Sedimentary Rock Notes

Sedimentary Rock Classification

• Sedimentary rocks contain records of past life and environments.
• Studying sedimentary rocks helps understand earth's history, past environments, and life on earth.

Clastic Sedimentary Rocks

• Clastic sedimentary rocks are composed of fragments of pre-existing rocks and minerals.
• These fragments have been transported, deposited, and lithified.
• Clastic implies the rock is composed of discrete grains (clasts).
• Clast Formation: Pre-existing rocks undergo weathering and erosion, creating rock and mineral fragments.
• Transportation: Grains are transported by wind, water, or ice.
• Deposition: Sediment grains accumulate in various environments such as beaches, oceans, lakes or rivers.
• Lithification: Compaction and cementation turn loose sediment into solid rock.
• Compaction: Weight of overlying sediments reduces pore space.
• Cementation: Minerals precipitate from groundwater, binding grains together.

Clast Composition

• Clasts consist of rock fragments and mineral grains.
• Terrigenous clastic rocks are derived from pre-existing rocks.
• Clastic rocks can also form from the accumulation of shells and skeletons. In which case, they would be called carbonate rocks, not terrigenous.
• Clast Types:
  • Framework grains: Larger pieces of sedimentary debris (gravel, sand, silt).
  • Matrix: Fine-grained material surrounding larger clasts.
  • Cement: Chemical glue holding the rock together (e.g., calcite $CaCO<em>3$, iron oxide $Fe</em>2O<em>3$, silica $SiO</em>2$).

Cement Types

• Calcite cement: Causes the rock to fizz in hydrochloric acid.
• Iron oxide cement: Gives the rock a reddish-brown, pink, red, or orange colour.
• Silica (quartz) cement: Does not fizz in hydrochloric acid and lacks reddish-brown coloration; recognized by elimination.

Grain Size Classification

• Terrigenous sedimentary rocks are classified based on clast size.
  • Gravel: > 2 mm
  • Sand: 1/16 - 2 mm
  • Mud (Silt): 1/256 - 1/16 mm
  • Clay: < 1/256 mm
• Wentworth scale provides a more detailed grain size classification.
  • Gravel: Boulder (>256 mm), Cobble (64-256 mm), Pebble (2-64 mm), Granule (2-4 mm)
  • Sand: Very coarse (1-2 mm), Coarse (0.5-1 mm), Medium (0.25-0.5 mm), Fine (0.125-0.25 mm), Very fine (0.0625-0.125 mm)
  • Silt: Coarse (0.031-0.0625 mm), Medium (0.016-0.031 mm), Fine (0.008-0.016 mm), Very fine (0.0039-0.008 mm)
  • Clay: <0.0039 mm

Sorting and Grain Shape

• Sorting refers to the uniformity of grain sizes in a sediment or sedimentary rock.
  • Well-sorted sediment has grains of similar size.
  • Poorly sorted sediment contains a wide range of grain sizes.
• Grain shape is important in classifying sedimentary rocks.
• Grains may be rounded or angular.
• Angularity suggests minimal transport.
• Rounding suggests extensive transport.

Conglomerate and Breccia

• Conglomerates are sedimentary rocks containing rounded gravel-sized clasts.
• Breccias contain angular gravel-sized clasts.
• Mudstones contain mud-sized particles (silt and clay).
• Siltstones contain silt-sized particles.
• Claystones contain clay-sized particles. Mudstones are those rocks with a mixture of silt and clay.

Arenites

• "Aren" designates sand-sized sediment.

Sandstone Classification

• Sandstones are classified based on sand grain composition: quartz arenite, arkose, litharenite.
  * Quartz Arenite: Composed mainly of quartz grains.
  * Arkose: Contains significant feldspar grains, often pink or white; feldspar suggests minimal weathering.
  * Litharenite: Composed predominantly of rock fragments.
• Graywacke is a "dirty sand"; loosely defined term for sandstone with rock fragments and a clay-rich matrix, best used loosely.

Shale and Claystone

• Shale and claystone are fine-grained rocks composed of clay minerals, mica, and quartz.
• Grains are too small to be seen without magnification; rock feels smooth, not gritty.
• Shale is fissile, splitting into thin, flat layers.
• Claystone is massive and breaks irregularly.
• Shale/claystone color indicates composition:
  • Black shale contains organic matter (bituminous shale)
  • Red shale contains iron oxide.
  • White Claystone (kaolin) composed of mineral kaolinite ; lacking organic matter and iron oxide;.
• Mud is a mixture of silt and clay.
  • Mudstones contain both silt and clay and are referred to as mudshales if fissile

Chemical and Biochemical Sedimentary Rocks

• Chemical, biochemical, and organic sedimentary rocks that are formed within the basin of deposition are referred to as intrabasinal.
• These include chemical precipitates (travertine, rock salt, gypsum) and accumulated remains of organisms (limestone from shells).

Evaporites

• Evaporites are chemical precipitates formed by mineral precipitation from evaporating water.

Travertine $CaCO_3$

• Travertine forms by evaporation at springs or caves.
• It is often light-colored, porous rock.
• It may show ring-like patterns and is formed at evaporation locales (springs, caves).

Rock Gypsum CaSO_4

H_2O

• Rock gypsum forms from seawater evaporation.
• It is soft and can be scratched with a fingernail.
• Anhydrite $CaSO<em>4$ originates from altered gypsum. When water is removed from gypsum CaSO4
  H2O, it becomes anhydrite $CaSO</em>4$.

Rock Salt $NaCl$

• Rock salt (halite) is glassy and crystalline.
• It has a salty taste.
• Halite precipitates from seawater after approximately 90% evaporation.

Siliceous Sedimentary Rocks

• Siliceous sedimentary rocks are dominated by silica $(SiO_2)$, which precipitates from solution within the basin of deposition. (They do not include quartz sandstones, which are extrabasinal in origin.)

Diatomite

• Diatomite is a soft, white, powdery rock composed of siliceous skeletons of microscopic algae (diatoms).
• It is distinguished from chalk by its non-reaction with hydrochloric acid, it can also be distinguished from kaolinite by its low density.

Chert $(SiO_2)$

• Chert is a very fine-grained silica sediment of chemical or biochemical origin.
• Some chert contains siliceous skeletons of microorganisms known as radiolarians.

Ironstones

• Ironstones are sedimentary rocks dominated by iron-bearing minerals.
• They include banded iron formations, hematite, and goethite.
• Oolitic ironstones consist of oolites containing iron oxides

Carbonate Rocks

• Carbonate rocks (limestone and dolostone) are composed of carbonate minerals, which have a $CO_3^{-2}$ group in their chemical formula.
• Carbonate Minerals: Calcite and aragonite $(CaCO<em>3)$, dolomite $CaMg(CO</em>3)_2$
• Often referred to as calcareous

Limestone Composition & Types

• Limestone is composed of calcite or aragonite.
• It is generally gray, but also can be tan or white.
• Transports calcium carbonate mud away from reefs

Dolomite

• Dolostone
• Calcium in limestone is replaced by Magnesium to form dolomite.

Micrite & Fossiliferous Limestone

• Micrite is fine-grained limestone composed of lime mud.
• Fossiliferous limestone (skeletal limestone) contains fossils of plants, animals, or algae.

Coquina

• Coquina is a type of fossiliferous limestone made up of fossil shells
• It is porous and light colored and the shells are commonly broken, abraded, and fairly well-sorted
• It is a calcirudite, shells are gravel-sized
• Foraminiferal limestone is formed from foraminifera shells.

Oolitic Limestone

• Oolitic limestone is composed of oolites.
• Oolites are tiny concentric spheres of calcium carbonate formed by aragonite precipitation in warm, shallow seas under the influence of blue-green algae.
• Structures larger than 2 mm are pisolites not considered oolites.
• Oolitic limestone is calcarenite
• Intraclastic limestone is limestone breccia with a lime mud matrix; consists of gravel-sized limestone chips redistributed by tides
• Calci rudite

Pelleted & Crystalline Limestone

• Pelleted limestone has small aggregates of microcrystalline calcite (pellets).
• They are fecal in origin (excrement of marine invertebrates)
• Crystalline Limestone: Typically consists of a coarse mosaic of intergrown calcite crystals

Organic Sedimentary Rocks: Coal

• Organic sedimentary rocks are composed of carbon-rich organic material derived from plant remains.
• Coal comes from plant fragments
• Plant fossils in coal indicate deposition in freshwater swamps.

Peat

• Peat is sediment composed of plant fragments.
• Coal is its lithified equivalent.
  *Peat transforms to Lignite which increases to bituminous coal as burial depth and temperature increase, then finally to anthracite

Coal Types

• Lignite: Is a soft, black, brownish coal-like material
• Bituminous Coal: Sooty with greater depth and duration of burial as well as at higher temperatures
• Anthracite Coal: (Metamorphic) Hard, shiny coal.

Sedimentary Rock Classification

• Sedimentary rocks are classified based on whether they have visible grain (Table 4.3).
• Sedimentary rocks without visible grains are classified by reaction to hydrochloric acid (Table 4.4).
  • First, examine whether grains in the rock are visible. If grains are visible, then determine whether clasts / allochems are larger or smaller than 2mm and then read descriptions to identify the sedimentary rock.
  • If grains are not visible, then test the rock with hydrochloric acid. Read descriptions to identify.