Sedimentation & Stratigraphy: Carbonates

What is a limestone?

rocks composed mainly of calcium carbonate (CaCO₃)

The rocks and sediments which eventually lithify to form limestones are called…?

carbonates or calcareous material

How to differentiate from Calcite and Quartz in the field?

the former has a Mohs’ hardness of 3

What is substitution?

when elements with the same charge and similar ionic radii can switch for each other in atomic lattice

What kind of ions can substitute for calcium to create two different forms of calcite in nature?

magnesium

What is the magnesium content for low magnesium calcite?

<4%

What is the magnesium content for high magnesium calcite?

11-19%

How does aragonite and calcite differ?

they have the same chemical composition, but different mineral forms

What is dolomite composed of?

CaMg(CO₃)₂ - calcium magnesium carbonate

How to distinguish dolomite from calcite and aragonite?

HCl test - it doesn’t react

What are skeletal fragments?

carbonate sediments that are whole or broken pieces of the hard body parts of organisms that use calcium carbonate minerals as part of their structure

What are Ooids?

spherical bodies of calcium carbonate <2mm in diameter

How do Ooids form?

precipitation of calcium carbonate around the surface of the sphere

What is oolitic limestone?

a rock made up of carbonate ooids

What are Pisoids?

concentrically layered carbonate particles >2mm in diameter, often more irregular in shape

What are Peloids?

fecal pellets of marine organisms that can be abundant in some carbonate deposits

What are Intraclasts?

fragments of calcium carbonate material that has been partly lithified and then broken up and reworked to form a clast which is incorporated into the sediment

What are aggregate grains?

carbonate grains consisting of several fragments cemented together

What are grapestones?

aggregate grains that are comprised of a collection of rounded grains

Fine grained calcium carbonate particles <4μm across are referred to as?

lime mud, carbonate mud, or micrite

What fossil is this?

crinoid

What fossil is this?

echninoids

What fossil is this?

calcareous sponges

What fossils are these?

belemnites

What fossil is this?

foraminifera

What fossil is this?

gastropods

What fossils are these?

brachiopods

What are bivalve molluscs?

distinctive layered shell structure consisting of two or three layers of calcite, aragonite, or both

What are gastropods?

molluscs with a calcite or aragonite layered structure that are distinctive for their coiled form

What are brachiopods?

shelly organisms with two shells made up of low magnesium calcite and a two-layer structure of fibrous crystals

What are echinoids and crinoids?

both construct their body parts out of whole low magnesium calcite crystals, the former in plates and the latter in stem disks

What are foraminifera?

small, single-celled marine organisms which range from a few tens of microns in diameter to tens of milimetres across

What are bryozoa?

a group of single-celled colonial organisms that are made of aragonite, high mg calcite, or a mix of the two