Seds - Carbonates

Question 1

Calcite (as a major carbonate mineral)

Answer 1

• CaCO3
• Trigonal - Rhombohedral
• Most common carbonate mineral found in the rock record
• Layered
• Central Ca ion
• 2 Cleavage planes
• Not most common carbonate mineral being made today

Question 2

Aragonite (as a major carbonate mineral)

Answer 2

• CaCO3
• Orthorhombic
• Most common carbonate mineral being made today
• Metastable – alters to calcite over time
o Why calcite is most abundant
• Less stable but more resistant (mechanically stronger) – why found in shells (corals and molluscs as its preferred through natural selection

Question 3

Dolomite (as a major carbonate mineral)

Answer 3

• (Ca.Mg(CO3)2)
• Not calcium carbonate – calcium magnesium carbonate
• Not depositional – forms through alteration of calcite
• Calcium is replaced by magnesium
o Ratio
 Below 50% = high magnesium calcium
 Above 50% = dolomite
• Meteoric waters the most likely supply of magnesium.
• Increased resistance to weathering.
• Iron causes buff colour in instances
• Wont dissolve under acid

Question 4

Breakdown of identifying major carbonate minerals:

Answer 4

•	Aragonite
o	Shimmery
o	Allochems
•	Calcite
•	Dolomite
o	Buff + rhomboidal
o	Authochems

Question 5

Allochems (grains):

Answer 5

Ooids:
•	Near spherical, smooth looking
•	Almost always calcite
•	Thin section
o	Very round
o	Different makeup of void, radial and nucleation spaces
•	Are a category

Peloids:
•	Similar to ooids
•	No internal structures
•	Amalgamation of mud
•	Mainly faecal matter or parts of mud
•	Can be subdivided into pellets (faecal) and intraclasts (mud)

Skeletal Grains:
• Fossil accumulations
• Bioclastic

Forams:
• Tiny skeletons from single celled organisms
• Large (can reach 1cm)
• Growth chambers
• Leads to multi-chambers
• Round and smooth – more structures than ooids

Cocoliths:
• Very small – microscopic – cant see with optical microscope
• Each plate is coccolith – together is coccolithophore – living organism
• Formation of chalk

Question 6

Autochems (matrix):

Answer 6

Sparry cements:
•	Crystals that cement grains together
o	Grains don’t touch
o	Cement without needed to dissolve grains – so much calcite
	Less chemical compaction in carbonates
	Less common sutured and long contacts

Micritic cements:
• Muddy material
• Either washed in or diagenetic
• Muddy matrix which cements parts together

Question 7

Cathodoluminecsecene

Answer 7

bombarding samples with electrons – raising energy levels – when they fall they release light based on wavelength
• Shows growth of carbonates is done in stages and not all at once
o Could show history
 Burial history for example

Question 8

Lime-mud

Answer 8

• Mudstone when fine grained mud minerals are not silicate but carbonate minerals
• Precipitation by algae mostly

Question 9

Evaporitic carbonates

Answer 9

o First thing to be precipitated out of seawater during evaporation
o Minor carbonates form
o Calcite and dolomite

Question 10

• Biogenic carbonates

Answer 10

o Main place they are produced
o Organisms alter seawater minerals and precipitate carbonate
o Carbonate production rates higher than many clastic processes. (Reef growth 1.5 – 6 m/kyr)
 Faster than other geology because it is biological precipitation
• Leads to thick carbonate deposition
o Temperature and light primary controls on deposition
 Shallow clear water is optimum
 Muddy water blocks light and provides too many nutrients so algae begin to grow and block the sun further
o Three main types of depositional environment

Question 11

Three main types of biogenic depositional environments

Answer 11

	Tropical
•	Shallow, clear water
•	Reefs
•	Biologically controlled
	Cold water
•	Biologically controlled
•	Too cool for biogenic processes but some carbonate is precipitated
	Mud-mound micrite
•	Carbonate mixed in mud
•	Abiotic

Question 12

Problems with sequence stratigraphy and carbonates

Answer 12

• Ideal sequence stratigraphy controlled by accommodation space
• Carbonates are a problem as they don’t erode away

Question 13

Carbonate sequence stratigraphy

Answer 13

• Early lithificiation/ skeletal framework allows for resistance to erosion.
• Rapid sediment production fills TST.
• Water is going up but hasn’t reached top stage
• Making accommodation space at maximum speed
• Carbonates will follow sea level growth and lead to thick carbonate packages
• High stand + lots of accommodation space
• Limestones are biogenic so the high sea level can drown the carbonate
• Falling stage systems tract
• Sea level goes down, leaving carbonate exposed
• Fine detail of carbonate is removed, and top layer is cemented by enrichment of calcite fluid
• Now left with a protruding resistant layer – limestone pavements

Question 14

Framework support for reefs:

Answer 14

• Skeletal frameworks allow for the building of competent sedimentary rocks before burial.
• Alters diagenetic sequence compared to clastic sediments
• Can resist mechanical compaction because its already rigid
• Carbonates don’t have a reduction in porosity because they are tough mechanically
• Not tough chemically; chemical compaction more significant

Question 15

What do carbonates tell us about the depositional environment?

Answer 15

• T controlled by photic zone (~80 m depth)
• Coral reef type
• C (Cool water) shows similar responses to clastic systems
• Carbonate ramp type
• M (Mounds) can build high slopes due to biogenic cements
• Mounds as mud becomes sticky from inclusion of carbonate