scholarly journals Marine Geology of the Turnagain Area

2021 ◽  
Author(s):  
◽  
Keith Brian Lewis
Keyword(s):  
Continental Shelf ◽  
Sea Level ◽  
Continental Slope ◽  
Late Quaternary ◽  
Outer Part ◽  
Shelf Break ◽  
Seismic Profiles ◽  
Sediment Samples ◽  
Near Shore ◽  
Upper Slope

<p>The Turnagain Area covers the continental shelf and slope off the east coast of North Island, New Zealand between Napier and Castlepoint. Its late Quaternary stratigraphy, tectonic history, sedimentation and foraminiferal distribution are described with the aid of continuous seismic profiles, sediment samples and cores. Results are presented in seven papers and a chart. The first three papers deal mainly with sub-bottom layers revealed by continuous seismic profiles; the next three papers describe dried sediment samples and cores and the last paper is a study of foraminifera in alcohol-preserved sediment samples. The topics discussed in each of the seven papers are as follows: 1. stratigraphy, sedimentation rates and origin of present topography on the continental shelf and upper slope; 2. rates of tectonic processes; 3. slumping; 4. distribution of sediments; 5. ages of indurated sediments; 6. ash horizons and rates of deposition on the lower part of the continental slope. 7. the distribution of living and dead foraminifera. The chart shows bathymetry and nature of sediment at the seabed. The sediments beneath the sea have been folding since Miocene times in the same way as marine sediments on the adjacent land. On the seabed anticlinal crests are preserved as ridges and banks and synclines form depressions. The present land area is rising and much of the seabed is sinking; the zero isobase between then is situated on the inner continental shelf. It has been at about the same position throughout Late Quaternary times, being always close to the dividing line between net erosion and net deposition. Rates of tilting have ranged from 2 to 36 microdegrees/thousand years and rates of vertical movement from +1.7 to -1.5 m/thousand years. Seaward of the zero isobase the continental shelf and upper slope has been built upwards and outwards by prisms of sediment, each prism representing a phase either of low sea level or of high sea level. Prisms deposited during periods of glacially lowered sea level are at their thickest beneath the upper slope; prisms deposited during periods of relatively high sea level are at their thickest beneath the continental shelf. Parts of the youngest prism on the upper slope have slumped on gradients as low as 1 [degree]. The topography and sediments formed during the last 20 thousand years have received the most attention. The present continental shelf if a composite feature. The inner part has been formed by wave-planation of hard rock near shore and deposition of the latest prism of sediment offshore. The outer part and the shelf break were formed by wave-planation and by deposition during the last low sea level about 20 thousand years ago. At that time the shelf break ranged in depth from about 40m to about 70m, being shallowest where eroded into soft sediment and deepest where deposited beyond the seaward edge of erosion. In adjacent areas the shelf break was probably formed at depths of less than 20m being eroded into hard rock. The inner part of the wave-planed surface formed at that time is now deeply buried by the latest prism of sediment but the outer part is covered by only a thin veneer. The outer shelf is still essentially a drowned low sea level feature. At the thickest part of the prism on the mid continental shelf, rates of deposition above an 8 thousand year old seismic reflector range from about 1 to about 4 m/thousand years, being most rapid south of major rivers. Rates are too slow to be measured at some places near the shelf break and at ridges on the continental slope. In depressions on the continental slope, sedimentation rates are indicated by the depth of the 3.4 thousand year old Waimihia ash and range from 0.36 m/thousand years in a depression relatively near land to 0.02 m/thousand years in the depression furthest from land. Sediments range from fine sand near shore to clayey fine silt on the lower slope. Many sediments are bimodal because they were deposited as a mixture of floculated and unfloculated grains. Rapidly deposited sediment on the continental shelf is predominantly detrital sand and silt; slowly deposited sediment near the shelf break and on ridges consists mostly of volcanic ash, foraminifera, and glauconite Muddy sediment in continental slope depressions contains sandy turbidite layers. Different environments are characterised by sediment types and foraminiferal faunas that can be matched in Tertiary Rocks.</p>

scholarly journals Marine Geology of the Turnagain Area

2021 ◽  
Author(s):  
◽  
Keith Brian Lewis
Keyword(s):  
Continental Shelf ◽  
Sea Level ◽  
Continental Slope ◽  
Late Quaternary ◽  
Outer Part ◽  
Shelf Break ◽  
Seismic Profiles ◽  
Sediment Samples ◽  
Near Shore ◽  
Upper Slope

<p>The Turnagain Area covers the continental shelf and slope off the east coast of North Island, New Zealand between Napier and Castlepoint. Its late Quaternary stratigraphy, tectonic history, sedimentation and foraminiferal distribution are described with the aid of continuous seismic profiles, sediment samples and cores. Results are presented in seven papers and a chart. The first three papers deal mainly with sub-bottom layers revealed by continuous seismic profiles; the next three papers describe dried sediment samples and cores and the last paper is a study of foraminifera in alcohol-preserved sediment samples. The topics discussed in each of the seven papers are as follows: 1. stratigraphy, sedimentation rates and origin of present topography on the continental shelf and upper slope; 2. rates of tectonic processes; 3. slumping; 4. distribution of sediments; 5. ages of indurated sediments; 6. ash horizons and rates of deposition on the lower part of the continental slope. 7. the distribution of living and dead foraminifera. The chart shows bathymetry and nature of sediment at the seabed. The sediments beneath the sea have been folding since Miocene times in the same way as marine sediments on the adjacent land. On the seabed anticlinal crests are preserved as ridges and banks and synclines form depressions. The present land area is rising and much of the seabed is sinking; the zero isobase between then is situated on the inner continental shelf. It has been at about the same position throughout Late Quaternary times, being always close to the dividing line between net erosion and net deposition. Rates of tilting have ranged from 2 to 36 microdegrees/thousand years and rates of vertical movement from +1.7 to -1.5 m/thousand years. Seaward of the zero isobase the continental shelf and upper slope has been built upwards and outwards by prisms of sediment, each prism representing a phase either of low sea level or of high sea level. Prisms deposited during periods of glacially lowered sea level are at their thickest beneath the upper slope; prisms deposited during periods of relatively high sea level are at their thickest beneath the continental shelf. Parts of the youngest prism on the upper slope have slumped on gradients as low as 1 [degree]. The topography and sediments formed during the last 20 thousand years have received the most attention. The present continental shelf if a composite feature. The inner part has been formed by wave-planation of hard rock near shore and deposition of the latest prism of sediment offshore. The outer part and the shelf break were formed by wave-planation and by deposition during the last low sea level about 20 thousand years ago. At that time the shelf break ranged in depth from about 40m to about 70m, being shallowest where eroded into soft sediment and deepest where deposited beyond the seaward edge of erosion. In adjacent areas the shelf break was probably formed at depths of less than 20m being eroded into hard rock. The inner part of the wave-planed surface formed at that time is now deeply buried by the latest prism of sediment but the outer part is covered by only a thin veneer. The outer shelf is still essentially a drowned low sea level feature. At the thickest part of the prism on the mid continental shelf, rates of deposition above an 8 thousand year old seismic reflector range from about 1 to about 4 m/thousand years, being most rapid south of major rivers. Rates are too slow to be measured at some places near the shelf break and at ridges on the continental slope. In depressions on the continental slope, sedimentation rates are indicated by the depth of the 3.4 thousand year old Waimihia ash and range from 0.36 m/thousand years in a depression relatively near land to 0.02 m/thousand years in the depression furthest from land. Sediments range from fine sand near shore to clayey fine silt on the lower slope. Many sediments are bimodal because they were deposited as a mixture of floculated and unfloculated grains. Rapidly deposited sediment on the continental shelf is predominantly detrital sand and silt; slowly deposited sediment near the shelf break and on ridges consists mostly of volcanic ash, foraminifera, and glauconite Muddy sediment in continental slope depressions contains sandy turbidite layers. Different environments are characterised by sediment types and foraminiferal faunas that can be matched in Tertiary Rocks.</p>

scholarly journals Changed Late Quaternary Marine Environments on Atlantic Continental Shelf and Upper Slope

1988 ◽  
Vol 30 (3) ◽  
pp. 251-269 ◽  
Author(s):  
K.O. Emery ◽  
A.S. Merrill ◽  
E.R.M. Druffel
Keyword(s):  
Continental Shelf ◽  
Late Quaternary ◽  
Atlantic Coast ◽  
The United States ◽  
Sea Levels ◽  
Sediment Samples ◽  
Sediment Types ◽  
Continental Shelves ◽  
Environmental Histories ◽  
Upper Slope

About 2000 large sediment samples were collected during the early 1960s throughout the continental shelf off the Atlantic coast of the United States to establish and map sediment types including sediments relict from times of glacially low (and subsequently higher) sea levels. In about 510 of these samples we found fossil shells of mollusks remaining from environmental conditions different from those at present. Publications and collections by others contain about 70 additional samples having relict mollusks. Some of these shells indicate lower sea levels, others colder water, and still others warmer water than is now present. Radiocarbon measurements from earlier studies by us and others established the dates of colder water (late Pleistocene), and we made additional measurements to learn the dates of warmer water (about 1000 to 2000 yr B.P.). The results show reasonably enough that continental shelves are the sites of relict faunas as well as of sediments that indicate changed and complex environmental histories.

scholarly journals Stability of a buoyancy-driven coastal current at the shelf break

2002 ◽  
Vol 452 ◽  
pp. 97-121 ◽  
Author(s):  
C. CENEDESE ◽  
P. F. LINDEN
Keyword(s):  
Continental Shelf ◽  
Continental Slope ◽  
Unstable Mode ◽  
Shelf Break ◽  
Coastal Current ◽  
Surface Currents ◽  
Flat Bottom ◽  
Vertical Boundary ◽  
Axisymmetric Disturbances ◽  
A Current

Buoyancy-driven surface currents were generated in the laboratory by releasing buoyant fluid from a source adjacent to a vertical boundary in a rotating container. Different bottom topographies that simulate both a continental slope and a continental ridge were introduced in the container. The topography modified the flow in comparison with the at bottom case where the current grew in width and depth until it became unstable once to non-axisymmetric disturbances. However, when topography was introduced a second instability of the buoyancy-driven current was observed. The most important parameter describing the flow is the ratio of continental shelf width W to the width L* of the current at the onset of the instability. The values of L* for the first instability, and L*−W for the second instability were not influenced by the topography and were 2–6 times the Rossby radius. Thus, the parameter describing the flow can be expressed as the ratio of the width of the continental shelf to the Rossby radius. When this ratio is larger than 2–6 the second instability was observed on the current front. A continental ridge allowed the disturbance to grow to larger amplitude with formation of eddies and fronts, while a gentle continental slope reduced the growth rate and amplitude of the most unstable mode, when compared to the continental ridge topography. When present, eddies did not separate from the main current, and remained near the shelf break. On the other hand, for the largest values of the Rossby radius the first instability was suppressed and the flow was observed to remain stable. A small but significant variation was found in the wavelength of the first instability, which was smaller for a current over topography than over a flat bottom.

Late Quaternary Evolution of the Northern Aegean Shelf

1989 ◽  
Vol 32 (1) ◽  
pp. 36-50 ◽  
Author(s):  
C. Perissoratis ◽  
D. Mitropoulos
Keyword(s):  
Sea Level ◽  
Late Quaternary ◽  
Shelf Area ◽  
Seismic Profiles ◽  
Fine Grained ◽  
Nestos River ◽  
River Bed ◽  
Ephemeral Lakes ◽  
Evros River ◽  
River Mouths

AbstractA dense network of 3.5 kHz and Uniboom seismic profiles revealed the geological evolution of the Ierissos-Alexandroupolis Shelf area of the northern Aegean during the latest sea-level rise. Near the end of the Pleistocene, ca. 14,000 yr B.P., the sea was at about − 120 m, and almost 5300 km2 of shelf was exposed to subaerial erosion. Two permanet lakes existed in the Ierissos and Kavalla Gulfs, as well as a number of ephemeral lakes scattered throughout the rest of the area. Kavalla Gulf was drained by the Nestos River which joined the Strymon River at the outer Strymonikos Plateau. On the flat Samothraki Plateau were dune fields, marshes, and a number of minor seasonal rivers, while the Evros River flowed east of the plateau. By ca. 13,000 yr B.P. the sea had risen to − 70 m and covered 30% of the previously exposed shelf. The lakes at Ierissos and Strymonikos Gulfs were shallower and the sea approached to within about 5 km of them. Kavalla Gulf and the adjacent eastern Strymonikos Plateau lay 10 m above sea level, while the sea intruded along river mouths at the Samothraki Plateau. During this transgression of the sea river-bed gravels and sands were covered by silts and clays. At the Pleistocene-Holocene transition, ca. 10,500 yr B.P., the sea lay at about − 50 m and in areas of steep slope the coast was close to its present position. At Kavalla Gulf, the sea advanced along the paleochannel of the Nestos River, and the island of Samothraki was separated from the mainland. By ca. 7500 yr B.P. the sea was only 15 m below its present level and the northeastern Aegean shelf assumed nearly its present morphology. The Nestos River changed course to the east and Thassos Island was separated from the mainland. Coarse sediment formed wedges nearshore, whereas fine-grained sediments were distributed widely by current action. In many areas, relict sediments are present.

scholarly journals Ocean processes at the Antarctic continental slope

2014 ◽  
Vol 372 (2019) ◽  
pp. 20130047 ◽  
Author(s):  
Karen J. Heywood ◽  
Sunke Schmidtko ◽  
Céline Heuzé ◽  
Jan Kaiser ◽  
Timothy D. Jickells ◽  
...  
Keyword(s):  
Southern Ocean ◽  
Continental Shelf ◽  
Continental Slope ◽  
Climate Models ◽  
Water Mass ◽  
Ocean Model ◽  
Shelf Break ◽  
Small Scale ◽  
Exchange Processes ◽  
The Antarctic

The Antarctic continental shelves and slopes occupy relatively small areas, but, nevertheless, are important for global climate, biogeochemical cycling and ecosystem functioning. Processes of water mass transformation through sea ice formation/melting and ocean–atmosphere interaction are key to the formation of deep and bottom waters as well as determining the heat flux beneath ice shelves. Climate models, however, struggle to capture these physical processes and are unable to reproduce water mass properties of the region. Dynamics at the continental slope are key for correctly modelling climate, yet their small spatial scale presents challenges both for ocean modelling and for observational studies. Cross-slope exchange processes are also vital for the flux of nutrients such as iron from the continental shelf into the mixed layer of the Southern Ocean. An iron-cycling model embedded in an eddy-permitting ocean model reveals the importance of sedimentary iron in fertilizing parts of the Southern Ocean. Ocean gliders play a key role in improving our ability to observe and understand these small-scale processes at the continental shelf break. The Gliders: Excellent New Tools for Observing the Ocean (GENTOO) project deployed three Seagliders for up to two months in early 2012 to sample the water to the east of the Antarctic Peninsula in unprecedented temporal and spatial detail. The glider data resolve small-scale exchange processes across the shelf-break front (the Antarctic Slope Front) and the front's biogeochemical signature. GENTOO demonstrated the capability of ocean gliders to play a key role in a future multi-disciplinary Southern Ocean observing system.

scholarly journals The East Australian Current, upwellings and downwellings off eastern-most Australia in summer

2017 ◽  
Vol 68 (7) ◽  
pp. 1208 ◽  
Author(s):  
G. R. Cresswell ◽  
J. L. Peterson ◽  
L. F. Pender
Keyword(s):  
Continental Shelf ◽  
Continental Slope ◽  
Wind Stress ◽  
Satellite Images ◽  
East Australian Current ◽  
Inner Shelf ◽  
Per Se ◽  
Flowing Stream ◽  
Inner Continental Shelf ◽  
Upper Slope

The complex influences of the East Australian Current (EAC) and winds on the waters of the continental shelf were addressed with a ship survey, moored and drifting instruments, satellite images and wind and sea level measurements. The study revealed intrusions of continental slope water reaching the inner continental shelf when the EAC was near the shelf edge and wind stress was near zero or upwelling favourable (northerly). The process was the onshore movement of a southward flowing stream of water originally from the continental slope. One event was captured near Cape Byron and Evans Head when these waters upwelled to the surface. When the wind stress turned northward, it reversed the inner shelf current and drove downwelling. Variations in the wind stress also modulated the strength of the EAC out across the shelf to the upper slope. The strength of the EAC per se varied with a time scale of 2–3 months; these variations decreased in amplitude westward until they were undetectable at the inner shelf. The EAC had a subsurface speed maximum of up to 1.6ms–1 at 100–150-m depth above the continental slope and was seen to accelerate with both time and distance southward along the 190-km length surveyed by the ship.

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