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1.
浙江桐庐地区上奥陶统堰口组主要为一套灰色细砂岩、粉砂岩与深灰色-灰黑色泥岩、粉砂质泥岩的韵律性薄互层, 底部发育2～3层灰色中-厚层的砾岩, 其砂泥岩薄互层中发育有典型的脉状、波状和透镜状层理及韵律层理。这种砾岩过去一直被认为是底砾岩, 其上的砂泥岩韵律性薄互层被认为是典型的古代潮坪沉积。野外调查并结合区域地质资料分析结果表明, 堰口组沉积环境应为较深水斜坡环境,而非浅水海岸环境;其底部的砾岩应为富基质的碎屑流沉积,而不应是底砾岩;中上部的砂泥岩韵律性薄互层应为较深水斜坡上的内潮汐沉积,而不是浅水潮坪沉积。这一内潮汐沉积具有典型的对偶层双向递变层序, 可能为内潮汐的大潮和小潮周期性变化的结果,这是在中国首次识别出的古代内潮汐沉积。 相似文献
2.
The Late Cretaceous Gürsökü Formation represents the proximal fill of the Sinop–Samsun Forearc Basin that was probably initiated by extension during the Early Cretaceous. The succession records sedimentation in two contrasting depositional systems: a slope-apron flanking a faulted basin margin and coarse-grained submarine fans. The slope-apron deposits consist of thinly bedded turbiditic sandstones and mudstones, interbedded with non-channelized chaotic boulder beds and intraformational slump sheets representing a spectrum of processes ranging from debris flow to submarine slides. The submarine fan sediments are represented by conglomerates and sandstones interpreted as deposited from high density turbidity currents and non-cohesive debris flows. The occurrence of both slope apron and submarine fan depositional systems in the Gürsökü Formation may indicates that the region was a tectonically active basin margin during the Late Cretaceous. 相似文献
3.
近二十年来,对沉积物重力流及其沉积物的研究已引起国内外沉积学界的广泛重视,无论在理论研究还是在实际应用方面,均取得了不少成功的经验,著述甚多。1987年以来,笔者对秦岭造山带中的刘岭群进行了详细的观察研究,首次在上泥盆统桐峪寺组发现一套以颗流、浊流为特征的重力流沉积组合。这一发现对整个刘岭群的沉积环境与沉积相研究具有重要意义,为秦岭造山带的形成与演化及大地构造格局研究提供了有力的沉积学依据。 相似文献
4.
Coarse-grained submarine fan and slope apron deposits in a Cretaceous back-arc basin, Antarctica 总被引：1，自引：0，他引：1
J. R. INESON 《Sedimentology》1989,36(5):793-819
The Cretaceous of west James Ross Island, Antarctica represents the proximal fill of a late Mesozoic back-arc basin that was probably initiated by oblique extension during the early development of the Weddell Sea. The succession records sedimentation in two contrasting depositional systems: a laterally persistent slope apron flanking the faulted basin margin interrupted both spatially and temporally by coarse-grained submarine fans. Slope apron deposits are dominated by thinly interbedded turbiditic sandstones and mudstones (mudstone association), interspersed with non-channelized chaotic boulder beds, intraformational slump sheets and isolated exotic blocks representing a spectrum of mass-flow processes from debris flow to submarine gliding. Localized sand-rich sequences (sandstone-breccia association) represent sandy debris lobes at the mouths of active slope chutes. The submarine fan sediments (conglomerate association) are typified by coarse conglomerates and pebbly sandstones, interpreted as the deposits of high-density turbidity currents and non-cohesive debris flows. Three assemblages are recognized and are suggested to represent components of the inner channelled zone of coarse-grained submarine fans, from major fan channels through ephemeral, marginal channels or terraces to levee or interchannel environments. The occurrence of both slope apron and submarine fan depositional systems during the Early and Mid-Cretaceous is attributed to localized input of coarse arc-derived sediment along a tectonically active basin margin. Periods of extensive fan development were probably linked to regional tectonic uplift and rejuvenation of the arc source region; cyclicity within individual fan sequences is attributed to migration or switching of fan channels or canyons. Slope apron sedimentation was controlled largely by intrabasinal tectonics. Local unconformities and packets of amalgamated slide sheets and debris flow deposits probably reflect episodic movement on basin margin faults. Differential subsidence across the basin margin anchored the basin slope for at least 20 Myr and precluded basinward progradation of shallow marine environments. 相似文献
5.
贺兰奥拉槽早古生代深水重力流体系的沉积特征和充填样式 总被引：6，自引：1，他引：6
位于鄂尔多斯西缘的贺兰构造带为一中元古代一古生代的奥拉槽。在区内的中寒武和中奥陶统中识别了一套巨厚的深水重力流沉积,其中包括下斜坡滑塌泥石流复合体、浊积扇以及碳酸盐岩斜坡扇裙等沉积类型。主要的相单元包括充填沟道或进入扇面形成的泥石流钙质角砾岩和砾岩、充填辫状水道的多层叠置的砂岩和砂砾岩、上叠扇的砂、泥岩互层以及浊积砂屑或含砾砂屑灰岩等。在中奥陶世该奥拉槽发展成一深水一半深水海槽,沿盆地西侧发育有浊积扇,而东侧仅有碳酸盐岩滑塌扇裙。它们可能是沿深水盆地两侧深大断裂产生的陡坡或水下断崖分布的,代表了早古生代贺兰奥拉槽在强烈沉陷期特定的深水盆地充填。 相似文献
6.
ALAN P. HEWARD 《Sedimentology》1978,25(4):451-488
Vertical sequence analysis within 1500-2500 m thick coarse-grained coalfield successions allows six sedimentary associations to be distinguished. These are interpreted in terms of depositional environments on, or related to alluvial fans which fringed a fault bounded source region. (i) Topographic valley and fanhead canyon fills: occurring at the bases of the coalfield successions and comprising sporadically reddened, scree, conglomeratic thinning and fining upward sequences, and fine-grained coal-bearing sediments. (ii) Alluvial fan channels: conglomerate and sandstone filled. (iii) Mid-fan conglomeratic and sandstone lobes: laterally extensive, thickly bedded (1-25 m) and varying from structureless coarse conglomerates and pebbly sandstones, to stratified fine conglomerates and cross-bedded sandstones. (iv) Interlobe and interchannel: siltstones, fine-grained sheet sandstones, abundant floras, thin coals and upright trees. (v) Distal fan: 10 cm-1.5 m thick sheet sandstones which preserve numerous upright trees, separated by silt-stones and mudstones with abundant floras, and coal seams. The sheet sandstones and normally arranged in sequences of beds which become thicker and coarser or thinner and finer upwards. These trends also occur in combination. (vi) Lacustrine: coals, limestones, and fine-grained, low-energy, regressive, coarsening upward sequences. Proximal fan sediments are only preserved in certain basal deposits of these coalfields. The majority of the successions comprise mid and distal alluvial fan and lacustrine sediments. Mid-fan depositional processes consisted of debris flows and turbulent streamflows, whilst sheetfloods dominated active distal areas. A tropical and seasonal climate allowed vegetation to colonize abandoned fan surfaces and perhaps resulted in localized diagenetic reddening. Worked coals, from 10s cm-20 m thick, occur in the distal fan and lacustrine environments. These alluvial fan deposits infill‘California-like’basins developed and preserved along major structural zones. In many of their characteristics, in particular the occurrence of thinning and fining, and thickening and coarsening upward sequences and megasequences, these sediments have similarities to documented ancient submarine fan deposits. 相似文献
7.
The Middle Albian Ondarroa turbidite system is a coarse grained, deep water unit which outcrops in the north-eastern part of the Basque-Cantabrian region, west of the Pyrenees. It is about 18 km long and 7 km wide, and shows an unusual ‘L’shape resulting from both a direct morphotectonic confinement and the presence of nearby shallow water carbonate buildups. Eight main facies have been distinguished within this turbidite system: (1) clast-supported conglomerates; (2) mud-supported conglomerates; (3) slump deposits; (4) normally graded pebbly sandstones; (5) cross stratified sandstones; (6) interbedded graded sandstones and mudstones; (7) interbedded non-graded sandstones and mudstones: and (8) mudstones. Inner system, middle system, outer system and basin plain divisions have been distinguished. The inner turbidite system is characterized by stacked channel fill conglomerates and lesser sandy turbidites and mudstones. The middle system consists of sandy and conglomeratic fining upwards sequences, normally several metres thick. The outer system has alternating non-channellized sandstones and mudstones, without any predictable vertical arrangement. The basin plain is characterized by mudstone-siltstone laminations and lesser, randomly occurring thin bedded sandy turbidites. Three main channel fills make up the inner turbidite system. Although all of them can be compared with the valley channel fills of the modern Mississippi Fan, and thus their bases can be interpreted as sequence boundaries, only the lowermost and the uppermost channel bases are documented as allocyclic boundaries. The Ondarroa turbidite system was deposited in an immature passive margin subjected to transtensional movements. It filled a composite pull apart depression with coarse clastics derived from a narrow platform to the north of the present outcrops which was invaded by fan deltas. A major pattern of sinistral strike-slip faulting linked to the opening of the Bay of Biscay is invoked to explain the Ondarroa turbidite system appearance and its tectonic confinement. 相似文献
8.
The Sakoa Group is the lowermost stratigraphical succession of the Karoo Supergroup and the oldest sedimentary unit in Madagascar, spanning the Late Carboniferous through Early Permian epochs. The Sakoa Group is exposed in the southern Morondava Basin. It is predominantly a siliciclastic sequence comprising seven lithofacies associations: (1) diamictites; (2) conglomeratic sandstones; (3) sandstones; (4) interbedded thin sandstones and mudstones; (5) mudstones; (6) coals; and (7) limestones. These facies represent deposition in the early extensional stages of continental rift development. The sediments were deposited predominantly on alluvial fans, and in braided to meandering stream and overbank environments. Locally lacustrine and coal swamp environments formed in low areas of the basin floor during rift initiation. Subsidence rates remained fairly constant throughout the Early Permian and were accompanied by a gradual reduction in relief of the basin margins and an increased geomorphic maturity of the fluvial systems flowing across the basin floor. Near the end of the Early Permian the southern Morondava Basin was inundated by a marine transgression , which resulted in deposition of the Vohitolia Limestone. Subsequent tectonic uplift and erosion resulted in a regional unconformity between the Sakoa Group and the overlying Sakamena Group. 相似文献
9.
鄂尔多斯盆地西缘砾质冲积扇沉积学特征 总被引：10，自引：2，他引：10
晚三叠世鄂尔多斯西缘冲积扇沉积主要由碎屑流、颗粒流、片流及河道砾岩等组成。文中讨论了各类砾岩的沉积学特征,同时指出,不同成因的砾岩,其粒度分布曲线形态及某些粒度参数亦不同.据此本文提出了根据-系列新的粒度分布曲线形态和粒度参数Mdf,Rgs和Mdf/Rgs等甄别冲积扇中不同类型砾岩的设想。 相似文献
10.
《Journal of Asian Earth Sciences》1999,17(1-2):233-246
Sandstones, located in the Kuching area, western Sarawak, are known as the ‘Plateau Sandstones’ (of possible Eocene to early Miocene age). However, based on a number of factors, including: (i) anomalous kerogene compositions; (ii) proximity of the on-lap surface; and (iii) palaeocurrent direction (generally to the NNE), it is thought that the sands exposed on the Bako Peninsula are unrelated to the Plateau Formation (located to the south of the Bako Penisula) and therefore a new name is coined; the Bako Sandstones, which form a subgroup of the Bako Sandstone Group. The Bako Sandstones form the Bako Peninsula, a flat-topped cliffed plateau which extends into the South China Sea at a latitude of 1°30′N. The plateau has a gently dipping surface, sloping northwards from a height of about 300 to 150 m. The sandstones form a succession of very thick bedded sandstones (up to 6 m thick), with lenses of conglomerates and subordinate sandy mudstones. The sandstones consist of pebbly coarse-medium grained sands, interbedded with polymictic pebble conglomerates. The sandstones are mainly lithic arenite, poorly to moderately sorted and consist of subangular to subrounded grains. Isolated pebbles are common throughout the sandstones. The most common structure in both sandstones and conglomerates is cross-bedding; planar cross-bedding and trough cross-bedding, together with thick sequences of climbing ripples. These structures suggest extensive tractional transport, forming both ripple and dune structures along the base of the channel. The geometry of the sands is either (i) lensoidal, or (ii) tabular, with the channel-fill interpreted as scour-fill channels or migrating dunes, respectively. Both types are commonly stacked vertically or amalgamate laterally to form thick interconnected units. The conglomeratic lenses, scour-fill features and rip-up shale clasts are related to higher energy erosional events, whilst the mud-draped ripples, ripple rejuvenation surfaces and two-tiered channel margins indicate a lower energy and stasis period. Slope instabilities at the channel margin are inferred from the slump structures present and shale clast slurries. The sandstones at Bako are thought to have formed within a braided channel environment (subject to exposure, from the presence of mud cracks within the formation). 相似文献
11.
ABSTRACT It is important to understand the exact process whereby very large amounts of sediment are transported. This paper reports peculiar conglomerate beds reflecting the transition of submarine debris flows into hyperconcentrated flows, something that has been well documented only in subaerial debris-flow events until now. Voluminous debris flows generated along a Cretaceous submarine channel, southern Chile, transformed immediately into multiphase flows. Their deposits overlie fluted or grooved surfaces and comprise a lower division of clast-supported and imbricated pebble–cobble conglomerate with basal inverse grading and an upper division of clast- to matrix-supported, disorganized conglomerate with abundant intraformational clasts. The conglomerate beds suggest temporal succession of turbidity current, gravelly hyperconcentrated flow, and mud-rich debris flow phases. The multiphase flows resulted from progressive dilution of gravelly but cohesive debris flows that could hydroplane, in contrast to the flow transitions in subaerial environments, which involve mostly non-cohesive debris flows. This finding has significant implications for the definition, classification, and hazard assessment of submarine mass-movement processes and characterization of submarine reservoir rocks. 相似文献
12.
M. Julleh Jalalur Rahman Peter Faupl M. Mustafa Alam 《International Journal of Earth Sciences》2009,98(8):1971-1980
The Bengal Basin, in the north-eastern part of the Indian subcontinent, contains a thick (± 22 km) early Cretaceous-Holocene
sedimentary succession. The Neogene succession in the Sylhet Trough of the basin reaches a thickness of more than 6 km of
which the Surma Group contains important sandstone reservoirs. Lithologically, the group consists of a succession of alternating
shales, siltstones, sandy shales and sandstones, with minor conglomerates. This research work is a sedimentological analysis
of the subsurface Neogene succession encountered in the petroleum exploration wells in the Sylhet Trough of the Bengal Basin.
Detailed lithologic logs of the cores, based on considering texture and sedimentary structure, permit a subdivision into eight
lithofacies, e.g., a shale-dominated facies, interbedded fine sandstones and mudstones, ripple-laminated sandstones, parallel-laminated
sandstones, massive sandstones, cross-bedded sandstones, cross-bedded sandstones with pebble/granule lag and conglomerates.
Characteristic sedimentary structures of the Surma Group, such as flaser-, wavy- and lenticular-bedding, bipolarity of ripple
cross-stratification, evenly laminated sand/silt-streaked shales, reactivation surfaces within cross-bedded sandstone sets,
mud-drapes on foreset laminae and herringbone cross-stratification as well as small-scale vertical sequences (several fining-upward
cycles) are diagnostic for tidal influence. On the basis of the lithofacies associations and prograding character of the deposits
revealed from the electrofacies associations, the Surma Group sediments have been interpreted as representing deposits of
tide-dominated deltaic depositional setting. 相似文献
13.
Glacial influence on Neoproterozoic sedimentation: the Smalfjord Formation, northern Norway 总被引：1，自引：0，他引：1
ABSTRACT There is much debate regarding the intensity and geographic extent of glaciation during the Neoproterozoic, particularly in response to recent geochemical work suggesting that the Neoproterozoic earth was at times ice covered from equator to poles (the ‘Snowball Earth’ hypothesis). A detailed sedimentological analysis of the Neoproterozoic Smalfjord Formation of northern Norway was conducted in order to determine the extent and intensity of glacial influence on sedimentation. In the Tarmfjorden area, the Smalfjord Formation consists of a stacked succession of diamictites interbedded with fine‐grained laminated mudstones containing rare outsized clasts. Diamictites and interbedded mudstones are interpreted as the product of subaqueous mass flows generated along the basin margin. In the Varangerfjorden area, chaotically interbedded diamictites, conglomerates and sandstones are overlain by a thick succession of stacked sandstone beds; onediamictite unit at Bigganjargga overlies a striated pavement. The Varangerfjorden outcrops appear to record deposition on a subaqueous debris apron. Although diamictites contain rare striated and faceted clasts, suggesting a glacial sediment source, their origin as subaqueous mass flows prevents the interpretation of ice mass form or distribution. Rare lonestones may be associated with floating ice in the basin, which may be of glacial or seasonal origin. Glacial ice may have contributed poorly sorted glacial debris to the basin margin, either directly or through fluvioglacial systems, but there is no evidence of direct deposition by ice at Varangerfjorden or Tarmfjorden. The overall fining‐upward trend identified in the Smalfjord Formation and overlying Nyborg Formation is consistent with depositional models of rift basin settings. This fining‐upward trend, the predominance of mass flow facies including breccias associated with scarps and the evidence for extensional tectonic activity in the region suggest that tectonic activity may have played an important role in the development of this Neoproterozoic succession. The Smalfjord Formation at Tarmfjorden and Varangerfjorden does not exhibit sedimentological characteristics consistent with severe glacial conditions suggested by the snowball Earth hypothesis. 相似文献
14.
综合岩心观察、薄片鉴定、砂砾岩厚度及百分含量等资料,对东营凹陷北带沙四段砂砾岩的沉积特征、成因类型及沉积演化模式进行了系统研究。结果表明,东营凹陷北带沙四段砂砾岩储层中块状层理、粒序层理、平行层理、叠覆冲刷构造、交错层理发育,同时变形构造、泥岩撕裂屑、重荷模和火焰构造常见。碎屑颗粒表现为无分选-中等分选、棱角状-次棱角状到次圆状的近物源沉积特征。岩相类型以块状层理砾岩相、递变层理砾岩相、递变层理砂质砾岩相、交错层理砾质砂岩相、平行层理砾质砂岩相、块状层理砾质砂岩相、块状砂岩相、平行层理砂岩相、块状层理砂岩相、变形层理砂岩相、泥质撕裂变形砂岩相、薄层透镜状或压扁层理砂岩相为主。综合古地貌、砂砾岩沉积特征及沉积机制等,认为东营凹陷北带沙四段主要发育近岸水下扇、湖底扇和扇三角洲等成因类型的砂砾岩扇体。在沉积演化上,沙四下亚段沉积时期,胜坨地区和民丰地区发育近岸水下扇沉积,利津地区发育扇三角洲沉积,扇体规模较小,横向连续性较差;沙四上亚段沉积时期,胜坨地区和民丰地区广泛发育近岸水下扇-湖底扇沉积,利津西部地区发育扇三角洲沉积,扇体规模较大,横向连片发育。 相似文献
15.
ABSTRACT The Wagwater Trough is a fault-bounded basin which cuts across east-central Jamaica. The basin formed during the late Palaeocene or early Eocene and the earliest sediments deposited in the trough were the Wagwater and Richmond formations of the Wagwater Group. These formations are composed of up to 7000 m of conglomerates, sandstones, and shales. Six facies have been recognized in the Wagwater Group: Facies I-unfossiliferous massive conglomerates; Facies II—channelized, non-marine conglomerates, sandstones, and shales; Facies III-interbedded, fossiliferous conglomerates and sandstones; Facies IV—fossiliferous muddy conglomerates; Facies V—channelized, marine conglomerates, sandstones, and shales; and Facies VI—thin-bedded sheet sandstones and shales. The Wagwater and Richmond formations are interpreted as fan delta-submarine fan deposits. Facies associations suggest that humid-region fan deltas prograded into the basin from the adjacent highlands and discharged very coarse sediments on to a steep submarine slope. At the coast waves reworked the braided-fluvial deposits of the subaerial fan delta into coarse sand and gravel beaches. Sediments deposited on the delta-front slope were frequently remobilized and moved downslope as slumps, debris flows, and turbidity currents. At the slope-basin break submarine fans were deposited. The submarine fans are characterized by coarse inner and mid-fan deposits which grade laterally into thin bedded turbidites of the outer fan and basin floor. 相似文献
16.
贵州平坝白垩纪地层及沉积特征 总被引：1，自引：0，他引：1
通过对平坝一带区域地质的调查,认为这套白垩系砾岩、紫红色泥岩为山麓—河湖相沉积,是燕山期构造控制的山前盆地沉积。在紫红色泥岩中首次发现介形类Ｔａｌｉｃｙｐｒｉｄｅａｓｐ．、Ｔ．ｌａ－ｔｉｏｖａｔａ、Ｃｙｐｒｉｓｓｐ．,证明其属上白垩统扎佐组,而不是侏罗系。根据沉积学研究将其划分为４种沉积类型,并圈定了各类型空间展布。砾岩,砂岩属碎屑流沉积产物,而紫红色泥岩为湖相沉积,两者在很大程度上属同时异相沉积。 相似文献
17.
A middle Pleistocene coarse‐grained canyon fill succession (the Serra Mulara Formation) crops out in the northern sector of the Crotone Basin, a forearc basin located on the Ionian side of the Calabrian Arc and active from the Serravallian to middle Pleistocene. This succession is an example of coarse‐grained submarine canyon fill, which consists of a north‐west to south‐east elongated body (4·25 km long and up to 1·5 km wide) laterally confined by a deep‐water clayey and silty succession and located behind the modern Neto delta (north of Crotone). The thickness of the unit reaches 178 m. The lower part of the canyon fill is dominated by gravelly to sandy density‐flow deposits containing abundant bivalve and gastropod fragments, passing upward into a succession composed of metre‐scale to decimetre‐scale density‐flow deposits forming sandstone–mudstone couplets. Sandstone deposits are mostly structureless and planar‐laminated, whereas the clayey layers record hemipelagic deposition during quieter phases. This succession is overlain by another composed of thicker structureless sandstones alternating with layers of interlaminated mudstones and sandstones, which contain leaf remnants and fresh water ostracods, and are linked directly to river floods. The canyon fill is overlain by gravelly to sandy continental deposits recording a later stage of emergence. Facies analysis, together with micropalaeontological data from the hemipelagic units, suggests that the studied canyon fill records, firstly, a progressive gravel material cut‐off during deposition due to an overall relative sea‐level rise, leading to a progressive increase in the entrapment of sediment in fluvial to shallow‐marine systems, and secondly, a generalized relative sea‐level lowering. This trend probably reflects high‐magnitude glacio‐eustatic changes combined with the regional uplift of the region, ultimately leading to emergence. 相似文献
18.
Fluvial styles recorded by the uppermost part of the Neoproterozoic ‘Rivieradal sandstones' succession of eastern North Greenland reflect variations in rate of generated accommodation space and possibly climatic changes. Three facies associations, arranged in two genetic sequences, are recognised within the succession. The lower sequence initially records little available accommodation space. A high degree of reworking results in sheet-like, high-energy, bed-load-dominated, braided river deposits lacking recurrent facies patterns. As accommodation space increases upwards through the sequence, reduction in reworking is recorded by the development of fining- and thickening-upward muddy fluvial cycles. Evidence of desiccation or prolonged periods of drought are absent within the deposits and climate was probably relatively humid. Channel deposits in the lower sequence reflect mixed-load, braided fluvial systems with stable channel banks and floodplains, and the gradient appears to have been low to moderate. These features are generally considered favourable for the establishment of meandering river systems, but channels, nevertheless, retained an overall braided character and their deposits show no evidence of meandering. Despite indications of a climatic setting without significant periods of drought sediments indicate that large fluctuations in discharge occurred within the mixed-load streams and this is suggested to be the main cause for the absence of meandering. The swift and rather dramatic response of the fluvial systems to changes in precipitation, probably resulted from rapid runoff rates caused by the absence of vegetation. The upper sequence shows an initial return to shallow, sandy braided river deposition recording little available accommodation space. A subsequent increase in the rate of generated accommodation space is indicated by the presence of alternating sheet sandstones and sand-streaked mudstones with abundant desiccation cracks. The sheet sandstones show evidence of high-energy, unconfined ephemeral fluvial flash-flood deposition, while the mudstones are interpreted to represent muddy floodplain deposits. The change in fluvial style, combined with the widespread evidence of desiccation, suggest an evolution towards a more semi-arid climate in the upper sequence. This climatic change could account for the reduced clastic input seen in the overlying marine succession which culminated in carbonate platform deposition. The present study suggests that even under conditions considered favourable for the formation of meandering streams, these will rarely occur in Proterozoic deposits due to the lacking influence of vegetation. Although meandering deposits cannot be ruled out as having formed in pre-vegetational times, the conditions for their formation appear to have been even more restricted than previously realised. 相似文献
19.
ILFRYN PRICE 《Sedimentology》1977,24(4):529-546
The Othris Mountains of eastern Greece contain a calcareous continental margin/ocean basin sequence exposed in a stack of Cretaceous thrust sheets. Upper Triassic to Lower Cretaceous shelf, submarine fan and basinal successions overlie shallow marine units of Lower Triassic and Permian age. In off-shelf sequences the older sediments are separated from the younger by a horizon of alkaline ‘early-rifting’ basalts. Ophiolites overthrust the marginal sequence. Pre-rifting sediments are represented by a varied suite of limestones and clastics resting on metamorphic basement and include distinctive, green lithic arenites. In the thrust sheet immediately over the para-autochthonous shelf sequence, pre-rifting sediments are separated from the rift basalts by an intermittent horizon of calcareous sandstones and conglomerates reworked from uplifted basement and older sediments. Textural and petrographic immaturity suggests that these are probably deposits derived from fault scarps, produced in an early phase of rifting. Above the basalts in the same sheet is a suite of calciclastic sediment-gravity-flow deposits, apparently sedimented on a submarine fan. Progressive downslope modification of calcirudites suggests deposition from evolving, high concentration flows. Massive calcarenite facies (? grain flows) are unusually abundant; a possible reflection of a shallow palaeo-shelf break since provenance and palaeocurrent evidence proves the clastic carbonates to have been derived from a calcareous shelf. In addition to limestone lithoclasts the calcirudites, but not the massive calcarenites, contain fragments of pre-rifting lithologies including the distinctive arenites. Since the shelf sequence in Othris is totally nondetrital these clasts imply derivation of coarse sediment from an off-shelf position; probably the walls of a submarine canyon. This may have occurred either by direct erosion of wall rock, or by reworking of material from an older clastic sequence. In the latter case the inferred fault-scarp deposits are a likely source. 相似文献
20.
The Late Eocene-Early Oligocene sedimentary fill of the Lemnos Island, NE Greece, is represented by a submarine fan and shelf
deposits. Turbidites in the system occur as a laterally isolated body, with one sediment influx center present. The influx
center is a proximal distributary channel that occupies a position approximately in the fan’s center and displays the coarsest
sediment in the study area. It also suggests in association with the main palaeocurrent direction toward NE a curved shape
for the fan. The stratigraphic succession of the submarine fans indicates that their sedimentation started during the base
level fall and completed shortly after the base level rise. As a consequence, the study area was filled by turbidites that
correspond to forced regressive, lowstand normal regressive, and transgressive genetic units. The progradational bedsets,
within the basal part of the turbidite deposits, recorded the history of the base level fall. The mixed progradational/aggradational
style of the upper part of the submarine fan system suggests that the regression of the shoreline is driven by sediment supply
during a period of base-level rise at the shoreline, or at a time of baselevel stillstand. The overlying shelf facies consist
of thick to medium bedded sandstones and mudstones, which display a general thinning upward trend. The base of the mudstone
facies that overlie the thick-bedded, amalgamated sandstones corresponds to a transgressive surface. This surface separates
the low-stand deposits (thick-bedded sandstones) from the high stand deposits (mudstone facies), suggesting that deposition
of shelf facies occurred during a transgressive system tract. 相似文献