Week 3,4,5 - Carbonates

Conditions for high carbonate production

Light

Nutrients

Warm water

High salinity

Shallow subtropical shelves

Lithofacies

Modern sediments reflect different physical environments (water depth, energy, nutrient supply)

Biogenic and/or abiogenic sediment composition

Analysis reconstructs depositional environments and processes

Order of tidal zones

Peritidal (Supratidal → Intertidal)

Subtidal

Features of the peritidal zone

Range <2m

Low faunal diversity

Subtidal sediment source

Regions of intertidal channels

Algal mats

Structures depend on climate

Why does the peritidal zone have low faunal diversity

2x a day organisms go from being under saline water to exposure to the air -> stressed environment -> lower biodiversity

How do textures in clays form

Clays expand when wet then contract when dry leading to mudcracks, desiccation polygons, fenestrae, lamination and bioturbation

Features of the subtidal zone

Ponds and tidal channels

High biodiversity with high abundance of skeletal fauna

Bioturbation

Fining upwards and levee deposits

Peritidal zone in an arid climate

Halite crusts, gypsum, anhydrite

Dolomite

Algal mats

Dessication cracks

Windblown deposits

Formation of karsts

Low pH water percolates through limestone -> dissolution of limestone -> infill of sandstone above -> diagenesis

Features of open shelves and lagoons

Moderate-low energy environments

Muddy sediments, bioclasts

Often burrowed

Separation based on degree of connection to the open ocean

Features specific to open shelves

Normal salinity

Abundant and diverse fauna

Features unique to lagoons

Elevated salinity

Abundant but low diversity fauna which can tolerate salinity fluctuations

Micritisation

The action of cyanobacteria in the clasts of carbonate sediments retract nutrients from theing carbonate

Features of sand bars and shoals

High energy

High carbonate production

Grainy sediments

Important reservoirs

Restrictions on echinoderms

Must be normal salinity

Implications of cross stratification

Current was present as there was energy moving the sediment

Appearance of bivalves in thin section

Line of symmetry between two shells

Long and curved grains

Appearance of gastropods in thin section

Variable shape

Common shallow marine and terrestrial

Corals in samples

Rugose + tabulate (Paleozoic) - high Mg calcite with high preservation potential

Scleractinian corals - aragonite skeletons with lower preservation potentials

Septa -> Rugose/Scleractinia

Tabulae -> Tabulate

Warm clear waters

Echinoderms in samples

Crinoids, starfish, sea urchins

Internal calcite skeleton

Normal salt content

Foraminifera

Small calcitic organisms

Benthic

Cambrian - recent

Algae in samples

Green -> visible filaments and no ordered internal structures, productive <15m depth

Red algae -> tiny rectangular pores, greater depths

Ooids

Spherical grains with concentric layers surrounding the nucleus

<2mm

Usually marine

High energy environments

Oncoids

Coated grains with concentric but irregular layers

Lack distinct nucleus

Usually shallow marine

Peloids

Structureless grains

100-500 microns

Well rounded and ellipsoidal

Derived from clasts

Grain aggregates

Form by algal binding and cementation

Binding and grapestone stage → lump stage

Intraclasts

Reworked partially lithified local material from within the basin

Extraclasts

Lithified material from outside the immediate depositional area

Lime mud

Micrite (calcium carbonate crystals <62 microns)

Originally aragonite or calcite replaced by low Mg calcite

Fills pores, borings and burrows to form rocks

Dunham classification

Organic/skeletal reefs

Rigid calcareous organisms

Matrix or skeleton supported

Deposited in warm or cold water

Able to withstand high energy wind/wave action

Reef/mud mounds

Inorganically and/or biogenically constructed

Lack a rigid skeletal framework

Unable to withstand high energy wind/wave action → deeper water settings

‘Fine grained, mud (micrite)-dominated build-ups

Stability provided by matrix, limited cementation

Organic components include bivalves, corals, sponges, bryozoa, microbes, stromatoporoids

Heterotrophic and biologically influenced/induced carbonate precipitation

Low topographic relief

Bioherm

Massive reef structure which forms distinct topography

Biostrome

Massive core which passes laterally consistent with the bedding

Constructive processes

Biological processes through direct growth, baffling and binding

Destructive processes

Wave damage and biological destruction

Cementation

Early cementation from seawater

Sedimentation

Accumulation of biogenic matter and reed derived detritus

4 stages of reef building

Pioneer

Colonisation

Diversification

Domination

Corals in high sedimentation rate areas

Long branches working upwards to reach the light

Corals in high energy rate areas

Strong and robust to withstand wind and wave action

Areas of a reef

Back reef

Reef flat

Reef front

Fore reef

Reef flat

Depths up to 10m

Reworked reef debris

Carbonate sand

Bioerosion

Reef crest

Encrusting organisms

Bioerosion

Periodic subaerial exposure

Bindstones and framestones

Reef front

Extensive coral growth seaward of reef crest

High energy zone

Low preservation potential

Forereef

Transition to basin

Sedimentation from grabity flows

Rudist

Cretaceous bivalve which appears like a coral

Basin deposition

Deep water

Low energy

Settling out of suspended material

Main extrinsic force causing lateral migration of facies

Sea level fluctuation

Transgression

Shallow water environments get progressively deeper

Regression

Deep water environments get progressively shallower

Eustacy

Over time sea level changes relative to the center of the Earth

Driven by the volume of water in the ocean due to change in glacial ice volume

Volume of ocean basins related to tectonic spreading rates

Time scale >10^6 years

Subaerial exposure

When sea levels drops below the top of the system

Formation of karsts on the surface from the slightly acidic rainwater leading to dissolution of carbonates

Effect of influx of silicilastics on reef growth

Suspended mud limits light penetration in the water column → inhibits carbonate formation

Mud settles slower than sand

What forms chalk

Coccoliths (cretaceous onwards)

Hothouse climates

High tectonic spreading rate

Dispersed continents

High sea level

Increased CO2

Calcite dominated seas

Icehouse climates

Low spreading rate

Supercontinents

Decreased CO2

Aragonite seas