Sedimentary rocks and the origin of sedimentary strata

• Sedimentary rocks are those rocks which form at or near the earth's surface primarily through: • Deposition of weathered silicate material by water, wind, or ice detrital, clastic, ter

Sedimentary Rocks and the

Origin of Sedimentary Strata

Basins to Bedding

• Sedimentary rocks are those rocks which form at or near the

earth's surface primarily through:

• Deposition of weathered silicate material by water, wind, or ice (detrital, clastic, terrigenous)

• Direct inorganic chemical precipitation from water

• Precipitation by organic processes

• Three end-member types:

particles, shell fragments

• Authigenic (form within basin)

but locally reworked

• O= Orthochemical

• Primary chemical precipitation

from dissolved ions

• Authigenic (form within basin of

deposition), no reworking

IO= Impure orthochemical IA= Impure allochemical

• IA: Impure Allochemical

• Very fossiliferous shale,

sandy fossiliferous or oolitic

Sedimentary Rocks: Terrigenous

• Terrigenous (clastic, detrital)

sediments and rocks

• Also called siliciclastic since

most particles are silicate

sedimentary basins

• Buried and lithified by

• Compaction

• Cementation

Sedimentary Rocks: Allochemical

• Allochemical (mainly carbonate )

sediments and rocks

• Dominantly biologic origin (shells

or bones)

• Carbonate systems develop where

siliciclastic sourcelands are low

and/or very distant

• The water is shallow marine

• Climates are tropical to

subtropical

Sedimentary Rocks: Orthochemical

• Orthochemical (chemical precipitate) sediments and rocks

• Dominated by limestones and

dolostones of precipitate origin

• Also includes evaporites, chert,

and iron formations

• Precipitate from marine or

non-marine waters due to chemical changes

Sedimentary Depositional Environments

• In geology depositional environments are defined by

processes and products

• Physical processes determine:

• Grain size, sorting, rounding

• Bedding style (including sedimentary structures) and geometry

• Biological processes determine:

• Fossil content

• Biological disruption of original stratification

• Chemical processes determine:

• Types of minerals formed at the site of deposition and during burial

• Study of modern depositional environments used to

infer how ancient rocks formed (“present is key to

past”)

Sedimentary Depositional Environments: Main Types

• Continental (above sea level)

• Fluvial (stream); stream channel and floodplain

• Glacial; direct deposits and outwash

• Marine (below sea level)

• Shallow sea (shelf) and reefs

• Submarine canyons (submarine “deltas”)

• Pelagic environments; abyssal plains

Sedimentary Basins

• Sedimentary rocks form in basins

• Areas of the earth’s surface subject to long term (millions to

tens of millions of years) subsidence resulting in space to

accommodate sediment (not subject to erosion)

 Convergent plate setting

and active plate

boundaries:

 Puget trough

 Divergent plate

boundaries:

 Passive; Atlantic coast basin

 Rift Basins; East African Rift

Terrigenous Clastic Basin

Carbonate Basin

• Simple model and

classification

Sedimentary

Basins and

Rocks

Siliciclastic Rocks: Components

• F-M-C-P

• F ramework Grains

• >0.05 mm allogenic mineral grains, rock fragments

• Residual from weathering

• Detrital M atrix

• <0.05 mm (clay, quartz, feldspar, carbonates, organics, oxides)

• Chemical weathering products

• C ement

• Authigenic, post-depositional orthochemical component

• Precipitated from circulating pore fluids (silica, carbonate, Fe-oxide, clay, feldspar, other oxides, zeolite, salts)

• P ores

• Primary (~40%) or secondary due to leaching/dissolution

• Classification based on (1) texture, (2) composition

Siliciclastic Rocks: Texture

Classification

• Grain Size

• Uden-Wentworth grain size scale

• Phi = -log2 (grain diameter in mm)

• naturally occurring groups

• Gravel ~ rock fragments

• Sand ~ individual mineral

grains (particulate residues)

• Mud ~ particulate residues

+/- chemical weathering products

• Clay ~ chemical weathering

products (clay minerals, etc.)

Siliciclastic Rocks: Texture

• Statistical/graphic presentation of texture

• Quantitative assessment of the % of different grain sizes

in a clastic rock

• Mean: average particle size

• Mode: most abundant class size

Siliciclastic Rocks: Texture

• Sandstones, siltstones, and shales

• G (gravel; >2 mm) - S (sand) - M (matrix; <0.063

mm)

• Conglomerates and breccias

• >30% gravel; indicates high transport energy

• Further classification based on composition

Siliciclastic Rocks: Sandstone

• Basic classification based on proportions of

• Mineral grains (dominantly

quartz)

• Matrix (clay to silt-sized clastic

material filling spaces between grains

Siliciclastic Rocks: Sandstone

• Many classification schemes, but most based on relative proportions of framework grains

• Relative abundance a function of mineral grain’s

 Availability, Chemical Stability, Mechanical Durability

• Anything Possible, most common:

• Quartz :

• monocrystalline, polycrystalline; ig, met, or sed source

• mechanically & chemically stable, abundant

• Feldspar:

• K-spar (sandine, microcline), Plag (Na-Ca)

• Abundant and somewhat stable (often altered)

• Rock (Lithic) Fragments:

• All kinds (including limestone/dolomite RF’s)

• Abundant, less stable (depending on dep conditions)

• Also accessory (minor abundance) “heavy” minerals

Siliciclastic Rocks: Sandstone

Siliciclastic Rocks: Sandstone

• Sandstone composition is tied to source area and tectonic setting

• Ternary System for Sandstone classification

Siliciclastic Rocks: Mudrocks

• Most abundant of all sedimentary rocks

• Composed of silt & clay-sized particles

• Dominated by clay minerals (kaolinite, smectite, illite)

• Also quartz, feldspar, carbonate, organic matter, others

• Composition modified by diagenetic processes

• Variable color

• Gray-black = presence of organic matter

• Red-brown-yellow-green = oxidation state of Fe

Siliciclastic Rocks: Mudrocks

Siliciclastic Rocks: Conglomerates

• Coarse-grained siliciclastic rock with muddy or sandy matrix

• Breccia: angular clasts in sandy matrix

• Diamictite: clasts in muddy matrix

through which clastic

sediment is transported and

in which some sediment is

deposited

• End product is relatively

“mature” sediment

 Sediments are chemically and mechanically stable in composition (high

temp, unstable minerals are not present)

 Sediments are well sorted into the end member sizes of sand and clay.

 Sandstones at the end of the long system are mature quartz arenites

Terrigenous Clastic

Depositional Environments

• The siliciclastic source

land is proximal to (close

to) the basin

• Commonly observed in

tectonically active

regions

• Sediments across the

entire system are

mineralogically and

texturally immature

• They are generally poorly

sorted and range in size

from gravel to coarse

sand

• Make up 10-15% of sedimentary rocks

• Excellent indicators of depositional environments; integral to study of past environments and earth history

• Important reservoirs for oil and gas

• Carbonates (>50% primary carbonate minerals)

Carbonate Sediment: Origin

• Most primary carbonate sediments form as biogenic particles in shallow marine environments (secreted as shells of invertebrates and algae)

• Warm water (tropical; 30 o N to 30 o S latitude)

• Shallow shelf; within the photic zone (mostly <10-20 m)

• Also accumulate in deep water (pelagic oozes)

• Inorganic precipitates from sea water also occur

• Can form in continental settings (lacustrine, desert, soil, springs)

Carbonate Rock Constituents

• Carbonate rocks mainly composed of:

• Lime mud (<0.004 mm)

• Largely fragmental algae remains, also chemical precipitate

• Crystalline carbonate material (>0.004 mm)

• Forms by precipitation (often as cement) or recrystallization

• Formed by organisms in situ

• Bound together by precipitated material

Carbonate Rock Constituents

• Implies deposition in a low

energy environment just

like in terrigenous

mudstone

Carbonate Rock Constituents

• Sparite (cement):

• Clear granular (“sugary”) carbonate crystalline

orthochemical material

• Formed in interstitial pore spaces of carbonate sediment

• Cement in pores indicates original void space

• Also commonly forms during diagenesis

• Recrystallized allochems

or micrite

Carbonate Rock Constituents

• Allochems: Intraclasts

• Reworked, early lithified carbonate fragments

• irregularly-shaped grains that form by syndepositional

erosion of partially lithified sediment

Carbonate Rock Constituents

• Layers precipitated onto a

grain during wave agitation

• Pisolites - same as oolites,

but >2 mm

• Oncolites - spheroidal

stromatolites (> 1-2 cm)

Carbonate Rock Constituents

Carbonate Rock Constituents

• Allochems: Skeletal particles (bioclasts)

• whole microfossils, whole megafossils, broken shell fragments

• Marine invertebrates: algae, forams, corals, bryozoans, brachiopods,

gastropods, mollusks, ostracods, etc.

• Standard microfacies (fossil fragment type -> environment)

Carbonate Rock Classification

• Based on depositional texture (mainly

proportion of allochems)

• Two main classification schemes:

• Folk

• % and type of allochem

• Micrite vs sparite matrix

Carbonate Rock Classification: Dunham

waters • Mud-bearing vs mud-free sediment

• Grain vs mud support

• Original components bound (biologically)

• Depositional texture recognizable

Carbonate Rock Classification: Dunham

• Presence or absence of lime mud; is there any mud at all Calm

waters allow for the accumulation of lime mud and indicates the absence of current induced agitation

• Grain Support: self supporting framework

• fluid circulation, diagenesis

• Grain kind: standard microfacies types

• Grain size, rounding, and coating: hydrologic interpretations

• Biogenically ppt masses bound at time of deposition:

• Boundstone

• organic framework

• laminations not consistent with gravity (stromatolite)

• roof over sediment filled cavities

Carbonate Depositional Systems

• In the warm, clear, shallow

water organisms create

• Also, particles created indirectly by biological or chemical activity

• Oolitic, pelletal, and intraclastic allochems are also produced locally, depending on conditions

Carbonate Depositional Environments

• Generic rimmed carbonate shelf platform – basin margin

Collaborative Activity

1 You have two sandstones (Table, handout)

A Plot the normalized proportions of Q, F, and L on the ternary diagram.

B For each sandstone:

1 Classify it (give it a compositional name and indicate arenite vs wacke)

2 Determine the most likely tectonic setting from which it originated, and give your evidence

3 Determine the depositional environment (general - long system, short system; be more specific if you can) in which it most likely formed, and give your evidence

2 You have three carbonates (handout)

A Based on the description, for each carbonate:

1 Give it a compositional classification under both the Folk and Dunham schemes (and indicate allochemical vs orthochemical)

2 Describe the depositional environment as best you can and give your evidence