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 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. DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS

SEDIMENTATION AND SEDIMENTARY ROCKS 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. DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS

SEDIMENTATION AND SEDIMENTARY ROCKS 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. DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS

SEDIMENTATION AND SEDIMENTARY ROCKS Shallow Marine Environments DEPOSITIONAL or SEDIMENTARY ENVIRONMENTS

SEDIMENTATION AND SEDIMENTARY ROCKS Deep Marine Environments DEPOSITIONAL or SEDIMENTARY 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 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. Walther’s Law of Succession of Facies

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, OHShowangunks, 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 pre- existing 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  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.