Geology: Sedimentary Rocks

Carbonate Mound

A domal structure built by the accumulation of calcareous organisms, which generally exhibits low biodiversity. Can form anywhere. End of Permian era - greatest mass extinctions. Algae and brytozoans survived. Relatively few around today, organisms that secrete CaCO3 live there. Low biodiversity (sponges, brytozoans) organisms that don't need nutrients in water.

Coal

A readily combustible rock that contains >50% by weight and >70% by volume of carbonaceous material, formed from the compaction of altered plant materials such as peat (organic Carbon, organic material). Peat is compressed organic material containing about 50% carbon (peat bog, swampy environment, little oxygen, not very rocky-looking). Lignite is low-grade coal containing about 70% carbon - dull black or brown coloration (powers turbines for electricity). Bituminous is mid-grade coal containing 80% carbon - dark coloration (20% sand/silt/clay, burns more readily). Anthracite is metamorphosed coal containing >90% carbon - dark black coloration (burns clean, meta-sedimentary rock rides the border, highest grade of coal). Coal is both chemical precipitate and clastic. Turn it into higher and higher grades of coal. Peat is burned to heat homes in Scotland and Ireland. Lignite is not super efficient to burn. Bituminous coal in East Texas is found by strip-mining the surface. Coalify past anthracite to make graphite and then diamond after long, long, long periods of time.

Carbonate Reefs

A ridge-like or domal structure built by the accumulation of calcareous organisms, which generally exhibits high biodiversity. Corals, brytozoans grow up, deposit CaCO3 out of butts. Produce environment favorable for fish and other marine organisms to live in. Tropical areas need warm shallow environments that are good for lots of photosynthetic organisms that need the sun. Inside of reef has ooids and pellets.

Examples of Biochemical Sedimentary Rocks

Aragonite: CaCO3 - limestone Calcite: CaCO3 - limestone They have the same chemical formula, but calcite is a more stable form than Aragonite. Dolomite: Ca0.5Mg0.5CO3 - dolomite (add Magnesium to Calcite) Aragonite -> Calcite -> Dolomite

Secondary Precipitates

Broad group of minerals that precipitate as a result of changes in the physical and chemical environment. Precipitation is often organically mediated. Secondary precipitates have already been precipitated and don't form directly out of water bodies. Limestone caves precipitate CaCO3 which forms stalactites and stalagmites. Water from the subsurface creates a massive void. Small pool grows up through time forming rings of precipitation. Cave formations, travertine deposits are from hydrothermal environments.

Carbonate Shoals

Calcareous deposition in relatively shallow water forming a ridge, bank or bar of unconsolidated material. Will happen inside barrier reefs. Low-energy wave front moves and creates ooids and soft sedimentary shoals. Moving back and forth. White sand is oolitic material.

Chemical Sedimentary Rocks: More Extensive List

Calcite: CaCO3 - limestone Dolomite: Ca0.5Mg0.5CO3 - dolomite (calcite + Mg) Dolomite problem is that there is NOT a ton precipitating from the sea. Potash: K2CO3 (pump water out of ground into shallow lake then evaporate more and precipitate Potash). Halite: NaCl - rock salt (salts rise up creating oil and gas space). Sylvite: KCl - bitter salt (orange color). Gypsum (Anhydrite): CaSO4; 2H2O CaSO4 (Gypsum is the hydrated from, Anhydrite is put deep in the ground away from water).

Chert

Chert is made of microcrystalline Quartz - SiO2 (microscopic planktonic organisms build skeleton out of this). Accumulation of siliceous shells, usually in deep water. Carbonates dissolve at about 1.8 km depth (carbonate compensation depth). Chert Types are flint and jasper. Flint is a dark coloration due to organics (Oxygen poor then it's black). Jasper is red coloration due to iron oxides (oxidized iron). Knives can be made from chert because its conchoidal fracture scratches glass. Silica is concentrated at deep areas - calcium carbonate has a compensation depth which is where the temperature is cold. Pelagic rain produces the most fine carbonate material where few is silica. Constantly living and dying, then rain out and accumulate on ocean floor through time in a Silicious ooze. Cryptocrystalline silica is hard-to-see quartz.

Classification

Classification: based on the composition of the grains in the rock, not the cements or matrix. The grains give geologist a better idea of the surface processes that transported/deposited the sediment (tell us what happened at the surface). Reconstruction of past environments = paleogeography or paleoenvironments (reconstruction of old environments). Quartz sandstone with calcite cement. Sand is important, grains matter, NOT the matrix. Quartz Sandstone originally deposited in beach environments; clay fossils from marine environments. Nac used to be underwater. Detail environmental change through time. Millions of years after deposition, cementation can happen long after it has been compacted.

Clastic, Biochemical, Chemical Sedimentary Rocks

Clastic Sedimentary Rocks: Made of clastic particles, smaller and smaller fragments. Biochemical Sedimentary Rocks: Derived from organisms, fossils, incorporate minerals into skeleton. (Algae CaCO3) Chemical Sedimentary Rocks: Formed whether or NOT we had life, evaporated precipitates (salt left behind crystallizes into sedimentary rocks).

Eolian Sedimentation

Clastic sediments deposited and modified by wind processes; subaerial deposition. Moving wind doesn't move very large things unless there is a hurricane or tornado. Don't expect to see cobbles or boulders being moved. Quartz Sandstone is abrasive. Strong wind = sand, silt, and clay being moved, clay requires the weakest wind to move it. White sands in Utah and New Mexico are a very soft sand made from Gypsum which has a hardness of 2, is not abrasive, is smooth, and can be used as a lubricant.

Aqueous Sedimentation

Clastic sediments deposited in a liquid environment as a result of fluid flow; subaqueous deposition. Rivers, streams, and oceans. Different sedimentary environments. Fluvial. Beaches is where we dump out are minerals. High energy waves concentrate sand at the beach. Sand on the beach and then silt farther out and clay even farther out. Size decreases from source to sink. Speed of flowing water can infer the rocks being deposited. Delta rivers meet oceans then sand drops out first.

Glacial Sedimentation

Clastic sediments resulting from glacial modifications to the Earth surface. Ice sedimentation is characteristic of glaciers - deposited by glaciers. It can move any-sized particles from house-sized boulders that move very slowly. Rocks scraped over by glaciers have glacier striations which are carved out lines on underlying rocks. Glaciers carry sediment until they melt and then everything is deposited at once and is called glacial till. U-shaped valleys are left behind.

Gravitational Sedimentation

Clastic sediments, subaerial or subaqueous, deposited primarily as a result of collapse or slope failure. Earth wants gentle sloping features. Under air or under water a steep cliff with high gravitational potential energy (mountain). Ex: Avalanache (ice deposits minerals) landslide (steep hill, slope fails). Soluble bedrock dissolves and form underground caves, doesn't support overlying material and collapses. Mix of all different sizes of materials.

Clay/Silt Sized Grains

Clay/Silt Sized Grains is a collective term for mudstones or shales (types of rocks). Claystone is mostly clay and siltstone is mostly silt. Composed of mix of clays - sheet silicates and silts, Al, K, Na - extremely fine grained quartz. Often containing sand, carbonate (calcite, dolomite) and organic material (kerogen). Accumulate in low-energy environment and form shale that has hydrocarbons. Fissile: breaks into sheets and slabs, related to clay content (because clays are deposited in sheets/layers, just like biotite and muscovite. Smaller sediments have increased surface area and decreased volume. Rate of chemical weathering increases. The more platy, then the more clay-like and the more fissile it is.

Sand Sized Grains

Collective term is a sandstone or arenite (predominantly sand-sized particles). Sandstone formations are often the reservoirs for water, oil, and (natural) gas due to porous nature. Arkose is also sandstone (but not quartz). Siltstone, clay are so small that there is no space for holding resources. Sandstone has enough space to hold resources. Sand is way bigger in comparison to silt and clay is impossibly tiny.

Diagenesis

Diagenesis: (dia-passing through the ground; gen-birth) The changes that occur after sediments are buried, but before they are metamorphosed or exposed at the surface (multiple processes). Compaction - increased pressure on the grains of the rock, packs grains together (smaller grains can be more compressed); water squeezed out. Cementation - mineral growth in pore spaces to "glue" the sediment together (precipitate minerals that make it hard/will last longer). Diagenetic changes occur and lithify the sediments into a rock. Diagenetic processes modify sediment into rock. NOT well-cemented then they will fall apart easier. Ocean water infiltrates between grains (rich in salt and elements then mineral precipitates).

Gravel

Gravel - clast size > pebble. Conglomerate Gravel is composed of rounded clasts that are peble size or larger. Breccia Gravel is composed of angular clasts that are pebble size or larger. They are closer to the source, and farther from the sink. Conglomerate has rounded, smooth pieces, like those found at La Nana and Banita Creek, it is a lithic conglomerate. Breccia has angular fragments and is less common. They can be created from landslides, impact events (meteor strikes earth), felsic/intermediate explosive erosions, and fault breccias (sliding past each other, grinding up rocks).

Sedimentary Rocks: Preface

Metamorphic - deep in Earth. They provide us with fossils, history of past life, oil, hydrocarbons, natural gas, and electricity from coal deposits. Formed at the surface, record history of surface of planet. Layers stack up over time. Lithified sediments hold our resources (water, drinking water comes from the ground).

Clastic Sedimentary Rocks

Mudrock include mudstone (clay content = silt content, they are about equal), claystone (clay content > dominant to silt content), siltstone (silt content > clay content), sandstone (>90% sand), marl (clay, silt, and carbonate mud), and shale (mudstone and claystone). Siltstone is gritty while shale is smooth. Rub siltstone against your teeth. Shouldn't use shale. Marl is carbonate which effervesces in HCl and forms in the marine environment.

Sedimentary Rocks

Only 8% of the volume of the crust (mostly igneous). Covers almost all of the earth's surface about 75% from thin veneers to thick sequences. Reservoir for important resources: water (drinking, irrigation), energy resources (vast quantities), and fossils (see ancestry through time). Ocean is primarily sedimentary rock, gulf coast floods often and the number one source of freshwater is groundwater. Found at the crust - small volume in relation to Earth. Rocks break down at the surface then sediments form sheet covering the land. It can be thick and cover 16,000 feet and shows the history of sedimentation.

Sandstone

Quartz Sandstone has >50% Quartz (Quartz Arenite). Arkose Sandstone has >50% Feldspar (granitic body). Lithic Sandstone has >50% Lithics (rocks/other rocks). Arkose is the base at Enchanted Rock. Black sand beaches in Hawaii will become lithic sandstones.

Sediment Size Categories

Sediment comes in all sizes: Gravel: Boulders (256mm and up) largest, high energy, Cobbles (64-256mm), Pebbles (2-64mm) Sand: 0.0625-2mm (can see these) Silt: 0.002-0.0625mm (small, less flat) Clay: 0.002mm and smaller (<.002mm), platy, low energy, very very small, can't discern individual clays. Strict size delineations. Banita and La Nana creek has silt as most of their grains.

Sediment Size

Sediment size: sediment found depends on energy in the environment. Size of grains is related to the energy/velocity of water or wind moving the sediment. Size decreases from source to sink. Hjulstrom curve shows the velocity of fluid vs. sediment size. Lower velocity then smaller grains are moved. Cohesive (static cling) smallest size but high energy to move. Mountain streams have the highest energy and can move boulders. Water flow/steepness decreases as reaches ocean. High-energy can move boulders (increased size). Low-energy can more clays and silts in the ocean (decreased size). Sand on beach is in the middle. Same velocity of water to remove boulders as silt and clay (hard to wash off of stuff). Source to sink (things end up here) sediment size decreases.

Clastic Sedimentary Rocks

Sedimentary rocks composed primarily of fragments derived from the weathering of preexisting rocks and transported (eroded) to their place of deposition. Get eroded from source to sink. Could get stuck in desert. Holds oil, gas, and water, shale is found in deeper waters.

Chemical Sedimentary Rocks

Sedimentary rocks formed by direct chemical precipitation from solution. Don't need life to form, often in bad conditions; hot, saline. Chemical precipitation of CaSO4 forms gypsum in Lake Nac. Utah. On shorelines, banded carbonite called Travertine forms from hydrothermal material. Directly out of evaporating water bodies. Water drains into lake, but not out, evaporates and leaves salt.

Biochemical Sedimentary Rocks

Sedimentary rocks formed by direct secretion from organims or as a result of changes in the environment resulting from environmental modifications by organisms. Could be formed because of animals. Ions used by organisms - coral, arthropods, marine organisms. Product of chemical weathering. Sea grass, algae create local environments to be favorable for precipitates. Use them to build body structure (skeleton) calcite and sea grass.

What is a sedimentary rock?

Sedimentary rocks: Rocks composed of lithified sediments. Sediment: material derived from pre-existing rocks through weathering processes. (materials that were weathered and eroded from pre-existing rocks). Two Types: Detrital: detrital/clastic sediment - solid pieces of rock (broken, product of mechanical/physical weathering). Chemical Sediment: minerals precipitated from solution or biogenically mediated by organisms. (chemical weathering, evaporate water, change chemical composition). Quartz is vitreous, plagioclase and biotite. Mechanically weather through abrasion ends up on beach from rivers. Then it's pushed down into the Earth, compacted, and organisms die and stack up, minerals are precipitated into the sediment to make Quartz Sandstone.

General Terms (classification)

Skeletal grain: shells of calcareous organisms (protect castings, branchiopods, snails, clams, corals). Carbonate mud: clay size particles (micrite, fine-grained). Coquina: shell hash (broken up shell material, high-energy wave environment breaking into smaller particles). Ooid: concentrically banded, spherical grains (low-energy environment, rolling of grains in shallow tropical water and organism/algae attaches based on mineral and precipitates composition CaCO3). Circular deposits of carbonate material. Peloid (pellet): spherical grains (usually feces), not have concentric banding, formed by fish poo/crab poo then become large deposits of grains. Algae blooms precipitate aragonite needles (clay-sized). Forming in same conditions then size of particles does NOT matter.

Sabkha

Supratidal (above high tide) environment of deposition (deposit salts) in arid or semi-arid regions, characterized by high evaporation rates and evaporite deposits. Ancient sabkha deposits. Deposit, crust of halite, sabkha.