Magnetostratigraphic Scale Research Articles

Magnetostratigraphy of the Permian–Triassic Boundary in the Lower Reaches of the Vetluga River, East European Platform, Nizhny Novgorod Region

We present the results of detailed paleomagnetic studies of six Permian-Triassic boundary sections in the central part of the East European Platform, located in the lower reaches of the Vetluga River: Astashikha, Voskresenskoe, Znamenskoe, Prudovka, Sosnovka, and Sukhoborka. The paleomagnetic data, which meet the modern quality standards for laboratory processing, together with the results of biostratigraphic studies, allow for the development and substantiation of magnetostratigraphic scales for each section, as well as for their correlation and the construction of a magnetic polarity scale for the Permian-Triassic sedimentary complex of the Vetluga River. Rock-magnetic characteristics are determined for each of the studied sections, and paleomagnetic poles of the East European Platform are calculated for the Late Permian and Permian-Triassic boundary.

Study of marine Paleogene diatom assemblages from the Kamchatka region: results of the last decade

Review of results obtained on fossil diatom assemblages studied from marine Paleogene sections in Kamchatka region in the last decade are presented. The first data for Kamchatka on direct correlation of Eocene diatom assemblages with complexes of calcareous plankton (the Il’pinskii Peninsula stratigraphic section, northeast Kamchatka), and on correlation of Oligocene diatom assemblages with the magnetostratigraphic scale (the Kvachina Bay section, west Kamchatka) are emphasized.

Верхнедевонские отложения (франский ярус) северо-запада Русской платформы: палеомагнитные данные

Paleomagnetic determinations for the Devonian strata of the Main Devonian Field, available in the international paleomagnetic data base, do not make it possible to construct both detailed magnetostratigraphic scales and reliable trajectories of the apparent polar wander path of the paleomagnetic pole. This is primarily due to the insufficient amount of data that determine modern reliability criteria. Obtaining more complete series of reliable paleomagnetic determinations is one of the important tasks of paleomagnetic studies. The paper presents new paleomagnetic determinations from a collection of rocks of the Frasnian stage of the Upper Devonian (Ilmen and Bureg Beds of the Semiluky Formation), sampled on the southern shore of Lake Ilmen, east of the village. Korostyn in 2009. Magnetomineralogical studies were carried out to determine the magnetic minerals of carriers of natural remanent magnetization. Three characteristic components of natural remanent magnetization have been identified. Two components correspond to the Late Paleozoic magnetization reversal (in the Carboniferous and Permian times). The third, bipolar component of the Frasnian age passes the reverse polarity test of the geomagnetic field.. The obtained position of the paleomagnetic pole by the bipolar component of the natural remanent magnetization is consistent with the available data on the section of the Late Upper Devonian of the Main Devonian Field.

Magnetostratigraphy and cyclostratigraphy of the Tithonian interval from the Vaca Muerta Formation, southern Neuquén Basin, Argentina

A cyclostratigraphic and paleomagnetic study was carried out in the lower part of the Vaca Muerta Formation, Los Catutos Member (Lower to lowermost Upper Tithonian) at the southern region of the Sierra de la Vaca Muerta, Neuquén Basin, west-central Argentina. The Vaca Muerta Formation is known for its unconventional resource potential and its exceptional behaviour as a world-class source rock. The studied section is c. 90 m thick and is characterized by decimetre-scale rhythmic alternations of marls, mudstones and limestones, showing a well-ordered hierarchy of cycles. Elementary cycles, bundles and superbundles were recognized with frequencies within the Milankovitch spectrum band. A total of 280 standard specimens collected from 35 paleomagnetic sampling horizons, dated with ammonites, were analyzed. The quality and primary origin of the characteristic magnetization was proved through the corresponding reversal test. A magnetostratigraphic scale was thus constructed that comprises 5 reverse and 4 normal polarity intervals, spanning the Andean Virgatosphinctes andesensis (ex mendozanus) (Lower Tithonian) to Windhauseniceras internispinosum (lowermost Upper Tithonian) ammonite Zones. Polarities were correlated with the GPTS 2016 using the equivalences between the Andean and Standard ammonite zones which, allowed to interpret that Los Catutos section was deposited between M22r to M20n Subchrons. By means of the correlation between the cyclostratigraphic and magnetostratigraphic scales, it was possible to date the Tithonian succession with unprecedented precision. Rock magnetic studies indicate that the remanence is carried by titanomagnetite.

Biostratigraphy and magnetostratigraphy of the upper Tithonian–Berriasian of the Crimean Mountains

Our data from studies over many years on upper Tithonian–Berriasian strata of the Crimean Mountains are summarised. Their zonal subdivision has been significantly refined using ammonites and foraminifera, as well as on the basis of calpionellid, ostracod and dinocyst distributions. We have been able to document the presence of all standard Tethyan zones in the Berriasian, i.e., the Jacobi, Occitanica and Boissieri Zones, identified on the basis of foraminifera, ostracods and dinocysts. Based on calpionellids (families Chitinoidellidae and Calpionellidae) in Eastern Crimea, three standard zones were identified: Chitinoidella (Dobeni and Boneti Subzones; Tithonian), Crassicollaria (Remanei and Massutiniana subzones; Tithonian) and Calpionella (Alpina and Elliptica subzones; Berriasian). Tithonian and Berriasian calpionellid assemblages were identified in Southwestern Crimea. A magnetostratigraphic scale for the upper Tithonian–Berriasian has been also developed, thus corroborating a continuous succession of magnetic chrons from M20 through M14. The existence of the M16n.1r subchron (‘Feodosiya’) is substantiated; it should be included into the Geomagnetic Polarity Time Scale. The base of polarity chron M18r appears to be the most likely choice among other palaeomagnetic intervals to determine the lower boundary of the Cretaceous System, because it is close to the base of the Grandis Subzone in Tethyan sections and the Chetae Zone in Boreal sections.

Magnetostratigraphy of the Upper Jurassic–Lower Cretaceous from Argentina: Implications for the J-K boundary in the Neuquén Basin

A systematic sedimentologic and paleomagnetic study was carried out in the Vaca Muerta Formation, cropping out in the northern Neuquén Basin, west-central Argentina. The studied section is c. 280 m-thick and represents a carbonate ramp system bearing ammonites that indicate Late Jurassic–Early Cretaceous ages. The Vaca Muerta Formation is one of the most important unconventional hydrocarbon reservoirs in the world and its thorough study has become a relevant target in Argentina. The J-K boundary is comprised within this unit, and although it is well-dated through biostratigraphy (mainly ammonites), the position of particularly the boundary is yet a matter of hot debate. Therefore, the systematic paleomagnetic and cyclostratigraphic study in the Vaca Muerta Formation was considered relevant in order to obtain the first Upper Jurassic–Lower Cretaceous magnetostratigraphy of the southern hemisphere on the first place and to precise the position of the J-K boundary in the Neuquén Basin, on the other. Biostratigraphy is well studied in the area, so that paleomagnetic sampling horizons were reliably tied, particularly through ammonites. Almost 450 standard specimens have been processed for this study distributed along 56 paleomagnetic sampling horizons that were dated using ammonites. Paleomagnetic behaviours showed to be very stable, and their quality and primary origin have been proved through several paleomagnetic field tests The resultant magnetostratigraphic scale is made up of 11 reverse and 10 normal polarity zones, spanning the Andean Virgatosphinctes mendozanus (lower Tithonian) to Spiticeras damesi Zones (upper Berriasian). These polarity zones were correlated with those of the International Geomagnetic Polarity Time Scale 2012 and 2016 through the correlation between Andean and Tethyan ammonite zones. Cyclostratigraphy on the other hand, proved to be quite consistent with the magnetostratigraphy. Through the correlation of the resultant paleomagnetic and cyclostratigraphic data, it was possible to date the section with unprecedented precision, and therefore, to establish the position of the Jurassic-Cretaceous boundary. The paleomagnetic pole calculated from the primary magnetization is located at: Lon = 191.6°E, Lat = 76.2°S, A95 = 3.5°, indicating a c. 24° clockwise rotation for the studied section, which is consistent with structural data of the region.

Magnetostratigraphy of the Middle–Upper Cambrian Verkhnyaya Lena Group and Lower Ordovician Ust-Kut formation in the southern Siberian Platform

The available paleomagnetic data on the Verkhnyaya Lena Group from different areas of the southern Siberian Platform are revised. The group rests unconformably upon the Lower Cambrian strata and is overlain by Lower Ordovician rocks, which determines conditionally the age of its red-colored deposits. Paleomagnetic correlation of composite sections through the region using defined zones of normal and reversed magnetic polarity serves as a basis for development of the magnetostratigraphic scale for the Verkhnyaya Lena Group. The scale includes nine magnetic zones, which play the role of markers; seven of them are traceable in all the examined sections of the southern Siberian Platform. By the distribution of zones with normal (N) and reversed (R) polarity, the magnetostratigraphic scale is subdivided into three parts. Its lower part is represented by reversed polarity, which is characteristic of the second half of the Lower Cambrian. The middle part is characterized by frequently alternating zones with normal and reversed polarity corresponding to the Middle Cambrian. The upper part of the scale corresponds to the interval of reversed polarity characteristic of the Upper Cambrian and Lower Ordovician. The Middle–Upper Cambrian boundary is located near the last N–R reversal of the geomagnetic field in the Cambrian. The magnetostratigraphic scale includes nine orthozones united into three superzones, which are attributed to two hyperzones of magnetic polarity.

New data on the most ancient early quaternary glaciation in Gornyi Altai

The data presented in this work introduce substantial corrections into the Upper Cenozoic stratigraphy and geological history of Gornyi Altai. They provide evidence for the most ancient Early Pleistocene glaciation in this region. This follows from finds of faceted boulders buried in the Bashkaus Formation. Pale-omagnetic investigations revealed in the latter a wide zone of negative polarity corresponding to the Matuyama Chron in the magnetostratigraphic scale and made it possible to correlate the formation with the Lower Pleistocene in the standard stratigraphic scale. It is shown that global cooling at the beginning of the Quaternary Period stimulated development of glaciation on the southern slope of the Kuraiskii Range exceeding in size its present-day scale.

LATE CARNIAN-EARLY NORIAN AMMONOIDS FROM THE GSSP CANDIDATE SECTION PIZZO MONDELLO (SICANI MOUNTAINS, SICILY)

A small collection of ammonoids from the Upper Triassic Scillato Formation at Pizzo Mondello (Agrigento, Sicily) is studied. The specimens were collected in a framework of a project aimed at providing an integrated high-resolution bio-chronostratigraphic support to the Upper Carnian-Norian magnetostratigraphic scale defined at this site, that is located in an historical area from which G.G. Gemmellaro collected the Upper Triassic of ammonoids monographed at the beginning of the XX century. The specimens from Pizzo Mondello were bed-by-bed sampled and represent the first collection of Upper Triassic ammonoids described from Sicily since Gemmellaro time. Quite several levels of the Pizzo Mondello section yielded ammonoids, but very few levels have provided more than one specimen. Due to the scarcity of specimens the taxonomic analysis has been complex because Gemmellaro, who described 166 new species, did not explain the variability of many of his taxa. Sixsteen taxa belonging to eleven genera are described. They include Placites sp. ind. , Discotropites plinii (Mojsisovics), Anatropites sp., Microtropites cf. paronai , Metathisbites cf. affinis , Hyattites aff. praefloridus , Projuvavites boehmi (Gemmellaro), Projuvavites inflatus (Gemmellaro), Gonionotites cf. italicus , Gonionotites aff. recuperoi , Dimorphites noricus n. sp., Dimorphites selectus Mojsisovics, Dimorphites sp., Pregriesbachites n. gen. , P. bukowskii (Gemmellaro), Discophyllites insignis . Among the new taxa, Dimorphites noricus n. sp. formalizes the nomen nudum “ Dimorphites n. sp. 1” quoted in literature for several years, that is index of the lowest subzone of the Norian stage. The small collection document the Discotropites plinii and Gonionotites italicus subzones of the uppermost Carnian Spinosus Zone and the Dimorphites noricus and D. selectus subzones of the Jandianus Zone, the first zone of the Lower Norian. This chronostratigraphic classification firmly tie the Pizzo Mondello succession to the Tethyan chronostratigraphic scale, and is crucial for the calibration of the Halobia and conodont bioevents identified in the section. The chronostratigraphic correlations based on ammonoids of the Pizzo Mondello section with Feuerkogel (Austria), Jomsom (Nepal), West Union Canyon (Nevada, USA) and Black Bear Ridge (British Columbia, Canada) is discussed. These are the most complete ammonoid-bearing sections in the world spanning the Carnian/Norian boundary, but all of them show a discrete distribution of ammonoid-bearing beds across the boundary. Ammonoid will not provide the primary marker event for the definition of the GSSP of the Norian, but they are crucial for the selection of the most significant events.

Magnetostratigraphy of Upper Permian sediments in the southwestern slope of Pai-Khoi (Khei-Yaga River section): Evidence for the global Permian-Triassic crisis

The study of rock samples from the Upper Permian Khei-Yaga River section revealed an r-n-r-n-r magnetic polarity succession based on the prefolding characteristic component of natural remanent magnetization. With account for stratigraphic and previous magnetostratigraphic data on Lower Triassic rocks from the Khei-Yaga River section, the examined strata of the Pechora Group (Silova Formation) may be compared with magnetic zones R1P2u and N1P3t in the magnetostratigraphic scale of European Russia. The gap in the paleomagnetic record, which corresponds in the examined section to the interval of the mid-Severodvinian Stage to basal Induan Stage, Zone N1T included, is estimated to be 10 Ma long. It is assumed that this hiatus represents one of the local signs of the global Permian-Triassic crisis.

Relation between geomagnetic field intensity and peculiarities of its variations with inversion frequency

The assumption that the variation of the ancient geomagnetic field (GF) is governed by processes in the Earth’s core is usually accepted without question. Numerical modeling of the evolution of αω -dynamo showed that change in the characteristics of the GF (paleointensity variations and inversion frequency) should correlate [1, 2]. However, the search for the correlation of paleointensity and inversion frequency has not been successful up to the present time owing to the lack of paleomagnetic data [3, 4]. Our reconstructions of paleointensity in the Jurassic and Cretaceous based on sedimentary rocks widened significantly the information dataset about this characteristic of the geomagnetic field. The summary of the author’s data on paleointensity obtained for the sedimentary rocks and world data based on thermomagnetized rocks provided a new basis for constructing the dynamics of the GF intensity and investigating the correlation between the characteristics of the magnetic field. Reconstruction of the fragments of GF intensity was based on the analysis of sedimentary rock collections gathered on the Russian Plate and adjacent territories [5, 6]. Previously, these collections were used for constructing the Middle Jurassic‐Cretaceous regional magnetostratigraphic scale [7]. The results of determinations of paleointensity based on thermomagnetized rocks were taken from the database (DB) at http://www. brk.adm.yar.ru/palmag/index.html. The information about inversions is mainly taken from [8]. The Albian‐ Coniacian interval is an exception. The frequency of the Albian‐Coniacian inversions was plotted from the more complete data in [7].

Magnetostratigraphy of Paleogene deposits in Kamchatka

Paleomagnetic characteristics of several Paleogene sections in Kamchatka (Il’pinskii Peninsula, Bering Island, Chemurnaut Bay, Mametcha Bay) are considered. The sections are correlated with due account for biostratigraphic data, and possible correlation of magnetic polarity zones distinguished in the sections with the international magnetostratigraphic scale is presented.

Geomagnetic field reversals: Main results and basic problems

[2] The geomagnetic field reversals seem to be among the most interesting phenomena discovered by the science in the 20th century. Naturally, the process of their operation attracted the attention of geoscientists who began to study them at the end of the 1950s to the beginning of the 1960s [Momose, 1958; Nomura, 1963; Petrova and Rybak, 1963; Sigurgeirsson, 1957; Van Zijl et al., 1962]. The characteristics of the geomagnetic field during its reversals, that is, during its transitions from one polarity to another, were and still are of great interest to the geoscientists, because the phenomenology of the processes of the destruction and reconstruction of the stationary magnetic moment, apparently reflecting the characteristics of the processes that violate the mechanism of its generation and reconstruction, might provide information for the operation of the dynamo mechanism, might help to understand its physical essence and broaden our knowledge about the structure of the Earth’s deep envelopes, and of the processes operating in them in the scale of geological time. Moreover, the discovery of the fact that each individual inversion has its own distinctive features which can be used to recognize it in the paleomagnetic rock sequences, would increase the value of the magnetostratigraphic scale. The second line of research did not give the expected results: inversions turned out to be indistinguishable in terms of the patterns of their operations, which are necessary for using them in stratigraphic studies. Yet, the work done in this line of research resulted in the

The Middle Triassic magnetostratigraphy from the Peri-Tethys basin in Poland

The paper presents the first complete magnetic polarity scale obtained for the Middle Triassic of the northern Peri-Tethys area. The Roetian and Muschelkalk deposits of southern Poland (Upper Silesia, the Holy Cross Mountains) were studied paleomagnetically. The obtained paleopole ‘A’ (52°N, 143°E) fits well to the Middle Triassic (ca. 240 Ma) segment of the Stable European APWP. A set of various quality characteristic directions isolated in 106 samples was used for construction of the composite polarity scale. This Peritethyan scale has been tied with conodont zonation and referred to the scale erected earlier in the Tethyan realm. Comparison of both scales indicates that the whole Roetian succession in the southern Polish basin should be correlated with the latest Olenekian. The normal marine sedimentation ceased in the Polish basin as early as in the late Fassanian time. The obtained magnetostratigraphic scale combined with the sequence stratigraphic framework enables a reliable chronological correlation of the late Olenekian–Ladinian succession over the entire Peri-Tethys Basin.

Magnetostratigraphy of the lower Triassic volcanics from deep drill SG6 in western Siberia: evidence for long-lasting Permo-Triassic volcanic activity

The deep drill hole SG6 in western Siberia (66°N, 78.5°E) penetrated 1.1 km of lower Triassic basalts, which are possibly an extension of the central Siberian Permo– Triassic flood basalt province. About 300 samples of these basalts were progressively demagnetized and measured. Principal component analysis often shows multiple magnetizations carried by haematite and magnetite. The corrected mean inclinations are +77° and −77° for the haematite component. A magnetostratigraphic scale was derived and showed a N–R–N–R–N succession. This is quite different from the Noril’sk and Taimyr typical polarity scale, R–N. The basalts found in the SG6 deep drill hole are slightly younger than those of central Siberia and Taimyr. They correspond to middle–upper Induan age, whereas the Noril’sk and Taimyr sections correspond to an uppermost Permian and lower Induan age. Altogether they indicate that, after a high output rate of volcanic material near the Permo–Triassic boundary, this activity slowed down drastically on the Siberian platform and Taimyr, but persisted for several million years in western Siberia.

BIOCHRONOLOGY OF SELECTED MAMMALS, MOLLUSCS AND OSTRACODS FROM THE MIDDLE PLIOCENE TO THE LATE PLEISTOCENE IN ITALY. THE STATE OF THE ART

The Authors have elaborated four range charts of mammalian (large and micro), molluscs and fresh-water and brackish ostracodes faunas, for the selected Plio-Pleistocene fossiliferous localities of the Italy. A new Mammal Age (Aurelian) correlatable to late Middle and Late Pleistocene has been defined. Inside this age two Faunal Units (Torre in Pietra and Vitinia) have been defined as characteristic for Early and Middle Aurelian, while no gisements have been chosen for the late Aurelian. Biochronological units are calibrated on magnetostratigraphic and isotopic scales and by radiometric datings.

Permian to Early Triassic magnetostratigraphy from the Central European Basin in Poland: Implications on regional and worldwide correlations

Permian–Early Triassic rocks sampled in 15 outcrops (93 hand samples) and 11 unoriented boreholes (630 hand samples) from the Polish part of the Central European Basin were studied paleomagnetically. In spite of extensive remagnetization, affecting 50% of samples from the boreholes, a magnetic polarity sequence was compiled. The polarity pattern is not only common for many sections from the investigated area but also convergent with the global synthetic magnetostratigraphic scale. The Lower Rotliegend (Early Permian) rocks contain a reversed polarity record, while the Upper Rotliegend (Late Permian)–Early Triassic sediments are characterized by mixed polarity magnetization. Within the Buntsandstein part of that mixed megazone 14 magnetic zones were found. Buntsandstein sedimentation started with a normal polarity zone, which may be compared with that of Early Griesbachian defined in several places in the world. The polarity record noted in the Middle Buntsandstein sediments could indicate significant stratigraphic gaps or condensations near the Middle Buntsandstein and Röt boundary. The comparison of the Upper Buntsandstein polarity with the Early–Middle Triassic magnetostratigraphic data from other basins suggests that the majority or even all the Röt sediments should be included in the Anisian.

Ancient platform carbonates with well-developed Lofer cyclicity: new candidates for magnetostratigraphy and geodynamically oriented palaeomagnetism

SUMMARY Young platform carbonates seem to preserve continuous records of the magnetic field. In contrast, ancient ones often fail to yield a meaningful palaeomagnetic signal. The frequent failure of the palaeomagnetic method in the platform carbonates of the AlpineMediterranean zone resulted in an investigation as to what extent the original lithology is responsible for this situation. The material selected for the study represents Late Triassic platform facies with welldeveloped Lofer cycles (metre-scale peritidal-subtidal cycles) of the Transdanubian Central Range in Hungary, with evidence for minimal late diagenetic, tectonic or secondary alteration processes. By selecting an area where the expected declinations for the Late Triassic are rotated in a counterclockwise manner by a significant amount, relative to the present north, we hope to recognize ancient natural remanence on the basis of both consistency and departure from the Earth's present field direction. For the study, different members of Lofer cycles were drilled at two sections (13 km apart). 71 and 38 samples, respectively, were taken from the two sections and fully oriented in the field. Using standard palaeomagnetic processing techniques, ancient natural remanence was isolated in all 'A', 'B' and 'C' members of the Lofer cycle, although different members were characterized by different magnetic parameters ('C' members were diamagnetic with very low NRM intensity; 'A' members had positive susceptibility and relatively strong NRM, and 'B' members intermediate properties). The mean palaeomagnetic directions isolated for the two sections, based on 58 and 30 samples, respectively, are D = 306, I = 47, k = 43, ay5 = 3 and D = 300, I = 52, k = 26, ay5 = 5 before and D, = 31 3, I, = and D, = 307, I, = 52 after tectonic correction. Both normal and reversed polarities were observed, and there were several polarity reversals, even in short sections. These results suggest that platform carbonates with Lofer cycles are likely to preserve primary remanence, thus they are suitable for magnetostratigraphy studies, if free from karstic cavities and fissures. The presence of 'A' members is very favourable for the preservation of the original magnetization (since general water circulation is inhibited by these members), but these may represent condensed intervals, thus the sedimentary record in these members is not necessarily complete. Despite the above limitations and also the possibility of gaps at sequence boundaries, platform carbonates are good candidates for future definition of magnetostratigraphic scales, for example in the Late Triassic; they also promise palaeomagnetic results of considerable tectonic importance, since platform carbonates with Lofer cycles are widespread in the Alpine-Mediterranean zone.

Magnetostratigraphy of Vindhyan Supergroup

Abstract In the absence of conventional radiometric dating and fossil evidence, magnetostratigraphy is considered to be a very powerful tool to correlate rock formations. Often the magnetozones are used as bench marks in correlation of rocks as the geomagnetic field reversals are ubiquitously synchronous. The Vindhyan sedimentation in the Indian stratigraphy represents a very important time period between 1400-400Ma with lithounits quite suitable for recovering the geomagnetic field signatures. With the recently obtained results from the Senui Group, palaeomagnetic field during the main Vindhyan Groups namely the Semri, Kaimur, Rewa and Bhander periods is available. It is quite interesting to note that all the formations investigated from these groups reveal both normal and reversed polarities of the palaeomagnetic field. Using this information a magnetic polarity time scale (GPTS) is constructed for the Vindhyan Supergroup during the late Proterozoic. When correlated with the Russian magnetostratigraphic scale for the Riphean period, it is noticed that the geomagnetic field during the late Proterozoic is similar to that of the Phanerozoic with superchrons at some periods and frequent reversals at others throughout the Precambrian.

Revised magnetostratigraphy of the Plio-Pleistocene sedimentary sequence of the Olduvai Formation (Tanzania)

The magnetic stratigraphy of the Plio-Pleistocene sedimentary formation of the Olduvai Gorge (Tanzania), which contains a well known series of prehistoric pebble cultures and hominid remains, has been reinvestigated. Sixty one sites were sampled along 90 metres of stratigraphic section extending from the base of Bed I into the Masek Beds. Palaeo- and rock-magnetic analyses reveal that magnetizations are stable, carried by low coercivity magnetic minerals (titano-magnetites) and are likely to be of depositional origin. A revised magnetostratigraphic scale is proposed. The base of the major normal magnetozone, N1, interpreted as the Olduvai sub-chron, lies ca 6 m below Tuff IA, i.e. significantly below the previously defined position. This revision, associated to 40Ar/39Ar ages recently determined on the Tuffs of Bed I, supports the validity of the astronomical calibration of the geomagnetic polarity time scale for the Olduvai sub-chron. The correlation of the normal magnetozone N2 with Jaramillo (or Jaramillo + Cobb Mountain), is suggested by the discovery of a reverse magnetozone (R3) in the uppermost part of the Pleistocene sequence (Masek Beds), and implies that the fossiliferous sequence of the Olduvai Formation was entirely deposited before the Brunhes/Matuyama boundary.