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1.
贵州赤水地区位于四川盆地西南缘,晚白垩世时期该地区沉积了一套厚达1300m的陆相地层。本文通过地表露头的古流向野外观测和室内分析,详细研究赤水地区晚白垩世沉积充填过程及构造意义。赤水地区晚白垩世早期辫状河的古流向为自北东向南西,表明碎屑物源主要来自盆地北侧和东侧。根据物源、地层分布及区域地质背景推断,赤水地区晚白垩世的陆相沉积盆地属于陆内前陆盆地,陆内造山带位于盆地东侧。晚白垩世陆内前陆盆地的形成,可能受控于此阶段华南的构造挤压事件形成的陆内造山作用。  相似文献   
2.
The interpretation of fluvial styles from the rock record is based for a significant part on the identification of different types of fluvial bars, characterized by the geometric relationship between structures indicative of palaeocurrent and surfaces interpreted as indicative of bar form and bar accretion direction. These surfaces of bar accretion are the boundaries of flood‐related bar increment elements, which are typically less abundant in outcrops than what would be desirable, particularly in large river deposits in which each flood mobilizes large volumes of sediment, causing flood‐increment boundary surfaces to be widely spaced. Cross‐strata set boundaries, on the other hand, are abundant and indirectly reflect the process of unit bar accretion, inclined due to the combined effect of the unit bar surface inclination and the individual bedform climbing angle, in turn controlled by changes in flow structure caused by local bar‐scale morphology. This work presents a new method to deduce the geometry of unit bar surfaces from measured pairs of cross‐strata and cross‐strata set boundaries. The method can be used in the absence of abundant flood‐increment bounding surfaces; the study of real cases shows that, for both downstream and laterally accreting bars, the reconstructed planes are very similar to measured bar increment surfaces.  相似文献   
3.
Differential earth movements occurred during Eocene, Miocene, and late Caino‐zoic times. The faulting formed basins of sedimentation, led to dissection of land‐surfaces in some localities and burial in others, and faulted the Cainozoic sediments.

Laterite and silcrete cap remnants of relict landsurfaces of two different ages. Laterite formed before the Eocene; it was faulted and dissected during the Eocene in the north but continued to develop until the Miocene in the south. Silcrete formed from Eocene to Miocene times; its dissection was promoted by late Cainozoic tectonism.

Since laterite and silcrete formed on the same strata in warm, very moist environments, lithology and climate are not important genetic factors causing laterite to form at one time and silcrete at another. Only base levels of erosion differed. The silcrete surface was largely developed by streams flowing into mid‐Cainozoic lacustrine basins, whereas there is no evidence that these drainage conditions prevailed for laterite formation.  相似文献   
4.
Cryptic sequence boundaries in braided fluvial successions   总被引:2,自引:0,他引:2  
In braided fluvial deposits, consisting of monotonous successions of sandstone or conglomerate, it may be difficult to distinguish regionally significant bounding surfaces (sequence boundaries) from autogenic channel-scour surfaces. Major surfaces may be characterized by erosional relief and draped by lag deposits, but not all sequence boundaries show these characteristics. Other clues to the presence of a major surface are sharp changes in detrital composition, shifts in regional palaeocurrent trends and evidence of early diagenesis of the sandstones immediately below the sequence boundary. Examples of these attributes of cryptic sequence boundaries are illustrated from three Mesozoic units in the Colorado Plateau area of the United States. In the Chinle Formation (Triassic), near Moab, Utah, angular intraformational unconformities overlie sandstones showing evidence of early diagenesis. In the Castlegate Sandstone (Upper Cretaceous) of east-central Utah, a cryptic sequence boundary can be discriminated from other erosion surfaces by the evidence of detrital petrography and early diagenesis. Palaeocurrent data indicate changes in regional palaeoslope at two sequence boundaries within this unit. Evidence of early diagenesis is also present at a sequence boundary in the Kayenta Formation (Jurassic) of westernmost Colorado.  相似文献   
5.
The anisotropy of magnetic susceptibility is a well-known geological proxy in revealing the directional tectonic and sedimentological features of rocks, although it can be ambiguous in situations where these two factors co-occur. This paper demonstrates the usefulness of the anisotropy of magnetic susceptibility in determining palaeotransport directions in turbiditic rocks that underwent subsequent thrusting and folding. This study demonstrates that the magnetic lineation is largely unsuitable as a palaeocurrent direction proxy, and suggests that the imbrication of magnetic foliation is better in such cases. Moreover, the anisotropy of magnetic susceptibility results were analyzed in reference to a joint and fold study within the framework of the regional structural geology. Magnetic fabric investigations were conducted in the eastern part of the Outer Western Carpathians (south-east Poland). During the study, a total of 191 oriented palaeomagnetic samples were collected from three outcrops (Nasiczne, Dwernik and Hoczew) in the Krosno Beds, Silesian Unit. For the purpose of sedimentological analysis, 121 m of turbidite successions were documented and 126 directional sedimentary structures were measured. The magnetic anisotropy of sandstones revealed typical sedimentary fabrics, often overprinted by variably intense tectonic deformation. Oblate susceptibility ellipsoids from Nasiczne showed tilt coherent with the palaeoflow direction, whereas the rocks from Dwernik and Hoczew contained triaxial magnetic fabric developed during compressional palaeostress. This paper suggests that medium-grained and coarse-grained sandstones, preferably with high mica content, are the most suitable for palaeotransport reconstructions among the studied lithologies.  相似文献   
6.
The Kanmantoo Group of South Australia is a thick (~7–8 km) succession of predominantly clastic marine sedimentary and metasedimentary rocks that were deposited very rapidly in a localised basin (Kanmantoo Trough) during the Early Cambrian. Despite structural complexity and varying grades of metamorphism, a surprising amount of primary sedimentological information is still available. Although a variety of depositional facies are represented, the group is dominated by parallel, sharp‐based, mineralogically immature sandstone interbedded with mudstone. The sandstone beds are most commonly fine to medium grained, massive and lacking in obvious grading except at the top. Single beds often reach several metres in thickness and amalgamation of beds is not uncommon. We argue that these sandstone beds could be the products of sustained high‐density turbidity currents. Triggering mechanisms for such turbidity currents remain uncertain, but they may have been initiated as hyperpycnal flows during catastrophic flood events at the mouths of high‐load‐capacity rivers, or from the failure of unstable buildups of sediment on delta slopes. Palaeocurrent studies from sole marks suggest a southerly source, which was probably an active orogenic terrain in formerly contiguous Antarctica. It is likely that a major delta complex lay at the southern end of the basin.  相似文献   
7.
The Palaeoproterozoic Capricorn Formation near Ashburton Downs in northwestern Australia formed during the latter stages of the convergence of the Pilbara and Yilgarn Cratons. Palaeocurrent and facies analyses show that the southwesterly derived sediments were deposited in terrestrial environments and in a lake or shallow sea with a shoreline trending southeast. Intraformational debris flows suggest instability during sedimentation. Zircon grains from an accretionary lapilli tuff, dated at 1804 ± 7 Ma by the SHRIMP U—Pb method, show that the Capricorn Formation was deposited at the same time as granitic plutons were intruded in the Gascoyne Complex to the south and west. Although the Capricorn Formation was deposited with marked angular unconformity over the turbiditic Ashburton Formation, both formations could have been deposited in a foreland basin on the northeast flank of the growing Ashburton Fold Belt.  相似文献   
8.
The early Cretaceous fill of the forearc/intra-arc Byers Basin includes a 600- to 900-m-thick interval of marine slope apron deposits, the President Beaches Formation. This is a predominantly argillaceous succession within which coarser-grained deposits are largely confined to lenticular packages of low width–thickness ratios. The entire formation was deposited in mid- to late-Berriasian times, coincident with a pulse of regional arc expansion, at minimum mean accumulation rates of 120–225 mm 1000 years–1. The mudstones are finely laminated, with a restricted benthic macrofauna and minimal bioturbation, indicating relatively inhospitable sea-floor environments. Sand-rich packages occur as 7- to 30-m-thick channel-fill units composed chiefly of classical medium-grained turbidites, in some cases associated with complex high-concentration turbidity current deposits and minor mud-rich debrites. These sand-bodies are apparently elongate along (normal to) the NW-facing palaeoslope implied by slump-fold axes (and the strike of the volcanic arc). Similarly, palaeocurrent indices show a consistent arc-parallel, NE-directed trend, suggesting that transport processes were strongly influenced by the structural 'grain' of the irregular slope morphology. Slope instability is recorded by widespread slump and soft-sediment collapse features promoted by a combination of steep sea-floor gradients and relatively high rates of sedimentation. A lack of systematic vertical facies trends indicates that this was not a progradational or well-organized system, despite high rates of sediment supply. However, the strong systematic relationship between palaeocurrents and palaeoslope suggests a promising basis for evaluating organization in otherwise poorly ordered slope apron depositional systems.  相似文献   
9.
M Moros  A Kuijpers 《Marine Geology》2002,192(4):393-417
High-resolution physical, mineralogical, sedimentological and micropalaeontological studies were carried out on North Atlantic cores from the Reykjanes Ridge at 59°N and from the region southwest of the Faeroe Islands. All core sites are situated along the pathway of Iceland-Scotland Overflow Water (ISOW) and the various parameters measured display similar features. Previously identified carbonate oscillations [Keigwin and Jones (1994) J. Geophys. Res., 99, 12397-12410] in the time span back to the Marine Isotope Stage 5-4 transition and Late Glacial lithic events [Bond and Lotti (1995) Science, 267, 1005-1010], such as the Heinrich ice-rafting events, are all represented in the core records. Long-term trends and higher-frequency changes in ISOW intensity were reconstructed on the basis of various independent proxy records. The long-term trends in circulation match theoretical orbitally forced insolation changes. Our observed links between ice-rafted detritus (IRD) input, variations in sea surface temperature (SST) and circulation at greater depth point to the need to re-examine the origin of IRD events. We suggest that these events may have been triggered by enhanced, partly sub-surface, heat transport to the north. Enhanced northward heat transport may have caused bottom melting of floating outlet glaciers and ice shelves, leading to increased iceberg discharge and ice sheet destabilization. This discharge resulted in lower SST’s and a lower temperature over Greenland. Thus, as shown by our records, this scenario implies a temporary de-coupling of surface processes and circulation at greater depth. A key feature is the occurrence of a saw-tooth pattern in the marine data, which is similar to the Greenland ice core records. Moreover, the ‘warming’ theory of IRD events would explain the observed ‘out-of-phase’ relationship between the Greenland and Antarctic ice core records and also the rapid establishment of higher temperatures over Greenland immediately after the cold phases (stadials) of the Dansgaard-Oeschger cycles.  相似文献   
10.
The chemistry and. mineralogy of calcretes and associated calcareous fine earth from the lower River Murray Basin and Yorke Peninsula in South Australia suggest that the carbonate material was deposited as a continuous unit and subsequently modified. At each site examined the Ca/Mg ratio progressively decreases with increasing depth, and where data are available, a similar decrease in the Ca/Sr ratio is observed. This sequence is independent of the hardness or physical condition of the carbonate. In some places the carbonate from the lowest horizons is nearly pure dolomite. The change in Ca/Mg ratio with depth is considered to be due to leaching, probably during periods of the past when the climate was less arid and the rainfall penetrated to about 2 m.

In the lower Murray Basin the Ca/Mg ratio of the surface calcrete horizon progressively increases from Tailetn Bend N to Sedan, and the silicate clay fraction changes from sepiolite at Tailem Bend, to palygorskite at Murray Bridge and illite at Sedan. The bedrock also varies, from magnesium‐rich amphibolites at Tailem Bend to granite at Sedan and, further, there is an apparent relation between the chemistry of the non‐carbonate fraction of the calcretes and the underlying rock. The original carbonate deposit probably formed in lakes of different chemistry, and has been modified by solution and precipitation.  相似文献   
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