Introduction to siliciclasticsandstones

Geosciences 443September 15, 2010

Pettijohn, Potter, and Seiver, 1972

Cambrian qtz arenite, WI

Miocene arkose, So. CA

Eocene wacke, central CA

Pettijohn, Potter, and Seiver, 1972

Pettijohn, Potter, and Siever, 1972

Pettijohn, Potter, and Siever, 1972

Neogene Ridge Basin, CA

Lv in Holocene sand, Lake Malawi Cht and Lci in Permian Powwow congl., W TX

Tertiary Six Mile Creek Fm, Ruby Mtns, MT

The matrix problem

Psuedomatrix – deformed LsCambrian Vaughan ss, W. Tx

Clay cementEocene Domengine ssVallecitos syncline, CA

Major controls on sandstone composition

• Source composition (dependent on tectonic setting)• Modification during weathering (climate)• Modification during transport (sorting, compositional

stabilization)• Modification during burial (introduction of cements,

deletion of labile grains)

Basu et al., 1975

Qm, Cambrian, WI

Qp, Miocene Orinda sandstone, E SF Bay

Stable craton provenance

Cambrian qtz arenite, WI

Continental Block provenance

Neogene Peeler ss,Ridge Basin, CA

Continental Block provenance

Neogene Peeler sandstone, Ridge Basin, CA

Recycled Orogen Provenance

Permian Powwow cngl, W TX

Miocene Orinda ss, East SF Bay

Magmatic Arc Provenance

Cretaceous Sites sandstone,Cache Ck, CA

Holocene sand, Mono Lake, CA

Major controls on sandstone composition

• Source composition (dependent on tectonic setting)• Modification during weathering (climate)• Modification during transport (sorting, compositional

stabilization)• Modification during burial (introduction of cements,

deletion of labile grains)

Modification of sandstone composition by climate

Suttner and Dutta, 1986

Suttner and Dutta, 1986

Eocene Domengine sandstone, Vallecitos syncline, CA

Major controls on sandstone composition

• Source composition (dependent on tectonic setting)• Modification during weathering (climate)• Modification during transport (sorting, compositional

stabilization)• Modification during burial (introduction of cements,

deletion of labile grains)

Pettijohn, Potter, and Seiver, 1972

Petrographic analysis of siliciclastic sandstones

Important considerations:1) Fabric of sample – spatial relationships among several grains

Examples: grain vs. matrix support; nature of grain contacts (point contact, concavo-convex; long contact; sutured contact; overgrowths)

2) Texture of grains – size, shape relations of grainssorting, rounding, sphericityConcept of textural inversion = textural anomaly

well-rounded sand grains floating in silt or clay matrixextremely well-sorted but very angular grainsfiner grains better rounded than coarser grains

3) Composition of framework grains: more on this later4) Composition and nature of interstitial material: diagenetic contributions

Major terrigenous constituents

• Detrital constituents: quartz, feldspar, chert, accessories, lithicgrains; biogenic components (may not be derived from outside basin of deposition)

• Chemical consituents: cements– Carbonate and silica– Sulfate, halite, clay

• Authigenic grains: minerals forming in situ during diagenesis (note: these can be reworked as detrital components).– Feldspars– Hematite– Glauconite– zeolites

QuartzQuartz types• Plain and monocrystalline – volcanic• Polycrystalline and undulose – metamorphic rocks• Undulose with abundant inclusions and bubbles – plutonic• Unstrained polycrystalline – vein quartz• Very finely crystalline quartz – chert (salt and pepper appearance in

xp)• Fan-like quartz with radial extinction – chalcedony• Isotropic opal may rarely form cement in very young sandstones

Other considerations• Elongate, lineated quartz – probably metamorphic• Monocrystalline quartz more common in mature sandstones• Polycrystalline quartz more common in immature sandstones• Embayed quartz common in shallow intrusives and volcanic rocks –

phenocryst formed early moves to lower P,T conditions and becomes unstable

• Fluid inclusions more common in plutonic rocks; may form in lines

Quartz overgrowths

• Chemical addition of quartz onto original detrital grain• Very common cement type• Samples typically ‘sparkle’ in sunshine• Overgrowth may be strained as is detrital grain• Overgrowths may become abraided if sandstone is weathered and

individual grains (+ overgrowth) are reworked. Good evidence ofsedimentary recycling.

Feldspars• K-spars include: orthoclase, microcline, anorthoclase, sanidine, adularia

(low T, diagenetic)• Plagioclase solid solution series from albite (An0) to anorthite (An100)• Perthite is a mixed (intergrown) plagioclase in a K-spar host• Antiperthite is Kspar in plag host• Graphic granite and myrmekitic texture: quartz blebs in Kspar crystal –

indicators of a plutonic source• metamorphic and igneous feldspars both common – look for mineralogic

or lithologic associations such as garnet, metamorphic rock fragments• Feldspar is less durable than quartz – perfect three-directions of

cleavage• Feldspar commonly resides in finer grain size fraction because of

cleavage• K-spars more common than plagioclase in early Pz sandstones –

(function of chemical weathering environment prior to land plantevolution?)

Lithic fragments

• Provide clearest and most direct provenance indicators• Fine-grained and may be difficult to interpret (e.g., chert may be

sedimentary or devitrified volcanic glass)• Factors influencing presence and type of lithic fragments:

– Source type– Distance from source (e.g., schist fragments restricted to close to

source)– Generally more prevalent in coarser fractions– Durability influenced by climate– Some are highly susceptible to weathering and diagenesis (e.g.,

volcanic lithics)– Intraclasts derived from within basin

Volcaniclastic rocks

• Very important volumetrically, especially in orogenic belts• Eruption type related to composition: explosive = silicic; ‘quiet’

eruptions = more mafic.• Volcaniclastic – may be air or water lain; later reworked at surface.

When is volcanic tuff a sedimentary tuff?• Tephra – fragmented volcanic material, usually glass-rich• Useful to recognize three tuff end members: vitric, crystal, lithic

– Vitric tuffs: texture most characteristic; pumice fragments, shard structures; flow banding; devitrification or alteration of glass during diagenesis can result in formation of zeolites, clays, celadonite (green), or palagonite (brown) and release of large volumes of silica

– Crystal tuffs: crystals can be most any volcanic mineral– Lithic tuffs: variety of lithic fragments can be incorporated into

rock, either during eruption or during subsequent reworking of volcanic sand.

Saint Peter sandstoneOrdovician, southern WI

Permian Powwow cngl, W TX

Miocene Orinda ss, East SF Bay

Holocene sand, Mono Lake, CA

Granitic rock fragment

Lv in Holocene sand, Lake Malawi