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1.
Monterey Fan is the largest modern fan off the California shore. Two main submarine canyon systems feed it via a complex pattern
of fan valleys and channels. The northern Ascension Canyon system is relatively inactive during high sea-level periods. In
contrast, Monterey Canyon and its tributaries to the south cut across the shelf and remain active during high sea level. Deposition
on the upper fan is controlled primarily by the relative activity within these two canyon systems. Deposition over the rest
of the fan is controlled by the oceanic crust topography, resulting in an irregular fan shape and periodic major shifts in
the locus of deposition.
Margin setting represents fan and/or source area 相似文献
2.
Monterey Fan is the largest modern fan off the California shore. Two main submarine canyon systems feed it via a complex pattern of fan valleys and channels. The northern Ascension Canyon system is relatively inactive during high sea-level periods. In contrast, Monterey Canyon and its tributaries to the south cut across the shelf and remain active during high sea level. Deposition on the upper fan is controlled primarily by the relative activity within these two canyon systems. Deposition over the rest of the fan is controlled by the oceanic crust topography, resulting in an irregular fan shape and periodic major shifts in the locus of deposition. 相似文献
3.
During the Quaternary, the Mac. Robertson shelf of East Antarctica was deeply eroded by glaciers and currents exposing the underlying basement, resulting in a scalped shelf. Major geomorphic zones are: (1) high-relief, ridge and valley topography (200–1400 m); (2) smooth sea floors associated with low-energy, depositional shelf valleys and basins (400–800 m); (3) low-relief, planated banktops (100–200 m); and (4) iceberg gouged and current reworked seaward-bank margins and upper slope (200 to < 630 m). About 90% of the shelf's surface has net erosional conditions and about 10% is net depositional. The sedimentary processes and deposits may be common to large areas of the East Antarctic margin. 相似文献
4.
Andrés Maldonado Alberto Palanques Belén Alonso Kim A. Kastens C. Hans Nelson Suzanne O'Connell William B. F. Ryan 《Geo-Marine Letters》1985,5(3):157-164
The Valencia Fan developed as the distal fill of a deep-sea valley, detached from the continental slope and the main sedimentary source. A survey of side-scan sonar, Sea Beam and reflection seismics shows that the sediment is largely fed through the Valencia Valley. The upper fan comprises large channels with low-relief levees, and the middle fan has sinuous distributary channels. Depositional bedforms predominate on the valley floor and levees, and erosional bedforms are common in the valley walls. A change to slope on the fan apex and the presence of volcanoes on the upper fan are the main factors influencing fan-growth pattern. 相似文献
5.
《Marine and Petroleum Geology》2003,20(6-8):649-676
High-resolution multichannel 2-D and 3-D seismic data, primarily from upper fan reaches of near-seafloor channel-levee systems on the Niger Delta slope and in the Arabian Sea, reveal a high level of detail and architectural complexity. Several architectural elements are common to each system examined in this study. They include inner levees, outer levees, erosional fairways, channel-axis deposits, rotational slumps blocks, and mass transport deposits. Although the scale of individual systems varies significantly, similarities in first-order architectural elements and their configurations suggest that common depositional processes are involved regardless of scale differences.Most of the channel-levee systems examined in this study are characterized by a basal erosional fairway that is bordered by outer levees of varying thickness. Together these elements define the base and margins of the channel-belt, where channel-axis deposits and inner levees are the dominant architectural elements. Vertical, sub-vertical, and lateral stacking patterns of sinuous and/or meandering channels create seismic facies that range from narrow to wide zones of high amplitude reflections (HARs) with chaotic to continuous and shingled to horizontal reflections. Some HARs appear as isolated or stacked asymmetric to symmetric u- and v-shaped reflections, referred to here as channel-forms. Channel-belts evolve within the confines of the scalloped erosional fairway walls (flanked by outer levee), and are similar in morphology to meander-belts in fluvial systems, but commonly have a greater component of vertical aggradation. Detailed study of one particular channel-levee system on the Niger Delta slope shows a period of incision followed by three distinct phases of channel development during its aggradational history. Each fill phase corresponds to a different channel stacking architecture, planform geometry, and nature of terrace development, with important implications for reservoir architecture. In some cases, multiple phases of inner levee growth are observed, each intimately linked to the channel migration and aggradation history. Channel sinuosity evolves dynamically, with some meander loops undergoing periods of accelerated meander growth at the same time that others show little lateral migration. 相似文献
6.
Bathymetric sections across the most recent valley just above and below the transition of upper to middle fan show a northward
shift of the thalweg between the confines of the levees. This small-scale migration was followed by a levec break on the higher
north side. Part of the bottom currents, which normally would have been contained within the levees, spilled over and spread
laterally in a northward direction. This levee break caused a canyonward shift of the boundary between the upper and mid fan,
and the diverted flow croded smaller valleys in the western part of the fan. 相似文献
7.
Shallow 3D seismic data show contrasting depositional patterns in Pleistocene deepwater slopes of offshore East Kalimantan, Indonesia. The northern East Kalimantan slope is dominated by valleys and canyons, while the central slope is dominated by unconfined channel–levee complexes. The Mahakam delta is immediately landward of the central slope and provided large amounts of sediments to the central slope during Pleistocene lowstands of sea level. In the central area, the upper slope contains relatively straight and deep channels. Sinuous channel–levee complexes occur on the middle and lower slope, where channels migrated laterally, then aggraded and avulsed. Younger channel–levee complexes avoided bathymetric highs created by previous channel–levee complexes. Levees decrease in thickness down slope. Relief between channels and levees also decreases down slope.North of the Mahakam delta, siliciclastic sediment supply was limited during the Pleistocene, and the slope is dominated by valleys and canyons. Late Pleistocene rivers and deltas were generally not present on the northern outer shelf. Only one lowstand delta was present on the northern shelf margin during the upper Pleistocene, and sediments from that lowstand delta filled a pre-existing slope valley complex and formed a basin-floor fan. Except for that basin-floor fan, the northern basin floor shows no evidence of sand-rich channels or fans, but contains broad areas with chaotic reflectors interpreted as mass transport complexes. This suggests that slope valleys and canyons formed by slope failures, not by erosion associated with turbidite sands from rivers or deltas. In summary, amount of sediment coming onto the slope determines slope morphology. Large, relatively steady input of sediment from the Pleistocene paleo-Mahakam delta apparently prevented large valleys and canyons from developing on the central slope. In contrast, deep valleys and canyons developed on the northern slope that was relatively “starved” for siliciclastic sediment. 相似文献
8.
Belén Alonso Kim A. Kastens Andrés Maldonado Alberto Malinverno C. Hans Nelson Suzanne O'Connell Alberto Palanques William B. F. Ryan 《Geo-Marine Letters》1985,5(3):141-148
The northern continental slope off the Ebro Delta has a badland topography indicating major slope erosion and mass movement of material that deposits sediment into a ponded lobe. The southern slope has a low degree of mass movement activity and slope valleys feed channel levee-complexes on a steep continental rise. The last active fan valley is V-shaped with little meandering and its thalweg merges downstream with the Valencia Valley. The older and larger inactive channel-levee complex is smoother, U-shaped, and meanders more than the active fan valley. 相似文献
9.
Hans Nelson 《Marine Geology》1976,22(2):129-155
The asymmetrical Astoria Fan (110 × 180 km) developed off the Columbia River and Astoria submarine canyon during the Pleistocene. Morphology, stratigraphy, and lithology have been outlined for a Pleistocene turbidite, and a Holocene hemipelagic sedimentary regime to generate geologically significant criteria for comparison with ancient equivalent deposits. Both gray silty clay of the Late Pleistocene and olive-gray clay of the Early Holocene are interrupted by turbidites. The few deeply incised fan valleys of the more steeply sloping upper fan contain thick, muddy and very poorly sorted sand and gravel beds that usually have poorly developed internal sedimentary structures. The numerous shallower fan valleys and distributaries of the flatter middle and lower fan contain thick, clean, and moderately sorted medium to fine sands that are vertically graded in texture, composition and well-developed internal sedimentary structures. Tuffaceous turbidites (containing Mazama ash, 6600 B.P.) can be traced as thick deposits (ca. 30–40 cm) throughout the Astoria Channel system and as thin correlative interbeds (ca. 1–2 cm) in interchannel areas. Similarly, sand/shale ratios are high throughout the fan valleys and the middle and lower fan areas of distributaries, but are low in the upper-fan interchannel areas.These depositional trends indicate that high-density turbidity currents carry coarse traction loads that remain confined in upper but not lower fan valleys. Fine debris selectively sorts out from channelized flows into overbank suspension flows that spread over the fan and deposit clayey silt. A high content of mica, plant fragments, and glass shards (if present) characterizes deposits of the overbank flows, a major process in the building of upper fan levees and interchannel areas.In the Late Pleistocene, turbidity currents funneled most coarse-grained debris through upper channels to depositional sites in middle and lower fan distributaries that periodically shifted, anastomosed and braided to spread sand layers throughout the area. At this time, depositional rates were many times greater (>50 cm/1000 years) than in the Holocene (8 cm/1000 years).During the Holocene rise of sea level, the shoreline shifted, the Columbia River sediment was trapped, and turbidity-current activity slackened from one major event per 6 years in the Late Pleistocene, to one per 1000 years in the Early Holocene, to none since the Mt. Mazama eruption (ca. 6600 B.P.). Turbidites became muddier and deposited as thick beds within main channels, in part explaining Holocene deposition rates three times greater there (25 cm/1000 years) than in interchannel regions. Turbid-layer debris, funneled through channel systems and trapped from flows off the continental terrace, also contributed to rapid sedimentation in valleys; however, less than 2% of the suspended sediment load of the Columbia River has been trapped in fan valleys during the Holocene.By the Late Holocene, continuous particle-by-particle deposition of hemipelagic clay with a biogenous coarse fraction was the predominant process on the fan. These hemipelagites contain progressively more clay size and less terrigenous debris offshore, and are finer grained, richer in planktonic tests and dominated by radiolarians compared to the foraminiferal-rich Pleistocene clays. The hemipelagic sedimentation of interglacial times, however, is insignificant compared to turbidite deposition of glacial times. 相似文献
10.
Peter T. Harris Andrew Heap Vicki Passlow Michael Hughes James Daniell Mark Hemer Ole Anderson 《Marine Geology》2005,220(1-4):181-204
The formation of incised valleys on continental shelves is generally attributed to fluvial erosion under low sea level conditions. However, there are exceptions. A multibeam sonar survey at the northern end of Australia's Great Barrier Reef, adjacent to the southern edge of the Gulf of Papua, mapped a shelf valley system up to 220 m deep that extends for more than 90 km across the continental shelf. This is the deepest shelf valley yet found in the Great Barrier Reef and is well below the maximum depth of fluvial incision that could have occurred under a − 120 m, eustatic sea level low-stand, as what occurred on this margin during the last ice age. These valleys appear to have formed by a combination of reef growth and tidal current scour, probably in relation to a sea level at around 30–50 m below its present position.
Tidally incised depressions in the valley floor exhibit closed bathymetric contours at both ends. Valley floor sediments are mainly calcareous muddy, gravelly sand on the middle shelf, giving way to well-sorted, gravely sand containing a large relict fraction on the outer shelf. The valley extends between broad platform reefs and framework coral growth, which accumulated through the late Quaternary, coincides with tidal current scour to produce steep-sided (locally vertical) valley walls. The deepest segments of the valley were probably the sites of lakes during the last ice age, when Torres Strait formed an emergent land-bridge between Australia and Papua New Guinea. Numerical modeling predicts that the strongest tidal currents occur over the deepest, outer-shelf segment of the valley when sea level is about 40–50 m below its present position. These results are consistent with a Pleistocene age and relict origin of the valley.
Based on these observations, we propose a new conceptual model for the formation of tidally incised shelf valleys. Tidal erosion on meso- to macro-tidal, rimmed carbonate shelves is enhanced during sea level rise and fall when a tidal, hydraulic pressure gradient is established between the shelf-lagoon and the adjacent ocean basin. Tidal flows attain a maximum, and channel incision is greatest, when a large hydraulic pressure gradient coincides with small channel cross sections. Our tidal-incision model may explain the observation of other workers, that sediment is exported from the Great Barrier Reef shelf to the adjacent ocean basins during intermediate (rather than last glacial maximum) low-stand, sea level positions. The model may apply to other rimmed shelves, both modern and ancient. 相似文献
11.
Dives in Lockheed's submarine “Deep Quest” followed the floor of the fan valley and outer gorge of Coronado Canyon, southwest of the California—Baja California border. Both the fan valley and outer gorge have meandering courses. Basin depressions and intermittent steep descents characterize the outer gorge. No indications were found of recent activity of turbidity currents at either location, but a sand layer 30 cm below the surface is interpreted as related to a lower stand of sea level at the close of the last ice age. Abundant organisms were seen on the valley floor and in the overlying water. 相似文献
12.
The Bengal Fan is the largest submarine fan in the world, with a length of about 3000 km, a width of about 1000 km and a maximum thickness of 16.5 km. It has been formed as a direct result of the India–Asia collision and uplift of the Himalayas and the Tibetan Plateau. It is currently supplied mainly by the confluent Ganges and Brahmaputra Rivers, with smaller contributions of sediment from several other large rivers in Bangladesh and India.The sedimentary section of the fan is subdivided by seismic stratigraphy by two unconformities which have been tentatively dated as upper Miocene and lower Eocene by long correlations from DSDP Leg 22 and ODP Legs 116 and 121. The upper Miocene unconformity is the time of onset of the diffuse plate edge or intraplate deformation in the southern or lower fan. The lower Eocene unconformity, a hiatus which increases in duration down the fan, is postulated to be the time of first deposition of the fan, starting at the base of the Bangladesh slope shortly after the initial India–Asia collision.The Quaternary of the upper fan comprises a section of enormous channel-levee complexes which were built on top of the preexisting fan surface during lowered sea level by very large turbidity currents. The Quaternary section of the upper fan can be subdivided by seismic stratigraphy into four subfans, which show lateral shifting as a function of the location of the submarine canyon supplying the turbidity currents and sediments. There was probably more than one active canyon at times during the Quaternary, but each one had only one active fan valley system and subfan at any given time. The fan currently has one submarine canyon source and one active fan valley system which extends the length of the active subfan. Since the Holocene rise in sea level, however, the head of the submarine canyon lies in a mid-shelf location, and the supply of sediment to the canyon and fan valley is greatly reduced from the huge supply which had existed during Pleistocene lowered sea level. Holocene turbidity currents are small and infrequent, and the active channel is partially filled in about the middle of the fan by deposition from these small turbidity currents.Channel migration within the fan valley system occurs by avulsion only in the upper fan and in the upper middle fan in the area of highest rates of deposition. Abandoned fan valleys are filled rapidly in the upper fan, but many open abandoned fan valleys are found on the lower fan. A sequence of time of activity of the important open channels is proposed, culminating with formation of the one currently active channel at about 12,000 years BP. 相似文献
13.
The 0.5- to 2-km thick Quaternary Laurentian Fan is built over Tertiary and Mesozoic sediments that rest on oceanic crust.
Two 400-km long fan valleys, with asymmetric levees up to 700-m high, lead to an equally long, sandy, lobate basin plain (northern
Sohm Abyssal Plain). The muddy distal Sohm Abyssal Plain is a further 400-km long. The sediment supplied to the fan is glacial
in origin, and in part results from seismically triggered slumping on the upper continental slope. Sandy turbidity currents,
such as the 1929 Grand Banks earthquake event, probably erode the fan-valley floors; but thick muddy turbidity currents build
up the high levees.
Margin setting represents fan and/or source area 相似文献
14.
Edward T. Baker 《Marine Geology》1976,21(2):67-79
Analysis of 121 in-situ nephelometer profiles recorded in the waters above Nitinat Deep-Sea Fan during three cruises between 1971 and 1974 reveals a substantial degree of correlation between fan topography and the vertical distribution of suspended particulate matter within 150 m of the sea floor. Profiles recorded above different topographic provinces may be generalized into two structural classes which have retained their characteristic features throughout the 4-year observation period: the levee type, in which a thin bottom nepheloid layer consists of a single steep scattering gradient sharply capped by the overlying clearer water; and the fan-valley type, in which a bottom nepheloid layer of variable thickness and prominent internal layering dissipates only gradually into the overlying clearer water. Although the source of the particles which form the bottom nepheloid layer has not yet been definitely identified, the multi-year stability of scattering intensity, thickness, and internal structure imply that it is maintained by some combination of continually operative processes, rather than by infusions of material from episodic events such as turbidity currents. 相似文献
15.
The 0.5- to 2-km thick Quaternary Laurentian Fan is built over Tertiary and Mesozoic sediments that rest on oceanic crust. Two 400-km long fan valleys, with asymmetric levees up to 700-m high, lead to an equally long, sandy, lobate basin plain (northern Sohm Abyssal Plain). The muddy distal Sohm Abyssal Plain is a further 400-km long. The sediment supplied to the fan is glacial in origin, and in part results from seismically triggered slumping on the upper continental slope. Sandy turbidity currents, such as the 1929 Grand Banks earthquake event, probably erode the fan-valley floors; but thick muddy turbidity currents build up the high levees. 相似文献
16.
17.
Wonn Soh 《Progress in Oceanography》2003,57(1):109-124
Sagami Bay is a deep-water foreland basin with an average sedimentary rate of approximately 0.1 g/cm2/year. It is an appropriate area to study for better understanding of sedimentary processes in a setting with a high sedimentation rate. Seven multiple core samples, 30-50 cm thick, were obtained from Sagami Bay. Four of the core samples were taken from the Tokyo submarine fan system (Tokyo canyon floor, Tokyo fan valley and its levee, the distal fan margin). Two samples were obtained from the Sakawa fan delta and the adjacent topographic high. The remaining one was from an escarpment of the Sagami submarine fault. Variations in chemical composition can be recognized at every coring site. They show two different sediment sources: the sediments of the Tokyo submarine fan system and those from Sakawa fan delta. Further, there are differences in chemical composition between canyon floor and levees even within the Tokyo submarine fan system. The results suggest that the sedimentary process is strongly controlled not by vertical particle settling but by a hyperpycnal flow process. The proxies obtained from the core samples do not reflect conditions in the water column immediately overlying the sea floor. Rather, they are controlled by conditions on the adjacent continental shelf or/and shallow basins, which are the areas of primary accumulation. 相似文献
18.
Side-scan sonar records collected over an area of the North Aleutian Shelf, approximately 250 km west of the head of Bristol Bay, Alaska, identified widespread evidence of active sea floor erosion processes, including sediment transport. Thousands of sea floor depressions, many linear and some containing rippled floors, were identified in water depths of 30 to 90 m. The depressions cover approximately 40 percent of the area surveyed. The sea floor depressions are interpreted to be erosional features, and in conjunction with a field of sand waves, exemplify the dynamic nature of the ocenographic processes active on this area of the sea floor. 相似文献
19.
A. V. Ilyin 《Oceanology》2010,50(2):240-253
Due to the complex transformation of the Earth’s crust in the rift valley, the morphology of the newly formed crust is changed
by that of the province of rift mountains. The main factors of the variability of the morphological structure are as follows:
the tectonomagmatic cyclicity of the geodynamic processes at the spreading centers and the isostatic uplift of the rift valley
floor. The interchange of magmatic and tectonic cycles determines the difference in the bathymetric levels of the isostatic
equilibrium at the edges of the rift valley slopes and the beginning of the formation of the topography of the province of
rift mountains. This relief represents an indepth system of ridges and valleys rhythmically interchanging in the lateral direction.
The morphology of the province of rift mountaines becomes the morphology of the acoustic basement throughout the ocean floor,
except for the continental margins and areas of intraplate tectonics and volcanism. 相似文献
20.
南海西部灾害性地质研究 总被引：5，自引：0，他引：5
灾害地质学(hazard geology)是研究对海底工程,特别是海洋石油工程能够产生直接危害,或具有潜在性危害的地质因素的特征及分布规律的科学。在过去几十年的海上石油开发中,由于事先未能对灾害性地质进行详细调查而造成重大损失的事件不乏其例。1973年3月,墨西哥湾一钻井平台,因浅层天然气喷发引起火灾,数千万美元的仪器设备毁于一旦。1977年南海莺歌海盆地作业的一架自升式钻井平台,在水深75m处插桩时,由于埋藏古河道的影响,地层分布不连续，两只柱腿插在古河岸上,至海底以下3m即稳定,另一只桩腿落入古河床,插入21m尚不稳定,致使钻井平台倾斜,后被迫移位才免遭于害。灾害性地质问题的研究已成为海洋石油和天然气开发成败的关键问题之一,因而引起了国内外有关部门的极大重视。1985-1987年,中国科学院海洋研究所受南海西部石油公司的委托,对珠江口以西至北部湾东部的广大海域,进行了大规模的灾害性地质、工程地质的普查及井位调査。调查中先后使用了“科学一号”科学考察船和“南海502”、“南海503”等工程物探及工程地质调查船,在海上进行了七千余公里的综合性工程物探测量,为研究调查区灾害性地质问题积累了丰富的资料。 相似文献