CHAPTER 6 SEDIMENTARY ROCKS

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 rocks

1) 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

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Round Fragments = CONGLOMERATE Angular Fragments = BRECCIA

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(pronounced Brechia)

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(“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 (SiO 2 ), calcite (CaCO 3 ), iron oxide (Fe 2 O 3 ), 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.

2) Organic

– Contains the cemented (lithified) remains of once living things – Sometimes contain fossils – Ex) coal

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

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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

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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

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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.