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1.
Facies analysis focussing on coarse-grained sediments has been carried out on more than 2500 m of drill cores from seven wells from southern margins of the North German Basin (NGB). The NGB forms a central element of the Southern Permian Basin (SPB). The wells exposed conglomerates and sandstones of the Rotliegend Grüneberg and Parchim Formations deposited in the Kotzen Basin and the Barnim Basin.17 lithofacies types have been grouped into six lithofacies associations. The studied successions are dominated by fluid gravity flow deposits (hyperconcentrated flows and stream flows) of alluvial fan and alluvial plain systems. Maximum particle size/bed thickness plots (MPS/BTh) support the interpretation as fluid gravity flow deposits. The MPS and BTh data have also been used to differentiate coarsening–thickening and fining–thinning trends of the fan systems.The dominance of water-rich mass flow processes together with sedimentary structures such as dewatering structures and outwashed tops suggests the presence of wet-type fans and plains under semi-humid to semi-arid seasonal climates in the central SPB. The investigated sediments show variation in clast composition subsequent to deep erosion processes on basin margins and changes of source areas. Synsedimentary normal faults and clastic dykes have been interpreted as indicators of tectonic activity of grabens itself and its frames. On a larger scale, then evolution from a half-graben to a graben is apparent for the Tuchen Sub-basin at least. The progradational/retrogradational cycles of the studied alluvial fan systems document combined local tectonic movements and influences of climatic changes. However, our data did not allow for a clear distinction between climatic and tectonic signals. Furthermore, a one-to-one correlation of fan cycles with depositional trends in the NGB basin centre would appear to be oversimplistic.  相似文献   

2.
The Andean piedmont of Mendoza is a semiarid region covered by extensive and partially vegetated dune fields consisting of mostly inactive aeolian landforms of diverse size and morphology. This paper is focused on the San Rafael plain (SRP) environment, situated in the distal Andean piedmont of Mendoza (34° 30′S), and reports the sedimentology and OSL chronology of two representative exposures of late Quaternary deposits, including their paleoenvironmental and paleoclimatic significance. Eleven facies, including channel, floodplain, fluvio–aeolian interaction, and reworked pyroclastic and aeolian deposits, were described and grouped into two facies associations (FA1 and FA2). FA1 was formed by unconfined sheet flows, minor channelized streams and fluvial–aeolian interaction processes. FA2 was interpreted as aeolian dune and sand-sheet deposits. OSL chronology from the SRP sedimentary record indicates that between ca. 58–39 ka and ca. 36–24 ka (MIS 3), aggradation was governed by ephemeral fluvial processes (FA1) under generally semiarid conditions. During MIS 2, the last glacial maximum (ca. 24–12 ka), a major climatic shift to more arid conditions is documented by significant aeolian activity (FA2) that became the dominant sedimentation process north of the Diamante–Atuel fluvial system. The inferred paleoenvironmental conditions from the SRP sections are in broad agreement with regional evidence.  相似文献   

3.
A marginal marine carbonate environment, giving away to an alluvial one, was established during Messinian time on Alonnisos Island, the footwall upland of the Southern Marginal Fault of the Sporades Basin (SMFS). Analysis of the evolving depositional systems, with emphasis on their sedimentation processes, faulting patterns and palaeopedological factors, has permitted an interpretation of the simultaneous controls of tectonism and climate. The carbonate sediments were deposited in a shallow marine environment formed along a faulted continental margin under warm and semi-arid climatic conditions. Faulting consisted of NE-trending dextral reverse faults and NW-trending strike slip faults, produced by WNW-directed compression. The basement structural elements affected the spatial distribution of the offshore and shoreface facies, whereas fifth-order cycles of sea-level change were responsible for the development of metre-scale, shallowing-up cycles. The compressional structures were subsequently reactivated by NNE extension. This tectonic inversion, together with a global sea-level fall, triggered alluvial fan sedimentation. Fan sedimentation was disrupted by long periods of non-deposition and soil formation under warm climatic conditions. Three distinct units are recognized in the fan: a lower unit consisting of clast-poor debris flows, attributed to semi-arid–humid periods; an intermediate unit of clast-rich sheetfloods and channel flows, deposited during arid periods; and an upper unit consisting of matrix-rich sheetfloods related to a return to semi-arid–humid conditions. We interpret that the water-flow processes responsible for deposition were most prevalent on fans of arid and semi-arid climates, whereas debris-flow processes were more typical of climates with higher rainfall. As the extension proceeded during the Plio-Quaternary time, the main tectonic activity of the Sporades Basin was taken up by the SMFS causing significant footwall uplift. Due to this process, Alonnisos Island was elevated above the Pliocene highstand and became an area starved of Quaternary sedimentation. Copyright © 1998 John Wiley & Sons, Ltd.  相似文献   

4.
The Piedmont Zone of the Indo-Gangetic Plain contains numerous, laterally coalescing small alluvial fans. The Latest Pleistocene–Holocene 30 km long Gaula Fan can be divided into gravelly proximal fan (0–14 km down-stream), gravel-sand rich mid fan (14–22 km) and sand–mud dominated distal fan (22–30 km). The fan succession is composed of two fan expansion cycles A and B. Separated by an undulatory erosional contact of regional extent, cycle A is characterized by river borne clast-supported gravelly deposits, and the overlying fan expansion cycle B by matrix-supported gravely debris flows. The main process behind fan development has been lateral migration of channels over the fan surface probably due to rapid sedimentation caused by increased sediment supply, and the fluctuating water budget in response to changing climate. The water laid expansion cycle A represents a humid phase. The debris flow deposits of expansion cycle B suggest a dry phase. Approximately between 8 and 3 Ka, cycle B also indicates a phase of tectonic instability in the Siwalik Hills forming the mountain front. The tectonic activity caused incision of rivers into the fan surface, and in turn resulted in reduced fan-building activity. At present the fan surface is accreting by sheet flow processes.  相似文献   

5.
Alluvial fans are usually constructed through episodic flood events. Despite the significance of these ephemeral floods on the morphodynamics of alluvial fans, depositional responses to the variations in flood conditions are still poorly documented. This greatly limits the ability to interpret ancient sedimentary successions of fans and the associated flood hydrodynamics. The Quaternary Poplar Fan from endorheic Heshituoluogai Basin provides an optimal case for addressing this issue. Based on the variations in facies associations and flood conditions, three depositional stages – namely; lobe building stage, channel building stage and the abandonment stage – are identified. During the lobe building stage the Poplar Fan is predominately constructed through incised channel flood, sheetflood and unconfined streamflood, with coeval development of distal surficial ephemeral ponds. The channel building stage is characterized by the development of gravelly braided rivers. However, only scour pool fill deposits are preferentially preserved in the Poplar Fan. During the abandonment stage, erosional lags and aeolian sands randomly occur throughout the fan, while gully deposits can only be found in the distal fan. The distinctive facies architecture of the Poplar Fan is likely to be the result of periodicity of climate fluctuations between wetter and drier conditions during the Late Pleistocene to Holocene. The ephemeral floods formed under wetter conditions usually show high discharge and sediment concentrations which facilitate the lobe building processes. During the drier periods, only gravelly braided rivers can be developed through ephemeral floods as the intensity and frequency in precipitation, discharge and sediment concentrations of the flood flows significantly decrease. The abandonment stage of the fan may occur between recurring flood episodes or during the driest periods. Furthermore, the long-term (105 to 106 year) geomorphic evolution of the Poplar Fan shows the influence of tectonic activities. The ongoing thrust uplift tectonic activities have caused destruction of the fan but can also facilitate the fan-head trench/incision of the fan, which in turn facilitate the progradation of the fan. This study proposes a new depositional model for alluvial fans constructed through episodic flood events, which shows the character of both sheet-flood dominated and stream-flow dominated end members of alluvial fans. These findings supplement the understanding of the variability of the alluvial fans and provide means to characterize rock record of alluvial fans and their associated flood and climate conditions.  相似文献   

6.
Irene Zembo 《Sedimentary Geology》2010,223(3-4):206-234
The sedimentary record of the Val d'Agri basin is of great importance for understanding the Quaternary tectonic activity and climatic variability in the Southern Apennines. Changes in tectonic controls, sediment supply and climatic input have been identified. The interval from ～ 56 to ～ 43 ka was associated with asymmetric subsidence restricted to the north-eastern actively faulted margin of the basin and development of axial braided river and transverse alluvial fan systems. Short-lasting Mediterranean-type pedogenesis between ～ 43 and ～ 32 ka (MIS Stage 3) coexisted with progradation–aggradation of the southern alluvial fan deposits and southwards tilting of the basin floor. Aggradation ended with consumption of accommodation space after 32 ka. During a subsequent stage of decline of vegetation cover, possibly as a consequence of climatic cooling (probably MIS Stage 2), active progradation of alluvial fans occurred. Breakthrough of the basin threshold and entrenchment of the drainage network must therefore be attributed to a latest Pleistocene to Holocene age. The first stages of basin opening and fill, predating ～ 56 ka have only been inferred by stratigraphic considerations: the earliest lacustrine sedimentation should be middle Pleistocene or older in age. The following south-eastward basin widening allowed progradation of alluvial fan systems, which completely filled the lacustrine area (tentatively late middle Pleistocene). Pedogenesis in “Mediterranean-like” climate conditions caused the final development of a highly mature fersiallitic paleosol at the top of the fan surfaces, in areas of morpho-tectonic stability, plausibly during MIS Stage 5. The study results demonstrate the potential of applying a multidisciplinary approach in an intermontane continental settings marked by a relative rapid and constant tectonic subsidence and a high rate of sediment supply during the Pleistocene glacial–interglacial cycles.  相似文献   

7.
The Piedmont Zone is the least studied part of the Ganga Plain. The northern limit of the Piedmont Zone is defined by the Himalayan Frontal Thrust (HFT) along which the Himalaya is being thrust over the alluvium of the Ganga Plain. Interpretation of satellite imagery, Digital Terrain Models (DTMs) and field data has helped in the identification and mapping of various morphotectonic features in the densely forested and cultivated Piedmont Zone in the Kumaun region of the Uttarakhand state of India. The Piedmont Zone has formed as a result of coalescing alluvial fans, alluvial aprons and talus deposits. The fans have differential morphologies and aggradation processes within a common climatic zone and similar litho-tectonic setting of the catchment area. Morphotectonic analysis reveals that the fan morphologies and aggradation processes in the area are mainly controlled by the ongoing tectonic activities. Such activities along the HFT and transverse faults have controlled the accommodation space by causing differential subsidence of the basin, and aggradation processes by causing channel migration, channel incision and shifting of depocentres. The active tectonic movements have further modified the landscape of the area in the form of tilted alluvial fan, gravel ridges, terraces and uplifted gravels.  相似文献   

8.
In the Eastern Cordillera of Colombia, a new structural model constrained by field data, paleontologic determinations, and interpretations of seismic reflection profiles is proposed. The model implies 70 km of shortening, including reactivation of basement structures as inverse faults in both flanks of the chain. These faults propagated within the lower Cretaceous strata, inducing passively rooted and transported thrust sheets as the successive basement faults were reactivated. Two structural styles are identified in the western flank: (1) positive flower structures in a transpressive regime, which affected rocks older than upper Paleocene and were unconformably covered by post–late Paleocene sediments, and (2) compressive structures during the late Miocene–Recent Andean phase. Presently, WNW-ESE compression reactivates Late Paleocene structures, which locally affect Andean trends. In the western margin of the Eastern Cordillera, the Cambao thrust takes up most displacement, whereas the Bituima fault takes only a minor part. To the south, this relationship reverses, suggesting complementary behavior by the Bituima and Cambao faults, as well as a transfer zone. This suggestion explains the southward termination of the Guaduas syncline as a structure related to the Cambao fault, whereas the Bituima fault increases its displacement southward, generating the Girardot foldbelt that takes over the structural position of the Guaduas syncline.  相似文献   

9.
The Devonian Old Red Sandstone Ridgeway Conglomerate Formation crops out in Pembrokeshire, SW Wales. It was deposited as part of a dryland alluvial fan, axial fluvial valley deposystem. It conformably overlies the mid Lochkovian Freshwater West Formation and probably predates deposition of the Lower Cosheston Group Mill Bay Formation indicating an Early Devonian (latest Lochkovian to earliest Pragian) age, rather than a Middle Devonian age as suggested by previous workers. It therefore represents the youngest preserved formation of the Milford Haven Group south of the Ritec Fault. The Formation thickens drastically into the Ritec Fault, indicating its control on sedimentation. The half‐graben topography initiated deposition of a hangingwall alluvial fan that was sourced from a southerly Lower Palaeozoic/Precambrian provenance within the present‐day Bristol Channel. The Formation is heterolithic in nature, with deposits on the fan reflecting a mixture of processes. Conglomerates were deposited primarily by laterally extensive sheetfloods, and as bars in low‐relief, laterally accreted channels. Sandstones were also predominantly deposited by sheetfloods. Gritty mudrocks in comparison demonstrate deposition by cohesive debris flows. The fan prograded northward and interfingered with a low‐gradient, high‐sinuosity fluvial channel system dominated by inclined and non‐inclined heterolithic stratification. Thinly laminated mudstone and sandstone interbeds were deposited in ephemeral fan‐toe and axial valley lakes that may have developed during sub‐humid climatic episodes. The lacustrine heterolithic association has associated matgrounds and possible ‘algal roll‐up’ structures. Calcretized peetee structures and root traces comprise a lake margin calcrete association. Fan gravels prograded into the axial fluvial valley during periods of increased sediment flux that may represent semi‐arid conditions and/or episodes of tectonic activity. Calcretes of varying development were established in both the fan and axial valley zones. Calcretes with lower stages of development are more proximal to the Ritec Fault reflecting decreased soil residence times and high deposition rates within the axial valley. More strongly developed soil profiles on the fan may indicate sequence boundaries associated with low sediment flux, or increased soil residence time due to active fan‐channel migration (the pedofacies concept). Groundwater calcretes have sharp‐based and layer‐bound calcrete profiles. Gully‐bed cements are locally developed within the fan gravels. Copyright © 2007 John Wiley & Sons, Ltd.  相似文献   

10.
Morphological and sedimentary records at the exit of Brahmaputra River at Pasighat in the NE Himalaya inform about the climate–tectonic interplay during the past ca. 15 ka. The geomorphology of the area comprises (1) fan terrace T3, (2) a high‐angle fan (3) terrace T2, (4) terrace T1 and (5) a low‐angle fan. Geomorphic consideration suggests that the fan terrace T3 and high‐angle fans are the oldest units and were coeval. The low‐angle fan is the youngest geomorphic unit. Sedimentological studies and optically stimulated luminescence chronology suggest that (i) fan terrace T3 formed between 13 and 10.5 ka and comprised multiple events of debris flows separated by the aggradation as channel bars in a braided river environment; (ii) the high‐angle fan formed during 15–10 ka and comprises channel bar aggradation in braided river conditions; (iii) terrace T2 formed during 10–8 ka due to aggradation in a braided channel environment with lesser events of debris flows; (iv) terrace T1 formed during <7 and 3 ka took place as bars of the braided river. Sudden coarsening of the sediment indicated a tectonic rejuvenation in the provenance region between 7 and 3 ka; and (v) the low‐angle fans dated to <3 ka formed due to aggradation in a small tributary joining the Brahmaputra River. This implies a phase when the main channel of the Brahmaputra did not flood regularly and the tributaries were actively aggrading. The sedimentation style and incision of these geomorphic units responded to contemporary climatic changes and uplift in the Siwalik range along the Himalayan Frontal Fault. Copyright © 2008 John Wiley & Sons, Ltd.  相似文献   

11.
广西横县六景火车站南侧,角度不整合覆盖下泥盆统莫丁组和那叫组下段,被新近系大面积覆盖而呈两部分远距离割裂状态的一套“二元结构”特征显著的红色泥砂基底碳酸盐岩粗巨砾岩及其横向相变部分,长期以来未被将其作为一个冲积扇的有机统一体进行研究。经笔者两次实地踏勘,系统采样和分析,以现代山麓冲积扇鉴别特征为主,辅以古冲积扇特征,综合分析认为该套沉积系列相关分割露头分别为一古近系冲积扇所残存扇根、扇中和扇缘。该冲积扇为一典型的古近纪早期干旱气候条件下,母岩区为陡峻碳酸盐岩山体的古山麓冲积扇。  相似文献   

12.
In the Atacama Desert, northern Chile, some ephemeral channels are developed in the Plio‐Quaternary alluvial sequence that caps the Neogene Atacama Gravels Formation. Geomorphological studies and high‐resolution digital elevation data (GPS) along a structural transect in the Central Depression are used to document modern growth history of subtle folding and faulting in the fore‐arc region. Outcrop data of the most recent deposits are combined with observations of warped and faulted late Quaternary pediments, alluvial fans and terrace surfaces to propose unsuspected neotectonic processes on the western flank of the Domeyko Cordillera. Neotectonic process recognition is here based largely upon the interpretation of alluvial landforms, drainage organisation and evolution as the intermittent river network shows systematic patterns of course deflections, successive incisions or deposition processes as it encounters the fault scarps or folds in the superficial deposits. This area presents both N–S‐trending active vertical faults in the topographically higher pampas, and N–S‐trending active folding in the lower pampas. These faults seem to accommodate E–W extension and compression that could be related to uplift of the western Andean margin within a compressive context. Uplift may have taken place unevenly over the past few million years after the deposition of the superficial alluvial surfaces that cap the Neogene Atacama Gravels. Copyright © 2003 John Wiley & Sons, Ltd.  相似文献   

13.
Source rock gold deposits in Colombia include Proterozoic unconformity-related paleoplacers, Palaeozoic stratabound Au/Ag-Cu-Pb-V-U red bed-type ores, and Mesozoic/Tertiary hydrothermal vein systems of Cordilleran magmatic arcs. The younger metallogenic episode occurred some 8 Ma ago in the Western Cordillera. Epithermal deposits are generally small (> 10,000–1 Mt grading 5–10 g/t Au) and the precious-base metal (Cu, Zn, Fe) association was probably deposited within the temperature range 180–300 °C. Widespread palaeoplacers in Quaternary terraces with enrichment in old channels, as well as extensively dredged alluvial deposits in presently active sediments, are connected with ongoing Andean uplift and erosion. Palaeochannel and alluvial gold concentrations represent the most promising targets for future exploration. An additional output could also derive from the clastic-hosted, stratiform deposits of the shield and E Cordillera.  相似文献   

14.
The integration of geomorphic mapping, soil stratigraphy, and radiocarbon dating of alluvial deposits offers insight to the timing, magnitude, and paleoclimatic context of Holocene fan sedimentation near Yuma, Arizona. Mapping of 3400 km2 indicates about 10% of the area aggraded in the late Holocene and formed regionally extensive alluvial fan and alluvial plain cut-and-fill terraces. Fan deposits have weakly developed gravelly soils and yielded a date of 3200–2950 cal yr BP from carbonized wood. Alluvial plain deposits have weakly developed buried sandy soils and provided a date of 2460–2300 cal yr BP from a terrestrial snail shell. Precipitation records were analyzed to form historical analogues to the late Holocene aggradation and to consider the role of climatic variability and extreme hydrologic events as drivers of the sedimentation. The historical precipitation record indicates numerous above-average events correlated to the Southern Oscillation Index (SOI) in the region, but lacks any significant reactivation of alluvial fan surfaces. The timing of aggradation from 3200 to 2300 cal yr BP correlates well with other paleoclimatic proxy records in the southwestern U.S. and eastern Pacific region, which indicate an intensification of the El Niño-Southern Oscillation (ENSO) climatic pattern and rapid climate change during this period.  相似文献   

15.
Optically Stimulated Luminescence (OSL) enables the chronology of the late Pleistocene evolution for the Val d'Agri intermontane basin of Southern Apennines to be defined in the frame of Mediterranean geodynamic and climate changes. Quartz sand from braided floodplain and alluvial fan depositional systems was analyzed using the coarse-grained, single-aliquot regenerative-dose (SAR) technique. The obtained optical ages are mostly consistent with other assessments (radiocarbon, tephrochronology) and stratigraphic constraints. OSL allows for the dating to 56–43 ka of an asymmetric subsidence stage that forced alluvial fan progradation, filling of a former lacustrine area, and development of an axial alluvial plain. A short period of Mediterranean-type pedogenesis, recorded at the top of the prograding-aggrading fans (OSL age bracket 43–32 ka), corresponds with MIS 3. During the subsequent stage of decline of vegetation cover, possibly corresponding to MIS 2, the latest progradation of alluvial fans occurred. The subsequent uplift and breakthrough of the basin threshold during the latest Pleistocene and Holocene induced entrenchment of the drainage network. The results presented here provide an example of the usefulness of OSL dating in intermontane continental settings where other geochronological constraints are scarce.  相似文献   

16.
The Rı́mac and Chillón Rivers eroded deep valleys on the Lima coastal plain during the Late Miocene (before ca. 5.3 Ma), due to at least 485 m of uplift produced by the Nazca Ridge, combined with a sea level lowstand of around −50 m. The main paleo-Rı́mac channel along the southeastern boundary of the alluvial cone was apparently deflected by the Lima Anticline and reached the sea in the vicinity of Morro Solar, whereas the paleo-Chillón ran largely parallel to the anticline, breaching it to enter the Pacific at present-day Magdalena. These valleys were filled by fine-grained sediments, possibly during marine transgression at 1.7 Ma, which was followed by uplift and regression to below present sea level. Meltwater surges from the Andean Cordillera during subsequent interglacial stades caused an accumulation of coarse, reworked glacial moraine in the Rı́mac and Chillón fans, forming the Lima Conglomerate and drowning the Lima Anticline. The Rı́mac and Chillón Rivers subsequently migrated north and westward, possibly in response to tectonic tilting of the landscape, causing silt and mud to accumulate in abandoned channels along the southeastern boundary of the fan.  相似文献   

17.
柴达木盆地处于古亚洲构造域和特提斯－喜马拉雅构造域的结合部，构造应力大而复杂，导致盆内地势起伏大，加上西南暖湿气流受喜马拉雅山系阻隔难以进入境内，盆内气候干旱，最终导致盆地内冲积扇极为发育。通过对大柴旦地区大头羊煤矿、鱼卡河、波门河和八里沟四个冲积扇的实地考察，共观测到3个亚相8个微相：扇根亚相沉积物最粗，分为古沟道、主水道和主水道间微相；扇中亚相沉积物偏细，成熟度增高，分为辫状水道、辫状水道间和纵坝微相；扇缘亚相沉积物最细，流体能量最低，分为水道径流和片流微相。不同沉积微相其沉积特征差异较大，认为古沟道、主水道和辫状水道微相具有较好的储集性能。勘探表明，冲积扇沉积与储层有着密切的关系，其内形成的油藏具有“自我保护”的能力；另外，冲积扇的形成很可能导致上覆地层形成扇背斜油藏，也可能导致下伏基岩形成基岩风化壳油藏。  相似文献   

18.
Ancient stream-dominated (‘wet’) alluvial fan deposits have received far less attention in the literature than their arid/semi-arid counterparts. The Cenozoic basin fills along the Denali fault system of the northwestern Canadian Cordillera provide excellent examples of stream-dominated alluvial fan deposits because they developed during the Eocene-Oligocene temperate climatic regime in an active strike-slip orogen. The Amphitheatre Formation filled several strike-slip basins in Yukon Territory and consists of up to 1200 m of coarse siliciclastic rocks and coal. Detailed facies analysis, conglomerate: sandstone percentages (C:S), maximum particle size (MPS) distribution, and palaeocurrent analysis of the Amphitheatre Formation in two of these strike-slip basins document the transition from proximal, to middle, to distal and fringing environments within ancient stream-dominated alluvial-fan systems. Proximal fan deposits in the Bates Lake Basin are characterized by disorganized, clast-supported, boulder conglomerate and minor matrix(mud)-supported conglomerate. Proximal facies are located along the faulted basin margins in areas where C:S = 80 to 100 and where the average MPS ranges from 30 to 60 cm. Proximal fan deposits grade into middle fan, channelized, well organized cobble conglomerates that form upward fining sequences, with an average thickness of 7 m. Middle fan deposits grade basinward into well-sorted, laterally continuous beds of normally graded sandstone interbedded with trough cross-stratified sandstone. These distal fan deposits are characteristic of areas where C:S = 20 to 40 and where the average MPS ranges from 5 to 15 cm. Fan fringe deposits consist of lacustrine and axial fluvial facies. Palaeogeographic reconstruction of the Bates Lake Basin indicates that alluvial-fan sedimentation was concentrated in three parts of the basin. The largest alluvial-fan system abutted the strike-slip Duke River fault, and prograded westward across the axis of the basin. Two smaller, coarser grained fans prograded syntaxially northward from the normal-faulted southern basin margin. Facies analysis of the Burwash Basin indicates a similar transition from proximal to distal, stream-dominated alluvial fan environments, but with several key differences. Middle-fan deposits in the Burwash Basin define upward coarsening sequences 50 to 60 m thick composed of fine-grained lithofacies and coal in the lower part, trough cross-stratified sandstone in the middle, and conglomerate in the upper part of the sequence. Upward-coarsening sequences, 90–140 m thick, also are common in the fan fringe lacustrine deposits. These sequences coarsen upward from mudstone, through fine grained, ripple-laminated sandstone, to coarse grained trough cross-stratified sandstone. The upward-coarsening sequences are basinwide, facies independent, and probably represent progradation of stream-dominated alluvial-fan depositional systems. Coal distribution in the Amphitheatre Formation is closely coupled with predominant depositional processes on stream-dominated alluvial fans. The thickest coal seams occur in the most proximal part of the basin fill and in marginal lacustrine deposits. Coal development in the intervening middle and distal fan areas was suppressed by the high frequency of unconfined flow events and lateral channel mobility.  相似文献   

19.
通过天山北缘白杨河及玛纳斯河现代辫状河沉积作用为主的洪积扇的研究,认为此类洪积扇以活动的辫流带和洪水 期才发生沉积的漫洪带为特征。扇面上不能全区同时发生沉积,而是选择扇面低势能区发生沉积。辫状河道呈条带状分 布,与常规砾质辫状河沉积特征类似,并向扇缘散开,沉积的砾石条带物性好。洪积扇的发育受构造、气候、水系的共同 控制,这些条件决定了洪积扇的沉积特征及发育规模。准噶尔盆地西北缘三叠系古代洪积扇以辫状河道沉积作用为主,是 发育在盆地边缘低坡度洪积扇,更倾向于辫状河沉积特征。  相似文献   

20.
The ∼20 m thick coarse-grained clastic succession in the basal part of Palaeoproterozoic Par Formation, Gwalior Group has been investigated using process-based sedimentology and deductive palaeohydraulics. Bounded between granitic basement at its base and shallow marine succession at the top, the studied stratigraphic interval represents products of an alluvial fan and its strike-wise co-existent braided river system that possibly acted as a tributary for the fan. Detailed facies, facies association analysis allowed identification of two anatomical parts for the fan system viz. proximal and mid fan. While thin proximal fan is represented by products of rock avalanche and hyperconcentrated flows with widely varying rheology, the mid fan is represented by products of sheet floods and flows within streamlets. The interpretation found support from palaeoslope estimation carried out on the fluvial part of the mid fan that plot dominantly within the alluvial fan field demarcated by Blair and McPherson (1994). Dry climatic condition suggested from dominance of stream flow over mass flow deposition within the Par alluvial fan. Strike-wise, the fan is discontinuous and juxtaposed with a braid plain system. In contrast to the fluvial part of fan system, the palaeoslope data from the braid plain system dominantly plot within the ‘natural depositional gap’ defined by Blair and McPherson. A raised palaeoslope for the river systems, as suggested from Proterozoic braid plain deposits around the Globe, is found valid for the Par braid plain system as well. From preponderance of granular and sandy sediments within the alluvial fan and braid plain systems and a pervasive north-westward palaeocurrent pattern within the fluvial systems the present study infers a gently sloping bevelled source area in the south-southeast of the basin with occurrence of steep cliffs only locally.  相似文献   

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