Chapter 3 the classification of clastic sedimentary rocks

Chapter 3: The Classification of Clastic Sedimentary RocksA very basic classification of all sedimentary rocks is based on the type of material that is deposited and the modes of deposit

Chapter 3: The Classification of Clastic Sedimentary Rocks

A very basic classification of

all sedimentary rocks is

based on the type of material

that is deposited and the

modes of deposition

Grain Size 1

(mm)

Sediment

> 2 Gravel Rudite Cobble, pebble, well

Silt or clay

1 For the purposes of this general classification we will assign the rock or sediment name shown if more than 50% of the particles are in the range shown More

detailed classification schemes will limit terms on the basis of different proportions

of sediment within a given grain size

A simple classification of terrigenous clastic rocks and sediment is based

on the predominant grain size of the material:

Classification based on grain size

Classification of Sandstones

Most sandstone classifications are based on the composition of the rock.Dott’s classificaton scheme is used in most courses at Brock

It is based on the relative proportions of:

Quartz

Feldspar

To classify sandstones using Dott’s scheme the first step is to determine composition of the rock.

Point counting is a method whereby a thin section on a petrographic

microscope is examined by stepping across the thin section at equal

intervals and identifying the material (quartz, feldspars, rock fragments

or matrix) that lies immediately beneath the cross hairs Counting 250

to 300 grains will accurately yield the proportion of each component

Component Number of Proportion

Grains counted (%)

Quartz 73 26

Feldspar 56 20

Rock fragments 34 12

Matrix 118 42

Total: 281 100

A first order classification is based on the proportion of matrix that is present:

% matrix Rock Name

< 15

15 - 75

>75

Arenite

Wacke or Graywacke Mudstone

Example Point Count Data:

Component Number of Proportion

Grains counted (%)

Quartz 73 26

Feldspar 56 20

Rock fragments 34 12

Matrix 118 42

Total: 281 100

A first order classification is based on the proportion of matrix that is present:

% matrix Rock Name

< 15

15 - 75

>75

Arenite

Wacke or Graywacke

Mudstone

Example Point Count Data:

Total: 100

To classify Arenites and Graywacke’s on the basis of their specific

compositions the data must be “normalized” to 100% quartz, feldspars and rock fragments

The next step is to plot the

normalized data on a ternary

diagram to determine the specific field in which the data fall

The next step is to plot the

normalized data on a ternary

diagram to determine the specific field in which the data fall

If the proportion of matrix is less than 15% plot the data and use

Dott’s diagram for the

classification of arenites

If the proportion of matrix is less than 15% plot the data and use

Dott’s diagram for the

classification of arenites

If the proportion of matrix is less than 15% plot the data and use

Dott’s diagram for the

classification of arenites

If the proportion of matrix is less than 15% plot the data and use

Dott’s diagram for the

classification of arenites

This classification is based on the major component of most sandstones and provides a basis for a consistent nomenclature for sandstones.

Specific types of rock fragments may also be important in determining the history of the sediment

Fragments of limestone or dolomite are simply classed as “rock

fragments” using Dott’s scheme

Such grains break down rapidly with transport so that their presence

suggests that the sediment was deposited very close to the area that it was produced

The names can be modified to reflect other components of the rock:

e.g., Calcareous quartz arenite: a quartz arenite with a calcite cement

I Genetic Implications of Sandstone Composition

In addition to providing a basis for sandstone nomenclature, the

composition of a sandstone also indicates something of its history

a) Maturity of a sandstone

Maturity refers to the cumulative changes that particles go through as it

is produced by weathering and is transported to a final site of

deposition

Given that the source rocks for many sediments are pre-existing

sedimentary rocks, a very mature sediment may have been through the rock cycle several times

Clastic sedimentary rocks can be made up of

Sediment texture and mineralogical composition all reflect the maturity

of a sediment

Most changes are related to transport distance, nature of weathering at the site of sediment formation and number of passes through the rock cycle

i) Textural Maturity

Changes in grain size and shape

Increasing textural maturity

Increased sorting

Demir, 2003, Downstream changes in bed material size and shape

characteristics in a small upland stream: Cwm Treweryn, in South Wales,

Yerbilimleri, v 28, p 33-47

From: Gomez, Rosser, Peacock, Hicks and Palmer, 2001, Downstream fining in

a rapidly aggrading gravel bed river

Water Resources Research, v 37, p

1813-1823.

The name of a sandstone tells you something of its maturity.

E.g., a Quartz arenite has less than 15% matrix and is better sorted than

a Quartz graywacke

The quartz arenite is more mature (greater transport distance and/or

more times through the rock cycle) than the Quartz graywacke

ii) Compositional Maturity

Compositional maturity is reflected by the relative proportion of

physically soft or chemically unstable grains

The fewer the soft or unstable grains, the more mature the sediment

What is the relative stability of minerals?

Bowen’s Reaction series shows the sequence in which minerals crystallize from a cooling magma

Mineral stability can also be shown using Bowen’s Reaction series:

The earliest minerals to crystallize are the least stable

Quartz is the most stable of the common mineral; it resists chemcial

weathering and is the most common mineral in most sedimentary rocks

Potassium feldspar is also common but

Muscovite is relatively soft and breaks down during transport

The stability of rock fragments varies with

The most “mature” sediment would be made up of 100% quartz

The “average” sandstone contains 12% feldspars

This reflects the fact that many sandstones are made up of particles that have been through several passes of the rock cycle

b) Provenance of a sediment

The Provenance of a sediment is inferred from aspects of composition

that reflect the source rock and tectonic and climatic characteristics of the source area for the sediment

Provenance: where something originated.

The source rock of a sediment and the tectonic setting are closely linked: the tectonic setting determines the relative abundance of different types

of rock that is available for weathering and the production of clastic

sediment

e.g., An arkosic sandstone (rich in feldspars) would have a source area that is rich in granites

i) Tectonic setting

e.g., a sandstone with abundant volcanic and low grade metamorphic rock fragments.

Island arc setting

Quartz arenite: sedimentary source rocks; uplifted sediments in an

orogenic belt

Not foolproof! These are two very different tectonic settings

ii) Climate

Climate exerts a strong control on the type of weathering that takes place

in the source area of a sediment; this, in turn, influences composition

Cold, arid climate: predominantly physical weathering, producing

abundant detrital grains (unaltered mineral grains and rock fragments).Sandstones produced in such settings will be relatively immature,

depending on the source rocks

Warm, humid climate: chemical weathering predominates.

Unstable minerals removed from the sediment that is produced by weathering

Will produce a more mature sediment than a cold climate

Plot of the feldspar content

in sands in eastern and

southern North America

Overall, there is a reduction in the proportion of feldspar in sands

towards the south

Several factors at work:

Source rocks: in the north are more granitic source rocks whereas in

the south the major source rocks are Paleozoic sedimentary rocks

Climate: colder in the north so that physical weathering is important,

producing immature sediment

Warmer in the south so that chemical weathering produces a more mature sediment

Many sediments were produced during glaciation which only breaks down source rocks by physical processes

Transport distance: the south has many rivers that have transported

sediment over long distances, increasing the maturity of the sands (e.g., Colorado River, Rio Grande, Mississippi River)

II Genetic Classification of sedmentary rocks

Classification on the basis of how the rocks were deposited

Commonly independent of composition, grain size, etc

b) Turbidites

Rocks made up of sediment

that was deposited from a

turbidity current.

http://cima.uprm.edu/~morelock/8_image/7turb.jpg

Turbidity currents are subaqueous flows of water and sediment that

flow down slope under the influence of gravity

Turbidites are characterized

by a particular association

of sedimentary structures

They may include sediment

ranging from silt to gravel

in size and have a wide

variety of compositions

Note that this classification

is independent of

depositional environment:

c) Storm Beds (Tempestites)

The lithified deposits of

storms influencing a shallow

marine environment

Independent of grain size or

lithology

III Which classification should you use?

This depends on the purpose of the study that you are participating in

Genetic classification of sedimentary rocks requires a knowledge of the depositonal setting and cannot normally be made on the basis of hand

Conglomerate A rudite

composed predominanty of rounded clasts.

Rounded clasts may indicate considerable distance of transport from source The significance will vary with the lithology of the clast (i.e., limestone clasts will become round a short distance from their source whereas

quartzite will require much greater transport).

Rudites are classified on the basis of particle shape, packing and

composition

Classification of Rudites

http://www.geographyinaction.co.uk/Assets/Photo_albums/Seven/pages/Conglomerate_jpg.htm

Generally indicates that the clasts have not traveled far from their source or were

transported by a non-fluid medium (e.g., gravity or glacial ice).

A rudite composed predominantly of angular clasts.

Breccia

Commonly refers to sediment deposited from glaciers or sediment gravity flows, particularly debris flows.

A rudite composed of poorly sorted, mud to gravel-size sediment, commonly with angular clasts.

Diamictite

http://www-eps.harvard.edu/people/faculty/hoffman/Snowball-fig11.jpg

Note: in the following the rock names are given for rudites consisting of rounded clasts

(conglomerates) but the term conglomerate may be replaced with the term "breccia" if the clasts comprising the rock are angular.

Orthoconglomerate

(clast-supported

conglomerate)

Clast-supported framework is typical

of gravels deposited from water flows in which gravel-size sediment predominates Open framework suggests an efficient sorting mechanism that caused selective removal of finer grained sediment Closed framework suggests that the transporting agent was less able to selectively remove the finer fractions

or was varying in competence, depositing the framework-filling sediment well after the gravel-size sediment had been deposited.

A conglomerate in which all clasts are in contact with other clasts (i.e., the clasts support each other) Such conglomerates may have no matrix between clasts (open framework) or spaces between clasts may be filled by a matrix of finer sediment (closed framework).

Orthoconglomerate with open framework

A conglomerate in which most clasts are not in contact;

i.e., the matrix supports the clasts.

conglomerate

Conglomerates that include clasts from a wide-variety of source rocks, possibly derived over a wide geographical area or a smaller but geologically complex area.

Oligomictic

conglomerate Suggests that the source area was nearby or source rock extended over wide geographic

area.

A conglomerate in which clasts include several different rock types.

A conglomerate in which the clasts are made up of only one rock type.

conglomerate

http://www.yuprocks.com/ilist/ic1.html

A conglomerate in which clasts are derived locally from within the

depositional basin (e.g., clasts composed of local muds torn up by currents;

such clasts are commonly termed "rip-up clasts" or

"mud clasts").

Deposition in an environment where muds accumulated Muds were in very close proximity to the site of deposition as the clasts would not withstand considerable transport

Conglomerate A conglomerate in which clasts are exotic (i.e., derived from Clasts derived from a distant source.

outside the depositional basin)

Clasts are normally very well rounded and well sorted.

Classification of Lutites

For our purposes, familiarity with terminology will suffice:

Shale: The general term applied to this class of rocks (> 50% of particles are

finer than 0.0625 mm).

Lutite: A synonym for "shale".

Mud: All sediment finer than 0.0625 mm More specifically used for

sediment in which 33-65% of particles are within the clay size range (<0.0039 mm).

Silt: A sediment in which >68% of particles fall within the silt size range

(0.0625 - 0.0039 mm).

Clay: All sediment finer than 0.0039 mm.

Fissility: Refers to the tendency of lutite to break evenly along parting planes The greater the fissility the finer the rock splits; such a rock is said to

be "fissile".

Siltstone: A rock composed largely of silt size particles (68-100% silt-size)

Mudstone: A bocky shale, i.e., has only poor fissility and does not split finely.

Argillaceous

sediment: A sediment containing largely clay-size particles (i.e., >50%).

Argillite: A dense, compact rock (poor fissility) composed of mud-size

sediment (low grade metamorphic rock, cleavage not developed).

Psammite: Normally a fine-grained sandstone but sometimes applied to rocks of

predominantly silt-size sediment.

Lutite terms based on

proportion of clay, degree of

induration and thickness of

stratification

Terminology related to stratification and fissility (parting).