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The paleomagnetism of a saprolitic paleosol developed on Archean granite and of an overlying hematitic breccia in the basal Lorrain Formation of the Cobalt Group (Huronian Supergroup, ∼2.4–2.3 Ga) near Ville-Marie, Quebec, has been investigated to further constrain the paleolatitude of the Lorrain Formation and the subjacent glaciogenic Gowganda Formation. Only the breccia, whose hematite may be a product of weathering prior to or shortly after deposition, yielded useable results with stepwise thermal demagnetization. A stable component A (maximum unblocking temperature, T ub , of ∼675°C) carried by hematite is directed shallowly to the east-northeast ( D =59.5°, I =2.4°, α 95 =6.7°, n =47, dip-corrected), and a less stable component B (maximum T ub of ∼580°C) carried by magnetite is directed steeply down to the northeast ( D =52.1°, I =70.3°, α 95 =2.8°, n =75, dip-corrected). Component A is ascribed to a chemical remanent magnetization (CRM) acquired when the hematitic breccia was deposited or soon thereafter. Component B is interpreted as a thermochemical overprint related to the Penokean Orogeny at 1.9–1.8 Ga. Dip-corrected specimen mean directions give the pole positions at latitude = 57.1°N, longitude = 338.3°E (dp = 4.1°, dm = 4.8°) for component B, and latitude = 21.1°S, longitude = 213.1°E (dp = 3.3°, dm = 6.7°) for component A. The more easterly directions for components A and B relative to comparable early and overprint components previously determined for the Lorrain and Gowganda formations at Elliot Lake and Desbarats 260–360 km southwest of Ville-Marie imply relative rotation of ∼30° or more about a vertical axis within the Southern Province since the Penokean Orogeny. The results suggest that the hematitic breccia and the Ville-Marie paleosol formed at a paleolatitude of 1.2°±3.4°, in accord with our earlier inference of equatorial paleolatitudes for the Lorrain and Gowganda formations and Huronian glaciation. Paleomagnetic data from several continents suggest that ferruginous weathering horizons formed over a wide range of paleolatitudes in the Proterozoic.

To further our understanding of the paleolatitudes of Precambrian glaciations, a paleomagnetic study has been conducted on the glaciomarine Coleman Member of the Gowganda Formation and the conformably overlying deltaic Firstbrook Member and Lorrain Formation (Huronian Supergroup, ∼ 2.4-2.3 Ga) in Ontario, Canada. Many of the rocks, notably the gray and grayish red argillic facies, display unstable magnetization or overprints carried by magnetite and/or hematite that give negative fold tests. Some red sandstones, however, have stable high-temperature (∼ 690°C) components marked by very shallow northerly or southwesterly directions. Five sites in pale red sandstone from the Coleman Member near Elliot Lake give a dip-corrected A direction of D = 216.3°, I = 5.5° (α95 = 6.7°, n = 30), and 17 sites in Lorrain red sandstone near Desbarats give a dip-corrected A direction of D = 5.4°, I = 5.5° (α95 = 5.9°, n = 75). Unfavorable structural attitudes rendered fold tests inconclusive. These very shallow directions may be ascribed to early chemical remanent magnetization (CRM) because (a) some specimens display polarity reversals, (b) such directions are unknown, either as original or as overprint components, in post-Huronian rocks of the region, and (c) a prior site in Coleman red beds gave a high-temperature, shallow NNW direction that is not significantly different from Lorrain A and which a positive conglomerate test suggested is not an overprint. The very shallow directions are near primary directions for the slightly older (2.45 Ga) Matachewan dyke swarm in the region. Lorrain argillic rocks at five sites give a normal component B (T = 670°C, D = 18.6°, I = 59.6°, α95 = 6.1°, n = 16, dip corrected) and a reverse component E (T = 500°C, D = 190.9°, I = −60.9°, α95 = 7.3°, n = 14, in situ), which are close to a prefolding direction carried by magnetite previously identified in Coleman gray argillites (Morris A). However, a primary origin for Morris A is dubious because of the widespread occurrence of overprints carried by magnetite in Coleman gray argillites and because the fold test employed folds formed during the Penokean Orogeny at 1900-1800 Ma. Our results suggest that Morris A/Lorrain B and Lorrain E may data from immediately before and just after Penokean folding, respectively. Hence Coleman-sandstone A and Lorrain A may be better estimates of original directions. Those components and Matachewan paleomagnetic data constrain Huronian paleolatitudes and suggest that Huronian glaciation occurred within 11° and possibly within 4° of the paleoequator.

A new palaeomagnetic study has been conducted on haematitic shales and siltstones of the late Neoproterozoic Bunyeroo Formation in the Adelaide fold belt (Geosyncline), South Australia, which host an extensive horizon of shock-deformed rock fragments and microtektite-like material of probable impact origin. Thermal step demagnetisation of 116 samples of red shale and siltstone from six sections (sites) revealed a high-temperature component with a bedding-corrected site-mean direction of remanence of D = 56.6°, I = 29.3° (α95 = 10.7°) that gives a pole at 18.1°S, 16.3°E (dp = 6.5°, dm = 11.8°). The high-temperature component provides a positive tectonic-fold test (99% level of confidence). The Bunyeroo high-temperature remanence direction is near the remanence direction (D = 50°, I = 40°) indicated by modelling the subsurface magnetic source of the central high-amplitude anomaly at Acraman, Australia's largest confirmed meteorite impact structure 220–350 km west of the Adelaide fold belt, and also is close to the mean direction (D = 48.3°, I = 54.7°, α95 = 5.2°) determined for surface melt rock from Acraman. Statistical tests show that the virtual geomagnetic poles indicated by the directions for the subsurface central magnetic source and surface melt rock at Acraman may be regarded as subsets of the Bunyeroo palaeomagnetic pole position, indicating that the three pole positions are statistically indistinguishable. The results imply that the subsurface magnetic source and surface melt rock acquired their remanence in the ambient geomagnetic field during cooling, after the impact when structural disturbance had ceased, while the Bunyeroo Formation was accumulating. The agreement among the various remanence directions argues strongly that the ejecta horizon in the Bunyeroo Formation was derived from Acraman. The present findings confirm that the Acraman impact occurred in the late Neoproterozoic, about 590 Ma, which is the age of the Bunyeroo Formation provided by Rb Sr whole-rock shale dating of equivalent and contiguous strata in the Adelaide fold belt region. The Bunyeroo palaeomagnetic data give a palaeolatitude of ∼ 15°, indicating a low palaeolatitude for the Acraman impact and supporting other findings that the Adelaide fold belt occupied low to equatorial palaeolatitudes during the late Neoproterozoic.

New palaeomagnetic analyses have been carried out for the Neoproterozoic (650-600 Ma) Elatina Formation, an important redbed unit of the Marinoan glaciogenic sequence in the Adelaide Geosyncline, South Australia, and flat-lying equivalent facies on the adjacent cratonic Stuart Shelf and Torrens Hinge Zone. The Marinoan rocks display strong evidence of marine glacial deposition, and coeval periglacial sand wedges in permafrost regolith on the Stuart Shelf indicate in-situ cold climate near sea level and marked seasonality. The palaeomagnetic data define a palaeopole for the formation and indicate that Marinoan glaciation, including permafrost, grounded glaciers and marine glacial deposition, occurred near the palaeoequator. Rocks analysed include 97 oriented outcrop samples from 15 sites at three widely separated sections (∼ 65–115 m thick) of gently folded and unmetamorphosed sandstone, siltstone and tillite spanning the Elatina Formation in the Central Flinders Zone of the Adelaide Geosyncline, soft-sediment folds from tidal rhythmites, and 60 specimens from 54 core samples from six deep drillholes on the Stuart Shelf and Torrens Hinge Zone. The most stable remanence components were only completely demagnetised by 685°C, indicating that haematite is the likely carrier of the remanent magnetisation. This conclusion is supported by the presence of ultrafine haematitic pigment coating clastic grains and filling interstices in the rocks. The observation of mixed polarities within some sandstone samples suggests that such lithologies acquired their remamence as chemical remanent magnetisation (CRM). A positive fold test on syndepositional soft-sediment folds in tidal rhythmites confirms that the rhythmites acquired a detrital remanent magnetisation (DRM) by the settling of haematite grains from suspension in quiet waters. Concordant palaeomagnetic directions determined for the rhythmites and other facies of the Elatina Formation show that the formation acquired its CRM close to the time of deposition. The existence of polarity reversals within a stratigraphic section and within some samples therefore argues strongly for the identification of a dipole field axis and the sufficient averaging of secular variation to define a palaeopole for the formation. The palaeopole derived from the oriented sample results (structurally corrected) is located at 52.4°S, 347.1°E ( d p = 3.7 °, d m = 7.4°); this pole position is consistent with the Neoproterozoic apparent polar wander path for Australia. The overall formation mean direction determined from the sample results (structurally corrected) has a declination of 197.3° and an inclination of −5.3° (α 95 = 7.4°), and indicates a mean palaeolatitude of deposition of 2.7° ± 3.7°N. These results accord with the virtual geomagnetic pole previously determined for tidal rhythmites of the Elatina Formation and provide the strongest evidence yet for the equatorial palaeolatitude of Neoproterozoic glaciation. The palaeomagnetic results, together with geological observations for Marinoan glaciogenic rocks, therefore confirm the Neoproterozoic (pre-Ediacaran, ≥ 590 Ma) climatic paradox in South Australia: frigid strongly seasonal climate, permafrost, and grounded glaciers near sea level in equatorial palaeolatitudes . Resolution of this paradox may illuminate Precambrian planetary dynamics and the change in global state during the Ediacaran.

In the geological past when the Earth's decelerating rotation passed through the critical length of day of ∼ 22.2 h, the free nutation or ‘tipping’ of the fluid core (for a core shape in hydrostatic equilibrium) would have resonated precisely with the Earth's retrograde annual forced nutation caused by solar torques. This annual resonance may have caused orders-of-magnitude amplification of core rotation and velocity of core motions, leading to temperature increase and heat release near the core-mantle boundary, instability of the D″ thermal boundary layer at the base of the mantle, and the upwelling of deep mantle plumes. Neoproterozoic palaeotidal data indicate 21.9 ± 0.4 h/day at ∼ 620 Ma, thus placing the critical length of day of ∼ 22.2 h and core resonance at 530 ± 100 Ma. A core shape departing by ±5% from its equilibrium value places resonance in the interval 530 ± 250 Ma. The one distinctive, major tectonothermal event in the interval 530 ± 250 Ma was that of ‘late Pan-African’ age, which affected much of Gondwana during the late Neoproterozoic-early Palaeozoic. This tectonothermal activity, which peaked between ∼ 570 and ∼ 500 Ma, was marked by high temperatures at mid-crustal levels and elevated heat flow, widespread granulite facies metamorphism and crustal reworking and anatexis, commonly in the absence of obvious pervasive deformation, extensive epeirogenic uplift and extrusion of flood basalts; such features indicate an intense crustal heating event that may be ascribed to mantle plumes and hotspots. Furthermore, flood volcanism and rifting in Laurentia during the late Neoproterozoic-early Palaeozoic suggest the coeval upwelling of mantle plumes that led to continental breakup including the opening of the Iapetus Ocean. The apparent global peak in plume activity between ∼ 570 and ∼ 500 Ma may reflect an increase in core-mantle heat flux and instability of D″, caused by the annual resonance of the fluid core (for a core shape in or near hydrostatic equilibrium). The core evidently underwent stronger resonances with the semi-annual and 1/3-annual forced nutations at widely spaced times during the Archaean, when the length of day was approximately 17.7 ± 0.4 h and 15.4 ± 0.4 h, respectively; these two resonances probably caused much greater increases in the velocity and amplitude of core motions and in core-mantle heat flux than occurred during the annual resonance. Proterozoic palaeorotational data suggest that the very strong semi-annual resonance may correlate with the worldwide tectonothermal event at ∼ 2700 Ma; this event is marked by widespread mantle and crustal anatexis, granitoid intrusion and the extrusion of flood basalts and komatiites, and has been ascribed to the upwelling of high-temperature plumes from the core-mantle boundary. The hypothesis that major tectonothermal reworking events may have resulted from thermal plumes generated at the core-mantle boundary in response to resonances of the fluid core for a tidally decelerating Earth may be tested by the acquisition of high quality palaeorotational data for the early Palaeoproterozoic and Archaean.

Previous palaeomagnetic study of a ∼ 10 m thick rhythmite member of the Elatina Formation, part of the Late Proterozoic (∼ 650 Ma) Marinoan glacial succession in South Australia, argued strongly for a low palaeolatitude (∼ 5°) of deposition. However, a fold test was thwarted by the approximate alignment of the remanence with the axis of tectonic folding. Here we report results of a fold test on soft-sediment slump folds with a wavelength of 14–22 cm in the Elatina rhythmites, the axes of which are approximately perpendicular to the direction of remanence. Fifty-one standard palaeomagnetic specimens sampled across the slump structure gave α95 = 3.7° prior to unfolding, α95 = 1.8° with 67% unfolding, and α95 = 2.4° with 100% unfolding. The results indicate that the remanence was acquired very soon after deposition; apparently it was acquired prior to soft-sediment slumping and then slightly sheared by the disturbance. The Elatina pole must be considered a virtual geomagnetic pole because the tidal rhythmite member studied represents only 60–70 years of deposition. However, the mean pole position is similar to other Late Proterozoic poles for South Australia, which implies that the very low inclination (< 10°) for the Elatina rhythmites does not record a geomagnetic excursion or reversal but does indeed indicate deposition in low palaeolatitudes. The similarity of many of the Late Proterozoic pole positions is evidence also that inclination error is not significant. The low palaeolatitude of Late Proterozoic glaciation is one of the major enigmas in contemporary Earth science, raising questions concerning the nature of the geomagnetic field, climatic zonation, and the Earth's rotational parameters in Late Proterozoic time.

The Mallowa Salt of the early Palaeozoic Carribuddy Group, Canning Basin, Western Australia, is a halite-mudstone-anhydrite-dolomite evaporite sequence of Late Ordovician to Early Silurian age deposited in barred marginal-marine to ephemeral saltpan and saline mudflat environments. BHP-Utah Minerals' potash exploration well Gingerah Hill No. 1 obtained drill core of a 477 m stratigraphic interval of the Mallowa Salt, which permitted the study of geochemical variations in a thick halite sequence contemporaneous with Late Ordovician-Early Silurian glaciation on other continents. A strip of uniform width was continuously ground from the core and the powdered rock bagged at 1 m intervals for geochemical assay. The resulting geochemical stratigraphic series display conspicuous cyclicity: several orders of cycles ranging from ∼ 3 m to100 + m stratigraphic thickness are discernible in the plots of minor constituents Br, MgO and K2O in extracted soluble salts and also in the plots of Na2O and total salts content. Fast Fourier transform (FFT) of the Br, MgO and K2O series gives principal spectral peaks in the 0.00–0.10 cycles/m frequency band at ∼ 250, 113, 35.4, 22.1 and 19.7 m, as well as peaks at ∼ 65 m and between 2.8 and 13.5 m. FFT of the Na2O and total salts series shows dominant spectral peaks at 220–259 and 109–114 m. If the strongest spectral peak at 113 m is taken to represent the relatively stable eccentricity period of 100 ka and a constant net rate of accretion assumed for long sections, the other main periods in the Mallowa Salt as exemplified by the Br spectrum would be31.3 ± 3.0,19.6 ± 1.1 and17.4 ± 1.1 ka. These figures are consistent with predicted Late Ordovician-Early Silurian (440 Ma) periods for obliquity (30.5 ka) and precession (19.3 and 16.4 ka) based on evolutionary change in the Earth-Moon system. Additional spectral peaks identified with implied periods of approximately 206–233, 57.5, 8–12 and 2.5–3.5 ka also may be of palaeoclimatic relevance. The relative amplitudes and structure of spectral peaks in the 0.00–0.10 cycles/m frequency band support the identification of climatic oscillations forced by orbital cycles. The data indicate a precession-eccentricity-dominated pattern, which accords with the know low palaeolatitude (

The comparison of 14 C ages determined for pedogenic carbonate and coexisting organic carbon obtained from arid zone calcareous soils and paleosols indicates that the carbonate dates are on average at least 3600 radiocarbon years too old. This is attributed to an initial low 14 C content of soil carbonate and minimal secondary contamination by environmental 14 C within the arid zone.