Transcript Sedimentary Geology - Lamont

SEDIMENTARY GEOLOGY OVERVIEW
Andrew S. Madof
Orals Review - 2007
January 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
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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
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)
Classification Of Sedimentary Rocks
DETRITAL (TERRIGENOUS) SEDIMENTARY ROCKS:
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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 & Recrystallization
• 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])
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 and Mudcracks
• 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
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