Palaeointensity In the Pelagic Realm: Marine Sediment Data Compared With Archaeomagnetic and Lake Sediment Records

Summary. Four box cores collected from the Ontong—Java plateau during the Eurydice expedition have been used to make relative geomagnetic palaeo-intensity measurements. Rock magnetic measurements on the sediments show that they are characterized by a uniform magnetic mineralogy, and that they are suitable for relative intensity estimates. These are obtained by normalizing the NRM by an ARM imparted in a low DC bias field. the palaeoceanographic event known as the preservation spike is used to establish a crude time-scale for the record so that it may be compared with other data from the same region, and also with global palaeointensity estimates. the marine sediment data are quite similar to Australian intensity data from lake sediments and archaeomagnetic sources, but as might be expected exhibit some obvious differences from the global record.     

Deposition and provenance of the Early Pleistocene Siliceous Member in Westbury Cave,Somerset, England

The Early Pleistocene is an important interval in the Quaternary period as a time not only of climatic and environmental change, but also of key events in human evolution. However, knowledge of this period in northwest Europe is hampered by the limited extent of deposits of this age. Westbury Cave in the Mendip Hills of Somerset preserves an understudied example of fossil-bearing Early Pleistocene sediments, with rare potential to inform our understanding of British Early Pleistocene stratigraphy and landscape evolution outside the East Anglian Crag Basin. This study identifies the processes responsible for deposition of the Early Pleistocene Siliceous Member in Westbury Cave, thereby aiding taphonomic and palaeoenvironmental interpretations of associated fossil assemblages. New excavations revealed over ten metres of Siliceous Member stratigraphy, dominated by fine-grained silts/clays with interbedded sands and gravels, interpreted as being deposited within a subterranean lake or flooded conduit with fluvial input. All sediments sampled were reversely magnetised and are assigned to the Matuyama Reversed Chron. Lithological analysis of gravel clasts revealed variable components of durable non-local and non-durable local clasts. Gravels containing the latter are interpreted as distal talus slope deposits, and those lacking non-durable lithologies as stream or flood deposits. However, it remains unclear from available data whether apparently non-local clasts were sourced from long distance or stem from a more local, now denuded catchment. Siliceous Member bio- and magnetostratigraphy suggest that deposition occurred late in the Early Pleistocene, a period apparently otherwise unrepresented in the UK.     

济阳坳陷弱磁性碳酸盐岩岩心裂缝古地磁定向研究

裂缝走向是影响裂缝性油藏有效开发的关键参数之一,但在非定向岩心上,裂缝走向无法确定。古地磁分析是一项有效的岩心定向技术,该方法在火山岩及一些剩磁强度较大的泥质岩石中取得了较好的应用效果,但它很少应用于弱磁性的碳酸盐岩岩心定向。论文对济阳坳陷富台潜山的弱磁性碳酸盐岩岩心样品开展了系统的等温剩磁和热退磁测试,成功分离出粘滞剩磁并确定了该区的裂缝发育方向：优势发育北东东和北北西两组方向,3 组次要方向为北西西向、北西向和北北东向;测试结果与FMI资料及构造应力场分析结果一致。本研究拓展了古地磁定向技术在弱磁性岩心中的应用。     

黄土高原北部晚新近纪“吴起古湖”的古地磁年代学与古环境记录

晚新近纪以来中国北方堆积的风成红粘土-黄土序列是古气候信息的良好载体, 在黄土高原北部的吴起地区存在一套湖相沉积物, 层位上介于第四纪黄土与晚新近纪红粘土之间, 但该古湖存在的具体时限尚不清楚. 我们利用岩石磁学及古地磁年代学方法并结合多种气候指标对代表“吴起古湖”的土佛寺剖面进行了初步研究, 结果表明湖相沉积物中特征剩磁的载体主要为磁铁矿和赤铁矿, 在此基础上得到的磁极性序列能很好地与国际地磁极性年表对比, 剖面起始于4.28 Ma B. P.左右, 湖相沉积层起始于3.0 Ma B. P.左右, 结束于1.2 Ma B. P.左右. 岩性、磁化率、粒度和沉积速率变化均反映出这一时期该地区环境变化的总体趋势是由暖湿向冷干发展, 期间“吴起古湖”的演化大致经历了三个阶段: 3.0~2.5 Ma B. P.为古湖形成时期, 水体相对较深; 2.50～2.05 Ma B. P.期间受古气候变化的影响湖水明显变浅; 2.05~1.20 Ma B. P.时古湖中仍有一定量的水体存在, 但随后逐步缩小并最终消亡. 该地区气候条件在1.2 Ma B. P.左右的恶化是导致吴起古湖消亡的主要因素, 区域构造活动引起的北洛河溯源侵蚀并切穿湖盆可能也是原因之一.     

Tectonic evaluation of the Indochina Block during Jurassic-Cretaceous from palaeomagnetic results of Mesozoic redbeds in central and southern Lao PDR

Rock magnetic and palaeomagnetic studies were performed on Mesozoic redbeds collected from the central and southern Laos, the northeastern and the eastern parts of the Khorat Plateau on the Indochina Block. Totally 606 samples from 56 sites were sampled and standard palaeomagnetic experiments were made on them. Positive fold tests are demonstrated for redbeds of Lower and Upper Cretaceous, while insignificant fold test is resulted for Lower Jurassic redbeds. The remanence carrying minerals defined from thermomagnetic measurement, AF and Thermal demagnetizations and back-field IRM measurements are both magnetite and hematite. The positive fold test argues that the remanent magnetization of magnetite or titanomagnetite and hematite in the redbeds is the primary and occurred before folding. The mean palaeomagnetic poles for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous are defined at Plat./Plon. = 56.0°N/178.5°E (A₉₅ = 2.6°), 63. 3°N/170.2°E (A₉₅ = 6.9°), and 67.0°N/180.8°E (A₉₅ = 4.9°), respectively. Our palaeomagnetic results indicate a latitudinal translations (clockwise rotations) of the Indochina Block with respect to the South China Block of −10.8 ± 8.8° (16.4 ± 9.0°); −11.1 ± 6.2° (17.8 ± 6.8°); and −5.3 ± 4.7° (13.3 ± 5.0°), for Lower Jurassic, Lower Cretaceous, and Upper Cretaceous, respectively. These results indicate a latitudinal movement of the Indochina Block of about 5–11° (translation of about 750–1700 km in the southeastward direction along the Red River Fault) and clockwise rotation of 13–18° with respect to the South China Block. The estimated palaeoposition of the Khorat Plateau at ca. 21–26°N during Jurassic to Cretaceous argues for a close relation to the Sichuan Basin in the southwest of South China Block. These results confirm that the central part of the Indochina Block has acted like a rigid plate since Jurassic time and the results also support an earlier extrusion model for Indochina.