Magnetic Polarity Stratigraphy Research Articles

Magnetostratigraphy of Neogene Andean foreland-basin strata, lat 33 S, Mendoza Province, Argentina

Neogene nonmarine strata in the southernmost part of the Precordillera thrust belt of the Andes of Argentina accumulated during the Miocene and Pliocene in response to eastward advance of thrust sheets located to the west. To link the sequence of deformational events with the sedimentary record, five stratigraphic sections in the Mendoza Province were dated using magnetic polarity stratigraphy calibrated with 40 Ar- 39 Ar dates from interbedded tephras. These data establish age control on tectonic activity, volcanism, and deposition. The existence of a backarc foreland basin that developed prior to or ca. 16 Ma is recorded by the oldest strata, the ca. 15.7–12.2 Ma (Baksi time scale) Marino Formation. Deposition of this unit is linked to a major phase of thrust activity in the central part of the Aconcagua fold-and-thrust belt, in the Principal Cordillera. Initiation of deposition coincides with an eastward shift of the subduction-related magmatic foci to the Argentine Principal Cordillera. The overlying ca. 11.7–9.0 Ma La Pilona Formation is thought to record the initiation of exhumation of the Frontal Cordillera to the northwest of the study region. An extensive ash-rich unit, the 8.9 (locally 9.5?) to 8.7 Ma Tobas Angostura Formation, is correlated with the latest and probably the largest extrusive episode in the Principal Cordillera. Chronologic data constrain the timing of oil generation in the Cuyo basin and development of prospective closures. The new data also require that land mammalian fossils found in the Rio de los Pozos Formation are late Miocene, rather than Pliocene, as previously believed.

Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal

Research Article| March 01, 2000 Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal T. P. Ojha; T. P. Ojha 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar R. F. Butler; R. F. Butler 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar J. Quade; J. Quade 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar P. G. DeCelles; P. G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar D. Richards; D. Richards 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar B. N. Upreti B. N. Upreti 2Department of Geology, Tri-chandra Campus, Ghantaghar, Kathmandu, Nepal Search for other works by this author on: GSW Google Scholar Author and Article Information T. P. Ojha 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 R. F. Butler 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 J. Quade 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 P. G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 D. Richards 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 B. N. Upreti 2Department of Geology, Tri-chandra Campus, Ghantaghar, Kathmandu, Nepal Publisher: Geological Society of America Received: 13 Aug 1998 Revision Received: 18 Mar 1999 Accepted: 26 Mar 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (3): 424–434. https://doi.org/10.1130/0016-7606(2000)112<424:MPSOTN>2.0.CO;2 Article history Received: 13 Aug 1998 Revision Received: 18 Mar 1999 Accepted: 26 Mar 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T. P. Ojha, R. F. Butler, J. Quade, P. G. DeCelles, D. Richards, B. N. Upreti; Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal. GSA Bulletin 2000;; 112 (3): 424–434. doi: https://doi.org/10.1130/0016-7606(2000)112<424:MPSOTN>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The middle Miocene–Pliocene Siwalik Group was deposited in the Himalayan foreland basin in response to uplift and erosion in the Himalayan fold-thrust belt. Results of thermal demagnetization experiments on samples from the Siwalik Group in central and western Nepal demonstrate that laminated siltstones yield paleomagnetic data useful for tectonic and magnetostratigraphic studies. Sandstones and paleosols of the Siwalik Group, however, generally display highly erratic paleomagnetic behavior during thermal demagnetization. On the basis of these observations, siltstones from a well-exposed, 2423-m-thick section of the Siwalik Group in Khutia Khola, far western Nepal, were sampled for magnetic polarity stratigraphy. The Siwalik Group is composed of informal lower, middle, and upper members. Correlation of the resulting polarity stratigraphy with the geomagnetic polarity timescale indicates that the exposed section spans 13.30 to 7.65 Ma. The lower-middle Siwalik boundary occurs at 11.05 Ma, near the beginning of chron C5n. The rate of sediment accumulation increases upsection, similar to rate changes previously observed in the Pakistan Siwalik Group, and probably in response to increasing proximity of the Himalayan thrust belt. In the Khutia Khola section, a discordant declination indicates that this region has rotated about a vertical axis 16.6° counterclockwise with respect to the Indian subcontinent. Measurements of δ13C in paleosol carbonate indicate the predominance of C3 plants until 7.65 Ma, and the clear presence of C4 plants higher in the undated portion of the section. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal

Research Article| March 01, 2000 Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal T. P. Ojha; T. P. Ojha 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar R. F. Butler; R. F. Butler 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar J. Quade; J. Quade 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar P. G. DeCelles; P. G. DeCelles 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar D. Richards; D. Richards 1Department of Geosciences, University of Arizona, Tucson, Arizona 85721 Search for other works by this author on: GSW Google Scholar B. N. Upreti B. N. Upreti 2Department of Geology, Tri-chandra Campus, Ghantaghar, Kathmandu, Nepal Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (3): 424–434. https://doi.org/10.1130/0016-7606(2000)112<424:MPSOTN>2.0.CO;2 Article history received: 13 Aug 1998 rev-recd: 18 Mar 1999 accepted: 26 Mar 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation T. P. Ojha, R. F. Butler, J. Quade, P. G. DeCelles, D. Richards, B. N. Upreti; Magnetic polarity stratigraphy of the Neogene Siwalik Group at Khutia Khola, far western Nepal. GSA Bulletin 2000;; 112 (3): 424–434. doi: https://doi.org/10.1130/0016-7606(2000)112<424:MPSOTN>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract The middle Miocene–Pliocene Siwalik Group was deposited in the Himalayan foreland basin in response to uplift and erosion in the Himalayan fold-thrust belt. Results of thermal demagnetization experiments on samples from the Siwalik Group in central and western Nepal demonstrate that laminated siltstones yield paleomagnetic data useful for tectonic and magnetostratigraphic studies. Sandstones and paleosols of the Siwalik Group, however, generally display highly erratic paleomagnetic behavior during thermal demagnetization. On the basis of these observations, siltstones from a well-exposed, 2423-m-thick section of the Siwalik Group in Khutia Khola, far western Nepal, were sampled for magnetic polarity stratigraphy. The Siwalik Group is composed of informal lower, middle, and upper members. Correlation of the resulting polarity stratigraphy with the geomagnetic polarity timescale indicates that the exposed section spans 13.30 to 7.65 Ma. The lower-middle Siwalik boundary occurs at 11.05 Ma, near the beginning of chron C5n. The rate of sediment accumulation increases upsection, similar to rate changes previously observed in the Pakistan Siwalik Group, and probably in response to increasing proximity of the Himalayan thrust belt. In the Khutia Khola section, a discordant declination indicates that this region has rotated about a vertical axis 16.6° counterclockwise with respect to the Indian subcontinent. Measurements of δ13C in paleosol carbonate indicate the predominance of C3 plants until 7.65 Ma, and the clear presence of C4 plants higher in the undated portion of the section. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Paleomagnetism of the Nijo Group and its implication for the timing of clockwise rotation of southwest Japan.

A paleomagnetic study was conducted for well-dated volcanic rocks from the Middle Miocene Nijo Group in the Mt. Nijo area, southwest (SW) Japan. Samples were collected from 33 sites. Detailed stepwise demagnetization with both alternating-field and thermal techniques revealed well-determined site-mean directions from 29 sites. Tilt correction was applicable to 13 sites, yielding an overall mean direction of D=352.2°, I=51.7°, and α95=7.4° and a paleomagnetic pole at 84.3°N, 35.5°E, and A95=8.5°. The bedding-tilt test passed with 95% probability. Although the reversal test was negative, the presence of dual polarities and a study of angular standard deviation of virtual geomagnetic poles imply that the overall mean direction averages out, to a certain extent, the past geomagnetic secular variation. Magnetic polarity stratigraphy determines the age of the Nijo Group; the younger age limit is 14.6 Ma (top of polarity subchron C5ADr) and the older age limit is 15.2 Ma (base of polarity subchron C5Bn.2n). Comparison of the paleomagnetic pole with those from the Asian continent shows no rotation in the Mt. Nijo area since 14.6 Ma, suggesting that SW Japan has not experienced rotation since that time. We conclude that the clockwise rotation of SW Japan, thought to have occurred in Miocene time in relation to the opening of the Japan Sea, had already ended by 14.6 Ma. Two clockwise-deflected directions of the Middle Miocene rocks (Muro Pyroclastic Flow Deposit and Kumano Acidic Rocks) in the Kii Peninsula are probably instantaneous recordings of a geomagnetic field excursion or transition.

Age and provenance of loess in West Qinling

Magnetic polarity stratigraphy of loess in West Qinling (Mts.) demonstrates that loess began to deposit in West Qinling at ca. 800 kaBP. Its material may come from the nearby Tibetan Plateau to the west. This suggests that a sharp change in air circulation and surface environment on and around the Tibetan Plateau occurred at that time.

Revised geochronology of the Casamayoran South American Land Mammal Age: climatic and biotic implications.

Isotopic age determinations (40Ar/39Ar) and associated magnetic polarity stratigraphy for Casamayoran age fauna at Gran Barranca (Chubut, Argentina) indicate that the Barrancan "subage" of the Casamayoran South American Land Mammal "Age" is late Eocene, 18 to 20 million years younger than hitherto supposed. Correlations of the radioisotopically dated magnetic polarity stratigraphy at Gran Barranca with the Cenozoic geomagnetic polarity time scale indicate that Barrancan faunal levels at the Gran Barranca date to within the magnetochronologic interval from 35.34 to 36.62 megannums (Ma) or 35. 69 to 37.60 Ma. This age revision constrains the timing of an adaptive shift in mammalian herbivores toward hypsodonty. Specifically, the appearance of large numbers of hypsodont taxa in South America occurred sometime between 36 and 32 Ma (late Eocene-early Oligocene), at approximately the same time that other biotic and geologic evidence has suggested the Southern high latitudes experienced climatic cooling associated with Antarctic glaciation.

Paleomagnetic polarity reversals in Marinoan (ca. 600 Ma) glacial deposits of Australia: Implications for the duration of low-latitude glaciation in Neoproterozoic time

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Orbital tuning of geomagnetic polarity time-scales

Milankovitch climate cyclicity and magnetic polarity stratigraphy are being successfully combined as a powerful geochronometer in the astronomical polarity timescale (APTS). The APTS for 0{5.23 Ma has been rapidly accepted as the denitive chronology for the Pliocene and Pleistocene against which even high precision radiometric dating is now calibrated. Extensions of astronomical calibration to the late Miocene (5.23 Ma to ca. 10 Ma) in Mediterranean and Pacic marine sections agree to within about a 100 ka eccentricity, which amounts to an uncertainty of only ca .1 % of the age. Orbital eccentricity periods are thought to remain stable over very long times and thus provide the possibility of precise relative age control in the preNeogene. An APTS, for example, has been developed in a thick lacustrine section of Late Triassic age (ca. 202{233 Ma) on the basis of the 404 ka orbital eccentricity cycle modulating the expression of the precession climate cyclicity. Finally, there has been renewed speculation about an obliquity-modulated precessional geodynamo based on periodicities in relative palaeointensity data from Ocean Drilling Program sediment cores.

Paleomagnetism of the Distal Member of the New Glasgow Formation, Nova Scotia, Canada: New Constraints on Magnetic Polarity Stratigraphy near the Base of the Kiaman Superchron

The ca. 315–260‐Ma Kiaman Reversed (Permo‐Carboniferous) Magnetic Superchron is an important time marker that can be used for correlation of stratigraphic sequences from a local to a global scale. Earlier workers, from a paleomagnetic study of the Joggins stratigraphic section of the western Cumberland Basin of Nova Scotia, demonstrated that the base of the superchron lies above redbeds of the lowermost portion of the late Namurian–Westphalian A Boss Point Formation. Although a detailed sequence of reversals was observed below this point, no normal magnetic polarity zones were recognized above it. In our study, samples were collected from redbeds of the distal member of the Westphalian A New Glasgow formation of the eastern Cumberland Basin, which directly overlie the gray sandstones of the Boss Point Formation. Most sampling sites carry a normal polarity magnetization, probably acquired at about the time of deposition. Therefore, the base of the Kiaman Reversed Superchron lies within or above the New Glasgow formation. Either the top of the Boss Point Formation is diachronous between the two areas, or, more likely, the base of the superchron is significantly higher in the stratigraphic section than previously recognized, a conclusion that is important for both intrabasin and intercontinental stratigraphic correlations. It implies that the pattern of pre‐Kiaman magnetic reversals reported from below the Boss Point Formation in the Joggins section is significantly older than previously thought and, therefore, cannot be used for comparison with magnetic reversal patterns that closely underlie the base of the superchron at other localities in the Cumberland Basin or on other continents.

Magnetostratigraphy of Plio-Pleistocene sediments in a 1700-m core from Osaka Bay, southwestern Japan and short geomagnetic events in the middle Matuyama and early Brunhes chrons

A magnetic polarity stratigraphy spanning more than the past 3.2 Myr was determined for a long 1545 m continuous sedimentary sequence of marine, fluvial, and lacustrine deposits from the Osaka Basin, southwestern Japan. Additionally two short geomagnetic reversal events were identified: a short event at about 0.69 Ma lasting for about 7 kyr in the early Brunhes chron around the lower boundary of a marine clay deposited during a period of eustatic high sea level during the marine oxygen isotope stage 17; and another short Matuyama event (1.60–1.58 Ma) correlated with Stage 54 event, and the early stage of the Sangiran Excursion. Accumulation rate was 0.3–0.4 mm/yr from 3 to 2 Ma, and then increased by about 50% with a rate of 0.51 mm/yr during the Olduvai subchron. High values of >0.59 mm/yr were maintained from the Upper Olduvai boundary to the Brunhes–Matuyama boundary (from 1.77 to 0.78 Ma), with a peak at 0.74 mm/yr during the Jaramillo subchron. These changes in accumulation rate probably reflect the history of tectonic subsidence of the drilling site. The magnetic polarity stratigraphy provides a chronological framework for sedimentological, paleoenvironmental, tephrochronological, and tectonic studies of the Osaka Basin.

The magnetic polarity stratigraphy of a Hauterivian/Barremian carbonate sequence from southern Italy

SUMMARY Remanence directions, measured at 2 cm intervals along a composite 88 m bore-core, enable mean palaeomagnetic poles to be defined at 13.6°S, 25.2°W and 13.6°N, 154.8°E. The directions of remanence vary very smoothly away from each palaeomagnetic pole, extending more than 90° from them. This raises doubts about the physical meaning of polarity definitions based on the distance between virtual and mean palaeomagnetic poles. For practical purposes, intermediate polarity is defined as directions whose virtual poles lie more than 25° from the mean pole, enabling at least five normal subchrons to be specified within the upper predominately reversed quarter of the core and 11 reversed subchrons within the lower predominantly normal three-quarters of the core. The stratigraphic thickness between these subchrons shows a very high linear correlation (r>0.99) with the stratigraphic thickness of other terrestrial sequences and the distances between marine polarity sequences of comparable age. The analysed sequence contains wavelength spectra which, when transformed to the temporal realm, match periodicities determined for three marine magnetic anomaly profiles of similar age. These also match planetary orbital periodicities for the Cretaceous. These observations suggest that secular variations and polarity transitions are driven by common core processes whose surface expression is influenced by changes in the planetary orbits. Such detailed geomagnetic features enable far greater reliability in establishing magnetostratigraphic correlations and also enable them to be dated astronomically.

Paleomagnetic evidence that the central block of Salinia (California) is not a far‐traveled terrane

New paleomagnetic results from Late Cretaceous (75–85 m.y.) red beds on the central block of Salinia indicate that Salinia was located within 6° (in latitude) of its current cratonal North American position during the Late Cretaceous (after correction for Neogene San Andreas Fault transport). The red beds formed as alluvial‐fan overbank deposits with hematite cement deposited directly on Salinian granites in the La Panza Range. Paleomagnetic analysis shows two components of magnetization in the red beds, a low‐blocking‐temperature present‐day overprint residing in goethite and a high‐blocking‐temperature (&gt;600°) component residing in hematite. The hematite magnetization is a chemical remanent magnetization which formed soon after deposition during pedogenesis. The bedding‐corrected hematite remanence contains a magnetic polarity stratigraphy with antipodal normal and reversed directions. Twenty‐three Class I sites (α95 &lt; 20°) have an average hematite direction with inclination =54.4° and declination = 18.2° (α95 = 6.1°) after structural correction. These paleomagnetic data suggest that Salinia resided at about 35°N latitude during the Late Cretaceous, within 6° of its current location adjacent to cratonal North America. By contrast, a summary of paleomagnetic data from the Peninsular Ranges terrane and the Sur‐Obispo terrane, which are currently outboard of Salinia, shows northward transport of these terranes of 12° to 22° relative to their current locations in North America since the Cretaceous. The offsets increase systematically away from the craton with the most outboard Sur‐Obispo terrane (which is composed of accretionary prism and distal forearc material) showing the largest degree of northward translation.

Étude magnétostratigraphique préliminaire sur l'âge du bassin de Rudgai-Sibi (Nord-Est du Balouchistan, Pakistan) et ses implications tectoniques

A first magnetostratigraphic study was carried out in Neogene fluvial formations of the Rudgai (Sibi) basin in the Sulaiman Fold Belt. In order to date these units more precisely and to understand the tectonic implications for the formation of this basin, a total of 59 samples were analysed in the laboratory. The results show that a unique correlation is possible between the observed magnetic polarity stratigraphy and the magnetic polarity stratigraphy scale. This allows us to place the sequence between 16.5 and 14.8 Ma and to show that the molassic sediments are well diachronic between the Rudgai (Sibi) basin (16.5 Ma) the basins of the Potwar Plateau (10.1 Ma).

Neogene magnetic polarity stratigraphy of some fluviatile Siwalik sections, Nepal

SUMMARY The magnetostratigraphy of five new sections through the Neogene Siwalik Group of Nepal is presented. Rock magnetic experiments and detailed thermal demagnetization experiments prove that haematite is the carrier of a primary DRM or a PDRM. After stepwise thermal demagnetization of specimens from all sections, directions of characteristic remanent magnetization were obtained. The results show a positive reversal test and exhibit inclination errors of about 20d. Due to gaps in exposures and extremely variable demagnetization behaviour of the haematite-bearing sediments, the resulting polarity sequences are poorly defined for some parts of the sections. However, double-sampled parts of some sections yield similar results. A correlation with the Surai Khola section (Appel, Rosler & Corvinus 1991) is proposed, based on rock magnetic parameters. AMS results from three of the sections prove the existence of primary sedimentary magnetic fabrics in these sections with systematic orientations of minimum and maximum susceptibility axes. Correlation with a standard polarity timescale indicates that the ages of all new sections lie within the age limits of the Surai Khola section.

Paleomagnetism of Upper Triassic continental sedimentary rocks from the Dan River–Danville rift basin (eastern North America)

A magnetic polarity stratigraphy and a corresponding paleomagnetic pole position are reported from 113 sampling sites representing 3000 m of Upper Triassic continental sedimentary rocks that crop out in the Dan River‐Danville basin of North Carolina and Virginia. Characteristic magnetizations isolated by thermal demagnetization for either the hematite-bearing red siltstones or the interbedded magnetite-bearing gray to black mudstones of the Leakesville Formation are indistinguishable in mean direction and pass reversal tests. The magnetic polarity sequence consists of 11 magnetozones that vary from ∪100 m to 800 m in thickness and can be uniquely correlated within biostratigraphic constraints to magnetochrons E9n to E14n of the Newark geomagnetic polarity time scale. According to this correlation, the sampled section is the age equivalent of the uppermost Stockton, the entire Lockatong, and the lowermost Passaic formations of the Newark basin, and represents ∪7.5 m.y. of deposition. The late Carnian Dan River‐Danville paleopole is located at 55.4°N 100.1°E (A 95 = 1.9°), which is not significantly different from paleopoles reported from essentially coeval rocks in the Newark basin. Considering that the Dan River‐Danville and Newark basins are ∪600 km apart, the close agreement of the coeval paleopoles argues strongly for the overall tectonic coherence of these rift basins with respect to each other and, most probably, with respect to cratonic North America. Discordant latest Triassic paleopoles from the southwestern United States, which have tended to be attributed to fast apparent polar wander for North America in the Late Triassic and predict anomalously high paleolatitudes for eastern North America, are best accounted

A terrestrial auxiliary stratotype point and section for the Plio-Pleistocene boundary in the Turkana Basin, East Africa

The Turkana Basin of northern Kenya and southern Ethiopia preserves an extensive record of terrestrial sediments and fossils dating from the Oligocene through the Holocene. Of particular significance is the Plio-Pleistocene Omo Group, which documents changes in environments and faunas through the Pli-Pleistocene. Omo Group strata have been intensively studied for nearly three decades, and a tightly constrained stratigraphic framework has been developed. The lithostratigraphic sequence shows that a fluvial system dominated the basin for most of the past five million years, with six relatively short but significant intervals of lacustrine deposition. The sequence is punctuated by some 130 tephra, most of which have been geochemically characterized, and can be used as a basis for local and inter-basinal correlation. These tephra reach as far as the Gulf of Aden and the Indian Ocean, providing a means of tightly correlating the marine and terrestrial records. Isotopic dating and magnetic polarity stratigraphy allow good temporal control on the sequence, with over 30 control points in the interval from 4.35 to 0.6 Ma. Extensive paleontological collections from this context provide critical calibration of East African biostratigraphic markers. The Plio-Pleistocene boundary can be closely constrained in the Omo Group deposits based on magnetic polarity stratigraphy. This allows the boundary to be related to components of the stratigraphic framework recognized in the terrestrial sequence there, and thence related to other East African terrestrial localities by a variety of correlation tools. Establishment of the Omo Group sequence as an Auxiliary Stratotype Point for the Plio-Pleistocene boundary will thus allow close relation of these terrestrial localities on the basis of tephro- and biostratigraphy, the most useful correlation tools in the East African terrestrial record.

Middle Triassic magnetostratigraphy and biostratigraphy from the Dolomites and Greece

Magnetostratigrapic and biostratigraphic data across the Anisian/Ladinian (Middle Triassic) boundary were obtained from the Frötschbach/Seceda section from the Dolomites region of northern Italy, and the Vlichos section from the Greek island of Hydra, where the Aghia Triada published section was also resampled. The Frötschbach/Seceda section includes two radiometrically dated (UPb) tuff levels and covers two of the three chief candidates for the position of the base of the Ladinian, namely at the base of the Secedensis Zone or the subsequent Curionii Zone. The Aghia Triada section yields biochronological evidence for the base of the Secedensis Zone, whose significance is, however, critically discussed in the light of the magnetostratigraphic correlation with Frötschbach/Seceda. The Vlichos section can be correlated with Aghia Triada and Frötschbach/Seceda by means of magnetic polarity stratigraphy and sparse fossil occurrences. The satisfactory correlation of the magnetozones allows us to construct a composite geomagnetic polarity sequence tied to Tethyan ammonoid and conodont biostratigraphy for about a 2.4 Myr interval across the Anisian/Ladinian boundary.

Miocene calibration for calcareous nannofossils from low-latitude Ocean Drilling Program sites and the Jamaican conundrum

Prior work with marine sedimentary sections exposed on Jamaica had been interpreted as evidence for a curious pattern of diachrony between the tropics and subtropics in the ages of certain biostratigraphic zonal boundaries of middle to late Miocene age. We offer an alternative to the Jamaican low-latitude nannofossil calibration, using a combination of paleomagnetic, biostratigraphic, and lithostratigraphic data derived from tropical sections recovered by the Ocean Drilling Program (ODP) in the eastern Pacific and western Atlantic Oceans. Our nannofossil calibration is based on magnetic polarity stratigraphy obtained from the Pacific Ocean and is consistent with the notion that Milankovitch periodicities governed cyclic changes in the lithology of sediments from the Ceara Rise, a sea-floor high in the Atlantic Ocean located offshore of the Amazon delta. Our results thus appear to be representative of the tropics and so call into question the ages that had been assigned to nannofossil zonal boundaries based on results from Jamaican sections.

2.5-million-year-old stone tools from Gona, Ethiopia.

The Oldowan Stone tool industry was named for 1.8-million-year-old (Myr) artefacts found near the bottom of Olduvai Gorge, Tanzania. Subsequent archaeological research in the Omo (Ethiopia) and Turkana (Kenya) also yielded stone tools dated to 2.3 Myr. Palaeoanthropological investigations in the Hadar region of the Awash Valley of Ethiopia, revealed Oldowan assemblages in the adjacent Gona River drainage. We conducted field work in the Gona study area of Ethiopia between 1992 and 1994 which resulted in additional archaeological discoveries as well as radioisotopic age control and a magnetic polarity stratigraphy of the Gona sequence. These occurrences are now securely dated between 2.6-2.5 Myr. The stone tools are thus the oldest known artefacts from anywhere in the world. The artefacts show surprisingly sophisticated control of stone fracture mechanics, equivalent to much younger Oldowan assemblages of Early Pleistocene age. This indicates an unexpectedly long period of technological stasis in the Oldowan.

Magnetic polarity stratigraphy of late Oligocene to middle Miocene mammal-bearing continental deposits in Central Anatolia (Turkey)

Magnetic polarity stratigraphy of late Oligocene to middle Miocene mammal-bearing continental deposits in Central Anatolia (Turkey)