Sedimentary Rocks Notes

Ch 7 Goals

• Learn what a sedimentary rock is

• Learn the different types

• Learn how we identify them

• Learn the various structures created by sed rocks and what it tells us

• Learn the different depositional environments

Sedimentary Rocks Are a Thin Cover

• Sedimentary rocks form a thin layer or cover over the Earth's basement rocks (igneous and metamorphic rocks).

What is a Sedimentary Rock?

• A sedimentary rock forms at or near the surface by one of the following processes:

  • Loose clasts cementing together

  • The growth of mounds of shells

  • Accumulation of organic matter

  • Precipitation of minerals from surface-water solutions

• Occur ONLY in the upper crust

• They form a cover that “buries” underlying igneous and metamorphic rocks (aka basement rocks)

• Most importantly – they tell the story of what is happening at Earth’s surface

• Also contain mineral deposits

Different Types of Sedimentary Rocks

• Clastic

• Biochemical

• Organic

• Chemical

1 - Clastic Sed Rocks

• Clastic sedimentary rocks are created by several processes operating at Earth’s surface:

  • Weathering: disintegration of bedrock into separate grains

  • Erosion: removal of grains

  • Transportation: dispersal of solid particles and ions by gravity, wind, water, and ice

  • Deposition: settling out of the transporting fluid

  • Lithification: transformation into solid rock

Creating Clastic Sedimentary Rocks

• Weathering

• Erosion

• Deposition

Lithification

• Two part process:

  • Step 1: Compaction

    • Air or water between sediment grains is squeezed out due to the weight of the overburden; grains fit together more tightly.

  • Step 2: Cementation

    • Minerals precipitate from groundwater and fill spaces between clasts.

    • This forms a cement that binds grains together.

    • Minerals are usually quartz and calcite

Classifying Clastic Sed Rocks

• Clast size – what is the diameter of the grains?

• Clast composition – rock fragments or individual minerals?

• Angularity & sphericity – smooth or rough corners?

• Sorting – are the clasts roughly the same size?

• Character of cement – different cements result from different minerals

Classifying Clastic Sed Rocks

• Points to understand:

  • Grain size is a measure of the size of fragments or grains. Size ranges from very coarse to very fine

    • gravel

    • sand

    • silt

    • clay

  • As transport distance increases, grain size decreases.

  • Grain composition refers to the mineral makeup of sediment grains.

Classifying Clastic Sed Rocks

• Mineral composition yields clues about the original source rock. A variety of different clast compositions (or a lack thereof) hints at source area and transport processes.

• Clasts may be comprised of individual mineral grains or rock fragments containing several mineral types. Smaller particles are more likely to be composed of single mineral grains.

Classifying Clastic Sedimentary Rocks: Maturity

• Feldspar weathers to clay; clay gets washed away.

• Lithic clasts break into individual grains.

Clastic Sedimentary Rocks: Breccia

• Breccia—coarse, angular rock fragments

• Angularity indicates the absence of rounding by transport

• These are deposited relatively close to clast source

Clastic Sedimentary Rocks: Conglomerate

• Conglomerate—rounded rock clasts

• Clasts are rounded as flowing water wears off corners and edges

• These are deposited farther from the source than breccia.

Clastic Sedimentary Rocks: Arkose

• Arkose—sand and gravel with abundant feldspar

• Commonly deposited in alluvial fans

• Feldspar indicates short transport and arid conditions.

Clastic Sedimentary Rocks: Sandstone

• Sandstone—clastic rock made of sand-sized particles

• Quartz is the most common mineral in sandstones

Clastic Sedimentary Rocks: Shale and Mudstone

• Fine clastics are composed of silt and clay

• Silt-sized sediments are lithified to form siltstone

• Clay-sized particles form mudstone or shale

Clastic Sedimentary Rocks: Diamictite and Wacke

• Diamictites are large clasts in a muddy matrix, and wacke is sand and rock fragments in a muddy matrix.

2 – Biochemical Sed Rocks

• Organisms in marine environments extract dissolved ions from seawater to build their shells

  • When they die, the shell remains

  • This material can lithify and become biochemical sedimentary rock

Biochemical Sedimentary Rocks: Limestone

• Limestone is a sedimentary rock made almost entirely of calcite or aragonite.

• These minerals are the most common materials used by organisms that make seashells.

• Limestone often preserves the shells of fossil organisms, sometimes in great abundance

Biochemical Sedimentary Rocks: Limestone

• Biochemical limestone forms in a unique depositional environment: warm, tropical, shallow, clear, $O_2$-rich, marine water.

• The $CaCO_3$ in limestone comes from shells from a diversity of organisms (plankton, corals, clams, snails, etc.).

• There are several textural varieties of biochemical limestone, including fossiliferous limestone, micrite, and chalk.

Biochemical Sedimentary Rocks: Chert

• Chert: rock made of cryptocrystalline quartz derived from opalline silica ($SiO_2$) from the skeletons of some marine plankton

• Chert can occur in beds or as nodules

• It has all of the properties of quartz, including hardness and conchoidal fracture

3 - Organic Sed Rocks: Coal and Oil Shale

• Organic sedimentary rocks deal with the “soft parts” left behind from living organisms

  • If well preserved and quickly buried, it can become coal and oil shale

4) Chemical Sedimentary Rocks: Evaporites

• These are sedimentary rocks that formed by the precipitation of minerals from water solutions

  • These have a crystalline texture from their original precipitation

  • May grow new crystals during a process called recrystallization

Chemical Sedimentary Rocks: Evaporites

• Evaporites are derived from evaporation of large volumes of sea or lake water, and evaporite minerals include halite (rock salt) and gypsum.

Bedding and Stratification

• Rocks

Formation of Bedding

• Over time layers of silt, gravel, and then silt are deposited which form bedding planes. These turn into siltstone, conglomerate and siltstone respectively.

Stratigraphic Formations

• The surface between two units is called a contact.

Ripple Marks and Dunes

• Water flowing over loose sediment creates bedforms, which directly reflect flow velocity and grain size. Bedforms in ancient sediments are useful indicators of environmental conditions.

• Ripple marks are cm-scale ridges and troughs that develop perpendicular to flow in sandy sediments.

• Dunes are larger scale (50 cm to over 100 m) versions of ripple marks.

Cross Bedding

• Cross beds are created by ripple and dune migration.

• Sand moves up the gentle side and piles up at the crest.

• Then, it slips down the steep face.

• The slip face moves downcurrent and is buried by the next avalanche of sand.

• The slip faces are preserved as cross beds.

Turbidity Currents

• Turbidity currents are formed in deep basins that receive periodic pulses of turbid water.

• Such pulses might result from an earthquake shock loosening sediment on a slope.

• As pulse wanes, water loses velocity and grains settle.

• The coarsest material settles first, medium next, then fine.

• This process forms graded beds (coarse to fine upward).

Depositional Environments

• Continental

  • Glacial

  • Mountain stream

  • Alluvial fan

  • Desert

  • River

  • Lake

• Coastal

  • Delta

  • Swamp

  • Beach

  • Estuary

  • Reef

• Marine

  • Continental shelf

  • Turbidity current

  • Submarine fan

  • Deep-sea current

Glacial Environments

• In glacial environments, sediments are created, transported, and deposited by the actions of moving glacial ice.

• Ice carries and dumps every grain size.

• A common feature of this environment is glacial till, a poorly sorted mixture of all grain sizes, gravel, sand, silt, and clay.

Mountain Stream Environments

• In mountain stream environments, water carries large clasts during floods.

• During low-flow conditions, cobbles and boulders are immobile.

• Coarse conglomerate is a characteristic of this setting.

Desert Environments

• Sand-dune environments develop where there is an abundance of wind-blown, well- sorted sand.

• Dunes move according to the prevailing winds and result in uniform sandstones with gigantic cross beds.

River Environments

• River environments preserve evidence of channelized sediment transport.

• Sand and gravel fill concave-up channels that often scour into previously deposited floodplain fines.

• Fine sand, silt, and clay are deposited on nearby floodplains.

Lake Environments

• Lake environments result from large ponded bodies of freshwater.

• Gravels and sands are trapped near shore.

• Well-sorted muds are deposited in deeper water.

• They are often capped with wetland muds.

• Deep lake muds may show varves, thin stripes of alternating finer and coarser sediment reflecting seasonal changes in sedimentation.

Marine Delta Environments

• Delta face

• Fluvial sand and silt

• Shallow-marine mud and silt

• Silt, interbedded with mudflows and turbidites

Shallow-Marine Clastic Environments

• Shallow- marine clastic deposits are comprised of fine sands and silts that accumulate in quieter waters offshore.

• The sea floor in these settings supports active biotic communities.

Shallow-Water Carbonate Environments

• Shallow-water carbonate environments develop in tropical, warm,
  clear, shallow, normal salinity, marine water.

Deep-Marine Environments

• Deep-marine deposits accumulate fines that settle out far from land.

• The skeletons of planktonic organisms make chalk or chert; fine silt and clay lithifies into shale.