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MARIANA I. OLARIU CORNEL OLARIU RONALD J. STEEL ROBERT W. DALRYMPLE ALLARD W. MARTINIUS 《Sedimentology》2012,59(2):356-378
The first sandstone unit of the Esdolomada Member of the Roda Formation (hereafter referred to as ‘Esdolomada 1’) was formed by a laterally‐migrating, shelf tidal bar. This interpretation is based on detailed mapping of the bedding surfaces on the digital terrain model of the outcrop built from light detection and ranging data and outcrop photomosaics combined with vertical measured sections. The Esdolomada 1 sandbody migrated laterally (i.e. transverse to the tidal currents) towards the south‐west along slightly inclined (1.6° to 4.6°) master bedding surfaces. The locally dominant tidal current flowed to the north‐west. This current direction is indicated by the presence of stacked sets of high‐angle (average 21°) cross‐stratification formed by dunes that migrated in this direction, apparently in an approximately coast‐parallel direction. The tidal bar contains sets and cosets of medium‐grained cross‐stratified sandstone that stack to reach a thickness of about 5·5 m. Individual cross‐bed sets average about 50 cm thick (with a range of 10 to 70 cm) and have lengths of ca 130 to 250 m in a direction perpendicular to the palaeocurrent. Set thickness decreases in the direction of migration, towards the south‐west, and the degree of bioturbation increases, so that the cross‐bedded sandstones gradually change into highly bioturbated finer‐grained and thinner‐bedded sandstones lacking any cross‐stratification. The rate of thinning of individual dune sets as they are traced down any obliquely‐accreting master surface is some 40 cm per 100 m (0·004) for the older, thicker sandstones, whereas the younger, thinner beds thin at a rate of 15 cm over 100 m (0·0015). The tidal bar has a sharp base and top and is encased in finer‐grained bioturbated, marine sandstones. The Esdolomada bar crest was oriented north‐west to south‐east, parallel to the tidal palaeocurrents and to the nearby palaeoshoreline, but built by lateral accretion towards the south‐west. Lateral outbuilding generated a flat‐topped bar with a measured width of about 1700 m, and a preserved height of 5·5 m. The bar, disconnected from a genetically related south‐westward prograding delta some 2 km to the north‐east, developed during the transgressive phase of a sedimentary cycle. The tidal bar was most probably initiated as a delta‐attached bar at the toesets of the delta front and during transgression evolved into a detached tidal bar. 相似文献
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《International Geology Review》2012,54(13):1508-1521
Twenty Cretaceous shale samples from two wells in the Orange Basin of South Africa were evaluated for their source rock potential. They were sampled from within a 1400 m-thick sequence in boreholes drilled through Lower to Upper Cretaceous sediments. The samples exhibit total organic carbon (TOC) content of 1.06–2.17%; Rock-Eval S2 values of 0.08–2.27 mg HC/g; and petroleum source potential (SP), which is the sum of S1 and S2, of 0.10–2.61 mg HC/g, all indicating the presence of poor to fair hydrocarbon generative potential. Hydrogen index (HI) values vary from 7 to 128 mg HC/g organic carbon and oxygen index (OI) ranges from 37 to 195 mg CO2/g organic carbon, indicating predominantly Type III kerogen with perhaps minor amounts of Type IV kerogen. The maturity of the samples, as indicated by T max values of 428–446°C, ranges from immature to thermally mature with respect to oil generation. Measured vitrinite reflectance values (%Ro) of representative samples indicate that these samples vary from immature to mature, consistent with the thermal alteration index (TAI) (spore colour) and fluorescence data for these samples. Organic petrographic analysis also shows that amorphous organic matter is dominant in these samples. Framboidal pyrite is abundant and may be indicative of a marine influence during deposition. Although our Rock-Eval pyrolysis data indicate that gas-prone source rocks are prevalent in this part of the Orange Basin, the geochemical characteristics of samples from an Aptian unit at 3318 m in one of the wells suggest that better quality source rocks may exist deeper, in more distal depositional parts of the basin. 相似文献
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《International Geology Review》2012,54(11):1377-1394
The Guerrero terrane is composed of Middle Jurassic–Lower Cretaceous arc assemblages that were rifted from the North American continental mainland during Late Jurassic–Early Cretaceous back-arc spreading within the Arperos Basin, and subsequently accreted back to the continental margin in the late Aptian. The Sierra de los Cuarzos area is located just 50 km east of the Guerrero terrane suture belt and, therefore, its stratigraphic record should be highly sensitive to first-order tectonic changes. Two Upper Jurassic–Lower Cretaceous clastic units were recognized in the Sierra de los Cuarzos area. The Sierra de los Cuarzos Formation is the lowermost exposed stratigraphic unit. Petrographic data and U-Pb zircon ages suggest that the Sierra de los Cuarzos Formation was derived from quartz-rich sedimentary and igneous sources within the North American continental mainland. The Sierra de los Cuarzos Formation is overlain by the Pelones Formation, which is composed of volcanoclastic sandstones derived from a mix of sources that include the mafic arc assemblages of the Guerrero terrane and quartz-rich sedimentary and volcanic rocks exposed in the continental mainland. The provenance change documented in the Sierra de los Cuarzos area suggests that the Pelones Formation was deposited when the Arperos Basin was closed and the Guerrero terrane was colliding with the North American continental mainland. Based on these data, we interpret the Pelones Formation as the syn-tectonic stratigraphic record associated with the accretion of the Guerrero terrane. 相似文献
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物源问题的研究对于了解和确定研究区有利沉积相带及储集砂体的分布、寻找有利油气储集层具有重要意义。胜坨地区沙四上勘探程度相对较低,前人关于目的层段的物源体系的研究成果很少。为了进一步查清研究区沙四上沉积时期砂体成因和物源方向,更好地预测砂体的分布,本文综合运用地震、测井、地球化学等资料,采用古地貌分析法、沉积砂体岩屑类型对比法、砂岩百分含量法和沉积物特征元素等方法探讨了胜坨地区沙四上亚段砂体的物源方向,并结合特征元素物源指数判别公式对沙四上亚段物源体系实现了定量评价。综合分析认为研究区沙四上亚段具有多物源体系特征,除了直接来自于胜北断层和坨94断层上升盘的物源外,还有沿着胜北断层与坨94断层转换带处的物源体系。 相似文献
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Geochemistry and Provenance of Maastrichtian Clastic Rocks in the Dikmendede Formation of Orhaniye in Kazan-Ankara-Turkey Region 总被引:1,自引:0,他引:1
An integrated petrographic and geochemical study of the sandstones of the Maastrichtian-aged in the Orhaniye (Kazan-Ankara-Turkey) was carried out to obtain more information on their provenance, sedimentological history and tectonic setting. Depending on their matrix and mineralogical content, the Maastrichtian sandstones are identified as lithic arenite/wacke. The Dikmendede sandstones derived from types of provenances, the recycled orogen and recycled transitional. The chemical characteristics of the Dikmendede sandstones, i.e., fairly uniform compositions, high Th/U ratios (>3.0), negative Eu anomalies (Eu/Eu* 0.72–0.99) and Th/Sc ratios (mostly less than 1.0), favor the OUC (old upper continental crust) provenance for the Dikmendede sandstones. The SiO2/Al2O3, Th/Sc (mostly <1.0) and La/Sc (<4.0) ratios are; however, slightly lower than typical OUC, and these ratios may suggest a minor contribution of young arc-derived material. The rare earth element (REE) pattern, and La/Sc versus Th/Co plot suggests that these sediments were mainly derived from felsic source rocks. The Dikmendede sandstones have high Cr (123–294 ppm) and Ni (52–212 ppm) concentrations, Cr/Ni ratio of 1.93, and a medium correlation coefficient between Cr and Ni and corresponding medium to high correlation of both (Cr and Ni, respectively) elements with Co. These relationships indicate a significant contribution of detritus from ophiolitic rocks. As rare earth element data are available for the Dikmendede sandstones, the Eu/Eu* is compared with LaN/YbN. Samples plot in the area of overlapping between continental collision, strike-slip and continental arc basins. The predominantly felsic composition of the Dikmendede sandstones is supported by the REE plots, which show enriched light REE, negative Eu anomaly and flat or uniform heavy REE. The Dikmendede sandstones have compositions similar to those of the average upper continental crust and post-Archean Australian shales. This feature indicates that the sediments were derived mainly from the upper continental crust. The Dikmendede sandstones have chemical index of alteration (CIA) values of 28–49, with an average of 40 indicating a low degree of chemical weathering in the source area. The compositional immaturity of the analyzed sandstone samples is typical of subduction-related environments, and their SiO2/Al2O3 and K2O/Na2O ratios and Co, Sc, Th and Zr contents reflect their oceanic and continental-arc settings. The Dikmendede sandstones were developed as flysch deposits derived from mixed provenance in a collision belt. 相似文献