Carbonate Rocks

Strong link with the “biologic world” carbonate sediments are born, not made (James, 1984)

“Organic world” has stronger and more complex relationship with the environment than the inorganic one.

A qz grain is a qz grain at the present time as well as in the Cambrian. An actual reef works in a different way than a Cambrian reef.

The study of carbonate rocks allows paleoenvironmental recostructionof high detail

Why carbonate rocks are special?

Lateral transport of carbonate sediments is usually very limited .

Why carbonate rocks are special?

The composition of a carbonate rock gives very often precise information on the depositional environment.

The composition of a siliciclastic rock does not give in general information of the original depositional setting

Why carbonate rocks are special?

Carbonates can be the final product of different processes

mechanical deposition

chemical precipitation in situ growth (build-up)

Why carbonate rocks are special?

Classification of carbonate rocks

Fundamental to understanding sedimentary rocks is the distinction of three basic components: GRAINS (termed allochems by Folk), MATRIX (carbonate mud), and porosity or porosity-filling CEMENT. Typical carbonate rocks are plotted here on a ternary diagram using those three end members.

Ternary Plot of Major Limestone Constituents

Carbonate Rock-Petrography

Carbonate Rocks are made of :

Grains or allochems Matrix or mud Cement

matrix cement grains grains

Grains non-skeletal grains

skeletal grains/bioclasts

non-skeletal grains

Carbonate Rock-Petrography

skeletal grains

Carbonate sediments and rocks (lithified sediment hardened by diagenetic

processes, such as cementation, compaction, mineral recrystallization)

Carbonate rock classification: Dunham (1962) based on

DEPOSITIONAL TEXTURE: information about energy of the depositional environment.

MOST IMPORTANT and COMMONLY in USE!!

Carbonate rock classification: Folk (1951) based on grain composition and presence of micrite or cement: information about

the origin of the grains and of the depositional environment. NOT CURRENTLY IN USE!!

Carbonate Classifications

Dunham (1962) Limestone Classification

The distinction as to whether a rock is supported by matrix (mud) or framework (grains) is fundamental to this scheme. The four basic terms for normal (non-crystalline) limestones can be further modified with terms describing constituent grains or other features. The most difficult aspect of this classification, in many cases, is in deciding whether a rock with large and irregular (skeletal or intraclastic) grains, or one that has undergone substantial post-depositional compaction, was originally mud- or grain-supported.

Dunham classification is based on depositional texture

Embry& Klovan (1971) Limestone Classification

In Embry and Klovan (1971) scheme,for biogenic limestones, the term “floatstone” replaces Dunhamʼs “packstone”. For coarser, grain-supported biogenic limestones, the term “rudstone” was coined. In addition, an organically bound rock can be termed a “bafflestone”, a “bindstone”, or a “framestone”, depending on the nature of the organic structures. These last three terms, in particular, have been widely adopted by those working on reefs, bioherms, and other biogenic carbonates.

EMBRY & KLOWAN (1971)

Dunham (1962) Limestone Classification

1. Original components bound together at the time of deposition: BOUNDSTONE 1a. Organisms build a rigid framework: FRAMESTONE (corals) 1b. Organisms act as baffle: BAFFLESTONE (algae, bryozoans) 1b. Organisms encrust and bind: BINDSTONE (Stromatolites, red algae)

Carbonate rock classification: Dunham (1962) + Embry and Klovan (1972)

2. Original components not bound together at the time of deposition 2a. Less 10% grains > 2mm -When mud-supported and grains are < 10%: MUDSTONE -When mud-supported and grains are > 10%: WACKESTONE -When grain-supported with micrite matrix filling the pore space: PACKSTONE -When grain-supported, no micrite matrix (sediment) but cement (precipitate) or unfilled pore space: GRAINSTONE 2b. More 10% grains > 2mm -Matrix supported: FLOATSTONE -Grain supported: RUDSTONE

Peloidal grainstone with intraclasts and bioclasts

Bioclastic grainstone

Lime mudstone

Bioclastic Packstone with intaclasts

Oolithic-bioclastic Grainstone

Bioclastic Packstone

Oolithic Grainstone

Bioclastic Wackestone

Bioclastic Packstone with intraclasts

Bioclastic Grainstone

Oolithic-peloidal Packstone

Boundstone

Algal boundstone (bindstone) Finely laminated, algal boundstone (Stromatolite).

Coral boundstone (framestone)

From Baccelle & Bosellini (1965)