Sedimentary Rock

Sedimentary Rocks

What Do We See

J only 5% of Earth’s crustJ cover about 75% of surfaceJ raw material (sediment) of

sedimentary rocks - weathering or erosion of pre-existing rock

J basic sources:

detritalnon-detrital

Sediments and Sedimentary Rocks

detrital - mineral/rock fragmentsdetritus or clasts - produced by mechanical weathering or erosiondetrital sedimentary rocks

sandstoneshale

mudstone


non-detrital - precipitate from solution (chemical weathering)inorganic or biochemical processes chemical sedimentary rocks

limestonehalite

gypsum


... what’s involved in the

physical transport of sediment?

place of weathering to place of deposition:

angular particles become rounded sorted by size and compositionagents of sediment transport water (primary mechanism),

gravity, wind, and icesediment deposition processes (sedimentation): loss of velocity, evaporation and/

or chemical interactions

Transport and Deposition

carried in suspension by:ice: any size particlewater: small gravel and smaller size particleswind: sand and smaller sizes

abrasion and rounding removes cornerssorting separates like-sized particles

Detrital Transport

deposition = decrease in energy or velocitysettling rate depends on:

particle size

density

Detrital Deposition

... what about

chemical transport of sediment?

results from saturation due to:

increase of available ions through oversupply or evaporationchange in temperature or other chemical conditions

Chemical Deposition

... in what types of

environments is sediment deposited?

Deep Marinedeep ocean - submarine fans (turbidites)

Depositional Environments

Marine - shallow ocean (continental shelf) lagoon reef barrier island


Transitional Beach Delta



Continental (terrestrial) fluvial (streams) eolian (deserts) lacustrine (lakes) glacial

... how is sediment

converted into solid rock?

conversion of sediment into rock results from:

1. Compaction2. Desiccation3. Cementation4. Crystallization

Lithification

1) Compaction - sediment grains packed together through burial

reduction in volume by up to 40%results from pressure or weight of overlying sediments

Lithification

2)Desiccation - loss of water from sediment pore spaces

results from compaction and evaporation in air

Lithification

3)Cementation - minerals precipitated from sediment pore fluids bind particles

common cements - calcite (CaCO3) and quartz (SiO2), dolomite [(CaMg)CO3], iron oxides (Fe2O3) and hydroxides [FeO(OH)]

Lithification

4)Crystallization - formation of interlocking crystals in chemical sediments

Lithification

... what special

features characterize sedimentary

rocks?

both detrital and chemical sedimentary rocks can show fragmented texture

separated by clast sizeWentworth Scale of particle size commonly used: gravel > 2 mm sand 1/16 to 2 mm silt 1/256 to 1/16 mm clay < 1/256 mm

Grain Sizes

degree to which clast or particle sizes similar

Sorting

occurs during transport with particles separated by: a) grain size - largest particles

settle first b) composition - densest minerals

settle first

Sorting

poorly-sorted - mix of particle sizes

Sorting

well-sorted - if primarily one particle size

angularity or roundness depends on how far rock transported:a) Roundness -

degree of edge and corner removal

b) Sphericity - degree to which fragment approaches spheroid shape

Clast Shape

Non-clastic Textures

subdivided according to crystal size :

fine-grained (< 1 mm)medium-grained (1-5 mm)coarse-grained (> 5 mm)

interlocking crystals shown only by chemical sedimentary rocks

... what are the different

types of sedimentary

rocks?

Clastic Sedimentary Rocks

conglomerate - gravel-sized rounded particles (>2 mm) surrounded by fine-grained material:

deposition in high-energy environmentcharacteristic of streams and beaches


Conglomerate

material not transported great distances

sedimentary breccia - particles > 64 mm and angular:


Sedimentary Breccia

sandstones - sand-sized particles (0.06-2 mm):

mainly quartz, some feldspar and rock fragments: 1) Quartz Sandstone - almost pure quartz 2) Arkose - quartz and more than 25%

feldspar 3) Graywacke - “Dirty” sandstone containing

quartz, feldspar, rock, and clay

both arkose and graywacke lack of transport


Mudrocks - low-energy environment: 1) Siltstones

silt-sized quartz and feldspar particles (≤ 0.06 mm)

2) Mudstones silt- and clay-sized particles (≤ 0.004 mm) clay, quartz, feldspar, calcite, and dolomite

3) Claystones - mostly clay 4) Shales

most abundant detrital rock rock breaks into layers (parallel to bedding)


Rounded rock fragments Conglomerate

Angular rock fragments Breccia

Quartz predominates Quartzsandstone

Quartz with considerablefeldspar

Arkose

Dark color, quartz withconsiderable feldspar, clay androck fragments

Graywacke

Splits into thin layers Shale

Breaks into clumps or blocks Mudstone


organic (biochemical) or inorganic activity

1) limestone/dolostone (carbonate rock) - biochemical or inorganic rock of calcite or

dolomite 2) evaporite (rock salt, rock gypsum) - halite,

sylvite, or gypsum inorganically precipitated 3) chert - microscopic quartz deposited by

biochemical or inorganic processes 4) coal - biochemical material (land plants that

grew in swamps)

Chemical Sedimentary Rocks


Clastic ornon-clastic

Calcite, CaCO3 Limestone


Dolomite,CaMg(CO3)2

Dolomite(dolostone)

Inorganic Non-clastic Microcrystallinequartz, SiO2

Chert

Non-clastic Halite, NaCl Rock salt

Non-clastic GypsumCaSO4•2H2O

Rock gypsum


Calcite, CaCO3 Limestone

Biochemical Non-clastic Microcrystallinequartz, SiO2

Chert

Non-clastic Altered plant remains Coal


... what can sedimentary rocks tell us

anyway?

evidence preserved in sedimentary rocks allows events and processes which formed them to be inferred

specific depositional environment of ancient sedimentary rocks indicated by sedimentary structures

Sedimentary Structures


usually horizontal or sub-horizontal layering

Bedding or Stratification

Laminated Beddingclosely spaced parallel layering


Graded Bedding progressive decrease in grain size upward through bed

rapid deposition

top of bed indicator


non-horizontal bedding

Cross-Bedding

wind or water currentsindicate top of bed and paleocurrent directions


small waves on surface of sediment caused by water or wind movementused as top of bed indicatortwo forms:

a) wave-formed ripple marks

b) current-formed ripple marks


Ripple Marks

symmetric ripples

formed in surf zone

Wave-formed Ripple Marks


water or air moves in one directioncurrent indicator


Current Ripple Marks

polygonal cracks

contraction during desiccation of silt- and clay-sized sedimentshallow water depositiontop indicator


Mud Cracks

provide no clues about deposition:

nodules

concretions geodes


Secondary Structuresform long after rock formation

parallel bedding

irregular, knobby-surfaced mineral body

different composition than surroundings


Nodules

dehydration cracks filled by material of different composition


Septarian Nodules

Concretions

usually sphericalcharacterized by concentric layeringcomposed of silicic material


in limestone and shale

roughly spherical hollow structures

pocket of water surrounded by silica deposition

large quartz or calcite crystals grow inward


Geodes

preserved plant and animal remains

clues about ancient life, evolution, and depositional environmentbasis for constructing geologic calendarallow rocks to be correlated in timetwo types

Fossils

most dissolved and replacedsilica and pyrite (e.g., petrified wood):

mold - cavitycast - filled mold

Fossils

…. so what do these structures tell us about how

and where sediments are

deposited?

different sedimentary rocks deposited in different places at same timechange in composition and nature laterally due to changes in depositional environmentrock assemblages reflect particular depositional environment

Sedimentary Facies

may result from sea level fluctuations: 1) Marine Regressions

sediment deposition during falling sea leveldeep marine deposits overlain by shallower marine and nearshore deposits

2) Marine Transgressionssediment deposition during sea level risesandstone (nearshore) overlain by shale (shallow marine) and limestone (deeper marine)

Sedimentary Facies

why are such variations important

and why worry about where

sediments are deposited anyway?

Petroleum/Natural Gas/Oil Shales/Tar Sands

migrate upward until reach surface or trappedpetroleum trap - permeable reservoir rock and impermeable cap rock

Sedimentary Hosted Resources

Uranium ores occur in fluvial sandstones containing organic matter uranium transported in groundwater until reducing conditions cause precipitation

Banded iron formations largest source of iron ore deposited in shallow seas iron precipitated from seawater by oxygen addition from algae (photosynthesis) or bacteria (respiration)


other sediment or sedimentary rock derived resources:

building materials

sand and gravel - constructionlimestone - cementgypsum - wallboard and plasterclay - bricksquartz - glass


halite - seasoning and preservativeclay - ceramics and kitty litterphosphates - fertilizers, matches, and preservatives diatomite - filtrationcoal - energy and coke in steel production


other sediment or sedimentary rock derived resources:

Sedimentary Rock

Documents