No description available.

The anisotropy of magnetic susceptibility (AMS) was investigated in three Chinese Loess Plateau sedimentary sections along a W–E transect. Previously published models assumed that winter monsoons were responsible for the magnetic fabric formation of loess sequences. In our new interpretation, the stronger summer monsoons from the southeast played the major role in magnetic fabric orientation in the studied west and central parts of the Chinese Loess Plateau. The AMS was generated during the rainy summer monsoon when the sedimentary particles including magnetite were rearranged, settled, and fixed. We reconstruct the summer paleomonsoon routes for the last 130 kyr. These winds prevail from SE to NW but appear to be affected by regional topographic factors. Evidence in the world’s ocean current system indicates an abrupt cooling from 34.1 to 33.6 Ma across the Eocene-Oligocene boundary (EOB) at 33.9 Ma. I describe and date with magnetostratigraphy a section from the north Junggar Basin (Burqin section). Three fossil assemblages revealed through the EOB (34.8, 33.7, and 30.4 Ma) demonstrate that perissodactyl faunas were abruptly replaced by rodent/lagomorph-dominant faunas during climate cooling, and the changes in mammalian communities were accelerated by aridification in central Asia. Paleomagnetic studies of two sections of the northern Junggar Basin, China, are presented from Burqin and Tieersihabahe. Our paleomagnetic results demonstrate counterclockwise tectonic rotations in Burqin and Tieersihabahe (–17.2 ± 9.6° and –11.8 ± 6.1°, respectively) as well as considerable northward latitudinal displacement (12.2 ± 6.5° and 9.7 ± 4.1°, respectively) with respect to Europe. These results are consistent with the motions of contiguous blocks in the same geological time intervals (India, north and south China, Tarim, Amuria, and Kazakhstan). No significant intracontinental shortening or vertical-axis rotation is observed for the Junggar block from 40 Ma to 20 Ma. Our results reveal that the major compression and rotation between Junggar and northern Europe occurred after 20 Ma due to continuous penetration of India into Asia. We interpret the uplift of the Altay Mountains and the formation of the Lake Baikal rift system to be due to such intracontinental compression and relative rotations. / Geophysics

Marine palynomorph assemblages from the Campanian-Maastrichtian Bearpaw Formation, consisting of 34 genera and 69 species of dinoflagellate cysts and 8 forms of acritarchs, were recovered from a composite section in the Cypress Hills and the Research Council of Alberta Castor borehole in southern Alberta, Canada. Correlation of the sections with magnetostratigraphy and radiometric dates of bentonites within the Bearpaw Formation provided a chronostratigraphic control that enabled correlation between the two sections and the Campanian-Maastrichtian Boundary Global Stratotype Section and Point at Tercis les Bains, France. Correlation of first and last occurrence data of key taxa in the boundary section at Tercis with the same taxa in the studied sections support the placement of the Campanian-Maastrichtian Boundary at the transition between magnetochrons 32n.1n and 31r within the <i>Baculites baculus</i> ammonoid range zone of the Western Interior of North America.<p>Semi-quantitative analyses of the assemblages record the transgressive-regressive episodes of the Bearpaw cyclothem. Increases in the relative abundances of gonyaulacacean cysts, numbers of dinocysts compared to terrestrial palynomorphs and assemblage diversity correlate with transgressive episodes, with the richest assemblages occurring during periods of open, offshore to neritic conditions correlated with the cyclothems transgressive peak.<p>Two new species of dinoflagellate cyst are described. The first, Dinoflagellate sp. 1, is a novel taxon, while the second, Downiesphaeridium sp. A, is a chorate cyst similar to Mesozoic forms previously ascribed to <i>Cleistosphaerdium diversispinosum</i> Davey et al. emend. Eaton et al.

Marine palynomorph assemblages from the Campanian-Maastrichtian Bearpaw Formation, consisting of 34 genera and 69 species of dinoflagellate cysts and 8 forms of acritarchs, were recovered from a composite section in the Cypress Hills and the Research Council of Alberta Castor borehole in southern Alberta, Canada. Correlation of the sections with magnetostratigraphy and radiometric dates of bentonites within the Bearpaw Formation provided a chronostratigraphic control that enabled correlation between the two sections and the Campanian-Maastrichtian Boundary Global Stratotype Section and Point at Tercis les Bains, France. Correlation of first and last occurrence data of key taxa in the boundary section at Tercis with the same taxa in the studied sections support the placement of the Campanian-Maastrichtian Boundary at the transition between magnetochrons 32n.1n and 31r within the <i>Baculites baculus</i> ammonoid range zone of the Western Interior of North America.<p>Semi-quantitative analyses of the assemblages record the transgressive-regressive episodes of the Bearpaw cyclothem. Increases in the relative abundances of gonyaulacacean cysts, numbers of dinocysts compared to terrestrial palynomorphs and assemblage diversity correlate with transgressive episodes, with the richest assemblages occurring during periods of open, offshore to neritic conditions correlated with the cyclothems transgressive peak.<p>Two new species of dinoflagellate cyst are described. The first, Dinoflagellate sp. 1, is a novel taxon, while the second, Downiesphaeridium sp. A, is a chorate cyst similar to Mesozoic forms previously ascribed to <i>Cleistosphaerdium diversispinosum</i> Davey et al. emend. Eaton et al.

No description available.

O Cretáceo inferior é caracterizado por mudanças drásticas em todos os envelopes terrestres, envolvendo desde variações geodinâmicas a importantes mudanças paleoclimáticas. Entretanto, há poucos registros desses eventos no hemisfério sul, sendo o DSDP-511 o principal deles. Com objetivo de gerar melhores vínculos temporais para os registros do Cretáceo Inferior no hemisfério sul, foram realizados estudos paleomagnéticos e de magnetismo ambiental em alta resolução espacial em uma secção do sítio DSDP-511 (núcleos 58 e 59), a qual corresponde ao intervalo entre 508,77 a 523,94 mbsf (meters below sea floor) e abrange os estágios Barremiano e Aptiano. A compreensão dos eventos paleoclimáticos neste sítio não é trivial, uma vez que, além de ambiguidade nas idades inferidas pela bioestratigrafia, apenas parte do evento de anoxia oceânica OAE 1a está registrada devido a uma lacuna de amostragem. Foram identificados dois principais portadores magnéticos através da análise de curvas de aquisição e desmagnetização de magnetização remanente anisterética (ARM) e magnetização remanente isotérmica (IRM). O mineral de baixa corecividade é a magnetita, confirmada a partir da desmagnetização de IRM triaxial. Sua origem é provavelmente detrítica. O portador de alta coercividade foi interpretado como hematita. A contribuição relativa dos portadores magnéticos apresentou uma variação acentuada no intervalo entre 512 e 514 mbsf, sendo manifestada através de um pico em todos os parâmetros de magnetismo ambiental. Esta variação está associada também à maior concentração de minerais magnéticos neste intervalo e coincide com um marcado aumento na temperatura da superfície dos oceanos. Após desmagnetização progressiva e identificação da componente magnética característica foram definidas duas zonas de polaridade normal e uma zona de polaridade reversa (518,9 e 518,16 mbsf). Além disso, foi definida uma zona discreta de polaridade reversa em 509,20 mbsf. Os resultados encontrados apresentam diferenças significativas quando comparadas com estudos anteriores no mesmo testemunho. Uma datação Re-Os em folhelhos negros de 125,3 ± 2,2 Ma entre 519,01 e 519,40 mbsf foi obtida recentemente e forneceu um vínculo absoluto de idade para o intervalo estudado. O padrão de reversões encontrado foi compatível com a presença dos chrons M0r e ISEA. O modelo de idades proposto permite reposicionar o evento OAE 1a entre as profundidades 513,5 a 518 mbsf, incluindo a parte não recuperada do testemunho. Esta interpretação também compatível com dados de TOC e de 13C obtidos recentemente por outros autores. / The lower Cretaceous is characterized by drastic changes in the Earth, from geodynamic variations to important paleoclimatic changes. However, there are few records of these events in the southern hemisphere, the DSDP-511 being the most comprehensive one. In order to provide better temporal constraints for the lower Cretaceous records in the southern hemisphere, paleomagnetic and high-resolution environmental magnetism studies were performed in a section of the DSDP-511 site (cores 58 and 59), which corresponds to the interval between 508.77 to 523.94 mbsf (meters below sea floor) and covers the Barremian and Aptian stages. The correct assessment of paleoclimatic events at this site is not trivial because, in addition to the ambiguity at the ages inferred by biostratigraphy, only part of the OAE 1a (oceanic anoxic event) was recovered. Two main magnetic carriers were identified through the analysis of acquisition and demagnetization of anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) curves. The low coercivity mineral is magnetite, as attested by triaxial IRM demagnetization. Its origin is likely detrital. The high coercivity carrier was interpreted as hematite. The relative contribution of magnetic carriers has a strong variation in the interval between 512 and 514 mbsf, being manifested by a peak in all parameters of environmental magnetism. This variation is also associated with the higher concentration of magnetic minerals and coincides with a marked increase in the sea surface temperature in this interval. After stepwise demagnetization and identification of the characteristic magnetic component, two zones of normal polarity and a zone of reverse polarity (518.9 and 518.16 mbsf) were defined. In addition, a discrete zone of reverse polarity was defined at 509.20 mbsf. The results obtained present significant differences when compared with previous studies in the same core. A Re-Os dating on black shales of 125.3 ± 2.2 Ma between 519.01 and 519.40 mbsf was obtained recently and provided an absolute constraint onto which the polarity zones could be matched. The pattern of reversions found is compatible with the presence of chrons M0r and ISEA. The proposed age model allows repositioning the OAE 1a event between depths 513.5 to 518 mbsf in the core and overlap the unrecovered part. This interpretation is also compatible with TOC and 13C data recently obtained by other authors.

O Cretáceo inferior é caracterizado por mudanças drásticas em todos os envelopes terrestres, envolvendo desde variações geodinâmicas a importantes mudanças paleoclimáticas. Entretanto, há poucos registros desses eventos no hemisfério sul, sendo o DSDP-511 o principal deles. Com objetivo de gerar melhores vínculos temporais para os registros do Cretáceo Inferior no hemisfério sul, foram realizados estudos paleomagnéticos e de magnetismo ambiental em alta resolução espacial em uma secção do sítio DSDP-511 (núcleos 58 e 59), a qual corresponde ao intervalo entre 508,77 a 523,94 mbsf (meters below sea floor) e abrange os estágios Barremiano e Aptiano. A compreensão dos eventos paleoclimáticos neste sítio não é trivial, uma vez que, além de ambiguidade nas idades inferidas pela bioestratigrafia, apenas parte do evento de anoxia oceânica OAE 1a está registrada devido a uma lacuna de amostragem. Foram identificados dois principais portadores magnéticos através da análise de curvas de aquisição e desmagnetização de magnetização remanente anisterética (ARM) e magnetização remanente isotérmica (IRM). O mineral de baixa corecividade é a magnetita, confirmada a partir da desmagnetização de IRM triaxial. Sua origem é provavelmente detrítica. O portador de alta coercividade foi interpretado como hematita. A contribuição relativa dos portadores magnéticos apresentou uma variação acentuada no intervalo entre 512 e 514 mbsf, sendo manifestada através de um pico em todos os parâmetros de magnetismo ambiental. Esta variação está associada também à maior concentração de minerais magnéticos neste intervalo e coincide com um marcado aumento na temperatura da superfície dos oceanos. Após desmagnetização progressiva e identificação da componente magnética característica foram definidas duas zonas de polaridade normal e uma zona de polaridade reversa (518,9 e 518,16 mbsf). Além disso, foi definida uma zona discreta de polaridade reversa em 509,20 mbsf. Os resultados encontrados apresentam diferenças significativas quando comparadas com estudos anteriores no mesmo testemunho. Uma datação Re-Os em folhelhos negros de 125,3 ± 2,2 Ma entre 519,01 e 519,40 mbsf foi obtida recentemente e forneceu um vínculo absoluto de idade para o intervalo estudado. O padrão de reversões encontrado foi compatível com a presença dos chrons M0r e ISEA. O modelo de idades proposto permite reposicionar o evento OAE 1a entre as profundidades 513,5 a 518 mbsf, incluindo a parte não recuperada do testemunho. Esta interpretação também compatível com dados de TOC e de 13C obtidos recentemente por outros autores. / The lower Cretaceous is characterized by drastic changes in the Earth, from geodynamic variations to important paleoclimatic changes. However, there are few records of these events in the southern hemisphere, the DSDP-511 being the most comprehensive one. In order to provide better temporal constraints for the lower Cretaceous records in the southern hemisphere, paleomagnetic and high-resolution environmental magnetism studies were performed in a section of the DSDP-511 site (cores 58 and 59), which corresponds to the interval between 508.77 to 523.94 mbsf (meters below sea floor) and covers the Barremian and Aptian stages. The correct assessment of paleoclimatic events at this site is not trivial because, in addition to the ambiguity at the ages inferred by biostratigraphy, only part of the OAE 1a (oceanic anoxic event) was recovered. Two main magnetic carriers were identified through the analysis of acquisition and demagnetization of anhysteretic remanent magnetization (ARM) and isothermal remanent magnetization (IRM) curves. The low coercivity mineral is magnetite, as attested by triaxial IRM demagnetization. Its origin is likely detrital. The high coercivity carrier was interpreted as hematite. The relative contribution of magnetic carriers has a strong variation in the interval between 512 and 514 mbsf, being manifested by a peak in all parameters of environmental magnetism. This variation is also associated with the higher concentration of magnetic minerals and coincides with a marked increase in the sea surface temperature in this interval. After stepwise demagnetization and identification of the characteristic magnetic component, two zones of normal polarity and a zone of reverse polarity (518.9 and 518.16 mbsf) were defined. In addition, a discrete zone of reverse polarity was defined at 509.20 mbsf. The results obtained present significant differences when compared with previous studies in the same core. A Re-Os dating on black shales of 125.3 ± 2.2 Ma between 519.01 and 519.40 mbsf was obtained recently and provided an absolute constraint onto which the polarity zones could be matched. The pattern of reversions found is compatible with the presence of chrons M0r and ISEA. The proposed age model allows repositioning the OAE 1a event between depths 513.5 to 518 mbsf in the core and overlap the unrecovered part. This interpretation is also compatible with TOC and 13C data recently obtained by other authors.

Este trabalho teve por objetivo investigar a escala temporal envolvida na deposição de ritmitos permocarboníferos da Bacia do Paraná (Subgrupo Itararé), expostos no Parque do Varvito (Itu, SP) e pedreira Itaú (Trombudo Central, SC), através de dados paleomagnéticos e de anisotropia de suscetibilidade magnética. Os estudos paleomagnéticos revelaram componentes de magnetização estável para ambas as coleções, com indicações de que a magnetização remanente é devida a portadores magnéticos (principalmente magnetita e hematita) de origem detrítica. A componente de magnetização característica, em ambos os casos, é de polaridade reversa, e foi identificada nos dois portadores magnéticos principais. A investigação da mineralogia magnética foi feita através de várias técnicas, incluindo espectroscopia Mossbauer, curvas termomagnéticas, curvas de histerese e ZFC/FC, além de microscopia ótica e eletrônica. Os dados paleomagnéticos e de anisotropia de susceptibilidade magnética permitiram a composição de séries temporais, que foram submetidas a análise espectral por transformada de Fourier de Lomb-Scargle para séries desigualmente espaçadas. Os espectros de potência resultantes foram posteriormente comparados com os espectros de séries de espessura individual das unidades litológicas, o que possibilitou a investigação de sinais harmônicos, sobre a qual foram propostas inferências a respeito das escalas temporais de sedimentação. Esta etapa do trabalho revelou escalas milenares para o domínio do tempo nos espectros de potência, indicando o registro dos ciclos orbitais de Milankovitch, bem como quase-periodicidades associadas à variabilidade solar e feedback do sistema oceânico-atmosférico, para todas as análises. Este conjunto de dados sugere o caráter não-anual de deposição dos ritmitos, ao contrário do proposto por alguns autores. O pólo paleomagnético calculado para a seção de Rio do Sul é compatível com o pólo do Permocarbonífero para a Bacia Sanfranciscana (MG), indicando que o intervalo de tempo envolvido na deposição dos sedimentos é suficientemente longo para eliminar os efeitos da variação secular do campo geomagnético. Alguns resultados adicionais desta tese proveram informações especiais: (i) Foram observados ciclos de indução solar (ciclos de Hallstattzein e de Suess) e a sugestão de ciclo compatível com o ciclo glacial-interglacial de Bond, recentemente sugerido como de origem solar, o que apontaria para uma possível feição harmônica, dominante sobre o sistema oceânico-atmosférico, e que poderia operar sobre períodos geológicos mais antigos; (ii) registro da remanência magnética possivelmente controlado por fatores mecânicos na sucessão de Itu, provavelmente relacionados a correntes de turbidez; (iii) indícios da presença de magnetossomas possivelmente rompidas e/ou oxidadas, em especial para a sucessão de Itu. / This work aims to investigate the timescale of the sedimentation of Permocarboniferous rhythmites from Paraná Basin (Itararé Subgroup) cropping out at the Parque do Varvito, (Itu, SP), and at the Itaú quarry, (Trombudo Central, SC) by means of paleomagnetic and anisotropy of magnetic susceptibility (ASM). The paleomagnetic study revealed stable magnetization components for both sections, which are carried by magnetic carriers (mainly magnetite and hematite) of detritic origin. The characteristic magnetization direction for each section is of reverse polarity and is found in both magnetite and hematite. Several techniques were used in the investigation the magnetic mineralogy including Mossbauer spectrometry, thermomagnetic curves, hysteresis and ZFC/FC curves, optical and electronic microscopy. Although not confirmed, some data pointed to the possibility of magnetic biomineralization with disrupted or oxidized mineral chains. The paleomagnetic and ASM data allowed the construction of time series that were submitted to spectral analysis by the Lomb-Scargle Fourier transform for unevenly spaced series. The resulting power spectra were then compared with the thickness spectrum. Based on the harmonic signals identified in all time series it was possible to infer deposition rates for both Itú and Itaú rhythmites. This procedure pointed to millennial timescale, and periodical signal compatible to Milankovitch orbital forcing, as well quasi-periodicities related to solar variability and feedback of ocean-atmospheric system, were identified in both sections. Such evidences strongly point to the non-annual sedimentation character, in opposition to previous hypothesis. The paleomagnetic data from the Rio do Sul rocks allowed the calculation of a paleomagnetic pole which is in agreement to the one recently obtained for the Santa Fé Group, Sanfranciscana Basin (MG), indicating that the time interval comprised by the Rio do Sul rhythmites is long enough for eliminating the secular variation effects. Results from this work provide also indications for the record of (i) solar forcing cycles (Hallstattzein and Suess cycles); (ii) spectral peak values of 1,5-ky and 6,0 - 8,0 ky cycle, which is compatible to the glacial-interglacial Bond cycle observed in Quaternary geological records. Such result would extend to ancient times the atmosphere-ocean interaction pattern.

Este trabalho teve por objetivo investigar a escala temporal envolvida na deposição de ritmitos permocarboníferos da Bacia do Paraná (Subgrupo Itararé), expostos no Parque do Varvito (Itu, SP) e pedreira Itaú (Trombudo Central, SC), através de dados paleomagnéticos e de anisotropia de suscetibilidade magnética. Os estudos paleomagnéticos revelaram componentes de magnetização estável para ambas as coleções, com indicações de que a magnetização remanente é devida a portadores magnéticos (principalmente magnetita e hematita) de origem detrítica. A componente de magnetização característica, em ambos os casos, é de polaridade reversa, e foi identificada nos dois portadores magnéticos principais. A investigação da mineralogia magnética foi feita através de várias técnicas, incluindo espectroscopia Mossbauer, curvas termomagnéticas, curvas de histerese e ZFC/FC, além de microscopia ótica e eletrônica. Os dados paleomagnéticos e de anisotropia de susceptibilidade magnética permitiram a composição de séries temporais, que foram submetidas a análise espectral por transformada de Fourier de Lomb-Scargle para séries desigualmente espaçadas. Os espectros de potência resultantes foram posteriormente comparados com os espectros de séries de espessura individual das unidades litológicas, o que possibilitou a investigação de sinais harmônicos, sobre a qual foram propostas inferências a respeito das escalas temporais de sedimentação. Esta etapa do trabalho revelou escalas milenares para o domínio do tempo nos espectros de potência, indicando o registro dos ciclos orbitais de Milankovitch, bem como quase-periodicidades associadas à variabilidade solar e feedback do sistema oceânico-atmosférico, para todas as análises. Este conjunto de dados sugere o caráter não-anual de deposição dos ritmitos, ao contrário do proposto por alguns autores. O pólo paleomagnético calculado para a seção de Rio do Sul é compatível com o pólo do Permocarbonífero para a Bacia Sanfranciscana (MG), indicando que o intervalo de tempo envolvido na deposição dos sedimentos é suficientemente longo para eliminar os efeitos da variação secular do campo geomagnético. Alguns resultados adicionais desta tese proveram informações especiais: (i) Foram observados ciclos de indução solar (ciclos de Hallstattzein e de Suess) e a sugestão de ciclo compatível com o ciclo glacial-interglacial de Bond, recentemente sugerido como de origem solar, o que apontaria para uma possível feição harmônica, dominante sobre o sistema oceânico-atmosférico, e que poderia operar sobre períodos geológicos mais antigos; (ii) registro da remanência magnética possivelmente controlado por fatores mecânicos na sucessão de Itu, provavelmente relacionados a correntes de turbidez; (iii) indícios da presença de magnetossomas possivelmente rompidas e/ou oxidadas, em especial para a sucessão de Itu. / This work aims to investigate the timescale of the sedimentation of Permocarboniferous rhythmites from Paraná Basin (Itararé Subgroup) cropping out at the Parque do Varvito, (Itu, SP), and at the Itaú quarry, (Trombudo Central, SC) by means of paleomagnetic and anisotropy of magnetic susceptibility (ASM). The paleomagnetic study revealed stable magnetization components for both sections, which are carried by magnetic carriers (mainly magnetite and hematite) of detritic origin. The characteristic magnetization direction for each section is of reverse polarity and is found in both magnetite and hematite. Several techniques were used in the investigation the magnetic mineralogy including Mossbauer spectrometry, thermomagnetic curves, hysteresis and ZFC/FC curves, optical and electronic microscopy. Although not confirmed, some data pointed to the possibility of magnetic biomineralization with disrupted or oxidized mineral chains. The paleomagnetic and ASM data allowed the construction of time series that were submitted to spectral analysis by the Lomb-Scargle Fourier transform for unevenly spaced series. The resulting power spectra were then compared with the thickness spectrum. Based on the harmonic signals identified in all time series it was possible to infer deposition rates for both Itú and Itaú rhythmites. This procedure pointed to millennial timescale, and periodical signal compatible to Milankovitch orbital forcing, as well quasi-periodicities related to solar variability and feedback of ocean-atmospheric system, were identified in both sections. Such evidences strongly point to the non-annual sedimentation character, in opposition to previous hypothesis. The paleomagnetic data from the Rio do Sul rocks allowed the calculation of a paleomagnetic pole which is in agreement to the one recently obtained for the Santa Fé Group, Sanfranciscana Basin (MG), indicating that the time interval comprised by the Rio do Sul rhythmites is long enough for eliminating the secular variation effects. Results from this work provide also indications for the record of (i) solar forcing cycles (Hallstattzein and Suess cycles); (ii) spectral peak values of 1,5-ky and 6,0 - 8,0 ky cycle, which is compatible to the glacial-interglacial Bond cycle observed in Quaternary geological records. Such result would extend to ancient times the atmosphere-ocean interaction pattern.

Les bassins sédimentaires d'Hamakoussou et du Mayo Oulo-Léré sont des bassins à sédiments continentaux. Ils se situent en territoire camerounais, dans le prolongement de la branche de Yola de direction Est-Ouest du Fossé de la Bénoué dont la mise en place est liée à l'ouverture de l'Atlantique Sud à partir de la dislocation du Gondwana. L'âge de ces bassins est basé sur des études biochronologiques les situant dans le Crétacé inférieur. La série sédimentaire est sensiblement la même dans les deux bassins, elle est constituée de dépôts fluviatiles à fluviolacustres montrant une alternance de grés, de siltite et d'argilite.Des prélèvements paléomagnétiques ont été effectués sur des niveaux fins situés dans la partie supérieure de la série sédimentaire de chaque section. Dans le bassin sédimentaire d'Hamakoussou, 50 échantillons sur 11 sites d'épaisseur égale à 69 m, ont été prélevés dans la localité de Djallou et 78 échantillons sur 20 sites d'une épaisseur de 511 m dans la localité d'Ourokessoum. Dans le bassin du Mayo Oulo-Léré, 116 échantillons sur 45 sites sur une épaisseur de 478 m ont été prélevés dans la localité de Tchontchi.La désaimantation progressive au champ alternatif et à la température de ces échantillons montre que les séries sédimentaires de ces bassins portent une aimantation primaire. Les directions d'aimantation ont permis de déceler une tectonique régionale marquée par un mouvement de rotation autour de l'axe vertical et un mouvement de translation des blocs. Les minéraux magnétiques porteurs de cette aimantation sont de faible, moyenne et de forte coercivité. Une séquence de trois polarités a été déterminée le long de chaque section du bassin d'Hamakoussou : une polarité inverse et deux polarités normales. Dans le bassin du Mayo Oulo-Léré, la section Mayo a livré deux polarités dont une normale et une, inverse. La séquence de polarités obtenue pour chaque coupe a été corrélée avec l'échelle de temps de polarités magnétiques. Les trois polarités des coupes du bassin d'Hamakoussou ont été corrélées avec les chrones M1 et M3 avec un âge compris entre 125 Ma et 128,11 Ma. La séquence d'inversions de la coupe du Mayo Oulo-Léré a été corrélée avec le chrone M1 avec un âge compris entre 125 Ma et 127,61 Ma. Le taux de sédimentation des dépôts du bassin d'Hamakoussou varie entre 5,5 cm/ma et 40,5 cm/ma et est de 38 cm/ma dans la section du Mayo Oulo-Léré. / Hamakoussou and Mayo Oulo-Léré sedimentary basins are extension of the executive branch of East-West Yola Benue Trough, whose formation is related to the opening of the South Atlantic from the dislocation of Gondwana, in the Cameroonian territory. An age range of lower Cretaceous to Barremian-Hauterivian limit has been assigned based on biochronological studies. The sedimentary sequence in both basins is composed upward finnings fluivial to fluviolacustrine deposits. The deposits present an alternation of fine grained sandstone, siltstone and mudstone overlying coarse sandstone which is underlain by micro-conglomerate to conglomerate facies at the base.A magnetostratigraphic study has been carried out on a fine sediments in the upper part of each sections. 50 samples from about 69.03 m thickness, were collected from 11 sites at Djallou and 78 samples, about 511.03 m thickness, were collected from 20 sites at Ourokessoum localities in the Hamakoussou basin ; and 116 samples, about 478.19 m thickness, collected from 45 sites at Tchontchi locality in the Mayo Oulo-Lere basin.Specimens subjected to progressive alternating field and thermal demagnetization show that the sedimentary sequences have a primary magnetization. The directions of magnetization indicate a regional tectonic marked by a rotation and translation block. Rock magnetic investigations reveal the presence of both high and low coercivity minerals. A sequence of three polarities was determined along each section of the Hamakoussou basin: one reversal polarity and two normal polarities, whereas two polarities: (normal and a reversal) were determined along Mayo section in the Mayo Oulo-Lere basin The three polarities sequences from the two sections from Hamakoussou basin are correlated with M1 and M3 Chrons and suggest an age between 125 and 128, 11 My. While the two polarities sequences from the Mayo Oulo-Lere basin are correlated with M1 Chron, and suggest an age between 125 and 127.61 My. The sedimentation rates of Hamakoussou basin deposits vary between 5.5 cm/kyr and 40.88 cm/kyr and 38.26 cm/kyr in the section of Mayo Oulo-Lere basin.