Paleomagnetic Section Research Articles

Magnetostratigraphy of the upper Jurassic and Lower Cretaceous deposits of the Bolshekhetskaya structural terrace

Relevance. The confinement of hydrocarbon deposits to certain stratigraphic horizons, poor knowledge of the northeast of Western Siberia and adjacent territories, including the magnetostratigraphic method, with their oil and gas potential (over the past 10 years, four unique hydrocarbon deposits have been discovered here), the boreal nature of the deposits, and the need for remote magnetostratigraphic correlations. Aim. To create a magnetostratigraphic reference section of the northeast of Western Siberia. Objects. Sections of deep wells of the Pendomayakhskaya, Vostochno Suzunskaya, Vostochno Lodochnaya and Gorchinskaya areas of the Bolshekhetskaya structural terrace, which exposed the Lower Cretaceous and upper parts of the Upper Jurassic (Yanovstan Formation) deposits. Methods. Paleomagnetic studies were carried out according to standard methods and included sampling of core samples with an "up-down" orientation, sample preparation (kappametry, marking of samples, photographing, sawing), measurement of magnetic susceptibility, temporary cleaning (the samples were located along the field during a week, then a week-against the field with determination of residual magnetization), complete demagnetization by alternating magnetic field, plotting of complete demagnetization, mass cleaning, determination of components of natural residual magnetization, assessment of paleomagnetic stability, identification of forward and reverse polarity zones, construction of paleomagnetic sections. To construct the magnetostratigraphic sections given in this article, the paleomagnetic and biostratigraphic data are linked. For this purpose, magnetochrons were identified using biostratigraphic analysis. Results. The authors have compiled a composite magnetostratigraphic section of the boundary Jurassic-Cretaceous deposits of the Bolshekhetskaya structural terrace and identified four magnetozones with subzones of direct and reverse polarity, compared with the Berriasian ammonite scale. The high information content of the results of magnetostratigraphy research under boreal conditions was noted.

Identification of Growth Strata: New Insights From Anisotropy Corrected Paleomagnetic Data

AbstractGrowth strata are important for understanding regional tectonic evolution. However, it is difficult to identify growth strata in the field, because of subsequent intensive modification and erosion. To address this problem, we studied two paleomagnetic sections at the stable deposition setting and the growth end of the Miocene growth strata of the Gengma Basin. Average inclinations of the two sections show a systematic deviation of 4.7° ± 5.2°. The anisotropy of magnetic susceptibility results for the growth end of the section A demonstrate a sedimentary fabric. However, the tilt‐corrected K3 axis deviates to the southwest from the normal direction of the bedding by 5.4° ± 3.8°, the magnetic foliation deviates to the equatorial plane by 5.3° ± 4.7°. The K3 axis is perpendicular to the bedding at the stable deposition setting. The deviation of the K3 axis from the normal direction of bedding is interpreted as the growth angle, which can be used to correct the paleomagnetic inclination data.

Lithological-magneto-palynological characteristics of the Upper Miocene, Pliocene and Gelasian deposits of the Cenozoic reference section of the Dnieper-Donetsk depression (Ukraine)


 
 
 Well 11 is located near the village of Velika Lanna, Krasnograd District, Kharkiv Region, and at the current stage of research is the most representative section of the continental Upper Cenozoic deposits of the Dnieper-Donets depression. For the first time, a comprehensive description of the Upper Miocene, Pliocene, and Pleistocene deposits obtained from the results of paleopedological, palynological, and paleomagnetic studies is presented for the research region. Characteristic features of 13 climatolites (pedocomplexes and intersoil clays) including five upper Miocene, three lower Pliocene, two upper Pliocene and two climatolites correlated with the Gelasian of the International Stratigraphic Scale were determined. According to the results of miner- alogical studies, three mineralogical strata such as montmorillonite-kaolinite, hydromica-montmorillonite, and hydromica were traced for the deposits of the section. The mineralogical boundaries are confined to the levels of the Salgir–Liubimivka and Siversk–Beregove climatoliths. The spore-pollen complexes of all studied deposits are described. It was established that the complexes characterizing each studied climatolith differ in taxonomic composition, the ratio of the pollen of the main groups of plants, and the presence of characteristic taxa. Changes in the composition of spore-pollen spectra characterizing all soils of pedocomplexes as well as intersoil clays and embryonic soils formed during short-term warming at the time of the cool stages of the Miocene, Pliocene, and Gelasian periods were traced. The described spore-pollen complexes and spectra were compared with those from the deposits of the same age in adjacent regions, which allowed identifying the general and regional features of the compared complexes. A detailed record of changes in the geomagnetic field during the formation of the Upper Miocene-Pleistocene deposits was obtained, which enables to construct a paleomagnetic section reflecting the areas with normal, reverse and anomalous polarity. According to the nature of distribution of such areas, the section is tied to the Cox scale. All these materials allowed carrying out the stratigraphic dissection of the Miocene-Gelasian deposits of the studied section, as well as to substantiate the feasibility of defining it as a reference for the Miocene-Pleistocene deposits of the Dnieper-Donets depression.
 
 

New data on stratigraphy (palynomorphs, ostracods, paleomagnetism) of Cenozoic continental deposits of the Ishim plain, Western Siberia

New micropaleontological and paleomagnetic data were obtained by studying core samples of Cenozoic continental deposits from two boreholes drilled in the south of Tyumen oblast (Western Siberia). Palynological assemblages in deposits of the Tavda (upper part), Novomikhailovka, Turtas, Abrosimovka, Tobolsk, Smirnovka, and Suzgun formations were described. Deposits of these formations are enriched in spore-pollen assemblages, which can be correlated with assemblages of regional palynozones of the West Siberian Plain. Ostracods were described in Quaternary deposits. On the basis of biostratigraphic and paleomagnetic data, the Late Eocene (Priabonian)–Holocene age of deposits was substantiated. For the first time, beds with dinocysts of genus Pseudokomewuia were identified in deposits of the Turtas Formation (Upper Oligocene) of the Ishim lithofacial area. In total, nine regional magnetozones were distinguished in the paleomagnetic section. On the basis of palynological and paleomagnetic data, sections of two boreholes were correlated, and hiatuses in sedimentation were revealed. A large hiatus is at the Eocene-Oligocene boundary (Western Siberia): the Lower Oligocene Atlym Horizon and Miocene–Pliocene and Eopleistocene sediments are missing. The Oligocene interval of the section is represented in a reduced volume.

Paleomagnetism and magnetostratigraphy of Upper Cretaceous and Cretaceous-Paleogene boundary intervals, southern Kulunda basin (West Siberia)

The study presents new paleomagnetic data on the Upper Cretaceous and Cretaceous-Paleogene boundary intervals of the southern Kulunda basin (Alei area), which were obtained from core samples collected from a 305-m-thick section penetrated in two wells. The paleomagnetic sections of each well were compiled and correlated based on the characteristic remanent magnetization (ChRM). Paleomagnetic, geological, stratigraphic, and paleontological data were used to compile the Upper Cretaceous and Cretaceous-Paleogene magnetostratigraphic section of the southern Kulunda basin. The magnetostratigraphic section consists of five magnetozones, one normal polarity zone, and four reversed polarity zones spanning the Upper Cretaceous and Lower Paleogene. The lower part of the Gan’kino Horizon, showing normal polarity, forms a single normal polarity magnetozone N. The upper part of the Gan’kino Horizon comprises two reversed polarity magnetozones (R1km and R2mt). The Talitsa and Lyulinvor Formations of Lower Paleogene age correspond to two reversed polarity magnetozones (R1zl and R2i). The compiled Upper Cretaceous and Lower Paleogene magnetostratigraphic section was correlated with the geomagnetic polarity time scale. Two options were considered for correlating the lower normal polarity part of the section with geomagnetic polarity time scale of Gradstein.

Paleomagnetism of the Paleogene-Neogene continental sediments of the Om’ basin (southern West Siberia)

The paper presents the results of detailed paleomagnetic studies of the Paleogene-Neogene continental sediments stripped by borehole 8 in southwestern West Siberia (Russkaya Polyana district, Omsk Region), near the Kazakhstan frontier. According to the previous biostratigraphic data, the sediments under study formed from Rupelian to Ruscinian. The results of stepwise thermal demagnetization and alternating-field demagnetization were used to carry out a component analysis of natural remanent magnetization, which revealed characteristic (primary) remanent magnetization (ChRM). The compiled paleomagnetic section, which includes seven regional horizons and same-named formations (Oligocene Atlym, Novomikhailovka, and Zhuravka Formations and Neogene Abrosimovka, Beshcheul, Tavolzhan, and Novaya Stanitsa Formations), was compared with the Cenozoic polarity scale for the West Siberian Plate. This made it possible to assess the completeness of the geologic section of the Paleogene and Neogene continental sediments in borehole 8 and to record the magnetozones and their fragments missing from the magnetostratigraphic section (for some intervals, in absolute chronology). The comparison shows that the magnetostratigraphic section of the studied sediments at the edges of the Om’ basin is approximately twice shorter than that of the basin center.

Paleomagnetism of the Cretaceous sediments of the southern West Siberian Plate (from well 8 core studies)

The paper presents magnetobiostratigraphical data on the Cretaceous sediments stripped by well 8 in the southern side of the Om’ basin within the Om’–Lar'yak facies zone (southern West Siberia). The biostratigraphical data show that the sediments under study formed in the Albian–Maastrichtian. The componential analysis of natural remanent magnetization, based on thermal demagnetization and demagnetization with an alternating magnetic field, revealed the characteristic magnetization component. This confirms the paleomagnetic data used to compile the Cretaceous paleomagnetic section of the well. A paleomagnetic description was obtained, and a magnetobiostratigraphic key section of the Cretaceous sediments of the well was compiled on the basis of comprehensive data. It comprises five Upper Cretaceous regional horizons and same-named formations (Pokur, Kuznetsovo, Ipatovo, Slavgorod, Gan'kino), which have not been studied paleomagnetically at all in West Siberia. The magnetobiostratigraphic section comprises Albian–Maastrichtian stratigraphic units (43.5 Myr) and consists of three magnetozones. For example, the Pokur, Kuznetsovo, and Ipatovo Formations (total thickness 210 m), which show normal polarity with small reversed-magnetization horizons, form one long normal-polarity zone, N(al-st). The Slavgorod and Gan'kino Formations (total thickness 75 m), separated by a sedimentation gap, form two reversed-polarity magnetozones, R1(km) and R2(mt).Reference datums (paleontologically well-constrained magnetozones) were used to correlate the magnetobiostratigraphic section with the common magnetostratigraphic and magnetochronologic scales. The long normal-polarity magnetozone N(al-st), spanning the Albian, Cenomanian, Turonian, Coniacian, and Santonian, matches the Dzhalal hyperzone and chron C34 (∼112–83.6 Ma). The reversed-polarity zones, spanning most of the Campanian (R1(km) (Slavgorod Formation)) and Maastrichtian (R2(mt) (Gan'kino Formation)), match chrons C33(r) and C31(r) in the absolute chronology (∼83.6–80 and 71–68.5 Ma, respectively) with a gap two chrons long (C33(n), C32).

New sedimentological, bio-, and magnetostratigraphic data on the Jurassic-Cretaceous boundary interval of Eastern Crimea (Feodosiya)

The first compiled composite section comprises continuous succession of upper Tithonian-lower Berriasian strata (Jacobi Zone) from different isolated outcrops of the Feodosiya area. Based on new magnetostratigraphic and sedimentological data, the paleomagnetic section is correlated with succession of M20r, M19n, M19r, M18b chrons and M18n.1r Subchron (“Brodno”). The thorough complex bio- and magnetostratigraphic correlation of the upper Tithonian-lower Berriasian interval (Jacobi Zone) carried out through the Western Tethys and Eastern Paratethys provided grounds for first defining age analogs of the Durangites Zone in the Crimean Mountains and specifying location of the boundary between the Jurassic and Cretaceous systems, as well as for determining late Tithonian age of strata in the Dvuyakornaya Bay section barren of fossils.

Stratigraphic, paleomagnetic, and palynological data on the Paleogene–Neogene continental sediments of southwestern West Siberia

The paper presents magnetostratigraphic data on Paleogene–Neogene continental sediments stripped by borehole 8 at the southern periphery of the Om’ basin, in the Ishim–Irtysh interfluve, on the border with northern Kazakhstan. Pollen assemblages and zones have been distinguished, and a paleomagnetic section of borehole 8 has been compiled. The section consists of 11 magnetozones, which are associated with pollen zones and paleocarpological data. The absence of some paleomagnetic and pollen zones from the section suggests that the geological record is incomplete, because the formations in the studied sediments are partly eroded. A dinoflagellate community was first recorded in Upper Oligocene–Lower Miocene sediments at the periphery of the Om’ basin. The composition and ecologic characteristics of this community cannot give an unambiguous answer to the question whether seawater could penetrate southwestern Siberia through the Turgai Sea. The composition of pollen and spores from the Lower Oligocene sediments (Novomikhailovka Formation) and from the Chilikty Formation (northern Kazakhstan) is similar and reflects the development of Turgai flora in a huge territory. Later, in the second half of the Miocene, broad-leaved/coniferous forests were replaced by small-leaved ones. In the late Miocene, the latter were replaced by forest steppes with some arid flora.

Revised Chronostratigraphy and Biostratigraphy of the John Day Formation (Turtle Cove and Kimberly Members), Oregon, with Implications for Updated Calibration of the Arikareean North American Land Mammal Age

Although the Arikareean North American land mammal age was first typified in the Great Plains, the succession there contains significant unconformities, a generally poor magnetic record, relatively sparse radioisotopic calibration, and a major faunal hiatus. In the John Day Valley of central Oregon, however, is a thick, remarkably complete sequence of Oligocene through early Miocene strata (the John Day Formation) potentially amenable to addressing these shortcomings and long known to harbor one of the richest records of mid‐Tertiary mammals in North America. Since Prothero and Rensberger’s first magnetostratigraphic study of the John Day Formation in 1985, new advances in geochronology, together with a more comprehensive suite of paleomagnetic sections keyed to new radioisotopic and biostratigraphic data, have greatly enhanced chronostratigraphic precision. In our attempt to refine John Day chronostratigraphy, we sampled nearly 300 sites for magnetostratigraphy over a 500‐m‐thick interval and used several radioisotopically dated volcanic tuffs for our correlation with the geomagnetic polarity timescale. Many of the rocks analyzed showed unusual magnetic behavior, possibly due to the known zeolitization in this region, thereby precluding an abundance of class 1 polarity determinations. Nevertheless, preliminary results indicate that the Turtle Cove Member stratigraphically upward through the lower Kimberly Member extends from late chron C12n through C7n.1r, or from about 30.6 to 24.1 Ma. Intensive radioisotopic and magnetostratigraphic characterization of these strata provides a framework by which the associated biostratigraphy is assessed for biochronological significance relative to fossiliferous successions of the Great Plains, in turn resulting in reassessment of Arikareean subbiochron (Ar1–Ar4) boundaries. We present a revision of those boundaries that differs from their traditional timing as a hypothesis for testing in other locations.

Vertebrate paleontology, biostratigraphy, geochronology, and paleoenvironment of Qaidam Basin in northern Tibetan Plateau

Qaidam Basin is the largest terrestrial basin of the Tibetan Plateau and has the most continuous sedimentary record in the Cenozoic. Although constituting the first known fossil vertebrates of the Tibetan Plateau and discovered in early scientific expeditions in the 1930s, mammalian fossils from Qaidam Basin have not played a significant role in studying the basin stratigraphy. Our explorations in the Qaidam Basin during the past 8 years have resulted in the first well documented collection of fossil vertebrates from there. In addition to being the best organisms for terrestrial biostratigraphy in the Cenozoic, the new fossil vertebrates offer a unique window into the ancient biotas that chronicle the late Cenozoic uplift of the plateau and associated environmental changes. Here we establish for the first time a faunal sequence of early Oligocene to early Pliocene age. Four mammalian faunas are recognized: Olongbuluk Mammal Fauna (middle Miocene), Tuosu Mammal Fauna (early late Miocene), Shengou Mammal Fauna (early late Miocene), and Huaitoutala Mammal Fauna (early Pliocene). All four mammal faunas can be directly or indirectly tied to a paleomagnetic section in the Olongbuluk–Huaitoutala area, permitting robust calibration of the chronology of these faunas. At least four fish faunas are also found: Lulehe Fish Fauna (late early to early late Oligocene), Shengou Fish Fauna (early late Miocene), Eboliang Fish Fauna (late late Miocene to early Pliocene), and Yahu Fish Fauna (early Pliocene). The fish faunas represent the only records of their respective times in the Tibetan Plateau and thus by themselves cannot be used for age determination. However, these fish faunas are either integrated into paleomagnetic sections or directly tied to mammal faunas of known ages, permitting independent assessment of their chronology. Zoogeographically, Qaidam mammal faunas show increasing endemism through time, presumably related to topographic barriers formed by the growth and uplift of the Tibetan Plateau and to climatic changes triggered by the same tectonic process. Fishes that were trapped in internally drained basins developed pachyostotic morphologies caused by hypersaline environments, which are associated with the increasing aridity of the Tibetan Plateau since early Pliocene.

Officer deliver “State of the Section” address

On 1 July 2002, officers of the AGU Geomagnetism and Paleomagnetism (GP) Section and the GP Section Executive Committee began a new 2‐year term. The officers, listed here, take this opportunity to address the GP membership about the state of the section and where it is headed.

Outstanding student paper awards

The Geomagnetism & Paleomagnetism Section presented four outstanding student paper awards at the 2001 Fall Meeting in San Francisco, California, last December.

Gathering recognizes contributions of former Section President

To celebrate the sixtieth birthday of Rob Van derVoo, AGU's President and President‐elect of its Geomagnetism and Paleomagnetism Section, 1988–1992, a workshop was recently held in an intimate conference setting in Ann Arbor, Michigan. The purpose was to celebrate Rob's many contributions to the fields of paleomagnetism and tectonics as he reached this milestone. Some 30 people attended and were treated to 18 presentations, many of which have recently been published or are currently in review for a special issue in his honor. While great emphasis was placed on paleomagnetism as the only quantitative tool for generating paleogeographic reconstructions for pre‐Mesozoic time, a recurring theme within the meeting was the integration of paleomagnetic results with those from other disciplines—a hallmark of Rob's research efforts at the University of Michigan. The meeting also provided an opportunity for students and colleagues of Rob's to indulge in more “speculative” ideas, and the presentations were accompanied by wide‐ranging and “spirited” discussions during both the “formal” sessions and the highly enjoyable evening social activities.

Outstanding student paper awards

The Geomagnetism & Paleomagnetism Section presented three outstanding student paper awards at the 2001 Spring Meeting in Boston, Massachusetts, last June

Outstanding student paper awards

The Geomagnetism & Paleomagnetism (GP) Section presented four outstanding student paper awards at the 2000 Fall Meeting in San Francisco, California, last December.

Outstanding student paper award

The Geomagnetism & Paleomagnetism Section presented an outstanding student paper award to Arlo B. Weil at the 2000 Spring Meeting in Washington, D.C., last June.

Outstanding student paper award

The Geomagnetism & Paleomagnetism Section presented an outstanding student paper award to Stefanie Brachfeld at the 1999 AGU Spring Meeting in Boston, Massachusetts, last June.Stefanie presented a paper titled “Secondary Normalization for Removing Magnetic Grain Size Bias from Relative Paleointensity Data: New Data from the North American Holocene.”

Outstanding student paper awards

The Geomagnetism & Paleomagnetism (GP) Section presented two outstanding student paper awards at the 1998 AGU Fall Meeting in San Francisco, California, last December.

Outstanding student paper award

The Geomagnetism and Paleomagnetism (GP) Section presented the outstanding student paper award to Michael Winklhofer at the 1998 Spring AGU Meeting, in Boston, Mass., last May.Michael received his degree in geophysics at the University of Munich where he is currently working toward a Ph.D. His dissertation topic examines the possibilities of how the Earth's magnetic field can be detected by animals using biogenic ferromagnetic material.