Palaeomagnetic Analysis Research Articles

Middle Cambrian high magnetic reversal frequency (Kulumbe River section, northwestern Siberia) and reversal behaviour during the Early Palaeozoic

We present a magnetostratigraphic study of Middle Cambrian sediments along the Kulumbe River (northwestern Siberia). The deposits are ∼850 m thick and lie within the Mayan stage as constrained by trilobite fossils. Palaeomagnetic analyses reveal two magnetic components. The first component, parallel to the direction of the present day field at the site, is isolated in the low to middle temperature range. A high temperature component (HTC) is then isolated up to 580 or 680°C, carried either by magnetite and/or by hematite. The HTC defines a sequence of 28 magnetic polarity intervals, some of them being defined by only one sample. Considering that the studied section has a duration of ∼5 Myr, we propose that the magnetic reversal frequency was high (∼4–6 reversals per Myr) during part of the Middle Cambrian, among the highest values known within the Phanerozoic. The reversal frequency may have been roughly similar during the Lower Cambrian. We further suggest a drastic decrease of the magnetic reversal frequency between the Lower–Middle Cambrian and the end of the Tremadoc (Ordovician) when a superchron probably occurred. However, this behaviour is still challenged by other scenarios which depend on the chosen Early Palaeozoic time scale and on the reliability of some magnetostratigraphic data.

Palaeomagnetism, rock magnetism and geochemistry of Jurassic dykes and correlative redbeds, Massachusetts, USA

Jurassic diabase dykes, sills and sedimentary rocks in central Massachusetts were sampled for palaeomagnetic analysis. The intrusions fall into three of the chemical types for eastern North American diabases: high TiO2 quartz-normative (Holden); low TiO2 quartz-normative (Ware); and high Fe2O3 quartz-normative (Pelham–Loudville). The characteristic magnetizations in the majority of intrusive samples unblock between 550 °C and 580 °C, with Curie temperatures in a discrete interval between 556 °C and 580 °C. The dominant remanence in the diabases is carried by C1 to C3 oxidation-exsolved titanomagnetite occurring as euhedral grains, as fine needles or dust in the matrix, as devitrifed glass, and as fine magnetite-ilmenite-silicate symplectite. In some dykes, titanomagnetite was further modified by deuteric oxidation during post-magmatic cooling, creating titanomaghematite and/or a granulation of the magnetite. Palaeopoles for the three diabase groups are: Holden, 60.1°N, 80.5°E, A95 = 4.1°; Ware, 73.5°N, 85.8°E, A95 = 3.9°; and Pelham–Loudville, 65.3°N, 95.6°E, A95 = 4.1°. These data are combined with samples from two stratigraphic sections through the Early Jurassic part of the Sugarloaf Formation in the Deerfield Basin representing both fine-grained mudstones and coarser arkoses. These haematite-dominated rocks reveal several components of magnetization, a steep recent field direction, an intermediate secondary diagenetic overprint direction in both mudstones and arkoses, and a high-temperature shallow primary direction found only in the mudstones. Palaeopoles for the Sugarloaf Formation are: mudstones, 57.7°N, 81.3°E, A95 = 9.1°; and arkoses, 75.1°N, 131.6°E, A95 = 5.9°. Based on the new palaeomagnetic data reported here, the North American plate in the Middle Jurassic was at higher palaeolatitudes than indicated by the present North American apparent polar wander path.

Dolomitic breccia veins as evidence for extension and fluid flow in the Dalradian of Argyll

Veins of red dolomite occur extensively in the Dalradian rocks of Argyll, Scotland and adjacent areas. The veins represent brittle extensional deformation, preferentially reactivating Caledonian quartz veins. The dolomite is associated with reddening of the adjacent Dalradian country rock, which it partially replaced. Dolomite was also precipitated in overlying Old Red Sandstone, and probably dates to late Carboniferous–early Permian. Fluid inclusion studies show that the veining involved moderate- temperature (75 to 115 °C) fluids. Stable isotope data suggest that these fluids were basinal brines. Traces of chalcopyrite, paragenetically late in the veins, may reflect the mineralization which occurs more widely in the Dalradian rocks of Argyll. The red colour of the dolomite is due to abundant haematite crystallites that grew in the dolomite crystal fabric. Palaeomagnetic analysis yields a consistent late Permian–early Triassic age for the haematite growth in the dolomite veins and the reddened Dalradian country rocks. This age represents the time of haematite precipitation from iron-rich dolomite that may have been related to deep oxidizing weathering. Gold anomalies associated with reddened basement rock must be of this age or younger.

Palaeomagnetic dating of dolomitization and Mississippi Valley-type zinc mineralization in the Mascot-Jefferson City district of eastern Tennessee: a preliminary analysis

Palaeomagnetic analysis of 576 specimens (∼400 in detail) from 37 sites in four mines of the Mascot-Jefferson City Mississippi Valley-type zinc district indicates that the sphalerite mineralization in the Lower Ordovician carbonate host rocks was emplaced during the early Pennsylvanian at 314±6Ma during the Alleghenian Orogeny, and that peak dolomitization or peak recrystallization of the host rocks occurred about 20My earlier.

Palaeomagnetic Studies of Burned Rocks

An assemblage of rocks, such as comprising a fireplace, will acquire a remanent magnetization upon cooling parallel to the ambient geomagnetic field and thus will share a common direction of magnetization. Detailed palaeomagnetic analyses of oriented samples will reveal whether these rocks have remained in situ since the last heating event or whether these rocks have moved or represent discard material. Thermal demagnetization makes it possible in many cases to estimate the maximum temperature which a given rock has experienced. The direction of magnetization can be used to reposition rocks to their cooling position and the cooling history of boiling stones can be reconstructed for some rocks. This paper presents results obtained from burned rocks from various archaeological sites.

Magnetic fabric and palaeomagnetic analyses of the Plio–Quaternary calc–alkaline series of Aegina Island, South Aegean volcanic arc, Greece

The South Aegean volcanic arc represents the magmatic expression of active subduction of the African plate beneath Eurasia in the region of Greece. Plio–Quaternary calc–alkaline (dacite and andesite) lava flows exposed on the island of Aegina have been sampled for palaeomagnetic analyses to determine whether the western arc area has experienced vertical axis rotations. In addition, these units allow an assessment of the use of magnetic fabric (anisotropy of magnetic susceptibility; AMS) data as indicators of lava flow directions in silicic extrusive rocks, providing a complement to the existing literature on AMS of basaltic flows. The magnetic mineralogy of both dacites and andesites is dominated by fine-grained magnetite/Ti-poor titanomagnetite. Sampled flows are characterised by oblate magnetic fabrics at site level which are co-axial to flow surfaces, although most sites also contain specimens with prolate AMS ellipsoids. Where magnetic lineations are developed, they are systematically aligned either parallel or orthogonal to average (exposure-scale) lava flow directions. Magnetic lineations in these rocks, therefore, may be interpreted in terms of flow directions only when the latter are known a priori. Observed variability in the directions of remanent magnetisation recorded by the Aegina lava flows may be explained wholly in terms of geomagnetic variations. Several sites record spot readings of the geomagnetic field during polarity transitions. Scatter of directions at the remaining sites is consistent with that expected from secular variation. Post-eruption tilting of magnetisation vectors is shown to be limited. The in situ mean direction of magnetisation of the non-transitional sites is indistinguishable from the geocentric dipolar field used as a reference direction for these young (<5 Ma) rocks. This indicates that the island has not experienced significant Plio–Quaternary tectonic rotations, in contrast to other parts of the Aegean volcanic arc. The magnetic data also further constrain the timing of volcanic activity in the western part of the South Aegean arc, when used in conjunction with existing radiometric and stratigraphic ages. The most recent volcanic products are shown to correlate with the Pleistocene part of the Matuyama polarity epoch. The Aegina volcanic centre has, therefore, been inactive for at least the last 720 000 yr, in contrast to the historical activity further to the east on Santorini.

A palaeomagnetic analysis of rapakivi intrusions and related dykes in the Fennoscandian Shield

In the Fennoscandian Shield (Baltica) there are seven major rapakivi plutons and fifteen minor ones ranging in age from ca 1.66 to 1.50 Ga. These plutons are distributed in a broad WNW zone and if the most eastern pluton is excluded there is a westward trend of decreasing age of the intrusions. A palaeomagnetic study has been performed on 4 minor plutons (Rödö, Mårdsjö, Norsjö and Mullnäset) and associated dykes in central Sweden. The results were combined with palaeomagnetic data from other rapakivi complexes in Fennoscandia in order to test if a stationary hot spot may be the origin of these anorogenic intrusions. Plotting the pole positions of this study together with poles of other complexes, poles calculated from rapakivi rocks and related dykes in Finland are located at somewhat lower latitudes and more eastern longitudes than poles of corresponding rocks in Sweden, probably reflecting an APW related to the general age differences between the plutons. The palaeolatitudes for the Fennoscandian Shield at the time of the rapakivi intrusions are restricted to a latitudinal range between ca 16° south and 27° north and there is a weak trend of increasing palaeolatitude with decreasing age of the rocks. A trend of gradually changing palaeolatitudinal positions has also been observed for the intrusion of Proterozoic anorthosite-rapakivi plutons in the Ukranian Shield. Such differences in palaeolatitudes is not expected in case of a single stationary hot spot being the source of the rapakivi intrusions, as the rock then should carry a magnetization reflecting the same latitudinal position.

Human cannibalism in the Early Pleistocene of Europe (Gran Dolina, Sierra de Atapuerca, Burgos, Spain)

Human remains belonging to at least six individuals were found in an exploratory excavation made at the site of Gran Dolina (Sierra de Atapuerca, Burgos, Spain). These remains were recovered from the Aurora Stratum of Unit TD6. This stratum has a thickness of approximately 30cm. The area of the exploratory excavation is about 7m2. According to palaeomagnetic analyses, Unit TD6 shows reversed polarity, which is considered to belong to the Matuyama chron. This unit is immediately below TD7, where the Matuyama–Brunhes boundary has been detected, indicating an age of around 780,000 years BP.There is no specific distribution, treatment, or arrangement of the human remains, which were found randomly mixed with abundant faunal remains and stone tools. Most of the faunal and human fossil bones from the Aurora Stratum have human induced damage. Stone tool cutmarks are frequent, and peeling (a type of fracture similar to bending a fresh twig between the hands) provides a specific breakage pattern together with percussion marks and chopmarks. Both nonhuman and human remains show similar intensive exploitation. Slight differences, however, have been observed between fauna and humans (e.g., peeling frequent in humans, rare in fauna), that appear related to different musculature, weight, and bone structure. The characteristics of this fossil assemblage suggest that it is solely the result of consumptive activities as there is no evidence of ritual or other intention. The possibility of distinguishing between dietary vs. survival cannibalism is discussed here.

Palaeomagnetic studies in the Permian Basin of Largentière and implications for the Late Variscan rotations in the French Massif Central

Detailed geological observations and palaeomagnetic analyses were carried out in the Largentière Stephano–Autunian basin and on the Stephanian deposits of the Alès coalfield, both located at the southeastern margin of the French Massif Central. Because of unfavourable rock types, the Alès Stephanian deposits did not yield any results. The palaeomagnetic pole (164.9°E, 45.4°N, K = 89, A95 = 4.1°) deduced from a study of the Autunian sediments of the Largentière Basin agrees very well with the reference pole for stable Europe. The Lodève–Largentière area, that is the southeastern border of the Massif Central, has been stable since Early Permian time with respect to stable Europe, whereas the western part (the Saint-Affrique Rodez Basin and, probably, the Brive Basin) has been rotated counterclockwise.

A NEW PERSPECTIVE ON THE ZONATION AND CORRELATION OF BARREN STRATA: AN INTEGRATED HEAVY MINERAL AND PALAEOMAGNETIC STUDY OF THE SHERWOOD SANDSTONE GROUP, EAST IRISH SEA BASIN AND SURROUNDING AREAS

The integration of data from high‐resolution heavy mineral and palaeomagnetic analyses of samples from nine East Irish Sea wells, and from onshore sections at St Bees Head and the North Cheshire Basin, has generated a temporally‐constrained heavy mineral stratigraphy and regional correlation within the Triassic Sherwood Sandstone Group. These results have shown that the combined application of these two techniques offers an effective tool for extending the zonation and correlation of barren strata into uncored wells. The lateral persistence of the heavy mineral zones, and the significant differences in the distributions of the heavy mineral varieties between individual zones, may reflect tectonic events and, to a lesser extent, climatic variations.Palaeomagnetic results indicate that the original magnetisation of the analysed samples was acquired as a detrital remanent magnetisation and that the characteristic remanent magnetisation in the red beds was acquired during and shortly after deposition. Magnetopolarities have been interpolated using the available data from which a composite reference section has been constructed.All well sequences exhibit a regionally persistent three‐fold zonation (Zones A, B and C, locally with subzones), defined by systematic changes in characteristic heavy mineral suites. Using a common sample set, the independently erected heavy mineral stratigraphy was tied to specific magnetostratigraphic Chrons. Heavy mineral Zone A coincides with a period of dominantly normal polarity, fixed by a 244 Ma Chron of Middle Scythian age. A change in polarity from normal to reversed at &gt; 243 Ma coincides with the boundary of Zones A/B. Zone B is Middle to Upper Scythian and shows a dominantly reversed polarity. A well‐defined Chron of 242.7 Ma has been detected close to the boundary of Zones B/C. Zone C is Anisian in age and is characterised by a dominantly normal polarity.

Rock magnetism and palaeomagnetism of greigite-bearing mudstones in the Italian peninsula

Magnetic measurements were carried out on a collection of samples from different geological settings throughout the Italian peninsula. The samples display magnetic properties that indicate the presence of ferrimagnetic iron sulphide minerals. Paired samples were also investigated to compare the palaeomagnetic behaviour with respect to different demagnetization treatments (thermal, static and tumbling alternating field (AF) demagnetization). Greigite (Fe 3S 4) was positively identified in most samples. In agreement with previous studies of greigite, the sediments display: thermal decomposition of the magnetic carriers at temperatures above ca. 230°C, resulting in decreased magnetic susceptibility values and maximum unblocking temperatures ( T ub) mostly in the range 320–350°C; high saturation isothermal remanent magnetization (SIRM) to low-field magnetic susceptibility ( k) ratios (SIRM/ k > 10 kA/m); hysteresis ratios that are typical for single domain (SD) grains ( M rs/ M s>0.5 and ( B 0) cr/( B 0) c<1.5, where M rs is the saturation remanence, M s is the saturation magnetization, ( B 0) c is the coercive force and ( B 0) cr is the coercivity of remanence); moderate coercivity, with ( B 0) cr values that range between 52 and 81 mT and a tendency to acquire a significant rotational remanent magnetization (RRM). We also found that greigite-bearing sediments are particularly sensitive to field impressed anisotropy. We propose a new magnetic parameter that can be used as a rapid mean to screen a rock sample for the presence of greigite, which relies on its SD behaviour in field impressed anisotropy experiments. In greigite-bearing sediments, the magnetic susceptibility measured in a given direction is increased by the application of a relatively high magnetic field (0.9 T in our experiments) at right angles to this direction. The susceptibility increase is directly correlated to the concentration of greigite in the sediment. The comparative palaeomagnetic analysis of standard demagnetization techniques indicates that thermal demagnetization, in close steps up to 350–380°C, is clearly the most effective treatment for the isolation of the remanent magnetization carried by greigite, whereas AF demagnetization treatments display significant acquisition of unwanted magnetic remanences (gyroremanent remanent magnetization (GRM) and RRM) at fields above ca. 40 mT.

Late Weichselian pollen stratigraphy, clay-varve chronology, radiocarbon chronology, and palaeomagnetic secular variations at Farslycke, Blekinge, southern Sweden

Pollen analysis, AMS 14C measurements and varve counting, carried out on a sequence of Late Weichselian lacustrine sediments in Blekinge in southeastern Sweden, show that the area was deglaciated during the middle part of the Bølling, at c. 12,500 14C years BP + c. 200 varve years. This date fits well with the regional deglaciation chronology. The deglaciation was accompanied by a typical late glacial vegetation succession. The varve chronology for the site is correlated to the regional chronology for Blekinge, and the ice recession rate is calculated to c. 100 m/year. The palaeomagnetic secular variation analyses show a characteristic westerly declination swing at c. 11,500-10,300 14C years BP, which is in accordance with curves from other sites in southern Sweden.

The Site of Ain Hanech Revisited: New Investigations at this Lower Pleistocene Site in Northern Algeria

New archaeological investigations have been carried out at the Oldowan site of Ain Hanech in northeast Algeria. The archaeological horizons are delineated and positioned relative to the regional stratigraphy. Preliminary palaeomagnetic analysis reveals normal polarity for the Oldowan occurrences that may be correlated with the Olduvai subchron on the indication of the vertebrate faunal biozonation. Broken up bones associated with stone artefacts and contained in a silty deposit mixed with sand and gravels were excavated from two archaeological sites. The bone assemblage includes equids, bovids, elephant, and rhinoceros, and dominated byEquus tabeti. Taphonomic patterns indicate that bones were buried rapidly over a period of years and while the site was occupied. The lithic artefacts are fresh and represent a coherent assemblage, including small debitage. The artefacts, made of limestone and flint, include flaked cobbles, whole flakes, various fragments, and retouched pieces. A microwear study, undertaken on pieces made of flint, indicates the use of two simple flakes and of one denticulate for meat processing. The stone artefact assemblage may be considered as a North African variant of the Oldowan.

Palaeomagnetic dating of the west Cumbrian hematite deposits and implications for their mode of formation

SUMMARY Palaeomagnetic analysis of six orientated hematite samples from the Florence Mine, Egremont, west Cumbria is described. A total of 21 specimens were subjected to incremental thermal demagnetization and the components of magnetization isolated by principal components analysis. All the specimens displayed a single component of magnetization with no evidence of either multicomponent magnetizations or significant low-temperature secondary magnetic components. The mean direction of the characteristic remanence based on the isolated components has a declination of 203°, and an inclination of −13° (α 95 = 10°, k = 12) with the corresponding pole position at 38.6°S, 333.5°E ( D p = 5.2, D m = 10.3). A single primary component is responsible for the characteristic remanent magnetism (ChRM) directions. The initial susceptibilities and magnetic intensities are compatible with petrographic observations that the samples comprise virtually pure hematite. The ChRM originated as chemical remanent magnetism when the hematite precipitated at temperatures over 100°C. Comparison of the calculated pole position for the west Cumbrian hematite deposits with the apparent polar wander path for Britain south of the Iapetus Suture reveals that the ore deposits formed during the Late Carboniferous and Early Permian. The west Cumbrian hematite deposits therefore are not precipitates of a brine expelled from the Triassic sediments of the East Irish Sea Basin. The hematite may have precipitated from a brine expelled from older sediments (Devonian?), a fluid associated with igneous activity resulting from Permian rifting in the Irish Sea area, or they may be associated with ingress of surface-derived fluids during the Permian.

Block rotations and continental extension in the central Aegean Sea: palaeomagnetic and structural evidence from Tinos and Mykonos (Cyclades, Greece)

The Aegean Sea, floored by an attenuated continental crust, is a Mediterranean-type back-arc basin formed above the subducting African plate. We investigated post-12 Ma block rotation in relation to back-arc extension in the area of the western Cyclades (central Aegean Sea). On Tinos island, NW-trending dacitic dikes and a Miocene monzogranite penetrated an Alpine metamorphic series. KAr analyses of 6 dike samples yielded an average age of 11.5 ± 0.4 Ma. The palaeomagnetic analyses of the dikes and the monzogranite reveal 23° vertical-axis clockwise rotation. We assume that the dacitic dikes intruded vertical (as they are today) and that no tilt correction is required. This is supported by the fact that no inclination anomaly is revealed when the palaeomagnetic data is compared with a tilt-corrected pole position from Kimi (Evia). The absence of tilting implies that post-12 Ma crustal thinning in the area of Tinos island was minor. On Mykonos, a low-angle normal fault dipping 30° to the NE juxtaposes a series of Oligo-Miocene molasse sediments above a 10–12-Ma granite. Published palaeomagnetic data revealed a considerable amount of inclination flattening and 22° clockwise rotation of the footwall granite. We show that tilting predated vertical-axis rotation, and that the (now) low-angle normal fault originated steeper, dipping ca. 54°. Planar-rotational fault geometry indicates extension of ca. 60%. In spite of the significant variations in the magnitude of post-12 Ma upper crustal extension across the Tinos-Mykonos transect, crustal thickness below the entire area changes only slightly. This indicates that upper-crustal levels were decoupled from the lower crust. The widespread Middle Miocene granitic magmatism in the Cyclades suggests that flow in the lower crust may have had a dominant role in maintaining the smooth Moho. Assuming that the western Cyclades constitute a set of NW-striking crustal blocks, it is estimated that vertical-axis rotations increased the N-S dimension of the area by ca. 31% (e.g. 25 km). The exact timing of this clockwise rotation is not well constrained, but it postdated the formation of low-angle normal faults and tilting, and may have taken place in the Late Miocene.

Palaeomagnetic dating of sub-Torridon Group weathering profiles, NW Scotland: verification of Neoproterozoic palaeosols

A palaeoplain that cuts across Lewisian Gneiss in the Cape Wrath area, northwest Scotland, is weathered to depths of 1–6 m beneath fluvial red conglomerates and sandstones of the Applecross Formation of the early Neoproterozoic Torridon Group. Weathered Lewisian rocks are enriched in K 2 O and Fe 2 O 3 , and contain hematite as an alteration product of biotite and magnetite. Palaeomagnetic analysis of the weathering profiles has been carried out to constrain the age of alteration. Thermal step demagnetization of 87 oriented core specimens of biotite-gneiss and amphibolite spanning four profiles at Poll a’ Mhurain and Sheigra reveal a stable, high-temperature component with a mean direction of remanence of D= 149.1°, I =57.2° (α 95 = 5.0°) (corrected for dip of the overlying Applecross strata) that gives a pole at 9.8°S, 199.2°E (dp = 5.3°, dm=7.3°). This direction differs significantly from that for little-weathered Lewisian at those two localities and also from directions for the late Mesoproterozoic Stoer Group. The remanence direction for the weathered rocks is, however, near the dip-corrected high-temperature remanence direction (D= 122.3°, I =50.5°, α 95 =10.8°) obtained by us from the thermal step demagnetization of 39 sandstone specimens of the Applecross Formation in the Cape Wrath area. Furthermore, the direction for the weathered rocks agrees closely with other dip-corrected results for the Torridon Group, such as the detrital remanent magnetization (DRM) mean direction (D= 148.3°, I =59.9°, α 95 =13.8°) for the Applecross Formation on Rum and that for the main outcrop of the Aultbea Formation ( D =139.7°, I =52.6°, α 95 = 14.5°). The concordance of palaeomagnetic results for the sediments and weathered rocks implies that the weathering profiles in the Cape Wrath area acquired their remanence penecontemporaneously with deposition of the Applecross and Aultbea formations. The palaeomagnetic results therefore argue strongly that the weathering profiles are palaeosols of Applecross age ( c. 980 Ma) that formed at a palaeolatitude of 38° and constitute the oldest known palaeosols in the British Isles—views that the weathering is substantially older or younger than Applecross deposition cannot be sustained. Our findings carry implications for Applecross palaeoclimate and source and strengthen the conclusion that atmospheric oxygen was a substantial fraction of its present abundance by early Neoproterozoic time.

Uranium/thorium dating of ferricretes from mid- to late pleistocene glacial sediments, western Tasmania, Australia

The uranium/thorium (U/Th) dating method was applied to pedogenic/diagenic ferricretes developed within glacial drifts from the northern West Coast Ranges of Tasmania. The dates derived from the method are, by definition, minima for the sediments, with the ferricretes being secondary mineral deposits which appear in the form of bands, horizons or nodules comprised of indurated masses of iron oxy(hydr)oxides developed post-depositionally within the host sediment or soil. The absence of such deposits in well-identified drift sediments of the last glaciation suggests the development of ferricrete horizons in glacial drifts occurred during earlier interstadials and interglacials. This has been confirmed by the uranium/thorium dating of ferricrete bands within glacigenic sediments obtained from three drill cores from the Boco Valley. The ages derived were found to be consistent with estimates for the timing of glaciation derived from palaeomagnetic analyses, radiocarbon dating and relative dating techniques. The results show that the uranium/thorium method can be used to successfully date ferricretes obtained from within glacial sediments of ages <350000 yr and provides valuable information regarding the minimum age of the host sediment. © 1997 John Wiley & Sons, Ltd.

New palaeomagnetic results from the Kerguelen Islands

New palaeomagnetic analyses have been carried out in the Kerguelen Islands on 32 lava flows of well-established age (20–22 Ma). Combined with previous studies, they yield a reliable pole for the Lower Miocene for the Antarctic plate: N = 59 flows. 349.9°E, 83.5°S, A95 = 6.1°. A reversal sequence reversed-normal has been identified in the Port Jeanne d'Arc section.

Soft-sediment deformation structures from the Marinoan glacial succession, Adelaide foldbelt: implications for the palaeoaltitude of late Neoproterozoic glaciation

A suite of soft-sediment deformation structures occurs in tidal rhythmites of fine sand to silt grade in the late Neoproterozoic (∼600 Ma) Elatina Formation, which is part of the Marinoan glaciogenic succession in the Adelaide foldbelt, South Australia. The structures include: (1) sets of asymmetrical cuspate ridges (γ = 13–50 cm, h = 3–5 cm) formed on bed surfaces and underlain by folds affecting as much as 60 cm thickness of strata; (2) symmetrical and interference ripple forms (γ = 3–15 cm, h = ≤1.5 cm) mostly confined to the troughs between the cuspate ridges and which are underlain by folds, involving up to 20 cm thickness of strata, that commonly parallel the undulations of the bed surface but in places have steepened limbs; (3) rill marks on the flanks of cuspate ridges and some ripple forms. The crests of the cuspate ridges and ripple forms commonly were draped and locally eroded and truncated during overall vertical accretion.The cuspate structures are interpreted as gravity slide deposits that formed after transformation of surficial sediment to a hydroplastic state, possibly by the cyclic stresses generated by storm waves, and its sliding on tidal-delta slopes. The ripple forms resulted from continuing wave activity and were maintained by draping and vertical accretion from unidirectional currents and locally by deposition of supercritical cross-lamination. The further deformation of the cuspate folds, as revealed by palaeomagnetic analyses of the structures, implies additional sliding and/or the differential loading of hydroplastic sediment in the troughs between the cuspate ridges.This study confirms that positive palaeomagnetic fold-tests on several cuspate folds indicate a primary origin for the shallow palaeomagnetic inclination (−5.3°) of the Elatina Formation and hence the equatorial palaeolatitude of late Neoproterozoic glaciation in South Australia.

North America during the Lower Cretaceous: new palaeomagnetic constraints from intrusions in New England

SUMMARY Six Late Aptian and Albian intrusions in Maine, New Hampshire and Vermont, dated between 113 and 100 Ma, were sampled for palaeomagnetic analysis. The majority of samples unblock between 550 C and 580 C, and Curie temperatures fall in a discrete interval between 556 C and 580 C. This indicates that the dominant remanence carrier is a TRM and is carried by titanomagnetite, which crystallized from the magma or was produced by deuteric oxidation alteration during post-magmatic cooling. Titanomagnetite occurs as discrete Cl-C3 grains, as exsolution lamellae in the silicates, and as fine reaction rims on the silicates. The palaeomagnetic results are combined with additional data from four Barremian and Early Aptian intrusions in southern Maine to produce nine reliable virtual geomagnetic poles. These possess a higher latitude than most published poles for the Lower Cretaceous. Ar4O/Ar3, ages on biotite and hornblende from these intrusions demonstrate three intrusive pulses at approximately 120 Ma, 111 Ma and 100 Ma, constraining the ages of the calculated VGPs. The corresponding palaeopoles are 72.3 ON, 198.2 E, A,, = 3.3 ; 74.5 ON, 195.2 E, A,, = 3.8; and 76.6 ON, 167.5 E, A,, = 5.3. Based on new palaeomagnetic data reported here, significant changes to the North American Mesozoic APW path are necessary. These changes constrain the palaeoposition of the North American plate in the Lower Cretaceous.