Sedimentary Rocks

Sediments and Sedimentary Rocks

ï Clastic:

ñ Composed of particles of pre-existing rocks

ï Chemical/Biochemical:

ñ Precipitated from water, sometimes with the aid of marine organisms

General Process for Creating Sedimentary Rocks

ï Weathering (physical & chemical)

ï Erosion:Ý removal of weathered products from source rocks

ï Transportation: particles carried to new locations via wind, water or ice

ï Deposition (sedimentation): particles settle out of transporting agent to form layers of sediment on land or in water

ï Compaction (diagenesis): sediments buried & compacted due to weight of overlying sediments accumulating above

ï Lithification (diagenesis): Buried sediments cemented together to form rocks

Currents as Agents of Transport

ï Transportation of sediments accomplished by currents of water or wind

ï Currents can modify the transported sediment three ways:

ñ Mineralogy (compositional evolution)

ñ Grain sizes

ñ Grain shapes

Grain sorting

Changes in the Size and Shape of Grains During Transport

Sedimentary Environments

Nonmarine (Terrestrial) Environments

Transitional Environments

Deltaic Environments

Marine Environments

Bathyl and Abyssal Environments

Sedimentary Structures

Cross Bedding

Bedded material are inclined at angles due to deposition by wind or water

Grains Deposited on Steeper, Down-current Slopes of Sand Dunes or Sand Bars

Graded Bedding

Grains in a sedimentary layer range in size from coarsest at the bottom to finest at the top

Graded Beds

ï Fine grained at top

ï Coarse grained at bottom

Graded Bedding Reflects a Waning Current That Deposited the Sediment

Ripples

Small ridges of sand or silt deposited by wind or water currents

Ripples Can Be Preserved in Sedimentary Rocks

Bioturbation Structures

Relicts of burrows or tunnels produced by burrowing organisms in the soft sediment

Bioturbation Structures Preserved in Sedimentary Rocks

Bedding Sequences

Patterns in the interbedding of sandstone, shale and other sedimentary rocks indicative of sedimentary environment

Diagenesis

Physical and chemical changes occurring in buried sediments during alteration and lithification (conversion to sedimentary rock)

Changes in Sediment During Burial

ï Gradual compaction

ñ Grains squeezed closer together

ñ Pore spaces between grains start to close

ñ Fluids within pore spaces expelled

ñ Precipitation of cementing materials (quartz, calcite, hematite, etc.)

ï The process of converting sediments into rocks is termed lithification

Types of Sandstone

Chemical & Biochemical Sediments and Rocks

Chemical and Biochemical Sediments

The dissolved products of weathering are eventually precipitated from water (usually seawater) by chemical and biochemical reactions

Chemical Sediments

ï Chemical sediments may include minerals like:

ñ Calcite

ñ Dolomite

ñ Gypsum

ñ Halite

ñ Quartz

Biochemical Sediments

ï Remains of organisms and/or minerals precipitated as a result of biological processes:

ñ Usually composed of calcium carbonate (calcite/aragonite)

ñ Silica can also be precipitated by certain marine organisms

Bioclastic Particles

ï Composed of fragments or detritus of biologically precipitated calcium carbonate

ï Three stage process

ñ Formation of calcium carbonate through biological activity

ñ Break-up of the calcium carbonate

ñ Transport and deposition of particles to another location

ï Bioclastic particles may consist of:

ñ Shells

ñ Shell fragments

ñ Detritus of biochemically precipitated carbonate rocks

ï Bioclastic sediments can then be cemented together by precipitated calcite to form limestone

Fossiliferous Limestone

Coquina

Coral Reef

Distribution of Modern Reefs

Inorganic Calcium Carbonate

ï Calcium carbonate can also be precipitated directly from water without the aid of marine organisms

ï Forms when water becomes supersaturated with dissolved calcium and bicarbonate ions

ÝÝÝÝÝÝÝÝÝ Ca²⁺ + 2HCO₃^- = CaCO₃ + H₂CO₃

Dolostone

ï A Ca-Mg carbonate rock consisting of the mineral dolomite

ï Generally does not form as a primary precipitate from seawater

ï Ca in carbonate sediments and limestone partly replaced by Mg ions from water passing slowly through pores of sediment

ÝÝÝÝÝÝ 2CaCO₃ + Mg²⁺ = CaMg(CO₃)₂ + Ca²⁺

Conversion to Dolomite: Supratidal Flats

Halite (Rock Salt)

Bedded Gypsum

Siliceous Sediments and Rocks

ï Composed primarily of SiO₂

ï May precipitate in the deep sea with the aid of organisms to form opal

ï Also forms through alteration of rocks

ñ Chert is composed of microcrystalline quartz

ñ Chalcedony a fibrous variety of microcrystalline quartz

ñ Agates form in rock cavities due to precipitation of silica from fluids

Coal, Oil and Gas

ï Formed by the chemical breakdown of organic matter buried to higher temperatures and pressures:

ñ Coal (swamp vegetation)

ñ Oil (microscopic marine organisms)

ñ Gas (vegetation and microscopic organisms)