Staff of the Meteorological Branch stationed on the island were not only extremely helpful but also made our stay particularly pleasant. Medioli of the Department of Geology, Dalhousie University, was a member of the expedition and participated in the collection of samples and in mapping. Since Sable Island provides an opportunity to conduct a controlled study of the interaction of wind and water on an isolated sand body, criteria valuable in distinguishing adjacent depositional environments, especially beaches and dunes, are investigated.
The central 'core' of the island consists of sand dunes that extend continuously for 27 kilometers from east to west. Dunes are on average 6 to 15 meters high, with steep dune slopes, free of vegetation, with sea views on both sides of the island. Dune slopes, covered with sparse vegetation, slope gently towards the center of the island, forming a sheltered hollow.
The dune ridge south of the lake is almost destroyed, resulting in a large beach flat. A lens or wedge of fresh, brackish water underlies most of the island. Numerous freshwater ponds occur in the central part of the island where the water table lies close to the surface.
Texture.—Size analysis of 138 samples was made with a slightly modified version of the Woods Hole Rapid Sediment Analyzer (Schlee, 1966).
SABLE ISLAND
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Samples weighing between 5 and 10 grams were used and the settling time converted to equivalent size <£.
RELICT SANDS ON SABLE ISLAND PALEOSOL
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Note the dark linear patches of heavy mineral concentrations, aligned subparallel to the dune and beach, resulting from winds blowing to the northeast.
SANDS ABOVE AND BELOW THE PALEOSOL
INTERPRETATION
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OPAQUE H
7 SEDIMENT TRANSPORT, SABLEISLAND JAMES & STANLEY II .. and Dryden, 1946) indicate that reworking of paleosands has destroyed or removed the unstable species and in the process made. Iron-stained grains of palaeosand can be used as one of the mineralogical tracers to indicate dominant directions of sediment transport.
SEDIMENT DISTRIBUTION ON SABLE ISLAND GENERAL
LATERAL TEXTURAL DISTRIBUTION
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SABLE ISLAND SEDIMENTARY FACIES
INTERPRETATION OF TEXTURAL DISTRIBUTION The entire northern portion of the island contains relatively coarse
LEGEND
LATERAL MINERALOGICAL DISTRIBUTION
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HEAVY MINERALS
Rock fragments.—Lithic fragments are concentrated in areas rich in pebbles and shell fragments, suggesting that the lithic grains were formed in part by abrasion of pebbles. The highest concentrations of heavy substances occur at the bottom of the dunes along the beaches, especially on the southern edge of the island, east of the meteorological station. In any case, this is not a true reflection of the overall heavy mineral assemblage, but rather garnet and magnetite, which account for about 70 percent of the heaviest mineral fractions.
Garnet, the most productive of the heavy substances, comprises 48 to 75 percent of the non-opaque fraction. This relatively resistant, dense and large mineral species (sp. gr.4.0) probably accumulates in the southern and eastern margins of the island in areas where there is continuous movement of the coarser, well-sorted sediment. The distribution of magnetite and zircon, both dense, resistant minerals, follows that of garnet, indicating that they also accumulate in the same areas as salag.
This is the result of differences in specific gravity, shape and inherent size of the three mineral species. Tourmaline (fig. 11), the lightest of the group (sp. gr. 3.1), is concentrated on the northern beach and dunes east of Lake Wallace. The distribution of rutile is irregular and does not comprise more than 2 percent of the transparent weight.
Hornblende, hypersthene, and kyanite, slightly lighter and less stable minerals, are grouped together because they show nearly identical distributions (Fig. 11). They are mainly concentrated on the northern part of the island, where they are most common. The rest of the heavy mineral types, including staurolite, andalusite, epidote and alterite, are irregularly distributed but have a slight tendency to concentrate on the northern part of the island.
RELATION OF MINERALOGY TO GRAIN SIZE
ROCK FRAGMENTS
The southwestern part of the island consists of clean sand with concentrations of heavy sand in the form of lag deposits. The northeastern margin is rich in iron-stained quartz and less dense heavy particles, indicating that sediment has been transported to this area. In summary, the sediment on the island proper appears to be moving from southwest to northeast.
The break at 1000 microns in the rock frequency distribution curve indicates that this size range. The reduction of rock fragments in smaller-sized grains is due to the relatively greater movement of smaller grains and because lithic grains are more easily abraded than quartz grains of the same size. The reduction of iron-stained quartz in the finer sand fraction is also due to the fact that the smaller grains undergo relatively greater movement, resulting in the removal of their hematite coating.
DISTINGUISHING BETWEEN BEACH AND DUNE SANDS GENERAL
A break at 1000 microns in the frequency distribution curve of rock fragments indicates this range in magnitude. is the grain saltation threshold for wind speeds on Sable Island. SEDIMENT TRANSPORT, SABLE ISLAND JAMES & STANLEY IO,. 1964), however, used Folk and Ward parameters calculated from both sieves and settling tubes and found that skew yielded inconclusive results.
TEXTURE
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MINERALOGY
MEAN GRAIN SIZE
SORTING
SKEWNESS
KURTOSIS
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ENVIRONMENTAL FACTORS AFFECTING MORPHOLOGY AND SEDIMENT TRANSPORT
METEOROLOGY
ORIGIN AND MAINTENANCE OF DUNES
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EFFECT OF SEASONAL WINDS
ROLE OF VEGETATION
SEDIMENT TRANSPORT AND EVOLUTION OF SABLE ISLAND
RECENT CHANGES IN ISLAND SHAPE
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PROPERTIES OF SURFACE WAVES
SUMMER
Previous assessments of the movement and wasting of the island have been based on observations taken over a relatively short period of time. In the last 200 years, the island appears to have lost 9 miles on the west side and gained 11 miles on the east side.
NEARSHORE SEDIMENT MOVEMENT
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SABLE ISLAND MAPS
SEDIMENT MOVEMENT
Our study suggests that Sable Island is not destined to be eroded and disappear, as indicated in most previous works. Storms cause significant changes in the island's periphery and morphology, and changes in the length, shape and orientation of the terminal bars are particularly notable. However, changes in the total length of the island are only temporary, as the cyclical movement of sand causes both accretion and retreat of the coastline.
Sable Island has narrowed somewhat over the past three centuries, but has maintained its approximate length, arc, and geographic position on the Sable Island Bank during this period.
SUMMARY
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