Low Eustatic Sea Level Research Articles

Coupled biostratigraphy and chemostratigraphy at Lanna, Sweden: A key section for the Floian–lower Darriwilian interval (Lower–Middle Ordovician)

The Lanna area in the province of Närke, south-central Sweden, hosts a succession of cool-water carbonate strata that are largely devoid of tectonic and diagenetic alteration, and span the Floian through lower Darriwilian Global stages (Ottenbyan–Kundan Regional stages). In this study we assess the integrated biostratigraphic and chemostratigraphic framework for the Ordovician sequence at Lanna, utilizing trilobites and conodonts coupled with bulk-rock carbon and oxygen isotopes (δ13Ccarb and δ18Ocarb). The local succession is thin but relatively complete; biostratigraphic details are similar to coeval successions in Baltoscandia. The new high-resolution δ13C and δ18O data are broadly consistent with other chemostratigraphic records at the regional scale, but notable differences in δ13C are obvious compared to global records. δ13C trends diverge markedly in the Dapingian and intra- and inter-regional differences become increasingly pronounced through younger strata. This may largely be explained by an overall low eustatic sea level, which led to more widespread oceanic restriction of epeiric seas. As a result, δ13C records became more influenced by local variables, muddling the global δ13C signal. This, in turn, suggests that sea level played a larger role than long-term changes within the global carbon cycle for regulating local–regional δ13C gradients in the Middle Ordovician. Regardless of the underlying cause(s), the documented variability poses significant challenges for using δ13C as a tool for detailed stratigraphic correlations. The limited expression of many named ‘carbon isotope excursions’ (CIEs) in the studied interval indicate that they are artefacts of local overprinting rather than reflective of changes to the global carbon cycle. Thus, these CIEs are of questionable correlative importance and validity. Although the scatter in numeric values is relatively large, long-term δ18O trends are overall consistent at the global scale, tentatively supporting the notion that climate cooled during the studied time interval.

Orogens of Big Sky Country: Reconstructing the Deep‐Time Tectonothermal History of the Beartooth Mountains, Montana and Wyoming, USA

AbstractArchean rocks exposed in the Beartooth Mountains, Montana and Wyoming, have experienced a complex >2.5 Gyr thermal history related to the long‐term geodynamic evolution of Laurentia. We constrain this history using “deep‐time” thermochronology, reporting zircon U‐Pb, biotite40Ar/39Ar, and zircon and apatite [U‐Th(‐Sm)]/He results from three transects across the basement‐core of the range. Our central transect yielded a zircon U‐Pb concordia age of 2,805.6 ± 6.4 Ma. Biotite40Ar/39Ar plateau ages from western samples are ≤1,775 ± 27 Ma, while those from samples further east are ≥2,263 ± 76 Ma. Zircon (U‐Th)/He dates span 686.4 ± 11.9 to 13.5 ± 0.3 Ma and show a negative relationship with effective uranium—a proxy for radiation damage. Apatite (U‐Th)/He dates are 109.2 ± 23.9 to 43.6 ± 1.9 Ma and correlate with sample elevation. Multi‐chronometer Bayesian time‐temperature inversions suggest: (a) Cooling between ∼1.90 and ∼1.80 Ga, likely related to Big Sky orogeny thermal effects; (b) Reheating between ∼1.80 Ga and ∼1.35 Ga consistent with Mesoproterozoic burial; (c) Cooling to ≤100°C between Mesoproterozoic and early Paleozoic time, likely reflecting continental erosion; (d) Variable Paleozoic–Jurassic cooling, possibly related to Paleozoic tectonism and/or low eustatic sea level; (e) Rapid Cretaceous–Paleocene cooling, preceding accepted proxies for flat‐slab subduction; (f) Eocene–Miocene reheating consistent with reburial by Cenozoic volcanics and/or sediments; (g) Post‐20 Ma cooling consistent with Neogene development of topographic relief. Our results emphasize the utility of multi‐chronometer thermochronology in recovering complex, non‐monotonic multi‐billion‐year thermal histories.

Quaternary subsidence history of Xisha Islands (northern South China Sea): Evidences from the reef-bank system

Subsidence is a potentially important factor influencing the evolution of carbonate platforms. Yet, few studies have been carried out on the quantitative reconstruction of subsidence history of carbonate platforms in the South China Sea. Using paleo-water depth constraints from core XK-1, drilled in the Xisha Islands, northern South China Sea, the tectonic and total subsidence history of Xisha Islands during the Quaternary is quantitatively reconstructed in this study. Slow tectonic subsidence between 1.945 and 0.988 Ma was related with magmatic intrusion beneath the Xisha Islands, while the relatively high tectonic subsidence rate between 0.988 and 0.781 Ma and the subsequent gradual decline in tectonic subsidence rate between 0.781 and 0.07 Ma were linked to the thermal cooling process. A rapid tectonic uplifting process after 0.07 Ma occurred as a result of compression and volcano eruption. Total subsidence rate showed an increase from the stage of 1.945–0.988 Ma to the stage of 0.988–0.781 Ma, and a gradual decline during the stage of 0.781–0.07 Ma. After 0.07 Ma, there was a rapid uplifting in response to the tectonic movement. The high eustatic sea level and intermediate total subsidence rate from 1.945 to 1.0 Ma caused the domination of bank deposits on the Xisha carbonate platform. From 0.988 to 0.65 Ma, the long-term eustatic sea-level lowering and the relatively high total subsidence rate favored the production of thick reef units. By contrast, the low eustatic sea level and the slow total subsidence between 0.65 and 0.42 Ma forced the accommodation space to be limited, resulting in formation of thin reef-bank deposits and occurrence of exposure horizons. During the stage of slow subsidence, the high eustatic sea level around 0.4 Ma allowed the last presence of thick reef unit in the Quaternary section, after which the lower eustatic sea level reduced the accommodation space again and only thin bioclastic bank deposits and iron-stained exposure horizon occurred. Around 0.07 Ma, the Xisha Islands were continuously exposed due to the uplifting and low eustatic sea level. The strong winter monsoon caused a heterozoan factory nearby the Xisha Islands, and blew the carbonate sediments onshore forming aeolianites. Our results proves that the total subsidence has indeed greatly influenced the evolution of Quaternary carbonate system in the Xisha area. The long-term (at scale of Myr) sea-level lowering could facilitate the flourishment of corals, and a moderate total subsidence in the context of secular sea-level lowering would provide more effective accommodation space for the continuous reef accretion, favoring the formation of thick reef units in the tropical areas.

Late Quaternary variations in the Oxygen Minimum Zone linked to monsoon shifts as seen in the sediment of the outer continental shelf of the eastern Arabian Sea

Multiproxy records of benthic foraminifera, pteropods, and total organic carbon (TOC) are reported from the late Pleistocene-Holocene interval of Core SK291/GC17, located off the coast of Goa (water depth 182 m), eastern Arabian Sea. Findings help unravel significant shifts in monsoon behavior and associated changes in the Oxygen Minimum Zone (OMZ). During the Younger Dryas (11900–11,600 cal yr BP) and ~ 11,600 to 9400 cal yr BP, the coast was closer to the studied core location because of the low eustatic sea level. This interval shows a higher abundance of benthic foraminifera preferring oxic bottom water condition over the suboxic group. At ~8000 cal yr BP, the decreased abundance of pteropod species Limacina trochiformis as well as lower values of TOC suggest the presence of a dry phase of the monsoon. The interval from ~7800 to 6400 cal yr BP was deposited under favorable bottom water conditions, as documented by a higher number of species (S), Information Function (Hs) and species richness of the benthic foraminiferal assemblage. The middle Holocene (~ 6200 to 4200 cal yr BP) interval is marked by a significant increase in the number of epipelagic pteropods caused by higher surface productivity and by a decreasing abundance of mesopelagic pteropods caused by the shoaling and intensification of the OMZ. The oxic group decreased drastically while the dysoxic group of benthic foraminifera increased during this interval due to the intensified OMZ. After ~4200 cal yr BP, the oxic assemblages of benthic foraminifera and pteropods suggest low productivity coinciding with the 4.2 ka dry event observed in the Indian monsoon records. The intervals with oxic assemblages of benthic foraminifera align with Bond events. Further the oxic assemblages show high frequency cycles centered at 692, 440 and 358 yr driven by solar variability, while Uvigerina peregrina , a benthic foraminifer sensitive to OMZ variability, shows high frequency cycles of 403 and 745 yr. • OMZ history of the eastern Arabian Sea during ~13,000 to 3400 cal yr BP • Winter monsoon was relatively stronger during the Younger Dryas cold interval. • The OMZ intensified and shoaled during ~6200 to 4200 cal yr BP. • Pteropod data indicates presence of a dry phase of the monsoon after ~4200 cal yr BP.

Origin and fate of sedimentary organic matter in the northern Bay of Bengal during the last 18 ka

The Northern Bay of Bengal (NBoB) is a globally important region for deep-sea organic matter (OM) deposition due to massive fluvial discharge from the Ganges-Brahmaputra-Meghna (G-B-M) rivers and moderate to high surface productivity. Previous studies have focused on carbon burial in turbiditic sediments of the Bengal Fan. However, little is known about the storage of carbon in pelagic and hemipelagic sediments of the Bay of Bengal over millennial time scales. This study presents a comprehensive history of OM origin and fate as well as a quantification of carbon sediment storage in the Eastern Bengal Slope (EBS) during the last 18ka. Bulk organic proxies (TOC, TIC, TN, δ13CTOC, δ15NTN) and content and composition of total hydrolysable amino acids (THAA) in a sediment core (SO188-342KL) from the EBS were analyzed. Three periods of high OM accumulation were identified: the Late Glacial (LG), the Bölling/Alleröd (B/A), and the Early Holocene Climatic Optimum (EHCO). Lower eustatic sea level before 15ka BP allowed a closer connection between the EBS and the fluvial debouch, favoring high terrestrial OM input to the core site. This connection was progressively lost between 15 and 7ka BP as sea level rose to its present height and terrestrial OM input decreased considerably. Export and preservation of marine OM was stimulated during periods of summer monsoon intensification (B/A and EHCO) as a consequence of higher surface productivity enhanced by cyclonic-eddy nutrient pumping and fluvial nutrient delivery into the photic zone. Changes in the THAA composition indicate that the marine plankton community structure shifted from calcareous-dominated before 13ka BP to siliceous-dominated afterwards. They also indicate that the relative proportion of marine versus terrestrial OM deposited at site 342KL was primarily driven by relative sea level and enlarged during the Holocene. The ballasting effect of lithogenic particles during periods of high coastal proximity and/or enhanced fluvial discharge promoted the export and preservation of OM. The high organic carbon accumulation rates in the EBS during the LG (18–17ka BP) were 5-fold higher than at present and comparable to those of glacial upwelling areas. Despite the differences in sediment and OM transport and storage among the Western and Eastern sectors of the NBoB, this region remains important for global carbon sequestration during sea level low-stands. In addition, the summer monsoon was a key promotor of terrestrial and marine OM export to the deep-ocean, highlighting its relevance as regulator of the global carbon budget.

Post-glacial sea-level change along the Pacific coast of North America

Sea-level history since the Last Glacial Maximum on the Pacific margin of North America is complex and heterogeneous owing to regional differences in crustal deformation (neotectonics), changes in global ocean volumes (eustasy) and the depression and rebound of the Earth's crust in response to ice sheets on land (isostasy). At the Last Glacial Maximum, the Cordilleran Ice Sheet depressed the crust over which it formed and created a raised forebulge along peripheral areas offshore. This, combined with different tectonic settings along the coast, resulted in divergent relative sea-level responses during the Holocene. For example, sea level was up to 200 m higher than present in the lower Fraser Valley region of southwest British Columbia, due largely to isostatic depression. At the same time, sea level was 150 m lower than present in Haida Gwaii, on the northern coast of British Columbia, due to the combined effects of the forebulge raising the land and lower eustatic sea level. A forebulge also developed in parts of southeast Alaska resulting in post-glacial sea levels at least 122 m lower than present and possibly as low as 165 m. On the coasts of Washington and Oregon, as well as south-central Alaska, neotectonics and eustasy seem to have played larger roles than isostatic adjustments in controlling relative sea-level changes.

Stagnant lid convection and the thermal subsidence of sedimentary basins with reference to Michigan

The thermal subsidence of basins formed above thick continental lithosphere differs from that of young passive margin basins and of young oceanic crust in that stagnant lid convection supplies significant heat flow from the asthenosphere. The lithosphere eventually approaches thermal equilibrium where the convective heat added to its base balances the heat lost by conduction to the surface. This paper presents a simple parameterization that quantifies these effects for modeling basin subsidence. The convective heat flow scales with the current lithosphere thickness squared while the conductive heat flow scales inversely to current lithospheric thickness. The predicted thermal subsidence rate scales to the difference between the conductive and convective heat flows and wanes gradually over hundreds of millions of years. The formalism can be modified to represent thermal subsidence where plume material has ponded within a catchment of locally thinned lithosphere. The base of the plume material forms a stable stratification that suppresses convective heat flow from below while heat continues to conduct to the surface by conduction. The predicted initial thermal subsidence rate scales with the large difference between conductive and zero convective heat flow. It is thus much greater than beneath lithosphere underlain by ordinary asthenosphere for a given amount of total eventual thermal subsidence. The paper compares thermal subsidence predictions from the models with and without plumes with sedimentation data from the Michigan basin. Observed initial Late Cambrian through Lower Devonian sedimentation in the Michigan basin is rapid as expected from the plume model, but the Ordovician sedimentation rate is slower than before and after. It is conceivable that this irregularity in the sedimentation curve is associated with low eustatic sea level and sediment‐starved conditions at the basin center in the Ordovician and Early Silurian periods, as opposed to irregular tectonics. Sedimentation poorly resolves a long tail of gradual subsidence that may extend to the present.

Biosignals from the EA Field, Nigeria

Abstract The EA Field study was undertaken to determine reservoir connectivity across growth faults. In this high subsidence area, conventional biostratigraphy does not have sufficient resolution to provide a production-scale time frame. Consequently, a new tool had to be developed which would provide such accuracy. This new high-resolution correlation tool, Biosignals, is based on the relationship ‘pollen-parent plant-ecology’. Pollen, thus related, have been grouped into ‘vegetation’ zones, which have been logged per well and summarized on saw blade diagrams. Sequences of vegetation zones could be recognized, showing an upward trend from spore dominated, via swamp, rain forest, savanna to montane. A number of potential regional time lines could be recognized: (1) a climate curve; (2) a change from the freshwater alga Pediastrum to the brackish water alga Botryococcus ; and (3) the top of frequent fungi. Based on this study the following conclusions may be drawn: (i) vegetational stacking patterns reflect fourth-order sequence stratigraphy; (ii) Biosignals suggest the tidal channels in well EA-13 to be lowstand deposits, although many of them belong to truncated sequences (only lowstand systems tract (LST) and transgressive systems tract (TST) preserved); (iii) on a third-order scale, the sandy middle part of the sequence (EA-1, 6000–6500 feet) correlates with dry climatic conditions and to low eustatic sea level. The two regionally extensive maximum flooding surfaces correlate with humid climatic intervals and eustatic high sea level; and (iv) the new high-resolution correlation tool provides a detailed biostratigraphical subdivision, within the existing conventional biozonation, and thus enables correlation of reservoir bodies across growth faults.

Evolution of the Cenozoic sedimentary wedge during rifting and seafloor spreading west of the Stappen High, western Barents Sea

The Cenozoic sedimentary wedge west of the Stappen High is divided into five main stratigraphic units. From oldest to youngest these are: unit R, unit B, unit TeC, unit TeD and unit TeE. Unit R was deposited in a faulted, relatively deep, marine basin prior to the onset of seafloor spreading in the northernmost part of the Norwegian-Greenland Sea. The unit was probably deposited from early Eocene to early Oligocene time, with the Stappen High as a major source area. The base of unit B represents the onset of a second episode of faulting, erosion and subsidence west of the Stappen High. The unconformity is probably related to the early Oligocene (anomaly 13) change in relative plate movements to a more west-northwest trend causing rifting, and later seafloor spreading, in the northern Norwegian-Greenland Sea. A period of heavy erosion on the paleocontinental slope and shelf-break occurred in mid-Miocene time (15.5 Ma, the base of unit TeC). The relatively deep erosion west of the Stappen High was probably related to compression and uplift along northeast-southwest oriented faults combined with a period of low eustatic sea level. The mid-Miocene to intra-Pleistocene parts of the Cenozoic sedimentary wedge (units TeC and TeD) are dominated by complex lowstand fan deposits and prograding sequences deposited during gradual thermal subsidence of the margin. Several unconformities related to relative sea-level changes are identified within this part of the wedge. The most important sequence boundaries are the 5.5 Ma base of unit TeD boundary and the intra-Pleistocene (0.8 Ma) base of unit TeE boundary which probably represent the onset of extensive ice-sheet formation on the Barents shelf.

Last Glacial loess and early Last Glacial vegetation history of Wairarapa Valley, New Zealand

Abstract The upper Quaternary stratigraphic and physiographic record of Wairarapa Valley, southern North Island, is one of the most complete sequences for this time period in New Zealand. Wairarapa Valley is situated within an active tectonic belt, and opens to the south on a coastline that has been rapidly deformed during the Quaternary. Warm climate and high eustatic sea level are reflected by a sequence of five well-preserved marine benches, and by lacustrine environments in Wairarapa Valley. Cold climate (and low eustatic sea level) are reflected by erosion, fluvial aggradation, and loess production. Three loess units deposited on Last Interglacial marine benches are widely preserved around the coastal part of the valley, and on fluvial aggradation gravels older than the Last Interglacial, further inland. At several other sites, the three loess units are preserved conformably on lacustrine sediments containing pollen and diatoms. At one site, Bidwill Hill, a well-preserved pollen and diatom flora reco...

Reworked Neritic Fossils in Upper Mesozoic and Cenozoic Central Pacific Deep-Sea Sediments Monitor Sea-Level Changes

Upper Mesozoic and Cenozoic pelagic sediments in the central Pacific Ocean contain occasional horizons with reworked and displaced fossils produced by organisms that once lived in neritic shallow-water environments. The flux of neritic fossils was restricted to eight intervals of low eustatic sea level during Late Mesozoic and Cenozoic times. They were eroded from shoals along the flanks of volcanic highs which often supported tropical islands and which since then have subsided.

Stratigraphy and Structure of Southern Blake Plateau, Northern Florida Straits, and Northern Bahama Platform from Multichannel Seismic Reflection Data

Approximately 2,100 km of 24-fold multichannel seismic reflection data reveal much about the subsurface geology for a large part of the continental margin east of Florida. Discordance between the westward-dipping pre-breakup sediments and the eastward-sloping basement along the edge of the Blake Plateau is interpreted as an effect of a splinter of continental margin derived from the African plate by a spreading-center jump in the Middle Jurassic. Early rifting centered under the main part of the Blake Plateau became inactive, as a spreading-center jump shifted the active rift to east of the present Blake Escarpment along the Blake Spur magnetic anomaly. In the northern Florida Straits the data reveal that the breakup unconformity, underlain by Triassic-Lower Jurassic(?) arkosic volcaniclastics, extends from southern Florida to the western Bahama Banks. These volcaniclastics are associated with the rift-crust of intermediate nature formed just prior to and during breakup of the North American and African continental plates. Back-reef platform deposits of limestones, dolomites, and evaporites of Late Jurassic to Albian age extend from the Blake-Bahama Escarpment westward beneath Florida. These deposits formed what once was a megabank extending over a wider area than the present smaller isolated Bahama Banks. The formation of the Florida Straits and Bahama channels occurred during the Cenomanian transgression. Only on the present Bahama Banks and Florida platform did shallow-water carbonate deposition persist to maintain a shallow-bank environment. Evidence of recurring scour by current erosion is found in the Florida Straits. Erosional events apparently occurred in the middle Cenomanian, middle Paleocene, early-middle Eocene, and Eocene-Oligocene, which coincidentally are times of lower eustatic sea level according to Vail et al (1977a). This evidence of Florida current scour indicates that the current was present as far back as the Cenomanian. Major faulting appears to have dropped the Northeast Providence Channel relative to the western Bahamas after the Albian. Submarine erosion and bank buildup created the channels and smaller relief features like Great Abaco Knoll beginning in about the Cenomanian. A carbonate bank margin and reef complex was present along the Bahama Escarpment since the Middle Jurassic. Apparently these organic buildups seeded on originally shallow structural relief on oceanic basement created during the spreading-center jump to the position of the Blake Spur magnetic anomaly. The bank margin apparently has retreated at least 15 km from a Late Jurassic-earliest Cretaceous position now marked by a bench below the Au unconformity. Active faulting occurred along the Great Abaco fracture zone at least through the Late Cretaceous and perhaps into the Tertiary. These relatively young tectonic events, together with the post-Albian faults in Providence Channel, indicate interactions between the Atlantic and Caribbean plates and that extensions of faulting have taken place far to the northeast of Cuba and the Greater Antilles.

Platform subsidence mechanisms and “eustatic” sea-level changes

The boundaries between major world-wide sequences of sediments on continental platforms are due primarily to either eustatic changes or to systematic uplift of the continental interior. If thermal contraction of the lithosphere controls basin subsidence, basins would continue to subside during times of low eustatic sea level. Calculations indicate that significant gaps in the geological record could be produced by modest eustatic sealevel changes even in rapidly subsiding basins. For example, a 95 m withdrawal could have produced the sub-Devonian unconformity in the Michigan Basin. The vertical amplitude of eustatic variations can be determined directly from the depth of paleotopographic valleys (135m for sub-Pennsylvanian of Illinois Basin) and from the present elevation of sediments deposited during highstands (300 m above present sea level). The Illinois Basin aparently continued to subside during the sub-Pennsylvanian regression.

Thermal Contraction and Flexure of Mid-Continent and Atlantic Marginal Basins

Summary Thermal contraction of the lithosphere is a probable cause of the gradual subsidence indicated by sediments of mid-continent basins and Atlantic continental shelves. The subsidence is complicated by time dependent regional isostatic compensation since adjacent parts of the lithosphere are mechanically coupled, and since creep in the lithosphere may relieve accumulated stress. Thus, if more subsidence occurs at point A than nearby point B, point A would be buoyed up and point B dragged down. Relaxation of this coupling during a later period of more gradual subsidence would produce uplift at B and downwarp at A. The absence of younger beds over local minima of subsidence such as the Florida arch, the flanks of the Michigan basin, and the Atlantic coastal plain (USA) can thus be explained. Variations in the subsidence rate due to exponential decay of the thermal anomaly or to starved basin-evaporite depositional sequences can produce observable effects. Analytic models of the Michigan basin and the Atlantic coast (USA) are compatible with previously estimated parameters: thermal decay time of the lithosphere, 50 My; flexural parameter of the lithosphere beneath air, 200 km; and viscosity of the lithosphere, 1025 poise. The effects of flexure are not clearly evident in Silurian evaporite deposition in the Michigan basin and it is probable that an extended time was required for the evaporite sequence to accumulate. The cause of the thermal heating event which precedes subsidence is unclear for mid-continent basins although bulk replacement of the uppermost mantle is necessary. The heating events may be associated with periods of slow sea-floor spreading (when slabs exert a tensional force on the lithosphere) and hence low eustatic sea level. There is little direct evidence that an initial heating event actually occurred in the Michigan basin immediately before the start of subsidence.

Étude des grands bancs de sable du Sud-est de la mer Celtique

Two surveys (continuous seismic reflection and surface-sediment sampling) carried out in 1970 by B.R.G.M. (Bureau de recherches géologiques et minières) and B.E.I.C.I.P. (Bureau d'études industrielles et de coopération de l'Institut français du pétrole) on the southeastern part of the Celtic Sea have enabled: (1) an accurate mapping of the bathymetry; (2) the reconnaissance of several great sand banks characterizing this shelf; some new ones have been discovered; the general bank orientation is parallel to the NE-SW trend of the Western Approaches; average size: length = 50 km, width = 6 km; height = 40 m; (3) the recognition of a sand-wave field to the W of the survey area, similar to the La Chapelle Bank sand waves; and (4) the study of the superficial structure of the shelf. From these results we may assume the following sequence for the Quaternary history of the shelf: 1. (1) Post-Pliocene levelling of the shelf (Early Pleistocene). 2. (2) Gullying into this surface related to a very low eustatic sea level at −240 m at least. 3. (3) Filling-up of the submerged channels by a transgressive stage. 4. (4) Preflandrian regression down to a shore-line at about −110 to −120 m; deposition of mixed (detritic + bioclastic) sediments on the submerged (outer) part of the shelf; beginning of the accumulation of the sand banks, resulting from and parallel to NE-SW dominant tidal currents, stronger than today. 5. (5) Sea-level rise (Flandrian transgression) during which the constructional process of the sand banks is reduced. 6. (6) Present situation with decrease of the current intensity (tide, internal waves .) which remains sufficient to allow the formation of sand waves; in some parts of the shelf, the latter are organized in a field pattern (zone de l'Espérance, La Chapelle Bank), owing to the morphological effect of canyon heads and to the shelf-break shape favouring the current action.