Sedimentary Rocks

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Identifying depositional environments

Rock type, sedimentary structures, fossils

Rock type

Grain size, roundness, etc.

Sedimentary structures

Arrangements of grains in 3D

Fossils

Remains of past life

What environment might create a conglomerate?

Something that has strong enough energy to smooth the rock but will slow to allow deposition

Wind, waves

Sandstones

2-0.06mm

Forms in many environments: beach, river, lake, desert

Formation clues: sorting, particle, shape, composition

Siltstones

0.004-0.006

Lakes, lagoons, deltas. All low energy, calmer environments relatively far along tributaries (more toward the end)

Mudstone (<0.004mm but not compacted)

Tidal flats, lakes, deep sea

Shale (<0.004mm but not compacted)

Same environment as mudstones, but in high sediment areas

Compaction comes from weight of overlying sediments

Breaks into thin, parallel layers

Most common sedimentary rock (~70%)

Most common environment, most common rock (deep sea)

Red coloration in rock

Indicate presence of iron oxides

Black coloration in rock

Indicated low oxygen levels, because organic matter breaks down in high O2

Mud cracks

Indicate drying after deposition, need environment that gets wet and dries out & needs to be low energy (tidal flats and seasonal lakes for example)

Ripple marks

Shallow water, 2 types A-symmetrical and symmetrical (waves create this with back and forth motion)

Graded bedding

Usually signal a decrease in transport energy (rate of flow) as time passes. Everything that is large settles quickly because water can’t hold it, everything that is small settles slowly

Cross Bedding

Layering within a stratum and at an angle to the main bedding plane

Tells us the direction of wind/water movement

Air or water

Weaker forces (wind) make bigger cross beds

Water always forms small cross beds because it pushes sediment to its death quickly

Chemical sedimentary rocks

Inorganic and Biochemical

Inorganic sedimentary rocks

Formed when minerals precipitate out of an aqueous solution

Biochemical sedimentary rocks

Formed when an organism extracts the dissolved material out of water (then die and are buried)

What is critical for forming chemical sedimentary rocks

Chemical weathering (dissolution) turns limestone into soluble carbonate ions

Water rich with carbonate ions becomes supersaturated

Ions precipitate to a solid slate as a carbonate mineral —> formation of limestone

Evaporites

A decrease of the solvent — water

Usually form in lakes (occasionally and always on the edges), marshes, and lagoons

Salts and sulfates precipitated out of evaporating water (halites, gypsum, carbonates)

BIFs

Sadly no longer being created, Fe ions were oxidized, account for 50% of global ion reserves, form in the deep sea and occasionally continental shelves.

Limestones

Caves, shallow marine, lakes

Cherts

Most often biochemical but also precipitation. Color most often comes from trace minerals (except black).

Coquina

A bunch of sharp shells, not much travel distance— basically shelly breccia

Tidal zones, beaches, barrier islands

Limestones

Corals, bivalves, other echinoderms make their bodies out of CaCO3

When they die they are buries and lithified

Coral reef

Chemical sediment produced by living things, form in nutrient rich water, shallow areas with sunlight, and warm temperatures/tropical waters