Sedimentary Rhythm Research Articles

Test Investigation on Liquefied Deformation Structure in Saturated Lime‐Mud Composites Triggered by Strong Earthquakes

AbstractThree identical model boxes were made from transparent plexiglass and angle iron. Using the method of sinking water and according to the sedimentary rhythm of saturated calcium carbonate (lime‐mud) intercalated with cohesive soil, calcites with particle sizes diameters of ≤ 5 μm, 10–15 μm and 23–30 μm as well as cohesive soil were sunk alternatively in water of three boxes to build three test models, each of which has a specific size of calcite. Pore water pressure gauges were buried in lime‐mud layers at different depths in each model, and connected with a computer system to collect pore water pressures. By means of soil tests, physical property parameters and plasticity indices (Ip) were obtained for various grain‐sized saturated lime‐muds. The lime‐muds with Ip ranging from 6.3 to 8.5 (lower than 10) are similar to liquid saturated silt in the physical nature, indicating that saturated silt can be liquefied once induced by a strong earthquake. One model cart was pushed quickly along the length direction of the model so that its rigid wheels collided violently with the stone stair, thus generating an artificial earthquake with seismic wave magnitude greater than VI degree. When unidirectional cyclic seismic load of horizontal compression‐tension‐shear was imposed on the soil layers in the model, enough great pore water pressure has been accumulated within pores of lime‐mud, resulting in liquefaction of lime‐mud layers. Meanwhile, micro‐fractures formed in each soil layer provided channels for liquefaction dewatering, resulting in formation of macroscopic liquefaction deformation, such as liquefied lime‐mud volcanoes, liquefied diapir structures, vein‐like liquefied structures and liquefied curls, etc. Splendid liquefied lime‐mud eruption lasted for two to three hours, which is similar to the sand volcano eruption induced by strong earthquake. However, under the same artificial seismic conditions, development of macroscopic liquefied structures in three experimental models varied in shape, depth and quantity, indicating that excess pore water pressure ratios at initial liquefaction stage and complete liquefaction varied with depth. With size increasing of calcite particle in lime‐mud, liquefied depth and deformation extent increase accordingly. The simulation test verifies for the first time that strong earthquakes may cause violent liquefaction of saturated lime‐mud composed of micron‐size calcite particles, uncovering the puzzled issue whether seafloor lime‐mud can be liquefied under strong earthquake. This study not only provides the latest simulation data for explaining the earthquake‐induced liquefied deformations of saturated lime‐mud and seismic sedimentary events, but also is of great significance for analysis of foundation stability in marine engineering built on the soft calcium carbonate layers in neritic environment.

The Seismic Sedimentology Application Research on Junggar Basin

Depending on different seismic frequencies and reflection interface information of sedimentary bodies, the core technologies involved in sedimentology had been introduced into clastic transpressional basins developing in Junggar Basin in the ways of the macro (seismic geomorphology) and micro (seismic lithology) to design a data interpretation process in double domain (time domain and Wheeler domain) in minimum isochronal strata unit. The isochronal horizon slice technique suitable for a variety of sedimentary models had been improved. The combination calibration and conversion of drilling data and seismic data were first realized according to unchangeable double domains time and depth relationship. A set of reservoir description technique was preliminary formed to be applied to lithologic identification, palaeosedimentary environment recovery, reservoir evolution and favorable reservoirs description basing on the analysis of stratigraphic sedimentary rhythm, which had been applied to the center of Junggar Basin and confirmed to be a worthwhile technique depending on seismic sedimentology for accurate reservoir description.

Formation Conditions and Sedimentary Characteristics of a Triassic Shallow Water Braided Delta in the Yanchang Formation, Southwest Ordos Basin, China.

A large, shallow braided river delta sedimentary system developed in the Yanchang Formation during the Triassic in the southwest of the Ordos basin. In this braided delta system, abundant oil and gas resources have been observed, and the area is a hotspot for oil and gas resource exploration. Through extensive field work on outcrops and cores and analyses of geophysical data, it was determined that developments in the Late Triassic produced favorable geological conditions for the development of shallow water braided river deltas. Such conditions included a large basin, flat terrain, and wide and shallow water areas; wet and dry cyclical climate changes; ancient water turbulence; dramatic depth cycle changes; ancient uplift development; strong weathering of parent rock; and abundant supply. The shallow water braided river delta showed grain sediment granularity, plastic debris, and sediment with mature composition and structure that reflected the strong hydrodynamic environment of large tabular cross-bedding, wedge cross-bedding, and multiple positive rhythms superimposed to form a thick sand body layer. The branch river bifurcation developed underwater, and the thickness of the sand body increased further, indicating that the slope was slow and located in shallow water. The seismic responses of the braided river delta reflected strong shallow water performance, indicated by a progradation seismic reflection phase axis that was relatively flat; in addition, the seismic reflection amplitude was strong and continuous with a low angle and extended over considerable distances (up to 50 km). The sedimentary center was close to the provenance, the width of the river was large, and a shallow sedimentary structure and a sedimentary rhythm were developed. The development of the delta was primarily controlled by tectonic activity and changes in the lake level; as a result, the river delta sedimentary system eventually presented a “small plain, big front” character.

Stability and tightness evaluation of bedded rock salt formations for underground gas/oil storage

The presence of interfaces has a critical influence on the stability and tightness of underground gas/oil storages. In China, these energy storages are constructed mainly in bedded salt formations and are widely distributed. Therefore, it is necessary to study the sedimentary rhythm and mechanical characteristics of the interfaces between beds. The petrologic study of core samples from exploratory wells in Yunying Salt Mine, Hubei Provence, China, reveals a sedimentary rhythm and multiple interfaces of bedded salt formations: (1) Mudstone–anhydrite–glauberite–Glauber’s salt–salt rock–mudstone are periodically deposited in sequences. Chemical deposition, the predominant formation mechanism, and mechanical deposition appear alternately and result in the formation of sedimentary multicycles; (2) depending on whether a certain component of the rhythm is lacking or not, interfaces are divided into sequential depositional interfaces and intermittent depositional interfaces. Depending on the deposition frequency, interfaces can also be classified into dominant interfaces and secondary interfaces. Depending on whether the mineral composition changes gradually or abruptly, interfaces can be divided into gradual transition interfaces and discrete discontinuous interfaces. Based on the classification of the interfaces together with scanning electron microscope analyses, the cementation state and strength of the interfaces are discussed. Both field investigations and laboratory tests show that the strengths of the chemical deposition interfaces are higher than those of the mechanical deposition interfaces. Specifically, interfaces of mudstone and rock salt are mostly weak. Taking into account the presence of weak interfaces, the strength of interbedded salt formations around caverns can be represented by a U-shaped lower-bound strength envelope curve, which means that shear failure may occur more easily at the haunches of the cavern. Therefore, when setting the range of the internal operating pressures, more attention should be paid to the shear strength of the weak interlayers and weak interfaces, in particular those at the haunches of the cavern, to ensure the stability and tightness of underground gas/oil stores.

Astronomical cycle origin of bedded chert: A middle Triassic bedded chert sequence, Inuyama, Japan

Astronomical forcing is one of the main drivers of climate change, and astronomical cyclicity recorded in sediments provides a clue to understand the dynamics of the global climate system. Bedded cherts consist of rhythmic alternations of chert and shale beds. Although previous studies have hypothesized that the origin of bedded chert is related to astronomical cycles (e.g. Fischer, 1976; Hori et al., 1993), conclusive proof remains elusive. To explore this possibility, we established a continuous, high-resolution lithostratigraphy of middle Triassic bedded chert in Central Japan. The average duration of each chert–shale couplet is 20 kyr, similar to that of the precession cycle. Spectral analysis of a bed number series of thickness variations in chert beds was performed assuming that each chert–shale couplet represents a 20-kyr precession cycle. The results reveal cycles involving approximately 200, 20, 5, and 2–3 beds, corresponding to periodicities of approximately 4000, 400, 100, and 40–60 kyr, respectively. By further assuming that the 20-bed cycle represents a 405-kyr eccentricity cycle of constant and stable periodicity, we converted the bed number series to a time series. Spectral analysis of the time series revealed distinct periodicities of 3600, 117, 97, and 38 kyr, in addition to 405 kyr. Besides 3600 kyr, these periodicities agree well with the 120, 95, and 37 kyr periodicities for eccentricity cycles and the obliquity cycle during the Triassic. Moreover, we detected amplitude modulation of the approximately 100-kyr cycle of thickness variations in chert beds with a 405-kyr periodicity, which may correspond to amplitude modulation of 100-kyr climatic precession cycle with the 405-kyr periodicity. The approximately 3600-kyr periodicity described above and 1800-kyr periodicity manifested as the amplitude modulation of the 405-kyr cycle are correlated to present-day long-term eccentricity cycles of 2400 and 4800 kyr evolved by chaotic behavior of solar planets. Collectively, these similarities in the periodicities of dominant cycles, their hierarchy, and the nature of amplitude modulation of cycles in chert bed thickness with those of astronomical cycles strongly support the hypothesis that the sedimentary rhythm of bedded chert is paced by astronomical cycles, thereby indicating the potential of bedded chert as a template for a Mesozoic cyclostratigraphy.

シーケンス層序学 その可能性を求めて 石油探鉱における堆積リズム解析の可能性 第四紀日本海海洋循環ダイナミックスの復元を例として

Sedimentary rhythm analysis is a method to study sedimentary cycles through the systematic sampling and analysis of the sequences, derivation of multiple paleoenviron-mental parameters from the analytical data, and examination of the frequencies in variation of these parameters and their phase relationships. Through sedimentary rhythm analyses, it is possible to study the dynamics of paleo-ocean circulation. In this short paper, I would like to show the application of this method to reconstruct dynamic behaviour of the surface and deep water circulation within the Japan Sea corresponding to the glacial-interglacial sea-level oscillations during late. Quaternary. Reconstruction of this sort is useful to understand the paleoenvironment as well as mechanism(s) of the petroleum source rock deposition within marginal seas.

Sedimentary rhythm of the Nakayama Formation (Middle Miocene∼Early Pliocene) in the Sado Island

Sedimentary rhythm of the Nakayama Formation (Middle Miocene∼Early Pliocene) in the Sado Island

Aspetti Della Biocenosi Coralligena in Due Stazioni Dei Bacini Occidentale Ed Orientale Del Mediterraneo

Abstract Facies of the coralligenous biocoenose in two stations of the Western and Eastern Mediterranean Basins. — The benthic comunities of circalittoral region have been studied by diving technique in the Western and Eastern Mediterranean Basins. The facies of the coralligenous biocoenose which show a larger difference between the two Basins are: 1) the communities of non calcareous algae in biotopes with constant direction depth streams and 2) the precoralligenous facies. These communities in the Aegean Sea are characterized by Sargassum linifolium, Cystoseira corniculata v. laxior, Caulerpa prolifera, Udotea petiolata, Dasycladus clavaeformis and by sponge Calix nicaeensis; in the Thyrrenean Sea by Laminaria rodriguezii, Cystoseira spinosa, Neurocaulon grandiflorum, Fauchea repens, Halymenia dichotoma, Callymenia reniformis and by the sponge Axinella cannabina. The environmental factors which determine these different facies in the circalittoral region of the two Mediterranean Basins are chiefly the temperature, the light intensity and the sedimentary rhythm.