Barremian-Aptian Boundary Research Articles

Ontong Java Plateau eruption as a trigger for the early Aptian oceanic anoxic event

The Early Cretaceous Ontong Java Plateau was emplaced at almost the same time as marine biotic changes that culminated in oceanic anoxic event 1 (OAE1a). A causative link between these events has been suggested, but direct evidence has been lacking until now. New Os isotope measurements across the Lower Aptian “Selli Level” black shale deposited during OAE1a in central Italy reveal two negative excursions in marine 187 Os/ 188 Os ratios within a period of 2 Ma starting above the Barremian-Aptian boundary and ending just above the Selli Level horizon, suggesting an order-of-magnitude increase in the global fl ux of unradiogenic Os. The results are consistent with early and major phases of eruption of the Ontong Java Plateau. The latter phase is estimated to have been as short as ~1 Ma and may have induced widespread oceanic stratifi cation that triggered OAE1a.

Diachronous pervasive remagnetization in northern Iberian basins during Cretaceous rotation and extension

Northern Iberia exposes a series of Mesozoic sedimentary basins, whose final formation history is tied to a rotation episode of Iberia during the Aptian, related to the opening of the Bay of Biscay. Many of these basins experienced widespread remagnetization, previously loosely tied to a ‘Mid-Cretaceous thermal event’. Here we make use of the improved apparent polar wander path (APWP) of Iberia to narrowly constrain the age of their remagnetization to show that several basins experienced a regional diachronous remagnetization. We apply the small circle intersection (SCI) method to reconstruct the paleomagnetic field direction during remagnetization in the Organyà Basin in the Southern Pyrenees, Spain, where the age of remagnetization is stratigraphically well-dated to around the Barremian–Aptian boundary, and where the paleomagnetic direction during remagnetization has been well-established. The resulting direction of D/ I = 316.8 ± 5.7°/54.8° ± 3.3° (where the uncertainty in declination is approximated by α 95 / cos I and the error in inclination by α 95) corresponds closely to a recently improved APWP of Iberia, providing support for the applicability of this method. Application of the SCI method allows us to show that the older portion of the remagnetized beds was already tilted by ~ 15–20° to the South during the remagnetization as a result of half-graben formation in the Organyà Basin. Moreover, the positive outcome of the SCI technique enables us to argue that previously published results from three other basins—the Cabuérniga and Cameros Basins and the Iberian Range—can be straightforwardly compared to the APWP. Hence, we show that the four pervasively remagnetized basins under consideration all acquired their remagnetization at different times. The remagnetization events were thus confined to these sedimentary basins on an individual ‘per basin’ scale—albeit occurring on a regional scale over a period of at least ~ 10–15 Myr. Therefore, we propose that the Cretaceous remagnetization events in northern Iberia are related to the extensional tectonic rifting. The pervasive remagnetization is most likely basin confined because there is no apparent overall regional or temporal trend among the basins. Reasonable explanations for remagnetization are those that can occur on the scale and within the context of individual sedimentary basins. These include the interplay of burial depth and an elevated geothermal gradient to reach the diagenetic temperatures required for magnetite-producing reactions. In the Iberian situation, there is no need to invoke more speculative regional Iberia-wide mechanisms.

Toward age determination of the M0r (Barremian–Aptian boundary) of the Early Cretaceous

We carried out integrated paleomagnetic and geochronologic investigations on Cretaceous lava flows at the Mashenmiao-Zhuanchengzi (MZ) section in Yixian, Liaoning Province, northeast China in seeking to understand the onset of the magnetic polarity chron M0r and the associated Barremian–Aptian boundary (BAB), which has been reported to be 125.0 ± 1.0 Ma [Ogg, J.G., Agternerg, F.P., Gradstein, F.M., 2004. The Cretaceous period. In: Gradstein, F.M., Ogg, J.G., Smith, A.G. (Eds.). A Geologic Time Scale. Cambridge University Press, UK, pp. 344–383]. Stepwise thermal or alternating field demagnetization indicates that all lava flows in the studied section were reversely magnetized. 40Ar/ 39Ar ages obtained from three lava flows are 121.2 ± 1.3 Ma, 120.2 ± 1.5 Ma and 122.0 ± 1.3 Ma, respectively, with a weighted mean age of 121.2 ± 0.5 Ma (2 σ). In combination with previous studies, we argue that these lavas are M0r in age, which therefore is probably some 4 Myr younger than estimated in the 125 Ma of the most recent geologic time scale.

The rotation of Iberia during the Aptian and the opening of the Bay of Biscay

The Cretaceous rotation of the Iberian plate in a geodynamical active setting between Africa and Europe is contemporaneous with the opening of the northern Atlantic Ocean, and provides fundamental, temporal and kinematical constraints on the evolution of the Bay of Biscay and the Pyrenees. Here, we report new paleomagnetic data from 50 sites (1109 drilled cores) from marine sediments of the Cretaceous Organyà Basin in the southern Pyrenees, to constrain the timing and magnitude of the rotation of Iberia. Berriasian to Barremian directions in exclusively limestone lithology are remagnetised, in agreement with previous Organyà Basin studies. The Aptian to Cenomanian marls and limestones in the Organyà Basin, however, provide primary magnetisations asserted by rock magnetic experiments and positive fold tests. Our results constrain the rotation of Iberia to be finished just before the Albian. Paleomagnetic data cannot well constrain the onset of the rotation, but sea-floor anomaly studies restrict the onset of the rotation to anomaly M0, or the Barremian–Aptian boundary. This implies that the entire rotation of Iberia happened in one single phase, and is confined to the Aptian. This is in contrast to earlier studies that suggested a two-phase rotation of Iberia, but we argue that the large errors in published ages are not in disagreement with our results. In summary, we constrain the ∼35° rotation of Iberia to the Aptian period which implies a rotation rate of 2.7–5.1°/Myr, depending on the (controversial) duration of the Aptian.

Assessment of diachronism of biostratigraphic boundaries by magnetochronological calibration of zonal scales for the Lower Cretaceous of the Tethyan and Boreal belts

Complex bio- and magnetostratigraphic studies ofLower Cretaceous deposits in the central Volga region,Northern Caucasus, and other regions carried out by theauthors of the present communication for more than10 years resulted in the detailed correlation of biostrati-graphic zonal scales between each other (mainly upperHauterivian–Aptian) and with the “standard” scale ofthe Western Mediterranean [1, 2]. Paleomagnetic inver-sions are known to be synchronous for all rocksequences on the earth regardless of the latitude of theirformation. Therefore, correlation of regional magneto-and biostratigraphic scales (magnetochronological cal-ibration) allowed us to establish and assess quantita-tively for the first time the asynchronous nature of theseries of stratigraphic boundaries, which were regardedas synchronous for different paleogeographic belts.The paleomagnetic correlation of Barremian–Aptian boundary deposits of the Russian Plate [7],Northern Caucasus [6], Mediterranean region [4], andEngland [9] makes it possible to assess the asynchro-nous nature of the Deshayesites volgensis Zone base inthe lower Aptian (Fig. 1). In the Northern Caucasus, theboundary between the D. weissiformis and D. volgensiszones is confined to the subzone of the reversed sign(the analog of M0 Chron). In the central Volga region,the base of the D. volgensis Zone and its West Europeananalog the D. forbesi Zone occurs above M0 within thedirect polarity interval. The time shift obtained by interpo-lating the available isotopic dates is ~10

Evolution of the marine stable carbon-isotope record during the early Cretaceous: A focus on the late Hauterivian and Barremian in the Tethyan realm

Abstract In order to improve our understanding of the relationships between the late Hauterivian oceanic anoxic Faraoni event, contemporaneous platform drowning along the northern Tethyan margin and global environmental change in general, we established high-resolution δ 13 C and δ 18 O curves for the late Hauterivian and the entire Barremian stage. These data were obtained from whole-rock carbonate samples from the Veveyse de Châtel-Saint-Denis section (Switzerland), the Fiume-Bosso section and the nearby Gorgo a Cerbara section (central Italy), and the Angles section (Barremian stratotype, France). We observe an increase of 0.3‰ in mean δ 13 C values within sediments from the middle Hauterivian Subsaynella sayni ammonite zone to the Hauterivian–Barremian boundary; δ 13 C values remain essentially stable during the early Barremian. During the latest early Barremian and most of the late Barremian, δ 13 C values increase slowly (until the Imerites giraudi zone) and the latest Barremian is characterized by a negative trend in δ 13 C values, with minimal values at the Barremian–Aptian boundary. During the earliest Aptian, δ 13 C mean values start to rise again and attain + 2.25‰. We interpret the evolution of the δ 13 C record as resulting from the interaction between changes in the carbon cycle in the Tethyan basin and the adjacent platforms and continents. In particular, changes towards warmer and more humid conditions on the continent and coeval phases of platform drowning along the northern Tethyan margin may have contributed to enhance the oceanic dissolved inorganic carbon (DIC) reservoir which may have pushed the δ 13 C record towards more negative values and exerted a general attenuation on the δ 13 C record. From this may have come the general change from a heterozoan to a photozoan carbonate platform community, which influenced the evolution in δ 13 C values by increasing the export of aragonite and diminishing export of dissolved organic carbon into the basins.

Early Cretaceous magnetic stratigraphy in the APTICORE drill core and adjacent outcrop at Cismon (Southern Alps, Italy), and correlation to the proposed Barremian-Aptian boundary stratotype

Lower Cretaceous mainly white and/or gray pelagic cherty limestones have been sampled in the APTICORE drill core and in the adjacent outcrop at Cismon, Southern Alps, Italy. Magnetic polarity stratigraphy of the drill core indicates that the top of the section is in the Cretaceous long normal polarity interval, 26 m above the reverse polarity zone correlative to polarity chron CM0. The base of the drilled section, at a stratigraphic depth of 116.71 m, is in a reverse polarity zone correlative to CM9. The coeval stratigraphic interval has been sampled in the adjacent outcrop in order to tie the drill core record to the outcrop. The correlation of polarity zones to polarity chrons is aided by the known correlation of nannofossil events to polarity chrons in other Italian land sections. New magnetostratigraphic sampling of the proposed Barremian-Aptian boundary stratotype section at Gorgo a Cerbara (Umbria, Italy) improves the definition of CM0 in this section and facilitates correlation to Cismon. Revised nannofossil biostratigraphy based on quantitative analyses greatly increases the stratigraphic resolution of the Barremian-Aptian boundary interval both at the proposed boundary stratotype (Gorgo a Cerbara) and at Cismon.

Generic nonlinear processes in self–exciting dynamos and the long–term behaviour of the main geomagnetic field, including polarity superchrons

When the main geomagnetic field undergoes a reversal in polarity it does so in no more than a few thousand years. Many hundreds of such reversals must have occurred over geological time (the age of...

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

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

Correlation of biostratigraphic and stable isotope events in the Aptian historical stratotype of La Bédoule (southeast France)

A high resolution carbonate carbon-isotope stratigraphy is established on bulk carbonates from the type locality of the Lower Aptian (= Bedoulian). This exceptionally expanded series permitted us to recognize two marked positive δ 13C excursions. Correlation with the biostratigraphic data shows that the first excursion commences immediately after the Barremian-Aptian boundary and reaches maximum values within the Early Bedoulian. A second, more significant, positive shift begins in the middle part of the Late Bedoulian and reaches its maximum near the end of the Late Bedoulian. Already known from other pelagic Tethyan sites and even from the Pacific Ocean, this Upper Bedoulian excursion is regarded as global. The lower shift seems to be only recorded in sections with high carbonate accumulation rates such as La Bédoule and may fall into a strongly condensed interval or hiatus within the previously studied sections.

The Barremian–Aptian turnover of biota in northwestern Europe: evidence from belemnites

`Neocomian' (Valanginian–Barremian) and `Gault' (Aptian–Albian) are lithostratigraphic terms used in the last century to divide the sub-Chalk Cretaceous of northern Europe. The division is based on significant palaeoceanographic and palaeogeographic changes in the early Aptian. These changes are linked to a major turnover of marine floras and faunas, which occurred simultaneously in the Boreal Realm, the Tethys and the Indo-Pacific Ocean. Various floral and faunal elements became extinct in the Aptian both in the Tethys and in the Boreal Realm, while on the other hand new genera and species evolved. Marked changes in the diversification rate were observed for organisms with a different mode of life (plankton, nekton, benthos). Apart from calcareous nannofossils, foraminifera and ammonites, this turnover also occurs in belemnites, and the suborder Belemnitina was completely replaced by the suborder Belemnopseina. The northwestern European Barremian is characterised by the very successful belemnite family Oxyteuthididae (Belemnitina), which was completely replaced by the genus Neohibolites (Belemnopseina) in early Aptian times. This Aptian turnover, which is expressed by a homogenisation of marine floras and faunas, is best explained by significant palaeoceanographic changes occurring in the early Aptian. The sea-floor spreading rate, which increased at the Barremian–Aptian boundary significantly, peaked in the late early Aptian. A mantle plume in the Pacific possibly caused an additional production of oceanic crust. Both processes caused a major sea-level rise. This in turn resulted in a major change of the palaeoceanographic setting and restricted marginal seas, like the northwestern European basins in Barremian times, became drowned. The more open oceanic setting of the Aptian enabled cosmopolitan floras and faunas to spread worldwide. The Aptian saw the radiation of planktonic foraminifera, which played only a minor role in pre-Aptian times. These palaeoceanographic changes, from restricted marginal seas to a more homogeneous pelagic setting, are considered to be the main cause for the significant changes in the composition of belemnite faunas.

Organic geochemistry and depositional history of the Barremian–Aptian boundary interval in the Lower Saxony Basin, northern Germany

The Barremian–Aptian boundary interval in northern Germany is characterized by a major lithologic turnover from dark coloured, organic matter-rich towards light-coloured, organic matter-lean sedimentary rocks. To derive information on the respective environmental conditions and their development through time, organic matter from this interval was studied by petrographic and geochemical methods. The study was performed on five cored boreholes each recovering the Barremian–Aptian boundary interval, and also on two research wells in the same area covering the Late Albian for comparative purpose.The geochemical and petrographical data allow the differentiation to two organofacies. The lower succession is characterized by variable organic carbon contents which reach maximum values up to 7% Corgin the paper shale units (‘Blättertone’) and in the early Aptian Fish Shale (‘Fischschiefer’; a possible product/equivalent of the Oceanic Anoxic Event 1a). Petrographically, the organic matter is mainly alginite and bituminite. It is moderately well preserved according to Hydrogen Index (HI) values between 150 and 450 mg HC/g Corgfor most of the samples. Then-alkane distribution reveals a contribution of both marine algae and terrestrial higher land plants. To explain these analytical results, a depositional model has been developed which is based on limited water exchange with the adjacent oceans having caused oxygen deficiency and deposition of numerous organic carbon-rich sediment layers. As revealed by HI values above 300 mg HC/g Corgand by ann-alkane maximum atn-C15, the organic matter in the Fischschiefer is mostly of marine origin. Furthermore, the 28,30-dinor-hopane detected in only one sample probably indicates slight variation in redox conditions during Fischschiefer deposition.The major change of environmental conditions began before Fischschiefer deposition, i.e., in the earliest Aptian. A decrease in organic carbon content and HI values and a higher ratio of reworked vitrinite and inertinite over primary vitrinite indicate gradually changing conditions. The observed environmental change is related to the Aptian transgression leading to a clear domination of terrigenous organic matter in the sediment above the Fischschiefer (HedbergellaMarl and above). Completely different oceanic conditions existed in the Lower Saxony Basin from then on until the Late Cretaceous, when organic matter-poor sediments were deposited under oxygen-rich water.

Calcareous nannofossils of the Barremian/Aptian boundary interval in NW Europe: biostratigraphic and palaeoecologic implications of a high resolution study

Detailed qualitative and quantitative studies of the distribution of calcareous nannofossils from six sections in NW Germany and the North Sea area covering the Barremian-Aptian boundary interval have been carried out. Fifteen nannofossil events (first occurrences, last occurrences, acmes) characterize the Upper Barremian to Lower Aptian sediments of NW Europe. Using the methods of quantitative stratigraphy a high resolution biostratigraphy based on calcareous nannofossil events is proposed. Most of these events can be recognized throughout NW Europe (NW Germany, North Sea) and allow a correlation throughout the North Sea. There is a general shift from endemic assemblages dominating in the Barremian towards more cosmopolitan ones in the Aptian.A distinctive pattern of nannofossil acmes can be recognized to repeat itself four times. These phases may represent marine incursions or minor transgressions enabling cosmopolitan (Tethyan) species to enter the Boreal Realm: (1) latest Barremian (before the first occurrence ofRucinolithus irregularis); (2) earliest Aptian (before the Fischschiefer); (3) Early Aptian (during the deposition of the Fischschiefer); (4) mid Early Aptian (after the deposition of the Fischschiefer).The Fischschiefer, an organic-rich sediment of Early Aptian age, deposited under anoxic conditions, is considered to be a regional phenomenon. The Boreal-Tethyan nannofloral homogenization in the Early Aptian can be explained by sea-level changes and regional phenomena such as enhanced transfer of nutrients into the Boreal Sea under warm and humid conditions of the mid Cretaceous climate.

The ‘Pseudocrioceras beds’ at La Bédoule (SE France) and the position of the Barremian-Aptian boundary in the historical lower Aptian stratotype

Marly limestones from La Bédoule (Bouches-du-Rhône, SE France), historical type-locality of the lower Aptian, contain in their lowermost part a rich ammonite fauna belonging to the heteromorphic genus Pseudocrioceras. Up to the present this level, corresponding to the Pseudocrioceras coquandi Zone of Busnardo (1984), was considered by ammonitologists as early Aptian. However, a detailed study of newly collected faunas from both this level and immediately under- and overlying beds leads us to reconsider the age assignment of this interval. Our results show that the Pseudocrioceras are first accompanied, in the upper part of the Martelites sarasini Zone, by a heteroceratid fauna showing a Barremian signature, and then coexist briefly with the first Deshayesitidae of the Aptian Deshayesites tuarkyricus Zone. Therefore, our proposal is to place the Barremian/Aptian boundary in the historical type-locality at the FAD (First Appearance Datum) of Deshayesites, being better defined and having a wider geographic extension in the Mesogean realm.

LATE BARREMIAN-EARLY APTIAN AMMONITES FROM THE MAIOLICA FORMATION NEAR CESANA BRIANZA (LOMBARDY BASIN, NORTHERN ITALY)

The ammonite faunas collected in three marly interbeds of the Maiolica limestone exposed near Cesana Brianza (Como province, Lombardy) are described in this paper. The age of the first and second interbeds is Late Barremian. In particular the first is tentatively ascribed to the sartousi zone and the second to the feraudi / giraudi zones. The new species Toxoceratoides sudalpinus sp. n. has been collected in the second level. The fauna of the youngest interbed does not allow us to recognize a specific ammonite zone, although it can be ascribed to the Barremian-Aptian boundary or even the basal Aptian. It is characterized by the occurrence of Leptoceratoidinae which were not reported before from this stratigraphic level.

Barremian-Aptian calcareous plankton biostratigraphy from the Gorgo Cerbara section (Marche, central Italy) and implications for plankton evolution

The Barremian-Aptian boundary interval of the Gorgo Cerbara section (Marche, central Italy) was revisited in order to improve stratigraphic correlations and investigate plankton evolution. A very close sampling at cm scale was carried out in the upper Maiolica and basal Scisti a Fucoidi formations. The latter formation includes the Livello Selli, the landward sedimentary expression of oceanic anoxic subevent OAE la. The study of closely spaced samples revealed that (1) the first diversification among planktonic foraminifera, marked by the appearance of the genus Globigerinelloides, occurred during the Barremian (this event was dated by means of ammonites); thus the first occurrence (FO) of both Globigerinelloides duboisi and Globigerinelloides blowi cannot be used to identify the Aptian; (2) the appearance of Rucinolithus irregularis is the biostratigraphic event which best approximates the Barremian-Aptian boundary; and (3) chron M0, being slightly younger than the FO of R. irregularis, is very close to this boundary. Semiquantitative and quantitative analyses of planktonic foraminifera, calcareous nannofossils and radiolarians show that the three groups fluctuate in abundance and assemblage composition throughout the studied interval. An important change in plankton distribution patterns occurs within the G. blowi foraminiferal Zone and C. litterarius nannofossil Zone, where planktonic foraminifera, radiolarians and calcareous nannofossils start to show large-scale, higher frequency fluctuations in abundance; fluctuations in the three groups are out of phase. These new plankton distribution patterns are interrupted by the Livello Selli, which is preceded and followed by ‘critical intervals’. The distribution and composition of all three planktonic groups lead to the interpretation of the ‘critical intervals’ as representing increased nutrient contents in the surface water and the Livello Selli as a very high fertility event. Among planktonic foraminifers the hedbergellids seem to indicate a more eutrophic habitat than the globigerinelloids. In calcareous nannofossil assemblages, the abrupt crisis affecting the nannoconids prior to deposition of the Livello Selli suggests that these nannofossils are characteristic of more oligotrophic conditions, whereas Zygodiscus erectus is indicative of a more eutrophic environment. Based on our data, the early Aptian Livello Selli has the same oceanographic significance as the latest Cenomanian Livello Bonarelli, i.e., it represents a high fertility event on a global scale.

M-sequence reversals recorded in DSDP sediment cores from the western Mid-Pacific Mountains and Magellan Rise

New paleomagnetic and paleontologic data from Pacific DSDP Sites 463 and 167 define the magnetic reversals that predate the Cretaceous Normal Polarity Superchron (K-N). Data from Mid-Pacific Mountain Site 463 provide the first definition of polarity chron M0 in the Pacific deep-sea sedimentary record. Foraminiferal biostratigraphy suggests that polarity chron M0 is contained entirely within the lower Aptian Hedbergella similis Zone, in agreement with foraminiferal data from the Italian Southern Alps and Atlantic Ocean. Nannofossil assemblages also suggest an early Aptian age for polarity chron M0, contrary to results from the Italian Umbrian Apennines and Southern Alps, which place polarity chron M0 on the Barremian-Aptian boundary. Biostratigraphic dating discrepancies caused by the time-transgressive, preservational, or provincial nature of paleontological species might be reconciled by the use of magnetostratigraphy, specifically polarity chron M0 which lies close to the Barremian-Aptian boundary. At Magellan Rise Site 167, five reversed polarity zones are recorded in Hauterivian to Aptian sediments. Correlation with M-anomalies is complicated by synsedimentary and postsedimentary sliding about 25 m.y. after basement formation, producing gaps in, and duplications of, the stratigraphic sequence. The magnitude and timing of such sliding must be addressed when evaluating the stratigraphy of these oceanic-rise environments.

Les calcaires urgoniens de géorgie

The Urgonian facies is related to the neritic depositsof the Georgian block and passes laterally into the bedded ammonitic limestones to the North. Urgonian sedimentation starts in the Middle Hauterivian and generally ends at the early Barremian; seldom it may last till the Lower-Upper Barremian boundary, and sometimes even later, but it never reaches the Barremian-Aptian boundary.

A BOREHOLE SECTION ACROSS THE BARREMIAN-APTIAN BOUNDARY (LOWER CRETACEOUS) AT SKEGNESS, LINCOLNSHIRE

Summary The Sutterby Marl (Upper Aptian) in a cored borehole sequence at Skegness, Lincolnshire, disconformably overlies clay containing a basal Aptian ammonite fauna ( Prodeshayesites bodei Subzone of the P. fissicostatus Zone). The junction of this latter clay, here termed the Skegness Clay, with the underlying Roach (Barremian) is conformable and provides the best section across the Barremian-Aptian boundary described to date in Britain.