Chp 6: Sedimentary Rocks

1. Rocks initially form from solidification of magma or lava

2. These rocks subjected to weathering and erosion processes (wind, running water, glaciers, etc) which break down those rocks into particles.

3. These particles are then transported and settle out to become ‘sediments.’

4. These sediments are compacted to form sedimentary rocks.

5. If these sedimentary rocks are subjected to enough pressure and heat they can then transform into Metamorphic Rocks….later Chp

…this process is known as The Rock Cycle……

Fig. 6-7, p.155

Derivation ofSediments fromPre-existing rocks-

II. Rock Types:Igneous rocks make up 90% by volume of the earth's crust. -Igneous rocks formed directly from molten material. -As this molten material cools it solidifies and hardens to become rock. - Intrusive igneous rock forms below the surface of the earth. -Extrusive igneous rocks form from molten material that has been forced out onto the surface of the earth (i.e. volcanoes).Sedimentary rocks form from the accumulation of eroded debris of other rocks or chemically from elements in seawater. -Sedimentary rocks make up 75% of all of the rocks exposed at the earth's surface. -most fossilized remains are found in sedimentary rocks. This makes sedimentary rocks useful in interpreting the earth's geologic history.Metamorphic rocks are formed from pre-existing rocks that have been altered as the result of intense heat and pressure. -Metamorphism increases the “crystallinity“ and hardness of the rock; sandstone changes to quartzite; shale changes to slate, limestone to marble.

Fig. 6-CO, p.148

Sedimentary rocks in The Valleyof the Gods, Utah

Some characteristics to note:

a. Layering in rocks is horizontalb. Erosion has exposed layers differentially by their lithology.c. Red color results from iron oxide(Fe02) cement.d. These rocks are mostly sandstone

III. Types of Sedimentary Rocks:

The type of sedimentary rock formed in an area reflects the environment in which it was deposited, this is referred to by geologists as the faciesof the rock.Since the facies of sedimentary beds tells the geologists so much information about the geologic past (paleoenvironments, paleoclimates, and past life forms), sedimentary rocks are emphasized in Historical Geology. There are 2 basic groups of sedimentary rocks:

1. Chemical Precipitates from the evaporation of seawater, or from the concentration of ions in water. These include rocks such as limestone and various salts such as Halite (NaCl), Sylvite (KCl), Gypsum (CaSO4), etc. The salts usually indicate periods of massive evaporation of aqueous environments.

2. Clastic Sedimentary Rocks are formed from the accumulation of debris from the weathering and erosion of other rocks. The 4 stages of the formation of clastic sedimentary rocks (“clastic” means "broken") are described on the following pages.

IV. The Four Steps for Formation of Sedimentary Rocks: 1. Physical and Chemical Weathering of the “Parent Rock. This results in the Parent rock being broken into fragments.

2. Transportation is the stage where the clastics are "moved“ (“transported”) from the source area by water, wind, gravity, or ice. -size of particle and distance moved very important. Larger particle requires more energy to move that particle.

-High- energy environments would include white water mountain streams that are capable of moving almost all sizes of particles. Low-energy environments include lagoons, lakes, deltas, swamps, etc., that are capable of moving only the smaller particles.

3. Deposition is the stage where the sediment is deposited in a particular geographic environment, which constitutes the sedimentary environment. -For example, large rocks formed on a mountain range would be carried down the steep gradient and deposited at the base of the mountain if the energy of the stream carrying them decreased when it reached the base of the mountain. Since the stream no longer has the high energy from the gradient, the large rocks are deposited in a manner indicative of a mountain stream environment. Sedimentary rocks can be interpreted to find out the environment in which they formed.

Sedimentary Environments can be divided into several categories:a. Shoreline and Coastal Environments b.“Fluvial” or Stream, River, and Delta Environmentsc. Alluvial Fans or deposits at the bases of mountainsd. “Aeolian” or “wind-borne” deposits There are numerous other sedimentary environments we will consider later….

4. Compaction is the final stage in the formation of a sedimentary rock. At this stagethe sediments are compacted due to the weight of the overburden (overlying sediments) and can be eventually “lithified” (turned to stone) as the particles are cemented together with substances such as Calcite (CaCO3), Silica (SiO2), or forms of Iron Oxide (i.e. Fe2O3), among other compounds..

Fig. 6-4, p.153

Depositional environments

Red sandstone in New Mexico-note parallel, horizontal bedding

Fig. 6-6, p.155

V. Properties of Clastic Sediments: These include certain characteristics of the sedimentary rock that give specific information about the environment of deposition. These include particle size, degree of roundness, degree of sorting, and color.

1. Particle Size: Clastic sediments are found in various sizes ranging from <1/256 mm to >256 mm. Refer to Figure 1. The Wentworth Scale of Particle Sizes. - The name of a particular sediment size is based on its particle size rather than its Chemical composition. For example, "sand" refers to particles having a size range Between 0.125mm – 0.5mm. There can be quartz sand such as that found along the Gulf Coast or there may be feldspar sands, gypsum sands, etc. -sediment size indicates the amount of energy operating in the depositional environment and is therefore a useful clue in determining what the sedimentary environment was. Boulders represent a high- energy environment such as a river channel while clays represent a low energy environment such as a floodplain or swamp.

2. Roundness: This is simply how “round” (or smooth) the particles in the rock are. -poorly rounded: angular, irregular shape, sharp edges. -well rounded: Particles are smooth and have no edges.The degree of roundness indicates either the amount of agitation the particles were subjected to before deposition, or the length of time it took to transport the particle.

The Wentworth Scale of Particle Sizes that is a list of sediment particle sizes and the names used to describe them:The Wentworth Scale of Particle SizesParticle Name Approximate Particle Diameter in millimeters Boulders greater than 256mm

Cobbles 1286432

Pebbles 1684

Granules2

Very Coarse Sand1.0

Course Sand Fractional Equivalents0.5 1/2

Medium Sand0.25 1/4

Fine Sand

Very Fine Sand 0.125 1/80.0625 1/160.0313 1/320.0156 1/64

Silt0.0078 1/1280.0039 1/256

Clay less than 1/256

Fig. 6-5, p.154

Lithification and classification of sedimentary particles

Fig. 6-11, p.157

a. Shale in Tenn: note how breaks along planes….called fissile…b. Mudstone in Glacier National Park

Fig. 6-8a, p.156

Microscopic view of sandstone-not individual grains, about 0.5mm what type of rounding is this-well rounded or poorly rounded?

Fig. 6-8b, p.156

Microscopic view of limestone- calcite crystals approx 1mm across

Fig. 6-10b, p.157

Arkose: sandstone that contains pebbles and sand size grains.Typically this rock has not been transported very far….

Fig. 6-10a, p.157

Quartz sandstone

Fig. 6-9b, p.156

Sedimentary breccia

Gravels deposited by swiftly flowing stream (a,b). PreCambrian (1 Billion year old conglomerate in Michigan (c) deposited same way today.

Fig. 6-22, p.172

Conglomerate: note the mixture of rounded grains and fine matrix

Fig. 6-9a, p.156

“Well rounded” particles: subjected to a high amount of saltation (bouncing along as they were transported) or being transported for a very long distance-such as from the center of a continent to its shoreline. Contact with other grains doing same thing… ‘Poorly rounded’ sediments: indicate either a low amount of agitation, or a short distance of transportation from the time the particle weathered away from their parent rocks.

A high-energy environment, allows for a long period of exposure to weathering, such as a beach or in a stream, is conducive to the formation to the formation of “well-rounded” sediments. On the other hand, a high-energy depositional environmentthat does not allow a long period of exposure to agitation, such as an alluvial fan, prevents the sediments from becoming “well-rounded”.

3. Sorting: refers to rock fragments separated according to particle size. -“poorly sorted” sediment would contain particles of varying size. This usually represents a rapid deposition as the result of a rapid decrease in the energy of an environment. found in alluvial fans at the base of a mountain. This results in a "dumping effect" of sediments at the base of the mountain (high- energy to low- energy). Poorly sorted and poorly rounded sediment is said to be "Immature". -“Well Sorted” sediment contains material that is made up primarily of all the same sized particles. This indicates that the rate of deposition is slow enough to allow the materials to be separated. This also means relatively high energy=beach. Sediment is said to be "Mature" if it is well rounded and well sorted.

Fig. 6-1a, p.150

Gravel on aBeach-looseaggregate ofsolids erodedfrom existingrocks.

Well Rounded rocks: edges are smooth and worn-NO sharp edges

Fig. 6-3a, p.152

Well rounded, well sorted deposit-smooth edges; approx same size

Fig. 6-3b, p.152

Angular, poorly sorted gravel: sharp edges, particles of many sizes

Fig. 6-3c, p.152

Fig. 6-6b, p.150

Gravel in outcrop-here is a sedimentaryrock-bound togetherby chemical cementfilling pore spaces.

Geologists call thisconglomerate

4. Color: The color of sediment can provide useful information about a sedimentary environment. In general, colors of sedimentary rocks can be interpreted in the following manner:

a.) Red, yellow, brown - oxidation conditions, probably marine in origin (RUST). b.) Black, gray, greenish-gray - reducing conditions, probably marine except for floodplains and swamps. Glauconite= marine deposition (green mineral) c.) Light gray or white - little iron present, either marine or non-marine; other characteristics of the rock must be considered such as the presence of fossils, the type of fossils, whether or not there is cross-bedding, etc.

VI. Chemical Precipitates: Chemically formed sediments are produced under various conditions, but generally speaking, when seawater becomes saturated with chemicals, they will precipitate out of solution. This is similar to when a lot of sugar is added to hot tea and then it is allowed to cool. Some of the sugar will "crystallize" or settle out of solution because the tea was "saturated" with sugar and it could not stay dissolved. Precipitates usually form only in low energy environments such as lagoons or deep-sea environments. Chemical Precipitates would not be found in high- energy environments.

Limestone and Dolostone – These “carbonate rocks result from the concentration and precipitation of Ca+, Mg+, and CO3- ions in the sea. A. Limestone - Ca CO3 (primarily calcite)- forms offshore from the precipitation of calcium and carbonate ions that have been dissolved off of the continents. Limestones may also be formed from the accumulation of microscopic calcareous tests (shells) of planktonic (or other aquatic level) micro-organisms. B. Dolostone - Ca,Mg (CO3)2 (primarily dolomite)- forms in a similar manner, but contains magnesium as well as calcium. Dolostone may start off as limestone and later is subjected to groundwater replacing Ca+ with Mg+. Or, some dolostones indicate having formed the calcium/magnesium carbonate all at once.

Table 6-2, p.158

Fig. 6-13a, p.160

Core of rock salt fromMichigan

Gypsum: evaporite

Fig. 6-13b, p.160

Fig. 6-14b, p.160

Thin layer of bedded chert

C. “Bioclastic sediments” are formed by living organisms. Many aquatic marine organisms produce shells or other protective coverings by secreting calcium carbonate (limestone) or calcium magnesium carbonate (dolomite). When these organisms die, their shells accumulate along the sea floor forming layers of broken shell fragments. Such material is biochemically produced and is ultimately broken by water action. They are then referred to as "bioclastic sediments". The sedimentary rock coquina is a good example of a bioclastic deposit. The availability of nutrients decreases the further from the shore therefore most marine organisms live in the coastal, shallow water areas. As the distance from shore increases, generally the number of marine organisms decreases. The facies of bioclastic sediments such as coquina usually indicates a lagoon to beachfront.

D. “Organic Rocks” form as the result of organics (such as vegetative matter) accumu-lating in low energy, reducing, anaerobic environments such as swamps. The material does not rot quickly and the volatiles are driven off leaving behind the carbon. A good example of an organic rock is coal. The first stage is called peat. As the peat gets compressed over time, it becomes lignite coal. As lignite becomes compressed, it becomes bituminous coal. As bituminous coal becomes compressed, it forms the metamorphic rock anthracite, the final stage of coal. Other types of organic rocks may form from accumulations of dead organisms (such as fish) in low energy lagoons.

Coquina-pile of shells….

Fig. 6-12b, p.159

Fig. 6-12c, p.159

Chalk cliff in Denmark-made up of skeletal remains of microscopic’organisms

Fig. 6-12d, p.159

Limestone with shells

Fig. 6-15a, p.161

Peat: plant remains,buried and compressed

Fig. 6-15b, p.161

Lignite: peat that has been buried deeper, compressedand heated.

Bituminous coal-about 80% carbon; dense, black, burns efficiently

Fig. 6-15c, p.161

Anthracite=highest grade coal, 95% carbon; yields more heat thanother types of coal

VII. Bedding or Layering of Sedimentary Materials:

Sedimentary rocks are deposited in layers known as "beds". The type of bedding will vary depending on the environment of deposition. Under normal conditions, beds are deposited in horizontal layers with the bedding planes (the line of contact between the beds) parallel to one another. "Cross-bedding" occurs when the surface of deposition is inclined (i.e. a delta) or a current is present (i.e. a stream). This type of bedding is called "cross-bedding" and is indicative of these environments. The types of currents that form cross-bedding strata are:

Aeolian - wind action Fluvial - river and stream action Marine in Origin - current action

Types of cross-bedding include planar - the bedding planes separating the cross-bedded units are parallel, wedge - the bedding planes are at an angle to one another and form a wedge; and trough - the bedding planes separating the cross-bedded units are curved.

Thick planar or wedge cross-bedding always indicates an aeolian (wind) deposit such as a sand dune in the desert. Thin planar or wedged units may be aeolian, fluvial, or marine. Because of this, other characteristics such as color must be used to determine the environment of deposition.

• Illustration of the principles of superposition– and original horizontality

Principle of Horizontality

• Horizontality: These sediments were originally

– deposited horizontally – in a marine environment

– This outcrop is Chattanooga Shale, Tennessee

• Illustration of the principles of superposition– and original horizontality

Principle of Superposition

• Superposition: The youngest– rocks are at the top

– of the outcrop

– and the oldest rocks are at the bottom

Jurassic Navajo Sandstone, Zion National Park, UT wind blown dune deposit. Note the thickness!! the cross bedding….

Fig. 6-23a, p.173

Fig. 6-17, p.164

a. Bedding in sand-shale layers.b. Fluvial cross bedding-originally deposited on sloping surface.c. Ancient cross beddingin Montana: hammer is30 cm long.

Ripple Marks: form in response to flow in one direction

Fig. 6-19, p.165

Cross bedding within a ripple

Current ripples in shallow stream Wave formed ripple marks in shallow seawater

To and fro motion of waves in shallow water

p.167

Lion monument in Lucerne, Switz

Carved in sandstones which illustrate cross bedding.

Also angular unconformity with rocks above….

1.Many times paleocurrents of water (and sometimes wind) can be traced by the ripple marks left in some sedimentary rocks indicating ancient river channels or beachfronts. 2. Mud cracks can also be preserved indicating ancient low energy mud flats. 3. Another type of bedding is known as graded bedding. This is where there is a gradation in the size of particles within a unit of deposition. Larger particles are found on bottom with successively smaller sediments on top. This type of bedding is formed by "turbidity currents", which are the sudden flows of material down the continental slopes. This causes the finer particles to be suspended in the water while the larger particles fall out and are deposited on the bottom with smaller and finer sediment on top.This results in a "gradation" in particle size. Graded bedding is deep water marine facies.

VIII. The Marine Lithofacies: This refers to the depositional sequence found in a cross section of a shore to deep-water environment. The usual sequences of rock types are:

1. Sandstone formed on beach areas2. Siltstone formed near-shore3. Claystone/Shale formed further out4. Limestone formed even further out in deeper waters

Mudcracks in ancient rocks in Montana-note cracks filled with sediment

Fig. 6-20b, p.166

Mudcracks form in clay rich rocks-due to drying and shrinking

Fig. 6-20a, p.166

Fig. 6-18, p.165

Formation of graded bedding-typically found in turbidites (deep sea)

Bedding

Fig. 6-21, p.166

Some features form long after rock is deposited: note the red-brown agatethat filled cavity, later filled by calcite crystals.

Transgression: - the advancement of the sea onto the land because of a worldwide increase in sea level or a subsidence of the landmass.Regression: - the retreat of the sea from the land due to a worldwide drop in sea level or the uplift of the land.Transgressional and Regressional sequences of strata can be used to interpret and retrace ancient coastlines.

Transgressional Sequence - reflects RISE in Sea Level

Regressional Sequence – reflects a FALL in Sea Level

Fig. 6-16, p.162

Marine Transgression Marine RegressionNote vertical succession of facies in each case-they are very different

Fig. 6-23b, p.173

Marine transgression, evidence in Grand Canyon: c. Muav Limestone b. Bright Angel shale a. Tapeats sandstone

• All sediments deposited in – glacial environments are collectively called drift

• Till is poorly sorted, nonstratified drift – deposited directly by glacial ice– mostly in ridge-like deposits called moraines

• Outwash is sand and gravel deposited – by braided streams issuing from melting glaciers

• The association of these deposits along with – scratched (striated) and polished bedrock – is generally sufficient to conclude – that glaciers were involved

Glacial Environments

• Moraines and poorly sorted till

Moraines and Till

• Origin of glacial drift

• Carbonate rocks are – limestone, which is composed of calcite– dolostone, which is composed of dolomite

• most dolostone is altered limestone• Limestone is similar to detrital rock in some

ways– Many limestones are made up of

• gravel-sized grains • sand-sized grains• microcrystalline carbonate mud called micrite

– but the grains are all calcite – and are formed in the environment of deposition, – not transported there

Carbonate Environments

• Some limestone form in lakes, – but most limestone by is deposited – in warm shallow seas– on carbonate shelves and– on carbonate platforms rising from oceanic depths

• Deposition occurs where – little detrital sediment, especially mud, is present

• Carbonate barriers form in high-energy areas and may be – reefs – banks of skeletal particles – accumulations of spherical carbonate grains known

as oolites • which make up the grains in oolitic limestone

Limestone Environments

Ooids: carbonate grains deposited in high energy environment-rounded!!

Fig. 6-12a, p.159

• The carbonate shelf is attached to a continent– Examples

occur in southern Florida and the Persian Gulf

Carbonate Shelf

• Carbonates may be deposited on a platform – rising from oceanic depths

• This example shows a cross-section – of the present-day Great Bahama Bank – in the Atlantic Ocean southeast of Florida

Carbonate Platform

Oil and natural gas traps: a. stratigraphic traps, b. structural traps, c. saltdome

Fig. 6-24, p.174

Mineral Resources in Sedimentary Rocks

Alaska pipeline taking crude oil from Prudhoe Bay south to terminal.

Fig. 6-25, p.177

Fig. 6-26, p.178

Economic Uses:

a. Iron ore mined from sedimentary rocks

b. Banded iron formation, Michigan. AlternatingLayers of red chert and silver colored iron minerals.

c. Iron ore mined and shaped into pellets.

Chp 6: Sedimentary Rocks- SummaryA. Sedimentary Rocks are derived from weathering of igneous rocks.

B. Two types of Sedimentary rocks: -Chemical precipitates: Calcium carbonate -Clastic rocks: sandstones, shales, etc.

C. 4 Steps to Formation of Sedimentary Rocks:-Erosion: various weathering processes-Transportation: air, glaciers, running water, etc-Deposition-Compaction: overburden, burial

D. Characteristics of Clastic Sedimentary Rocks -Roundness: well rounded vs angular

-Sorting: well sorted vs poorly sorted-Color: red=exposed to air; glauconite=green=marine-Particle Size (see following diagram)

Table 6-1, p.152

p.170-172

Fossils commonly occurIn Sedimentary rocks:

a. Dinosaur excavation in Wyoming.b. Paleontologists excavatingRhinoceros (foreground) andHorse (background).c. Mural showing animalsfound in La Brea Tar Pitsfrom Los Angeles

Chp 6: Sedimentary Rocks- Summary

II. Chemical Precipitates -Limestone: calcium carbonate - Dolostone: calcium and magnesium carbonate -Bioclastic deposits: form around remains of marine organisms. e.g. chalk, coquina (shells), -Evaporites: halite, gypsum, etc. Evaporitic conditions -Coal: peat, lignite, bituminous coal, anthracite

Bedding/Layering of Sedimentary Rocks -Aeolian: wind, cross bedding -Fluvial: cross bedding, laminations, etc. -Turbidites: graded bedding. -Marine: currents-ripples -Marginal marine to terrestrial: mud cracks

Laws of: Superposition and horizontality: oldest on bottom, flat

Chp 6-Sedimentary Rocks: SummaryChp 6-Sedimentary Rocks: Summary

Vertical Succession of Facies -Transgressive: sea level rise -Regressive: sea level fall

Economic Uses of Sedimentary Rocks: -iron ore from fluvial deposits -placer (fluvial) deposits: precious minerals-gold and silver -Drilling for hydrocarbons (oil and gas) stratigraphic traps, structural traps, salt domes -gravel pits for road use

Chp 6: Sedimentary Rocks- Aeolian dunes

Chp 6: Sedimentary Rocks- Ripples

Chert: notice the concoidal fracturing along bottom edge; hard rock, Composed of microscopic particles of quartz.

Fig. 6-14a, p.160

• Alluvial fans form best along the margins of desert basins – where streams and debris flows – discharge from mountains onto a valley floor – They form a triangular (fan-shaped) deposit – of sand and gravel

• The more central part of a desert basin – might be the site of a temporary lake, a playa lake, – in which laminated mud and evaporites accumulate

Alluvial Fans and Playa Lakes

Fig. 6-2, p.151

Sheep Rock, ORE.:a. Sedimentary rockscapped by thin lava flow at top.

b. Sedimentary rocksin Dry Fossil Beds NatlMonument.

c. Mammals that lived in Area from 37 to 55 millionYears ago:1.Titanotheres2. A carnivore3. Ancient horses4. Tapirs5. rhinoceroses

Ooids: carbonate grains deposited in high energy environment-rounded!!

Fig. 6-12a, p.159