Continental slope sedimentation in the Sheepbed Formation (Neoproterozoic, Windermere Supergroup), Mackenzie Mountains, N.W.T.

1996 ◽  
Vol 33 (6) ◽  
pp. 848-862 ◽  
Author(s):  
R. W. Dalrymple ◽  
G. M. Narbonne
Keyword(s):  
Sea Level ◽  
Continental Slope ◽  
Cold Water ◽  
Passive Margin ◽  
Slope Instability ◽  
Fine Grained ◽  
Windermere Supergroup ◽  
Mackenzie Mountains ◽  
Water Depths

The Sheepbed Formation (Ediacaran) is a 1 km thick, siliciclastic unit that overlies glacial deposits of the Ice Brook Formation and is overlain by carbonates of the Gametrail Formation. Observations in the Mackenzie Mountains indicate that the Sheepbed Formation accumulated in water depths of 1–1.5 km on a passive-margin, continental slope. The lower part of the formation consists predominately of dark mudstone. Fine-grained, turbiditic sandstone becomes more abundant upward, as does the scale and abundance of slope-instability indicators. Mesoscale facies successions (i.e., evidence of channels, lobes, and (or) compensation cycles) are developed in the upper half of the formation. The larger-scale changes are interpreted as reflecting a postglacial sea-level rise, followed by a relative fall and an increase in the rate of deposition. Contourites that may have been formed in response to the circulation of deep, cold water occur in the lowstand deposits. Their presence confirms previous speculation that the proto-Pacific Ocean was initiated at the beginning of Windermere deposition (ca. 780 Ma), not at the start of the Cambrian. The paleoflow direction toward the present-day northwest suggests that this part of Laurentia lay in the northern hemisphere. In situ Ediacaran megafossils are preserved on the soles of sandy turbidites; the deep-water setting indicates that these organisms were not photoautotrophs.

scholarly journals Impact of sea-level fluctuations on the sedimentation patterns of the SE African margin: implications for slope instability

2019 ◽  
Vol 500 (1) ◽  
pp. 267-276 ◽  
Author(s):  
Aaron Micallef ◽  
Aggeliki Georgiopoulou ◽  
Andrew Green ◽  
Vittorio Maselli
Keyword(s):  
South Africa ◽  
Sea Level ◽  
Coastal Region ◽  
Passive Margin ◽  
Slope Instability ◽  
Bottom Current ◽  
The South ◽  
Sea Level Fluctuations ◽  
International Ocean Discovery Program ◽  
Seismic Reflection Profiles

AbstractThe sheared-passive margin offshore Durban (South Africa) is characterized by a narrow continental shelf and steep slope hosting numerous submarine canyons. Supply of sediment to the margin is predominantly terrigenous, dominated by discharge from several short but fast-flowing rivers. International Ocean Discovery Program Expedition 361 provides a unique opportunity to investigate the role of sea-level fluctuations on the sedimentation patterns and slope instability along the South African margin. We analysed >300 sediment samples and downcore variations in P-wave, magnetic susceptibility, bioturbation intensity and bulk density from site U1474, as well as regional seismic reflection profiles to: (1) document an increase in sand input since the Mid-Pliocene; (2) associate this change to a drop in sea-level and extension of subaerial drainage systems towards the shelf-edge; (3) demonstrate that slope instability has played a key role in the evolution of the South Africa margin facing the Natal Valley. Furthermore, we highlight how the widespread occurrence of failure events reflects the tectonic control on the morphology of the shelf and slope, as well as bottom-current scour and instability of fan complexes. This information is important to improve hazard assessment in a populated coastal region with growing offshore hydrocarbon activities.

Tribes Hill–Rochdale formations in east Laurentia: proxies for Early Ordovician (Tremadocian) eustasy on a tropical passive margin (New York and west Vermont)

2011 ◽  
Vol 149 (1) ◽  
pp. 93-123 ◽  
Author(s):  
ED LANDING ◽  
JONATHAN M. ADRAIN ◽  
STEPHEN R. WESTROP ◽  
BJÖRN KRÖGER
Keyword(s):  
New York ◽  
Sea Level ◽  
Continental Slope ◽  
Appalachian Mountains ◽  
Passive Margin ◽  
Depositional Sequence ◽  
Early Ordovician ◽  
Layer Cake ◽  
Low Diversity ◽  
Early Late

AbstractSlow subsidence and tectonic quiescence along the New York Promontory margin of Laurentia mean that the carbonate-dominated Tribes Hill and overlying Rochdale formations serve as proxies for the magnitude and timing of Tremadocian eustatic changes. Both formations are unconformity-bound, deepening–shoaling, depositional sequences that double in thickness from the craton into the parautochthonous, western Appalachian Mountains. A consistent, ‘layer cake’ succession of member-level units of the formations persists through this region. The Tribes Hill Formation (late early Tremadocian, late Skullrockian, late Fauna B–Rossodus manitouensis Chron) unconformably overlies the terminal Cambrian Little Falls Formation as the lowest Ordovician unit on the New York Promontory. It was deposited during the strong early Tremadocian, or Stonehenge, transgression that inundated Laurentia, brought dysoxic/anoxic (d/a) slope water onto the shelf and led to deposition of the Schaghticoke d/a interval (black mudstone and ‘ribbon limestone’) on the Laurentian continental slope. The uniform lithofacies succession of the Tribes Hill includes a lower sand-rich member; a middle, dark grey to black mudstone that records d/a in eastern exposures; and an upper, shoaling-up carbonate highstand facies. A widespread (12000+ km2) thrombolitic interval in the highstand carbonate suggests the New York Promontory was rimmed by thrombolites during deposition of the Tribes Hill. Offlap and erosion of the Tribes Hill was followed by the relatively feeble sea-level rise of the Rochdale transgression (new) in Laurentia, and deposition of the Rochdale Formation. The Rochdale transgression, correlated with the Kierograptus Drowning Interval in Baltica, marks a eustatic rise. The Rochdale Formation represents a short Early Ordovician interval (early late Tremadocian, middle–late Stairsian, Macerodus dianae Chron). It correlates with a depositional sequence that forms the middle Boat Harbour Formation in west Newfoundland and with the Rte 299 d/a interval on the east Laurentian slope. The Rochdale has a lower carbonate with abundant quartz silt (Comstock Member, new) and an upper, thrombolitic (Hawk Member, new) high-stand facies. Tribes Hill and Rochdale faunas are mollusc-rich, generally trilobite-poor, and have low diversity, Laurentian faunal province conodonts. Ulrichodina rutnika Landing n. sp. is rare in Rochdale conodont assemblages. Trilobites are also low in diversity, but locally form coquinas in the middle Tribes Hill. The poorly preserved Rochdale trilobites include the bathyurid Randaynia, at least two hystricurid species and Leiostegium.

Surficial sediments of the western Gulf of Carpentaria, Australia

1987 ◽  
Vol 38 (1) ◽  
pp. 151 ◽  
Author(s):  
MR Jones
Keyword(s):  
Sea Level ◽  
Fluvial Sediments ◽  
Nearshore Zone ◽  
Sediment Sources ◽  
Surficial Sediments ◽  
Fine Grained ◽  
Marginal Areas ◽  
Waves And Tides ◽  
Water Depths ◽  
Low Rate

Modern sedimentation in the Gulf of Carpentaria is confined principally to marginal areas in water depths of less than 50 m. Within this zone, the distribution of sediments is irregular, being controlled by proximity to sediment sources and degree of exposure to waves and tidal activity. This is particularly so in the western gulf, where fluvial sediments supplied to Limmen Bight have been transported by waves and tides northwards beyond that embayment to accumulate in the sheltered environment of Blue Mud Bay. Limmen Bight is exposed to the south-easterlies, which produce sufficient wave action in the nearshore zone to prevent the widespread deposition of fine-grained terrigenous sediments. As a result, relict sand deposits occupy much of the floor of Limmen Bight in areas shallower than about 25 m. In deeper areas, terrigenous muds are deposited at a low rate over relict fluvial and marine sediments. The relict deposits accumulated in continental and nearshore marine environments during low-sea-level periods of the late Pleistocene.

Continental-slope instability triggered by seepage: An experimental approach

2020 ◽  
Vol 90 (8) ◽  
pp. 921-937
Author(s):  
Carolina H. Boffo ◽  
Tiago A. de Oliveira ◽  
Daniel Bayer da Silva ◽  
Rafael Manica ◽  
Ana Luiza de O. Borges
Keyword(s):  
Mass Transport ◽  
Sea Level ◽  
Continental Slope ◽  
Physical Simulation ◽  
Slope Instability ◽  
Shear Stresses ◽  
Relative Sea Level ◽  
Hypothetical Scenario ◽  
Experimental Apparatus ◽  
Continental Slopes

ABSTRACT Mass-transport complexes (MTCs), mass-transport deposits (MTDs), and associated facies and features are widely recognized in continental slopes around the world. In most current stratigraphic models of MTCs and MTDs, these submarine sediment failures are related to aquifer outflow (sapping, seepage) along continental slope fronts that originated during relative sea-level fall. We test a hypothetical scenario that is favored during early forced regression using reduced-scale physical simulation. A major underground subaerial hydraulic gradient is assumed to flow towards the basin depocenter as a function of relative sea-level fall. We developed an experimental apparatus with slope angles varying between 15 and 30° to test this concept. Hydraulic gradients, aquifer outflow velocities, and triggered collapses induced by the seepage effect were recorded at various positions of the slope. Analysis shows that steeper slope gradients require lower seepage velocities (and shear stresses) to trigger collapse, but gentler slopes remain unchanged. Experimental data are compatible with a seepage effect that could potentially trigger mass failure and the formation of MTCs during relative sea-level fall. The features produced in the experiment have geometries comparable to natural environments, and the experimental seepage velocities are of an order of magnitude similar to those monitored in submarine aquifers. The experimental results advance understanding of mass transport in continental slopes by introducing and testing new methods, and also provide new insights into potential submarine geohazard risks where tectonic uplift operates along some coastal regions.

scholarly journals Coherent sea-level fluctuations along the global continental slope

2006 ◽  
Vol 364 (1841) ◽  
pp. 885-901 ◽  
Author(s):  
Chris W Hughes ◽  
Michael P Meredith
Keyword(s):  
Sea Level ◽  
Continental Slope ◽  
Large Scale ◽  
The Arctic ◽  
Strongly Correlated ◽  
Long Distance ◽  
Tropical Regions ◽  
Sea Level Fluctuations ◽  
Wave Modes

Signals in sea-level or, more properly, sub-surface pressure (SSP; sea-level corrected for the inverse barometer effect) are expected to propagate rapidly along the continental slope due to the effect of sloping topography on wave modes, resulting in strongly correlated SSP over long-distances. Observations of such correlations around the Arctic and Antarctic are briefly reviewed, and then extended using satellite altimetry to the rest of the global continental slope. It is shown that such long-distance correlations are common, especially in extra-tropical regions. Simple correlations from altimetry cannot, however, establish the wave speed, or whether waves are responsible for the correlations as opposed to large-scale coherence in the forcing. A case study around South America is used to highlight some of the complications, and is found to strengthen the case for the importance of wave modes in such long-distance SSP coherence, although more detailed in situ data are required to resolve the cause of the correlations.

In situ acoustic properties of fine-grained sediments on the northern continental slope of the South China Sea

2020 ◽  
Vol 218 ◽  
pp. 108244
Author(s):  
Guanbao Li ◽  
Jingqiang Wang ◽  
Baohua Liu ◽  
Xiangmei Meng ◽  
Guangming Kan ◽  
...  
Keyword(s):  
South China Sea ◽  
Continental Slope ◽  
South China ◽  
The South China Sea ◽  
Acoustic Properties ◽  
The South ◽  
China Sea ◽  
Fine Grained

scholarly journals Provenance of the Incipient Passive Margin of NW Laurentia (Neoproterozoic): Detrital Zircon from Continental Slope and Basin Floor Deposits of the Windermere Supergroup, Southern Canadian Cordillera

Lithosphere ◽  
2021 ◽  
Vol 2021 (1) ◽  
Author(s):  
Thomas Hadlari ◽  
R. W. C. Arnott ◽  
W. A. Matthews ◽  
T. P. Poulton ◽  
K. Root ◽  
...  
Keyword(s):  
Continental Slope ◽  
Continental Margin ◽  
Detrital Zircon ◽  
Drainage System ◽  
Source Area ◽  
Passive Margin ◽  
Canadian Cordillera ◽  
The North ◽  
Windermere Supergroup ◽  
Slope Deposits

Abstract The origin of the passive margin forming the paleo-Pacific western edge of the ancestral North American continent (Laurentia) constrains the breakup of Rodinia and sets the stage for the Phanerozoic evolution of Laurentia. The Windermere Supergroup in the southern Canadian Cordillera records rift-to-drift sedimentation in the form of a prograding continental margin deposited between ~730 and 570 Ma. New U-Pb detrital zircon analysis from samples of the post-rift deposits shows that the ultimate source area was the shield of NW Laurentia and the near uniformity of age spectra are consistent with a stable continental drainage system. No western sediment source area was detected. Detrital zircon from postrift continental slope deposits are a proxy for ca. 676-656 Ma igneous activity in the Windermere basin, likely related to continental breakup, and set a maximum depositional age for slope deposits on the eastern side of the basin at 652±9 Ma. These results are consistent with previous interpretations. The St. Mary-Moyie fault zone near the Canada-U.S. border was most likely a major transform boundary separating a rifted continental margin to the north from intracratonic rift basins to the south, resolving north-south variations along western Laurentia in the late Neoproterozoic at approximately 650-600 Ma. For Rodinia reconstructions, the conjugate margin to the southern Canadian Cordillera would have a record of rifting between ~730 and 650 Ma followed by passive margin sedimentation.

scholarly journals From gravity cores to overpressure history: the importance of measured sediment physical properties in hydrogeological models

2020 ◽  
Vol 500 (1) ◽  
pp. 289-300 ◽  
Author(s):  
Davide Mencaroni ◽  
Jaume Llopart ◽  
Roger Urgeles ◽  
Sara Lafuerza ◽  
Eulàlia Gràcia ◽  
...  
Keyword(s):  
Gulf Of Mexico ◽  
Physical Properties ◽  
Slope Instability ◽  
Continental Margins ◽  
Submarine Slope ◽  
Fine Grained ◽  
Depositional Settings ◽  
Direct Measurements ◽  
Facies Characterization

AbstractThe development of overpressure in continental margins is typically evaluated with hydrogeological models. Such approaches are used to both identify fluid flow patterns and to evaluate the development of high pore pressures within layers with particular physical properties that may promote slope instability. In some instances, these models are defined with sediment properties based on facies characterization and proxy values of porosity; permeability or compressibility are derived from the existing literature as direct measurements are rarely available. This study uses finite-element models to quantify the differences in computed overpressure generated by fine-grained hemipelagic sediments from the Gulf of Cadiz, offshore Martinique and the Gulf of Mexico, and their consequences in terms of submarine slope stability. By comparing our simulation results with in situ pore pressure data measured in the Gulf of Mexico, we demonstrate that physical properties measured on volcanic-influenced hemipelagic sediments underestimate the computed stability of a submarine slope. Physical properties measured on sediments from the study area are key to improving the reliability and accuracy of overpressure models, and when that information is unavailable, literature data from samples with similar lithologies, composition and depositional settings enable better assessment of the overpressure role as a pre-conditioning factor in submarine landslide initiation.

How Artefacts Do Leadership: A Ventriloquial Analysis

2021 ◽  
pp. 089331892199807
Author(s):  
Jonathan Clifton ◽  
Fernando Fachin ◽  
François Cooren
Keyword(s):  
Organizational Communication ◽  
Recent Work ◽  
Distributed Leadership ◽  
Network Theory ◽  
Actor Network Theory ◽  
Fine Grained ◽  
Naturally Occurring ◽  
Human Actors

To date there has been little work that uses fine-grained interactional analyses of the in situ doing of leadership to make visible the role of non-human as well as human actants in this process. Using transcripts of naturally-occurring interaction as data, this study seeks to show how leadership is co-achieved by artefacts as an in-situ accomplishment. To do this we situate this study within recent work on distributed leadership and argue that it is not only distributed across human actors, but also across networks that include both human and non-human actors. Taking a discursive approach to leadership, we draw on Actor Network Theory and adopt a ventriloquial approach to sociomateriality as inspired by the Montreal School of organizational communication. Findings indicate that artefacts “do” leadership when a hybrid presence is made relevant to the interaction and when this presence provides authoritative grounds for influencing others to achieve the group’s goals.

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