The Q-value and resistance of the heliospheric resonator model for the 22-year solar cycle     

Lorna Small  Michael J. Rycroft 《Planetary and Space Science》1983,31(6):701-704

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Role of autoretreat and A/S changes in the understanding of deltaic shoreline trajectory: a semi-quantitative approach     

Tetsuji Muto  Ron J. Steel 《Basin Research》2002,14(3):303-318

ABSTRACT There is continued interest in how the rate of relative sea‐level rise [A ( > 0)] and the rate of sediment supply [S] function during the growth and evolution of deltaic shorelines. The theory of shoreline autoretreat, recently corroborated in flume experiments, claims that (1) A( > 0) and S can never be in equilibrium, and (2) shoreline or shelf‐edge progradation inevitably turns to retrogradation, when relative sea level is rising even modestly and even if A/S = const (> 0). Autoretreat arises because the area of the clinoform surface of the delta (or shelf edge) per kilometer of shoreline must increase as the relative sea level rises, and the delta (or shelf edge) progrades into deeper water. A finite sediment supply rate is thus liable to become inadequate to sustain progradation. The problem increases further as a rising sea level also greatly increases the delta‐plain volume that needs to be filled, further limiting the progradation of the system. The fundamental trajectory of shoreline migration is thus one characterized by a concave‐landward shape, even under the steady forcing of the basin. The magnitudes of A (> 0) and S, or A/S do not determine whether the landward turnaround of the shoreline is realized or not, but affect merely the length and height of the fundamental trajectory curve. Thus, any attempt to detect and interpret temporal changes in A and S from the observed stratigraphic record of shoreline trajectory needs first to take full account of the inbuilt autoretreat mechanism. We develop here a simple, semi‐quantitative method of reconstructing the basin conditions (A and S) from the stratigraphic record of prograding deltaic shorelines (or prograding shelf‐margin clinoforms) on the basis of the theory of shoreline autoretreat. The deterministic nature of the autoretreat theory is advantageous in managing this latter issue, because any expected or unexpected change emerges as some discrepancy from a trajectory that was predicted for the initial conditions. The autoretreat theory also provides a convenient graphical method of dealing with the uncertainty of the field data, and with evaluating the accuracy of any reconstruction. Our methodology has been developed to deal with the behaviour of deltaic shorelines, but is basically applicable to any clinoform system, the development of which is affected by relative sea level. The suggested method is applied to an Early Eocene (Ypresian) regressive shoreline succession in the Central Tertiary Basin on Spitsbergen. The studied regressive wedge developed as a delta‐driven, progradational shelf‐margin system under a regime of overall (i.e. long‐term) rise of relative sea level, but also suffered short‐term sea‐level falls associated with valley incisions on the coastal plain and shelf. On the assumption that S was constant or was steadily decreasing, the analysis of field data obtained from three sites within the basin suggests that the initial water depth in the basin was around 0.45 km, and that the overall relative sea‐level rise (c. 0.80 km) happened largely during an early time period and was followed by a longer period of much lower rate of rise. This pattern of relative sea‐level rise is consistent with the Palaeogene tectonic subsidence trend of the basin which was determined independently through a geohistory analysis. The uncertainty of the field data does not negate our reconstruction. The combined effects of autoretreat and A/S changes on a deltaic shoreline trajectory are confirmed through the development of an autoretreat‐based methodology. Conventional sequence stratigraphic models that assume a possible equilibrium condition between A and S are both conceptually misleading and insufficient to analyse basin conditions quantitatively. Sequence stratigraphic analyses of shorelines need to incorporate the autoretreat concept.  相似文献   

Pinch-out style and position of tidally influenced strata in a regressive–transgressive wave-dominated deltaic sandbody, Twentymile Sandstone, Mesaverde Group, NW Colorado     

Lars Seidler  & Ron Steel 《Sedimentology》2001,48(2):399-414

The Upper Cretaceous Twentymile Sandstone of the Mesaverde Group in NW Colorado, USA, has been analysed with respect to its pinch‐out style and the stratigraphic position of tidally influenced facies within the sandstone tongue. Detailed sedimentological analysis has revealed that the Twentymile Sandstone as a whole is a deltaic shoreface sandstone tongue up to 50 m thick proximally. Facies change character vertically from very fine‐grained, storm wave‐dominated shelf sandstones and mudstones to fine‐grained, wave‐dominated sandstones and, finally, to fine‐ to coarse‐grained tidally dominated sandstones. The pinch‐out style is characterized by a basinward splitting of the massive proximal sandbody into seven coarsening‐upward fourth‐order sequences consisting of a lower shaly part and an upper sandy part (sandstone tongue). These are stacked overall to reflect the regressive‐to‐transgressive development of the tongue. Each of the lower sandstone tongues 1–3 are gradationally based, very fine‐grained and dominated by hummocky cross‐stratification and were deposited on the lower to upper shoreface. Sandstone tongues 4 and 5 prograded further basinwards than the underlying tongues, are erosively based, fine‐ to coarse‐grained and mainly hummocky, herringbone and trough cross‐stratified. Especially in tongue 5, tidal indicators, such as bipolar foresets and double mud drapes, are common. These tongues were deposited as upper shoreface and tidal channel sandstones respectively. Sandstone tongues 6 and 7 retrograded in relation to tongue 5, are very fine‐ to fine‐grained and hummocky cross‐stratified. These tongues were deposited in lower shoreface to offshore transition environments. The two lower fourth‐order sequences were deposited during normal regressions during slowly rising or stable relative sea level and represent the highstand systems tract. The three succeeding fourth‐order sequences, which show succeedingly increasing evidence of tidal influence, were deposited during falling and lowstand of relative sea level and represent the falling stage (forced regressive) and lowstand systems tracts. The uppermost two fourth‐order sequences were deposited during rapidly rising sea level in the transgressive systems tract. The maximum tidal influence occurred during lowstand progradation, in contrast to most other published examples reporting maximum tidal influence during transgression.  相似文献   

Clinoform growth in a Miocene,Para‐tethyan deep lake basin: thin topsets,irregular foresets and thick bottomsets          下载免费PDF全文

Rattanaporn Fongngern  Cornel Olariu  Ronald J. Steel  Csaba Krézsek 《Basin Research》2016,28(6):770-795

Late Miocene lacustrine clinoforms of up to 400 m high are mapped using a 1700 km² 3‐D seismic data set in the Dacian foreland basin, Romania. Eight Meotian clinoforms, constructed by sediment from the South Carpathians, prograded around 25 km towards southwest. The individual clinothems show thin (10–60 m thick), if any, topsets, disrupted foresets and highly aggradational bottomsets. Basin‐margin accretion occurred in three stages with changing of clinoform heights and foreset gradients. The deltaic system prograded into an early‐stage deep depocenter and contributed to high gradient clinoforms whose foresets were dominated by closely (100–200 m) spaced 1.5–2 km wide V‐shaped sub‐lacustrine canyons. During intermediate‐stage growth, 2–4 km wide canyons were dominant on the clinoform foresets. From the early to intermediate stages, the lacustrine shelf edges were consistently indented. The late‐stage outbuilding was characterised by smaller clinoforms with smoother foresets and less indentation along the shelf edge. Truncated and thin topsets persisted through all three stages of clinoform evolution. Nevertheless, the resulting long‐term flat trajectory shows alternating segments of forced and low‐amplitude normal regressions. The relatively flat trajectory implies a constant base level over time and was due to the presence of the Dacian–Black Sea barrier that limited water level rise by spilling to the Black Sea. Besides the characteristic shelf‐edge incision of the thin clinoform topsets and the resultant sediment bypass at the shelf edge, the prolonged regressions of the shelf margin promoted steady sediment supply to the basin. The high sediment supply at the shelf edges generated long‐lived slope sediment conduits that provided sustained sediment transport to the basin floor. Clinothem isochore maps show that large volumes of sediment were partitioned into the clinoform foresets, and especially the bottomsets. Sediment predominantly derived from frequent hyperpycnal flows contributed to very thick, ca. 300–400 m in total, bottomsets. Decreasing subsidence over time from the foredeep resulted in diminishing accommodation and clinoform height, reduced slope channelization and smoother slope morphology.  相似文献   

Fisheries policy and the EEC: the democratic influence     

David I.A. Steel 《Marine Policy》1984,8(4):350-353

This article gives a Parliamentary view of how fisheries policy in the European Community may develop now that Europe has a Common Fisheries Policy. The focus should now change from the tension of Council of Ministers meetings to the proper development of the European Commission's executive role and the effective monitoring of the bureaucracy by the democratically elected European Parliament.  相似文献   

Spatiotemporal dynamics of water sources in a mountain river basin inferred through δ2H and δ18O of water     

Lillian M. McGill  J. Renée Brooks  E. Ashley Steel 《水文研究》2021,35(3):e14063

In mountainous river basins of the Pacific Northwest, climate models predict that winter warming will result in increased precipitation falling as rain and decreased snowpack. A detailed understanding of the spatial and temporal dynamics of water sources across river networks will help illuminate climate change impacts on river flow regimes. Because the stable isotopic composition of precipitation varies geographically, variation in surface water isotope ratios indicates the volume-weighted integration of upstream source water. We measured the stable isotope ratios of surface water samples collected in the Snoqualmie River basin in western Washington over June and September 2017 and the 2018 water year. We used ordinary least squares regression and geostatistical Spatial Stream Network models to relate surface water isotope ratios to mean watershed elevation (MWE) across seasons. Geologic and discharge data was integrated with water isotopes to create a conceptual model of streamflow generation for the Snoqualmie River. We found that surface water stable isotope ratios were lowest in the spring and highest in the dry, Mediterranean summer, but related strongly to MWE throughout the year. Low isotope ratios in spring reflect the input of snowmelt into high elevation tributaries. High summer isotope ratios suggest that groundwater is sourced from low elevation areas and recharged by winter precipitation. Overall, our results suggest that baseflow in the Snoqualmie River may be relatively resilient to predicted warming and subsequent changes to snowpack in the Pacific Northwest.  相似文献   

Facies and architectural variability of sub-seismic slope-channel fills in prograding clinoforms,Mid-Jurassic Neuquén Basin,Argentina     

Yuqian Gan  Ronald J. Steel  Cornel Olariu  Flávio De Almeida Jr 《Basin Research》2020,32(2):348-362

Most slope-channel outcrop studies have been conducted at continental margin-scale on seismic data. However, in foreland and back-arc deepwater settings, sub-seismic scale slope channels hold equally important information on deepwater sediment delivery, often in hydrocarbon-bearing provinces. One such slope-channel system is examined in Lower Jurassic prograding shelf-margin clinoforms in Bey Malec Estancia, La Jardinera area, southern Neuquén Basin, Argentina. In a 4 km wide, 300 m tall, slightly oblique- to depositional-dip section of Jurassic Los Molles Formation deepwater slope deposits, seven clinoform timelines were identified by isolated slope-channel fills with thicknesses less than 50 m. Sedimentary logs, satellite images, a digital elevation model and drone photogrammetry were used to map variations in downslope channel geometry and infill facies. The slope channels are filled with sediment density flow deposits: poorly sorted conglomeratic debrites, structureless sandy high-density turbidites and well-sorted, fine-grained, graded low-density turbidites. The debrite portion decreases downslope, whereas high- and low-density turbidites increase. A grain-size analysis reveals a broad downslope fining trend of turbidite and debrite beds within slope channels with increasing water depth, and some notable bypass of conglomeratic facies to the lowermost slope channels and basin floor fans. The architecture of the slope channels changes from lateral to aggradational infill downstream. The Bey Malec clinoforms and its slope channels add new knowledge on downslope changes for sediment delivery in relatively shallow (<500 m water depth), prograding-dominant deepwater basins. They also highlight one of very few outcropping examples of oblique-type clinoforms.  相似文献   

Micromorphology of ice keel scour in pebbly sandy mud and fine‐grained sands: Scarborough Bluffs,Ontario, Canada          下载免费PDF全文

Lorna D. Linch  Jaap J. M. van der Meer 《Sedimentology》2015,62(1):110-129

The keels of icebergs and ice‐pressure ridges plough through unconsolidated sea/lake sediments gouging out long grooves known as ice keel scour marks. Although the surface and (more recently) subsurface morphology of scours are well‐documented, little is known of the effect of grain size on the detectability, style and intensity of sub‐scour deformation. This investigation macroscopically and microscopically (two‐dimensional thin sections) examines suspected ice keel scour in: (i) glaciolacustrine pebbly sandy mud and (ii) fine‐grained sands at Scarborough Bluffs, Ontario, Canada. In this investigation, there is an almost identical suite of deformation structures (individual structures and overprinted structural patterns) to those identified in iceberg‐scoured clays from former Glacial Lake Agassiz (Manitoba, Canada); this confirms that deformation in the pebbly sandy mud and fine‐grained sands at Scarborough Bluffs is likely to be indicative of ice keel scour. Discrete differences in the detectability, style and intensity of deformation between the Scarborough Bluffs and Glacial Lake Agassiz sediments are probably a function of grain size in response to ice keel scour. This research provides additional information on the types of structures that are associated with sediment deformation by processes of ice keel scour in a variety of grain sizes. This information is particularly valuable to inform palaeoenvironmental reconstruction and offshore engineering in areas where ice keel scour occurs in a variety of grain sizes. It also demonstrates the potential value of micromorphology where, for example, the study of cores is necessary.  相似文献   

The role of tidal,wave and river currents in the evolution of mixed‐energy deltas: Example from the Lajas Formation (Argentina)          下载免费PDF全文

Valentina Marzia Rossi  Ronald J. Steel 《Sedimentology》2016,63(4):824-864

Many modern deltas show complex morphologies and architectures related to the interplay of river, wave and tidal currents. However, methods for extracting the signature of the individual processes from the stratigraphic architecture are poorly developed. Through an analysis of facies, palaeocurrents and stratigraphic stacking patterns in the Jurassic Lajas Formation, this paper: (i) separates the signals of wave, tide and river currents; (ii) illustrates the result of strong tidal reworking in the distal reaches of deltaic systems; and (iii) discusses the implications of this reworking for the evolution of mixed‐energy systems and their reservoir heterogeneities. The Lajas Formation, a sand‐rich, shallow‐marine, mixed‐energy deltaic system in the Neuquén Basin of Argentina, previously defined as a tide‐dominated system, presents an exceptional example of process variability at different scales. Tidal signals are predominantly located in the delta front, the subaqueous platform and the distributary channel deposits. Tidal currents vigorously reworked the delta front during transgressions, producing intensely cross‐stratified, sheet‐like, sandstone units. In the subaqueous platform, described for the first time in an ancient outcrop example, the tidal reworking was confined within subtidal channels. The intensive tidal reworking in the distal reaches of the regressive delta front could not have been predicted from knowledge of the coeval proximal reaches of the regressive delta front. The wave signals occur mainly in the shelf or shoreface deposits. The fluvial signals increase in abundance proximally but are always mixed with the other processes. The Lajas system is an unusual clean‐water (i.e. very little mud is present in the system), sand‐rich deltaic system, very different from the majority of mud‐rich, modern tide‐influenced examples. The sand‐rich character is a combination of source proximity, syndepositional tectonic activity and strong tidal‐current reworking, which produced amalgamated sandstone bodies in the delta‐front area, and a final stratigraphic record very different from the simple coarsening‐upward trends of river‐dominated and wave‐dominated delta fronts.  相似文献   

Co-management of sea turtle fisheries: Biogeography versus geopolitics     

Lisa M. Campbell  Jennifer J. Silver  Noella J. Gray  Sue Ranger  Annette Broderick  Tatum Fisher  Matthew H. Godfrey  Shannon Gore  John Jeffers  Corrine Martin  Andrew McGowan  Peter Richardson  Carlos Sasso  Lorna Slade  Brendan Godley 《Marine Policy》2009

Co-management between local communities and government agencies is promoted as a strategy to improve fisheries management. This paper considers the potential for co-management of sea turtle fisheries within four UK Overseas Territories (OTs) in the Caribbean, and for co-ordinated management among those territories. We focus on fisher incentives for engaging in co-management and on the potential to scale up co-management to a regional level. This paper presents data from Anguilla, British Virgin Islands, Montserrat, and Turks and Caicos Islands, where 110 turtle fishers participated in a socio-economic survey undertaken as part of the ‘Turtles in the UK Overseas Territories in the Caribbean’ project. Based on three established criteria for co-management (perceived crisis in stock, willingness to participate and community cohesion), results suggest that fisher support for co-management exists within each OT, but the extent of support for and views of specific management interventions varies among OTs. The implications of results for co-management in each territory, and for establishing co-ordinated management regimes in the region, are discussed in the context of current debates about the nature of resources and scalar (mis)matches between resource and management regimes.  相似文献