GEOL 238 Module 3 & 4 Flashcards

Origin of carbonate sediments

Origin of carbonate sediments

Carbonate sediments are born, not made

• Majority of carbonate sediments are precipitates from organisms that produce skeletons and shells

Two types of carbonate sediment factories

1. Pelagic factory

2. Benthic factory

Pelagic factory

The surface ocean; microscopic carbonate producers that fall to seafloor when they die

Benthic factory

The shallow seafloor

4 optimal conditions for a carbonate factory

1. Shallow

2. Warm

3. Nutrient rich

4. Sunlit

Allochems

Silt to gravel-sized carbonate grains in carbonate rocks

4 types of allochems

1. Skeletal Particles (can be benthic or planktic)

2. Coated grains (Ooids, Pisoids, oncolites)

3. Peloids

4. Intraclasts

Skeletal fragments

Derived from either:

1. Benthic organisms (live on seafloor) → ex. gastropod, mollusc, brachiopod, bryozoan, crinoid, coral, green algae

2. Planktic organisms (live in water column) → ex. gastropod, foraminifera

Peloids

Grains (usually fecal pellets) composed of microcrystalline calcite or aragonite

• Smaller than ooids and have no internal structure

Ooids

Coated carbonate grains with a nucleus

Pisoids

Similar to ooids; but are larger than 2mm and usually more irregular

• They may have an algal origin

Intraclasts

Fragments of lithified or partly lithified sediment that was transported only a short distance

Extraclasts

Fragments consisting of lithology not represented in the immediate depositional environment

Carbonate mud

Can form by direct precipitation or by breakdown of skeletal components (e.g. some green algae)

Stromatolites

Formed of microbial mats (e.g. cyanobacteria). The sediment gets trapped in fine filaments in mats, microbes grow up and around sediment to form new mat

• Algal laminations are crinkly

• Responsible for oxygenating our atmosphere!

Oncoids

Coated irregular layers bound by Cyanobacteria (microbial origin) formed in energetic subtidal environments

• Develop in a more energetic environment than stromatolites

2 methods of Dunham classification

1. Classification based on relative amounts of mud and allochems

2. Classification based on how organisms bind the rock

Diagenetic change

Chemical processes that occur post deposition

Why are limestones vulnerable to diagenetic change?

Because skeletal fragments are in equilibrium with seawater, but not with freshwater or groundwater

• After burial, may start dissolving allochems

  and precipitating cement

3 regions of the peritidal environment

1. Supratidal

2. Intertidal

3. Subtidal

Supratidal

More equivalent to backshore. Flooded during highest tides only

Intertidal

Exposed and flooded during tidal cycles

Subtidal

Below low tide

3 types of supratidal environments

1. Humid ‘algal marsh’

2. Well drained and elevated

3. Sabkha (arid)

Sabkha

An arid supratidal environment with intense evaporation

• Supratidal (only flooded during extremely high tides)

• Evaporation is so intense it pumps ground water through to continuously precipitate halite

2 types of intertidal environments

1. Low energy

2. High energy

4 types of subtidal environments

1. Lagoonal muds

2. Lime muds and sands

3. Lime sand shoals

4. Stromatolites

5. Reefs

2 features of humid supratidal environments

1. Mangrove swamps and algal marshes

2. Intertidal mud flats

2 features of tidal channels + tidal flats

1. Algal mats

2. Mangroves growing on channel levees

3. Desiccation cracks (even in humid settings)

4. Cracking of microbial mats

Location of carbonate sand beaches

At the front of the tidal flat in Bahamian tidal flat depositional environments

3 features of hypersaline subtidal environments

1. Stromatolitic reef

2. Flooded tidal flat

3. Low stromatolite domes and microbial mats

4 features of semi-arid intertidal environments

1. Microbial mats

2. Ponding seawater

3. Stressed stromatolites

4. Oolitic sand with wave ripples

Evaporite formation in semi-arid intertidal environments

Evaporation is just enough to precipitate gypsum (CaSO4 . 2H2O) within sediment

4 stages of evaporation of seawater

1. 50% of seawater evaporated → Calcite (CaCo3)

2. 80% of seawater evaporated → Gypsum (CaSO4 . 2H2O)

3. 90% of seawater evaporated → Halite (NaCl)

4. 95% of seawater evaporated → K + Mg salts

Where do evaporites form?

Arid peritidal systems

4 features of arid intertidal systems

Similar to tidal channels

1. No mangroves

2. Expansive microbial mats along channel margins

3. Gypsum and anhydrite

3 types of modern muddy peritidal cycles

1. Humid; normal marine salinity (Bahamas)

2. Semi-arid; hypersaline (western Australia)

3. Arid; normal marine salinity (Persian Gulf)

Humid; normal marine salinity (Bahamas) peritidal facies model

BOTTOM TO TOP

1. Subtidal: bioturbated, muddy

2. Low intertidal: microbial laminites and fenestrae, burrows

3. High intertidal: microbial laminites and fenestrae, dessication cracks

4. Supratidal: flat pebbles, microbial laminites and fenestrae, tree roots

Overall trend of peritidal cycles

Shallowing and coarsening upwards

Semi-arid; hypersaline (western Australia) peritidal facies model

BOTTOM TO TOP

1. Subtidal: stromatolites, bivalves, ooids

2. Low intertidal: stromatolites, bivalves, ooids

3. High intertidal: stromatolites, bivalves, ooids

4. Supratidal: microbial laminites and fenestrae, flat pebbles, gypsum crystals

Arid; normal marine salinity (Persian Gulf) peritidal facies model

BOTTOM TO TOP

1. Subtidal: bioturbated, muddy

2. Low intertidal: microbial laminites and fenestrae, burrows

3. High intertidal: microbial laminites and fenestrae, deep desiccation cracks

4. Supratidal: microbial laminites and fenestrae, anhydrite nodules, gypsum crystals

3 changes to peritidal cycles over time

1. Appearance of bioclasts (Early Paleozoic)

2. Decline of stromatolites

3. Evolution of angiosperms

4. Evolution of deep-burrowing crustaceans

Peritidal cycles

Shallowing upward cycles with stacked subtidal, intertidal, and supratidal deposits

2 sources of peritidal cycles

1. Autogenic → aggradation or shifting around of environments

2. Allogenic → driven by sea- level cycles

Location of pelagic carbonate factory

Open ocean; sediments end up in deep ocean basins

Location of benthic carbonate factory

Continental shelves and islands

3 factors determining where carbonates are found in the world’s oceans

1. Solubility differences between different carbonate minerals

2. Ocean circulation

3. Changes in seawater chemistry with depth

The amount of CO2 in seawater drives….

…. precipitation vs dissolution of calcium carbonate

Equation of precipitation vs dissolution of calcium carbonate

CaCO3 + H2 O + CO2 = Ca2+ + HCO3-

→ dissolution
← precipitation

CO2 dissolution and water temperature

Cold water can dissolve more CO2 than warm water

Cold water = dissolution of CaCO3

Lysocline

Water depth when we first start to see the corrosion/dissolution in carbonate particles

Carbonate Compensation Depth (CCD)

Water depth at which carbonate sediment no longer accumulates

Rate of accumulation = rate of dissolution

CO2 dissolution and thermohaline circulation

Seawater ages with time and older water has higher CO2 content

• Water at seafloor in N. Pacific is the oldest water in circulation system

Two sources of deepwater carbonate sediment

1. Pelagic factory

2. Periplatform (re-sedimented)

Pelagic factory

Long periods of suspension rain of calcareous phytoplankton and zooplankton

Periplatform (re-sedimented)

Sediment derived from the shallow-water carbonate factory (platform) and redeposited in deeper water

• This material is called a re-sedimented deposit

When did the pelagic factory evolve

• Didn’t get going in a considerable way until the Jurassic

• Carbonate mud in rocks older than Jurassic in deepwater formed on the platform (peri-platform ooze) and been re-sedimented

2 organisms which are sources of pelagic carbonates

1. Foraminifera

2. Coccolithophores (produce chalk)

Where does resedimented peri-platform ooze accumulate?

More in proximal slopes than distal slopes

Difference between oxic (aerobic) vs anoxic (anaerobic) environments

Oxic (aerobic) environments → bioturbated carbonates

• Complete exchange with overlying waters

Anoxic (anaerobic) environments → laminated black muds

• Little exchange with overlying waters

Formation of carbonate slope conglomerates

• Resedimented

• Coarser deposits found higher on slope

• Same transport processes as siliciclastic gravity flows

Carbonate platform margin

1. Platform edge

2. Peri-platform talus

3. Pelagic and re-sedimented carbonates (extending to toe of slope)

4. Basin

Evaporite

Minerals that precipitate from solutions when water evaporates, can form in marine and non-marine settings

Order of precipitation for evaporite compounds

Least soluble compounds are precipitated first

Calcium Carbonate → Calcium sulfate → Sodium Chloride → K and Mg salts

Calcium sulfates (most common evaporites)

Refers to gypsum and anhydrite

• Gypsum is hydrous form of calcium sulphate

• Anhydrite can form through direct precipitation or through dehydration of gypsum upon burial

• Anhydrite may become hydrated to gypsum

4 kinds of gypsum/anhydrite textures

1. Selenite crystals

2. Alabaster (fine-grained gypsum)

3. Fibrous gypsum

4. Nodular textures (chickenwire and enterolithic structures)

Formation of selenite crystals (gypsum/anhydrite texture)

When gypsum precipitates directly from seawater→ large selenite crystals

Formation of alabaster (gypsum/anhydrite texture)

When gypsum forms through rehydration of anhydrite → alabaster (fine-grained gypsum)

Formation of fibrous gypsum (gypsum/anhydrite texture)

Forms in veins

Formation of nodular textures (gypsum/anhydrite texture)

When gypsum grows within carbonate or clayey sediments → gypsum crystals will alter to anhydrite pseudomorphs

2 sub-textures:

1. Chickenwire structure

2. Enterolithic structure

Chickenwire structure (gypsum/anhydrite texture)

When gypsum nodules in carbonate/clayey sediments grow large enough to start coalescing and interfering

Enterolithic structure (gypsum/anhydrite texture)

When gypsum nodules in carbonate/clayey sediments demand more space as nodules grow, creating folded, contorted, ropy layers

3 kinds of halite deposits

1. Salt crusts (shallow water)

2. Laminated forms (deeper water; typically also

   include anhydrite/gypsum laminae)

3. Teepee structures (peritidal)

Teepee structures

Expansion of salt crusts due to mineral precipitation can cause buckling into inverted v-structures, common in peritidal environments

• Halite teepees → supra tidal setting

• Carbonate teepees → junction between water-table and sediment surface

Playa lakes (ephemeral lakes, also called salt pans)

Periodic, usually seasonal, wetting and drying in arid environments leads to alternations of evaporites and clastic sediment layers

• Can have smaller continental evaporite settings where water accumulates (e.g. between dunes)

Where do marine evaporites form today?

• Restricted coastal regions like salinas and coastal sabkhas

• Salinas (arid lagoons and salt pans) may precipitate layered gypsum and/or halite

• Fluctuations in salinity will result in variations in precipitated minerals and occasional carbonate layers

Where do evaporites form within sabkhas?

Within sediment above a saline water table with high evaporation rate

Ancient evaporites

Geologic evidence for thick accumulation of evaporites in deep marine basins in the past

• Evaporation of a column of seawater 1,000 m thick will produce ~15 m of evaporites

• Need no (or very limited) freshwater supply to basin and restricted opening to ocean so water can be replenished slowly

Formation of deepwater evaporites

Thick evaporite deposits require arid conditions and a silled basin (some kind of constriction (vertical or lateral) that impedes circulation of seawater)

• Sills can be tectonic features (folds, etc.) or reefs

Messinian salinity crisis

Today, more water is evaporating from the Mediterranean than is being delivered by rivers

• Causes of evaporation still debated, but the ‘crisis’

  lasted from ~ 6 Ma - 5.3 Ma

• This water is replenished by a current flowing through Strait of Gibraltar and a deeper current brings warm, salty water back out to Atlantic

Stratigraphy

The study of larger successions of sedimentary rocks and how time is recorded

3 principles defined by Nicholas Steno

1. Principle of Superposition

2. Principle of Original Horizontality

3. Principle of Lateral Continuity

Formation

A fundamental lithostratigraphic unit; a body of rock, identified by lithologic characteristics and stratigraphic position, that is mappable either at the surface or in the subsurface

Member

A lithostratigraphic unit next in rank below a formation (formations can be broken into one or more members)

Group

Consists of assemblages of formations

4 types of stratigraphic surfaces

1. Conformity

2. Disconformity

3. Angular unconformity

4. Nonconformity

Conformity

A surface that separates younger and older strata but along which there is no physical evidence of non-deposition

Disconformity

Strata concordant, but evidence of erosion with significant missing time

Angular unconformity

Older strata erosionally overlain by younger strata deposited at a different angle

Nonconformity

Younger strata overlying igneous or metamorphic rock

Facies

A package of sedimentary rock distinguishable by lithologic, structural, and biogenic aspects detectable in the field

Interactions between facies and stratigraphy

We want to recognize important stratigraphic surfaces (e.g., unconformities) and characterize the facies between those surfaces

Lithostratigraphy

Linking units of similar lithology and stratigraphic position

Chronostratigraphy

Linking units that were deposited at the same time (regardless of lithology)

• Foundation of sequence stratigraphy

Dunvegan formation

A clastic wedge (i.e. a delta complex) that built eastward into a shallow marine sea as a result of a phase of mountain building in the Rockies

Reefs

Shallow water carbonate deposit in the subtidal carbonate factory

2 kinds of platforms

1. Rimmed platform (barrier reef system)

2. Unrimmed platform (no barrier reef system; can be open shelf or ramp)

Patch reef

Small reefs that develop in a restricted lagoon environment

5 dating/correlation techniques in stratigraphy

1. Biostratigraphy

2. Absolute dating

3. Magnetostratigraphy

4. Chemostratigraphy

5. Sequence stratigraphy

Biostratigraphy

The use of fossil occurrences within the rock record to establish correlations between time-equivalent rock strata as determined by the presence of a particular fossil species