Magnetic Chron Research Articles

Tectonomagmatic setting of lava packages in the Mandla lobe of the eastern Deccan volcanic province, India: palaeomagnetism and magnetostratigraphic evidence

Abstract Flow-by-flow palaeomagnetic measurements of 37 lava flows in the 900 m-thick, isolated lava pile around Mandla in the eastern Deccan Volcanic Province (DVP) reveals multiple magnetic polarity events: implying C29n–C28r–C28n magnetostratigraphy. Magnetic polarity results when traced out from section to section, maintaining the order of superposition, show juxtaposition of lava packages with distinct characters near Deori (e.g. flows 1–4 abated against flows 5–14) and the Dindori areas. At Dindori and towards its south, the distinct lava packages (e.g. flows 15–27 and flows 28–37) are juxtaposed along the course of Narmada river. It is explained by the presence of four normal post-Deccan faults in the Nagapahar, Kundam–Deori, Dindori and Badargarh–Amarkantak sectors: thus, signifying structural complexity with vertical shifts or offset of 150–300 m. Magnetic chron reversals in conjunction with field and chemical data support these findings. Further, these lavas are compositionally akin to Bushe, Poladpur, Ambenali and Mahableshwar Formational lavas, and follow the same stratigraphic order as in the Western Ghats. Alternating field (AFD) and thermal demagnetizations (THD) isolate the normal mean direction of the Mandla lobe: D =344.5° and I =−30°, where D and I are the mean declination and inclination of the each lava flow (α 95 =8.2; K =72.6; N =17, where α 95 is the half-angle of the cone of 95% confidence about the mean direction, K is the precision parameter and N is the number of flows). The Virtual Geomagnetic Pole (VGP) position determined for these lavas, when compared with the Deccan Super Pole, indicates concordance with the main Deccan volcanic province, thus assigning a shorter period of eruption close to the Cretaceous–Palaeogene boundary (K/PB) for the eastern and western Deccan Traps.

Chapter 2 The structure of the Azores Triple Junction: implications for São Miguel Island

Abstract To date, a lack of reliable morphological and geophysical data has been a major limitation to understanding the tectonic and magmatic processes that shape the Azores Triple Junction and the Terceira Rift. This situation has changed recently: for the first time both areas are covered by high-quality swath bathymetry surveys and marine magnetic data with GPS quality positioning. This provides a good description of the surface morphology, and also of magnetic chrons that give fundamental information for the timing of spreading processes in a geological time frame. There is also a large amount of data from GPS stations that provide accurate estimations of present-day velocities for most of the islands. It is shown that only two main rift systems can be found on the plateau, the older one matching the Princess Alice Basin, and the newer one matching Terceira Rift; the shift between the two probably occurs close to c. 3 Ma. It is shown that extension is currently concentrated on the Terceira Rift, progressively attaching Graciosa and Terceira islands to Eurasia, while São Miguel is being strained by rifting. It is also shown that no right lateral strike-slip fault connects the Terceira Rift to the Mid-Atlantic Ridge.

Some data on the characteristics of the geomagnetic field at the Gauss–Matuyama magnetic chron boundary from the Pirnuar section, West Turkmenistan

Extremely scarce data have been published on the structure of the geomagnetic field during the Gauss and early Matuyama chrons until recently. Only a few papers contain information about the characteristics of the field during the Gauss–Matuyama reversal, derived by studying the terrestrial sediments. This motivated us to revisit the paleomagnetism of the sedimentary rocks of Akchagyl age in the section of the Pirnuar Valley in West Kopet Dag, for the first time studied by us in the late 1960s–early 1970s. These rocks are the analog of the top Piacenzian–bottom Gelasian and span the mentioned time interval. The reanalysis was conducted with the use of the state-of-the-art paleomagnetic techniques and modern magnetostratigraphic timescale. We have studied a number of the characteristics which enabled us to distinguish the rocks whose remanence is most likely to have a depositional origin. Based on the paleomagnetic characteristics of these rocks, we reconstructed the structure of the paleomagnetic field for the studied interval (~270 ka) of the initial stage of the Gauss–Matuyama reversal and revealed the excursions at the final and initial stages of the Gauss and Matuyama chrons. This analysis has significantly updated the time constraints of the rock sedimentation in the studied section and supported the locations of the virtual geomagnetic pole during the reversal, obtained previously.

Conjugate volcanic rifted margins, seafloor spreading, and microcontinent: Insights from new high‐resolution aeromagnetic surveys in the Norway Basin

AbstractWe have acquired and processed new aeromagnetic data that cover the entire oceanic Norway Basin located between the Møre volcanic rifted margin and the Jan Mayen microcontinent (JMMC). The new compilation allows us to revisit the structure of the conjugate volcanic (rifted) margins and the spreading evolution of the Norway Basin from the Early Eocene breakup time to the Late Oligocene when the Aegir Ridge became extinct. The volcanic margins (in a strict sense) that formed before the opening of the Norway Basin have been disconnected with the previous Jurassic‐Mid‐Cretaceous episode of crustal thinning. We also show evidence of relationships between the margin architecture, the breakup magmatism distribution along the continent‐oceanic transition, and the subsequent oceanic segmentation. The Norway Basin shows a complex system of asymmetric oceanic segments locally affected by episodic ridge jumps. The new aeromagnetic compilation also confirms that a fan‐shaped spreading evolution of the Norway Basin was clearly active before the cessation of seafloor spreading and extinction of the Aegir Ridge. An important Mid‐Eocene kinematic event at around magnetic chron C21r can be recognized in the Norway Basin. This event coincides with the onset of diking and increasing rifting activity (and possible oceanic accretion?) between the proto‐JMMC and the East Greenland margin. It led to a second phase of breakup and microcontinent formation in the Norwegian‐Greenland Sea ~26 Myrs later in the Oligocene.

Magnetostratigraphy, K–Ar dating and erosion history of the Hafrafell volcanics, SE-Iceland

Glacial erosion in volcanic terrain just west of the Öræfajökull stratovolcano, SE Iceland, has carved \>2-km-deep valleys. The ca. 2.8-km-thick stratigraphic sequence preserved in the mountain Hafrafell records evolution of landscape relief from relatively flat land at 4 Ma to a deeply dissected valley network today. Through geological mapping, magnetostratigraphy and K-Ar geochronology we establish that the area was first built up by lavas during the Gilbert chron, about 4 Ma. From about the same time we find the earliest evidence of glaciation. A 739-m-thick lava sequence formed, into which glaciers carved a ≥260-m-deep incision, the Hafrafell valley, during Matuyama time, \>2 Ma. The incision was subsequently filled with lava flows during upper Matuyama time, \<2 Ma. Mapping reveals 12 erosion surfaces, HR1–HR12, that formed during the last 4 Myr. The landscape evolution and erosion history of Hafrafell is divided into 6 stages with the first two occurring during the late Neogene, the Gilbert and Gauss magnetic chrons, when lava accumulation was slow and the landscape relatively flat. During stage 3, in lower Matuyama time, lava production increased by a factor of 2. During stage 4 the Hafrafell valley formed in upper Matuyama time. This stage marked clear development of more than 260-m-deep valleys. In stage 5 the Hafrafell valley was filled with subaerial and subglacial volcanic products. Finally, during Brunhes time, in stage 6, intense subglacial volcanism occurred near Hafrafell together with further valley network deepening to some 2-km-depth.

Repeat ridge jumps and microcontinent separation: insights from NE Arabian Sea

Microcontinents separate due to ridge jumps associate either asymmetric sea floor spreading or plume–ridge interactions. India separated from Seychelles at ∼64 Ma by asymmetric sea floor spreading initially when the spreading centre in the Mascarene Basin jumped towards the Indian sub-continent between magnetic chrons C29 and C28. The subsequent tectonics is difficult to comprehend since Laxmi Ridge-another microcontinent-formed during the later phase. Most of the studies considered the Laxmi Ridge as a sliver. Others considered it to be oceanic crust. High resolution, deep (∼25 km) seismic data reveals that (i) the ridge possesses > 15 km deep sea-ward dipping reflector (SDR) packages; (ii) normal faulted rift valleys devoid of syn-rift sedimentary packages; and (iii) axial magma chambers 5–7 km beneath the ridge top. Additionally, from 2D forward gravity models we deduce that the ridge most possibly comprises of high density (oceanic) crust. We conclude the Laxmi Ridge to be indeed composed of oceanic crust and a fossil spreading centre. We thus identified the ridge jumps and their relation to the Seychelles microcontinent separation.Previous numerical models suggest that the time required for a ridge jump is controlled by magmatic heating, spreading rate at the ridge, and plate ages. For repeated ridge jumps, the additional factor is the dynamic relation between the plume and lithosphere in terms of melt transfer and heating. We find that the medium spreading rates and high magmatic heating due to the Réunion plume and young plates favoured rapid and repeated ridge jumps towards the plume.

Distributed deformation close to the Azores Triple “Point”

Terceira Rift and the northern and southern branches of the Mid-Atlantic Ridge (MAR) form a triple junction close to 39°N known as the Azores Triple Junction. New swath bathymetric data are used to investigate the surface expression of faulting close to the triple junction, by the systematic mapping of MAR-generated abyssal hills. It is shown that close to the geometrical intersection between the three spreading axes there exists no single transform fault connecting Terceira Rift to the MAR but a distributed tectonic deformation area characterized by mesoscale brittle deformation close to the surface, covering approximately 90 km by 100 km, and almost no volcanism, which links Terceira Rift to the MAR, accommodating the relative displacement of the three plates close to the geometrically triple point. Magnetic chrons are used to compute the spatial variation of spreading velocity at the Mid-Atlantic Ridge and confirm the above interpretation: they show a progressive increase of spreading velocity along a single MAR segment, between pure “Nubian” at 38°30′N and pure “Eurasian” at 39°25′N, without the development of a transform fault that would integrate the Eurasia–Nubia plate boundary. The comparison of similar triple junctions where a slower axis joins two faster ridges, shows that the slower arm does not reach the “triple point” and that there is always a finite triple junction area, highly tectonized, in which size is dependent on the angle between the two faster arms and, consequently, on the relative spreading velocity of the slower arm.

Stratigraphy, 40Ar–39Ar dating and erosional history of Svínafell, SE-Iceland

The interplay of volcanism and erosion in the Svínafell massif, on the western slope of the Öræfajökull volcanic center, SE-Iceland, is traced with geological mapping, magnetostratigraphy and $^{40}$Ar–$^{39}$Ar age determinations. The volcanic strata are mainly of Quaternary age, i.e., geomagnetic chrons of lower Matuyama to upper Brunhes. The 1832 m thick sequence in Svínafell is composed of 37 discrete lithologic formations, assigned to seven volcano-stratigraphic groups beginning with the onset of volcanism in the Öræfajökull stratovolcano during lower Brunhes magnetic chron (C1n < 781 ka). A regional basin formed shortly before the initiation of volcanism, generating a depocenter for the plant-fossil bearing Svínafell sediments between 0.70 and 1.78 Ma. The Svínafell volcanic strata accumulated during a minimum of eight glacial and inter-glacial stages. We document the Svínafell erosion history and landscape evolution, including 12 erosion surfaces. Erosion has led to extended stratigraphic hiatuses and removal of thick volcanic sequences.

Management of ambiguities in magnetostratigraphic correlation

Magnetostratigraphy is a powerful tool to provide absolute dating of sediments enabling robust and detailed chronostratigraphic correlations. It is based on the correlation of a magnetic polarity column, observed and measured in a given sediment section, to a magnetic polarity reference scale where polarity changes are well dated via other independent methods. However, magnetostratigraphic correlations are loose as they are only constrained by binary magnetic chrons (i.e. normal or reversal) and their thickness, which are both defined from depth variations of the magnetic remanent directions. The thickness of a given magnetic polarity zone is a function of time and sediment accumulation rate, which may not be stationary, leading to ambiguities when performing the correlations.To address these ambiguities, a numerical method based on the Dynamic Time Warping algorithm is proposed. Magnetostratigraphic correlations are computed in order to minimise the local variation of the accumulation rate. The main advantage of the proposed method is to automatically provide a set of reasonably likely correlations. This set can then be scrutinised for further analysis and interpretation. However, the likelihood of a correlation should be handled carefully as it depends on the information content of the magnetostratigraphic section itself and remain ultimately valid by ancillary constraint. Nevertheless, the method gives consistent results on difficult synthetic cases that simulate abrupt variations of the sedimentation rate. Insights on true sections debated by previous authors are also given.

The volcanic margins of the northern Labrador Sea: Insights to the rifting process

A new interpretation of seismic reflection data on the continental margin of northern Labrador shows basement structures similar to those observed on other volcanic rifted margins. Seaward dipping reflections, inner flows, volcanic plateaus, and lava deltas are observed on these data. Magnetic chron 27n (∼61 Ma) is coincident with the volcanic plateaus, connecting these features to the Paleocene volcanism farther north in Davis Strait. Therefore, we are able to extend the region of volcanism about 500 km south along the margin. Similar structures are also observed on the conjugate west Greenland margin. Below the volcanic plateaus, gravity and wide‐angle seismic data show thick igneous crust, which is fairly symmetrically distributed across the conjugate margins. However, the geometry of the thinned continental crust is not symmetrical: a narrower zone of thinning is observed off northern Labrador. The thick igneous crust lies seaward of thinned continental crust, which exhibits little or no underplating by the magmatic event. This observation is compatible with recent models for the formation of nonvolcanic margins, followed by a magmatic event late in the rift evolution. In Mesozoic time rifting of the cold, thick cratonic lithosphere occurred between Greenland and North America with minimal volcanic activity. Then in Late Cretaceous time, the zone of rifting narrowed and localized seaward of the shelf, creating a relatively narrow zone of lower lithospheric thinning and mantle upwelling into which the Paleocene magmas were eventually channeled. This scenario is supported by studies of Mesozoic igneous rocks on land in coastal west Greenland and Labrador.

Stress fields acting during lithosphere breakup above a melting mantle: A case example in West Greenland

We characterize and map the stress fields acting during plate breakup along the West Greenland volcanic margin. The determination of interpolated stress fields is based on an inversion of fault-slip data sets and magma-driven fractures, crosscutting mainly an exposed inner seaward-dipping basaltic wedge (i.e., SDRi: inner Seaward Dipping Reflectors). This SDRi is segmented along-strike, with differently oriented segments. Relative chronology of stress fields is inferred from published age results on oriented dykes. We identify two distinct tectonic episodes (P1 and P2) with a P1–P2 change over at ~54Ma, i.e. during magnetic chron C24R. P1 is syn-magmatic and purely extensional. It is associated with the major crustal stretching event affecting the margin. P1 probably acted as early as the Late Palaeocene. This stress field was first homogeneous with the minimum principal stress σ3 trending ~N060E, defining a P1A stage. During development of the SDRi, σ3 locally reoriented to become orthogonal to each margin segment and, thus, to the continentward-dipping detachment faults bounding the SDRi (P1B). P1 is coeval with lithosphere breakup and is associated with an extension orthogonal to the Labrador–Baffin axis, which is inherited from the Mesozoic. A regional and radical change of σ3 to a ~NS trend takes place during P2, which follows on immediately from P1. P2 is also syn-magmatic. It is associated with only minor extension. σ3 runs parallel to the North American (NAM)/Greenland (GR) kinematic vector from C24R to C13. We establish therefore that the minimum horizontal stress σ3 for P1 and P2 is parallel to the relative displacement of Greenland related to NAM but not to its absolute displacement during the Tertiary. Taking into account those results as well as variations in magma chemistry from P1 to P2, we suggest that tectonic stresses at a volcanic margin could arise from the local dynamics of the melting mantle.

The age of the Sarmatian–Pannonian transition in the Transylvanian Basin (Central Paratethys)

A marked paleoenvironmental change took place at the beginning of the late Miocene in the Central Paratethys, with dominantly marine Sarmatian successions grading rapidly into mainly brackish Pannonian deposits. A long and excellently exposed section comprising the Sarmatian–Pannonian transition has been investigated at Oarba de Mureş in the Transylvanian basin (Romania). In this paper, we focus on both radiometric and magnetostratigraphic dating to provide a chronology for the Sarmatian–Pannonian transition in Transylvania. Two volcaniclastic layers, located approximately 40 m below the Sarmatian–Pannonian transition, yield excellent 40Ar/ 39Ar ages. The weighted mean plateau age for biotite and sanidine separates provided isotopic ages of 11.62 ± 0.12 Ma and 11.65 ± 0.13 Ma. This implies deposition during the magnetic chron C5r.2r, which is in agreement with the magnetostratigraphic results of the Oarba de Mureş composite section. Rock magnetic analyses indicate greigite as the main magnetic carrier, with characteristics very similar to the magnetosomal greigite found in the Carpathian foredeep. The newly obtained chronology at Oarba de Mureş constrains the age of the Sarmatian–Pannonian transition in the Transylvanian basin to 11.3 ± 0.1 Ma, slightly younger than the 11.61 Ma postulated in the Styrian and Vienna Basins.

Reevaluation of magnetic chrons in the North Atlantic between 35°N and 47°N: Implications for the formation of the Azores Triple Junction and associated plateau

In this paper we present a new magnetic compilation for an area of the North Atlantic located between 35°N and 47°N and up to anomaly 33r. We also present a strategy to pick magnetic isochrones and compute finite rotation poles. This technique is based on a continuous reduction to the pole technique and some basic assumptions regarding the direction of the remanent magnetization vector. A cost function that measures the misfit between interpreted and rotated isochrones and the systematic exploitation of the parameter space is used to compute the best set of finite Eulerian rotations for the chrons 5, 6, 6C, 11–12, 13, 18, 20, 21, 22, 23, 24, 25, 26, 28, 29, 30, 32, 33, and 33r. This set of chrons and poles is used to discuss the evolution of the North Atlantic close to Iberia and, in particular, the onset and early development of the Azores Triple Junction area. We show that the relative motion between the Eurasian and the African plates can be coherently described in terms of rigid plate kinematics, respecting both the anomalies shapes and the precise location of the main structural elements of the area: the Pico Fracture Zone, the East Azores Fracture Zone, and the Gloria Fault. We distinguish from the magnetic point of view two different areas of the Azores plateau: the South Azores domain where almost undisturbed NNW magnetic lineations can be found and the Azores domain close to the topographic highs and with no systematic magnetic stripping with the exception of a few recent lineations, probably Matuyama and Brunhes. We present an approximate reconstruction of the plate configuration after chron 18 to conclude that the attachment of Iberia to Eurasia was younger than previously thought (lower Miocene), triggering the formation of the Azores domain, in which stretching took place essentially in the last 20 Ma at an average rate of ∼3.8 mm/a, and progressively attaching the South Azores domain to the African plate by a northward progression of the triple junction.

What status for the Quaternary?

The status of the Quaternary, long regarded as a geological period effectively coincident with the main climatic deterioration of the current Ice Age, has recently been questioned as a formal stratigraphic unit. We argue here that it should be retained as a formal period of geological time. Furthermore, we consider that its beginning should be placed at the Gauss-Matuyama magnetic chron boundary at about 2.6 Ma, rather than at its current position at about 1.8 Ma. The Quaternary would be formally subdivided into the Pleistocene and Holocene epochs. The global chronostratigraphical correlation table proposed is enclosed at the back of this issue.

Geophysical insights and early spreading history in the vicinity of the Jan Mayen Fracture Zone, Norwegian–Greenland Sea

Using a new high-resolution aeromagnetic survey (JAS-05) that was acquired along the trend of the Jan Mayen Fracture Zone (JMFZ), west of the Vøring volcanic margin, we investigated the geodynamic framework of the early spreading evolution of the Norwegian–Greenland Sea. The tectonic structure, main faults and magnetic chrons have been reinterpreted based on new magnetic gridded data and integrated with bathymetry, gravity and seismic data. The new interpretation reveals more details about the early spreading history of the Norwegian–Greenland Sea in the vicinity of the JMFZ. Although anomalous melt production (seaward-dipping reflectors, underplating) associated with the breakup of the Mid-Norwegian margin has been described in many studies, we present data that suggest that significant magmatism continued episodically during the opening of the Norwegian–Greenland Sea along the trend of the JMFZ. The Vøring Spur (VS), an anomalous oceanic high, lying north of the eastern segment of the JMFZ exhibits a contrasting Bouguer gravity low and a complex magnetic signature. The gravity signature of the VS can be modelled and explained as an abnormal thick oceanic crust, which locally can reach up to 15 km. We propose that the thick oceanic crust (overcrusting) was syn-rift and formed during Mid- to Late Eocene. A plate reconstruction at Eocene time suggests that the VS could be part of a triple junction initiated during the breakup between the Vøring Marginal High and the Greenland part of the Traill–Vøring igneous complex, now located offshore East Greenland. Mantle upwelling beneath the early spreading ridge and/or local stress reorganisation could have induced transtension and lithospheric thinning along the JMFZ and magmatic activity would have increased locally along this ‘leaky transform’. We suggest that the Early Tertiary tectono-magmatic processes that operated in the Norwegian–Greenland Sea are similar to the processes involved in the modern triple junction evolution of the Azores Plateau region.

Determination of rapid Deccan eruptions across the Cretaceous‐Tertiary boundary using paleomagnetic secular variation: Results from a 1200‐m‐thick section in the Mahabaleshwar escarpment

Flow‐by‐flow reanalysis of paleomagnetic directions in two sections of the Mahabaleshwar escarpment, coupled with analysis of intertrappean alteration levels shows that volcanism spanned a much shorter time than previously realized. The sections comprise the upper part of magnetic chron C29r, transitional directions and the lowermost part of C29n. Lack of paleosecular variation allows identification of four directional groups, implying very large (40 to 180 m thick) single eruptive events (SEEs) having occurred in a few decades. Paleomagnetism allows temporal constraints upon the formation of 9 out of 23 thin red bole levels found in the sections to no more than a few decades; the two thickest altered layers could have formed in 1 to 50 ka. The typical volumes of SEEs (corresponding to magnetic directional groups) are estimated at 3000 to 20,000 km3, with flux rates ∼100 km3a−1, having lasted for decades. Flood basalt emission can be translated into SO2injection rates of several Gt a−1, which could have been the main agent of environmental change. The total volume of SO2emitted by the larger SEEs could be on the order of that released by the Chicxulub impact. Moreover, each SEE may have injected 10 to 100 times more SO2in the atmosphere than the deleterious 1783 Laki eruption. The detailed time sequence of SEEs appears to be the key feature having controlled the extent of climate change. If several SEEs erupted in a short sequence (compared to the equilibration time of the ocean), they could have generated a runaway effect leading to mass extinction.

Paleomagnetism of the Late Cretaceous intrusions from the Minusa trough (southern Siberia)

The paper summarizes paleomagnetic and rock-magnetic data on the Late Cretaceous diatremes and associated dikes from the Minusa trough located within the southwestern Siberian Platform. It is shown that the stable characteristic component of magnetization is superimposed magnetization (in physical sense). It is linked to Fe-rich titanomagnetite produced by the decay and oxidation of Ti-rich titanomagnetite derived from a primary magma. This process, however, coincides in time with the intrusion cooling, which is supported by paleomagnetic tests. Correlation of magnetic polarity with 39Ar/ 40Ar ages suggests that the acquired stable characteristic component of magnetization corresponds to magnetic Chrons C33-C32 and characterizes the Middle Campanian magnetic field (74–82 Ma). The mean paleomagnetic pole for this span is located at 82.8° N, 188.5° E, with α 95 = 6.1 and, within confidence intervals, coincides with the reference data from the European part of the Eurasian plate. The excellent agreement between virtual paleomagnetic poles testifies that the intraplate motions in the Mesozoic resulting in the crust deformation of Central Asia ceased in the late Cretaceous or were so small that elude detection by the paleomagnetic method.

New angles on South Atlantic opening

Existing models of relative motion between the South America and Africa plates significantly misrepresent the azimuth of Lower Cretaceous seafloor spreading in the South Atlantic Ocean. An improved model is derived from fits of fracture zones, magnetic reversal anomalies, and the edges of a rifted large igneous province at the Northeast Georgia Rise and Agulhas Plateau. An absolute date for this rifting event is not known but, by assuming least change in spreading rates during magnetic chron C34, rifting can be estimated to have occurred at ~100 Ma. This modelling demands a plate divergence history that involves diachronous opening of the South Atlantic, consistent with published estimates of the ages of breakup from sedimentary basins on the South American and African passive margins. The diachronous opening lasts approximately 40 million years, during which time it must be accommodated by significant intracontinental deformation. A reconstruction using the intra-C34 rotation also illustrates the earliest possibility for direct deep-water connection between the central and south Atlantic oceans. One further consequence of this model is that a total reconstruction derived from it and closure of the central Atlantic between North America and Africa suggests that the Venezuelan and Gulf of Mexico basins may be conjugates formed during the earliest opening of the central Atlantic, and not separate marginal basins.

Late Barremian to early Aptian calcareous nannofossil paleoceanography and paleoecology from the Ocean Drilling Program Hole 641C (Galicia Margin)

Calcareous nannofossil assemblages at Site 641C (Galicia Margin, North Atlantic) were investigated in order to determine changes in fertility and temperature of surface waters. Taxa such as Zeughrabdotus spp. <3.5 μm, Biscutum constans, Discorhabdus rotatorius and Diazomatolithus lehmanii, which thrived in higher fertility conditions, are particularly abundant across the CM0 interval as opposed to those with oligotrophic affinities such as Watznaueria spp. and Nannoconus spp., which are generally reduced in abundance. The abundances of nannoconids are much lower than those observed in Tethyan sections, indicating higher fertility conditions. Slumpings and low recovery prevent the identification of the onset of the “nannoconid crisis”, but a sharp drop in nannoconid abundances, observed prior to the CM0 interval, correlates with the “nannoconid decline” observed in several Tethys sections.The normalized ratio between low and high fertility taxa (Fertility Index) was used to characterize the nannofossil assemblages in terms of productivity changes. The highest values of the Fertility Index were observed across magnetic chron CM0. The paucity of cold water taxa such as Seribiscutum spp. and Repagulum parvidentatum suggests warm water conditions throughout the deposition of upper Barremian–lower Aptian sediments on the Galicia Margin.

Lower Cretaceous stage durations combining radiometric data and orbital chronology: Towards a more stable relative time scale?

We propose an alternative calibration of Lower Cretaceous stage durations constrained by direct absolute dating of each stage combined with orbital chronology. Ten glauconitic horizons sampled in the Vocontian basin (SE, France) from the base of the Lower Hauterivian to Upper Albian, yielded K–Ar ages from 123.3 ± 1.7 Ma to 96.9 ± 1.4 Ma, respectively. The relative duration of each stage has been derived by cyclostratigraphy previously obtained in the south-east France and central and south Italy basins. Using the GL-O standard from the Albian–Cenomanian boundary at 95.3 Ma as the anchor point, a cyclostratigraphic age for each stage boundaries has been extrapolated and thus compared with the K–Ar ages. This shows a very well-defined linear correlation which demonstrates the robustness of the proposed durations of the Lower Cretaceous stages. The estimated durations are 5.3 ± 0.4 my, 5.1 ± 0.3 my, 6.8 ± 0.4 my and 11.6 ± 0.2 my for the Hauterivian, Barremian, Aptian and Albian stages, respectively. It also shows that glauconite minerals are powerful radiochronometric tools, when precisely stratigraphically defined and carefully selected. Moreover, the large discrepancy of the estimated Aptian duration of more than 6 my between the most recent published time scale and this study highlights the problem of the radiometric calibration of the M-0 magnetic chron. Finally, the stage durations and boundary ages proposed here bring strong constraints towards the calibration of the Lower Cretaceous time scale. Such accurate temporal calibration is required before any relationship between major biological crises and magmatic emplacement, for instance, could be further investigated.