SEDIMENTARY GEOLOGY OVERVIEW

Andrew S. MadofOrals Review - 2007January 12, 2007

SEDIMENTARY GEOLOGY = Sedimentology (process) + Stratigraphy (response)

• Sedimentology = study of PROCESSES (i.e. production, composition, transport, and deposition of sediment)

• Stratigraphy = study of RESPONSES (i.e. inferring the controls on the spatial and temporal changes of strata) → exact processes that created the rocks can’t be know because only the rocks are left, not the processes

Sedimentation And Sedimentary Rocks

 Sedimentary Rocks:• Form 75% of the rocks exposed at the Earth’s

Surface

• Are the reservoirs for fossil fuels, iron and aluminum ores, and groundwater

• Record of Earth’s history

Sediment

• Sediment = loose, solid particles and can be: – Terrigenous = fragments from silicates (igneous and/or

metamorphics)

– Biogenic = fossils (carbonate - reefs; silicates - forams)

– Chemical = precipates (halite, gypsum, anhydrite, etc…) - note: with chemical sedimentary rocks, evaporation > precipitation and/or supersaturation in closed basins (lakes or oceans)

• Classified by particle size – Boulder - >256 mm

– Cobble - 64 to 256 mm

– Pebble - 2 to 64 mm

– Sand - 1/16 to 2 mm

– Silt - 1/256 to 1/16 mm

– Clay - <1/256 mm

Grain size (diameter) and grain-shape depend on:• Transport media: rivers (pebbles bounce on river bottom, sand moved in traction, and silt/clay suspended in water column); oceans and lakes (near-shore and deep-water systems); glaciers (sediment moved on glacier bottom); wind (sand dunes)• Distance from parent rock: the longer the distance traveled, generally the smaller and the more well-rounded the grains (due to higher kinetic energy)• Mineral hardness: the harder the parent rock, the longer it will take the sediments to erode (example: silicates are more resistant to weathering and erosion than feldspars, and this is why beaches are often comprised of sand, not feldspar-rich sediments)•Consider: sorting (= range of grain sizes) → winds sort well (meaning grain sizes are very similar); glaciers sort poorly (meaning there is a large spread of grain sizes in glacial deposits)

Grain size

Classification Of Sedimentary Rocks

DETRITAL (TERRIGENOUS) SEDIMENTARY ROCKS:• Mudstones• Sandstones• Conglomerates• Breccias

DETRITAL SEDIMENTARY ROCKS:• Classification Based On Particle Size

a) All detrital rocks are clastic

b) Sand and silt are predominantly quartz

c) Finer-sized particles of clay minerals

SANDSTONES:a) 25% of all sedimentary rocks

b) Sandstone particles (1/16-2 mm in diameter)

c) Practical uses of sandstones: buildings and reservoir for fossil fuels and groundwater

 

CONGLOMERATES AND BRECCIAS:a) Grain diameters larger than 2 mm

b) Conglomerates have rounded grains

c) Breccias have angular grains 

Breccia (from fault motion?)

Sandstone

MUDSTONES:

a) More than half of all sedimentary rocks

b) Contain the smallest particles (0.004 mm in diameter)

c) Environments of deposition: lakes, lagoons, deep ocean basins, river floodplains

d) Color variety of shale represents mineral composition

e) Practical uses of shale: bricks, ceramics, cement, and oil shale

 

Chemical Sedimentary Rocks

Inorganic Chemical Sedimentary Rocks

a) LIMESTONE (inorganic):

I) FORMATION

II) Oolitic Limestone

III) Tufa

IV) Travertine

Lithification = Turning Sediment Into Sedimentary Rock

• Diagenesis = Changes in the sediment due to increased heat, pressure, and circulating groundwater

• Lithification = Compaction + Cementation

• Compaction = Diagenetic process by which the weight of overlying materials reduces the volume of sedimentary body (decreases porosity)

•Cementation: Precipitation of dissolved ions in the pore space

a) calcium carbonate - CaCO3

b) silica - SiO2

c) iron compounds - Fe+2 and Fe+3

• Texture of Rock: Formed by compaction and cementation of sediment particles

• Recrystallization: recrystallization of certain unstable minerals into new, more stable minerals (this happens primarily in carbonates, when you start with carbonate mud [a.k.a. micrite] heat it up, then cool it to form larger grains [a.k.a. sparite]) 

Cementation & Recrystallization

Types of Sediment

Common Geological Environments

Locations of Subsurface Evaporites

Sedimentary Structures

• Bedding (stratification): arrangement of sediment particles into distinct layers

 A) Changes in sediment change bedding

 B) Changes in transport energy change bedding

 

• Normally graded bedding: sediment layer (formed by a single depositional event) in which particle size varies gradually with the coarsest particles on the bottom (note: inversely graded bed = fines on bottom and coarse grains on top )

 

•Cross-bedding: sedimentary layers deposited at an angle to the underlying set of beds

• Surface sedimentary features A) Ripple Marks: small surface ridges produced

when water or wind flows over sediment after it is deposited

  B) Mudcracks: occur on the top of a sediment layer

when muddy sediment dries and contracts

Cross-bedding and Mudcracks

Development of Cross-Bedding

Asymmetric and Symmetric Ripples

river or wind currents (uni-directional)

tidal currents (bi-directional)

Formation of Coal from Swamp Deposits

Formation of Coal from Swamp Deposits

Initial Deposits of Flat/Tabular Clay

Formation of Ooliths

Origin of Mud Cracks

Lithification of Sediments

Sedimentary Facies Formation

Sedimentary Facies Formation

Sediment in a Stream

Marine sedimentary environment

Landward Migration of Shoreline = Regression (regression can either form due to 1) lower sea level or 2) shoreline building basinward [a.k.a. progradation])

Graded Bedding = Vertical Decrease of Sediment Size

Turbidity Current = PROCESS

Turbidite = RESPONSE