Late Jurassic magnetic polarity sequence

Polarity reversal sequences have been observed in the Brushy Basin Member of the Morrison Formation (Upper Jurassic, Kimmeridgian-Tithonian age) at two sites in western Colorado separated by 80 km. The two polarity reversal sequences display a good correlation of relative lengths and number of polarity intervals and thus may provide a significant correlation tool in this laterally variable formation.The whole polarity sequence observed in the Morrison Formation correlates very well with the oldest portion of lineated anomalies of the sea floor. The sedimentary record on land and the anomaly pattern of the oceanic crust both show the same relative lengths and approximate number of polarity intervals, as well as ratio of reversed to normal polarity.     

Late Jurassic/Early Cretaceous magnetic stratigraphy from the sümeg section,Hungary

The pelagic limestones exposed at Sümeg appear to represent continuous deposition from Kimmeridgian through Berriasian. Detrital magnetite and haematite pigment are the carriers of remanence in the red and non-red limestones in the lower part, while magnetite becomes predominant in the upper half of the section. Thermal demagnetization has succesfully removed overprint magnetizations, and a well-defined magnetic stratigraphy has been obtained. The Late Jurassic/Early Cretaceous mixed polarity interval is correlated with the sequence of geomagnetic reversals derived from oceanic magnetic anomalies.     

Integrated Upper Carnian to Lower Norian biochronology and implications for the Upper Triassic magnetic polarity time scale

We summarize the ammonoid, conodont and halobiid biochronology of the Upper Carnian to Lower Norian, based on a discussion of data in the Alps, Sicily, Balkans, Turkey, Himalayas and Timor. With this integrated biostratigraphic scale, the Pizzo Mondello section (Sicily) can be recalibrated and the Carnian-Norian boundary more precisely located there. As a result, the magnetostratigraphy of this section is now in good agreement with previous results from Turkey, although the latter series are more condensed. Cross-correlation of available magnetostratigraphic data from marine Tethyan sections allow us to construct a composite Upper Carnian to Upper Norian geomagnetic polarity time scale (GPTS). This GPTS leads us to question previously proposed magnetobiostratigraphic and chronostratigraphic correlations within the Upper Triassic Newark non-marine sedimentary sequence.     

Late Cretaceous magnetic anomalies in the North Atlantic

An identification of anomalies 31–34 is presented for the North Atlantic. North of the Azores-Gibraltar Ridge this implies a revision of the identification of the magnetic anomalies older than anomaly 26. DSDP site 10 in the western North Atlantic appears to be located on the old end of anomaly 33. The relative spacings of anomalies 29–34 in the North and South Atlantic, North and South Pacific and Indian Oceans are compared and the estimated relative widths of the magnetic polarity intervals in the Late Cretaceous are revised.     

A paleoisthmus linking southern South America with the Antarctic Peninsula during Late Cretaceous and Early Tertiary

Paleontological and stratigraphical data indicate that a link between terminal southern South America and the Antarctic Peninsula probably was an irregularly narrow continuous land from Late Cretaceous (Campanian) through the Eocene period. In paleogeographical feature it is more like modern Isthmus of Panama. A paleoisthmus called the isthmus of Scotia is hypothesized as a type of connection between them. It lasted about 35 Ma and extended about 700–900 km long and 100–200 krn wide. This isthmus played an important role in biotic dispersal and migra-tion among South America, Antarctica, Australia and New Zealand blocks during this interval. Project supported by the Science and Technology Con~rnission, the Chinese Academy of Sciences and the National Natural Science Foundation of China (Grant No. 49572076).     

Late Cenozoic magnetic polarity stratigraphy in the Jiudong Basin,northern Qilian Mountain

Late Cenozoic sediments in the Hexi Corridor, foreland depression of the Qilian Mountain preserved reliable records on the evolution of the Northern Tibetan Plateau. Detailed magnetic polarity dating on a 1150 m section at Wenshushan anticline in the Jiudong Basin, west of Hexi Corridor finds that the ages of the Getanggou Formation, Niugetao Formation and Yumen Conglomerate are >11-8.6 Ma, 8.6-4.5 Ma and 4.5-0.9 Ma respectively. Accompanying sedimentary analysis on the same section suggests that the northern Tibetan Plateau might begin gradual uplift since 8.6-7.6 Ma, earlier than the northeastern Tibetan Plateau but does not suppose that the plateau has reached its maximum elevation at that time. The commencement of the Yumen Conglomerate indicates the intensive tectonic uplift since about 4.5 Ma.

     

Jurassic and Early Cretaceous Radiolaria of the Lower Amurian Terrane: Khabarovsk region, far east of Russia

New data on biostratigraphy, sedimentology and tectonics of the Russian Far Eastern region (Lower Amurian terrane) are presented. This study shows that sedimentary sequence of the terrane consists of interbedded Radiolaria-bearing siliceous and volcaniclastic sediments spanning an interval of over 90 million years. It is shown that accumulation of radiolarian deposits on an oceanic plate was associated with alkaline (intraplate) volcanism in the Jurassic, while the plate was drifting, and with some arc volcanism during the Early Cretaceous. The younger siliceous rocks contain volcaniclastic material and indicate that the studied sequence approached the trench in the Early Cretaceous (Hauterivian-Barremian) and became accreted in the late Albian–early Cenomanian. We describe and illustrate radiolarian species extracted from 21 samples. A taxonomic list of 194 taxa and nine plates of Jurassic–Early Cretaceous Radiolaria are presented.     

Paleomagnetism and radiochemical age estimates for Late Brunhes polarity episodes

Several reversed polarity magnetozones occur within deep-sea sediment core CH57-8 from the Greater Antilles Outer Ridge, within sediment of latest Pleistocene/Late Brunhes age. The uppermost reversed interval spanning 31 data points coincides with the X faunal zone of the Last Interglacial Period. Radiochemical dating of cores CH57-8 and KN25-4 has shown that all the reversed polarity magnetozones are significantly younger than the Brunhes/Matuyama boundary at 0.7 m.y. B.P. A variation of the excess²³⁰Th method was used, in which²¹⁰Po and²³⁸U were the actual radionuclides measured. In a third core from the Mid-Atlantic Ridge, our²¹⁰Po results were similar to those which others obtained earlier by direct²³⁰Th measurements.     

Geomagnetic reversal frequency since the Late Cretaceous

Models of geomagnetic reversals as a stochastic or gamma renewal process have generally been tested for the Heirtzler et al. [1] magnetic polarity time scale which has subsequently been superseded. Examination of newer time scales shows that the mean reversal frequency is dominated in the Cenozoic and Late Cretaceous by a linearly increasing trend on which a rhythmic fluctuation is superposed. Subdivision into two periods of stationary behavior is no longer warranted. The distribution of polarity intervals is visibly not Poissonian but lacks short intervals. The LaBrecque et al. [2] polarity time scale shows the positions of 57 small-wavelength marine magnetic anomalies which may represent short polarity chrons. After adding these short events the distribution of all polarity intervals in the age range 0–40 Myr is stationary and does not differ significantly from a Poisson distribution. A strong asymmetry develops in which normal polarity chrons are Poisson distributed but reversed polarity chrons are gamma distributed with indexk = 2. This asymmetry is of opposite sense to previous suggestions and results from the unequal distribution of the short polarity chrons which are predominantly of positive polarity and concentrated in the Late Cenozoic. If short-wavelength anomalies arise from polarity chrons, the geomagnetic field may be more stable in one polarity than the other. Alternative explanations of the origin of short-wavelength marine magnetic anomalies cast doubt on the inclusion of them as polarity chrons, however. The observed behavior of reversal frequency suggests that core processes governing geomagnetic reversals possess a long-term memory.     

Late Cretaceous genesis of the Kula plate

We propose that the Pacific-Kula ridge began spreading approximately 85 m.y. B.P., during Late Cretaceous time. Extrapolation of the Great Magnetic Bight backwards in time results in an implausible ridge configuration. This implies that plate velocity vectors for the Pacific, Kula, and Farallon plates were not constant during this interval. Evidence for splitting of the Kula plate from the Pacific plate along the Chinook trough is the relationship between the north-striking Amlia and Adak fracture zones, the Chinook and Emperor troughs, and the magnetic lineations south of the Aleutian trench. If this hypothesis is correct, it will require that Mesozoic reconstructions of the Pacific basin and their relation to Cenozoic reconstructions be re-examined. A previously unrecognized Mesozoic plate may be required. We propose calling this the Izanagi plate.     

地球磁极倒转与白垩纪超静磁带的非线性分析

地球磁场多次发生南北(正负)磁极位置的变换和白垩纪超静磁带(CNS)的异常现象,这已为大家所公认.但造成这种异常现象的原因,则是迄今未能很好解答的一个难题. 应用非线性理论对地球磁极倒转和白垩纪超静磁带进行了分析, 认为超静磁带事件意味着地球核幔相互作用和外核流体运动可能处于能量最低的状态,地球磁场系统通过不断地与外界交换物质和能量,维持一种空间或时间的有序结构.在121～83Ma期间,无外星撞击地球引起地磁极性倒转,可能是白垩纪超静磁带出现的原因之一.地球磁场极性的随机倒转具有混沌运动的自逆转特性,混沌理论给地磁极性倒转提出了一个简明的动力机制解释.     

Palaeomagnetism of Cretaceous and Jurassic volcanic rocks in west Sicily

The mean palaeomagnetic pole position obtained from Upper Cretaceous rocks in west Sicily is at 21°N, 100°E (A₉₅ = 15°), and at 38°N, 67°E (A₉₅ = 31°) obtained from Middle Jurassic rocks. These pole positions are completely different from comparable pole positions for southeast Sicily and Africa and imply a clockwise rotation of west Sicily since the Upper Cretaceous of about 90° relative to southeast Sicily and Africa and also a clockwise rotation of about 60° relative to “stable” Europe. The sense of rotation of west Sicily is opposite to any known rotation of other crustal blocks in the central Mediterranean.     

Late Jurassic ammonite evolution and paleoenvironment of the Russian Platform

The stratigraphic record shows a considerable decrease in ammonoid taxonomic diversity and distinct changes of ammonite shell morphology toward end of the Jurassic in the Central Russian Basin. By the end-Volgian, ammonites were represented by only two genera, belonging to a single family Craspeditidae, which differ markedly from previous ammonite families in their shell form. The end-Jurassic decrease in ammonoid biodiversity started in the mid-Volgian and is mainly correlated with shallowing of the shelf.     

Magnetic polarity during the Middle Jurassic as recorded in the Summerville and Curtis Formations

Three-quarters of the Summerville Formation and a small portion of the Curtis Formation of central Utah have been sampled at close stratigraphic spacing. The two formations constitute an intertonguing marine-marginal marine, tidal flat sequence representing middle Callovian time. The magnetic polarity sequence observed is dominantly reversed with a few relatively short normal intervals. The age of these formations corresponds to some portion of time within the Jurassic oceanic quiet zone. The fact that normal polarity was not prominent in these formations casts some doubt upon the hypothesis that a long period of normal polarity was the cause of the quiet zone. Furthermore, a literature compilation indicates that normal polarity did not dominate the Jurassic as had been thought. The pole position determined from the Middle Jurassic Summerville Formation slightly revises earlier results and suggests a tiny amount of apparent polar motion relative to North American between Late Triassic and Middle Jurassic time.     

Jurassic and Lower Cretaceous radiolarian zonation in Japan and in the western Pacific

Abstract A radiolarian zonal scheme for the entire Jurassic and Lower Cretaceous using biostratigraphic data from both Japanese Island sections and the western Pacific seafloor is documented. The zonation is applicable to low and middle paleolatitude portions of the Paleo-Pacific ocean. Radiolarian bio-events such as the evolutionary first appearance biohorizon, first occurrence biohorizon, and last occurrence biohorizon were used to define zones. The 11 zones proposed are, in ascending order, Parahsuum simplum, Trillus elkhornensis, Laxtorum(?) jurassicum, Tricolocapsa plicarum, Tricolocapsa conexa, Stylocapsa(?) spiralis, Hsuum maxwelli, Pseudodictyomitra primitiva, Pseudodictyomitra carpatica, Cecrops sep-temporatus, and Acanthocircus carinatus zones. Preliminary age assignments for these zones are presented.     

The extension of the magnetic time scale in sediments of the Central Pacific Ocean

The extension of the magnetic reversal record back to the early Miocene is presented. This record is pieced together with the aid of microfloral analysis from three low sedimentation rate siliceous deep sea cores from the Equatorial Pacific.Nineteen Magnetic Epochs are now recognized from the earliest Miocene to the Present. By correlating the micropaleontological data in our cores with selected foraminiferal datums from DSDP Leg IX we correlate these datums with the following magnetic epochs: the Pulleniatina Datum occurs in the lower part of Epoch 5, the G. acostaensis Datum occurs in Epoch 11, the G. nepenthes Datum occurs in Epoch 12, the Orbulina Datum at the Epoch 15/16 boundary and the G. dissimilis Datum in the lower part of Epoch 16. The Early/Middle Miocene boundary (Orbulina Datum) is tentatively placed at the top of Epoch 16.     

A Quaternary geomagnetic instability time scale

Reversals and excursions of Earth's geomagnetic field create marker horizons that are readily detected in sedimentary and volcanic rocks worldwide. An accurate and precise chronology of these geomagnetic field instabilities is fundamental to understanding several aspects of Quaternary climate, dynamo processes, and surface processes. For example, stratigraphic correlation between marine sediment and polar ice records of climate change across the cryospheres benefits from a highly resolved record of reversals and excursions. The temporal patterns of dynamo behavior may reflect physical interactions between the molten outer core and the solid inner core or lowermost mantle. These interactions may control reversal frequency and shape the weak magnetic fields that arise during successive dynamo instabilities. Moreover, weakening of the axial dipole during reversals and excursions enhances the production of cosmogenic isotopes that are used in sediment and ice core stratigraphy and surface exposure dating. The Geomagnetic Instability Time Scale (GITS) is based on the direct dating of transitional polarity states in lava flows using the ⁴⁰Ar/³⁹Ar method, in parallel with astrochronologic age models of marine sediments in which oxygen isotope and magnetic records have been obtained. A review of data from Quaternary lava flows and sediments gives rise to a GITS that comprises 10 polarity reversals and 27 excursions that occurred during the past 2.6 million years. Nine of the ten reversals bounding chrons and subchrons are associated with ⁴⁰Ar/³⁹Ar ages of transitionally-magnetized lava flows. The tenth, the Gauss-Matuyama chron boundary, is tightly bracketed by ⁴⁰Ar/³⁹Ar dated ash deposits. Of the 27 well-documented geomagnetic field instabilities manifest as short-lived excursions, 14 occurred during the Matuyama chron and 13 during the Brunhes chron. Nineteen excursions have been dated directly using the ⁴⁰Ar/³⁹Ar method on transitionally-magnetized volcanic rocks and these form the backbone of the GITS. Excursions are clearly not the rare phenomena once thought. Rather, during the Quaternary period, they occur nearly three times as often as full polarity reversals.     

New paleomagnetic and magnetic fabric results for Early Cretaceous rocks from the Turpan intramontane basin,east Tianshan,northwest China

~~New paleomagnetic and magnetic fabric results for Early Cretaceous rocks from the Turpan intramontane basin,east Tianshan,northwest China~~