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British Virgin Islands
Daria Jones
Virgin Gorda, BVI
James O’Brien
SEDIMENTATION AND SEDIMENTARY ROCKS
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
SEDIMENTATION AND SEDIMENTARY ROCKS
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.
SEDIMENTATION AND SEDIMENTARY ROCKS
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.
SEDIMENTATION AND SEDIMENTARY ROCKS
DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Marine Environments
Entirely oceanic environments.
Most sediments are clastic, some are organic.
Vary according to depth of water.
Shallow marine < 200 m.
Narrow band along continent.
Sandstone, mudstone, limestone
Deep marine - offshore.
Fine-grained wind blown clays
and organic, calcareous or siliceous oozes.
SEDIMENTATION AND SEDIMENTARY ROCKS
DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Shallow Marine Environments
SEDIMENTATION AND SEDIMENTARY ROCKS
DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Deep Marine Environments
SEDIMENTATION AND SEDIMENTARY ROCKS
DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS
Deep Marine Environments
SEDIMENTATION AND SEDIMENTARY ROCKS
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.
SEDIMENTATION AND SEDIMENTARY ROCKS
SEDIMENTARY FACIES
SEDIMENTATION AND SEDIMENTARY ROCKS
SEDIMENTARY FACIES
Walther’s 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.
SEDIMENTATION AND SEDIMENTARY ROCKS
SEDIMENTARY FACIES
SEDIMENTATION AND SEDIMENTARY ROCKS
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
SEDIMENTATION AND SEDIMENTARY ROCKS
DIFFERENCES IN CLIMATE AND WEATHERING
Slopes in Arid Regions
SEDIMENTATION AND SEDIMENTARY ROCKS
DIFFERENCES IN CLIMATE AND WEATHERING
Slopes in Humid Regions
Kope Formation, OH
Showangunks, NY
Maine Geological Survey
July 2002, Site of the Month
METAMORPHISM AND METAMORPHIC ROCKS
All rocks are susceptible to metamorphism.
Change occurs in response to conditions affecting them.
METAMORPHISM AND METAMORPHIC ROCKS
METAMORPHIC ROCKS
Generally form at conditions
between those that form
igneous and sedimentary
rocks.
METAMORPHISM is the
process by which heat, pressure,
and chemical reactions deep in
the Earth alter the mineral
content and/or structure of preexisting rock without melting it.
METAMORPHISM AND METAMORPHIC ROCKS
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.
METAMORPHISM AND METAMORPHIC ROCKS
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.
METAMORPHISM AND METAMORPHIC ROCKS
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.
METAMORPHISM AND METAMORPHIC ROCKS
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.
METAMORPHISM AND METAMORPHIC ROCKS
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.
METAMORPHISM AND METAMORPHIC ROCKS
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.
METAMORPHISM AND METAMORPHIC ROCKS
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.