FYI: LIMESTONES...sedimentary my dear Watson: Not Volcanic now:
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INTRODUCTION
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Sedimentary rocks are composed of materials derived from pre-existing igneous, metamorphic or sedimentary rocks. The decomposition or destruction of igneous, sedimentary, and metamorphic rocks by chemical and mechanical processes at surface pressures and temperatures results in two general products of weathering: solute and sediment.
A solute is that portion of the rock which is soluble in water and is transported in solution via rivers to the ocean.
Sediment is that portion of the rock which is insoluble in water and is transported as part of the load of the stream to the ocean.
Sedimentary rocks may be formed from the elements or compounds carried in solution in terrestrial or marine waters or from the sediment transported to the ocean via streams, winds, and glaciers. In general, sedimentary rocks formed
by the removal of elements from solution by evaporation or precipitation are referred to as chemical precipitates, and
from transported particles of sediment are referred to as terrigenous clastics.
The origin of elements in solution is the chemical decomposition of existing minerals into their component elements. Most elements in this state are easily taken into solution by ground-water or surface waters.
Once in solution, elements may be transported by rivers to the world ocean where they contribute to the salinity of ocean water or they may remain for a time in ground-waters or lakes. In either case, the potential exists for many elements to recombine as compounds as physical or biological conditions change, but the mechanism is always precipitation.
Minerals produced in this way will create a chemically derived sediment which may produce a sedimentary rock (see Table 5-2). The greatest volume of sedimentary rocks of this type was formed in the marine environment where minerals were either
precipitated inorganically from sea water to form a fine-grained sediment on the sea floor or
precipitated by marine organisms and used in the construction of skeletons.
Upon the death of such organisms, whole or fragmented skeletons accumulate as sediment on the sea floor. Precipitates may also develop in fresh water lakes, under hydrothermal conditions (e.g., around geyser vents and hot springs), or from ground-waters (e.g., cave deposits). Conditions for the formation of precipitates implies that the chemically derived sediment formed and accumulated within the region or basin where the corresponding sedimentary rock is found.
In the case of terrigenous clastic sediment, mixtures of various types of mechanically derived sediment from distant source areas are transported by water, wind, ice, or gravity from a source area and ultimately deposited in a sedimentary basin. During transport, the sediment is sorted by size class and specific gravity and rounded by abrasion.
After sufficient accumulation, sediment of either type may be lithified by compaction and cementation into sedimentary rock (see Tables 5-2 & 5-3). Compaction begins as soon as the weight of overlying sediment is sufficient to force mineral grains into closer and closer contact and into the pores or voids filled usually with water. The result is a significant loss in porosity and permeability, and sediment of lower volume but greater density. Cementation or the binding together of these mineral grains also begins very soon after deposition, and involves precipitation of calcite, iron-rich minerals, quartz or other minerals between the grains.
Sedimentary rocks assume a variety of types due to:
the composition of the sediment,
the distance and type of transport or precipitation,
the environment of deposition,
the amount of compaction, and
the amount and type of cementation.
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CHEMICAL PRECIPITATES
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Rock types in this group owe their origin to the inorganic or organic chemical precipitation of a variety of compounds from bodies of fresh, marine, or hypersaline waters. Processes of precipitation are varied, but fall into three major groups:
organically and inorganically precipitated calcium carbonate (CaCO3),
evaporation, and
inorganic precipitation of minerals other than calcium carbonate.
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Processes
The following is a brief description and discussion of the major processes responsible for various rock types formed by precipitation. Knowledge of the possible processes of formation of these rocks will aid in their identification and interpretation.
Organically controlled precipitation of calcium carbonate (CaCO3).
The organic and inorganic precipitation of calcium carbonate (CaCO3) is given special coverage because organisms provide sediment for the major rock type of the precipitate group. Calcium (Ca) and carbonate (C03) are abundant in sea water, and organisms have utilized the compound calcium carbonate (CaCO3) for the building of shells for 600 million years. Although the details of the process are complicated, in general an organism combines calcium from sea water with metabolic carbon and oxygen to precipitate calcium carbonate to form the shell and other skeletal hardparts. At death, the whole or partial disintegration of the shell or skeleton is a source of calcium carbonate sediment that accumulates on the sea floor and which has the potential to become limestone. This is the origin of most limestones and the reason that most limestones consist of fragments of shells or skeletons of marine life and large quantities of very fine particles derived from complete disintegration of skeletons of calcareous algae or from mechanical abrasion of fragments of shells or skeletons. After lithification, this fine-grained lime mud forms microcrystalline limestone or micrite.
Inorganic precipitation of calcium carbonate (CaCO3).
Direct, inorganic precipitation of calcium carbonate occurs in caves in the form of stalactites, stalagmites, etc., and around hot springs and geysers, but is uncommon under normal marine conditions. Sea water is saturated with calcium carbonate but usually it will precipitate only on preexisting grains such as fragments of shell, and then only in areas of strong wave and current action. By this process, small spherical grains called oolites are formed. Calcium carbonate may also precipitate directly from seawater to form cement between grains.
Precipitation by evaporation.
Evaporation is a common means of markedly increasing the salinity of water (measured as the concentration of dissolved matter in water), and of maintaining high levels of salinity. Under such conditions, concentrations of a number of elements are increased to the point that the water is said to be hypersaline, and elements will form various mineral precipitates (see Table 5-2). If the water is contained in a restricted basin where there is no net supply of fresh or less saline waters, the water remains at or near the condition of hypersalinity and precipitation proceeds. In shallow bodies of water, extremely small needles of calcium carbonate may be precipitated from sea water in this way and may later form the very fine-grained and dense limestone termed micrite or, more commonly, the fine-grained calcium-magnesium carbonate rock called dolomite. The other more common minerals and rock types associated with evaporation are referred to as evaporites. These are halite (NaCl) and gypsum (CaSO4.H2O).
Other precipitates by inorganic processes: Chert.
Sea water is normally undersaturated with respect to silicon, and the precipitation of silica (SiO2) to form chert is a special circumstance which can occur in one of two ways. Chert may form as a layered deposit on the floor of a sedimentary basin under conditions where the bottom waters are supersaturated with silicon. The source of silicon is usually from the chemical disintegration of freshly formed submarine volcanic material or through the release of silicon from the chemical breakdown of large numbers of siliceous skeletons of minute marine invertebrates. In either case, SiO2 can be precipitated as chert in bedded masses. Chert can also form as nodules or concretions under local conditions where the chemistry at the interface between sea water and already deposited sediment interacts to cause precipitation of SiO2 or by replacement and cementation within the sediment after burial.
Other precipitates by inorganic processes: Dolomite.
Unlike the evaporative form of dolomite, it is possible to produce dolomite from the postdepositional recrystallization and replacement of calcium carbonate in the presence of waters rich in magnesium. Chemically, dolomite [CaMg(CO3)2] which is a high-magnesium calcite, differs from calcium carbonate (CaC03) only by having magnesium replace calcium at some of the sites in the crystal lattice. A sufficient amount of replacement causes a structural change in the crystal habit and the mineral becomes recognizable as dolomite. Although possible, limestone units usually do not convert entirely to dolomite, but instead are a mixture of dolomite and calcite.
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Rocks formed by chemical precipitation
The types of rocks formed by precipitation are included in Table 5-2 and described below.
1. Limestone.
A sedimentary rock containing more than 50% carbonate of which more than 50% is calcium carbonate (calcite or aragonite). Limestone will effervesce rapidly in hydrochloric acid at room temperature, however, many terrigenous clastic rocks lithified with calcite cement will effervesce as well. The origin of limestone can be either organic or inorganic and it can be classified as one of three types according to texture acquired during deposition.
Micrite
Limestones in which micrite, originally fine, lime mud, either fills spaces between larger grains comprised of fossils, fossil fragments, oolites, etc., or composes most or all the rock. If the larger grains compose less than 10% of the rock, it is called simply a micrite. If larger grains compose more than 10% of the rock, the name of the most numerous grain is added as an adjective to the term micrite; e.g., fossiliferous micrite (fossils are the most numerous type of larger grain), oolitic micrite (oolites are the most numerous type of larger grain), sandy micrite (sand grains are the most numerous type of larger grain).
Sparite
Limestones in which there is very little or no micrite between grains comprised of fossils, fossil fragments, oolites, etc. This space is filled instead with a clear, crystalline, calcite cement called spar. Spar precipitates between grains either very soon following deposition or during compaction and burial. Adjectives are added to the term sparite to make the rock name more descriptive; e.g., oolitic sparite, fossiliferous sparite, etc.
Coquina
Spanish for cockle, shellfish. A limestone composed entirely or chiefly of mechanically sorted fossil skeletons or fragments of fossil skeletons, and that is weakly cemented. An organic precipitate.
2. Gypsum.
Greek for chalk. An evaporative rock consisting of hydrous calcium sulfate (CaSO4.H20), and frequently found associated with halite and anhydrite in thick extensive beds. The rock has the same general properties as the mineral gypsum.
3. Halite.
An aggregate of crystals of the mineral halite. It is native salt, and occurs in massive or granular form.
4. Dolomite.
A sedimentary rock consisting of more than 50% carbonate of which more than 50% is the mineral dolomite (compare with limestone). The rock occurs in crystalline and non-crystalline forms and usually represents the post-depositional replacement of limestone. Pure dolomite will effervesce very slowly in hydrochloric acid at room temperature.
5. Chert.
A hard, extremely dense or compact, dull to semivitreous, cryptocrystalline, sedimentary rock composed for the most part of cryptocrystalline silica (SiO2), usually in the form of fibrous chalcedony, and lesser amounts of microcrystalline or cryptocrystalline quartz and amorphous silica (opal). It has a tough, splintery to conchoidal fracture, and may be white or variously colored gray, green, blue, pink, red, yellow, brown, and black. The term flint is essentially synonymous, although it is normally used for the dark variety of chert.
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TERRIGENOUS CLASTIC ROCKS
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The term terrigenous clastic refers to a rock or sediment composed principally of broken fragments that are derived from preexisting rocks or minerals on the land surface, and that have transported individually for some distance from their places or origin. Rocks of this type may be deposited under:
shallow marine conditions,
deep marine conditions (sediment blown out to sea), or
on the land surface under the action of wind, water, or ice.
Because terrigenous clastic rocks directly represent the action of the transporting agents of water, wind, or ice, the size, degree of roundness and sorting (texture) of mineral grains and rock fragments provide much information regarding distance and type of transport from their place of origin and depositional processes. It is therefore important that you recognize the dominant texture of each terrigenous clastic rock for purposes of identification and interpretation. It is also important to remember that these rock types are formed of rock fragments and mineral grains taken directly from other rocks. The classification introduced here for terrigenous clastic rocks is based on:
the size of the particles or sediment size
the degree of sorting.
In addition, several common adjectives are used to expand the classification of clastic rocks to make the rock name more descriptive. These are listed in Table 5-3. The major rock types of Table 5-3 are listed and defined below.
1. Shale.
Teutonic or German for shell (meaning is misleading). A fine-grained, sedimentary rock formed by the induration of clay particles, and characterized by finely stratified structure (laminations) or a fissility that is approximately parallel to bedding. The grain size of shale is less than 1/256 mm in diameter. Induration is the hardening of rock or rock material by the action of heat, pressure, or the introduction of cementation material not contained in the original sediment. The term fissility is characteristic of shale and refers to the property whereby the rock splits easily into thin sheets or layers along closely spaced parallel surfaces. Shale can be red, brown, black, gray, green, or blue, and is the most abundant sedimentary rock.
2. Mudstone.
A fine-grained, indurated sedimentary rock composed of clay particles, but free of fissility or lamination and thus appears as massive, i.e., it is without internal structure. If a mudstone contains appreciable quantities of particles larger than 1/256 mm in diameter, the rock is considered gradational between mudstone and siltstone, and the adjective silty is applied to the term mudstone (see Table 5-3).
3. Siltstone.
An indurated or somewhat indurated silt having the texture and composition of mudstone, but in which the silt size particles ( > 1/ 256 mm < 1/16 mm) predominate over clay sized particles. The texture and composition of siltstone is essentially midway between mudstone and sandstone. Siltstones with a high proportion of clay size particles are termed muddy, and those with a high proportion of sand size particles are termed sandy.
4. Sandstone.
Medium-grained, clastic sedimentary rock composed of abundant rounded or angular fragments of sand size (>1/16 mm < 2 mm) set in a fine grained matrix (usually silt or clay) and more or less indurated by cementing material which is commonly silica, iron oxide, or calcium carbonate. The sand size particles usually consist of the mineral quartz, and the term sandstone, when used without qualification, indicates a consolidated terrigenous clastic rock containing 85-90% quartz sand. Sandstones can be silty or conglomeratic, and are the second most abundant sedimentary rock.
5. Conglomerate.
Latin for heaped, rolled, or pressed together. A coarse grained, terrigenous, clastic rock composed of rounded or subangular fragments larger than 2 mm in diameter (granules, pebbles, cobbles, boulders) set in a fine-grained matrix of sand, silt, or any of the common naturally cementing materials such as calcium carbonate, iron oxide, silica, or indurated clay. The rock or mineral fragments may be of varied composition and range widely in size, and are usually rounded and smoothed from transportation by water or wave action.
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ENVIRONMENTAL SIGNIFICANCE OF TEXTURE AND COMPOSITION
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Much can be learned about the physical and chemical conditions of environments of deposition of sedimentary rocks from their texture and composition.
Texture is an expression of the size, shape, and arrangement of grains in the rock, and
composition is the mineralogy of the mineral grains and of the enclosing matrix.
For example, the original sediment that was transformed into a limestone containing marine fossils was deposited in the marine waters of an ocean. In all probability, chemical conditions of salinity, oxygenation, carbon dioxide content and temperature were similar to shallow, warm regions of modern oceans. The general character of these conditions is inferred from the fossils and the knowledge derived from the study of modern environments which shows that limestone, in general, is formed only in warm waters.
Physical conditions at the site of deposition or environment can be inferred from the texture of the rock. Sparites, for example, have little or no fine-grained lime mud or micritic matrix. The absence of lime mud in the matrix generally indicates that the wave and current energy of the environment were high enough to keep mud-size particles of sediment in suspension and thus they were not deposited. From this observation it can be inferred that the sediment was deposited under conditions of shallow, agitated waters such as those associated today with shoals, bars, and beaches; the typical locations of high wave energy and strong currents.
Table 5-1 is a list of textures and compositions and their associated environmental significance to guide you in making inferences about environments of deposition for most sedimentary rocks you are asked to examine.
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Table 5-1
Environments of Deposition
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Texture/Composition
Significance
Abundant fine sediment;
clay, micrite, fine silt
Low wave or current activity; bay, lagoon, deeper water, flood plain, delta, etc.
Mostly coarse sediment with little or
no fine particles
High depositional energy; river channel, beach
Coarse particles with fine matrix
High energy with rapid deposition; alluvial fan, flood deposit, reef talus
Worn and rounded particles or oolites with little or no matrix
Moderate to high energy; bars, shoals, beaches
Marine or terrestrial fossils
Marine or terrestrial environments
Marine limestone
Warm and generally shallow water
Abundance of feldspar (arkose)
Deposition near source of sediment
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PROCEDURE FOR CLASSIFYING SEDIMENTARY ROCKS
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After working with sedimentary rocks, you will find that their classification is rather easy and, in most cases, obvious. At first, however, you may find some features confusing. The procedures outlined below were designed to guide you through this confusion. Use the Sedimentary Rock Lab Form provided to record information about the rock specimens, and Table 5-2 (Chemical Precipitates) or Table 5-3 (Terrigenous Clastics) to determine the name of the rock.
1. Test the rock with a drop of hydrochloric acid to determine its origin as chemical precipitate or terrigenous clastic. Then proceed to 2 or 3.
Reacts with acid.
Rapidly.
Many particles touch one another and are visible without use of a handlens.
Particles are mostly fossils, fossil fragments, or oolites = PRECIPITATE
Particles are mostly silicate minerals (usually quartz grains) or rock fragments = CLASTIC
Particles do not touch, and some are large enough to be visible without a handlens.
Particles are mostly fossils, fossil fragments, or oolites; the matrix is dense and usually white or gray, but may be black = PRECIPITATE
Particles may be fossils, silicate minerals or rock fragments; the matrix is gritty (grains visible with handlens) and usually brown, tan, red, gray = CLASTIC
Particles not visible without a handlens.
Dense to grainy or chalky appearance; usually white or gray but occasionally black = PRECIPITATE
Gritty; grains visible with handlens; usually tan, brown, red or gray = CLASTIC
Visibly crystalline; may see molds of fossils = PRECIPITATE
Slowly.
Fine to coarsely crystalline; usually tan or brown, may be gray; may see molds of fossils = PRECIPITATE
Very fine grained and not gritty; may be layered; usually tan, brown, red or gray = CLASTIC
Does not react with hydrochloric acid.
Crystalline; usually white or gray = PRECIPITATE
Dense to very finely crystalline; scratches glass; conchoidal fracture = PRECIPITATE
None of the above = CLASTIC
2. If the rock is a chemical precipitate and
reacts rapidly with acid = LIMESTONE
If there are abundant visible particles of fossils, fossil fragments, or oolites and they seem to touch one another, the rock may be a sparite. To confirm this, use a handlens to see if clear calcite spar is present between the grains. You will need to wet the surface to see the spar in most cases.
Particles do not touch or if the matrix is not spar = micrite
Rock is composed of loosely cemented fossil shells or shell fragments = coquina
Reacts slowly with acid = DOLOMITE
Does not react with acid and
is crystalline and
tastes salty = HALITE
is soft = GYPSUM
is dense or very finely crystalline, scratches glass, and breaks with conchoidal fracture = CHERT
3. If the rock is a terrigenous clastic, determine the texture, grain size, and particle composition and refer to Table 5-3 to further subdivide the classification.
After completing the Sedimentary Rock Lab Form for the specimens in the laboratory, determine the names from Tables 5-2 or 5-3, and, to the degree possible, indicate a general environment of formation following the guidelines given in Table 5-1.
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Table 5-2
Classification of Chemical Precipitates
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Process
Composition Sedimentary Rock
Organic
or
Inorganic
Calcium Carbonate
Collectively known as LIMESTONE
With or without larger grains MICRITE
Little or no micrite between grains cemented with calcite spar SPARITE
Composed entirely of whole or fragmented loosely cemented shells COQUINA
Evaporation
CaCO3 MICRITE
CaSO4.H20 GYPSUM
NaCl HALITE
CaMg(CO3)2 DOLOMITE
Other Inorganic
Processes
SiO2 CHERT
CaMg(CO3)2 DOLOMITE
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NOTE: One of more of the following adjectives should be added to the name of limestone and dolomite rocks to make the name more descriptive.
Fossiliferous (when whole fossils are contained in the rock) or fossil fragment.
Use name of fossil if known; e.g., coral micrite, oyster fragment sparite.
Oolitic
Sandy
Clayey
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Table 5-3
Classification of Terrigenous Sedimentary Rocks
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Sediment Size
Sedimentary Rock Name
CLAY Less than 1/256 mm, too small to observe, not gritty
SHALE
MUDSTONE
Gradational
Silty MUDSTONE
Muddy SILTSTONE
SILT 1/256 to 1/16 mm, some visible grains, gritty
SILTSTONE
Gradational
Sandy SILTSTONE
Silty SANDSTONE
SAND 1/16 to 2 mm, all grains are visible
SANDSTONE
Gradational
Conglomeratic SANDSTONE
Sandy CONGLOMERATE
GRAVEL 2 mm and larger
CONGLOMERATE
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NOTE: To make the name of the rock more descriptive, it is often necessary to add one of the following adjectives to the rock name.
Arkosic - the rock contains observable amounts of feldspar
Quartzose - the rock is essentially composed of all quartz
Siliceous - rock cemented with silica
Calcareous - contains calcite usually in the form of cement
Fossiliferous - rock contains abundant fossils
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