Cambrian Poles Research Articles

Palаeomagnetism of the Vendian traps of Volyn, southwestern margin of the East European platform. P. 1: palаeomagnetic directions and poles

In Earth’s geological evolution the Vendian is significant because: 1) it preceded the «Cambrian Evolutionary explosion» when the multicellular organisms became suddenly much more diverse; 2) it is associated with the global tectonic and palaeogeographic restructuring of supercontinents, a change in geomagnetic field generation and other global processes affecting different layers of the Earth. At the same time, recent publications indicate an extremely irregular behavior of the geomagnetic field in the Vendian, which significantly differs from the recent regime of the geomagnetic field generation. New information on the configuration and magnitude of the geomagnetic field is crucial for understanding the reasons that can cause different modes of geodynamo. The article describes the new results of palaeomagnetic studies of the Volyn traps — basalts and tuffs of the Volyn series with ages about 560—580 My. The determined palaeodirections and palaeopoles are in good agreement with the previously published data of other authors and substantially complement them. The ChRM-component of «low-titanium» basalts was isolated in high-temperature (>500 °C) and characterized as primary. The same samples were used for palaeointensity determinations which showed the extremely low values of the geomagnetic field — an order of magnitude lower than the present one. The ChRM-component of «high-titanium» basalts was yielded in the temperature range of 200—400 °C. No results were obtained from palaeointensity experiments, and the complex nature of remanent magnetization remains unclear. The new data are compared with the palaeo-magnetic data of the Vendian—Early Cambrian poles of the East European Platform determined by other authors. To test the hypothesis of an anomalous Ediacaran magnetic field characterized by a low dipole moment and a high inversion frequency, more data are necessary. These data can be obtained from the studies of a stratigraphically more complete section of the Volyn traps opened by boreholes. These results will be presented in the next part of the article.

Ediacaran–Cambrian paleogeography of Baltica: A paleomagnetic view from a diamond pit on the White Sea east coast

The controversial late Ediacaran to Cambrian paleogeography is largely due to the paucity and low reliability of available paleomagnetic poles. Baltica is a prime example of these issues. Previously published paleomagnetic results from a thick clastic sedimentary pile in the White Sea region (northern Russia) provided valuable Ediacaran paleontological and paleomagnetic data. Until recently, Cambrian-age rocks in northern Russia were known mostly from boreholes or a few small outcrops. A recent mining operation in the Winter Coast region exposed >60 m of red sandstone and siltstone of the Cambrian Brusov Formation from the walls of a diamond pit. Paleomagnetic data from these rocks yield two major components. (1) A single-polarity A component is isolated in ∼90% of samples between 200 and 650 °C. The corresponding pole (Pole Latitutde, Plat = 20°S; Pole Longitude, Plong = 227°E, α95 = 7°) agrees with the Early Ordovician reference pole for Baltica. (2) A dual-polarity B component is identified in ∼33% of samples, mostly via remagnetization circles, isolated from samples above 650 °C. The corresponding pole (Plat = 12°S; Plong = 108°E, α95 = 5°) is close to other late Ediacaran data but far from all younger reference poles for Baltica. We argue for a primary magnetization for the B component and the secondary origin of the other Cambrian poles from Baltica. This in turn requires a major reshuffling of all continents and blocks around the North Atlantic. The early stages of Eurasia amalgamation and models for the evolution of the Central Asian Orogenic Belt require revision.

The first magnetostratigraphic data on the stratotype of the Lopata Formation, Northeastern Yenisei Ridge: Problems of its age and paleogeography of the Siberian Platform at the Proterozoic–Phanerozoic boundary

New paleomagnetic and magnetostratigraphic data are presented for the stratotype of the Upper Riphean Lopata Formation (Teya River, Yenisei Ridge). The paleomagnetic pole calculated is significantly distinct from the Phanerozoic and Riphean poles of the Siberian Platform and is similar to the Late Vendian–Early Cambrian poles of the Madagascar Group. The stratigraphic range studied is characterized by an anomalously high frequency of geomagnetic inversions (15 zones of magnetic polarity), which is comparable with the inversion frequency of the Late Vendian sections of Baltica. These data, along with previous paleontological findings, indicate an age of the Lopata Formation of 555–540 Ma.

Anisotropy of thermoremanent magnetisation of Cryogenian glaciogenic and Ediacaran red beds, South Australia: Neoproterozoic apparent or true polar wander?

Determining the effects of compaction-related inclination shallowing of remanence directions is crucial for ascertaining the validity of low palaeolatitudes for Neoproterozoic red beds in South Australia that are central to the debate concerning low-latitude Proterozoic glaciation. The inclination correction (or flattening) factor, f, is defined as tan(ID)/tan(IF), where ID and IF are the inclinations of the measured detrital remanence and the ancient inducing field, respectively. The anisotropy can be estimated using anisotropy of magnetic susceptibility and the anisotropy of high-field isothermal remanence (hf-AIR). The elongation–inclination (E–I) method has also been used to infer inclination shallowing. We add the anisotropy of thermoremanent magnetisation (ATR) to these methods. For the late Cryogenian Elatina Formation arenites, which constitute the bulk of the Elatina data set, the inclination correction using f=0.738 derived from ATR increases the palaeolatitude of the Elatina Formation from 6.5±2.2° to 8.8±3.2°, which confirms that the Elatina glaciation occurred near the palaeoequator. Inclination corrections for the Ediacaran argillaceous Brachina and Wonoka formations, using f=0.35–0.38 derived from ATR, are significantly greater than for the more arenaceous Elatina Formation, which increases their palaeolatitudes from ~12° to ~30°. Carbonates from the basal Ediacaran Nuccaleena Formation yielded f=0.8 from ATR, which represents only a small palaeolatitude correction from 19° to 23°. The anisotropy results imply that the characteristic remanent magnetisations carried by all these units were acquired early as depositional remanent magnetisations, essentially at the time of deposition. The shift of the palaeopoles from argillaceous units indicating significantly higher palaeolatitudes introduces a distinctive loop into the late Cryogenian–Ediacaran–Cambrian pole path for Australia. This loop shows similarities with the North American pole path for this period, for which true polar wander (TPW) has been inferred. However, until ages of Neoproterozoic strata in South Australia are better constrained uncertainty persists on whether the similarities of the Australian and North American pole paths reflect TPW.

Cambrian paleomagnetism in the Bateni Ridge: evolution of the Kuznetsk Alatau island arc ( southern Siberia)

We present paleomagnetic data on the Cambrian volcanosedimentary complexes of the Bateni Ridge, which correspond to the evolution of the Kuznetsk Alatau island arc. The paleomagnetic poles which served as the basis for the apparent polar-wander path (APWP) were determined. The paleomagnetic data were compared with those on the Cambrian poles for other fragments of the Kuznetsk Alatau island arc. For example, the APWP of the Bateni fragment corresponds to that of the Kiya (Martaiga) fragment of the eastern slope of the Kuznetsk Alatau. The distribution of the paleomagnetic poles suggests that the relative drift of the Kuznetsk Alatau arc fragments and the entire island arc system resulted from large-amplitude strike-slips. In the Cambrian, this paleoarc moved from ∼10° N to ∼10° S and rotated more than 50° clockwise. In the paleomagnetic record, its accretion is reflected in the coincidence of the Late Cambrian–Early Ordovician poles on the eastern slope of the Kuznetsk Alatau with one another and the coeval poles for the Siberian craton. This coincidence also suggests that the general structure of the present Kuznetsk Alatàu formed as early as the Early Caledonian. Nevertheless, the differences in the position of the poles on the western and eastern slopes of the Kuznetsk Alatau suggest that strike-slip activity along the Kuznetsk–Altai deep fault (the major one in the region) continued in the Late Paleozoic and, probably, in the Mesozoic.

A Mesoproterozoic paleomagnetic pole from the Yangzhuang Formation, North China and its tectonics implications

A new paleomagnetic study has been carried out on Mesoproterozoic sediments in the Jixian area, North China. Detailed stepwise thermal demagnetization allows us to isolate three components from 141 oriented drill-core samples (14 sites). A low-temperature component (component A) identified from most of samples falls close to the present local earth field direction. A middle-temperature component (component B) is found only in 21 samples from four sites. The component B directions (after bedding correction) yield a paleopole position at 336.0°E, 10.2°N with A 95 = 17.1°, which is close to the Lower Cambrian pole of the North China Block. Characteristic remanent directions with dual polarities obtained from high-temperature (component C) pass fold test. The directions yield a pole position at 190.4°E, 2.4°N with A 95 = 11.9°. This result indicates that the North China Block was located in low latitude at ca. 1.35 Ga. Recently, several studies suggested that Laurentia, Baltica and Siberia were also situated at low latitudes from well dated paleopoles between ca. 1.5 and 1.25 Ga. This would imply that the North China Block, Laurentia and Baltica might drift together or close in the Mesoproterozoic.

Age of magnetization of Mesoproterozoic rocks from the Natal sector of the Namaqua-Natal belt, South Africa

We report paleomagnetic results from Mezoproterozoic rocks from the Margate and Mzumbe terranes of the Natal belt, KwaZulu-Natal, South Africa. The Port Edward pluton, a member of the Oribi Gorge Suite, yields a pole position at 7.4°S, 327.8°E, A 95 = 4.2°. The magnetic remanence of these magnetite-bearing rocks was not acquired soon after crystallization at ∼1025 Ma but only when the sampling area cooled below 500 °C at about 1005 Ma. Similar results have been reported from the western (Namaqualand) sector of the Namaqua-Natal Belt which suggests that the entire belt underwent a similar tectono-metamorphic history. The tonalitic Mzumbe Gneiss was intruded by a swarm of porphyroblastic metadolerite dykes. The magnetization of these Equeefa dykes and the gneissic wall rock is carried by titano-maghemite with Curie temperatures between 330 and 400 °C. Their magnetization was reset at about 530 Ma after burial and exhumation and they yield a pole position at 24.7°N, 17.4°E, A 95 = 10.6°. These results demonstrate the difficulty in finding rocks on the Kalahari craton that would yield pole positions to fill the data gap in the apparent polar wander path between 1.1 and 1.0 Ga, a critical time span for Rodinia reconstructions, and provide the first dated Cambrian pole position for the Kalahari craton.

Return to Black Mountain: palaeomagnetic reassessment of the Chatsworth and Ninmaroo formations, western Queensland, Australia

SUMMARY Palaeomagnetic results from late Middle Cambrian‐Early Ordovician carbonate sequences sampled at Black Mountain (Mt Unbunmaroo), Mt Datson and near Chatsworth Station (southeastern Georgina Basin) are presented. A palaeomagnetic reassessment of these carbonates was designed in an effort to constrain regional magnetization ages as results from an earlier study, conducted at Mt Unbunmaroo, play a pivotal role in a proposed Cambrian inertial interchange true polar wander (IITPW) event. Remanent magnetizations within these carbonates were found to be variably developed with most specimens displaying two of the five isolated components. Component PF, for which goethite is the identified remanence carrier, is thought to reflect a chemical remanent magnetization of recent origin. Component TR, held by haematite, has a palaeomagnetic pole consistent with the Tertiary segment of Australia’s apparent polar wander path (APWP) and most probably was acquired as a consequence of prolonged weathering during this period. The A component has a palaeomagnetic pole at 54.7 ◦ S, 262.3 ◦ E( dp = 2.3 ◦ , dm = 4.5 ◦ ) after unfolding. This direction, constrained by positive fold and reversal test statistics, is consistent with Australia’s Early Devonian APWP, perhaps reflecting a remagnetization event associated with the intracratonic Alice Springs Orogeny. A Late Ordovician‐Early Silurian remanence, component B, is also described; with 100 per cent unfolding the associated palaeopole lies at 8.0 ◦ S, 216.8 ◦ E( dp = 2.6 ◦ , dm = 5.1 ◦ ). A third Palaeozoic, and presumed primary or early diagenetic, component, C, also passes applied fold and reversal tests and has a palaeomagnetic pole at 48.6 ◦ N, 186.0 ◦ E( dp= 2.1 ◦ S, dm = 4.0 ◦ ). This palaeopole is dissimilar from younger magnetizations, is consistent with Cambrian poles from other parts of cratonic Australia and falls within a cluster of Middle‐Late Cambrian (515‐500 Ma) palaeopoles from other Gondwanan continents. The age attributed to the palaeopole associated with the C component, ∼510 Ma, provides a tight constraint on the younger boundary of the proposed Cambrian IITPW event and its agreement with other Gondwanan palaeopoles is incompatible with the IITPW hypothesis. Components A, B and C are analogous to palaeomagnetic results reported in the earlier investigation of this region, and a comparison of results from the two studies, coupled with rigorous statistical analyses of the new findings, is presented.

True polar wander, a supercontinental legacy : Evans D.A., Earth and Planetary Science Letters, 1998, 157/1–2 (1–8)

A palaeomagnetic study is reported of reddened facies (Fengtai Formation) of Neoproterozoic glacial rocks which underlie Early Cambrian rocks with disconformity in the Huabei (North China/Sino-Korean) Block. The diamictite (preferred age 620-600 Ma) carries a dual polarity remanence residing in hematite of the red matrix. The mean direction derived from 62 samples is DI = 205.9− 32.4° (α95 = 3.9°) yielding a pole position at 233°E, 62°N. Tests on the matrix deformed beneath dropstones suggest that remanence was fixed before full compaction, although clasts near the base of the formation are largely overprinted. Overlying Early Cambrian sediments of the Houjiashan and Yutaishan formations have a similar remanence also of dual polarity (DI = 205.5− 32.1°, α95 = 3.9°, 32 samples). The magnetisation in the diamictite is therefore interpreted to have been acquired during loading by the ice sheet and/or the overlying Cambrian succession. Red shales from the Liulaobei Formation (∼890-840 Ma) near the base of the Neoproterozoic succession in the Huabei Block yield a contrasting remanence of DI = 59.475.3° (α95 = 7.1°, 19 samples) equivalent to a pole position at 150°E, 43°N. Pole positions from this study accord with Lower-Middle Cambrian poles from the Australian and South China Blocks with North China sited adjacent to northeastern Australia in accordance with recent biogeographic and palaeogeographic models. Although the palaeolatitude derived from the Fengtai diamictite (17°) is Cambrian in age and probably later than the glaciation, the correlation with Australia implies that glaciation in North China took place in low palaeolatitudes (∼20°) and reinforces the view that late Neoproterozoic glaciation, at least at this perimeter of Gondwana, occurred at low latitudes. Neoproterozoic-Cambrian poles from North China, South China and Australia fail to conform to the popular Rodinia reconstruction with the latter blocks adjacent to western North America and also fail to support the view that Gondwana formed by the fusion of widely separated eastern and western segments during Pan African orogeny. Instead they accord to poles from Africa and South America in a precursor of Gondwana (Protogondwana). The subsequent Palaeozoic derivation of this supercontinent was probably achieved by intracontinental strike-slip motion during Pan African deformation following extinction of the Afro-Arabian arc.

Palaeomagnetism of Neoproterozoic glacial rocks of the Huabei Shield: the North China Block in Gondwana

A palaeomagnetic study is reported of reddened facies (Fengtai Formation) of Neoproterozoic glacial rocks which underlie Early Cambrian rocks with disconformity in the Huabei (North China/Sino-Korean) Block. The diamictite (preferred age 620-600 Ma) carries a dual polarity remanence residing in hematite of the red matrix. The mean direction derived from 62 samples is D I = 205.9 − 32.4° ( α 95 = 3.9°) yielding a pole position at 233°E, 62°N. Tests on the matrix deformed beneath dropstones suggest that remanence was fixed before full compaction, although clasts near the base of the formation are largely overprinted. Overlying Early Cambrian sediments of the Houjiashan and Yutaishan formations have a similar remanence also of dual polarity ( D I = 205.5 − 32.1° , α 95 = 3.9°, 32 samples). The magnetisation in the diamictite is therefore interpreted to have been acquired during loading by the ice sheet and/or the overlying Cambrian succession. Red shales from the Liulaobei Formation (∼890-840 Ma) near the base of the Neoproterozoic succession in the Huabei Block yield a contrasting remanence of D I = 59.4 75.3° ( α 95 = 7.1°, 19 samples) equivalent to a pole position at 150°E, 43°N. Pole positions from this study accord with Lower-Middle Cambrian poles from the Australian and South China Blocks with North China sited adjacent to northeastern Australia in accordance with recent biogeographic and palaeogeographic models. Although the palaeolatitude derived from the Fengtai diamictite (17°) is Cambrian in age and probably later than the glaciation, the correlation with Australia implies that glaciation in North China took place in low palaeolatitudes (∼20°) and reinforces the view that late Neoproterozoic glaciation, at least at this perimeter of Gondwana, occurred at low latitudes. Neoproterozoic-Cambrian poles from North China, South China and Australia fail to conform to the popular Rodinia reconstruction with the latter blocks adjacent to western North America and also fail to support the view that Gondwana formed by the fusion of widely separated eastern and western segments during Pan African orogeny. Instead they accord to poles from Africa and South America in a precursor of Gondwana (Protogondwana). The subsequent Palaeozoic derivation of this supercontinent was probably achieved by intracontinental strike-slip motion during Pan African deformation following extinction of the Afro-Arabian arc.

Implications for Gondwana of new Ordovician paleomagnetic data from igneous rocks in southern Victoria Land, East Antarctica

New paleomagnetic data presented here from the southern Victoria Land (SVL) region of East Antarctica further refine the Gondwana early Paleozoic apparent polar wander path. The results are based on paleomagnetic analyses of Early to Middle Ordovician granitoids and dike swarms from which a new SVL pole (23°E, 3.5°S, A95 = 5.9°) was calculated. The new SVL paleomagnetic pole agrees with less well‐determined Ordovician poles from other parts of East Antarctica indicating little or no translation/rotation across the East Antarctic craton since the Middle Ordovician. A new Gondwana ∼475 Ma mean pole (11°E 36°N, A95 = 7°, N = 4 studies, in African coordinates) has been calculated from African, Australian, and Antarctic Early and early Middle Ordovician paleomagnetic poles. Age reassignment of the Antarctic Sør Rondane paleopole (Zijderveld, 1968) places it into the Cambrian period and when combined with other Gondwana Cambrian poles results in a new Gondwana ∼515 Ma pole (7°E, 22°N, A95 = 9.5°, N = 7 studies, in African coordinates). The new Gondwana ∼515 Ma and ∼475 Ma poles, when compared with poles of similar age from Laurentia, allow paleogeographic reconstructions to be made that are in keeping with models predicting that Iapetus Ocean basin opening and closure may have been related first to rifting and then collision of Laurentia with Gondwana. The paleomagnetic data also suggest that most of West Gondwana moved toward lower latitudes between the Middle/Late Cambrian and the late Early Ordovician which may be reflected in the fossil record.

Palaeomagnetic constraints on the position of Laurentia from middle Neoproterozoic to Early Cambrian times

Abstract Most published palaeopoles from Laurentia indicate that the craton moved in equatorial regions during much of the Neoproterozoic-Cambrian period. However, recent palaeomagnetic results from the Mackenzie and Wernecke Mountains of northwestern Canada suggest that Laurentia passed over the South Pole sometime close to the Precambrian-Cambrian boundary. By this scenario, the craton remained in equatorial latitudes as part of a Neoproterozoic supercontinent until after the second and last glaciation recorded in Laurentia. It began its ascent to high southern latitudes on or before deposition of the Risky Formation, perhaps following rifting along the eastern margin of Laurentia at ∼ 615 Ma. High-latitude poles related to hematitization might evidence Laurentia's flight to higher latitudes (60–70°S). Hematitization appears to have occurred during a long hiatus in the geological record between deposition of the Risky Formation and the terminal Proterozoic Ingta Formation, and may evidence a final rifting event along the western margin of Laurentia in the late Neoproterozoic. A combined pole from the Ingta and Early Cambrian Backbone Ranges formations forms a bridge to Early Cambrian poles that trend from the Equator to mid-southern latitudes. Except for the excursion of Laurentia over the South Pole in the transition from the Proterozoic to the Phanerozoic Eons, Laurentia seems to have been at latitudes less than 60°S in the period from ∼ 900 to 500 Ma.

Paleomagnetism of the Catoctin volcanic province: A new Vendian‐Cambrian apparent polar wander path for North America

The Vendian/Cambrian segment of the Lauretian apparent polar wander path (APWP) has been poorly constrained and the subject of some controversy. The Catoctin volcanic province in central Virginia is well‐dated at 570±35 Ma (Rb‐Sr) and 597±18 Ma (U‐Pb) and therefore presented an excellent paleomagnetic target for resolving the Laurentian Vendian‐Cambrian APWP. A total of 206 samples from 32 sites were collected from the Catoctin basalts, feeder dikes and sills. The study revealed three ancient directions of magnetization. The youngest, C component, fails the fold test and yielded a characteristic in situ direction of D = 147°, I = +44° (k = 21, α95 = 9°). The corresponding paleopole falls along the Middle Ordovician segment of the Laurentian APWP and we consider this component to be the result of a Taconic remagnetization. The second component, the B component, is carried by hematite, exhibits dual‐polarities and passes a fold test. The tilt‐corrected B component characteristic direction is D = 92°, I = +17° (k = 16, α95 = 13°). The corresponding paleopole at 4°S, 193°E falls near a well‐established Late Cambrian (505 Ma) pole for Laurentia, and we consider this component to be a remagnetization during a Late Cambrian tectonic event in the central Appalachians. The third component isolated in the Catoctin basalts, the A component, yields a tilt‐corrected mean of D = 68°, I = +84° (k = 59, α95 = 9°). This component passes a fold and reversal test. A suite of samples was collected from two Catoctin feeder dikes and surrounding country rocks that yield a positive baked contact test. The A pole at 43°S, 128°E falls significantly away from previously proposed Vendian poles for Laurentia. A reevaluation of previous paleomagnetic studies from coeval rock units reveals similarly steep directions and leads us to propose a new APWP. This new APW track indicates that Laurentia was located near the pole during the interval 615–580 Ma and drifted rapidly (16 cm yr−1) toward its Late Cambrian equatorial position.

An Historical Perspective of the Early Palaeozoic APWP of Gondwana: New Results from the Early Ordovician Black Hill Norite, South Australia

While paleomagnetic pole paths and the palaeo-distribution of the continents is reasonably well understood for the past 200 Ma, there is an urgent need for more reliable Palaeozoic palaeomagnetic poles, especially from the Gondwana continents. The distribution of continents and the disposition and convergence of their margins is an important consideration in conceptual models of terrane accretion. Some continental distributions suggest only small oceans between Laurentia and Gondwana throughout the Palaeozoic, while others suggest a wide oceanic separation existed in the Devonian-Carboniferous. Current thoughts on the Gondwana Palaeozoic pole path may be grouped into, the autochthonous school, the allochthonous school and the Kanimblan overprinting push. The pole position from the Black Hill Norite, South Australia, suggests that some Australian Cambrian pole positions which plot nearby may be Early Ordovician overprints.

A palaeomagnetic study of two Precambrian-Cambrian dyke swarms from the Armorican Massif

Abstract The palaeomagnetic analysis of two dolerite dyke swarms, metamorphosed during the late stages of the Cadomian Orogeny, is reported. The Vazon dyke swarm of Guernsey, Channel Islands, has yielded a characteristic component of magnetization (D = 199°, I = 48°, a95 = 12°, n = 8 dykes) which is dual polarity, while six other sampled dykes record intermediate field directions. The magnetization is interpreted as being acquired shortly after dyke emplacement and the corresponding palaeomagnetic pole (lat, 9°S; long, 340°E) is in good agreement with previously determined Cambrian poles from Armorica. An emplacement age for the dykes of 560–540 Ma is therefore suggested on the basis of the radiometric ages assigned to these previously published results. The Dahouet dyke swarm of the Baie de St Brieuc was also sampled and has yielded two remanence directions (A and B) common to most sites. The A remanence (D = 198°, I = 2°, a95 = 7°, n = 7 dykes) yields a pole (lat, 37°S; long, 334°) which is interpreted as being an overprint acquired during the Variscan Orogeny and is consistent with Carboniferous results. The B remanence (D = 202°, I = 34°, A95 = 8°, N = 6 dykes) and its corresponding pole (lat, 20°S; long, 335°E) is more ambiguous in its interpretation as it lies between the known Cambrian APW path and a diffuse area of poles associated with overprinting during the Variscan orogeny. A tentative interpretation of this remanence as being Cambrian in origin is consistent with the suggestion that both dyke swarms were emplaced relatively closely in time, as suggested by their similar geological settings.

Paleomagnetism of Proterozoic and Cambrian Strata, Adrar de Mauritanie, CRatonic West Africa

Four paleomagnetic poles are reported from red beds of the Adrar de Mauritanie, cratonic West Africa. Thirteen localities were occupied, and 224 samples were collected in seven units from a middle Proterozoic to early Paleozoic sedimentary section. Two poles are reported from middle and late Proterozoic strata; a third pole is reported from Cambrian strata. The magnetizations for these reside in stable high‐temperature components, revealed by progressive thermal demagnetization analysis to 675°C. The fourth pole is interpreted to reflect a Permian overprinting in the Cambrian strata. Two Cambrian‐Ordovician units sampled contained directions too scattered to allow the calculation of any pole. The pole from middle Proterozoic strata (998±32 Ma; Rb‐Sr date on argillite), derived from four localities (52 samples, single polarity), plots at 48.7°N, 206.9°E (dp = 1.1°, dm = 2.9°). The pole from late Proterozoic strata (775 ± 52 Ma; Rb‐Sr date), derived from two localities (15 samples, mixed polarity), plots at 34.0°N, 246.9°E (dp = 4.6°, dm = 8.6°). Neither has correlatives in age in the paleomagnetic data set for cratonic West Africa, nor are there obvious correlatives with the paleomagnetic data set from the remainder of Africa. The pole that we consider to reflect a Cambrian direction comes from three localities (23 samples, mixed polarity), plotting at 0.3°S, 249.3°E (dp = 5.7°, dm = 11.5°). Its antiparallel pole corresponds with one group of Cambrian poles reported from Africa but differs markedly from another group of Cambrian poles. The late Paleozoic overprint direction obtained from three units (50 samples from four localities, single polarity) provides a (reversed polarity) pole that plots at 47.2°S, 42.6°E (dp = 1.5°, dm = 3.0°), which is in good agreement with Permian poles reported from elsewhere in Africa. In spite of the large number of poles already reported from Africa, additional data are needed before a reliable apparent polar wandering path can be constructed for the Paleozoic and Proterozoic of Africa.

Palaeomagnetism, palaeogeography and the multicomponent analysis of middle and upper Cambrian rocks of the Barrandian in the Bohemian Massif

With a view to the importance of the Barrandian in the Bohemian Massif for global palaeogeographic reconstructions, considerable attention has been devoted to palaeomagnetic stability tests and multicomponent analysis of rocks of the pre-Variscan formations, using substantially improved laboratory techniques. This study is devoted to samples of Upper Cambrian andesite and Middle Cambrian sediments. Alternating and thermal fields were used for magnetic cleaning. A thermal demagnetizer fitted with a newly developed inductor of high sensitivity, the ROCOMA (rotating coil magnetometer), with a sensitivity of 0.1 to 0.2 nT, was used. The whole newly built and fully tested MAVACS system (magnetic vacuum control system) guaranteed a high degree of compensation of the magnetic field in a volume of 5 1, to a value not exceeding 2 nT. As numerous laboratory experiments have shown, the high magnetic vacuum was necessary to derive the palaeomagnetic directions of rocks of the older formations affected significantly by the Variscan orogenic phase. The position of the Upper Cambrian pole (28.6°S, 131.2°E) should be considered tentative because it was derived using samples from only three andesite outcrops. Two other andesite outcrops preserved Carboniferous or Permo-Carboniferous palaeomagnetic directions. The position of the Middle Cambrian pole (11.3°S, 154.0°E) was determined using samples of sediments (mainly greywackes and shales) from ten outcrops, some of which represent extensive rock cross-sections. By combining multicomponent analysis with fold tests, the mean direction of the pre-Variscan magnetization could be derived. The very low values of the moduli of the natural remanent magnetization of the Middle Cambrian sediments, and the considerable amount of magnetically soft components, required special care to be taken in the laboratory procedures. The palaeozoogeographic zones of the Barrandian are related to palaeogeographic latitudes of about 40°S, derived from palaeomagnetic data. Cold water is characteristic for the Bohemian fauna, but arctic conditions can be ruled out due to the amount of primary red sediments, mostly of continental origin.

A paleomagnetic study of the layered mafic intrusion at Sept-Îles, Quebec

A paleomagnetic study of gabbroic and anorthositic members of the Sept-Îles layered mafic intrusion has isolated a remanence with direction D = 333°, I = −29 °(remanence A) and a corresponding pole of 141 °E, 20°N. The rocks are cut by numerous diabase dykes of unknown age. The remanence carried by the dykes has direction D = 188°, I = −85 °(remanence B), with a corresponding pole of 116°E, 59°S. Another remanence, statistically identical to B (D = 186°, I = −85°), is found at dyke contacts and in the local host rock; its associated pole, 115°E, 61°S, is identical to the one derived from the dykes. This overprint is significantly different from remanence A, and is obviously related to dyke emplacement. The location of the pole derived from remanence A is in excellent agreement with many other Cambrian poles from a variety of locations throughout cratonic North America. The rocks at Sept-Îles have been previously dated radiometrically at 540 Ma. Thus, all evidence indicates that remanence A is an original thermochemical remanent magnetization, acquired during initial cooling of the intrusion in the Cambrian and prior to the emplacement of the dykes.

Paleomagnetism of Early Cambrian redbeds on Cape Breton Island, Nova Scotia

Results are reported from steeply dipping red sandstones collected at nine sites (45 samples) of the Lower Cambrian Morrison River and MacCodrum formations on southeast Cape Breton Island, which forms part of the Avalon terrane. Demagnetization studies indicate minimal overlap of stability spectra in thermal, alternating-field, and chemical treatments. A stable component (hematite) with high unblocking temperatures (680 °C) is dominant at all sites. Its mean direction corrected for geologic tilt is D = 281°, I = +32°, k = 97, giving a paleomagnetic pole (pole M) at 20°N, 146°W (dp, dm = 3°, 6°). The corresponding in situ direction is D = 147°, I = +59°, k = 97, with a pole M′ at 2°S, 39°W (dp, dm = 6°, 8°). Poles M and M′ were compared with published Paleozoic poles for the Avalon Zone and cratonic North America to test whether the magnetization originated before or after the Taconic (Ordovician) folding. It could have been acquired during deposition or early diagenesis (Early to Middle Cambrian), in which case the paleolatitude differences between Morrison River–MacCodrum and age-equivalent cratonic sites require a minimum 20° post-Early Cambrian displacement between Avalon Zone and craton. This is, however, difficult to verify, since a fold test is not possible and well-dated Cambrian poles from Avalonia are lacking. Three or four of the many post-Cambrian results from the Avalon and Gander zones were found to be comparable with pole M′, so that alternatively the Morrison River–MacCodrum magnetization could be a postfolding, Late Ordovician–Early Silurian or Late Devonian overprint.

Paleomagnetic and petrographic investigation of the Taum Sauk Limestone, southeast Missouri

Paleomagnetic results from 17 sites in one quarry of the Cambrian (Dresbachian Stage) Taum Sauk Limestone (member Bonneterre Formation) reveal two components of magnetization. On the basis of thermal demagnetization, coercivity spectrum analysis, and petrographic evidence, both components reside in hematite. Sites can be divided into three groups based on behavior during thermal demagnetization and petrographic criteria. The first group consists of massive pink limestones and contains a component with declination 270°, inclination −11°, corresponding to a Cambrian pole at 175.6°E, 3.6°S. The second group of sampling sites consists of specimens with numerous burrows filled with hematite intergrown with calcite. The sites are dominated by the second component of magnetization which has declination 147° and inclination 10° and corresponds to a reversed pole position at 133.1°E, 37.3°N. A significant fraction of this component, which is interpreted to be Carboniferous in age, resides in hematite intergrown with calcite in the burrows. Petrographic relationships indicate that the calcite and hematite were acquired before dolomite, which places a maximum age on dolomitization. The third group consists of massive pink limestones with variable amounts of burrowing and contains both directions of magnetization in roughly equal proportions, with closely overlapping blocking temperature spectra. The presence of two stable directions of magnetization from one location with neither being dominant or even present throughout the quarry indicates the significant effect that diagenetic and sedimentologic processes may have on the magnetization of sedimentary units.