Download Sandstones

Sandstones
Sandstones

Framework fraction
– Silicate grains
• 1/16 to 2 mm

Matrix
– Cement and very fine-size material
• <~0.03 mm
Framework Mineralogy

Major Minerals
– Quartz
– Feldpars
– Clay minerals & fine micas
Accessory minerals
 Rock fragments
 Chemical cements

Quartz

Most Stable:
– Greatest resistance to chemical decomposition
– Multiple recycling
50-60% of framework fraction
 Monocrystalline
 Polycrystalline
 Undulatory extinction
 Origin: felsic plutonic rocks-granites,
metamorphic rocks, and older sandstones

Feldspars
10-20% of framework grains
 Alkali feldspars (K-feldspars)

– orthoclase, microcline, sanidine, anorthoclase
– More abundant in sandstones
– Plutonic or Metomorphic origin

Plagioclase feldspars
– Albite (Na), oligoclase, andesine, labradorite,
bytownite, anorthite(Ca)
– More abundant in sandstones derived from
volcanic rocks
– Felsic or continental crust origin
Clay minerals & fine micas

clay minerals
–
–
–
–

kaolinite group
illite group
smectite group
chlorite group
fine micas
– muscovite
– biotite
Accessory Minerals








<~1-2%
Include: muscovite, biotite, & heavy minerals
(specific gravity > 2.9)
Muscovite more stable than biotite, more abundant
Easy to concentrate
Stable nonopaque-zircon, tourmaline, rutile
Metastable nonopaque-amphiboles, pyroxenes,
garnet, apatite, epidote, topaz monazite
Stable opaque-hematite, limonite
Metastable opaque-magnetite, ilmentite,
leucoxene.
Rock Fragments






10-15% of framework mineral grains
Range from 0-95%
Igneous: cystalline colvanic rock and volcanic
glass are most common in sandstones
Metamorphic: metaquartzie, schist, phyllite, slate,
argillite, and less commonly gneiss clasts
Sedimentary: Chert-microcrystalline quartz
All preserved as sand-size fragments
Mineral Cements

Silicate Minerals:
–
–
–
–
–
Quartz
Microquartz (Chert)
Opal
Feldspars
Zeolites
Mineral Cements

Most common:
– quartz, calcite, clay minerals, and hematite

Also pyrite, gypsum, and barite can also
form cements under special geologic
conditions
Quartz Cements
form in environments of high energy
currents, such as beach deposits, marine
bars, desert dunes, and some fluvial
sandbars
 most of the quartz cements are derived from
the sands themselves or quartz sands

Quartz Cements


Overgrowths- rim of
cement where the
quartz cement is
chemically attached to
the crystal lattice of
existing quartz grains
Syntaxial- when
overgrowth retains
crystallographic
continuity of the grain
Mineral Cements

Carbonate Minerals:
–
–
–
–
Calcite
Aragonite
Dolomite
Siderite
Calcite Cements
patchy cement soluble in surface waters
 Often partially dissolved cements
 secondary porosity

Mineral Cements

Iron Oxide Minerals:
– Hematite
– Limonite
– Goethite
Hematite Cements
hematite cement indicates an oxidizing
environment during diagenesis
 most common oxidation state is Fe+2
 Fe+2 is brought near the surface where the
iron oxidizes to Fe+3 and can be carried
away by hydrous fluids
 Precipitation of Fe+3 forms hematite (Fe2O3)

Mineral Cements

Sulfate Minerals:
– Anhydrite
– Gypsum
– Barite
Sulfate Mineral Cements
Barite (BaSO4) can form if the fluids are
rich in Ba
 Gypsum (CaSO4.H2O) can from if the fluids
are oxidizing and rich in sulfur

Sulfate Mineral Cements

Sand Crystalscrystallographically
continuous crystals in
the cement when the
cements form near the
surface
Sands of the Gulf of Mexico
Sands in the Gulf of Mexico
I.
Eastern Gulf of Mexico - Kyanite + Staurolite
(32%) derived from metamorphic rock in
the Appalachian Mountains.
II.
Mississippi River Province - Augite (23%),
Hornblende (40%), Epidote (16%), and Garnet
(3%) derived from glacial deposits in upper
Mississippi River drainage.
III.
Central Texas Province - Hornblende (58%),
Epidote (17%), and Garnet (7%) but no
Augite. Mostly from Colorado River of Texas.
Sands in the Gulf of Mexico
VI.
Rio Grande Province - Epidote (15%),
Hornblende (23%), Augite (24%), and brown
hornblende from volcanic rocks (7%).
VII.
Mexican Province - There are few studies of
these sands, but they are expected to be similar
to Rio Grande Province, reflecting a volcanic
source.
QFL Classification
Arenites
Wackes
Characteristics of Sandstones tell
us:

Source area
– rock type
– current directions
– weathering environment

Transport
– medium, energy
– distance

Depositional environment
– marine or non-marine
– physical environment (beach, river, delta, etc.)
Increased Textural Maturity:
clay removal
 increased sorting
 increased rounding
 breakdown (absence) of unstable
fragments
 breakdown (absence) of unstable
minerals

Super-mature Sandstones:
Clean (no mud matrix)
 well-sorted
 well-rounded grains
 mostly quartz grains
 quartz arenites
 Cratonic, typically recycled, formed in
beach or other high energy environment

References
http://www.science.ubc.ca/~geol256/notes/c
h8_sands.html
 http://www.tulane.edu/~sanelson/geol212/sa
ndst&cong.htm
 http://www.geo.wvu.edu/~lang/Geol185/Pet
15-99SsClassn/sld001.htm
