Sed Strat exam 2

Terrestrial: alluvial fans

High energy (periodic), flashy environment

- Coarse sand- to boulder-sized clasts (angular to rounded)
Typical in arid deserts

  (immature)

- Requires continued uplift and erosion of highlands

Braided stream

No fines (too energetic)

      - Cross-bedding, imbrication

Reverse grading (debris flow)

      - Often dry, stream beds 

        fill after water recedes

Braided: upper plane bed

Channel walls absent (wide)

    - Higher energy (competence)

    - Higher gradient

    - Gravel and sand

Meandering: lower plane bed

- Channelized

    - Lower energy

    - Muds and suspended   

        load sedimentation

    - Lateral erosion and 

       deposition

Terrestrial: fluvial - meandering

Streamflow is slower and deeper – sets up spiral flow

Stream avulsion

high flow causes meander to breach its course, flowing on another part of floodplain

Point bar

accretes laterally, channel lag is buried by finer material (planar laminations: dunes and ripples),

- finally mud (if abandoned)

Levees and crevasse splays

Natural levees form on banks of streams to confine channel

During high flow, levee will breach to form a crevasse splay (x-bdd of bank sediment as water floods floodplain)

Capped by a coating of mud  (suspension load) as waters recede

(red shale, mudcracks, soils, organics and root traces)

Marine: Longshore drift

The transport of sediment sub-parallel to the shoreline, mainly in the surf and swash zone.

Peritidal coastal environments
Not river (sed) dominated

Marshy at high tide, emergent at low tide

Minimal relief; shows reworking by tides, no progradation, barrier islands formed by longshore drift

Freshwater and saltwater mix here (brackish)

Shoreface (or nearshore)

Wave dominated; susceptible to erosion (low preservation)

 

- Lower: fine sand (& mud), may be planar lam. ± bioturbation

- Middle: accumulates longshore bars due to backwash of breakers (med. sand, well sorted, shells , x-bdd)

- Upper: affected by plunging waves, trough x-bdd

Backshore

Above high water line: topped by the berm

 

- Wind dominates waves (except storms), dune fields

- Lots of burrowing (crabs and roots)

- Washover fans may break through dune line during storms

Why is limestone important?

Fossiliferous (evolutionary history)

Porosity – reservoir for hydrocarbons
Porosity – reservoir for water (aquifers)
Cement, concrete, building stone

Chemical records for ancient environments
Responsible for our current atmosphere

Carbonate sediments

Limited conditions where carbonate environments can form – (clear, warm, shallow, agitated seas)

Organism type depends on conditions

Carbonate precipitation

Bjerrum plot: concentrations of carbonate species under a range of pH values

Carbonate precipitation (Equation for carbonate formation in seawater)

Ca²⁺ + 2HCO₃^-  ⇌  CaCO₃ + H₂O + CO₂

Conditions that favor carbonate precipitation

All promote CO₂ loss

Temperature increase

- Pressure release (degassing)

- Agitation (degassing)
- Organic activity

- Low clastic input (“poisons” CaCO₃-producing critters)

- Light (photosynthesis)

- Above the CCD

Carbonate compensation depth (CCD)

“Darwin’s Line”

30ºN to 30ºS latitude: due to conditions in this range: shallow, warm, normal marine, little clastic input

The Carbonate Factory: autocyclic vs allocyclic models

Carbonate grains: allochems

Fossil, ooids,Interclast,Pellets

Carbonate classification
Folk scheme

Carbonate rocks are named using:

1)Main allochem type

●

2)Matrix or cement dominated

Carbonate mud = micrite

Carbonate classification Dunham scheme

1)Matrix or grain supported

2)Name modified based on allochem material

Carbonate classification
Embry-Klovan scheme

1)Autochthonous^* or allochthonous^#

●

2)* - Type of “activity” allochems were doing

  # - Matrix or grain

  supported

Carbonate

mineralogies

Carbonate equilibrium reactions

CO_2(aq) + H₂O <--> H₂CO₃

H₂CO₃ <--> HCO₃^- + H⁺

HCO₃^- <-->CO₃^2- + H⁺

Carbonate compensation depth (CCD)

Ooid formation

 - <2.0 mm diameter

   (>2.0 = pisoids)

 

- Form wave-agitated waters, <6 m depth

 

- Nucleus can be any particle

- Grow from repeated precipitation of aragonite needles (chemical)

  - Bio-mediated(?) with organic   compounds

Carbonate grains: coated grains

Ooids – sand-sized coated grains, wave agitation, some nucleus (qtz, shell, ooid, etc.)
Peloids: Structureless ellipsoidal sand-sized grains, often poop or micritized grains

Pisoids: Concentrically coated, large (>2mm) spherical(ish) grain, forms in soils

Oncoids: Large (≥cm) irregularly coated carbonate grains – cyanobacterial origins

Carbonate polymorphs

Distribution of carbonates today

Carbonate platform buildup

If carbonate production outpaces tectonic subsidence, carbonate sediments will quickly reach sea level and stop aggradation

Progradation

lateral transport of sediment off shelf

Peritidal Carbonates

Periodic desiccation (low energy, tidal flat)

Salinity/temp variations

Laterally extensive bedded limestone and dolostone

Algal mats abundant, but no reefs…

Supratidal Zone

Submerged only during spring tide or storms

- Desiccation features

- Algal mats

- Evaporites

- Birds eye (fenestrae) and ‘chicken wire’ textures

- Tepee structures

- Grades into eolian (carbonate) dunes

Intertidal Zone

- Daily tides and waves

-

- Rippled carbonate mudflats

- Algal mats/stromatolites

  (Shark Bay, Australia)

- Tidal channels

- ‘Breccia’ of shells and rip-ups

Subtidal Zone

- Always subaqueous, down to ~200 m (SWB)

- Normal marine

- Carbonate production keeps up, within photic zone

- Below wavebase, so mud can accumulate (but also huge clasts from storms)

Reef environments “real estate is everything”

- Positive relief structure , resistant carbonate buildup

- Built by organisms living in them

- Framework builders    

Modern – coral

  Ancient: clams, microbes, 

stromatoporoids,   archaeocyathans

Anatomy of a reef - Forereef

- Made up of reef debris and talus

-

- Large coral fragments (knocked off during storms)

- Bedding often inclined as it drapes the sides of the reef

Anatomy of a reef – Reef crest

- High-energy environment

- Coral lifestyles and form dictated by energy regime

- Reef crest grows up to essentially mean low tide level

Anatomy of a reef – Back Reef

- Protected, only most severe storms pummel this habitat

- More branching- and platy-lifestyles for corals

- High diversity, optimal living conditions (?)

Anatomy of a reef – Lagoon

- Low-energy, often mud-rich sediments

-

- Patch reefs, rapid vertical growth (branching or mounding typical, not platy)

- Colonized by sea grasses and other delicate benthos

Dolomite and Dolomitization

1.Based on solubility and thermodynamics dolomite should form in seawater and should be the most abundant carbonate mineral. 

2.About 50% of all carbonate rocks are dolomite.

3.Dolostone hosts many of the world’s base metal deposits.

4.Many petroleum carbonate reservoirs are dolostones.

5.Dolomite composition ranges from 42-55 mole % MgCO₃

6.Dolomite that forms in modern sedimentary environments is calcium rich and poorly ordered. Ordering refers to the relative purity of calcium versus magnesium in the cation layers between those of carbonate.

7.Stoichiometric well-ordered dolomite is thermodynamically more stable relative to calcium-rich poorly ordered dolomite.

Reaction pathways: Primary Precipitate

Ca⁺² + Mg⁺² + 2CO₃-² = Ca,Mg(CO₃)₂

Cementation process – destroys porosity

Reaction Pathways Replacement Reaction

2CaCO₃ + Mg⁺² = CaMg(CO₃)₂ + Ca⁺²

 Replacement texture, but creates porosity

Reaction Pathways: Replacement with volume conservation

(2 – X)CaCO₃ + Mg⁺² + XCO₃^-2 = Ca,Mg(CO₃)₂ + (1-X)Ca⁺²

Dolomitization Pathways:Cementation

Ca²⁺ + Mg²⁺ + 4HCO^-₃ à Ca,Mg(CO₃)₂ + 2CO₂ + 2H₂O 

Dolomitization Pathways: Replacement

 2CaCO₃ + Mg²⁺ à CaMg(CO₃)₂ + Ca²⁺

Dolomitization Pathways: Mimetic Replacement

(2-X)CaCO₃ + Mg²⁺ + XCO₃^2- à Ca,Mg(CO₃)₂ + (1-X)Ca²⁺

Reflux Dolomitization

Chemical Sediments

Iron Formation Model

Iron formation formation

Phosphorites

Francolite:

(Ca,Mg,Sr,Na)₁₀(PO₄,SO₄,CO₃)₆F₂₋₃

Phanerozoic phosphorite model

Precambrian phosphorite formation

Biogenic sediments through time

Chert formation – biogenic(?)

Castille Formation

Shallow evaporitic basin with laminated gypsum and calcite ppts

Eolian environments

Climate (arid)
Topography (rain shadows)

Latitude

Continentality

Milankovitch cycles

Eolian environments on Earth

30 degrees latitude where most deserts are

Coriolis Effect on wind patterns

NH clockwise(right)
SH counter clockwise(left)

Eolian systems

- Sand dunes (deserts and coastal systems)

- Can be paleolatitude indicators (10º–30º)

- Arid region indicator, shows wind direction

Dune environments

- Fine sand to silt

- Bedforms (x bdd)

- Intradune deposits (ponds and mudcracks, ± fossils, evaporites)

- Desert pavement ± ventifacts

Types of dunes

Barchan, Parabolic, Transverse, Longitudinal

What dictates dune type…

Transport in wind

- Dust devils pick up finer fraction

- Sand-sized particles move by saltation and grain collision.

   - Results in well-sorted,

     well-rounded, and pitted     

     grains (frosted)

Dunes are mainly quartz, but there are exceptions (gypsum)

Eolian cross-bedding

Can be up to 35 m high

Foresets dip at 20º – 30º

Loess

wind-blown silt deposits

Glacial processes

- Slow moving ice flow; picks up all grain sizes

- Pick up sediments, “scours” the terrain as the glacier moves over

- Alpine (mountain) vs. continental glaciers

Zone of accumulation

more snow falls than is lost

Zone of ablation

more snow lost.

Sediment transport in a glacier

The terminal moraine from the LGM

Glacial erosion

polishing, chattermarks, striations

Outwash Plain

Braided river deposits from meltwater (downstream)

Till and tillites

•Poorly sorted

•No internal stratification

•Called “diamictite” if of unknown source, or “tillite” if from a glacier

•Striations on clasts can help identify if the deposit is glacial or not (if striations present = glacial)

Glacial sediments in proglacial lakes

glacial flour varves

Glacial Deposits

Outwash plains have braided streams that carry sand and glacial flour, and have gravel bars of cobbles and boulders

Glacial Essentials

•Found where temps are cold, and summers don’t melt ice/snow

•Sequence: till at base, gravel bars or cross-bedded sands, then varves (possibly)

•Tills are poorly sorted, non-stratified; varves are layered; fossils possible including diatoms, leaves, some verts

Deltas

Delta facies models

Why do we have tides?

Delta Environments

Delta (D) – running water from streams enters standing water (ocean)
(Deposits more sediment than longshore drift can carry away)

Delta Facts

Found on passive margins.

1.  Needs a stable shelf on which seds can accumulate.

2.  No coastal mountain ranges; means large river basins that provide deltaic sediments can form

Deltas build outward (Progradation)

Delta plain = meandering floodplain, swamp, beach

Delta front = steeper, may show sands and channels

Prodelta = sloping down to open shelf

Delta shape depends on sed input, wave energy, tides…

Typical sequence shows coarsening upwards as delta progrades (builds seaward)…

Submarine fans

Bouma sequences (turbidites)

E – Suspended seds (mud and hemipelagic “organic snow”)

D – Laminated seds (LPB)

C – Ripples (x-bdd)

B – Laminated seds (UPB)

A – Massive to graded beds

Scour

Laterally and vertically adjacent (Walther’s Law) 

What causes turbidites?

High sedimentation from the shelf

Seismic events (seismites)

Storm events (tempestites)

Sea level fall

Turbidity currents in the “modern”

200 km³ of sediment more than 1100 km

40-60 mph

Lithostratigraphy

rock properties

Magnetostratigraphy

magnetic remanence

Biostratigraphy

– fossil assemblages (biozones)

Chemostratigraphy

chemical properties (isotopes)

Well logging

borehole properties

Seismic stratigraphy

subsurface acoustic properties