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Riverine tufas are freshwater carbonate deposits that precipitate in springs and streams distributed in limestone areas. They are noteworthy for their annual laminations that contain both dense and porous laminae and their fast precipitation rate. These features indicate that riverine tufa may be an excellent material for recording the secular variation of the geomagnetic field. Part of recently deposited tufas in Nagaya in Okayama Prefecture, Japan have a stable natural remanent magnetization (NRM), which is a reliable record of the average geomagnetic field direction. However, only the dense laminae in the recent tufa, however, carry a faithful record of the geomagnetic field direction; the porous laminae do not. The NRM of the porous laminae seems to be affected considerably by the dip direction of the slope on which the tufa is deposited. Based on the results of isothermal remanent magnetization acquisition experiments and microscopic observations, the dominant magnetic mineral in the recent tufa was determined to be magnetite. A Holocene palaeotufa, which was formed 2000-2800 years ago, was found to have no stable remanent magnetization. This is possibly due to the absence of dense layers where magnetic particles are tightly fixed. Since it is very important in palaeomagnetic secular variation studies to be able to reconstruct much older, continuous records of the variation of the geomagnetic field, tufa deposits are not ideal materials and their applicability for this purpose is rather low, although the further investigation is needed. [ABSTRACT FROM AUTHOR]

Previous palaeomagnetic studies have demonstrated that a large part of the South China Block (SCB) has behaved as a stable body since the Cretaceous. We undertook a palaeomagnetic investigation of Lower Cretaceous red sandstones at 24 sites within the Ganzhou and Xingguo basins in southern Jiangxi Province in China. The aim of this study was to further constrain the extent of the SCB, which has been stable since the Cretaceous. We isolated the characteristic directions of higher temperature components (HTCs) with an unblocking temperature from 650 to 700 °C, by progressive thermal demagnetization and principal component analysis. The optimal concentration of site mean HTC directions calculated using the direction-correction tilt test was achieved at 51.2 ± 32.4 per cent untilting, indicating syntilting magnetization, suggesting that the remanences were not acquired immediately after sedimentation. Most Cretaceous sedimentary basins on the eastern part of the SCB were controlled by fault movement (extensional basins). Most tilting in such an extensional basin is thought to have progressed contemporaneously with the structural and stratigraphic development of the basin. We concluded that the remanence acquisition of red sandstones on the Ganzhou and Xingguo basins occurred during synsedimentary tilting. We adopted 51.2 per cent untilted directions of the HTCs as the palaeomagnetic field directions during the Early Cretaceous. The mean palaeomagnetic pole (76.3°N, 224.3°E, α95= 3.3°), calculated using virtual geomagnetic poles from 23 sites (excluding site 20, which has an insufficient number of samples showing a stable magnetic component), is in good agreement with the Late Cretaceous palaeomagnetic poles from the same region and also with most of Cretaceous poles previously reported from the stable body of the SCB. This agreement indicates that the Jiangxi region has been part of the stable body of the SCB since the Cretaceous. This result also demonstrates that the tectonic influence of the India–Asia collision did not destroy the rigidity of most of the SCB. The global mean pole position (78.8°N, 214.4°E, α95= 2.6°), calculated using sixteen Cretaceous poles from the stable body of the SCB, is suitable for use as the reference Cretaceous palaeomagnetic pole of the stable SCB. [ABSTRACT FROM AUTHOR]

We present new palaeomagnetic results from Late Cretaceous red beds of the Paomagang Formation collected at 42 sites in the Yichang area (30.7°N, 111.7°E), middle–northern part of the South China Block. A high unblocking temperature component around 680°C was isolated from 26 sites by stepwise thermal demagnetization. Fold tests are positive at the 95 per cent confidence limit for 14 and 11 sites of the Paomagang and Wangdian areas, respectively. Normal and reversed polarity sequence found in the Wangdian area passed a reversal test at the 95 per cent confidence limit. The tilt-corrected mean direction for Yichang area is , corresponding to a palaeopole at 71.5°N, 280.0°E with . Comparison with the expected inclination from the 80 Ma Eurasian APWP pole indicates a inclination flattening of . Middle Cretaceous to Cenozoic NW–SE extension tectonics within the SCB contributes only 2.0° to the inclination flattening. While the massive samples of the Paomagang Formation show a moderate degree of anisotropy and steeper inclination , the laminated samples, which form more than 75 per cent of the studied samples, show a higher degree of anisotropy and shallower inclination value . A good correlation between remanence inclination and facies types indicates that depositional processes produced the observed shallow inclination in the Yichang area. [ABSTRACT FROM AUTHOR]

A core (305 cm long) taken from Lake Siling in central Tibet is subdivided on the basis of carbonate composition into a dolomite zone (275–255 cm depth), a dolomite–aragonite zone (255–245 cm depth), an aragonite zone (245–147.5 cm depth) and a hydromagnesite–aragonite zone (147.5–30 cm depth). Most core samples contain clay minerals, quartz and low-Mg calcite. Six samples on terraces near the lakeshore consist of hydromagnesite with heavy O and C stable isotopic ratios and low density. The hydromagnesite–aragonite zone is characterized by a strong correlation among δ18O (−4.9–2.3‰), δ13C (5.5–8.0‰) and particle density (2.40–2.68 g/cm3). This occurs when carbonates from the underlying aragonite zone, characterized by lighter O and C isotopic ratios and high density, are plotted against terrace samples of hydromagnesite. The correlation in the hydromagnesite–aragonite zone is attributed to the mixing of hydromagnesite with the aragonite zone sediments in varying proportions. When compared with available 14C TAMS ages, it appears that hydromagnesite was added to bottom sediments at about 5000 years bp. At that time, the lake began to shrink and formed lacustrine terraces because evaporation dominated over precipitation around the lake. The depositional rate of the hydromagnesite–aragonite zone (0.6 mm/year) was about six times faster than that of the aragonite zone (0.1 mm/year) due to formation of hydromagnesite within an arid climate. The plausible sources of four carbonate minerals are as follows: (1) aragonite is an inorganic carbonate from the lake water, (2) hydromagnesite is an evaporitic carbonate on terraces, (3) dolomite is a diagenetic carbonate and (4) low-Mg calcite is a detrital carbonate. [Copyright &y& Elsevier]