clastic sedimentology.docx

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Introduction

Carbonates are rocks composed mainly of calcium carbonate, CaCO3. Some examples of

common carbonate rocks are limestones and chalk. Carbonates form by precipitation from water;

either straight from the water, or induced by organisms, to make their shells or skeletons, andthey form in many environments (Figure 1).

F igure 1: Some typical environments that carbonates can form. Redrawn from Tucker and Wright (1990).

Mineralogy

There are three main minerals that form carbonates:

Calcite (CaCO3), which comes in high magnesium and low magnesium forms.

Aragonite (CaCO3), which has a different structure to calcite.

Dolomite (CaMg(CO3)2), a magnesium rich carbonate produced by diagenesis.

Only low magnesium calcite is stable at surface pressure and temperatures. It is therefore the

most common mineral in ancient carbonates. However, most modern carbonates are composed of aragonite as this is the mineral that most biological organisms create to make their shells or

skeletons. Examples of organisms that produce aragonite shells are bivalves (sea shells),

gastropods (snails) and Halimeda (a green algae). Organisms that produce a calcite shell include

brachipods (a rare type of sea shell) and ostrocods (a small crustacean).



Components

Carbonates can be made of several components. These are:

Bioclasts. Ooids.

Peloids.

Intraclasts.

Micrite.

Sparite.

Bioclasts

Bioclasts are fragments of dead sea creatures. These include shells (Figure 2) and corals.The creatures precipitate the carbonate in order to produce some kind of structure.

F igure 2: A bioclastic limestone. Here the bioclasts are bivalves of Carboniferous age.

Ooids

Ooids are rounded grains formed by precipitation of calcite around a nucleus to produceconcentric circles (Figure 3). They form in warm, shallow waters, with a strong tidal

currents. Wave action may also contribute to their near-spherical shape.

F igure 3: The structure of an ooid. Redrawn from Tucker and Wright (1990).

Peloids

Peloids are sand sized grains (100-150 micrometers) of micro-crystalline carbonate. They

are generally rounded or sub-rounded. They originate from fecal pellets, algae and mud



clasts. They are sometimes found clumped together, in a formation known as a

grapestone.

Intraclasts

Intraclasts are clast of other limestone that appear in younger limestones. They can bequite difficult to distinguish at times, as they may be made of a similar rock as that which

encases it. For example, hardgrounds can from when sea water flows through carbonatesediment, lithifying it rapidly. Subsequently, the hardground may be broken up andincorporated into the surrounding sediment.

Micrite

Micrite is microcystalline carbonate mud, with grains less than 4 micrometers.

Sparite

Sparite is coarser than micrite, with a grain size of more than 4 micrometers and is

crystalline.

Both micrite and sparite form the matrix or cement in carbonate rocks.

GRAIN COMPOSITION

There are two primary categories for the classification of carbonate grains: Non-Skeletal andSkeletal. Non-Skeletal grains can be entirely abiotically formed as in the case of coated grains,

grain aggregates and clasts, or both abiotically and biotically formed as in the case of peloids.

Within the Trenton limestone, however, the majority of readily observable sedimentary grains

are produced directly from the disarticulation and fragmentation of skeletal hard parts and areconsidered skeletal grains.

o Non-skeletal grains

non-skeletal grains can be produced either abiotically or biotically and can be classifiedinto one of the following four categories: coated grains, as in the case of ooids and

pisoids; grain aggregates, as in cemented groups or clusters of carbonate grains; clasts,

such as lithified pieces of previously formed carbonate rocks; or peloids, which are any

number of sand-sized, microcrystalline and often structureless carbonate grains.

Skeletal grains

By far the most-dominant carbonate grain type in the Trenton Limestone, skeletal

fragments from echinoderms, arthropods such as trilobites and ostracodes, bryozoans,

brachiopods, molluscs, sponges etc. make up significant portions of these limestones. Thetaphonomy or state of preservation of these fossil types helps to constrain the range of

depositional processes under which these carbonate grain producing organisms were

subjected after their death and leading up to their final burial.

MATRIX COMPOSITION

http://www.mcz.harvard.edu/Departments/InvertPaleo/Trenton/Intro/PaleoPage/TrentonFauna/Echinodermata/Echinodermata.htm


http://www.mcz.harvard.edu/Departments/InvertPaleo/Trenton/Intro/PaleoPage/TrentonFauna/Arthropoda/Arthropoda.htm


http://www.mcz.harvard.edu/Departments/InvertPaleo/Trenton/Intro/PaleoPage/TrentonFauna/Arthropoda/Trilobita/Trilobita.htm


http://www.mcz.harvard.edu/Departments/InvertPaleo/Trenton/Intro/PaleoPage/TrentonFauna/Arthropoda/Crustacea/Crustacea.htm


http://www.mcz.harvard.edu/Departments/InvertPaleo/Trenton/Intro/PaleoPage/TrentonFauna/Bryozoa/Bryozoa.htm


http://www.mcz.harvard.edu/Departments/InvertPaleo/Trenton/Intro/PaleoPage/TrentonFauna/Brachiopoda/Brachiopoda.htm


http://www.mcz.harvard.edu/Departments/InvertPaleo/Trenton/Intro/PaleoPage/TrentonFauna/Mollusca/Mollusca.htm












The next major category to consider when discriminating carbonate rocks such as the Trenton

Limestone, is matrix classification. This category focuses on the sedimentary material, or lack

thereof, found between grains within a rock. Generally sediments are classified as either grain-supported, where granular materials with readily observable size and shape dominate the rock, or

as matrix-supported where the material is generally too small to be distinguished as a grain.

o Matrix supported

In this case, matrix materials usually consist of microcrystalline or cryptocrystalline

carbonate sediments. These textural descriptors indicate that the crystal size and shape

is too small to be readily observable without the use of a petrographic microscope and

polished thin-sections.

As the compositional discrimination of matrices is highly-dependent ondiagenetic effects and requires microscopic analysis, matrix classifications of the

Trenton limestones is not a focus of the discussion here. One must, however, note

the presence or absence of matrix within a rock.

o Grain-supported

In the case of grain-supported carbonate rocks, this classification is used to define the

dominance of visible sedimentary grains within a limestone. Due to their large

concentration relative to cryptocrystalline grains, individual silt to sand-sized particles or larger grains are more likely to be in direct contact with one another. Thus in a grain-

supported limestone, there is a general lack of fine-grained micrite and larger carbonate

grains compose the majority of the rock framework.

CEMENTS

Although once sedimentary carbonate grains are deposited, their final transition to rock

represents an additional process that needs to be at least considered in the classification of most

limestones. The cementation of carbonate grains is generally a very complicated process andoccurs along several diagenetic pathways depending upon the environment of cementation, on

the specific mineralogic composition of the carbonate grains themselves, and the mineralogic

composition of fluids that flow through the sedimentary mass. The discussion here is only to briefly present the general desciption of cement types for classification purposes.

Classification

There are two main methods of classifying carbonate rocks; Dunham's and Folk's classification.

Below is Dunham's classification which uses the texture of the rock to classify the carbonate.

Dunham's classification is useful for field work.



CARBONATE ROCK CLASSIFICATION

Throughout the last half-century, studies of modern carbonate depositional environments and

their grain compositions and size distribution ranges have led to multiple classification schemesfor carbonate rocks. Such studies have used modern textural classifications such as grain size,

sorting, rounding, and grain composition and cement/matrix proportions to standardize thedescription of both modern and ancient carbonate rocks. These classification schemes, althoughslightly different in their nomenclatural development, remain very similar in their overall useage.

The most commonly used classification systems are either based on grainsize analysis or on theclassifications of Folk (1962), or Dunham (1962), where both focus on textural characteristics of

carbonate rocks. The following discussion will briefly introduce the conceptual usage of bothclassification schemes. Note that the intention of this discussion is to provide enough background

for the reader to understand the lithologic classification of the Trenton Limestones. For a more

detailed discussion of these classification systems, the reader is encouraged to review the

bibliography section of this webpage and select the pertinent references for more information.

o Grain-Size Classification

As is common in most sedimentary rocks, carbonate rocks can be classified according to the

dominant grain size in the rock. Using this classification requires that the grain-size be estimated

or measured exactly and then applied to a size-range chart for the establishment of a rock term.

A carbonate rock is usually classified in one of three main categories: calcilutite (those rocks

where the grain-size is 62 micrometers or smaller); calcarenite (those rocks where the grain-size

is between 62 micrometers and 2 millimeters); and calcirudite (those rocks where the grain-size

is greater than 2 millimeters).

The main strength of this classification system is to suggest a general association betweengrain-size, grain-sorting and deposition energy, however this system is most often limitedfor the description of most limestones.

o Folk Classification

As mentioned previously, the classification system of Folk (1959, 1962) relies mainly on textural

characteristics of limestones and carbonate sediments. Folk, recognizing the three maincompositional categories of carbonate rocks: grain composition, matrix composition, and cement

composition, suggested that a carbonate rock classification should encompass aspects of all of

these characteristics when trying to establish a rock name. The figure to the right shows Folk's

nomenclatural system using: grain types (he referred to them as "allochems"); matrix; andcements. In this scheme Folk uses the allochem types (shown in purple) as the prefix, and the

dominant matrix or cement composition as the suffix. For example, for a rock that contains a

micritic matrix and skeletal fragments, the associated name would be biomicrite.

Folk's nomenclatural system provides a rough framework for the naming of rock types, but he further

established that the range of matrix to cement occurs along a continuum from 0% matrix and 100%

cement or spar, through 100% matrix and 0% spar. To further differentiate rocks showing different



relative proportions of micrite and spar, he constructed the following diagram to relate the relative

ratios of each to textural categories. In this later scheme shown below, Folk introduces the percent

allochems, and degree of sorting, rounding and abrasion (shown in red) to name specific carbonate rock

types. In this classification system, he has added prefix modifiers to establish more refined rock names

(shown in blue). For example, if a limestone was shown to be dominantly of micritic matrix with

approximately 40 % of the mass of the rock composed of ooids, Folk would have applied the name

sparse oomicrite to communicate the rock's composition.

Dunham Classification

In an alternative scheme to that of Folk, Dunham (1962) proposed a similar carbonate rock classification

system utilizing some of the same principles used by Folk. In Dunham's nomenclature, he looked first at

textural considerations of a rock including whether texture was recognizable in the rock. He then looked

to see whether sedimentary materials were somehow bound as part of the depositional process. He was

interested in separating those rock types where biologic activity had trapped sedimentary materials, as

in the case of stromatolites. In his hierarchy, once the basic textural categories were assigned, he then

looked at the relative proportion of mud in the sample. If the rock had no mud and was dominated by

coarse-grained sediments, he classified these rocks as grainstones. However, if the sample containedany amount of mud it was then considered in percent relative to the number of grains. In this way, he

was designated mudstones as having less than 10% grains, wackestones with more than 10% grains but

less than the amount required to support the rock, and packstones where the sedimentary grains

supported the rock framework but still had appreciable quantities of mud. Dunham's rock names are

shown in blue below

Recognizable Depositional Texture Crystalline

Components not bound together Originalcomponents

boundtogether

Crystalline

Carbonate

Contains Mud Lacks mudand is grain

supported

Boundstone

Mud Supported GrainSupported

GrainStone

<10%

grains

>10% grains

Packstone



Mudstone Wackestone

Folk's classification is very useful for carbonates in thin section. It classifies carbonates based on

their components (see above) and the matrix/cement binding the components together.

Clasts

Interclast material

Micrite Sparite

Fossils Biomicrite Biosparite

Ooids Oomicrite Oosparite

Peloids Pelmicrite Pelsparite

Intraclasts Intramicrite Intrasparite

Conclusion

Carbonate rock are sedimentary rocks from from calcium carbonate, CaC0 3. Calcium carbonateis either the mineral aragonite, high magnesium carbonate or low magnesium carbonate. Only

low magnesium carbonate is stable at surface pressures and temperatures, but most modern

carbonate is aragonite. Carbonates are formed from various components, which include ooids,

peloids, clasts and biological remnants. Carbonates are classified by either Folk's or Dunham'sclassification schemes.

References

M. E. Tucker and P. Wright, 1990. Carbonate Sedimentology. Buy It.

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