Sedimentary Rocks

Chapter 3 Section 3

SEDIMENTARY ROCKS

• Sedimentary sedimentum Latin for ‘settling’• Rock formed from compressed or cemented layers

(DEPOSITS) of sediment.• Sometimes fossils accumulate and are

compacted/cemented together. • Contain fragments of older rocks from weathering.

Canyonlands Nat. Park, Utah

SEDIMENTARY FORMATION WITHIN ROCK CYCLE

Formation of Sedimentary Rocks1. Weathering, Erosion, and Deposition• Weathering= breaks rocks into sediments; first step in formation of sedimentary rocks (mechanical and chemical)• Erosion= water, wind, ice, gravity move weathered sediments• Deposition= when agents of erosion (listed above) lose energy sediments stop moving deposition

• Deposited according to size—largest first

2. Compaction and Cementation• After sediments deposited become lithified (turned to rock)

• Compaction= squeezes, or compacts, sediments; caused by weight of sediments

• Cementation= dissolved minerals are deposited in the tiny spaces among the sediments

Sandstone grains are cemented

Conglomerate rounded pebbles cemented together

Classifying Sedimentary Rocks• Classified into two main groups according to the

way they form:1. Clastic: made of weathered bits of rocks and

minerals• Most common minerals found in clastic = clay

(most abundant product) of chemical weathering and quartz (durable and resistant to chemical weathering• Grouped according to size of sediments:• Conglomerate = larger pebbles• Sandstone = sand sized grains• Shale = most common sed. rock; fine grained

Breccia: quartz; clastic

Classifying Sedimentary Rocks2. Chemical/Biochemical: form when dissolved minerals

precipitate (deposit in solid form) from water solutions• Precipitation generally occurs when H2O evaporates or boils off

leaving solid product.• Limestone = calcite and aragonite; clear, shallow marine waters accumulation of shell coral, algal and fecal debris or CaCO3 from water• Coquina=type of limestone biochemical – sediments of shells and skeletal remains settle

• Rock salt = halite (mineral form of NaCl); evaporation• Rock Gypsum = evaporation in shallow sea basins or salt lakes gypsum precipitates

Coquina

Rock Salt

Features

• Unique features of sedimentary rocks that are clues to how, when, and where they formed.• Layers law of superposition• Ripple marks: beach or stream bed• Fossils

Classification of Major Sedimentary

Rocks