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1.
The Big Sandy Formation in western Arizona consists of up to 65 m of clays, silts, sands and volcanic ashes that were deposited in a lacustrine paleoenvironment. Three paleomagnetic samples were collected from each of 54 sites spaced at stratigraphic intervals of no more than 5 m. After laboratory studies of pilot samples to determine their paleomagnetic characteristics, all other samples were measured for their NRM and then demagnetized in alternating fields of 150 Oe. After statistical filtering and a hierarchical site classification, 48 sites were used to interpret the magnetic polarity zonation.Vertebrate fossils from the Big Sandy Formation are collectively termed the Wikieup Local Fauna, and indicate a late Hemphillian Land Mammal “Age”. Three fission-track (zircon) dates from the Big Sandy Formation yielded a mean age of 5.5 ± 0.2 m.y. B.P. Using these radiometric data to calibrate the magnetic polarity zonation, it appears that the Big Sandy Formation spans late Epoch 6 to early Gilbert time (late Miocene to early Pliocene). The Wikieup Local Fauna is compared to two other roughly contemporaneous mammalian assemblages from New Mexico and Texas. Faunal differences previously thought to represent a very late Hemphillian age for the Wikieup Local Fauna are apparently related to ecogeographic variation and not time.  相似文献   

2.
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.  相似文献   

3.
Abstract The Upper Cenozoic sedimentary sequences drilled at Sites 1150 and 1151, Ocean Drilling Program Leg 186, enabled establishment of radiolarian zonation and calibration of the age of bioevents in the forearc area of the northern Japan Islands. The sequences were divided into nine zones from the Pleistocene Botryostrobus aquilonaris Zone to the Upper Miocene Lipmanella redondoensis Zone at Site 1150, and 11 zones from the Pleistocene Stylatractus universus Zone to the Middle Miocene Dendrospyris? sakaii Zone at Site 1151. These zones correlate successfully with the studied sequences of many of deep‐sea cores in the Northwest Pacific Ocean and with some sections of onshore Japan. Of 67 important radiolarian bioevents recognized during the study, 29 Pleistocene to Upper Miocene events were directly tied to the geomagnetic polarity time scale through the well‐defined paleomagnetic polarity records, and 21 Upper Miocene events were calibrated based on the diatom biostratigraphy. Of these events, 24 geographically widespread events were selected to test synchroneity and usefulness as time‐horizons within the mid‐to‐high latitude of the Northwest Pacific, involving eight other offshore and onshore sections. Examination showed that most of the zonal boundary events are synchronous within the considered region, and that many diachronous events, most of which are eliminated from the zonal scheme, are unreliable events linked to rare and sporadic occurrences of the species. Radiolarian biostratigraphy of the studied cores clearly indicates three major hiatuses in the Middle Pleistocene, Late Miocene and late Middle Miocene. The latter two hiatuses can be correlated to two global oceanic hiatuses, NH6 and NH3, respectively.  相似文献   

4.
Over 1000 m of fluvial molasse, exhibiting a stable detrital remanent magnetization, is exposed in a mammal-bearing sequence in the Upper Siwalik Group of the Pabbi Hills, Pakistan. The magnetic polarity chronology reveals that the sequence records a time period of 2.6 m.y., extending from the early Gauss Normal Epoch into the Brunhes Normal Epoch. During this period, sedimentation rates increased upward in time from 0.25 m/1000 yr to 0.45 m/1000 yr. The sudden disappearance of red beds and a change in the lithoclastic composition of basal channel sands suggests that about 800,000 years ago the primary source area began shifting from the metamorphic terrane of the Himalayan Orogen to a more local sedimentary terrane on the folded margins of the Himalayan foredeep. About 500,000 years ago the anticlinal Pabbi Hills attained surface expression. Uplift continued at a minimum rate of 1 m/1000 yr.A local Pliocene/Pleistocene boundary based on the Olduvai Normal Event is clearly recognized. Local fossil finds reveal thatEquus, diagnostic element of the Pinjor faunal zone, appeared locally about 1.8 m.y. ago and thatHipparion, a faunal element of the Tatrot and earlier faunal zones, persisted locally until at least 3.0 m.y. ago.  相似文献   

5.
Abstract Shipboard and shore‐based investigation on siliceous and calcareous microfossil biostratigraphy, magneto‐stratigraphy and tephrostratigraphy identified numerous datum events from the sedimentary sequences of Sites 1150 and 1151 drilled on the forearc basin of northern Japan by the Ocean Drilling Program Leg 186. Some 83 datum events were selected to construct new age–depth models for the sites. Based on the reliable magneto‐stratigraphy from the Pleistocene to the Upper Miocene, which were correlated to the standard geomagnetic polarity timescale, and on excellent records of diatom and radiolarian biostratigraphy throughout the sequences, the shipboard age model was revised. Major revisions referred to stratigraphic position of the Miocene–Pliocene boundary that has been shifted more than 200 m downward in each sequence. The age–depth relations of the forearc sites represent drastic changes in the sedimentation rate—extremely high (40 cm/k.y. on average) in the Early Pliocene and low (less than 2 cm/k.y. on average) in the Middle Miocene—and several hiatuses exist throughout the sequence. The drastic changes can be related mostly to changes in diatom sedimentation and the tectonics of the Japanese Island Arc. Local ages for some foraminiferal, calcareous nannofossil and radiolarian bioevents are estimated from the age–depth models at each site. These newly calibrated bioevents and biozones as well as established diatom biostratigraphy are incorporated into the updated magneto‐biochronologic timescale, which will contribute to an improvement in biochronologic accuracy of Neogene sediments in northern Japan and adjacent areas.  相似文献   

6.
Sea-floor spreading data from the Southwest Pacific have recently been used to predict the Cainozoic geological history along the Indo-Australian/Pacific plate boundary. Geologic and sedimentologic data pertaining to this plate boundary where it crosses southern New Zealand, as the Alpine Fault, are summarised and discussed. It is concluded that there is a close accord between the plate-tectonic predictions and South Island Cainozoic geological history. In particular, (1) no Cainozoic plate boundary traversed the New Zealand region prior to 38 m.y. B.P. (late Eocene); (2) transcurrent movement on the Alpine Fault took place largely between ca. 30 m.y. B.P. (middle Oligocene) and ca. 10 m.y. B.P. (late Miocene); and (3) the period 10 m.y. B.P. to present corresponds to a phase of oblique compression, continental collision, and mountain building along the Alpine Fault sector of the plate boundary. There is a close correlation between the sites and histories of Cainozoic sedimentation and this tectonic timetable.  相似文献   

7.
Fission-track dates and planktonic microfossil datum levels provide a revised chronology for the marine Late Cenozoic of southern California. In southern California, the Pliocene/Pleistocene boundary has been placed at the first appearance of Globorotalia truncatulinoides within the Pico Formation, Balcom Canyon, Ventura County. A fission-track age on glass shards from the Bailey Ash close to this level yields a result of 1.12 ± 0.36 m.y. B.P. (millions of years before present). In tropical deep-sea cores, however, G. truncatulinoides has been shown to evolve within the Gilsa paleomagnetic event with an estimated age of 1.8 m.y. B.P. Thus, the first appearance of G. truncatulinoides in southern California is cryptogenic and probably related to the delayed migration into this region of water-mass conditions suitable for this species.Two volcanic ashes from the upper part of the Malaga Formation, Malaga Cove, Los Angeles County, yielded fission-track dates on glass shards of 4.42 ± 0.57 m.y. B.P. (lower ash) and 3.364 ± 0.69 m.y. B.P. (upper ash). These dates, in addition to inferred paleomagnetic ages of planktonic microfossil datum levels suggest that the Delmontian Stage of California ranges in age from ～6 to ～3 m.y. B.P. Therefore, the Miocene/Pliocene boundary considered by Berggren and Van Couvering to be ～5 m.y. B.P. must lie in the lower Delmontian Stage but paleontologic criteria for its recognition in California are not yet available.  相似文献   

8.
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.  相似文献   

9.
The ages of reversals of the Earth's magnetic field have been dated accurately back to 3.4 m.y. ago. Between this time and the age of the Cretaceous-Tertiary boundary, dates for reversals have been calculated assuming a constant rate of sea-floor spreading in the South Atlantic Ocean. The presence of thick piles of lava flows in Iceland allows us to produce independent evidence for the ages of reversals back to 13.0 m.y. B.P. Because of the extreme regularity of extrusion of these lava flows, the measurement of their magnetic polarity allows us to correlate the lava flows which were extruded during the polarity intervals associated with sea-floor spreading anomalies. The measurement of many K-Ar ages on these lava flows also allows us to compare the ages of reversals assumed by the linear interpolation between the ages of 3.4 m.y. and the Cretaceous-Tertiary boundary at 66.5 m.y., with those suggested by the radiometric dates. We find that in general the assumption of constant spreading has been a good one, but suggest a small change in the ages of reversals, amounting to an increase of about 0.27 m.y. in ages of reversals between 8.5 and 13.0 m.y. ago.  相似文献   

10.
Palaeomagnetic and palaeontological studies on samples withdrawn from a 250-m-long unorientated core from Reggio di Calabria, Italy (38°N, 16°E) are described. The lower 200 m of core penetrated off-shore clays and visual examination of the split sections of core indicated no obvious breaks in deposition. After alternating field demagnetization in 200 Oe the palaeomagnetic inclination log shows the Brunhes/Matuyama boundary at 110 m below which the Jaramillo and Olduvai Events within the Matuyama Epoch are identified. This interpretation of the magnetostratigraphy is supported by palaeontological evidence. The rate of accumulation of clay is estimated to have ranged between about 60 and 190 mm/kyr with an overall average through the Matuyama Epoch of about 90 mm/kyr. Assuming that this average rate continued through the Brunhes Epoch, the age of the top of the clay unit is estimated to be about 90,000 yr B.P. About 5 m from the unconformable top surface of the clay, a split sequence of reversed inclinations is interpreted as a record of the Blake Event, and the overall average deposition rate implies that its duration may have been as long as 50,000 yr. No other reversed event is recorded by the palaeomagnetic inclination log through the Brunhes Epoch, though there are four horizons where shallow positive inclinations are recorded.  相似文献   

11.
A total of 163 cores have been taken from a maximum of 40 separate lavas in three separate sections of the Jökuldalur, southwest of Egilsstadir, Iceland, and subjected to paleomagnetic analysis and some K-Ar dating. Previous work on the sections by McDougall and Wensink (1966) led to the establishment of the Gilságeomagnetic polarity event, with an age of about 1.60 m.y., during the reversed polarity Matuyama epoch. This earlier study described a possible reversely magnetized lava separating the Gilsáevent from a second normal polarity lava, perhaps representing the Olduvai event. Such a possibility was subsequently a source of speculation from diverse sources concerning the polarity history for the lower Matuyama. The present study indicates clearly that there is no second normal polarity event represented in the sections. Only one normal polarity event is therefore represented in the lower Matuyama of the Jökuldalur, and the age of the lavas involved is confirmed to be approximately 1.58 ± 0.08 m.y. Because of uncertainties in the interpretation of the original K-Ar results from Olduvai Gorge, it is still not possible to be certain that the Olduvai and Gilsáevents are separated in time. An incidental result of the survey is evidence to show that, contrary to recent suggestions by Einarsson (1972), there is no substantial hiatus between the major lower parts of the section and lavas believed to represent extrusions after a regional tilting and peneplanation.  相似文献   

12.
New paleomagnetic data from shallow-marine sediments of the Ichishi Group suggest a clockwise tectonic rotation of Southwest Japan in the Middle Miocene. Samples have been collected from mud or tuff layers at 17 sites. Stability of remanent magnetization has been examined by using alternating field and thermal demagnetization. The polarity sequence, composed of four normal and seven reversed polarity sites, is correlated to Polarity Epoch 16 (15.2–17.6 Ma), based on micropaleontological assignment of the upper Ichishi Group to Blow's Zone N8. The mean paleomagnetic direction of the 11 sites shows an anomalous declination toward the northeast. This result suggests that Southwest Japan was subjected to a clockwise rotation through 45° since 16 Ma. The clockwise rotation can be explained by the drift of Southwest Japan associated with the spreading of the Japan Sea during the Middle Miocene.  相似文献   

13.
Investigation of four sections of Tithonian to Valanginian pelagic limestone have led to refinement of the correlation of calpionellid zones to the magnetic polarity time scale. The correlations are self-consistent but differ slightly from those previously published. The discrepancy with the published correlation from the Bosso section [1] has been resolved by re-evaluation of the biostratigraphy of this sequence.The revised correlation places the base of theChitinoidella Zone in the lower part of polarity chron CM21n, the base of Zone A near the top of CM20n, the A/B boundary at the base of CM18, theB/C boundary in the upper part of CM17, theC/D boundary at the top of CM16 and theD/E boundary at the top of CM14.  相似文献   

14.
Hiroki Hayashi 《Island Arc》2004,13(1):318-331
Abstract The present paper describes the general outline of Neogene paleoceanographic changes in the northwestern Pacific by means of planktonic foraminiferal assemblages. Planktonic foraminiferal fossils occur commonly in the upper Miocene to lower Pleistocene sediments of Hole 1151A, Ocean Drilling Program Leg 186 in the forearc basin off northeast Japan, with the exception of 11 barren intervals. These barren intervals are explained as a result of dissolution under organic decomposing processes. Three assemblages of planktonic foraminifers were identified by Q‐mode cluster analysis. The succession of the assemblages can be divided into four paleoceanographic stages: (i) warm‐temperate Tortonian; (ii) cold‐temperate Messinian to lower Pliocene; (iii) warm climatic optimum in the middle part of the Pliocene; and (iv) strong glacial–interglacial oscillation of the upper Pliocene to the lower Pleistocene. Three short warming events—namely, the late Miocene climatic optimum 3, the Miocene–Pliocene boundary and the middle Pliocene events—and a short cooling event of the late Miocene could be determined in the studied section of Site 1151.  相似文献   

15.
Tectonically, the large-scale right-lateral strike-slip movement along the Red River fault zone is char-acterized at its late phase with the southeastward extension and deformation of the Northwestern Yunnan normal fault depression on its northern segment, and the dextral shear displacement on its central-southern segment. Research of the relations between stratum deformation and fault movement on the typical fault segments, such as Jianchuan, southeast Midu, Yuanjiang River, Yuanyang, etc. since the Miocene Epoch shows that there are two times dextral faulting dominated by normal shearing occurring along the Red River fault zone since the Miocene Epoch. The fission track dating (abbrevi-ated to FT dating, the same below) is conducted on apatite samples collected from the above fault segments and relating to these movements. Based on the measured single grain’s age and the con-fined track length, we choose the Laslet annealing model to retrieve the thermal history of the samples, and the results show that the fault zone experienced two times obvious shear displacement, one in 5.5 ± 1.5 MaBP and the other in 2.1± 0.8 MaBP. The central-southern segment sees two intensive uplifts of mountain mass in the Yuanjiang River-Yuanyang region at 3.6―3.8 MaBP and 1.6―2.3 MaBP, which correspond to the above-mentioned two dextral normal displacement events since the late Miocene Epoch.  相似文献   

16.
Two large-diameter cores from the Greater Antilles Outer Ridge have confirmed the Blake episode 0.1 m.y. B.P. as a genuine paleomagnetic reversed interval at least in that region. The feature is clearly defined in more than 125 stably magnetized specimens of Last Interglacial abyssal brown clay, but its precise age and duration cannot yet be estimated reliably from these or other available data. During the reversed period, positions of the virtual geomagnetic pole lay about 20° from the south geographic pole, with polarity transitions traversing paths in the eastern hemisphere. Each core also recorded a normally polarized excursion within the Blake feature. Because the most acceptable Blake episode data presently span only a small geographic area, they are insufficient for distinguishing between global and local geomagnetic models for the feature.  相似文献   

17.
Tectonically,the large-scale right-lateral strike-slip movement along the Red River fault zone is characterized at its late phase with the southeastward extension and deformation of the Northwestern Yunnan normal fault depression on its northern segment,and the dextral shear displacement on its central-southern segment.Research of the relations between stratum deformation and fault movement on the typical fault segments,such as Jianchuan,southeast Midu,Yuanjiang River,Yuanyang,etc.since the Miocene Epoch shows that there are two times dextral faulting dominated by normal shearing occurring along the Red River fault zone since the Miocene Epoch.The fission track dating (abbreviated to FT dating,the same below) is conducted on apatite samples collected from the above fault segments and relating to these movements.Based on the measured single grain's age and the confined track length,we choose the Laslet annealing model to retrieve the thermal history of the samples,and the results show that the fault zone experienced two times obvious shear displacement,one in 5.5 ±1.5 MaBP and the other in 2.1±0.8 MaBP.The central-southern segment sees two intensive uplifts of mountain mass in the Yuanjiang River-Yuanyang region at 3.6-3.8 MaBP and 1.6-2.3 MaBP,which correspond to the above-mentioned two dextral normal displacement events since the late Miocene Epoch.  相似文献   

18.
Magnetic lineations in the Pacific Jurassic quiet zone   总被引：1，自引：0，他引：1  
Magnetic anomalies of low amplitude (<100 gammas) are present in the Jurassic magnetic quiet zone of the western Pacific Ocean. These small anomalies are lineated and can be correlated among the Phoenix, Hawaiian and Japanese lineation patterns. Thus, they represent seafloor spreading that recorded some sort of magnetic field phenomena prior to magnetic anomaly M25 at 153 m.y. B.P. The most likely possibility is that they represent a series of late Jurassic magnetic field reversals that occurred during a period of anomalously low magnetic field intensity. We propose a time scale of magnetic reversals between 153 and 158 m.y. B.P. to account for these anomalies and suggest that the dipole magnetic field intensity increased by a factor of about four from 160 to 140 m.y. B.P. in the late Jurassic.  相似文献   

19.
Thick sequences of relatively undisturbed Plio-Pleistocene sediments in the Wanganui Basin, North Island, New Zealand consist of well exposed silts, clayey silts, sandstones, rare limestones, and several tephras. Oriented specimens were collected from a section more than 2500 m thick and palaeomagnetic measurements were made using A.C. demagnetisations in fields up to 35 mT. With the aid of tephrochronology the age of the upper sequence is now well established and falls within the Matuyama epoch. The lower two-thirds of the section except for the basal 500 m is predominantly normally magnetised and is interpreted as a very extended sequence of the Gauss epoch. The lowest 500 m then represents the Gilbert epoch. The Plio-Pleistocene boundary, as defined at Vrica, Italy, falls within the upper part of the section studied, in the Upper Nukumaruan stage. For the first time a reliable correlation is made with the international boundary, using as intermediaries the palaeomagnetic and tephrostratigraphy of deep-sea cores from the southwest Pacific.

As a result of the high deposition rate (of the order of 1.2 m/ky) and the apparent lack of unconformities, the temporal resolution is high; short-lived magnetic events are detected, especially in the lower Matuyama and upper Gauss epochs. These generally correlate well with events reported from other extended sections.  相似文献   


20.
Detailed radiolarian biostratigraphy in the Plio-Pleistocene was analyzed by using samples from IODP Site U1340 that was drilled to a core depth of 604 m in the southern Bering Sea. A total of 227 species belonging to 102 genera were identified. Based on the distributions of the radiolarian index species at Site U1340, five radiolarian zones since the Pliocene were established in the southern Bering Sea for the first time, and 25 radiolarian bioevents were recognized. Their ages were estimated on the basis of the age-depth plot that was constructed by the synthetical datum of the effective biostratigraphic and magnetostratigraphic events. The radiolarian zones at Site U1340 were systematically compared with those in its adjacent regions since the late Early Pliocene, which further improved and interpreted the biostratigraphic datum as well as their correlations in the middle-high latitude of the North Pacific. In addition, the comparative results of radiolarian zones show that Botryostrobus aquilonaris Zone emended in this paper is equivalent to the upper part of the same zone defined by Hays, 1970, and Druppatractus irregularis-Dorydruppa bensoni Zone as well as Spongodiscus sp. Zone, newly proposed in this paper, are well correlated with Cycladophora sakaii Zone and Stylatractus universus Zone in the subarctic North Pacific, respectively.  相似文献   

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