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1.
松辽盆地白垩系砂岩长石碎屑的钠长石化作用   总被引：12，自引：0，他引：12  
松辽盆地白垩系砂岩以长石岩屑砂岩和岩屑长石砂岩为特征。砂岩中发育长石碎屑、岩屑等不稳定组分，而且随着埋藏深度加深（成岩作用加强）钾长石逐渐减少并最终在2700m以下消失。斜长石碎屑中钠长石组分逐渐增多，钙长石组分逐渐减少，最终形成纯钠端元的钠长石。在成岩过程中长石碎屑的钠长石化主要有3种方式：①由离子交代作用导致长石碎屑的钠长石化；②长石碎屑边缘钠长石次生生长；③与长石碎屑溶解伴生的新生钠长石作用。结合热动力学平衡原理分析，斜长石的钠长石化基本不受成岩温度和压力的制约，而钾长石的钠长石化需要较高的成岩温度和压力作用才能进行。因此，斜长石的钠长石化可见于成岩早期，而钾长石的钠长石化只发生于成岩晚期。  相似文献   

2.
Petrographic examinations and electron microprobe analyses of Proterozoic granitic rocks, SE Sweden aimed to characterize and unravel the mechanisms and conditions of plagioclase alterations. These alterations include saussuritization, albitization and replacement of plagioclase by K‐feldspar. The hydrothermal alterations, which are inferred to have occurred at ca. 250–400°C, resulted in concomitant formation of Al‐rich titanite, epidote, calcite, pumpellyite, prehnite and iron oxides. Replacement of plagioclase by K‐feldspar occurs in red‐stained zones, which have developed close to thin fractures owing to the precipitation of tiny Fe‐oxide pigment particles within the altered plagioclase, whereas saussuritized plagioclase has less systematic spatial relationships to these fractures. Albitization of plagioclase occurred in rocks that are poor in biotite compared to rocks that suffered extensive saussuritization. The chemical and textural characterization of various types of plagioclase alterations allows elucidation of the granitic hydrothermal systems. Features of feldspar alteration in the granitic rocks are similar to those encountered in feldspathic sandstones and should hence be considered in studies on diagenetic changes of siliciclastic successions during basin evolution. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   

3.
《Sedimentology》2018,65(4):1277-1300
Fluvially derived tuffaceous Chinle sandstones from Petrified Forest National Park provide a well‐preserved Late Triassic archive of climate information. Petrographic analysis of 38 Chinle sandstones provides new insight into the depositional history and evolution of palaeoclimate during Chinle deposition. This study focuses on the relationship between climate and meteoric diagenesis as a guide for constraining climate change in western equatorial Pangea during the Late Triassic. Petrographic analysis of Chinle sandstones reveals their wide range of textural attributes, as well as pedogenic and shallow burial diagenetic features that occurred during the Late Triassic. These diverse petrological characteristics are indicative of the evolving Late Triassic climate, when placed into a well‐constrained stratigraphic and geochronological framework. The stratigraphic succession is characterized by variations in the abundance of framework grains, detrital matrix, weathering intensity of feldspar and volcanic rock fragments, and the mineralogy of clay cements. Climate records from Chinle palaeosol geochemistry indicate a progression from wet to dry conditions. This trend is also reflected in the meteoric diagenetic features of Chinle sandstones. During deposition of the lower Chinle, elevated rainfall promoted the weathering of labile volcanic detritus to kaolinite, whereas feldspars (especially plagioclase) were partially or completely dissolved. In the upper Chinle, a trend towards drier conditions favoured the formation of smectite and less feldspar dissolution resulting in a higher abundance of well‐preserved plagioclase grains. Shallow burial meteoric weathering reactions in Chinle sandstones reflect the evolving climate during the Late Triassic.  相似文献   

4.
Major- and trace-element contents and Sr–Nd isotope ratios were determined in albitite, albitized and unaltered late-Variscan granitoid samples from the world-class Na-feldspar deposits of central Sardinia, Italy. The albite deposit of high economic grade has geological, textural, and chemical features typical of metasomatic alteration affecting the host granitoids. Albitization, locally accompanied by chloritization and epidotization, was characterized by strong leaching of Mg, Fe, K, and geochemically similar trace elements, and by a significant increase of Na. Ca, and P were moderately leached in the most metasomatized rocks. Other major (Si, Ti, Ca) and trace elements (U, Th, Y, and Zr), along with light (LREE) and middle (MREE) rare-earth elements, behaved essentially immobile at the deposit scale. The Nd-isotope ratios (0.512098 to 0.512248) do not provide information on the emplacement age of the unaltered late-Variscan granitoids. On the other hand, their Sr-isotope ratios fit an errorchron of 274±29 Ma (1σ error), in fair agreement with all published ages of Sardinian Variscan granitoids. The very low Rb content of albitized rocks precludes application of the Rb–Sr radiometric system to determine the age of albitization. The Sm–Nd system is not applicable either, because the 143Nd/144Nd ratios of albitized rocks and unaltered granitoids overlap. The overlap confirms that Sm and Nd were substantially immobile during albitization. On the other hand, the measured 87Sr/86Sr ratios of the albitized rocks are appreciably lower than those of the unaltered host granitoids, whereas, their initial Sr-isotope ratios are higher. This seems to suggest that a) albitization was induced by non-magmatic fluids rich in radiogenic Sr, and b) albitization occurred shortly after the granitoid emplacement. This conclusion is supported by Nd isotopes, because unaltered granitoids and albitites fit the same reference isochron at 274 Ma. The fluids acquired radiogenic Sr by circulation through the Lower Paleozoic metasedimentary basement. Specifically, it is estimated that Sr supplied by the non-carbonatic basement represents about 22 wt% of total Sr in albitite.  相似文献   

5.
塔里木库车坳陷中新生界储层砂岩成岩期钠长石化   总被引：7，自引：0，他引：7  
库车坳陷中新生界以陆相碎屑建造为主,在该区储层砂岩中,钠长石为广泛存在的自生矿物之一。显微结构证据显示,成岩期钠长石交代长石等碎屑,或呈细小自形晶填隙产出,在化学成份上,钠长石与被交代的长石存在过渡关系。在碎屑长石钠长石化过程中,母体相的溶解速率大于钠长石的沉淀速率,并伴有石英,高岭石等矿物的消耗,因此有助于次生孔隙的发育及储层物性的改善。从高岭石的垂向分布,以及钠长石相对含量与埋深的关系,可以看出钠长石化作用主要发生在探成岩(mescdiagenesis),对应于4 000-5 500m埋探,和90℃ ～110℃ 地温带 在库车和塔北地区,主要工业油气藏亦集中分布在这一深度带。  相似文献   

6.
The petrographic study of more than 250 dykes of lamprophyric and lamproid rocks of the eastern periphery of the Bohemian Mass shows that the basic plagioclase has been widely albitized, primary albite occurring quite exceptionally only. There are treated and statistically evaluated all the signs proving or supporting the idea of plagioclase albitization, as well as the microscopic signs distinguishing primary albite from the secondary one.  相似文献   

7.
Porosity and permeability may be preserved in deep sandstone reservoirs by clay coating (mainly chlorite) which limits quartz overgrowths. Chloritization around quartz grains results mainly from mineralogical transformations of pre‐existing clays. It is fundamental to study those clay precursors to better understand and predict the location and distribution of clay coatings in subsurface sandstones for petroleum or geothermal prospecting. This paper reports a high‐resolution analysis of the composition, distribution and fabric of clays along a modern estuary, the Gironde (south‐west France). The scale of the study ranges from thin sections, through sand bar bodies, up to the entire estuary. Results show that clays are detrital and deposited at the same time as sand grains despite strong hydrodynamic conditions. Clays bind to medium‐grained sands forming detrital clay grain coats. On average, 26% of detrital sand grains are coated along the entire length of the estuary. Coat thickness varies from 1 μm to more than 200 μm, and coat coverage exceeds 30% in some samples. The turbidity maximum zone position (surface water turbidity from 1 to 10 g l?1) in the estuary, which is controlled by seasonal variations in hydrodynamic processes, significantly impacts the location of the maximum clay content and the abundance of coated grains in sandy facies along the estuary. Flocculation in the water column results in the accumulation of suspended matter and contributes to the high turbidity in the estuary. Exopolymeric substances produced by diatoms are observed both in the field and by cryo‐scanning electron microscopy, suggesting that they may play a major role in binding detrital clay around sand grains. Finally, tidal bars and heterolithic point bars in the estuary funnel and estuarine channels are prime sedimentological targets for finding clay coatings and potentially good reservoir quality in deeply buried sandstones.  相似文献   

8.
Granitic rocks of Newfoundland are described on the basis of field observations and analysis of grain sequences of essential minerals and are compared with "ideal granite," a concept developed in investigation of Eurasian intrusives. Samples included ideal granites and granites affected by the earliest stages of metasomatism. All non-ideal granites have been albitized. Albitization is most intensive in Precambrian adamellites of the Avalon Platform, fine-grained granites of the Gander massif are albitized and silicified, and alkali granites of the Buchens Mine region are K-feldspathized and silicified. — Author.  相似文献   

9.
Importance of fracturing during retro-metamorphism of eclogites   总被引：3，自引：0，他引：3  
Presented textural and petrological data show that the deep to intermediate continental crust may fracture and that microfractures are the locus of fluid and mass transfer necessary for retrograde metamorphism. Kyanite eclogites from Ulsteinvik, Norway, underwent partial retrogression to granulite and amphibolite facies assemblages during near-isothermal exhumation from depths equivalent to more than 2.0 GPa at temperatures of 700–800 °C. Plagioclase-bearing assemblages, rich in hydrous phases, formed along margins of eclogite lenses and along mesoscopic fracture systems. Hydrated zones are from 1–50 cm thick, with adjacent wall-rock eclogite replaced by symplectites. At a low degree of reaction, the secondary minerals in the wall-rock are found along intra- and intergranular microfractures (typically 50–100 μm wide). Minerals filling the microfractures include orthopyroxene–plagioclase–spinel in garnet; plagioclase–sapphirine, plagioclase–corundum and plagioclase–spinel in kyanite; and diopside–plagioclase in omphacite. The microfractures are often arranged en echelon and are connected through microfaults. Releasing bends filled with amphibole and spinel form along microfaults in garnet. The faulting and fracturing caused localized chemical change in garnet: the damage zones close to faults are enriched in FeO and MnO with steep compositional gradients (8 wt% FeO over <20 μm). These FeO- and MnO-enriched zones form wedge-like structures around the tip of the faults (horsetail structures) and rose- or flame-like structures at sticking points along faults. They may represent examples of stress-induced chemical transport during fracture propagation. The change from dry to amphibole-bearing assemblages at the tip of the fracture, and fractures ending in splays of fluid inclusions trails, reflect the involvement of a fluid phase during fracture propagation. This suggests that the ‘dry’ granulite facies retrogression was also driven by fluid infiltration and that metamorphism at depth in collision zones may not be controlled by pressure and temperature alone.  相似文献   

10.
《地学前缘(英文版)》2020,11(3):835-842
Preservation bias may significantly impact the application of detrital zircon geochronology in reconstructing Earth surface processes.Here we compare detrital zircons from the actively eroding Murchison River channel in Western Australia with Ordovician fluvial sediments that have drained similar source rocks along the western margin of the West Australian Craton.In addition to standard analysis of detrital zircon age spectra we apply multivariate statistics to test the relation between 3-D grain shape,U-content and U-Pb ages,with the objective to quantify differences between both sample groups and track preservation along the transport pathway of the Murchison River.Our results show that zircon grains in modern river sands display an upstream trend toward larger surface areas,volume equivalent diameters and grain widths,as well as toward higher U-contents and lower apparent grain densities.3-D grain shape,size and age spectra of Murchison River zircons evolve consistently downstream,but even at the river outlet remain distinct from the Ordovician samples,as a less mature representation of source.We interpret Ordovician river zircons to represent a significantly depleted subset from which up to 22% of the zircon population may have been lost compared to the actively transported detrital load.This discrepancy between the characteristics of detrital zircons in modern active rivers and ancient fluvial Ordovician sandstones demonstrates a bias that could be relevant for other source-sink detrital transport systems throughout Earth history.  相似文献   

11.
An extinction-angle distribution was computed for randomly oriented grains of plagioclase feldspar with a known An-composition. Expressed by a histogram with eighteen columns, the results are shown by an 18×19 data array for nineteen types of low-temperature plagioclase. In a mixture of grains with various An-compositions, the extinction-angle distribution is given by a linear combination of the obtained data matrix with the frequencies of An-composition. If we have a sample containing randomly oriented grains of plagioclase feldspars, a frequency distribution of those with An-composition can be calculated after measuring their extinction angles. By taking into account the constraints of inequalities, the calculation is treated with the dual method of linear programming. The procedure provides for a simpler and more expedient method for determining the frequency distribution of An-composition. This method was tested by applying it to a model sample containing randomly oriented plagioclase grains with known An-composition frequencies, whose extinction-angle distribution was determined by the Monte Carlo method. An example of application of this method to Permian sandstones from central Japan is presented.  相似文献   

12.
This study deals with petrography and provenance of the Neogene reservoir sandstones encountered in the Kailas Tila, Titas, Bakhrabad and Shahbazpur Gas Fields of Bengal Basin. Framework grains are sand-sized to silt-sized particles of mainly detrital origin. The most common detrital grains are quartz, feldspars, and rock fragments. Mica occurred as minor and non-opaque heavy minerals found as minor accessories. Among the main detrital framework grains, quartz constitutes 51–60%, feldspar 3–15%, lithic fragments 8–22%. Sandstones encountered in the studied wells have been classified as sublithic arenite, feldspathic arenite and lithic arenite in order of abundance. Different triangular plots reveal that the Neogene sandstones of the studied wells exhibit a quartzolithic composition, low feldspar, very low volcanic grains and abundant sedimentary and low grade metamorphic lithic clasts indicating that the sands were derived from quartzose recycled orogen province, such as a fold thrust province or a collision suture zone. This study suggests that either the eastern Himalayas or Indo-Burman Ranges might act as the source of the sandstones of the studied wells of the Bengal Basin.  相似文献   

13.
ABSTRACT Two styles of feldspar alteration – carbonatization and albitization – were investigated using a cathodoluminescence (CL) technique. The nature of the alteration depends on the composition of the fluids. The infiltration of CO2-rich fluids causes decomposition of An-rich zones in plagioclase followed by the formation of secondary calcite, albite and white mica. K-feldspar is more resistant to CO2-induced alteration. The circulation of aqueous fluids results in decomposition of primary oligoclase into albite and clinozoisite. Secondary K-feldspar exsolved as small independent grains on the rim of the primary oligoclase, if the primary plagioclase was enriched in the orthoclase component. The fluids easily penetrate the crystals using, crystallographic plains, e.g., twinning or cleavage or simply along cracks. These migration pathways enable the fluids to enter the inner parts of the grain, which would otherwise not be affected by grain-surface alteration.  相似文献   

14.
DANIELA FONTANA 《Sedimentology》1991,38(6):1085-1095
The Upper Cretaceous Pietraforte Formation, an allochthonous unit of the Ligurian domain in the northern Apennines, provides a case study of the importance of detrital carbonate grains for provenance determination in sandstones. The Pietraforte Formation is composed of turbidite sandstones with subordinate conglomerate, deposited in an external sector of the Ligurian ocean, close to the Adriatic margin. The sandstones have a lithic composition, characterized by abundant sedimentary and metasedimentary rock fragments (35–56% of the terrigenous framework), little feldspar (<7%) that is almost exclusively plagioclase, and a high ratio of fine- to coarse-grained polycrystalline quartzose grains to total quartzose grains (average Qp/Qt=0.37). Carbonate rock fragments dominate the lithic association of both sandstones and conglomerates and provide the most detailed information for provenance determination. They are composed primarily of dolostones and a wide variety of limestones containing identifiable age-diagnostic microfossils. Fossils and rock textures of carbonate clasts document the erosion of Upper Triassic to Lower Cretaceous shelf and pelagic carbonate units which can be matched with Mesozoic rock types present in the Tuscan domain of the northern Apennines. Compositional results constrain the source of the Pietraforte Formation sandstones to the western margin of the Adriatic plate, from uplifted sedimentary and metasedimentary rocks of the Tuscan domain and its low-grade metamorphic basement. Coeval intrabasinal sources provided additional supplies to the depositional basin of the Pietraforte Formation; this intrabasinal supply consists of shelf carbonate allochems, planktonic foraminifera and argillaceous rip-up clasts. The presence of carbonate grains from shallow-water environments may indicate the existence during deposition of marginal shelf areas favourable for carbonate allochem production.  相似文献   

15.
Zeolites, calcite, quartz, kaolinite, chlorite and other authigenic minerals occur in the non-marine sandstones of the Lower Cretaceous Blairmore Group of the southern Alberta Foothills, Canada. Zeolites are restricted to plagioclase-rich sandstones and do not occur with kaolinite. Laumontite and barian-strontian heulandite (containing up to 6.9 weight percent BaO and 4.1 weight percent SrO) generally occur as pore fillings, but laumontite also occurs within albitized plagioclase. About 80 percent of plagioclase grains examined by electron microprobe are partly to completely albitized. A structural-stratigraphic reconstruction of the areas sampled indicate maximum burial depths, including tectonic thickening, in the range 4.7 to 7.8 Km, with P load ～1 to ～2 Kb, respectively. A comparison of mineral assemblages in Blairmore rocks with published experimental data suggest T did not exceed ～250°–280°C, depending on the load pressure. If P H2O∶P load, the presence of laumontite and albite suggest minimum T on the order of 150°–180°C. The lack of lawsonite suggests P-load <3Kb, which is compatible with structural-stratigraphic data. The occurrence of the alternative assemblages calcite-kaolinite-quartz and laumontite implies gradients in f CO2/f HO2. Assemblages containing laumontite presumably equilibrated with fluids having X CO2a Mg++/a (H+)2. X-ray diffraction studies on disordered graphitic material indicate a comparable degree of graphitization between Blairmore graphitic material and that found in zeolite facies assemblages elsewhere. If log f O2 were at least as low as that defined by quartz-magnetite-fayalite buffer, disordered graphite could not have equilibrated with fluids of the same composition as those equilibrated with laumontite or kaolinite.  相似文献   

16.
《Lithos》2007,93(1-2):89-106
The Salvezines Massif in northern French Pyrénées has undergone extensive albitization. The massif consists of gneisses and a leucogranite hosted in Paleozoic sediments (schists and carbonates). The leucogranite was emplaced at the end of the Hercynian orogeny. The granite underwent monazite fractionation during magmatic stages, which induced lowering of REE and Th contents in the most evolved rocks. Hydrothermal alteration during late magmatic stages is also identified through the development of a tetrad effect in the REE patterns of the granite and through the fractionation of ratios like Zr/Hf out of the CHARAC (CHarge RAdius Controlled) range. The granite underwent partial to complete sub-solidus albitization. Progressive metasomatic overprint induced metasomatic replacement of feldspars into albite and dissolution of quartz (episyenitization) together with new albite formation. Hydrothermal muscovite with a sheaf-like structure sometimes crystallised in albitites. Inherited muscovites from magmatic stage and newly formed muscovites from albitites have distinct chemical compositions with the latter being much more phengitic. The initial peraluminous chemistry of the leucogranite was lost during albitization and composition evolved towards a pure albite one (A/CNK = 1). Albitization induced gains in Na and Al, and losses in Si and K. Leaching of REE and U is also identified during albitization, as well as the non-CHARAC behaviour of some elements. δ18O values of quartz and muscovite in albitized granites are in the magmatic range (about 12 and 9.5‰, respectively). Feldspars acquired high δ18O values during albitization (up to 15‰). Muscovite grains from albitites have higher δ18O values than inherited ones and tend towards isotopic equilibrium with albite. The albitizing fluids have interacted with high-δ18O rocks (probably local carbonates) prior to alter granite. Muscovites inherited from magmatic stages display very characteristic disturbed 39Ar–40Ar age spectra with saddle-shapes when a muscovite associated with albitization provided a plateau age at 117.5 Ma for this alteration event. Albitization in the Salvezines Massif just preceded the regional talc/chlorite hydrothermal mineralization. Both events might then represent two outward signs of a huge hydrothermal system at the time of the rotation of the Iberian plate around Europe. The North Pyrenean Metamorphism is identified by 39Ar–40Ar analyses at ca. or younger than 100 Ma.  相似文献   

17.
《Applied Geochemistry》1994,9(5):479-490
Natural radionuclide migration in Miocene sedimentary rocks has been studied at the Tono U deposit in Japan, which is a potentially useful analogue of radioactive waste isolation in geological environments.Uranium series disequilibrium studies have revealed that natural radionuclides have been redistributed during the past 3.5 × 105 a in the U-mineralized zone. Permeability tests using core samples of the U-mineralized zone have shown that the microfabrics of sedimentary rocks, such as the connectivity of pores which control the groundwater movement, directly influence the redistribution of nuclides.Detailed observations using cathodoluminescence (CL) and electron probe microanalysis (EPMA) were carried out after a dye-impregnation test in order to improve the understanding of the importance of microfabrics in relation to nuclide migration. Results of the observations show that U has migrated within detrital grains, such as biotite and quartz, as well as between these grains. Uranium-series disequilibrium studies of each detrital quartz and biotite grain have been shown that these detrital minerals behave differently in respect of the migration of U, owing to their different textural properties. The detrital biotite flakes, especially along the (001)-cleavage planes, appear to have fixed U for a long period of time, whereas the U has remained mobile within the microfractures in detrital quartz grains.  相似文献   

18.
ABSTRACT Detrital alkali feldspars currently at burial depths of 3·2–3·5 km in the Upper Jurassic Humber Group of the Fulmar oilfield, UK North Sea, are overgrown and have been partially replaced by authigenic Or‐rich feldspar. Intracrystal microtextures suggest several different provenances for the detrital grains. The overgrowths are uniformly non‐cathodoluminescent and have occasional celsian‐rich zones. Transmission electron microscopy shows that they are composed of a microporous mosaic of subµm‐ to µm‐sized sub‐grains associated with barite, illite and pyrite. The subgrains are somewhat rounded but have an approximate {110} Adularia habit and display a faint modulated microtexture on the nanometre scale. They have triclinic symmetry, but the lattice angles depart only slightly from monoclinic symmetry. These features are characteristic of K‐feldspar precipitated relatively rapidly and at low temperature. Authigenic Or‐rich feldspar has also partially replaced microcline and perthitic albite within the detrital grains, often at a suboptical scale. Although, like diagenetic albitization, replacement by K‐feldspar is probably a very common diagenetic reaction, it has rarely been reported owing to difficulties in imaging the diagnostic textures with the scanning electron microscopy techniques used by most workers. The permeability of the subgrain microtexture may significantly hinder the use of feldspar overgrowths for K/Ar and 40Ar/39Ar dating of diagenesis, and the existence of suboptical, replacive authigenic K‐feldspar within detrital grains may significantly modify the apparent Ar ages of detrital grains. Similar subgrain microtextures in optically featureless quartz overgrowths are also illustrated.  相似文献   

19.
Examination of quartz from plutonic source rocks, sediments and soils, using both optical and scanning electron microscopes, reveals that the mineral is, almost always, cut by sets of partially healed, subplanar, subparallel microfractures dividing it into sheets typically one to a few micrometres thick. The structure, which originates in source rocks, is not due to crystal cleavage although it does show weak crystallo‐graphic influence in its directions. One to several sets of microfractures, intersecting at varying angles, can be present in any one quartz crystal. The microfractures vary in quality like cleavages; separation takes place readily along some, less so along others.

In superficial environments, the ease with which microfractures can be reopened largely controls the durability of quartz and the degree to which comminution can proceed. However, this type of breakage must cease when grains are one sheet thick (about 2–20 Mm in diameter). Hence quartz suddenly changes from being the dominant mineral in silt fractions to a normally minor constituent of clay fractions. The shape of grains depends largely on the number of (and angles between) microfrac‐ture sets present in the parent quartz. Most quartz grain surfaces represent fractures whose details characterize the pre‐emergent history of the quartz rather than sedimentary environments.  相似文献   

20.
The presence of a dispersed clast fraction in strata near the base of the Cosquer Formation in west Brittany, does not support a glacial origin for this unit. The upper 25 to 30 m of the underlying Kermeur Formation consists of a prograding sequence of very fine to fine sandstones deposited in a mid to distal current swept shelf setting. This sequence shows signs of slope instability, as do the supposed ‘glacial strata’ which overlie it. The upper two thirds of the Cosquer Formation contain spectacular slump-breccias. Smaller clasts within the laminated mudrocks at the base of the formation are associated with thin graded and non-graded sandstone laminae. They show no evidence of active penetration into underlying laminae other than can be explained by compaction. Larger clasts are confined to thicker massive beds, or disrupted units with marked internal contorted lamination. This, along with the abundance of slump features within the sequence suggests lateral emplacement by sediment gravity flows in a distal shelf-slope setting. Surface textures of sand grains within the formation are related to rock disaggregation along fractures developed during post-depositional deformation and are not related to glacial processes. Distinctive mineralogically immature, poorly-sorted aggregate sediment pellets, which have been considered as positive proof of glaciation, are not present.  相似文献   

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