Polarity Intervals Research Articles

Slow cooling of middle and lower oceanic crust inferred from multicomponent magnetizations of gabbroic rocks from the Mid‐Atlantic Ridge south of the Kane fracture zone (MARK) area

The remanent magnetization of gabbroic material of the Mid‐Atlantic Ridge south of the Kane fracture zone (MARK) area provides constraints on both the thermal structure and tectonic history of the lower crust in this slow spreading environment. The remanence of these gabbroic samples is often complex, with the juxtaposition of intervals of apparently normal and reversed polarity rocks over small spatial scales (tens of centimeters to a few meters). Moreover, several samples when thermally demagnetized have a reversed polarity magnetization component between higher and lower stability normal polarity components. Given the nominal age (∼1 Ma) of the crust, we suggest that this pattern of normal/reversed‐normal polarity most plausibly reflects emplacement and/or cooling through three successive polarity intervals, Jaramillo normal polarity interval (1.07–0.99 Ma), a portion of the Matuyama reversed polarity interval (0.99–0.78 Ma), and the Brunhes normal polarity interval (0.78 Ma to present). A small number of samples with three well‐defined magnetization components have magnetic characteristics compatible with a remanence carried by fine‐grained, possibly single domain, magnetite. Laboratory unblocking temperatures in these samples therefore allow estimation of lower crustal temperatures at the time of the Jaramillo/Matuyama (0.99 Ma) and Matuyama/Brunhes (0.78 Ma) polarity transitions. Together with depth estimates derived from fluid inclusion studies these results suggest that middle and lower crustal temperatures remained as high as ∼350°–475°C for a minimum of 0.21 m.y. after emplacement. We suggest that continued injection of liquid, in the form of sills or small magma bodies, over a broad region (half width of ∼3 km) is responsible for this slow cooling. In addition, inclinations of the highest stability component from these drill sites are remarkably similar to that expected from an axial geocentric dipole, suggesting that little, if any, resolvable tilt occurred during uplift of these rocks to the seafloor.

Up and down cascades: three-dimensional magnetic field model.

In our previous works we already have proposed a two-dimensional model of geodynamo. Now we use the same approach to build a three-dimensional self-excited geodynamo model that generates a large scale magnetic field from whatever small initial field, using the up and down cascade effects of a multiscale turbulent system of cyclones. The multiscale system of turbulent cyclones evolves in six domains of an equatorial cylindrical layer of the core. The appearance of new cyclones is realized by two cascades: a turbulent direct cascade and an inverse cascade of coupling of similar cyclones. The interaction between the different domains is effected through a direct cascade parameter which is essential for the statistics of the long-life symmetry breaking. Generation of the secondary magnetic field results from the interaction of the components of the primary magnetic field with the turbulent cyclones. The amplification of the magnetic field is due to the transfer of energy from the turbulent helical motion to the generated magnetic field. The model demonstrates a phase transition through the parameter characterizing this energy transfer. In the supercritical domain we obtain long-term intervals of constant polarity (chrons) and quick reversals; relevant time constants agree with paleomagnetic observations. Possible application of the model to the study of the geometrical structure of the geomagnetic field (and briefly other planetary fields) is discussed.

A short, reverse polarity interval within the Jaramillo subchron: Evidence from the Jingbian section, northern Chinese Loess Plateau

A high‐resolution paleomagnetic and rock magnetic investigation supported by scanning electron microscopy observations, X‐ray diffraction determinations, and anisotropy of magnetic susceptibility analyses has been carried out on a loess‐paleosol sequence from Jingbian (37.4°N, 108.8°E), at the northern extremity of the Chinese Loess Plateau. The results indicate that the magnetic assemblage is dominated by pseudo‐single‐domain magnetite with associated maghemite and traces of a higher‐coercivity phase, probably hematite. The Brunhes‐Matuyama and upper Jaramillo transitions have been carefully documented and a record of a short reverse polarity interval (SRI), entirely within the Jaramillo normal polarity subchron, has been obtained. The duration of the SRI, recorded over a stratigraphic interval of 48 cm, is 10.1 kyr (29 kyr if the polarity transitions are included). These results seem to exclude that rock magnetic variations are adversely affecting the recording process and compel us to reconsider the Jaramillo subchron as characterized by a pair of normal polarity intervals interrupted by a short reverse polarity interval.

Astronomically calibrated ages for geomagnetic reversals within the Matuyama chron

We present a magnetostratigraphic record from the western Philippine Sea that is tied to a marine δ18O record for the past 2.14 million years. The ages of geomagnetic reversals were astronomically calibrated by tuning the oxygen isotopic stratigraphy, yielding a chronology for the following subchrons: Matuyama/Brunhes boundary, 781 ± 3 ka (slightly above δ18O Stage 19.3); top of the Santa Rosa polarity interval, 920 ± 2 ka (Stage 23/24); base of the Santa Rosa polarity interval, 925 ± 1 ka (Stage 24); top of the Jaramillo subchron, 988 ± 3 ka (Stage 27); base of the Jaramillo subchron, 1072 ± 2 ka (Stage 31); top of the Cobb Mountain subchron, 1173 ± 4 ka (Stage 35/36); base of the Cobb Mountain subchron, 1185 ± 5 ka (Stage 36); top of the Olduvai subchron, 1778 ± 3 ka (Stage 63/64); base of the Olduvai subchron, 1945 ± 4 ka (Stage 71/72); top of the Reunion II subchron, 2118 ± 3 ka (Stage 80/81); and base of the Reunion II subchron, 2133 ± 5 ka (Stage 81). This astronomically calibrated chronology independently confirms the ages of major reversals in recently published astronomically calibrated polarity timescales for the Matuyama chron. It also provides the first astronomically calibrated dates for the lower and upper reversals associated with the Cobb Mountain and Santa Rosa polarity intervals, respectively.

Correction to “A geomagnetic record over the last 3.5 million years from deep‐tow magnetic anomaly profiles across the Central Indian Ridge” by Gaud Pouliquen, Yves Gallet, Jérôme Dyment, Philippe Patriat, and Chiori Tamura

High‐resolution records of the geomagnetic field intensity over the last 3.5 Myr provided by paleomagnetic analyses of marine sediments and volcanics have shown the occurrence of short‐lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep‐tow (∼500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well‐dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short‐wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific‐Antarctic Ridge. Our results suggest that in addition to polarity reversals, geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small‐scale magnetic anomaly, likely generated by several closely spaced excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near‐bottom results support the existence of three geomagnetic features between the Gauss‐Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A. ln subchron within the Gauss chron. This study emphasizes the potential of deep‐tow magnetic surveys in detecting fluctuations in geomagnetic field intensity and, in particular, short‐lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum.

Age and duration of activity at the Isle of Mull Tertiary igneous centre, Scotland, and confirmation of the existence of subchrons during Anomaly 26r

The duration of activity that produced the Isle of Mull Tertiary igneous centre has been constrained to 2.52±0.36 million years (1σ) using new high-resolution 40Ar/ 39Ar radiometric dating techniques. These new 40Ar/ 39Ar ages for Mull and also one from Skye, presented here, confine the duration of igneous activity to within one magnetic chron (3 million years within Chron C26r). However, the Mull and Skye Tertiary centres record a sequence of reversed and normal polarity intervals. Recalibration of tie points used to determine the magnetic polarity time scale cannot account for all of the observed changes in polarity from Mull and Skye. We propose that the new ages for the Mull and Skye centres indicate that these parts of the British Tertiary volcanic province developed fast enough to record short-period polarity intervals, which correspond to tiny wiggles in the marine magnetic anomaly record.

Revised tectonic implications for the magnetic anomalies of the western Weddell Sea

Ten years after the USAC ( U.S.– Argentina– Chile) Project, which was the most comprehensive aeromagnetic effort in the Antarctic Peninsula and surrounding ocean basins, questions remain regarding the kinematics of the early opening history of the Weddell Sea. Key elements in this complex issue are a better resolution of the magnetic sequence in the western part of the Weddell Sea and merging the USAC data set with the other magnetic data sets in the region. For this purpose we reprocessed the USAC data set using a continuation between arbitrary surfaces and equivalent magnetic sources. The equivalent sources are located at a smooth crustal surface derived from the existing bathymetry/topography and depths estimated by magnetic inversions. The most critical area processed was the transition between the high altitude survey over the Antarctic Peninsula and the low altitude survey over the Weddell Sea that required downward continuation to equalize the distance to the magnetic source. This procedure was performed with eigenvalue analysis to stabilize the equivalent magnetic source inversion. The enhancement of the Mesozoic sequence permits refining the interpretation of the seafloor-spreading anomalies. In particular, the change in shape and wavelength of an elongated positive in the central Weddell Sea suggests that it was formed during the Cretaceous Normal Polarity Interval. The older lineations in the southwestern Weddell Sea are tentatively attributed to susceptibility contrasts modeled as fracture zones. Numerical experimentation to adjust synthetic isochrons to seafloor-spreading lineations and flow lines to fracture zones yields stage poles for the opening of the Weddell Sea since 160 Ma to anomaly 34 time. The corresponding reconstructions look reasonable within the known constraints for the motions of the Antarctic and South America plates. However, closure is not attained between 160 and 118 Ma if independent published East Antarctica–Africa–South America rotations are considered. The lack of closure may be overcome by considering relative motion between the Antarctic Peninsula and East Antarctica until 118 Ma time, an important component of convergence.

The Global Boundary Stratotype Section and Point (GSSP) of the Toarcian-Aalenian Boundary (Lower-Middle Jurassic)

The Global Boundary Stratotype Section and Point (GSSP) for the Aalenian Stage, formally defined at the base of bed FZ107 in the Fuentelsaz section, Castilian Branch of the Iberian Range (Spain), has been ratified by the IUGS. Multidisciplinary biostratigraphical data, based on ammonites, brachiopods, ostracods, bivalves, foraminifera, calcareous nannofossils assemblages and palynomorphs, assure worldwide correlations; magnetostratigraphic data increase this correlation power. The position of the boundary coincides with the first occurrence of the ammonite assemblage characterized by Leioceras opalinum and Leioceras lineatum and corresponds with a normal polarity interval correlated with the up-to-date Jurassic magnetic polarity time scale (Gradstein and others, 1994; Ogg, 1995).

A sedimentary paleomagnetic record of the Matuyama chron from the Western Antarctic margin (ODP Site 1101)

A high-resolution paleomagnetic record for part of the Matuyama chron (0.7–2.1 Ma) is reported for Ocean Drilling Program Site 1101 (Leg 178), off the Antarctic Peninsula Pacific margin. A rock-magnetic investigation of 62 discrete samples revealed that the natural remanent magnetization (NRM) is carried by pseudo-single domain magnetite. Progressive alternating field demagnetization of 83 m of U-channels provided a polarity stratigraphy down to the Olduvai subchron. Two geomagnetic events preceding the Jaramillo subchron were identified, including the Cobb Mountain polarity interval. The bulk magnetic parameters vary by more than a factor of 20 over the entire time interval, but by less than a factor of 6 over the 0.7–1.1 Ma interval. This latter interval was selected for paleointensity determinations, which were carried out by normalizing the NRM by the anhysteretic remanent magnetization (ARM). Direct comparison of the Site 1101 paleointensity record with other curves available for the same time interval suggests a geomagnetic origin for features present in the record. A more quantitative comparison was achieved by means of a jackknife test performed on nine records of relative paleointensity over the 0.95–1.1 Ma interval. This test yielded no outlier for the period considered, confirming the geomagnetic character of the records. We have constructed a low-resolution stack revealing some of the characteristic paleointensity features of the Jaramillo subchron.

A geomagnetic record over the last 3.5 million years from deep‐tow magnetic anomaly profiles across the Central Indian Ridge

High‐resolution records of the geomagnetic field intensity over the last 4 Myr provided by paleomagnetic analyses of marine sediments have shown the occurrence of short‐lived low field intensity features associated with excursions or short polarity intervals. In order to evaluate the ability of marine magnetic anomalies to record the same geomagnetic events, we have collected six deep‐tow (∼500 m above the seafloor) and several sea surface magnetic anomaly profiles from the Central Indian Ridge across the Brunhes, Matuyama, and Gauss chrons (i.e., from the ridge axis to anomaly 2A). After removal of topography, latitude, and azimuth effects, we converted distances into time sequences using well‐dated polarity reversal anomalies as tie points. We calculated the average signal to test the robustness of the short‐wavelength anomalies. The resulting stacked profile is very similar to stacked sea surface and downward continued profiles from the Central Indian Ridge, the East Pacific Rise, and the Pacific‐Antarctic Ridge. Our results suggest that in addition to polarity reversals, to previously suggested geomagnetic events (subchrons or excursions) within the Brunhes and Matuyama chrons. A new small‐scale magnetic anomaly, likely generated by several closely spaced geomagnetic field intensity variations represent the major contributor to the detailed shape of recent marine magnetic anomalies in investigated areas. We observe a dense succession of microanomalies that are correlated excursions (Ontong Java 1 and 2, and Gilsa), is found after the Olduvai chron. The near‐bottom results support the existence of three geomagnetic features between the Gauss‐Matuyama boundary and Olduvai. They also suggest three geomagnetic events during the C2A.1n subchron within the Gauss chron. This study emphasizes the potential of deep‐tow magnetic surveys in detecting fluctuations in geomagnetic field intensity and, in particular, short‐lived excursions, a poorly constrained part of the geomagnetic field temporal variation spectrum.

Middle Cambrian high magnetic reversal frequency (Kulumbe River section, northwestern Siberia) and reversal behaviour during the Early Palaeozoic

We present a magnetostratigraphic study of Middle Cambrian sediments along the Kulumbe River (northwestern Siberia). The deposits are ∼850 m thick and lie within the Mayan stage as constrained by trilobite fossils. Palaeomagnetic analyses reveal two magnetic components. The first component, parallel to the direction of the present day field at the site, is isolated in the low to middle temperature range. A high temperature component (HTC) is then isolated up to 580 or 680°C, carried either by magnetite and/or by hematite. The HTC defines a sequence of 28 magnetic polarity intervals, some of them being defined by only one sample. Considering that the studied section has a duration of ∼5 Myr, we propose that the magnetic reversal frequency was high (∼4–6 reversals per Myr) during part of the Middle Cambrian, among the highest values known within the Phanerozoic. The reversal frequency may have been roughly similar during the Lower Cambrian. We further suggest a drastic decrease of the magnetic reversal frequency between the Lower–Middle Cambrian and the end of the Tremadoc (Ordovician) when a superchron probably occurred. However, this behaviour is still challenged by other scenarios which depend on the chosen Early Palaeozoic time scale and on the reliability of some magnetostratigraphic data.

Marine magnetic anomalies: evidence that ‘tiny wiggles’ represent short-period geomagnetic polarity intervals

Since the 1960’s, the geomagnetic polarity time scale, which is based on marine magnetic anomalies, has become fundamentally important in geochronology. Despite the importance of marine magnetic anomaly records, there has been longstanding uncertainty about the meaning of the smallest anomalies observed in these records. Small amplitude, short wavelength anomalies are frequently observed in marine magnetic anomaly records across fast-spreading oceanic crust (>50 mm/yr). The origin of these small-scale anomalies (referred to as ‘tiny wiggles’) has remained controversial over the last 30 years. ‘Tiny wiggles’ have been interpreted to represent either short-period polarity intervals or large-scale fluctuations in the ancient field intensity. We present palaeomagnetic evidence from a sedimentary record from the North Pacific Ocean, which demonstrates that two short, but clearly resolvable, polarity zones, in addition to a probable geomagnetic excursion, occur within Chron C5n.2n (9.92–10.95 Ma) where three ‘tiny wiggles’ have been reported on marine magnetic anomaly profiles. Relative palaeointensity data indicate that the field collapsed prior to and during the reversals (and during the excursion) but that it recovered to higher field intensities within the polarity intervals before collapsing to low values at the succeeding polarity transition. This indicates that some ‘tiny wiggles’ represent real short-period geomagnetic polarity intervals, while others may represent geomagnetic excursions. The existence of such short polarity intervals confirms the predictions of statistical analyses of geomagnetic reversal frequency and indicates that ‘tiny wiggles’ represent the maximum resolution of geomagnetic polarity intervals in marine magnetic anomaly records.

Paleomagnetism of the Late Triassic Petrified Forest Formation, Chinle Group, western United States: Further evidence of “large” rotation of the Colorado Plateau

Paleomagnetic poles and magnetostratigraphy were obtained from the late Carnian and early Norian Petrified Forest Formation. The remanent magnetization characteristics of this variegated claystones formation are correlated closely with stratal color; red strata display univectorial magnetizations directed to the origin of orthogonal axes plots between 300° and 630°C, whereas progressively greater overlap in magnetization component stabilities is exhibited by purple, blue, and green strata. Rock‐magnetic, petrographic, and microprobe investigations indicate that detrital titanohematite is the carrier of much of the characteristic magnetization, with a smaller contribution from magnetite in green and portions of the red strata. Eight magnetic polarity intervals were observed in three quarters of the formation (155 m) good correlation is observed with the coeval portion of the Newark Supergroup. Paleopoles from the late Carnian Blue Mesa and the early Norian Painted Desert Members of the Petrified Forest Formation are identical and agree well with other Chinle Group paleopoles from the Colorado Plateau; the tight cluster formed by all poles indicates that little or no apparent polar wander (APW) with respect to North America occurred during the Late Triassic (late Carnian through Rhaetian time). In addition, Chinle Group paleopoles derived from strata located on the North American (NA) craton also are tightly grouped and indicative of a paucity of significant APW through the Late Triassic. With that same observation from the coeval Newark Supergroup, three independent Late Triassic data sets from different tectonic settings in NA indicate minimal or no APW during more than 20 m.y. The combination of the vertebrate fauna and magnetostratigraphy within the Chinle Group provides a high degree of age control, allowing comparisons among coeval Colorado Plateau (CP) and NA craton paleopoles at about a 250–500 kyr level. CP Chinle Group paleopoles are displaced 9±3° clockwise from cratonic paleopoles. The Chinle Group is one of five sets of paleomagnetically sampled Mesozoic strata shown by magnetostratigraphy and/or biostratigraphy to have been deposited coevally on both the CP and the NA craton; paleopoles from the five sets indicate that CP paleopoles have been displaced a total of 10±3° relative to NA cratonic counterparts. Furthermore, the wealth of paleomagnetic and biostratigraphic data now available from both the Chinle and Newark strata demonstrates that nothing is sacred in NA tectonics: Newark Basin paleopoles are displaced 9±3° counterclockwise from western cratonic poles.

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.

Magnetostratigraphy of the Kavaalani section (southwestern Turkey): Consequence for the origin of the Antalya calcareous nappes (Turkey) and for the Norian (Late Triassic) magnetic polarity timescale

A magnetostratigraphic study of the Kavaalani section of uppermost Carnian to Upper Norian age, located in the Antalya Calcareous Nappes (southwestern Turkey), reveals nineteen polarity intervals. This pattern correlates very well with two other polarity sequences obtained from the same nappe system (Bolücektasi Tepe and Kavur Tepe) if these sections were deposited in the same (northern) hemisphere. This new interpretation changes our previous conclusions regarding the southern hemisphere origin of the magnetic remanence of the Kavur Tepe section. The paleomagnetic data obtained from the Kavur Tepe and the Kavaalani sections therefore reflect large (∼180°) internal rotations within the Antalya nappes. These nappes were likely formed close to the northern tip of the Arabian promontory. We propose a revised yet still preliminary version of the Norian magnetic polarity sequence.

Comment on the Lau Basin Cobb Mountain records by Abrahamsen and Sager

The Cobb Mountain Subchron is one of the few short polarity intervals for which transition records have been obtained both from a wide geographical distribution and from different types of paleomagnetic recorders. These records exhibit VGP paths that are remarkably similar. Given the wide geographic distribution of these sites, the similarity in VGP paths is most simply interpreted as the presence of very large scale, if not dipolar, symmetries in the transitional fields. Only the records from western California and the Lau Basin exhibit VGP paths that are markedly different from the others. New dates for the Punaruu lavas suggest that they recorded a new geomagnetic cryptochron rather than the Cobb Mountain [Singer, B.S., Hoffman, K.A., Chauvin, A., Coe, R.S., 1999. Dating transitionally magnetized lavas of the late Matuyama Chron: Toward a new 40 Ar/ 39 Ar timescale of reversals and events. J. Geophys. Res. 104, 679–693]. This new age indicates a revised correlation of some of the other California records whose ages agree better with the Punaruu cryptochron. A re-examination of the Lau Basin records reveals that much of the difference between these and the other records results from a bias introduced when the paleomagnetic data were smoothed. Reanalysis of these records shows that the Lau Basin VGP paths are very similar to those obtained from western Pacific, and the North Atlantic.

Astronomical forcing of sedimentary cycles in the middle to late Miocene continental Calatayud Basin (NE Spain)

A high-resolution cyclostratigraphic and magnetostratigraphic study was carried out on cyclically bedded successions of middle Miocene lacustrine to distal alluvial fan-floodplain deposits from the Calatayud basin, in northeast Spain. Eight (partially overlapping) subsections near the village of Orera are correlated in detail using distinct sedimentary cycle patterns or by following marker beds in the field. Together they form the Orera Composite Section (OCS). Sedimentary cycles are recognised on at least two different scales. The basic small-scale cycles in the OCS consist of an alternation of grey and, occasionally, red clays with white, dolomite-rich, carbonate beds. They are arranged in larger-scaled, so-called large-scale cycles based on repetitive changes in the overall carbonate–clay lithology. Two other, but less distinct, types of intermediate scale cyclicity are also recognised. In terms of depositional environment, the cycle hierarchy is interpreted to represent periodic lake expansion over the palaeo-alluvial fan-floodplain area. The palaeomagnetic results yield a reliable magnetostratigraphic record, which confirms the cyclostratigraphic correlations between the subsections in detail. Rock magnetic experiments reveal that haematite is the main magnetic carrier of the primary component. The magnetostratigraphy of the OCS is correlated straightforwardly with the geomagnetic polarity time scale. This resulted in an age of 10.7–12.8 Ma for the entire succession, which is supported by fossil micromammal findings. In addition, it also reveals the presence of two, possibly three, short new polarity intervals. Based on the number of cycles in the OCS the average periodicity of the basic small-scale cycles is approximately 23 000 yr, while the large-scale cycle indicates a periodicity of 400 000 yr. This suggests that these sedimentary cycles are controlled by astronomically induced climate changes causing lake-level fluctuations and thus resulting in the deposition of carbonate-clay cycles. The continental sequences of the OCS provide a unique opportunity to extend the astronomical polarity time scale into the middle Miocene. The overlap of such continental sequences in the Mediterranean area with time-equivalent astronomically induced marine sequences is fundamental for establishing marine–continental, bed-to-bed, correlations and for understanding regional climate change.

An age constraint on Gulf of California rifting from the Santa Rosalia basin, Baja California Sur, Mexico

Marine rocks of the Santa Rosalia basin, Baja California Sur, were sampled in a pilot study to determine their suitability for magnetostratigraphy and geochronology with the goal of providing an age constraint on Gulf of California rifting. Progressive demagnetization of samples from the Boleo Formation, the earliest marine sequence overlying the deeply eroded basement, reveals a high-coercivity characteristic remanent magnetization (ChRM) in addition to a low-coercivity overprint. The ChRM appears to be a primary magnetic remanence with stratigraphically bound normal- and reversed-polarity directions. A single ^(40)Ar/^(39)Ar isotopic age of 6.76 ± 0.90 Ma (2σ) was obtained for the cinta colorada, a tephra deposit of reversed paleomagnetic polarity within the Boleo Formation. The age of the cinta colorada is refined by calculating isotopic age probabilities for each of the reversed-polarity intervals of the geomagnetic polarity time scale (GPTS) in the ±2σ range 5.86–7.66 Ma. The interval with the highest probability is 6.57–6.94 Ma. In conjunction with the isotopic age, preliminary magnetostratigraphy of the Boleo Formation is correlated with the GPTS in order to further delineate the onset of marine sedimentation. The most likely correlation yields an age of 6.93–7.09 Ma (GPTS subchron C3Bn) for the base of the Boleo Formation and 6.14–6.27 Ma for the top. This correlation, indicating an average sedimentation rate of 28 ± 4 cm/k.y., could be significantly altered if a more thorough magnetostratigraphy proved the existence of additional polarity intervals in the Boleo Formation. However, even if the isotopic age of the cinta colorada is used as the only age constraint, the result is consistent with data from the northern Gulf of California and shows that rifting started much earlier than the ca. 3.6 Ma commencement of sea-floor spreading at the mouth of the Gulf of California. The deposition of the Boleo Formation was probably related to an early phase of gulf rifting caused by a change in Pacific–North American plate motions.

Assessing the fidelity of palaeomagnetic records of geomagnetic reversal

A major difficulty facing the study of geomagnetic polarity reversals lies in interpreting palaeomagnetic records of polarity transitions. These records are the sole source of information about what happens to the field as it reverses. In addition to comparison of records of the same reversal from different types of recorders to check for accuracy, we need some internal measure that can be used as a gauge of the temporal resolution provided by the data. This paper explores methods using the extent to which secular variation is recorded in the full polarity intervals bounding polarity transitions to estimate the temporal extent to which the transitional record should be interpreted. Cumulative dispersion and two autocorrelation methods are evaluated using datasets representative of different end–member palaeomagnetic records of secular variation and polarity transitions.

Stratigraphy and Magnetostratigraphic/Faunal Constraints for the Age of Sauropod Embryo-Bearing Rocks in the Neuquén Group (Late Cretaceous, Neuquén Province, Argentina)

The stratigraphy and age of a sauropod nesting ground containing the first definitive embryonic remains of sauropods preserved inside their eggs is analyzed. The fossil locality, called Auca Mahuevo, occurs in the Anacleto Member of the Río Colorado Formation in Neuquén Province, Argentina. The 5 m thick interval of overbank mudstones containing the fossilized eggs and embryos occurs near the middle of a 35 m sequence of thin, fluvial, concretionary sandstones and thicker units of silty sandstone. Flooding of shallow stream channels deposited overbank silt and mud on the eggs, killing the embryos and initiating the process of fossilization. Egg fragments containing patches of fossilized integument were found as float weathering out of the mudstone on local flats. Complete eggs containing embryonic bones and teeth were quarried from a steep ridge where the mudstone was exposed.Twelve paleomagnetic samples collected throughout the lower 30 m of the section establish the presence of a Reverse geomagnetic polarity interval. This constitutes the first magneto-stratigraphic characterization for this part of the Río Colorado Formation and for the late Cretaceous sequence of formations that comprise the Neuquén Group. Biochronologic age estimates for the Río Colorado fauna combined with the Reverse polarity determinations for the fossiliferous sediments in the Anacleto Member argue for an age younger than the long Cretaceous C34 Normal, which ends at the upper boundary of the Sartonian and older than the late Campanian. The Reverse interval containing the fossils at Auca Mahuevo is therefore considered to be early or middle Campanian in age, most likely correlative with C33R between 83.5 and 79.5 Ma.