British Virgin Islands Daria Jones Virgin Gorda, BVI James O’Brien

British Virgin IslandsDaria Jones

Virgin Gorda, BVIJames O’Brien

SEDIMENTATION AND SEDIMENTARY ROCKS

DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS

Analysis of sedimentary rock can provide clues to theirDEPOSITIONAL ENVIRONMENT.

Can then be used to interpret geologic history of a region.

Can includeContinental environmentsTransitional environmentsMarine environments


Continental EnvironmentsFound on landmasses.Most sediments are clastic.Include paleocurrent

direction or indicators.Plant and freshwater fossils

common.Includes rivers, lakes, caves,

deserts and glaciers.



Transitional EnvironmentsOccur 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.



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 claysand organic, calcareous or siliceous oozes.



Shallow Marine Environments



Deep Marine Environments




Deep Marine Environments


SEDIMENTARY FACIES

Adjacent sedimentary or depositional environmentsproduce 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.


SEDIMENTARY FACIES


SEDIMENTARY FACIES

Sedimentary facies move as environmental conditionschange.

Position of the beach will vary as position of sea levelchanges.Adjacent facies will stack up vertically over time.

Walther’s Law of Succession of Facies


SEDIMENTARY FACIES


DIFFERENCES IN CLIMATE AND WEATHERING

Arid RegionsSandstones and Limestones produce cliffs.Mudrocks make up slopes.

Humid RegionsSandstones produce cliffsMudrocks and limestones make up slopes



Slopes in Arid Regions



Slopes in Humid Regions

Kope Formation, OH Showangunks, NY

Maine Geological SurveyJuly 2002, Site of the Month

METAMORPHISM AND METAMORPHIC ROCKS

All rocks are susceptible to metamorphism.Change occurs in response to conditions affecting them.


METAMORPHIC ROCKS

Generally form at conditions between those that formigneous and sedimentary rocks.

METAMORPHISM is the process by which heat, pressure, and chemical reactions deep in the Earth alter the mineralcontent and/or structure of pre-existing rock without melting it.


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.


What Drives Metamorphism?

Rocks and minerals are most stable at the conditionsunder 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 weatheringWith metamorphism, clay are altered to become

micas with increasing heat and pressure.


Rocks remain solid during metamorphism.Minerals become unstable during metamorphism.

Bonds break in minerals allowing the creationof new minerals, stable at new conditions.

Not all the bonds break, that would be melting.

Circulation of ion-rich fluids enhances the movementwithin a rock undergoing metamorphism.The power of water strikes again.

Composition of the parent rock also influences the metamorphic rock that forms.


METAMORPHISM is the transformation of solidrock to form new minerals and textures.

Quartz will remain quartz, but size and shape may change.

Clays break down and elements and ions recombineto form new minerals.

With enough heat, atoms and ions can move within arock to form new minerals.

Composition of the rock may stay the same, but theminerals and texture may change.


FACTORS INFLUENCING METAMORPHISM

HEAT

Most important factor in driving metamorphism.Accelerates the pace of most chemical reactions.

Heat increases with depth in the EarthThermal Gradient

20-30C/kilometer depth74 F/mile depth

Rocks at depth are always at higher temperature.Rocks and minerals that form there are stable.



HEAT

Moving rocks from the surface to depth causes increasein heat and metamorphic potential.

Sources for Increase in Heat

Tectonic activity pushes rock from surface to depth.Sediments get buried.Contact with magma.



HEAT

Heat in the crust is produced by:Decay of radioactive isotopesUpward conduction of heat from the mantleFriction 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 withrising magma.

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British Virgin Islands Daria Jones Virgin Gorda, BVI James O’Brien