Sedimentology and Palaeontology Lecture 20: Carbonates

Question 1

Define Carbonate sediments

Answer 1

Any sediments or sedimentary rocks that are formed primarily of CaCO3 and/or CaMg(CO3)2.

Carbonates are generally produced within their
depositional environment.

Question 2

Describe the Carbonate polymorphs

Answer 2

Calcite
Mg-Calcite
Aragonite
Dolomite

Question 3

Define Biological, Biochemical and Chemical in the context of carbonate production

Answer 3

Biological = biomineralisation

Biochemical = precipitation of CaCO3 from ambient
water induced by biological activity

Chemical = direct inorganic precipitation of CaCO3 from ambient water

Question 4

Define Euryhaline and Stenohaline

Answer 4

Euryhaline = broad salinity tolerances (e.g. bivalves)

Stenohaline = narrow salinity tolerances (e.g.corals)

Question 5

Define Phototrophs and Heterotrophs

Answer 5

Phototrophs = obtain energy from the sun
They tend to live in equatorial regions.

Heterotrophs = obtain energy by consuming food
These tend to live in higher altitude regions

Question 6

Why is textural maturity not applicable to Carbonate sediments
/
Why is grain size and shape sorting not useful when interpreting Carbonate sediments

Answer 6

Textural maturity is not really applicable to carbonates because different taxa disintegrate at different rates.

Also different organisms different shell size and shape which affect grain size and shape, therefore making it much more unclear when interpreting

Question 7

Define skeletal grains

Answer 7

Skeletal grains are sediment grains which come from organisms (shells of organisms)

e.g. Bivalve, Brachiopod, Gastropod, Echinoderm, Foraminifera, Bryozoan, Red Algae, Coral

Question 8

Define Non skeletal grains

Answer 8

Non skeletal grains are Carbonate sediment grains which are not from shells of organisms.

Formed by biologically induced precipitation of CaCO3

Cyanobacteria = non-calcifying photosynthetic
filamentous micro-organisms - important agents of
CaCO3 production and destruction

Question 9

Describe the non skeletal grain type: Ooids

Answer 9

Ooids (name derived from Greek Ooin = egg)

Subspherical shiny coated grains (<2 mm diameter)

Detrital nucleus (e.g. shell fragment) and cortex of concentric layers (cyanobacterial biofilms induced precipitation of CaCO3)

Formation requires sustained energy and light

Look like eggs or white milk duds

Question 10

Define the non skeletal grain type: Oncoids

Answer 10

Oncoids

Grape shaped. Have nucleus like central pale bit. Have swirling lines around nuclues

Irregularly-shaped biologically
accreted grain (mm to cm size)

Detrital nucleus with a cortex of encrusters (e.g. cyanobacteria, algae etc.)

Reflect intermittent high energy and period rolling

Question 11

Define Micritisation

Answer 11

Micritisation = inwards destruction of grains by
cyanobacteria

Micritisation is visible in sediments when skeletal grains start to turn white and dull coloured

Question 12

Define micrite

Answer 12

Micrite is extremely fine sediment in carbonate sediments.

Micrite is 10 to 100µm, looks like a powder

Question 13

Define peloids and pellets

Answer 13

Peloids = fully micritized grain, allochems that are composed of micrite, irrespective of size, shape, or origin. The two primary types of peloids are pellets and intraclasts.

Pellets = faecal

Question 14

Define the non skeletal grain type: Grapestones

Answer 14

Grapestones = grain aggregates bound together by cyanobacteria and then micritized

Question 15

Define the non skeletal grain type: Intraclasts

Answer 15

Intraclasts = eroded and reworked fragments of part-lithified carbonate sediment

Question 16

Give the two main types of carbonate sediment classification systems

Answer 16

Folk classification
* Prefix = grain type(s) from least to most abundant (E.g. Biominerals, Ooids, Peloids, etc)
* Suffix = sparite cement or micrite matrix

Dunham classification
* Emphasis on textures
* A compositional prefix may be added