Under the effect of gravity, the least soluble limestone residues and the conglomerations of manganese formed by bacteria fell to the bottom of the caves or were carried away by flowing water. Depending on the hydrodynamic regime, these residues (clays, sands, gravels, pebbles) were to varying extents transported by water and were removed from or left behind in the caves. As a result, the sandy fractions of sediments deposited at the time of flows with the weakest hydrodynamism are mostly made up of these residues.

Dinoflagellate cyst biostratigraphy of the Opalinuston Formation (Middle Jurassic) in the Aalenian type area in southwest Germany and north Switzerland

Feist-Burkhardt, S. & Pross, J. 2010: Dinoflagellate cyst biostratigraphy of the Opalinuston Formation (Middle Jurassic) in the Aalenian type area in southwest Germany and north Switzerland. Lethaia, Vol. 43, pp. 10–31. In order to provide a detailed dinoflagellate cyst stratigraphy of the Lower Aalenian Opalinuston Formation from the Aalenian type area, 68 samples from four boreholes and one outcrop section were analysed. The sample localities are Hausen an der Fils and Wittnau in southwest Germany, Weiach in north Switzerland and Mont Russelin in the Swiss Jura Mountains. Dinoflagellate cyst assemblages were recovered from the Late Toarcian Aalensis Zone to the Late Aalenian Murchisonae Zone. The samples yielded rich, well-preserved and diverse assemblages with 51 dinoflagellate cyst taxa identified in total. The dinoflagellate cyst distribution data obtained from this study allow a high-resolution biostratigraphical subdivision of the lowermost Middle Jurassic Opalinuston Formation into four palynostratigraphical units. First and last occurrences, acmes and consistent presence of the species Batiacasphaera sp. A, Evansia cf. granochagrinata, Kallosphaeridium praussii, Nannoceratopsis triangulata, Phallocysta? frommernensis and Wallodinium laganum were selected as the criteria for defining these units. The obtained high-resolution palynostratigraphical scheme provides a basis for establishing and further refining early Middle Jurassic biostratigraphy in the Boreal and Tethyan realms. □Aalenian, biostratigraphy, dinoflagellate cysts, Germany, Jurassic, Switzerland, Toarcian.

Palaeoclimatic and biotic changes during the Aalenian (Middle Jurassic) at the southern Laurasian Seaway (Basque–Cantabrian Basin, northern Spain)

The uppermost Toarcian–lowermost Bajocian deposits have been studied in 10 sections located in the western part of the Basque–Cantabrian Basin (northern Spain). The studied 276 successive recorded ammonite assemblages allowed detailed biostratigraphical subdivision and the correlation of the sections at the ammonite zone and subzone scale. The foraminifers were studied in 96 samples and 55 diagenetically screened belemnite rostra were analyzed for C and O isotope. Facies and thickness distribution of the Aalenian sediments suggest that deposition took place in a nearly symmetrical sub-basin included into an intraplate shallow platform, developed over continental crust, on which flexure was the main control responsible for subsidence and basin evolution. Noteworthy negative δ 13C bel excursions, coinciding with seawater temperature changes, were recorded during the Bradfordensis Biochron, around the Concavum–Limitatum biochron boundary, and around the Aalenian–Bajocian boundary. A close relationship between the changes in seawater temperature and the biotic changes observed in the foraminiferal and in the ammonoids assemblages has been evidenced. During the Comptum Biochron, the δ 18O bel-based palaeotemperature shows a remarkable cooling interval with an average seawater temperature of 15.7 °C. This cooling favoured the immigration of species of foraminifers that thrived in platforms with colder seawater temperatures located north of the Basque–Cantabrian Basin. As a consequence, a strong increase in the diversity of the foraminiferal assemblages (28.5% of first appearances) occurred. This interval also coincides with the highest ammonoid abundance recorded during the Aalenian. A notable increase in temperature with peak values up to 24.3 °C, was measured during the Bradfordensis Biochron. This warming marks the beginning of progressive loss of foraminifer diversity and the decrease in the abundance of ammonoids. However, the increase in the relative abundances of some foraminiferal taxa such as the genus Spirillina during the warming phase is remarkable. Some of the foraminiferal species that had their last occurrence during this interval seem to be immigrants from NW Europe that arrived during the Comptum cooling interval, and that did not survive to rise of the seawater temperature. Another drop in temperature was recorded during the late Gigantea and the early Concavum biochrons. Coinciding with this cooling interval nearly 20% of the foraminiferal species disappeared while the ammonoid abundance increased. A new and significant warming episode occurred during the latest Aalenian (Limitatum Biochron), extending to the earliest Bajocian (Discites Biochron). Biotic response to this new Δ T of 2.5 °C is remarkable, marking one of the most important crises of the Aalenian. More than 30% of the foraminiferal species disappeared and no new appearances were recorded. Ammonoids show a decrease in the specimen abundance during the Limitatum–Discites warming, preceding a drastic decrease in the species abundance. The remarkable biotic changes linked to the variations of seawater temperature recorded in the Basque–Cantabrian Basin during the Aalenian, open the possibility that some of these faunal turnovers, specially marked in the benthic foraminifers, were of global extent and mainly forced by climate changes.

Aalenian carbon-isotope stratigraphy: Calibration with ammonite, radiolarian and nannofossil events in the Western Tethys

In this study a detailed carbon-isotope stratigraphy of four Aalenian sections well-dated with ammonites from the Median Subbetic palaeogeographic domain (external zones of the Betic Cordillera, southern Spain) is presented. Two of these represent the most complete Aalenian sections of the southern Iberian palaeomargin (western Tethys). These sections are rhythmic sequences made of marls and marly limestones containing rich ammonite fauna and nannofossils that enable an accurate biostratigraphic control at the subzone level. Moreover, the lower and upper Aalenian boundaries are well defined in these sections and therefore represent optimal sites to link the carbon-isotope curves to ammonite zones and subzones and nannofossil events. The uppermost Toarcian–lowermost Bajocian δ 13C values of bulk carbonates show evident fluctuations. The values are low (~ 1‰) in the Upper Toarcian (Mactra Subzone), increase slightly to ~ 2‰ in Lower Aalenian (Comptum Subzone), fall to a relative minimum (~ 1–1.5‰) in the Middle Aalenian (Bradfordensis Zone), reach a marked positive peak of 2.5–2.7‰ in the Upper Aalenian (Concavum Subzone), and then the values decrease again to 1.5–1.7‰ near the Aalenian/Bajocian boundary. Marine carbonate δ 13C curves in Subbetic and the Apennines shows similar variations but higher δ 13C values in the Apennines sections. A slight offset is also present between the sections from the same basin. This drift is interpreted by variations in carbon isotopic composition of sea-water DIC associated with different palaeoceanographic settings. Comparison of the δ 13C values of bulk carbonates and biotic events (ammonite diversity, and nannofossil and radiolarian abundances) reveals relationships between the carbon cycle and biotic evolution. The latest Toarcian–Aalenian turnovers (Toarcian/Aalenian boundary and Middle/Late Aalenian) follow low δ 13C values, where extinct taxa are replaced by new ones. Radiation episodes are associated with increasing δ 13C values and with sea-level rise. In the earliest Late Aalenian, concomitantly with a major δ 13C positive excursion and with the transgressive cycle, it is noted a major increase of the eutrophic calcareous nannofossil genus Biscutum and a sharp peak in radiolarian abundance. Although the Aalenian interval seems not to be characterized by major large amplitude negative or positive excursions, as observed in other Phanerozoic intervals, these data point to a close connection between perturbations in the global carbon cycle and the palaeoenvironmental changes that occurred, which in turn had important interactions with the evolution of pelagic biota.

Fertility changes in surface waters during the Aalenian (mid-Jurassic) of the Western Tethys as revealed by calcareous nannofossils and carbon-cycle perturbations

In Aalenian times, the South Iberian Palaeomargin was part of the westernmost Tethys Ocean. The Median Subbetic palaeogeographic domain of the Betic Cordillera was a relatively deep trough in the South Iberian Palaeomargin during the Early Jurassic–Late Cretaceous interval, where mainly pelagic and hemipelagic limestones and marls were deposited. A semiquantitative study of nannofossil assemblages was performed in sediments from the upper Toarcian–lowest Bajocian from two Median Subbetic sections (Agua Larga and Cerro de Mahoma). Nannofossil assemblages are composed mainly of cosmopolitan and Tethyan taxa. The NJ8a, NJ8b and NJ9 Zones as well as other useful biohorizons (FOs of Triscutum tiziense and Carinolithus magharensis and LO of Similiscutum finchii) were identified and directly correlated to ammonite zones. The analysis of the relative abundances of some common to abundant taxa including Biscutum, Carinolithus superbus, Crepidolithus crassus, Lotharingius, Schizosphaerella and Watznaueria display noticeable fluctuations that can be correlated between the two sections. The comparison of these fluctuations with the δ 13C carb curves and the interpretation of the palaeoecologic significance of some of these taxa provided an outline of the palaeoceanographic trophic regime throughout the interval studied. During the latest Toarcian–Early Aalenian, the high proportions of oligotrophic Schizosphaerella, moderately high proportions of C. crassus and low proportions of eutrophic Biscutum, correlate with low to moderate values in the δ 13C carb curves. Radiolarians display low abundance throughout this interval. This is interpreted as an interval where mesotrophic to oligotrophic and stable conditions occurred in surface waters. The Middle Aalenian, characterized by high proportions of Schizosphaerella and C. crassus and low proportions of Biscutum, correlates with low values in the δ 13C carb curves, and was interpreted to correspond to an episode when stable oligotrophic conditions occurred in surface waters. Radiolarians moderately increased throughout this interval. Finally, the Late Aalenian–earliest Bajocian interval, with lower proportions of Schizosphaerella and C. crassus, and higher proportions of Biscutum, also correlates with a significant positive excursion in the δ 13C carb curve, suggesting a shift from oligo- to eutrophic conditions in surface waters. This change in productivity is also revealed by a conspicuous increase in radiolarian abundance, at the same time as a quasi-complete replacement of Early Jurassic radiolarian fauna took place. The analysis of faunal-flora turnovers reveals a causal link between the global carbon-cycle and the pelagic response. This noticeable faunal-flora change throughout the Late Aalenian–Bajocian can be interpreted as a major biological response to the drastic modification in the western Tethys palaeogeography as consequence of the Atlantic opening, which in turn caused a new pattern in the oceanic circulation.

10.1007/s11506-008-1005-z

Stages in evolution of the Early Jurassic to Aalenian foraminifers and ostracodes are established based on the analyzed diversity dynamics of respective microfauna associations. Evolution of foraminifers is divided in two, the Hettangian-initial early Toarcian and the late early Toarcian-Aalenian stages, while the identical first stage in evolution of ostracodes has been followed by the late early Toarcian-Callovian stage. During the Jurassic, periodic migrations of foraminiferal and ostracod genera and species, which were interrelated with large transgressions and climatic changes, took place in the initial late and mid-late Pliensbachian, initial early Toarcian, and the late Toarcian-early Aalenian. Being isolated to the maximum extent in the second half of the late Aalenian, the Arctic basin lost connections with seas of northwestern Europe. The Early Jurassic to Aalenian biogeography of the Arctic basin is established based on the results of cluster analysis (group average link method, Jaccard coefficient, presence or absence of foraminifers and ostracodes genera) with due account for preceding and subsequent formation history of microbenthos structure in biochores. The distinguished biochores are ranked as realms and provinces of foraminifers and ostracodes. As is established, contours of the realms and provinces populated by different groups of microbenthos did not coincide and changed with time. Ecotones between the realms (e.g., the North Sea province) changed their localitization to be a part of the Arctic or Boreal Atlantic realms in different epochs. The Early-Middle Jurassic sedimentary successions of the Arctic basins reveal several levels of sharp taxonomic changes in composition of microbenthos under influence of the first-order abiotic factors.

Foraminifera from the Aalenian and the Bajocian GSSP (Middle Jurassic) of Murtinheira section (Cabo Mondego, West Portugal): Biostratigraphy and paleoenvironmental implications

The detailed study of benthic foraminifera in 28 samples from Late Toarcian–Aalenian–Early Bajocian hemipelagic sediments of the Murtinheira section (Cabo Mondego, Lusitanian Basin, West Portugal), has permitted the recognition of the assemblage composition and the analysis of their evolution throughout this time interval. The representatives of the Suborder Lagenina dominate all the assemblages and, in the majority of them, the most abundant species is Lenticulina münsteri. The Late Toarcian and Early Aalenian assemblages are very similar. From the Comptum Subzone, a gradual replacement of the Early Jurassic forms by characteristic Middle Jurassic taxa takes place. During the Late Aalenian–Early Bajocian interval, a significant number of last occurrences (LOs) of taxa are recorded. The assemblages composition, the estimation of the relative taxa abundances and the data obtained from the calculation of several diversity indexes, together with the previous sedimentological and paleontological works in this area, indicate that these assemblages were developed in a shelfal basin environment. This platform showed normal marine salinity values, good oxygenation and was always located above calcite compensation depth (CCD). However, the environmental conditions were not always the best for the development of rich and diverse foraminiferal assemblages. These conditions were probably unstable during the Late Toarcian (Aalensis Subzone)–Early Aalenian (lower part of the Comptum Subzone). From the Comptum Subzone to the lower part of the Limitatum Subzone (Late Aalenian), conditions were more stable and more favourable for the development of foraminiferal communities. However, in the Aalenian–Bajocian transition the environmental conditions changed again and affected the development of this group. In the Limitatum Subzone a decrease in the diversity of the assemblages was recorded. This fact seems to be related to a global perturbation in the carbon-cycle that could have affected the foraminifera, together with other marine and continental organism groups.

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.