Chron C29r Research Articles

Magneto-biostratigraphy of the Cicogna section (Italy): Implications for the late Paleocene–early Eocene time scale

We present the magnetostratigraphy and calcareous nannofossil biostratigraphy of the ~ 81 m thick upper Paleocene–lower Eocene Cicogna section from the Belluno Basin of NE Italy. The Cicogna section extends in stratigraphic continuity from Chron C25r to Chron C23r and from nannofossil Zone NP7/NP8 to NP12 (=CP6–CP10), thus covering an interval of time comprised between ~ 56.6 and ~ 52.2 Ma. Inferred sediment accumulation rates were relatively constant in the order of ~ 18 m/Myr, and were used to estimate the age of standard calcareous nannofossil events for comparison with, and improvements of, current time scales. We found several new and potentially useful biohorizons that help refining the chronology of this critical interval of time encompassing a long and biochronologically poorly known Chron C24r. We furthermore observed an apparent correlation between rock-magnetic properties of the sediments and global climate variability as revealed by a reference benthic δ 18O record from the literature, whereby higher relative amounts of oxidized magnetic phases such as hematite and/or maghemite seem to correlate to extreme warm periods like the PETM at ~ 55 Ma and the early Eocene from ~ 54 to ~ 52.5 Ma. Mechanisms able to explain this apparent coupling between rock-magnetic variability and long-term global climate are presently under scrutiny.

Petromagnetic and paleomagnetic characterization deposits at Mesozoic/Cenozoic boundary: The Tetritskaro section (Georgia)

Petromagnetic and magnetostratigraphic characteristics are obtained for the Tetritskaro section. The boundary layer at the Mesozoic/Cenozoic (K/T) boundary is fixed primarily by an abrupt rise in the para- magnetic magnetization (total Fe concentration) and, to a lesser degree, by an increase in the concentration of such magnetic minerals as goethite, hemoilmenite, and magnetite. The along-section distribution of titanomag- netite of volcanic origin and metallic iron of cosmic origin does not correlate with the K/T boundary and litho- logic properties of the sediments. The boundary of the Mesozoic and Cenozoic geological eras lies within the reversed polarity chron C29r and is marked by an abrupt rise in the geomagnetic field paleointensity and an instability of paleomagnetic direc- tions, rather than by a polarity change. The accumulation time of the boundary clay layer is about 1.5-2 kyr, while abrupt changes in the paleointensity and direction of the geomagnetic field encompass 30-40 kyr. Such long occurrence intervals of the events in question cannot be related to a short-term impact phenomenon.

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.

The Bajo Segura Basin (SE Spain): implications for the Messinian salinity crisis in the Mediterranean margins

The analysis of the Messinian and Pliocene stratigraphy of the Bajo Segura Basin (a marginal basin of the westernMediterranean) has revealed three synthems deposited in a high sea-level context: T-MI (late Tortonian-Messinian), MII (Messinian), and P (early Pliocene), bounded by two lowstand erosional surfaces (intra-Messinian and end-Messinian unconformities).With respect to the salinity crisis, we propose the following series of events: 1) pre-evaporitic or pre-crisis phase (T-MI synthem); 2) first sea-level fall and subaerial exposure (intra-Messinian unconformity), possibly related to the precipitation of the Lower Evaporites; 3) syn-evaporitic phase (MII synthem), recorded both by selenitic gypsum (Upper Evaporites) as well as by lagoon deposits (Lago-Mare); 4) second sea-level fall and subaerial exposure (end-Messinian unconformity), characterized by deeply incised palaeovalleys; and 5) post-evaporitic or post-crisis phase (P synthem), which coincides with the definitive restoration of open marine conditions in the basin. A combined biostratigraphic and magnetostratigraphic study revealed that all the events linked to the salinity crisis (from the end of the pre-evaporitic phase to the beginning of the post-evaporitic phase) occurred within the chron C3r (c. 5.9-5.2 Ma).

The Messinian–early Pliocene stratigraphic record in the southern Bajo Segura Basin (Betic Cordillera, Spain): Implications for the Mediterranean salinity crisis

The analysis of the Messinian and Pliocene stratigraphy of the southern Bajo Segura Basin (Betic Cordillera, Spain) has revealed three highstand sedimentary phases (Messinian I, Messinian II, and Pliocene) bounded by two lowstand erosional surfaces (intra-Messinian and end-Messinian unconformities). The Messinian I highstand phase is characterized by the progradation of coastal and shallow marine sandstones (La Virgen Fm) over slope and pelagic-basin marls (Torremendo Fm). After this first phase, a fall in sea level brought about the intra-Messinian unconformity, a subaerial erosional surface with local accumulations of karstic breccias and caliche-like carbonate crusts. The Messinian II highstand phase is represented by sandy beaches and muddy lagoons (Garruchal Fm) correlative with shallow marine evaporites (San Miguel Fm); this second phase records the intra-Messinian reflooding of the basin, which characterizes the salinity crisis in the marginal basins of the Mediterranean. A new sea-level fall resulted in the end-Messinian unconformity, of which the most significant feature is the presence of a broad palaeovalley, c. 200 m deep, which, along its course, completely eroded the deposits of the Messinian II phase and part of the deposits of the Messinian I phase. The Pliocene highstand phase begins with coastal and shallow marine conglomerates and sandstones (La Pedrera Fm) which fill the deep part of the above-mentioned palaeovalley. These bottom deposits evolved gradually upwards towards pelagic marls (Hurchillo Fm), over which shallow marine and coastal sandstones prograded (Rojales Fm). This third phase records the flooding of the basin at the beginning of the Pliocene, when the salinity crisis ended in the marginal basins of the Mediterranean. The combination of calcareous nannoplankton biostratigraphy and magnetostratigraphy has confirmed that both the end of the sedimentation of the Messinian I phase, as well as the two lowstand erosional surfaces (intra- and end-Messinian unconformities) and also the onset of the Pliocene phase occurred in the chron C3r (c. 5.9–5.2 Ma). Under the assumption of the classical model of a desiccated deep basin, either of the two aforementioned erosional surfaces, or even both, could be correlative with the evaporites deposited in the abyssal parts of the Mediterranean.

Early diagenetic greigite as a recorder of the palaeomagnetic signal in Miocene-Pliocene sedimentary rocks of the Carpathian foredeep (Romania)

SUMMARY During the Miocene‐Pliocene, the Carpathian region represented the westernmost part of the so-called Eastern Paratethys, a palaeobioprovince that covered central and eastern Europe as well as parts of southwest Asia. Previous palaeomagnetic investigations provide a highresolution magnetochronology for the sedimentary sequences of the Carpathian foredeep and indicate a marked transition in magnetic carriers from iron oxides to iron sulphides, in Chron C3r. Here, we demonstrate using detailed rock magnetic investigations and scanning electron microscope (SEM) analyses that the major magnetic iron sulphide mineral is greigite (Fe3S4). Thermomagnetic runs indicate an irreversible decrease in magnetization with increasing temperature up to 400 ◦ C and SEM observations indicate octahedral grain morphologies and Fe:S ratios that are indicative of greigite. Hysteresis loops have ‘rectangular’ shapes, which are typical of single domain behaviour, with coercivity and coercivity of remanence at values of Bc = 35‐45 mT and Bcr = 52‐67 mT, respectively. First-order reversal curve diagrams have contours that close around single domain peaks with Bc values of 45‐90 mT. Isothermal remanent magnetization component analysis reveals a small dispersion for the greigite component (dispersion parameter, DP ∼ 0.15 log units) indicating a narrow grain size distribution. A positive fold test, inclination shallowing and two positive reversal tests are additional arguments for the syn-depositional formation of this greigite. We thus argue that (most of) the greigite was formed under early diagenetic conditions, that is, within 1000 yr of deposition of the sediment in this setting, and that it thus can be considered as a reliable recorder of the palaeomagnetic signal in this situation. The appearance of greigite during Chron C3r (between 6.0 and 5.5 Ma) in the Carpathian foredeep is most likely related to regional tectonic and/or climatic events that reshaped the basin configuration and changed the palaeoenvironmental conditions.

The late Maastrichtian nannofossil record of climate change in the South Atlantic DSDP Hole 525A

The phytoplankton response (calcareous nannofossils) to the Late Maastrichtian climate evolution is investigated in the South Atlantic DSDP Hole 525A and compared to published geochemical and micropaleontological data. The results point to a succession of dramatic climatic fluctuations. “Cool-water indicators” ( Ahmuellerella octoradiata, Kamptnerius magnificus and Nephrolithus frequens) suggest cool surface water conditions prevailed during Chron C30n. At the top of C30n, their sudden drop in abundance, the last occurrence of B. constans and the concomitant increase in the tropical species Micula murus suggest warming and lower surface water productivity. An M. murus acme within Chron C29r reflects maximum warming. During the last 100 kyr of the Maastrichtian, the decrease in M. murus and increase in cool-water indicators reflect rapid cooling with the cool climate persisting over. The calcareous nannoplankton response to climate change correlate with similar findings in the Equatorial Atlantic Hole 1258A and parallels the stable isotope record of planktic and benthic foraminifera of DSDP Hole 525A as well as the decline in 187Os/ 188Os. Comparison of this marine record and the continental climate record in North America suggests a link between Deccan volcanism and the late Maastrichtian warm event.

Late Cenozoic paleomagnetism of West Siberian Plate

On the basis of combined (paleomagnetic, lithological, and paleontological) data, a scale of Neogene geomagnetic polarity is proposed for the West Siberian Plate (WSP). It comprises 17 large orthozones of normal and reversed polarity. The scale was compiled by comparing and correlating the Neogene key sections of the Kulunda and Baraba plains, Irtysh regions between Omsk and Pavlodar and near Tara, and Ob’ region near Tomsk. The reliability of paleomagnetic data is confirmed by component analysis of natural remanent magnetization and by a possibility of determining its primary component. In the studied Neogene rocks this is a high-temperature component related to magnetite, hematite, and maghemite, which decays at 420–675 °C. In the period from Early Miocene to Late Pliocene, the Late Cenozoic geomagnetic field reconstructed from NRM vectors for WSP rocks of Neogene age experienced 17 reversals (at the level of orthozone boundaries), with eight regimes of normal polarity and nine regimes of reversed polarity. Comparison of the WSP Neogene scale with Berggren’s scale provided absolute age boundaries of the Late Cenozoic series on the WSP regional stratigraphic scale. The 23.8 Ma boundary between the Oligocene and Miocene is recorded in the regional scale at the sole of the Lyamin beds of the Abrosimovka Formation, at the bottom of Orthochron R 1N 1aq, and the Miocene-Pliocene 5.2 Ma boundary (Chron C3r), in the Novaya Stanitsa Formation (top of the Cherlak beds).

Palaeoceanographic controls on reef deposition: the Messinian Cariatiz reef (Sorbas Basin, Almería, SE Spain)

AbstractThe Cariatiz section lies at the toe of the palaeoslope of the Messinian Cariatiz fringing reef, at the northern margin of the Neogene Sorbas Basin in SE Spain. Distal‐slope reef deposits in the upper part of the section can be traced laterally to the reef core of the last episodes of reef progradation. The underlying deposits are alternating diatomitic marl, marl and silty marl that intercalate with sandstone beds. Combined lithological changes, variations in proportions of warm‐water planktic foraminifera and δ18O values suggest that at least seven, probably precessional, cycles are recorded throughout the Cariatiz section. The correlation of seven cycles in the pelagic deposits to seven reef progradation cycles, and associated vertical shifts in reef facies, indicates relative sea‐level oscillations of several tens of metres. Biostratigraphic and palaeomagnetic data suggest that both the Cariatiz section and the fringing reef formed during the reverse polarity Chron C3r. Surface‐water temperatures seem to be the major factor controlling carbonate production in the reef system. Deposition of bioclastic calcirudite and calcarenite, with no active coral growth, took place at the lowest sea‐level within each reef cycle during temperature minima within each precessional cycle. Porites framework and reef‐slope deposits with Halimeda gravel, in contrast, formed during temperature rises and thermal maxima within precessional cycles.

Maastrichtian calcareous nannofossil biostratigraphy and paleoecology in the Equatorial Atlantic (Demerara Rise, ODP Leg 207 Hole 1258A)

Species richness, absolute and relative abundances of Maastrichtian calcareous nannofossils were analyzed in Hole 1258A. Absolute abundances of taxa were used for the biozonation of this hole and the record of successive bio-events. Distribution patterns of cool water taxa such as Ahmuellerella octoradiata, Gartnerago segmentatum and Kamptnerius magnificus underscore two major cooling events. These events occurred during Chron C31r and during undifferentiated Chrons C31n to C30n. The latter event appears to be accompanied by lowered productivity, as expressed by the lower abundances of fertility index taxa Biscutum constans and Zeugrhabdotus spp. An end-Maastrichtian warming event is recorded in Chron C29r by the sudden drop in abundance of cool water taxa and acme of tropical taxon Micula murus. The disappearance of B. constans just before this event and the significant increase of Micula decussata, respectively, suggest lowered productivity and high-stress environmental conditions prior to the K/P boundary (Cretaceous–Paleogene). These results correlate very well with the climatic scenario proposed by Li and Keller (1998a) in South Atlantic Hole 525A as suggested by stable isotopes and distribution patterns of planktic foraminifera.

Late Miocene to Early Pliocene chronostratigraphic framework for the Dacic Basin, Romania

New magnetostratigraphic results and calcareous nannofossil data of seven sections in the Dacic Basin of southern Romania, covering the Latest Maeotian to Early Romanian time span (6.4–4.0 Ma), allow a correlation with the current Astronomical Polarity Time Scale. This establishes a chronological framework for the Pontian and Dacian stages of the Paratethys and facilitates to correlate the Paratethys stages with those of the Mediterranean. The Maeotian–Pontian boundary is estimated at 6.15 ± 0.11 Ma, within subchron C3An.1n, while the age of the Pontian–Dacian boundary is 5.30 ± 0.1 Ma, within subchron C3r. The Odessian–Portaferrian and Portaferrian–Bosphorian substage boundaries can be placed at about 6.0 and 5.6 Ma, at the base and near the end of chron C3r, respectively. Consequently, the Pontian stage has a duration of approximately 0.85 Myr and is coeval with the upper half of the Messinian stage. The base of the Dacian stage corresponds to the onset of the Pliocene and is coeval with the Pontian–Kimmerian boundary of the Euxinic (Black Sea) Basin. The Dacian–Romanian boundary is recorded either at 4.58 ± 0.05 Ma, in subchron C3n.2n (Nunivak), or at 4.25 ± 0.05 Ma in subchron C3n.1n (Cochiti). The length of the Dacian stage is therefore about 0.7 Myr or approximately 1 Ma. The transition of the Getian into the Parscovian substage can be dated at ∼ 4.83 Ma within subchron C3n.3n (Sidufjall), or at ∼ 4.55 Ma within subchron C3n.2n (Nunivak). The occurrence of calcareous nannofossil assemblages in the lower Portaferrian and near the Portaferrian–Bosphorian transition, belonging to the upper part of the Discoaster quinqueramus–NN11 Zone, and in the middle Bosphorian, belonging to the Amaurolithus tricorniculatus–NN12 Zone, indicates the presence of ephemeral marine connections between the Paratethys Basin and the Mediterranean during the Late Messinian. Another marine influx, inferred from nannofossil species characteristic of the NN12 Zone and found in lower Dacian sediments near the top of subchron C3n.4n (Thvera), post-dates the basal-Pliocene transgression of the Mediterranean Basin.

Paratethyan–Mediterranean connectivity in the Sea of Marmara region (NW Turkey) during the Messinian

The Sea of Marmara region is thought to have been a gateway between Paratethys and the Mediterranean since the Middle Miocene, and is therefore an important control on water mass exchange between the two realms. The Miocene successions in the northeastern Aegean and northwestern Marmara regions indicate that the first Mediterranean marine transgression to affect these areas occurred during the late Serravallian. In the northeastern Aegean region, frequent marine incursions occurred during the Tortonian and Messinian stages. The Messinian stage in this area is represented by a package of brackish- to fresh-water carbonates with some marine sandstone–siltstone interbeds (Alçıtepe Formation), which conformably overlies the Tortonian Kirazlı Formation. The Messinian sequence is overlain with an erosional contact by a shallow marine siliciclastic sequence (Göztepe Formation) of Zanclean age. With its brackish- to fresh-water carbonates and broadly constrained age, the Messinian sequence is interpreted as being coeval with the Upper Evaporite–Lago Mare sequence observed in western Mediterranean basins. In the western Marmara region, the Pontian (Messinian) Alçıtepe Formation consists of bioclastic and oolitic limestones with basal clastic rocks. It conformably overlies the fluvio-lacustrine siliciclastic sediments of the Middle to Upper Miocene Kirazlı Formation and is overlain by fluvio-lacustrine sediments of the Kimmerian (5.5–3.2 Ma) Truva and Tevfikiye formations with an erosional contact. The bioclastic limestones of the Alçıtepe Formation in the western Marmara region contain a molluscan and ostracod fauna that are endemic to Paratethys. These fauna indicate deposition in a shallow, brackish- to fresh-water environment. Faunal and paleomagnetic analyses of a section of the Alçıtepe Formation at Yenimahalle (Çanakkale) confirm that the formation is of Pontian age and represents chron C3r (6.04–5.24 Ma). The ostracod analysis indicates that during deposition of the Alçıtepe Formation, salinity increased from brackish in the lower part to more saline conditions in the upper part. Ostracod valves have low 87Sr / 86Sr values relative to coeval Late Miocene ocean water. This indicates that exchange between the Sea of Marmara region and the global ocean was restricted throughout this period. Fossil and Sr-isotope evidence suggests, however, that there was a Paratethyan–Marmara connection during the deposition of the lower part of the Alçıtepe Formation, with Paratethyan influence reaching the north Aegean. Connection via Marmara between Paratethys and the Mediterranean was not re-established until the late Aktchagylian (Late Pliocene). The re-connection was caused by both increased activity on the North Anatolian Fault and global sea level rise.

Age and Correlation of Fossiliferous Late Paleocene–Early Eocene Strata of the Erlian Basin, Inner Mongolia, China

The Asian continent preserves a rich and diverse record of Paleogene mammal faunas and their evolution through time. The sequence of faunal succession is of key importance to our understanding of the origin and diversification of modern mammal groups, as phylogenetic data suggest that many major modern clades may be rooted in Asia. By calibrating the Asian fauna sequence within a chronostratigraphic framework, we can begin to compare patterns of succession on a global scale and constrain models for the origination and dispersal of modern mammal groups in the early Paleogene.The Erlian Basin of Inner Mongolia preserves Early Paleogene strata and mammal fossils assignable to the Gashatan, Bumbanian, and Irdin Manhan Asian Land Mammal Ages (ALMAs). We measured stratigraphic sections and analyzed the stable isotope composition of paleosol carbonates and paleomagnetic directions of rocks at three localities in the Erlian Basin. The data document patterns in lithology, carbon isotope composition, and magnetic polarity that are consistent at all three localities and allow us to present two constrained hypotheses for the correlation of the local stratigraphic sections. Within the resulting composite section, we are able to identify a secular decrease in the carbon isotope composition of paleosol carbonate that can be equated to a multimillion-year trend preserved in late Paleocene and early Eocene terrestrial and marine records. Using this trend and previously documented constraints on the age of the Bumbanian ALMA, the composite section is shown to correlate within the interval of time represented by chrons C26n–C24n of the Geomagnetic Polarity Timescale (GPTS). We outline three possible correlations of the sequence of magnetic polarity zones in our composite section to the GPTS and explore the biostratigraphic implications of these. All three possible correlations show that Gashatan faunas in Inner Mongolia occur within chron C24R, and the preferred correlation suggests that the Gashatan taxa may have persisted close to the Paleocene/Eocene boundary. If confirmed through further sampling, this result would imply that the first appearance of the modern mammal orders Primates, Artiodactyla, and Perissodactyla in Asia at the base of the Bumbanian ALMA did not significantly precede their first appearances in Europe and North America at the Paleocene/Eocene boundary. Fossil sites in the Erlian Basin promise to be central to resolving the debate about whether these clades lived and diversified in Asia before dispersing throughout the Northern Hemisphere at the Paleocene/Eocene boundary.

New chronostratigraphy of the Cretaceous–Paleogene boundary interval at Bidart (France)

The Cretaceous–Paleogene boundary section at Bidart (French basque coast) was prevented for a long time to be considered as a reference section for the K–T boundary due to a suspected hiatus comprising most of Chron C29r.In our new magneto- and biostratigraphic study of this section we discovered that the underlying primary magnetic polarity is not revealed in the vector directions but in a small, but consistent, intensity excursion during laboratory demagnetization trends. Magnetite and maghemite are the main carriers of remanence, with contribution of hematite to the magnetic mineralogy of the red-brown lowermost Danian and Maastrichtian marly limestones. Magnetite is the main magnetic mineral in the white to pink Danian limestones. The NRM of marls and marly limestones is characterized by: (1) a present-day field overprint, (2) a medium-temperature (300–400 °C) reversed component interpreted as a relic of the characteristic remanent magnetization (ChRM) carried by magnetite grains which are not completely oxidized into maghemite, and (3) a high temperature (up to 590 °C) normal component due to late diagenesis hematite growth. The NRM of Danian limestones is characterized by (1) a present-day field overprint and (2) a ChRM component of normal or reversed polarity.The ChRM directions allow a new correlation of the magnetic polarity sequence with the Geomagnetic Polarity Time Scale (GPTS): Chron C29r corresponds to the Maastrichtian unit and extends up to 3.5 m above the K–T boundary, and Chron C29n occurs at the top of the section. There is no evidence for a hiatus at or below the K–T boundary, and the section does not extend up to Chron C28r as previously suggested. These new chronostratigraphic assignments are in good agreement with the previous biostratigraphic data and from the new biostratigraphic analyses based on calcareous nannofossils presented in this paper.Our study demonstrates that the section of Bidart can be considered as a key section to study the K–T boundary event(s).

Untangling the Palaeocene climatic rhythm: an astronomically calibrated Early Palaeocene magnetostratigraphy and biostratigraphy at Zumaia (Basque basin, northern Spain)

The magnetostratigraphy of a 54-m-long section above the Cretaceous–Tertiary boundary at the sea-cliff section of Zumaia in the Basque basin (northern Spain) has been established. The section encompasses the entire Danian and the lower part of the Selandian stages as indicated by calcareous plankton biostratigraphy. The studied interval consists of (hemi)pelagic limestone–marl alternations in the form of couplets and bundles, which range from centimetre/decimetre to metre scale respectively and a few thin-bedded calcareous turbidites. The magnetostratigraphy, based on samples from about 200 stratigraphic levels, allows the identification of six reversal boundaries from chron C29r to C26r at a bed level. The spatial (or temporal) evolution of periodicities from a lithologically coded series is studied with the continuous wavelet transform technique. A preliminary age model based on the standard CK95 GPTS indicates that the basic lithologic carbonate–marl couplet corresponds to the 19–23-kyr precession cycle (21–31-cm cycle in the depth domain) and that a bundle cycle (usually groups of four to six basic couplets) with global periodicity centred at 1.22 m corresponds to the ∼110-kyr eccentricity cycle. We have tuned the bundle cycles to the Va03_R7 eccentricity orbital solution [Astrophys. J. 592 (2003) 620–630] following an initial match of a node of the ∼2.4-Ma eccentricity modulatory cycle in the target time series to particularly carbonate-rich bundles from the upper part of the Zumaia section that displays significant power of a 4.4-m-period cycle corresponding to the ∼404-kyr eccentricity cycle. Consistency between lithologic patterns and characteristics in the eccentricity target is reasonably met although the ∼404-kyr eccentricity cycle is not persistent throughout. The tuning, however, appears robust as it brings the age of the K/T boundary at ∼65.8 Ma. It is argued that a sea-level signal (tectonically driven?) is superimposed on the climatic forcing at the Milankovitch band masking the full expression of the low-frequency astronomical periods. We provide a cycle-tuned duration for all intervening Early Palaeocene polarity chrons and estimate relative ages for bioevents. The cycle-tuned chronology indicates that the CK95 GPTS overestimates the duration of chrons C28 and C27 by 20 and 26% respectively. Our data may prove useful to better constrain Early Palaeocene biostratigraphy of calcareous plankton and in the redefinition of the boundary between the Danian and Selandian stages.

40Ar/39Ar geochronology and paleomagnetic stratigraphy of the Lukeino and lower Chemeron Formations at Tabarin and Kapcheberek, Tugen Hills, Kenya

40Ar/39Ar single-crystal laser-fusion dating, K–Ar dating, and paleomagnetic reversal stratigraphy have been used to determine the chronostratigraphy of the Kabarnet Trachyte, Lukeino Formation, Kaparaina Basalt Formation, and Chemeron Formation at the sites of Kapcheberek (BPRP#77) and Tabarin (BPRP#77) in the Tugen Hills, Kenya. The succession ranges in age from 6·56–3·8Ma. The upper Lukeino Formation at Kapcherberek, including the fauna from the site BPRP#76, was deposited during chron C3r and can be constrained to the interval 5·88–5·72Ma. The Chemeron Formation at Tabarin includes at the base an ignimbrite and associated basal air-fall tuff with a combined age of 5·31±0·03Ma. Sedimentary and volcaniclastic rocks of the Chemeron Formation which unconformably overlie the ignimbrite record chrons C3n.2n through C2Ar. The combined40Ar/39Ar and paleomagnetic data constrain the age of this sequence to 4·63–3·837Ma. The age of the Tabarin mandible fragment (KNM-TH 13150) and associated fauna at site BPRP#77 in the Chemeron Formation is 4·48–4·41Ma, marginally older than similar early hominids from Aramis, Ethiopia. Basin subsidence appears to be defining an overall accumulation rate of about 17cm/ka over the 2·7Ma represented at Tabarin and Kapcheberek, despite episodes of rapid accumulation and hiatuses.

K/Ar and 39 Ar/ 40 Ar whole-rock dating of zeolite facies metamorphosed flood basalts: the upper Paleocene basalts of the Faroe Islands, NE Atlantic

Abstract The Paleocene flood basalts of the Faroe Islands form a central part of the North Atlantic Igneous Province, but have proven difficult to date because of very low-grade burial metamorphism in the chabazite-thomsonite to the laumontite zeolite zone. We present 17 replicated K/Ar and 8 Ar/Ar whole-rock analyses of basalts from the >3 km thick lower basalt formation, the age of which has been debated for years. Samples are from the massive core of thick, exposed flows, and two boreholes (Vestmanna-1 and Lopra-1). Six samples are drill cuttings. Extensive microprobe work and mass balance calculations show that roughly 60% of the potassium of the dated basalts resides in plagioclase, interstitial cryptocrystalline rhyolite and smectitic clay, the rest mainly forming thin rims of alkali feldspar on plagioclase. Six basalts fulfil the following criteria: (1) they are almost homogeneous in K and Ar (ages on different splits vary by <4 Ma); (2) the only low-temperature phase present is smectite (saponite±minor interstratified chlorite-smectite); and (3) max. c. 6% of total K occurs in smectite. This smectite replaced metastable interstitial glass during early burial and has a trivial effect on measured ages. The six basalts give mean K/Ar whole-rock ages of 56.5 ± 1.3 to 58.9 ± 1.3 Ma (1σ), which are interpreted as igneous ages consistent with mapped palaeomagnetic reversals and unpublished Ar/Ar dates. They suggest that the oldest drilled lavas erupted at 58.8 ± 0.5 Ma (1σ) in the later part of magnetochron C26r accumulating at >2 km Ma −1 , and that the volcanism came to a slow end at 56.4 ± 0.5 Ma in the beginning of chron C24r. The Lower Basalt Formation is overlain by 10m of coal-bearing sediments and 2 km syn-breakup lavas, deposited in early C24r (>55 Ma). The remaining 11 basalts are either inhomogeneous, carry > 10% of the total K in clay, have C/S > saponite or contain traces of zeolites, secondary quartz, dioctahedral smectite or celadonite as probable indicators of prolonged alteration, and they give low or variable K/Ar ages. The Ar/Ar analyses include five of these poor samples plus three of the first group. They give a plateau age of 55.7 ± 0.9 Ma (1σ) for two exposed flows assigned to chron C25n, but of 60–63 Ma for six drilled lavas assigned to C26r. We argue that the later Ar/Ar ages are too high due to 39 Ar recoil loss out of the sample (0–25%) or relocation during irradiation and should be ignored.

Latest Paleocene–earliest Eocene cyclostratigraphy: using core photographs for reconnaissance geophysical logging

Abstract I present a new method for reconnaissance cyclostratigraphic study of continuously cored boreholes: the generation of detailed sediment color logs by digitizing Ocean Drilling Program (ODP) core photographs. The reliability of the method is tested by comparison with the spectrophotometer color log for the uppermost Paleocene–lowermost Eocene section (Chron C24r) at ODP Hole 1051A. The color log generated from Hole 1051A core photographs is essentially identical to the spectrophotometer log. The method is applied to the Chron C24r section at Holes 1051A and 690B, producing the first high-resolution geophysical log for the latter section. I calculate astronomically calibrated durations between bio- and chemostratigraphic events within Chron C24r by correlating the cyclostratigraphies for Holes 1051A and 690B. These durations are significantly different from previous estimates, indicating that the chronology of events surrounding the Paleocene/Eocene boundary will have to be revised. This study demonstrates that useful geophysical logs can be generated from digitized ODP and DSDP core photographs. The method is of practical use for sections lacking high-resolution logs, as is the case for most lower Paleogene sections.

Cosmic markers, 40Ar/ 39Ar dating and paleomagnetism of the KT sections in the Anjar Area of the Deccan large igneous province

Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433–436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417–424] reported the discovery of iridium-bearing sediments sandwiched between basalt flows in the Anjar area (Kutch province, India). They concluded that the signature of the K/T impact had been recorded and that onset of volcanism in the Deccan traps preceded the K/T boundary, excluding the possibility of a causal connection. This paper reports complementary analyses of Anjar outcrops by a joint Indo–French team, where we focused on cosmic markers (iridium and spinels) in the intertrappean sediments and 40Ar/ 39Ar dating and paleomagnetism of the lava flows. Anomalous Ir concentrations (up to 0.4 ng/g) are confirmed, with up to three thin and patchy enriched layers which cannot be traced throughout the exposed sections. Despite careful search, no Ni-rich spinels were found. Eight basalt samples provided 40Ar/ 39Ar results, four on plagioclase bulk samples, four on whole rocks. Spectra for whole rocks all indicate some amount of disturbance, and ages based on plagioclase bulk samples seem to be consistently more reliable [Hofmann et al., Earth Planet. Sci. Lett. 180 (2000) 13–28]. The three flows underlying the Ir-bearing sediments are dated at ∼66.5 Ma, and two overlying flows at ∼65 Ma. Magnetic analyses (both thermal and by alternating fields) uncovered clear reversed primary components in the upper flows, and more disturbed normal components in the lower flows, with evidence for an additional reversed component. There are reports [Bajpai, Geol. Soc. India Mem. 37 (1996) 313–319; Bajpai, J. Geol. Soc. London 157 (2000) 257–260] that the intertrappean sediments contain uppermost Maastrichtian dinosaur and ostracod remains above the uppermost Ir-bearing level, and may not be mechanically disturbed. We propose the following scenario to interpret these multiple field and analytical observations. Deccan trap volcanism started within uppermost Maastrichtian normal chron C30N at ∼66.5–67 Ma in the Anjar area. Volcanism then stopped at least locally, and lacustrine sediments were deposited over a period that could be in the order of 1–2 Ma. The K/T bolide impact was recorded as a deposit of Ir, and possibly (though not necessarily) spinels. Volcanism resumed shortly after the K/T boundary, within reversed chron C29R, as witnessed by the three reversely magnetised overlying basalt flows dated ∼65 Ma. This was responsible for erosion and destruction of part of the uppermost sediments (including spinels if there were any) and heterogeneous and non-uniform redeposition of Ir at a number of underlying sedimentary levels. This was also responsible for the partial remagnetisation of the underlying flows. These findings generally confirm and complement those of Bhandari et al. [Bhandari et al., Geophys. Res. Lett. 22 (1995) 433–436; Bhandari et al., Geol. Soc. Am. Spec. Paper 307 (1996) 417–424], and are compatible with the occurrence of the K/T impact at the paleontological K/T boundary, and of Deccan trap volcanism straddling the boundary and starting before the impact. Anjar provides evidence for minor volcanism somewhat earlier than suggested by some authors, though still within normal chron C30N. There is no indication contradicting the view that the bulk of Deccan trap volcanism occurred over only three chrons (C30N, C29R, C29N) [Courtillot, Evolutionary Catastrophes: the Science of Mass Extinctions, Cambridge University Press, 1999; Courtillot et al., Earth Planet. Sci. Lett. 80 (1986) 361–374; Vandamme et al., Rev. Geophys. 29 (1991) 159–190].

Magnetic polarity stratigraphy of Siwalik Group sediments of Karnali River section in western Nepal

The remanent magnetization of siltstones and sandstones sampled at 476 levels/sites throughout a 3560 m thick molasse sequence belonging to the Siwalik Group (0–2015 m: Lower; 2015–3560 m: Middle) has been studied by stepwise thermal demagnetization. This section is exposed along the Karnali River in Nepal. The natural remanent magnetization (NRM) usually consists of two components: a viscous or thermoviscous component of recent field origin, and an ancient characteristic component (ChRM). The former component is of normal polarity and resides either in goethite (unblocking temperature < 150 °C; resistant to AFD up to 150 mT) or in maghemite (unblocking temperature 150–400 °C). Goethite contributes up to 90 per cent of the total intensity in the finer variegated muddy samples belonging to the lower half of the section. Maghemite content is significant in the grey mud-free lithologies from the upper half of the section. The main component, unblocked in the high-temperature range (commonly 610–680 °C) and believed to reside in haematite, presumably of mostly detrital origin, represents a characteristic remanence (ChRM). The tilt-corrected ChRM directions at individual sites show antipodal clusters (ratio of normal- to reverse-polarity sites: 0.62), and yield mean inclinations recording significant inclination shallowing—a feature well recorded in the Siwaliks. This ChRM is interpreted to represent a largely primary detrital remanence. The ChRM data from 430 sites yield the Karnali River magnetic polarity sequence, whose correlation with the geomagnetic polarity timescale (Cande & Kent 1995) suggests a depositional age of 16 Ma (younger than chron C5Cn.1n) to 5.2 Ma (around the top of chron C3r) for the 3560 m section sampled. Hence, the Karnali River exposes the oldest part of the Siwalik Group in Nepal. Estimates of the sediment accumulation rate (SAR) average to 32.9 cm kyr−1 for the 10.8 Myr time span of deposition.