鄂尔多斯盆地南部三叠系延长组湖相重力流沉积细粒岩及其油气地质意义

随着页岩油气勘探开发和相关领域研究的不断深入,细粒沉积物的搬运和沉积已成为当前沉积学研究的热点问题之一,但中国中生代湖泊环境中的泥质重力流沉积尚未引起应有的关注。通过岩心观察、薄片鉴定等手段及综合研究,分析了鄂尔多斯盆地晚三叠世湖相泥质重力流沉积特征,探讨了其形成机制与成因分类。鄂尔多斯盆地三叠系延长组湖相泥页岩结构类型多样,发育泥质块体流沉积、泥质碎屑流沉积、泥质浊流沉积和泥质异重流沉积等多种重力流沉积类型。按照泥质含量将重力流划分为砂质重力流、泥质重力流和混合重力流3种亚类,并根据成因将重力流划分为滑塌体、碎屑流、浊流及异重流等4种亚类;结合成因和泥质含量,将重力流沉积共划分为12种类型。滑塌岩、碎屑岩分布于三角洲前缘斜坡脚附近;浊积岩、异重岩广泛分布于三角洲斜坡至沉积中心。认为泥质沉积物可以在强水动力条件下搬运—沉积;重力流沉积细粒物质在湖相沉积中占据很大的比例;泥质重力流对泥页岩中的碎屑物质、黏土矿物及有机质的搬运和沉积起到重要作用,因而对于页岩油气的生烃、储集性能和压裂工艺研究具有重要意义。

Siliceous Cementation of Chlorite-Coated Grains in the Permian Sandstone Gas Reservoirs,Ordos Basin

Objective It has long been controversial that whether authigenic chlorite coatings in sandstone reservoirs can prevent precipitation of siliceous cements. It is commonly believed that chlorite coatings （also called chlorite films, chlorite linings, or chlorite rims） may prevent quartz overgrowth, and thus help the preservation of original pores in sandstone reservoirs. Recently, however, this assumption has been challenged by reservoir geologists. This dispute cannot be solved by mere analysis of thin sections, nor by chemical equations and diagenesis analysis. The main objective of the present contribution is to shed light on this problem on the basis of sandstone samples from the Permian Shanxi and Shihezi Formations in the eastern part of the Sulige gas field, Ordos Basin in central China.

A Marine or Continental Nature of the Deltas in the Early Cretaceous Lingshandao Formation--Evidences from Trace Elements

Objective A succession with well-developed soft-sediment deformation structures and well-exposed gravity-flow deposits in the Early Cretaceous Lingshandao Formation has caused considerable controversy on whether the nondisputed deltaic environment was marine or continental.This dispute cannot be solved by mere sedimentological analysis and,moreover,too few fossils are present to be decisive.

An Upward Shallowing Succession of Gravity Flow Deposits in the Early Cretaceous Lingshandao Formation,Western Yellow Sea

Objective Several well-exposed gravity flow deposits in the Early Cretaceous Lingshandao Formation provide a rare opportunity to study deep-water processes. The main objective of this work is to establish the spatial and temporal relationships between the various gravity flow deposits, and thus to establish their sequence stratigraphic patterns and depositional conditions.

Seismites resulting from high-frequency, high-magnitude earthquakes in Latvia caused by Late Glacial glacio-isostatic uplift

Geologically extremely rapid changes in attitude by glacial rebound of the Earth crust after retreat of the Scandinavian Ice Sheet at the end of the last Weichsetian glaciation influenced the patae- ogeography of northern Europe. The up（ift of the Earth crust apparently was not gradual, but shock-wise, as the uplift was accompanied by frequent, high-magnitude earthquakes. This can be deduced from strongly deformed layers which are interpreted as seismites. Such seismites have been described from several coun- tries around the Baltic Sea, including Sweden, Germany and Poland. Now similarly deformed layers that must also be interpreted as seismites, have been discovered also in Latvia, a Baltic country that was covered by an ice sheet during the last glaciation. The seismites were found at two sites： Near Vatmiera in the NE part and near Rakuti in the SE part of the country. The seismites were found in sections of about 7 m and 4.5 m high, respectively, that consist mainly of glaciofluvial and gtaciotacustrine sands and silts. At the Vatmiera site, 7 seismites were found, and at the Rakuti site these were even 12 seismites. The two sections have not been dated precisely up tilt now, but lithological correlations and geomorpho- logical characteristics suggest that the sediments at the Vatmiera site cannot be older than 14.5 ka. Because the accumulation of the section did not take more than about 1000 years, the average recurrence time of the high-magnitude （M≥4.5-5.0） earthquakes must have been maximally only 100-150 years, possibly only 6-7 years. The sediments at Rakuti must also have formed within approx. 1000 years （17-16 ka）, implying a recurrence time of high-magnitude earthquakes of maximaUy once per 100-200 years.

The response of stromatolites to seismic shocks： Tomboliths from the Palaeoproterozoic Chaibasa Formation, E India

It is demonstrated here for the first time how Palaeoproterozoic stromatolites survived seismic disturbance of their substrate. The stromatolites under study could have been cyanobacteria or any other photo-autotrophic microbes, which formed mats that covered a substrate of very fine-grained sandstones and mudstones of the Chaibasa Fm. in eastern India. The sediments represent a shelf environment. The local abundance of the stromatotites suggests that the tow-energy environment formed a suitable habitat. The common phases of tectonic quiescence were, however, occasionally interrupted by seismic shocks. These were sufficiently strong to deform the mat layers, the tower parts of which might already have been （semi-） consolidated. The mats became partly folded, partly faulted, and already consolidated parts of the stromatotite layers broke off. This can be deduced from the angular shapes of part of the broken-off fragments. It appears, however, that part of these fragments were stilt sufficiently soft to become rounded and deformed by rotting over the seafloor, probably under the influence of tidal currents. When come to rest, these fragments served as a new substrate for new generations of the microorganisms. These micro-organisms thus survived by continued growth on the reworked fragments and built up new stromatolites that may show an ‘angular disconformity＇ with the stromatotites of their substrate. It thus is shown that stromatotites have an adequate response to a sudden disturbance of their habitat, and that they survive earthquakes by colonization of broken-off fragments. We call the ‘healed＇ fragments ‘tombotiths＇ （tumbled stones）.

The hyperpycnite problem: comment

A recent review article("The hyperpycnite problem") by Shanmugam(Journal of Palaeogeography 7(3):197–238,2018) in this journal has some shortcomings, comes to scientifically incorrect conclusions, and calls for student education in a way that would significantly diminish the students’ changes for a geological career. Having studied--and still studying--hyperpycnites in the field, it seems only appropriate to inform the readers of this journal about our concerns regarding the review article mentioned above.Actually, the article by Shanmugam(Journal of Palaeogeography 7(3):197–238, 2018) is not a review paper but rather a comment, criticizing al researchers who have come to the conclusion, on the basis of both field research and theory,that hyperpycnal flows result in sediments(hyperpycnites) with characteristics that make it possible to distinguish them from deposits formed in another way. This is also the essence of Shanmugam’s comment: as he states himself, he is"an ardent critic of al genetic facies models".

“The response of stromatolites to seismic shocks: Tomboliths from the Palaeoproterozoic Chaibasa Formation,E India”: Reply

Our contribution about a newly recognized sedimentological feature, which we called tombolith,has raised some comments on the etymology of the term, the assumed lack of field data, the usage of a term that has a special meaning in stratigraphy, the origin of the seismic shocks that played a role, so-called basic tenets, the origin of soft-sediment deformation structures, and the role of liquefaction. We argue that most comments result from an insufficiently thorough reading of our text, whereas some other remarks concern aspects that we did not deal with. Finally, several remarks are based on outdated literature and from insufficient knowledge of the regional geology.

A new 'superassemblage' model explaining proximal-to-distal and lateral facies changes in fluvial environments,based on the Proterozoic Sanjauli Formation(Lesser Himalaya,India)

Facies analysis of fluvial deposits of the Proterozoic Sanjauli Formation in the Lesser Himalaya was combined with an architectural analysis. On this basis, a model was developed that may be applied to other fluvial systems as well, whether old or recent. The new model, which might be considered as an assemblage of previous models, explains lateral variations in architecture and facies but is not in all respects consistent with the standard fluvial models. The Sanjauli fluvial model is unique in that it deals with lateral facies variations due to shifts of the base-level along with fluctuations in accommodation space owing to changes in palaeoclimate.