Aptian Time Research Articles

Фораминиферы и биостратиграфия апта северного палеобиогеографического района Западной Сибири

Several (7) boreholes have been drilled between the territories of the Pur and Taz rivers which are assigned to the Northern Palaebi-ogeographical district on the base of finding Foraminifera. The marine Aptian deposits have been discovered in this district for the first time. The southern boundary of the Aptian foraminiferal finds in the sections of the wells lies between the Pur and Taz rivers. Based on these finds, one may think that the Aptian boreal transgression has spread in the West Siberian basin from the Arctic one. The Aptian assemblages have also been found in the territory of the Yjamal Peninsula. For the most part, transgression may have occurred in the Middle Aptian age because there exist many Foraminifera Assemblages which conditionally are assigned to this age. The author of the paper has studied the Aptian Foraminifera Assemblages from the section wells of the South-Russkaya area, the line sections of wells from the Taseiskaya area (well P-159), the West-Tambeiskaya area (wells 42, 45, 124), the North-Tambeiskaya area (wells 201, 205). In theese sections agglutinated quartz - siliceous Foraminifera related to the Aptian age were discovered. Foraminiferal Assemblages proved to be of a small systematic diversity and a quantity of specimen species along with their rather poor preservation. This made it difficult to determine comparatively well-preserved specimens. Content rocks are mainly grey aleuro-lites with the interlayers of dark-grey argillites and light-grey millstone grit of the Viculian Horizon. The systematic composition of most assemblages of the identified sections allows us to establish the deposits of the conditionally Middle Aptian age in the middle part of the Viculian Horizon. The deposits concerned are documented in the section of borehole 55 (the South-Russkaya area) in the depth intervals 1841,50-1834,30 m. In the assemblage of this section the species Trochamminoides aff. ivanetzi Podobina, Labrospira aff. rotunda Podobina, Haplophragmoides topagorukensis Tappan, Pseudoverneuilina (?) aptica Podobina sp. n., Siphogaudryina rayi (Tappan) are present. Rounded forms of the Labrospira, Haplophragmoides, Trochammina genera are prevalent in established Aptian assemblages. Almost all the foraminiferal assemblages in this and the other established sections are supposedly of the Middle Aptian age. Based on the Foraminifera finds, the Aptian transgression is supposed to have reached its maximum in the Middle Aptian time. In addition, conditionally early and late Aptian foraminiferal assemblages are established in solitary sections. In the well sections of the Yjamal Peninsula in the Viculian Horizon the Aptian microfauna - agglutinated quartz-siliceous foraminifera and ostracod nucleis are also discovered. The foraminifera in all the sections are not abundant, they are approximately of a similar systematic composition and of rather a poor preservation. The paper concerned presents factual data on the first discovered Aptian foraminiferal assemblages related to the rocks of the Viculian Horizon. All Aptian Assemblages (mainly the Middle Aptian ones) are illustrated in 8 accompanying photos (drawing) which to some extent allow us to form an opinion about their systematic composition and quantative content. In addition an illustration of Pseudoverneuilina (?) aptica Podobina sp. n. species are shown in the fig. 9. Also there are brief description of this species.

Revision of Toxoceratoides royeri (d'Orbigny, 1842) and its bearing on the systematics of the Aptian Acrioceratidae Vermeulen, 2004 (Ammonoidea, Ancyloceratina, Ancyloceratoidea)

The acrioceratid ammonite genus Toxoceratoides Spath has long been used to accommodate any late Barremian or Aptian species that show morphological similarities with its type species Toxoceratoides royeri (d'Orbigny). The revision of T. royeri and its allied species T. rochi Casey has convinced us that new genera should be introduced for some Aptian forms that were previously referred to Toxoceratoides. These are Klingerites gen. nov. (type species: Toxoceratoides? haughtoni Klinger & Kennedy), Richardreymentella gen. nov. (type species: Ancyloceras patagonicum Stolley), Immelites gen. nov. (type species: Tonohamites multituberculatus Immel & Guoxiong), Jenslehmannella gen. nov. (type species Jenslehmannella bangestanense gen. et sp. nov.). This contribution also reconsiders the taxonomic value of Tonohamites Spath, Colomboceratoides Etayo Serna and Raymondcaseyites Avram. The new taxonomic treatment proposed herein outlines the diversity and provincialism of the Acrioceratidae during Aptian times and questions the alleged cosmopolitan distribution of Toxoceratoides.

Cretaceous evolution of the Andean margin between 36°S and 40°S latitude through a multi‐proxy provenance analysis of Neuquén Basin strata (Argentina)

AbstractDuring the Cretaceous, the Neuquén Basin transitioned from an extensional back‐arc to a retroarc foreland basin. We present a multi‐proxy provenance study of Aptian to Santonian (125–84 Ma) continental sedimentary rocks preserved in the Neuquén Basin used to resolve changes of sediment drainage pattern in response to the change in tectonic regime. Sandstone petrology and U–Pb detrital zircon geochronology constrain the source units delivering detritus to the basin; apatite U–Pb and fission track dating further resolve provenance and determine the age and patterns of exhumation of the source rocks. Sandstone provenance records a sharp change from a mixed orogenic source during Aptian time (ca. 125 Ma), to a magmatic arc provenance in the Cenomanian (ca. 100 Ma). We interpret this provenance change as the result of the drainage pattern reorganisation from divergent to convergent caused by tectonic basin inversion. During this inversion and early stages of contraction, a transient phase of uplift and basin erosion, possibly due to continental buckling, caused the pre‐Cenomanian unconformity dividing the Lower from Upper Cretaceous strata in the Neuquén Basin. This phase was followed by the development of a retroarc foreland basin characterised by a volcanic arc sediment provenance progressively shifting to a mixed continental basement provenance during Turonian‐Santonian (90–84). According to multi‐proxy provenance data and lag times derived from apatite fission track analysis, this trend is the result of a rapidly exhuming source within the Cordillera to the west, in response to active compressional tectonics along the western margin of South America, coupled with the increasing contribution of material from the stable craton to the east; this contribution is thought to be the result of the weak uplift and exhumation of the foreland due to eastward migration of the forebulge.

Regional stratigraphy and subsidence of Orphan Basin near the time of breakup and implications for rifting processes

AbstractThe stratigraphic, subsidence and structural history of Orphan Basin, offshore the island of Newfoundland, Canada, is described from well data and tied to a regional seismic grid. This large (400 by 400 km) rifted basin is part of the non‐volcanic rifted margin in the northwest Atlantic Ocean, which had a long and complex rift history spanning Middle Jurassic to Aptian time. The basin is underlain by variably thinned continental crust, locally <10‐km thick. Our work highlights the complex structure, with major upper crustal faults terminating in the mid‐crust, while lower crustal reflectivity suggests ductile flow, perhaps accommodating depth‐dependent extension. We describe three major stratigraphic horizons connected to breakup and the early post‐rift. An Aptian–Albian unconformity appears to mark the end of crustal rifting in the basin, and a second, more subdued Santonian unconformity was also noted atop basement highs and along the proximal margins of the basin. Only minor thermal subsidence occurred between development of these two horizons. The main phase of post‐rift subsidence was delayed until post‐Santonian time, with rapid subsidence culminating in the development of a major flooding surface in base Tertiary time. Conventional models of rifting events predict significant basin thermal subsidence immediately following continental lithospheric breakup. In the Orphan Basin, however, this subsidence was delayed for about 25–30 Myr and requires more thinning of the mantle lithosphere than the crust. Models of the subsidence history suggest that extreme thinning of the lithospheric mantle continued well into the post‐rift period. This is consistent with edge‐driven, small‐scale convective flow in the mantle, which may thin the lithosphere from below. A hot spot may also have been present below the region in Aptian–Albian time.

Intraspecific variability and systematics in South American Syrotrigoniinae (Trigoniida, Bivalvia)

The systematics of the genus Syrotrigonia is revised in the light of the intraspecific variability of a large sample of Syrotrigonia sigeli from the Valanginian from Neuquén Basin, Argentina. The genus can be recognized by the presence of concentric or subconcentric costae surrounding the umbo, later on developing an inflection and finally resulting in a set of anterior horizontal to commarginal costae and another set of posterior sub-vertical costae. The anterior part of the area bears transverse, usually anteriorly concave, costae which direct towards the umbo on the escutcheon; initially they are continuous with those on the flank, but later on they usually alternate. General shell shape, the presence of an antecarinal sulcus, the junction pattern between both sets of costae and the number of horizontal costae relative to vertical costae are variable among species, while costae width or density and the development of horizontal vs. commarginal costae may vary highly within species. The presence of commarginal rugae developed on the whole surface of the shell is interpreted as a result of environmental perturbations. A brief biogeographical interpretation of the family Buchotrigoniidae is also outlined. Syrotrigonia probably originated in North America in the Tithonian, being widespread in the Pacific coast of South America by Early Cretaceous times; at least six (probably seven) South American species could be recognized. By Aptian times the taxon reached the Tethyan realm, its last record being Aptian or Albian in age. The frequent presence of different species in marginal marine deposits suggests a euryhaline lineage adapted to salinity variations, this may also be the cause of the frequent presence of commarginal rugae. Considering the high variability displayed in the analyzed material, large samples are needed to characterize new species within the group.

Understanding the Mesozoic kinematic evolution in the Cameros basin (Iberian Range, NE Spain) from magnetic subfabrics and mesostructures

Analysis of anisotropy of magnetic susceptibility (AMS) and brittle mesostructures (hydroplastic synsedimentary faults and tension gashes) is applied in this study in order to characterize the Mesozoic tectonic events in the Cameros basin (NW Iberian Range), formed between Tithonian and Albian times. Low-field AMS at room and low temperature (LF-AMS at RT and LF-AMS at LT, respectively) together with high-field AMS (HF-AMS) measurements allow separating ferro- and paramagnetic fabrics. The combination of LF-AMS at LT and HF-AMS torque measurements confirms the reliability of both procedures in terms of isolating the paramagnetic contribution to the AMS. Magnetic fabric results combined with the analyses of synsedimentary faults indicate a NW–SE extension direction during Aptian (and probably Barremian) times. This extension direction is perpendicular to the main extension direction (NE–SW) prevailing during early and late stages of basin evolution. It is also consistent with extension direction deduced from large-scale bending folds and tension gashes, developed after partial lithification. Cleavage development during Albian enhanced the orientation of the magnetic fabric in lithologies where the previous extensional magnetic lineation is coaxial with the expected one for compression.

Evidence of theropod dinosaurs from a Lower Cretaceous karst filling in the northern Sauerland (Rhenish Massif, Germany)

Theropod dinosaur teeth from a Lower Cretaceous karst filling in Devonian reef limestone (Massenkalk) located near the village of Balve on the northern margin of the Rhenish Massif, Germany, are described. Palynological evidence indicates that the karst filling took place during the Late Barremian to (Early) Aptian time interval. Palaeontological excavations carried out during several years at the Balve locality yielded remains of various animal groups, including sharks, amphibians, turtles, crocodiles, pterosaurs, herbivore dinosaurs, and, rarely, mammals. Of particular interest is an assemblage of different theropod dinosaur teeth, which has been characterized with the help of dental comparative morphometry. Based on these analyses, the teeth material from the Balve excavation can be assigned to five morphological groupings including dromaeosaurid and tyrannosauroid theropods. Morphological comparison of the theropod dinosaur teeth record with finds from other fossil localities in western and northwestern Europe of similar age shows clear similarities, but also some distinct differences.

High resolution δ13C stratigraphy through the Selli Oceanic Anoxic Event (OAE1a) in the Apulia carbonate platform: the Borgo Celano section (western Gargano Promontory, Southern Italy)

We show that the global-scale environmental perturbations leading to the Selli OAE1a were clearly recorded in the middle Bedoulian Apulia carbonate platform at Borgo Celano, in the western Gargano Promontory (Southern Italy). The studied section (Bari Limestones), which is 72.45 m thick and spans the Late Barremian p.p. -latest Early Aptian time interval, has been logged with an average resolution of 3 samples per metre for chemostratigraphic analyses and up to 7 samples per metre for facies analysis in the aim of detailed paleocologic and paleoceanographic investigation. It contains the complete biosedimentary and chemostratigraphic record across the Selli OAE1a, as suggested by a consistent match of the observed isotope excursions with those documented in the Cismon pelagic reference section and the lack of significant non-depositional or erosional hiatuses within the chemostratigraphic Selli level: biostratigraphic and sedimentary-diagenetic analyses document a continuous record of fully subtidal conditions. Furthermore, the Borgo Celano section allows correlation of the shallow- and deep-water records without intervening stratigraphic breaks. Upper Barremian-Lower Bedoulian, chlorozoan-type carbonates showing subtidal-intertidal Milankovitch-type cyclicities characterize the lower part of the Borgo Celano section (C2 pro parte isotopic segment). Starting from the latest C2 segment, a remarkable Crisis Interval (CI) about 4 m thick shows an increasing frequency of bivalves ( Chondrodonta and requieniids), the disappearance of green algae and a reduced diversity of foraminiferal assemblages (foramol-type carbonates). This interval is sharply overlain by an interval, about 3.70 m thick, that is largely dominated by “out-of-balance” microbial limestones almost devoid of metazoans, apart from sparse, small-sized requieniids and Chondrodonta , which accompanies the C3-middle C4 isotope segment. It is interpreted to reflect a sharp hyperthermal and acidification event induced by excess p CO 2 . Starting from the upper C4 segment, less severe but still threatening environmental conditions permitted the deposition throughout the lower C5 segment of peculiar shallow-water tropical-like carbonates, which are dominated by oncoidal floatstones and microbial wackestone/packstone emphasizing the enduring influence of benthic cyanobacteria during the Selli OAE1a. Upsection guilds are dominated by foraminiferal and peloidal wackestones/packstones with sparse bivalves and green algae (upper C5-lowermost C7 isotope segments). Similarly to other carbonate platforms of the central Tethyan Realm (e.g., Apennine and Adriatic), the resilience of the Apulia carbonate platform inner shelf to the environmental perturbations that led to the coeval drowning of its margin, the starvation of its slope and to the deposition of black shales in the bounding Ionian basin is a striking supraregional feature. The inception of the resilience of the Apulia carbonate platform inner shelf is coincident with the lithostratigraphic boundary in between the Borgo Celano and Chondrodonta and requieniids members of the Bari Limestones and physically corresponds in the field with the base of a significant, 1 m thick, Chondrodonta -rich marker bed. These observations confirm the value of routinely applied field and laboratory studies for precise chemostratigraphic and bio-chronostratigraphic correlations on a regional scale. Furthermore, chemostratigraphic data presented here corroborate recent paleoceanographic and stratigraphic models proposed for the Lower Aptian across the transition between the Apulia carbonate platform and the bounding Ionian basin, and provide further elements for regional reconstructions of historical patterns.

Secular change in northwestern Tethyan water-mass oxygenation during the late Hauterivian–early Aptian

The Mesozoic is punctuated by several extreme episodes of oxygen drawdown in the ocean, known as Oceanic Anoxic Events (OAE). Beside these discrete events, little is however known about longer-term oxygen level fluctuations and their controls. A high-resolution dataset of Rare Earth Elements (REE) content in carbonate deposited in the Vocontian Basin (SE France) has allowed to survey the evolution of the Cerium anomaly (Ce/Ce*) during part of the Early Cretaceous (latest Hauterivian–early Aptian). This ratio is used as a proxy for the degree of oceanic oxygenation. The Cerium anomaly is compared to the coeval evolution of relative sea level, organic-rich occurrences, and nutrient level proxies, in order to infer a cause-and-consequence relationship between these parameters and the evolution in sea-water oxygenation. The long-term evolution of the Cerium anomaly shows that northwestern Tethyan water masses have evolved from less oxygenated during the latest Hauterivian–early Barremian interval to more oxygenated during the late Barremian–earliest Aptian time, before being strongly oxygen-depleted during the early Aptian OAE 1a time interval. This trend is correlated with both the long-term trend in nutrient levels, as well as with the frequency of organic-rich layers within the rock record. On a medium-term scale (sub-stage level), there is a good correlation between the Cerium anomaly and tectono-eustatism (as deduced by the Mn content of the basinal carbonate and sequence stratigraphic observation on surrounding shallow-water areas). This suggests that sea-level changes, by altering ocean stratification and currents, as well as modulating the amount of nutrient delivered from the continent, exert control on oceanic oxygenation via changes in primary productivity.

Detrital zircon (U–Th)/He thermochronometry of the Mesozoic Daba Shan Foreland Basin, central China: Evidence for timing of post-orogenic denudation

The timing of the transition of a peripheral foreland basin from underfilled to overfilled provides important insights into the post-orogenic dynamics of its coupled orogenic wedge. Such information is lacking in the Mesozoic foreland basins surrounding the Qinling Orogen, central China, such as the Daba Shan Foreland Basin (DSFB), the northeastern part of the Sichuan Basin. In an effort to constrain the timing of maximum burial and onset of denudation, we report detrital zircon (U–Th)/He (ZHe) data from four deep boreholes in the DSFB. Results show that ZHe ages from the oldest foreland basin formations (Upper Triassic–lower Middle Jurassic) are mostly younger than their depositional age, whereas non-reset ZHe ages characterize younger strata. This suggests that the oldest strata attained maximum temperatures between ~130°C and 200°C (ZHe partial retention zone), which is consistent with previous independent paleotemperatures (organic matter reflectance and illite crystallinity) reported in two of the boreholes and another nearby. The fully reset ages apparently show the youngest peaks at ~80Ma and ~100Ma, which are interpreted as the minimum constraints on the timing of maximum basinal burial and subsequent denudation. Considering that the regional deposition has lasted to Aptian time, it is proposed that the onset of the basin exhumation in the DSFB started at Aptian–Albian time (~100–125Ma), heralding the termination of regional tectonic shortening in the hinterland (Southern Qinling).

Mapping the rise and demise of Urgonian platforms (Late Hauterivian - Early Aptian) in southeastern France and the Swiss Jura

Lower Cretaceous carbonate deposits historically called “Urgonian limestones” are widely exposed around the margins of the Vocontian basin in southeastern France and in the adjacent Swiss Jura. This paper presents the history of their rise, growth and sudden demise. Eleven maps were constructed for deposits ranging in age from the Late Hauterivian pro parte to the Early Aptian (Bedoulian) pro parte. Based on sequential interpretations, they illustrate the present geographical distribution of the inner platform facies (Urgonian limestones stricto sensu, with rudists), the outer platform facies (essentially bioclastic deposits) and the basinal facies (slope, hemipelagic, pelagic deposits). These maps depict only the final terms of each successive sequence (the late highstand intervals). Chronostratigraphy is constrained by ammonites found mainly in basinal deposits, by echinoids, by rudists and to a lesser extent by dinoflagellates and calcareous nannoplancton. Inner platform, outer platform and slope (talus) deposits are dated by rich assemblages of orbitolinids and dasycladalean algae. Currently 39 species of orbitolinids have been recognized and their ranges collated with those of the ammonites in the area.In the Jura and in Provence the oldest Urgonian deposits are dated early Late Hauterivian, thus showing the synchroneity of the onset of platform carbonates development on both the southern and northern margins of the basin. Thereafter, growth of the platforms led a clearly regressive shallowing-upward trend, resulting from a stepwise progradation toward the center of the Vocontian area, coordinated with cyclical exposures in the inner platform areas. The maximum reduction of the platform deposits occurred early in Late Barremian times, coeval with a noticeable turnover in the orbitolinids assemblages.Thereafter, carbonate platform deposition shifted toward the margins of the Vocontian basin. In Early Aptian time, a well-dated discontinuity of regional extent marks the sudden, almost synchronous disappearance of the Urgonian deposits.

The Shah Kuh Formation, a latest Barremian – Early Aptian carbonate platform of Central Iran (Khur area, Yazd Block)

The Shah Kuh Formation of the Khur area (Central Iran) consists of predominantly micritic, thick-bedded shallow-water carbonates, which are rich in orbitolinid foraminifera and rudists. It represents a late(est) Barremian – Early Aptian carbonate platform and overlies Upper Jurassic – Barremian continental and marginal marine sediments (Chah Palang and Noqreh formations); it is overlain by basinal deposits of the Upper Aptian – Upper Albian Bazyab Formation. The lithofacies changes at both, the base and top of the Shah Kuh Formation are gradational, showing that the formation is part of an overall transgressive sedimentary megacycle, and that the formational boundaries are potentially diachronous on larger distances. Analyses of facies and stratal geometries suggest that the Shah Kuh carbonate system started as a narrow, high-energy shelf that developed into a large-scale, flat-topped rudist platform without marginal rim or steep slope. The Shah Kuh Platform is part of a large depositional system of epeiric shallow-water carbonates that characterized large parts of present-day Iran during Late Barremian – Aptian times (“Orbitolina limestones” of NW and Central Iran, the Alborz and the Koppeh Dagh). Their biofacies is very similar to contemporaneous deposits from the western Tethys and eastern Arabia, and they form an important, hitherto poorly known component of the Tethyan warm-water carbonate platform belt.

New stratigraphic data on the Aptian of the Persian Gulf

During Aptian times the northeastern corner of the African plate, the Arabian craton, was in the tropics. Two-thirds of it was covered by a broad epeiric sea opening eastward into the Tethys Ocean. Carbonate sedimentation recorded several environmental perturbations caused by changes in relative sea-level and by interconnected coeval global events. A well in offshore Abu Dhabi was used as the reference because it was drilled through the whole of the interval of interest and cored strata dated Gargasian (middle Aptian sensu gallico, early late Aptian sensu anglico) downward to beds of Late Barremian age. The holostratigraphic approach employing biostratigraphy, lithostratigraphy and sequence stratigraphy, along with basic well log interpretation and δ13C-based chemostratigraphy, facilitated correlation with outcrops in Iran and Oman, and with other wells in Iraq, Qatar and the United Arab Emirates, over distances of several hundreds of kilometers. This approach made possible refinement of our regional model for this interval and the identification of a set of events that included several forced regressions, transgressions of varying importance (by extension in the literature those floodings of greatest magnitude have been called “drownings”), the Oceanic Anoxic Sub-Event 1a and a microbial Bacinella “bloom”.

Sequence-stratigraphic analysis of the Aptian deposits in the valley of the Mzymta River

In this work, based on a well drilled in the valley of the Mzymta River (northwestern Caucasus), the application of the sequence-stratigraphic method is demonstrated in this area for the first time. This method allows us to clarify the nature of numerous repetitions in the same interval of the observed geological sequence (modern, ancient landslide or synsedimentary landslide, and tectonic or eustatic). In addition, the geochemical characteristics of bituminous sediments are reported. The sequence studied in the well shows a complex polygenetic evolution of sediments since Aptian time and the oceanic anoxic event-1 (OAE-1).

Surfaces with a long history: the Aptian top Shu’aiba Formation unconformity, Sultanate of Oman

AbstractDiscontinuity surfaces in shallow‐marine carbonate successions may represent significant time gaps in the geological record of ancient epeiric‐neritic seas. Understanding the hidden geological information contained in major discontinuities is thus of key significance in palaeo‐environmental analysis, sequence stratigraphy, reconstructions of sea‐level change and basin evolution. In the present paper, the Aptian top Lower Shu’aiba Formation discontinuity in the Sultanate of Oman is taken as a prominent example of a regionally extensive (>100 000 km2) surface with a long (up to 10 Myr) and complex geological history. The top Shu’aiba discontinuity formed on the topographically elevated domain of the Oman platform and represents in essence the Late Aptian time interval. Coeval carbonates in the intrashelf Bab Basin and oceanic rim indicate forced regression and sequence‐wise, gradual down‐stepping. Available regional, sedimentological, sequence‐stratigraphic, petrographic, palaeontological and geochemical evidence from outcrops and cored wells in Oman is summarized, in part complemented by new data, and reviewed in a process‐oriented context. In the field, the discontinuity is expressed as a low relief, stained surface with evidence for a marine hardground stage being dominant. Indistinct features that indicate a transient meteoric precursor stage (isotope shifts, meteoric cements, circumgranular cracks, etc.) are present but their interpretation requires careful and detailed work. This feature is remarkable, as a series of relative sea‐level falls with amplitudes of up to several tens of metres from the Early to Late Aptian boundary to the end of the Aptian are reported from the Middle East and elsewhere. Despite the palaeogeographic position of the study area in the tropical climate zone, evidence of deep‐cutting karst features, characteristic for many long‐term exposure surfaces worldwide is scarce. Acknowledging the fact that the modern world offers no genuine analogues for the Lower Aptian carbonate system in Oman, morphological similarities between actualistic, wave‐eroded coastal terraces and the top Shu’aiba discontinuity are discussed critically. This discussion may imply that, during an exposure time of several million years, the top Shu’aiba discontinuity experienced repeated stages of shallow flooding and emergence, with each transgression removing portions of the underlying rock record. The data shown here exemplify the complexity of hiatal surfaces in epeiric‐neritic carbonates and may serve as a case example for other major discontinuities.

Lead isotopic record of Barremian–Aptian marine sediments: Implications for large igneous provinces and the Aptian climatic crisis

We present initial isotopic ratios of lead for Early Cretaceous (Barremian–Aptian) sections from Shatsky Rise (Pacific) and Gorgo a Cerbara (Italy). Our Pb isotopic data track an interval representing Oceanic Anoxic Event (OAE)-1a, which is characterized by quasi-global deposition of organic carbon-rich black shale. Pb isotopic compositions of sediments from Shatsky Rise decrease at the end of Barremian time, from radiogenic continental values to unradiogenic values, and subsequently remained less radiogenic until the end of early Aptian time. We explain the isotopic shift by a significant increase in supply rate of unradiogenic Pb, most likely due to massive volcanism. In contrast, the Pb isotopic compositions from the Italian section, which was situated at the western end of Tethys, are mostly identical to those of upper continental crust, showing no significant change in supply rate of unradiogenic Pb. The discrepancy between two sites is attributed to quiescent deep-submarine eruptions of Pacific large igneous provinces (LIPs) such as the Ontong Java Plateau (OJP), which severely limited dispersion of Pb-carrying particles out of the Pacific Ocean. Published Os isotopic data from the Italian section indicate two episodes of massive eruptions of OJP or contemporaneous Manihiki and Hikurangi plateaus starting from earliest Aptian time, slightly later than that indicated by the sedimentary Pb isotopic record from Shatsky Rise. Differences in isotopic variations between Pb and Os likely reflect differences in their chemical behaviors in the oceans, i.e., Pb isotopic compositions would have varied in response to local or regional changes in sediment provenances, whereas large-scale changes in Os inputs are required to explain variations in seawater Os isotopic compositions. Our Pb isotopic data, together with the published Os isotopic record, provide new evidence for the eruptive history of OJP together with contemporaneous Pacific plateaus and its environmental consequences, starting from end-Barremian time and extending through early Aptian time.

New constraints on timing of Hidalgoan (Laramide) deformation in the Parras and La Popa basins, NE Mexico

The Parras and La Popa basins of northeastern Mexico together contain at least three separate types of structures that were active during Maastrichtian through Early Eocene time. These structures include salt diapirs and salt welds, E-W trending anticlines detached within the Campanian Parras shale, and large NW-SE trending anticlines detached within the Jurassic salt layers. The oldest halokinetic structure, the La Popa salt weld, began to form by Late Aptian time. This structure is parallel to the later NW-SE contractional folds, but it pre-dates regional contraction. The earliest dated contractional structures in the Sierra Madre Oriental are E-W trending folds and faults found east of the La Popa syncline. These structures were active only briefly during the deposition of the lower Maastrichtian Muerto Formation. Very similar east-west structures in the northern Parras Basin contain growth strata of upper Maastrichtian and Paleocene age. Folding of upper Paleocene beds indicates that this shortening continued into the Eocene. Very Large amplitude NWSE trending anticlines in La Popa Basin also formed due to regional contraction, and they clearly re-fold the shallow-detached E-W set. Regional evidence suggests that all contractional deformation ceased around 40 Ma. Salt withdrawal may have continued after the cessation of contractional deformation. The contractional deformation in these basins is similar in structural style to the Sevier orogen in the U. S. A., but it is later and of shorter duration than the Sevier orogeny; it is time-equivalent to the Laramide orogeny, but it differs significantly from the Laramide structural style. Thus, the Mexican orogenic system differs from the U. S. A. orogens. This uniqueness was recognized by Guzman and De Cserna (1963) who named this deformational event in Mexico the Hidalgoan orogeny.

The Valais units in Savoy (France): a key area for understanding the palaeogeography and the tectonic evolution of the Western Alps

The Valais units in Savoy (Zone des Brèches de Tarentaise) have been re-mapped in great detail and are subject of combined stratigraphic, structural and petrological investigations summarized in this contribution. The sediments and rare relics of basement, together with Cretaceous age mafic and ultramafic rocks of the Valais palaeogeographical domain, represent the heavily deformed relics of the former distal European margin (External Valais units) and an ocean–continent transition (Internal Valais unit or Versoyen unit) that formed during rifting. This rifting led to the opening of the Valais ocean, a northern branch of the Alpine Tethys. Post-rift sediments referred to as “Valais trilogy” stratigraphically overlie both External and Internal Valais successions above an angular unconformity formed in Barremian to Aptian times, providing robust evidence for the timing of the opening of the Valais ocean. The Valais units in Savoy are part of a second and more external mid-Eocene high-pressure belt in the Alps that sutured the Briançonnais microcontinent to Europe. Top-N D1-deformation led to the formation of a nappe stack that emplaced the largely eclogite-facies Internal Valais unit (Versoyen) onto blueschist-facies External Valais units. The latter originally consisted of, from internal to external, the Petit St. Bernard unit, the Roc de l’Enfer unit, the Moûtiers unit and the Quermoz unit. Ongoing top-N D2-thrusting and folding substantially modified this nappe stack. Post 35 Ma D3 folding led to relatively minor modifications of the nappe stack within the Valais units but was associated with substantial top-WNW thrusting of the Valais units over the Dauphinois units along the Roselend thrust during W-directed indentation of the Adria block contributing to the formation of the arc of the Western Alps.

A Monospecific Assemblage of Terebratulide Brachiopods in the Upper Cretaceous Seep Deposits of Omagari, Hokkaido, Japan

The Campanian (Upper Cretaceous) seep carbonate at Omagari (Hokkaido, Japan) yields a monospecific association of the terebratulide brachiopod Eucalathis methanophila Bitner sp. nov. The association is the only occurrence of brachiopods known from the post-Early Cretaceous history of chemosynthesis-based communities. Unlike many earlier rhynchonellide-dominated hydrocarbon seep associations—which disappeared in Aptian times—this association is composed of chlidonophorid terebratulides. It is hypothesised here that large rhynchonellide brachiopods have been outcompeted from chemosynthesis-based associations by large chemosymbiotic bivalves (especially lucinids) and that this seep association containing numerous terebratulide brachiopods originated as a result of immigration from the background fauna settling in a seep that lacked numerous large bivalves but offered some hard substrates for brachiopod attachment. Some living chlidonophorids are known to settle around seep/vent localities or more generally i...

Cretaceous slab segmentation in southwestern Gondwana

AbstractThe Mesozoic Austral Basin of Patagonia, in southwestern Gondwana, experienced a major tectonic segmentation during Aptian times. Sometime between 121 and 118 Ma (Aptian), the northern part of the Austral Basin, known as the Aisén Basin or Río Mayo Embayment, was inverted, with the sediments overlain by calc-alkaline subaerial volcanic rocks of Aptian to Maastrichtian age. In the southern segment of the Austral Basin, known as the Magallanes Basin, predominantly marine sediments accumulated until Cenozoic times in a back-arc position, relative to a magmatic arc located to the west. The subduction-related N–S-trending volcanic chains of both segments were geographically displaced during Aptian to Late Cretaceous times. In the Aisén segment north of ~49–50° S, the volcanic chain was located further east than the coeval arc in the Magallanes segment. A transform fault connected the trenches of both segments, with the Aisén segment dipping at a shallower angle than the Magallanes segment.