British Virgin Islands

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British Virgin Islands Daria Jones
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Virgin Gorda, BVI James OBrien
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DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Analysis of sedimentary rock can provide clues to
their DEPOSITIONAL ENVIRONMENT. Can then be used
to interpret geologic history of a region. Can
include Continental environments Transitional
environments Marine environments
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DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Continental Environments
Found on landmasses. Most sediments are
clastic. Include paleocurrent direction or
indicators. Plant and freshwater
fossils common. Includes rivers, lakes,
caves, deserts and glaciers.
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DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Transitional Environments
Occur at boundary between ocean and land. Some
sediments are clastic, some are
organic. Influenced by tides, currents, and
breaking waves. Includes estuaries, deltas,
beaches and lagoons.
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DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Marine Environments
Entirely oceanic environments. Most sediments are
clastic, some are organic. Vary according to
depth of water. Shallow marine lt 200 m. Narrow
band along continent. Sandstone, mudstone,
limestone Deep marine - offshore. Fine-grained
wind blown clays and organic, calcareous or
siliceous oozes.
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DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Shallow Marine Environments
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DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Deep Marine Environments
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DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Deep Marine Environments
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SEDIMENTARY FACIES
Adjacent sedimentary or depositional
environments produce different sediment and rock
types. While different, they are deposited at the
same time. Represents a horizontal continuum. The
horizontal changes demonstrate the variability of
the adjacent environments. Vertical changes
reflect how environments have changed over time.
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SEDIMENTARY FACIES
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SEDIMENTARY FACIES
Walthers Law of Succession of Facies
Sedimentary facies move as environmental
conditions change. Position of the beach will
vary as position of sea level changes. Adjacent
facies will stack up vertically over time.
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SEDIMENTARY FACIES
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DIFFERENCES IN CLIMATE AND WEATHERING
Arid Regions Sandstones and Limestones produce
cliffs. Mudrocks make up slopes. Humid
Regions Sandstones produce cliffs Mudrocks and
limestones make up slopes
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DIFFERENCES IN CLIMATE AND WEATHERING
Slopes in Arid Regions
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DIFFERENCES IN CLIMATE AND WEATHERING
Slopes in Humid Regions
Kope Formation, OH
Showangunks, NY
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Maine Geological Survey July 2002, Site of the
Month
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All rocks are susceptible to metamorphism. Change
occurs in response to conditions affecting them.
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METAMORPHIC ROCKS
Generally form at conditions between those that
form igneous and sedimentary rocks. METAMORPHI
SM is the process by which heat, pressure, and
chemical reactions deep in the Earth alter the
mineral content and/or structure of pre- existing
rock without melting it.
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Most are buried beneath layers of sedimentary
rocks. Processes take place deep in the
Earth. Brought to the surface by tectonics and
erosion. Most of what we know about metamorphism
and metamorphic rocks comes from lab experiments.
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What Drives Metamorphism?
Rocks and minerals are most stable at the
conditions under which they form. Movement of
rocks causes instabilities to exist. New
minerals and rocks are formed in an attempt to
become stable. Increase the amount of change,
metamorphism occurs. Produce clays by
weathering With metamorphism, clay are altered
to become micas with increasing heat and
pressure.
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Rocks remain solid during metamorphism. Minerals
become unstable during metamorphism. Bonds break
in minerals allowing the creation of new
minerals, stable at new conditions. Not all the
bonds break, that would be melting. Circulation
of ion-rich fluids enhances the movement within
a rock undergoing metamorphism. The power of
water strikes again. Composition of the parent
rock also influences the metamorphic rock that
forms.
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METAMORPHISM is the transformation of solid rock
to form new minerals and textures. Quartz will
remain quartz, but size and shape may
change. Clays break down and elements and ions
recombine to form new minerals. With enough
heat, atoms and ions can move within a rock to
form new minerals. Composition of the rock may
stay the same, but the minerals and texture may
change.
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FACTORS INFLUENCING METAMORPHISM
HEAT
Most important factor in driving
metamorphism. Accelerates the pace of most
chemical reactions. Heat increases with depth in
the Earth Thermal Gradient 20-30?C/kilometer
depth 74 ?F/mile depth Rocks at depth are
always at higher temperature. Rocks and minerals
that form there are stable.
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FACTORS INFLUENCING METAMORPHISM
HEAT
Moving rocks from the surface to depth causes
increase in heat and metamorphic
potential. Sources for Increase in
Heat Tectonic activity pushes rock from surface
to depth. Sediments get buried. Contact with
magma.
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FACTORS INFLUENCING METAMORPHISM
HEAT
Heat in the crust is produced by Decay of
radioactive isotopes Upward conduction of heat
from the mantle Friction from subduction Heat
necessary for metamorphism is not attained
until a depth of 10 kilometers (6 miles). To
occur at the surface it requires contact
with rising magma.