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SEDIMENTARY ROCK Section 6.3

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Objectives Explain the processes of compaction and

cementation. Describe how chemical and organic

sedimentary rocks form. Describe how clastic sedimentary rock

forms. Identify seven sedimentary rock features.

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Sediment Loose fragments of

rock, minerals, and organic material that result from natural processes.

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Sedimentary Rock Characteristics

Determined by the source of the sediment, the way the sediment was moved, and depositional conditions.

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2 Main Processes Convert SedimentsInto Sedimentary Rock

Compaction

Cementation

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Compaction The weight of

overlying sediments causes pressure, squeezing the sediment, reducing the size of the pore (empty) space between sediment grains.

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Cementation

Water carrying dissolved minerals passes through the sediments and then forms small crystals between the rock fragments to hold the fragments together.

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Three Main Classes of Sedimentary Rocks

ChemicalOrganicClasticBased upon how the rocks form and

their composition.Each class is further subdivided based

on the shape, size, and composition of the sediments that form the rocks.

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Chemical Sedimentary Rocks

Rocks that form from minerals that were once dissolved in water.

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Precipitation (Settling)

Some minerals settle out of water as a result of a change in temperature. Some chemical limestones form when cool ocean currents lower the temperature of ocean water and calcite (calcium carbonate; CaCO3) precipitates and eventually solidifies on the ocean floor.

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An example: The Redwall Limestone of the Grand Canyon area

Great thicknesses of limestone can result. The topmost cliff is the Redwall Limestone. It averages 500 feet in thickness and is about 335 million years old.

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Evaporites

Dissolved minerals can be left behind when water evaporates. These minerals then form rocks called evaporites. Gypsum and halite (rock salt) are two examples.

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Organic Sedimentary Rocks Rock formed from the remains

of living things.

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LimestoneOrganic limestones formed from shells made up

of calcite from clams, oysters, snails, corals, and plankton.

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CoalFormed from

plants that are buried and compacted into matter that is mostly carbon.

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Chalk

A type of limestone made up of the shells of tiny, one-celled marine organisms that then formed a mud on the bottom of an ancient sea.White Cliffs of Dover

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Clastic Sedimentary Rocks

Made up of accumulations of rock fragments carried away from their source by gravity, water, wind, and/or ice.

Clastic sedimentary rocks are classified according to the kind and size of sediments that form them.

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Size Classifications

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ConglomerateFormed from rounded rock fragments

greater than 2 mm, held together by a matrix of sand, clay, and cement.

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Breccia (BRECH ee uh) Like a

conglomerate but formed from fragments which are angular and have sharp corners.

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SandstoneCoarse-grained

rock formed from sand 2 mm to 1/16 mm in diameter.

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ShaleFine-grained rock, some small rock

particles may be seen Usually clays, shows thin layering.

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MudstoneMade from mud, rock particles

too small to see, thick layers

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Clastic sedimentary rock formations can reach great thicknesses.

Navaho Sandstone, Utah

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Characteristics of Clastic Sediments

Determined by the way sediments were transported.

Four main agents: water, wind, ice, and gravity.

Both the distance the sediment is moved and the agent that moves the sediment determine the characteristics.

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Sorting

The tendency for air or water to separate sediments according to size.

Due to the change in the speed of the agent that is moving the sediment.

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Angularity

As sediment particles are transported they collide with each other and objects in their path.

In general, the farther sediments travel the finer (smaller) and smoother the particles of sediment become.

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Sedimentary Rock Features

These allow scientists to identify the depositional environment, the setting in which sediment is deposited.

Includes rivers, deltas, beaches, and oceans.

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Stratification Occurs when there is a change in the kind of

sediment being deposited. The layers, or beds, vary in thickness depending on how long each type of sediment was being laid down. Most beds are horizontal.

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Cross-beds Slanting layers that

form within beds. Usually form in sand dunes or river beds.

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Graded Bedding Different sizes and

shapes of sediment settle out in different levels in a bed (largest and most rounded on the bottom to smallest on top).

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Reverse Bedding Happens in

mudflows, in which the smallest grains are on the bottom and the largest grains are on top.

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Ripple Marks Formed from the

action of wind or water on sand or mud.

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Mud Cracks Formed when

muddy deposits dry and shrink to form cracks. Later sediments may fill in the cracks and preserve them in sedimentary rock.

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FossilsThe remains (hard

parts) or traces (impressions) of ancient plants and animals.

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ConcretionsLumps, or

nodules, of rock with a composition different from that of the main rock body.

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Geodes

Dissolved minerals can crystallize inside cavities (holes) in sedimentary rocks (igneous also).

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Assignment – Due Friday

Directed Reading 6.3 Key Terms Ternary Diagram &

Mineral Composition