Chapter 3: The Classification of Clastic Sedimentary Rocks

Chapter 3: The Classification of Clastic Sedimentary Rocks

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. Classification based on grain size A simple classification of terrigenous clastic rocks and sediment is based on the predominant grain size of the material: Grain Size1 (mm)

Sediment name Rock Name Adjectives >2 Gravel Rudite Cobble, pebble, well

sorted, etc. 0.0625-2 Sand Arenite Coarse, medium, well sorted, etc. < 0.0625 Mud

Mudstone or Lutite Silt or clay 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. 1 Classification of Sandstones

Most sandstone classifications are based on the composition of the rock. Dotts classificaton scheme is used in most courses at Brock. It is based on the relative proportions of: Martrix (fine-grained - <0.03mm - material that is associated with the sand grains). Quartz Feldspar Rock fragments (sand grains that are made up crystals of two or more different minerals). To classify sandstones using Dotts 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. 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 Arenite 15 - 75 Wacke or Graywacke Mudstone

>75 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 Arenite 15 - 75 Wacke or Graywacke

Mudstone >75 To classify Arenites and Graywackes on the basis of their specific compositions the data must be normalized to 100% quartz, feldspars and rock fragments. A. Total Rock Component Quartz A. Quartz, feldspars and rock fragments. Proportion

(%) 26 Component Quartz Proportion1 (%) 45 Feldspar 20

Feldspar 34 Rock fragments 12 Rock fragments 21 Matrix

42 a graywacke Total: Total: 100 100 Total Q, F, and Rf: 58 Calculated as the proportion of each component in the total rock divided by the

total proportion of quartz, feldspars and rock fragments (in this case that total is 58). 1 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 Dotts diagram for the classification of arenites. If the proportion of matrix is less than 15% plot the data and use Dotts diagram for the classification of arenites. If the proportion of matrix is less than 15% plot the data and use Dotts diagram for the classification of arenites.

If the proportion of matrix is less than 15% plot the data and use Dotts 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. The names can be modified to reflect other components of the rock: e.g., Calcareous quartz arenite: a quartz arenite with a calcite cement. 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 Dotts 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. 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 multicycled particles. i.e., have passed through the rock cycle several times. Each time through the cycle the sediment becomes more and more mature. 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 Increased rounding Increased sphericity From: Gomez, Rosser, Peacock, Hicks and Palmer, 2001, Downstream fining i a rapidly aggrading gravel bed river. Water Resources Research, v. 37, p. 1813-1823.

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. 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? Bowens Reaction series shows the sequence in which minerals crystallize from a cooling magma. Mineral stability can also be shown using Bowens 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 their mineralogy. The most mature sediment would be made up of 100% quartz grains. With increased transport and number of times through the rock cycle the less stable minerals are lost. The average igneous and metamorphic rocks contain 60% feldspars. 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 Provenance: where something originated. 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. i) Tectonic setting 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. A mountain chain adjacent to a convergent margin (e.g., modern Andes)?

An exposed craton (e.g., the Canadian Shield)? Not foolproof! These are two very different tectonic settings. 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. 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. Many sediments were produced during glaciation which only breaks down source rocks by physical processes. Warmer in the south so that chemical weathering produces a more mature sediment. 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. a) Tillite A rock that is made up of lithified till that was deposited from glacial ice. Normally very poorly sorted (mud to gravel-size particles) and the gravel is angular. b) Turbidites Rocks made up of sediment that was deposited from a turbidity current. 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: turbidites may be deposited in marine or non-marine settings (e.g., lakes). c) Storm Beds (Tempestites) The lithified deposits of storms influencing a shallow marine environment. Independent of grain size or lithology.

Genetic classification of sedimentary rocks requires a knowledge of the depositonal setting and cannot normally be made on the basis of hand specimens alone. III. Which classification should you use? This depends on the purpose of the study that you are participating in. Most studies aimed at determining ancient depositional environments can classify sandstones on the basis of grain size only. Studies that aim to reconstruct ancient tectonic settings require a detailed analysis of the composition of the sandstones. Some studies require compositional classification in order to understand the mechanical properties of the sandstone (e.g., if the study aims to determining excavation costs). Classification of Rudites

Rudites are classified on the basis of particle shape, packing and composition. 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). Breccia A rudite composed predominantly of angular clasts. 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). Diamictite A rudite composed of poorly sorted, mud to gravel-size sediment, commonly with angular clasts. Commonly refers to sediment deposited from glaciers or sediment gravity flows, particularly debris flows. 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 A conglomerate in which all (clast-supported clasts are in contact with other conglomerate) 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). 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. Orthoconglomerate with open framework. Paraconglomerate (matrix-supported conglomerate)

A conglomerate in which most clasts are not in contact; i.e., the matrix supports the clasts. Typical of the deposits of debris flows or water flows in which gravel size clasts were not abundant in comparison to the finer grain sizes. Polymictic

conglomerate A conglomerate in which clasts include several different rock types. 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

A conglomerate in which the clasts are made up of only one rock type. Suggests that the source area was nearby or source rock extended over wide geographic area. Intraformational conglomerate 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. Extraformational Conglomerate A conglomerate in which clasts are exotic (i.e., derived from outside the depositional basin). Clasts are normally very well rounded and well sorted. Clasts derived from a distant source.

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: planes.

Refers to the tendency of lutite to break evenly along parting The greater the fissility the finer the rock splits; such a rock is said to be "fissile". 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. Siltstone: A rock composed largely of silt size particles (68-100% silt-size)

Lutite terms based on proportion of clay, degree of induration and thickness of stratification. Terminology related to stratification and fissility (parting).

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