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
• 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.
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
• 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.