SEDIMENTARY ROCKS

CHAPTER 6

Materials that make up sediments

• Weathered Rocks

• Organic Material

• Mineral Fragments

Ways sedimentary rocks can form

• Compaction and cementation of sediments

• Evaporation of a solution

• Precipitate from a solution

Kinds of sedimentary rocks1) Clastic/Detrital

– Composed of fragments and small pieces of rock (clasts)

– sometimes show layering– Make up more than 85% of all sedimentary

rocks– Ex) sandstone, siltstone, shale

Fragments

• Round Fragments = CONGLOMERATE

• Angular Fragments = BRECCIA– (pronounced Brechia) – (“cc” is “ch” like Gucci)

Types of Clasts

• The formation of clastic rocks begins with the movement and relocation of fragments.

• The majority of these are moved by running water.

• Larger pebbles and gravels are often the first to be dropped and settle in shallow water near the shore.

• Next to settle are the smaller sands.• Finally, in calm water, the silts and clays.

Formation of Clastic Rocks

• Loose sediments become solid rock when sediments become cemented.

• Ocean water, lake water, and ground water all contain natural cements in the form of dissolved mineral.– Silica (SiO2), calcite (CaCO3), iron oxide (Fe2O3), and

clay minerals.

• When minerals fill in the spaces between sand grains, pebbles, or other rock particles, they bind the fragments together through cementation.

• The type of cement influences the rock’s color.

– Contains the cemented (lithified) remains of once living things

– Sometimes contain fossils– Ex) coal

2) Organic

3) Chemical

• Form by precipitation or evaporation of a solution

• Often contain crystals or appear “dried up”

• Ex) rock salt

4) Bioclastic

• Composed of broken shell fragments and similar remains of living organisms

• Ex) limestone

Features of Sedimentary Rocks• Stratification

– The arrangement of visible layers.– The most characteristic feature of sedimentary rocks.– Results from the change in the type of sediment being

laid down in one place.

• Fossils– The remains, impression, or any other evidence of a

plant or animal preserved in rock.– Occurs when a dead organism is buried by sediment

that gradually turns into rock.– The soft parts of the organism decay; the hard parts

become rock.

Stratification and Fossils

Features of Sedimentary Rocks

• Ripple Marks– Common feature of sedimentary rocks.– Sand patterns formed by the action of winds,

streams, waves, or currents.– Ripple marks are generally preserved in sandstone.

• Mud Cracks– Common feature of sedimentary rocks.– Develop when deposits of wet clay dry and contract.– The cracks are filled in with different solutions and

fossilize.– Generally form in shale.

Ripple Marks and Mud Cracks

Features of Sedimentary Rocks• Nodules

– Hard lumps of fine-grained silica– Found in limestone and chalk.

• Concretions– Round solid masses of calcium carbonate.– Found in shale.– Both Nodules and Concretions form when

minerals in a solution precipitate around a fragment in the clay sediment.

Nodules and Concretions

Geodes– Spheres of silica rock.– Generally found in limestones.– Groundwater creates cavities in limestone and

minerals in the groundwater concentrates in the cavities to form crystals.