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The Middle to Upper Ordovician siliciclastic succession in Central Bolivia provides excellent exposures of up to 1 m thick massive sandstone beds produced by liquefaction-induced sediment remobilisation. These fine-grained massive sandstones occur in shallow-marine nearshore facies that were deposited above storm wave base. Vertical to steeply inclined clastic dykes, which penetrate up to 1.5 m of the overlying sediment pile, feed into the basal parts of massive sandstone sheets. These dykes are interpreted as pathways used by liquefied sediment during upward-directed escape from a subsurface horizon. The relatively short lateral spacing of 5 to 25 m between the individual dykes initiated lateral coalescence of ejected individual sediment bodies. As a result, massive sandstone sheets formed and can be traced laterally over several kilometres. While undeformed sandy deposits contain between 5 to 10% dispersed clay the average clay content in massive sandstone sheets is ∼ 15%. The elongated, sometimes S-shaped form of the sandy, laminated fragments reflects squeezing during movement showing that they have been deformed under shear stress during flow. The axis of maximum elongation of the fragments indicates local flow direction. As the massive sandstone sheets are likely the products of seismic shocks they provide the first evidence of tectonic activity in this part of the basin during the Middle to Late Ordovician. 相似文献
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在淮北新元古代倪园组和九顶山组海相碳酸盐岩地层中，保存着许多燧石相微生物化石。通过对其形态结构、群落组合特征及埋藏保存状态研究，探讨其系统分类、生存环境及区域地层对比，确定这些微生物化石结构的原始性和群落的低分异性。结合相关地层中原始的光球状浮游微生物、Chuaria—Tawuia类型宏体碳质化石生物群及叠层石优势的特征考虑，这些微生物的生存时间是在南沱冰期之前。 相似文献
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Massimiliano Ghinassi 《Sedimentary Geology》2007,202(4):677-701
This study focuses on the lowstand and early transgressive systems tracts of a basin-fill sequence of lower Pliocene nearshore deposits in the Val d'Orcia Basin of the Northern Apennines, Italy. The basin at that time was a semi-enclosed marine embayment, and, in the study area, its margin was subject to highly variable subsidence along the depositional strike, attributed to a decrease in tectonic displacement. The nearshore succession in the more rapidly-subsiding segment of the basin is around 20 m thick, comprising three storeys of laterally-stacked Gilbert-type delta lobes overlain by a shoal-water delta, whereas the nearshore succession in the adjacent, more slowly-subsiding segment, is up to 9 m thick. This succession is characterised by alternation of shoreface and offshore deposits, moderately wave-worked and covered by shoal-water deltaic facies.These coeval nearshore successions consist of several transgressive-regressive cyclothems. The development and lateral variation of the cyclothems was controlled by the local subsidence rate and coastal topographic gradient. Some of the cyclothems are considered to be higher-order sequences and others to be parasequences, with the former passing laterally into the latter in the area where the sea-level fall was countered by fast local subsidence. Some of the bounding surfaces are of limited lateral extent, with two parasequences passing laterally into a single one.Coastal topography controlled particularly the thickness of transgressive deposits. In the low-gradient setting of a delta plain, the relative sea-level rises caused major landward shifts of the shoreline and reduced fluvial sediment supply, with the formation of a transgressive lag in sediment-starved conditions. In the high-gradient coastal setting of the non-deltaic zone, the shoreline shift was minimal and had relatively little impact on local sediment supply, which promoted an accretionary transgression.At the end of the lowstand stage, the rate of sediment accumulation in the non-deltaic nearshore zone was lower, allowing the onset of subsequent transgression to be recorded considerably earlier than in the deltaic nearshore zone. This diachroneity suggests that facies criteria alone may not necessarily be a reliable basis for the recognition of systems tract boundaries. 相似文献
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The well exposed outcrops of the upper Kimmeridgian shallow-marine carbonates at Jabaloyas (Iberian Chain, NE Spain) permit the evaluation of geophysical methods for the identification of sedimentary facies. Direct measurement of magnetic susceptibility in facies and detailed grids of magnetometry, electromagnetic multifrequency and ground-penetrating radar (50–500 MHz antennas) have been performed in two study areas where the upper Kimmeridgian rocks are nearly horizontal. Magnetometry indicates negative anomalies in residual magnetic field and vertical magnetic gradient related to reef pinnacles and faults. Electromagnetic data reveal that positive anomalies of apparent conductivity correlate with non-reefal facies. The areal distribution of magnetometry and EM data does not permit the unequivocal identification of pinnacles and faults at the studied area. By contrast, ground penetrating radar profiles and maps of relative reflectivity in two way travel time slices are useful for the identification of faults (hyperbolic anomalies) and reefal and non-reefal facies (radar facies A and B, respectively). The integration of geophysical data, mainly ground penetrating radar, has permitted the 3D reconstruction of reef pinnacles and its tectonic framework. 相似文献
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