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 Crystals- crystallographically 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/ch8_sands.html

http://www.tulane.edu/~sanelson/geol212/sandst&cong.htm

http://www.geo.wvu.edu/~lang/Geol185/Pet15-99SsClassn/sld001.htm