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中国城市分布特征及其影响因素 总被引：1，自引：0，他引：1
通过对221 BC-1911 AD年间中国城市分布特征及其影响因素的研究,发现：① 在整个研究阶段,中国城市分布的重心位于中东部地区,秦—唐时期重心向西南地区大幅移动,唐—元时期重心先东北方向移动后转向西南方向,元—清时期重心主要向北移动。以腾冲—瑷珲一线为界线分区研究发现,在整个研究阶段西部重心在南北及东西方向呈现出较大的波动趋势,东部重心呈现出与全国类似的运动轨迹。② 标准差椭圆分析表明全国及东西部地区城市分布经历了明显的分散—集聚—分散的变化趋势,其中西部地区最为明显。从城市分布的平均方向看,全国及东部地区具有一定的相似性,均以东北—西南为主要分布特征,西部地区是以西北—东南为主要分布特征。③ 从城市密度分布特征看,其空间连续性和自组织性不断加强且由空间相关性引起的结构性变异处于显著状态。从方向上来看,全方向上的均质化程度呈下降趋势,西北—东南方向各时期城市密度均质化程度相对较好,空间差异相对较小,而东—西方向差异最为明显。④ 分析不同时期城市设置的相关因素发现,221 BC-1911 AD年间,中国城市设置相对集中在地形平坦、气候适中且靠近河流及中心城市的地区。 相似文献
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电力资源在中国能源建设以及能源发展方式转变中处于中心地位,全面降低电力强度是实现资源、经济、环境可持续发展的关键所在。研究中国电力强度重心迁移路径以及区域差异可以厘清电力强度分布状况,对实现上述目标具有现实意义。本文运用重心理论研究了中国1985-2014年电力强度重心迁移路径,进而采用泰尔指数计算并分解了电力强度非均衡程度。研究结果表明：1985-2014年,电力强度总体上呈现出持续向西迁移的态势。期间,1985-1993、2003-2014年主要体现为向西北方向移动,1993-2003年则表现为向西南方向移动的特征。1985-2000年,区域间及区域内部差异对于重心迁移的贡献率各占50%;2000-2014年,演变推力主要来源于区域内部差异,贡献率均值为54.49%,内部差异则受西北、西南、华北地区影响最大。 相似文献
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M. Ali H. Oda A. Hayashida K. Takemura & M. Torii 《Geophysical Journal International》1999,136(1):218-228
We have conducted a palaeomagnetic study on three sediment cores obtained from a single site in Lake Biwa, central Japan. A total of 1430 discrete samples from three cores show strong and stable remanence, which is carried by pseudo-single-domain magnetite. Palaeomagnetic records from individual cores show little scatter of directions, and there is excellent correlation among the records from the site. A composite palaeomagnetic secular variation (PSV) record for the Holocene was constructed after fine-scale adjustment and stacking of the three records. The age model is based on two wide-spread tephra layers: Kawagodaira (3150 yr BP, calendar years) and Kikai-Akahoya (7250 yr BP). Our results show that the declinations and inclinations are strikingly similar to archaeomagnetic data from southwest Japan for the last 2000 years. The PSV record from Lake Biwa shows good correlation in directions with that of Beppu Bay, located about 500 km west of Lake Biwa, although the Beppu Bay record shows relatively smaller amplitudes of both inclination and declination, probably due to heavy smoothing of the initial results. When the Lake Biwa record is compared with the proposed type curve for southwest Japan, there is considerable disagreement throughout the Holocene. A re-analysis of the data sets and other information used to construct the type curve revealed that the discrepancy in the results was due to inadequate age determination for the cores used in previous studies. We conclude that the stacked PSV record from Lake Biwa meets the criteria required for a reliable curve. Moreover, our results resolve the complications found in the correlation of PSV records in southwest Japan and thereby contribute to a better understanding of Holocene PSV. 相似文献
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《Basin Research》2018,30(3):426-447
Integration of detrital zircon geochronology and three‐dimensional (3D) seismic‐reflection data from the Molasse basin of Austria yields new insight into Oligocene‐early Miocene palaeogeography and patterns of sediment routing within the Alpine foreland of central Europe. Three‐dimensional seismic‐reflection data show a network of deep‐water tributaries and a long‐lived (>8 Ma) foredeep‐axial channel belt that transported Alpine detritus greater than 100 km from west to east. We present 793 new detrital zircon ages from 10 sandstone samples collected from subsurface cores located within the seismically mapped network of deep‐water tributaries and the axial channel belt. Grain age populations correspond with major pre‐Alpine orogenic cycles: the Cadomian (750–530 Ma), the Caledonian (490–380 Ma) and the Variscan (350–250 Ma). Additional age populations correspond with Eocene‐Oligocene Periadriatic magmatism (40–30 Ma) and pre‐Alpine, Precambrian sources (>750 Ma). Although many samples share the same age populations, the abundances of these populations vary significantly. Sediment that entered the deep‐water axial channel belt from the west (Freshwater Molasse) and southwest (Inntal fault zone) is characterized by statistically indistinguishable age distributions that include populations of Variscan, Caledonian and Cadomian zircon at modest abundances (15–32% each). Sandstone from a shallow marine unit proximal to the northern basin margin consists of >75% Variscan (350–300 Ma) zircon, which originated from the adjacent Bohemian Massif. Mixing calculations based on the Kolmogorov–Smirnoff statistic suggest that the Alpine fold‐thrust belt south of the foreland was also an important source of detritus to the deep‐water Molasse basin. We interpret evolving detrital zircon age distributions within the axial foredeep to reflect a progressive increase in longitudinal sediment input from the west (Freshwater Molasse) and/or southwest (Inntal fault zone) relative to transverse sediment input from the fold‐thrust belt to the south. We infer that these changes reflect a major reorganization of catchment boundaries and denudation rates in the Alpine Orogen that resulted in the Alpine foreland evolving to dominantly longitudinal sediment dispersal. This change was most notably marked by the development of a submarine canyon during deposition of the Upper Puchkirchen Formation that promoted sediment bypass eastward from Freshwater Molasse depozones to the Molasse basin deep‐water axial channel belt. The integration of 3D seismic‐reflection data with detrital zircon geochronology illustrates sediment dispersal patterns within a continental‐scale orogen, with implications for the relative role of longitudinal vs. transverse sediment delivery in peripheral foreland basins. 相似文献
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国家森林乡村建设是实施乡村振兴战略和改善农村人居环境的重要措施,但森林乡村在不同尺度的分异特征及优化方向尚不明晰。本文综合运用探索性空间数据分析、耦合度模型和Voronoi图分析等研究方法,从全国、区域和行政尺度分析了国家森林乡村空间格局、数量规模和内部分异特征,并基于森林乡村与人口耦合关系及乡村可达类型识别,分析了全国森林乡村的分区格局及其优化方向。结果表明：全国森林乡村密度具有明显的东高西低分异特征,长江三角洲地区是森林乡村高密度集中分布最大的连片区。基于县域单元的省域森林乡村泰尔指数差异较大,东南区域森林乡村分布相对均衡,西南和东北区域内部差异显著。森林乡村空间可达性呈现自东向西的梯度递减,森林乡村与人口密度的耦合类型分布具有较强的内部分异性。文章基于森林乡村可达类型与耦合分区的组合分析,将全国划分为重点发展区、特色挖掘区、集聚提升区、适度引导区和功能转型区五类,明确了不同类型区森林乡村建设的重点方向,将为面向乡村振兴的全国森林乡村建设与分区优化提供决策依据。 相似文献
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Daniel J. Leathers 《自然地理学》2013,34(2):171-190
This study documents the spatial and temporal characteristics of northeast United States tornadoes and the synoptic patterns associated with their development. Daily 1200 UTC surface pressure, 500 mb height and 850 mb temperature data are used in a compositing analysis to indicate the general conditions on tornado-producing days during four quasi-seasonal periods. Temporally, two-thirds of all northeast tornadoes occur between the hours of 1800 UTC and 0000 UTC. Annually, greater than 75% occur during the four-month period from May through August. During the period of study (1950 through 1986) the region had an average of 30 tornado occurrences per year. Spatially, three preferred areas of tornadic development are identified across the northeast region. These areas include western and southeastern Pennsylvania and north-central Massachusetts. The general synoptic patterns associated with tornadic events in the northeast United States remain consistent throughout the year. The composite analyses indicate that the presence of a strong surface low pressure system moving through the Great Lakes, coupled with significant upper-level divergence associated with a vigorous shortwave feature and a cold frontal boundary, are the synoptic features most common during the initiation of tornadic storms in this region. [Key words: synoptic climatology, tornadoes, northeastern United States.] 相似文献
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This paper presents a case study of a cluster of tornadoes produced by tropical cyclone (TC) Ivan in the Mid-Atlantic region of the United States on 17–18 September 2004. Ivan produced 118 tornadoes between 15 and 18 September, but 57 of these occurred in a period of only 10 h on the last two days. The number of tornadoes that occurred in this event is anomalous when compared to regional tornado climatology and to other TC tornado events. Furthermore, a significantly large number of the tornadoes, when compared to other TC tornadoes, were rated strong according to the Fujita Scale, and they were anomalously long- and wide-tracked. Ivan was in a synoptically favorable environment when these tornadoes were produced, and the tornadoes occurred in a favorable local-scale environment near a front with spatially overlapping shear and buoyancy. 相似文献
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Analysis of data collected between 1950 and 1995 in the state of Indiana indicates that the state experiences an average of nearly 19 tornadoes each year, which occur on approximately 8 or 9 days during an average year. Analysis of storm dimensions indicates that the annual probability of a point location in Indiana being impacted by a tornado is 1.4 × 10-4 per year. The largest number of the more intense tornadoes (F2 and higher) typically occur from March to June, during the afternoon and evening, although tornadoes have been observed in each month of the year and each hour of the day. Inter-annual variability of tornadic events in Indiana is large and may reflect both physical causes (e.g., the variability of cyclone passages or jet stream location) and recording bias (e.g., increased public awareness during the 1950s). Using the data set from Indiana, statistical analyses of the temporal and spatial variability of tornadoes are undertaken. Analyses of tornado reports by county indicate that, in addition to area and population, surface roughness is a significant determinant of event frequency. Analyses presented herein also indicate that in contrast to previous studies, (1) no statistically significant hebdomadal cycle of tornado reporting is evident in the data and (2) the diurnal cycle of tornado records is evident for all F-scale classes and is not confined largely to the weak tornado categories. Analyses of the physical dimensions and F-scale ratings of tornadoes in Indiana indicate that there has been a decline in reported tornado intensity, width, and length between 1950 and 1995. These changes are in accord with evidence of a decreasing number of reports of strong and violent tornadoes across the United States over this period and appears to be principally a result of reporting biases at the beginning of the record. [Key words: tornado climatology, temporal and spatial variability, reporting bias.] 相似文献
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Summary. In order to contribute to the resolution of the problem of the plate tectonic character of the Caribbean, a palaeomagnetic study has been carried out on some Jamaican igneous rocks. Sixteen Late Cretaceous intrusives and lavas and one Late Miocene lava sampled in five sites have been investigated. Because of widespread maghaemitization of the predominantly large-grained deuteric class 1 titanomagnetites, some difficulty was experienced in identifying stable directions of magnetization in the Cretaceous rock units. Using thermal demagnetization technique, two distinct directions of magnetization were obtained, significantly different from those observed in contemporaneous North American rocks. Nine units yield a palaeomagnetic pole at 143.8°W, 44.1°N, referred to as 'normal', while seven units yield'equatorial'poles situated both east and west of Jamaica. It is not possible to decide which of the two directions of magnetization reflects the Late Cretaceous geomagnetic field in Jamaica, but from other evidence they appear to merit palaeotectonic interpretation. They are consistent with the plate tectonic behaviour of the Caribbean since Late Cretaceous and its motion from the southwest relative to the present day frame of reference, with a simultaneous large anticlockwise horizontal rotation of Jamaica. Late Miocene lavas, containing high-coercivity magnetic material, yield closely grouped directions giving a palaeopole at 152.4°W, 73.3°N, supporting the deductions made from the Cretaceous data. 相似文献
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A flexural model for the Paradox Basin: implications for the tectonics of the Ancestral Rocky Mountains 总被引：1，自引：0，他引：1
D. L. Barbeau 《Basin Research》2003,15(1):97-115
The Paradox Basin is a large (190 km × 265 km) asymmetric basin that developed along the southwestern flank of the basement‐involved Uncompahgre uplift in Utah and Colorado, USA during the Pennsylvanian–Permian Ancestral Rocky Mountain (ARM) orogenic event. Previously interpreted as a pull‐apart basin, the Paradox Basin more closely resembles intraforeland flexural basins such as those that developed between the basement‐cored uplifts of the Late Cretaceous–Eocene Laramide orogeny in the western interior USA. The shape, subsidence history, facies architecture, and structural relationships of the Uncompahgre–Paradox system are exemplary of typical ‘immobile’ foreland basin systems. Along the southwest‐vergent Uncompahgre thrust, ～5 km of coarse‐grained syntectonic Desmoinesian–Wolfcampian (mid‐Pennsylvanian to early Permian; ～310–260 Ma) sediments were shed from the Uncompahgre uplift by alluvial fans and reworked by aeolian‐modified fluvial megafan deposystems in the proximal Paradox Basin. The coeval rise of an uplift‐parallel barrier ～200 km southwest of the Uncompahgre front restricted reflux from the open ocean south and west of the basin, and promoted deposition of thick evaporite‐shale and biohermal carbonate facies in the medial and distal submarine parts of the basin, respectively. Nearshore carbonate shoal and terrestrial siliciclastic deposystems overtopped the basin during the late stages of subsidence during the Missourian through Wolfcampian (～300–260 Ma) as sediment flux outpaced the rate of generation of accommodation space. Reconstruction of an end‐Permian two‐dimensional basin profile from seismic, borehole, and outcrop data depicts the relationship of these deposystems to the differential accommodation space generated by Pennsylvanian–Permian subsidence, highlighting the similarities between the Paradox basin‐fill and that of other ancient and modern foreland basins. Flexural modeling of the restored basin profile indicates that the Paradox Basin can be described by flexural loading of a fully broken continental crust by a model Uncompahgre uplift and accompanying synorogenic sediments. Other thrust‐bounded basins of the ARM have similar basin profiles and facies architectures to those of the Paradox Basin, suggesting that many ARM basins may share a flexural geodynamic mechanism. Therefore, plate tectonic models that attempt to explain the development of ARM uplifts need to incorporate a mechanism for the widespread generation of flexural basins. 相似文献
12.
北部湾海岸带植被覆盖时空动态特征及未来趋势 总被引：1，自引：0，他引：1
分析北部湾海岸带植被覆盖动态变化趋势,能为开展海岸带植被生态环境监测提供决策。以2000―2011年SPOT-VEGETATION逐旬NDVI数据为基础,采用MVC（最大值合成法）、标准差、线性趋势分析（SLOPE）和Hurst指数等数理统计方法对研究区植被覆盖时空变化特征及未来趋势进行定量分析。结果表明：1）研究区植被覆盖整体上处于变好的状态,在年尺度上呈现出“波动―明显改善”的趋势,且海岸带东岸与西岸的植被变化趋势快于丘陵地区;在季节尺度上NDVI的增长速率为：秋季＞夏季＞春季＞冬季;在月尺度上NDVI在6―11月植被生长迅速,而在1―4月则生长缓慢;2）NDVI均值的空间分布规律自东北―西南中心线呈现出“两头高、中心地带低”的趋势,且NDVI均值自西向东的变化规律为-0.026/1N°,具有经向地带性特点;3）NDVI的Hurst指数值为0.306 5~0.995 3,平均值为0.777 4,反持续性序列（15.78%）＜持续性序列（84.22%）,未来总体植被覆盖呈现出持续性改善趋势。未来需要重点进行保护的植被区域主要集中在十万大山的西南部、钦江流域的上游以及合浦县的西南部。 相似文献
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Imbricate reflections commonly occur in the glacigenic section of seismic profiles from the Bjørnøya Trough. This was the main drainage pathway for fast‐flowing ice‐streams from the former Barents Sea and Scandinavian ice sheets. Industry three‐dimensional (3D) seismic data from the southern flank of the Bjørnøya Trough are used here to investigate these imbricate reflections. Integration of vertical seismic sections with 3D plan view images and attribute maps reveal that imbricate reflections at the SW Barents Sea Margin are mega‐scale sediment blocks with a glacigenic origin. Imbricate reflections in two regions to the east of the survey appear on plan‐view as well‐developed lineations of U‐shaped crescents; however, following detailed analysis of their location, geometry and relation to sailing direction during data acquisition, we can demonstrate that these are seismic artefacts. These artefacts are related to the straight parts of east–west‐trending plough marks on the sea floor, having a dip direction that is directly related to the sailing direction of the ship during seismic acquisition. By analysing both real glacigenic imbrications and false imbrications or artefacts, we are able to demonstrate the critical distinguishing criterion. 相似文献
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Seismic stratigraphy and paleogeographic evolution of Fairway Basin,Northern Zealandia,Southwest Pacific: from Cretaceous Gondwana breakup to Cenozoic Tonga–Kermadec subduction 
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下载免费PDF全文 Pierrick Rouillard Julien Collot Rupert Sutherland Francois Bache Martin Patriat Samuel Etienne Pierre Maurizot 《Basin Research》2017,29(Z1):189-212
We present the first comprehensive seismic‐stratigraphic analysis of Fairway Basin, which is situated on the rifted continent of Zealandia in the Tasman Sea, southwest Pacific, between Australia and New Caledonia. The basin is 700 km long, 150 km wide, and has water depths of 500–3000 m. We describe depositional architecture and paleogeographic evolution of this basin. Basin formation was concurrent with two tectonic events: (i) Cretaceous rifting during eastern Gondwana breakup and (ii) initiation and Cenozoic evolution of Tonga–Kermadec subduction system to the east of the basin. To interpret the basin history we compiled and interpreted 2D seismic‐reflection profiles and make correlations with DSDP boreholes and the geology of New Caledonia. Five seismic‐stratigraphic units were defined. The deepest and oldest unit, FW3, folded and faulted can be correlated with volcaniclastic sediments and magmatic rocks in New Caledonia that are associated with Mesozoic Gondwana margin subduction. Alternatively, given the basin location 200–300 km west of New Caledonia and inboard of the ancient plate boundary, the unit could have formed as Gondwana intra‐continental basin with no known correlative. The overlying unit FW2b records syn‐rift deposition, probably associated with Cretaceous Gondwana breakup. Subaerial erosion supplied terrigenous sediment into the deltas in the northern part of the basin, as suggested by the truncation surfaces on the basement highs and sigmoid reflector geometries within unit FW2b respectively. Above, unit FW2a records post‐rift sedimentation and passive subsidence as the Tasman Sea opened and the Fairway Basin drifted away from Australia. Subsidence led to the flooding of the basement highs and burial of wave‐cut surfaces. Eocene compressive deformation resulted in minor folding and tilting within the Fairway Basin and was associated with the formation of many diapiric structures. The top of unit FW2 is an extensive unconformity that is associated with erosion and truncation on surrounding ridges. Above this unconformity, unit FW1b is interpreted as a turbidite system sourced from topography created during the Eocene tectonic event, which we interpret as being related to Tonga–Kermadec subduction initiation. Pelagic carbonate sedimentation is now prevalent. Unit FW1a has progressively draped the basin during Oligocene to Pleistocene subsidence. Many small volcanic cones were erupted during this final phase of subsidence, either as a delayed consequence of subduction initiation, or related to Tasmantid and Lord Howe hotspot trails. The northern Fairway Ridge remains close to sea level and its reef system continues to supply carbonate detrital sediments into the basin, most likely during sea‐level lowstands. Fairway Basin contains a nearly continuous record of tectonic and paleoclimatic events in the southwest Pacific since Cretaceous time. Its paleogeographic history is a key piece in the puzzle for understanding patterns of regional biodiversity in the southwest Pacific. 相似文献
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Martin Nauton‐Fourteu Shane Tyrrell Andrew Morton Chris Mark Gary J. O&#x;Sullivan David M. Chew 《Basin Research》2021,33(1):342-363
Quartz‐rich sandstones can be produced through multiple sedimentary processes, potentially acting in combination, such as extensive sedimentary recycling or intense chemical weathering. Determining the provenance of such sedimentary rocks can be challenging due to low amounts of accessory minerals, the fact that the primary mineralogy may have been altered during transport, storage or burial and difficulties in the recognition of polycyclic components. This study uses zircon and apatite U‐Pb geochronology, apatite trace elements, zircon‐tourmaline‐rutile indices and petrographic observations to investigate the sedimentary history of mineralogically mature mid‐Carboniferous sandstones of the Tullig Cyclothem, Clare Basin, western Ireland. The provenance data show that the sandstones have been dominantly and ultimately sourced from three basement terranes: older Laurentian‐ associated rocks (ca. 900–2500 Ma) which lay to the north of the basin, peri‐Gondwanan terranes (ca. 500–700 Ma) to the south and igneous intrusive rocks associated with the Caledonian Orogenic Cycle (ca. 380–500 Ma). However, the multi‐proxy approach also helps constrain the sedimentary history and suggests that not all grain populations were derived directly from their original source. Grains with a Laurentian or a Caledonian affinity have likely been recycled through Devonian basins to the south. Grains with a peri‐Gondwanan affinity appear to be first cycle and are potentially derived from south/southwest of the basin. Taken as a whole, these data are consistent with input into the basin from the south and southwest, with the reworking of older sedimentary rocks, rather than intensive first‐cycle chemical weathering, likely explaining the compositional maturity of the sandstones. This study highlights the need for a multi‐proxy provenance approach to constrain sedimentary recycling, particularly in compositionally mature sandstones, as the use of zircon geochronology alone would have led to erroneous provenance interpretations. Zircon, together with U‐Pb geochronology from more labile phases such as apatite, can help distinguish first‐cycle versus polycyclic detritus. 相似文献
16.
To clarify broad-scale patterns and controls of treefall directionality from Hurricane Katrina, we examined fall directions across a 4,500 km2 landscape mosaic in southern Mississippi using georeferenced, planar-rectified aerial photographs. Analyses using directional statistics, measures of local spatial autocorrelation, and general linear modeling indicated that treefall was significantly directional for nearly all of our locations and constrained primarily by mesoscale surface wind directions and landscape setting. None of our plots exhibited fall angles consistent with damage caused by wind reversals following the passage of the storm or by microbursts or tornadoes spawned by the hurricane. When coupled with results from other studies focused on hurricane-caused damage, these results suggest that it is possible to develop empirical, landscape-scale models of wind impacts or stand vulnerability using basic site information (e.g., topography, soils), biotic conditions (e.g., land cover, forest attributes), and generalized, but readily available, estimates of surface wind flow patterns. 相似文献
17.
呼伦贝尔沙质草原碟形风蚀坑表面气流及其意义 总被引：6，自引：1，他引：6
呼伦贝尔沙质草原风蚀坑发育于宽单峰中能风况环境,是该区主要风沙地貌类型。对碟形坑表面气流的观测结果显示,气流进入风蚀坑后,在入风口扩散,风速开始降低至坑底达到最低;在坑后缘出风口汇集形成急流,风速剧增至坑后积沙顶部风速达到最高值。风蚀坑后缘在风沙流的磨蚀作用下后退,风蚀坑顺风向扩展,同时此处较大的剪切风速和输沙率使大量来自坑内的沙物质沉积到坑后草地上呈扇状蔓延。呼伦贝尔沙质草原风蚀坑在来自西北、西和西南方向风的交替作用下,向东南、东和东北方向扩展而生成。随着风蚀坑深度和水平尺度的增加,最终可能沿强风能方向或合成输沙方向扩展成为大型的槽形坑。 相似文献
18.
近40a新疆输沙势的分析 总被引：8，自引：1，他引：7
利用新疆100个气象站近40a来风的观测资料分析了南北疆合成输沙势的时空分布。分析表明: ①新疆风能高能地区较多, 其东部、北疆西部、西北部合成输沙势(RDP)较高, 南疆西部、西南部RDP较低, 北疆比南疆高。②北疆四季、年RDP从20世纪70年代以来波动减小, 南疆四季、年RDP在60、70年代达到最大, 之后到1998年振荡减小, 1998年后却陡然增大。南北疆RDP在春季最大, 夏季次之, 冬季最小, 在60年代较大, 70年代最大, 90年代最小。③南疆东部、西南、西北部RDP/DP(方向变率指数)较大, 中部、西部较小; 从东部到西部方向逐渐由偏西、西南向偏东、东南转变。北疆东部RDP/DP较大, 中部、天山中部和北麓平原一带在0.6~0.7之间, 西部、西北部有两个低值中心, 东北部在0.7~0.8之间; 方向以偏东南、东居多。 相似文献
19.
《Basin Research》2018,30(2):249-278
The Turonian‐Coniacian Smoky Hollow Member of the Straight Cliffs Formation in the Kaiparowits basin of southern Utah records a stratigraphic transition from isolated fluvial channel bodies to increasingly amalgamated channel belts capped by the Calico bed, a sheet‐like sand‐gravel unit. Characteristics of the Smoky Hollow Member are consistent with a prograding distributive fluvial system including: up‐section increases in average grain size, bed thickness, channel‐body amalgamation, a fan‐shaped planform morphology and a downstream increase in channel sinuosity. The system prograded to the northeast based on thickness and facies patterns, and palaeocurrent indicators. This basin‐axial sediment‐dispersal trend, which was approximately parallel to the fold‐thrust belt at this latitude, is supported by provenance data including detrital zircons and modal sandstone compositions indicating sediment derivation mainly from the Mogollon Highlands and Cordilleran magmatic arc to the southwest, with episodic input from the more proximal Sevier fold‐thrust belt to the west. Progradation occurred during a eustatic still‐stand, relatively stable climatic conditions, and continuous tectonic subsidence, thus suggesting increased extrabasinal sediment supply as a primary control on basin‐fill. Progradation of the Smoky Hollow Member fluvial system culminated in a ~2–3 My hiatus at the top of the lower Calico bed. Correlation with the Notom delta of the Ferron Sandstone, 80 km northeast in the Henry basin, is proposed on the basis of facies relationships and geochronology. The Calico bed unconformity is linked to regional tectonically driven tilting and erosion observed in both basins. 相似文献
20.
重庆巫山张家湾遗址2000年来的环境考古 总被引：15，自引：4，他引：15
通过对重庆巫山张家湾西汉遗址T911剖面进行粒度、磁化率等环境替代指标的分析研究，结果表明，汉代该区发达的农业、手工业生产使人们盲目砍伐树木，使该区植被严重退化，使生态环境恶化，降水集中、坡地表面松散沉积丰富、缺乏植被保护等使流水侵蚀、坡地径流发育，导致水土流失严重，该区的人类文明因此而衰落，同时研究发现，该区在竺可桢划分的中国4次温暖期与4次寒期扣的第2次寒冷期（公元初至公元600年）仍为温暖期，这是由于该区西北面与东南面均有高大山脉包围，一定程度上阻挡了来自北方的冷空气进入本区的缘故，该区气候演变在一定程度上可能受西南季风的影响，至少该区在汉代气候演变与西南季风的强弱有某种相关关系，作为一个相对独立的气候变化单元，研究其气候变化与人类文明发展的耦合关系具有重要的理论意义与实际意义。 相似文献