Introduction to Earth Science

Chapter 10 - Sedimentary Rocks & Processes

10.1
This chapter is about the origin and distribution of sedimentary deposits (sediments and sedimentary rocks). The word “sedimentary” refers to materials consisting of sediments or formed by deposition; the word sedimentary also applies to both the processes and the products of deposition (Figure 10-1).

Sedimentary rock is rock that has formed through the deposition and consolidation and solidification of sediment, especially sediment transported by fluids—including water (rivers, lakes, and oceans), ice (glaciers), and wind. These erosional processes grind down rocks into fragments and into smaller individual mineral grains. This materials is transported sorted by fluids, separating coarser fractions from finer fractions and by density.

Sedimentary rocks are often deposited in layers, and frequently contain fossils. Studies of sedimentary deposits reveal characteristics of the depositional environments at the time that the material accumulated. This information can help tell the history of geologic events and climate change of an area.
Click on images for a larger view throughout this website.
The Rock Cycle
Fig. 10-1. Sediments, sedimentary rocks and sedimentary processes are part of the Rock Cycle.
10.2

How do sediments become sedimentary rocks?

Sediments can become lithified into sedimentary rocks once they've been deposited in a stable setting where burial, compaction, and cementation can take place. The processes, collectively called lithification (or diagenesis), typically takes place slowly over time but rates depend on many factors including the chemistry of the sediments and groundwater passing through the sediment, and how quickly or deeply burial takes place. Deposits of unconsolidated sediments typically have high porosity—pores are open spaces between grains filled with gas or fluids (water or in some cases, petroleum). Compaction is the process of gravitation consolidation of sediments, decreasing the volume of pore space between particles of sediment and increasing hardness. Cementation involves processes that harden sediments through the precipitation of minerals in pore spaces between grains of rock and mineral fragments, binding them together (Figure 10-2). Common minerals that form cement include quartz, calcite, limonite, hematite, and clays. The cementing minerals are slowly deposited between grains by groundwater.
Cement fills in pore space between mineral grains
Fig. 10-2.
Cement fills in spaces between mineral grains.
10.3

Where do sedimentary rocks occur?

Sedimentary rocks are exposed throughout the world's continents, covering about half of the exposed land on the earth surface. This sedimentary cover blanketing continental areas was originally deposited mostly in coastal environments, in shallow seas flooding shallow continental basins, on continental shelves and in ocean basins along the margins of continents. These ancient sedimentary deposits are well exposed in mountainous regions (Figure 10-3). Most of these sedimentary rocks that blanket much of the continents formed in the last several hundred million years. Even more massive quantities of sediments occur along continental margins in ocean basins. These thick sequences of sedimentary deposits contain all the oil, natural gas, coal, and many mineral resources essential to our modern world. They also preserve the fossil record—fossils, preserved in sediments deposited at the times when ancient life forms lived and died. Sedimentary rocks preserve abundant evidence of ancient environments and changing landscapes through time.

The mile-thick sequence of sedimentary rock formations exposed by erosion in the Grand Canyon is an exceptional example of the sedimentary cover preserved on the North American continent (Figure 10-4). The sedimentary rocks exposed in the Grand Canyon represent sediments deposited in environmental setting ranging from shallow marine seaways to coastal sand dunes that accumulated long before the uplift of the Rocky Mountains and Colorado Plateau region.
Map of the world showing the location of volcanoes
Fig. 10-3. Map of geologic provinces of the world.
Sedimentary rock formations exposed in the Grand Canyon
Fig. 10-3.
Sedimentary rock formations exposed in the Grand Canyon.
10.4

Classification of Sediments and Sedimentary Rocks

Sediments and sedimentary rocks are classified in by origin of source material and by grain size.

Classified by origin: There are 4 sources of sediments:

a) Lithogenous sediments are sediments derived from terrestrial sources through weathering and erosion
b) Biogenous sediments are sediments produced by biological activity
c) Hydrogenous sediments are sediments precipitated directly from water
d) Cosmogenous sediments are materials that are derived from material entering the atmosphere from outer space.

Of the 4 types of sediments, lithogenous and biogenous sediments are the most abundant on earth today.

Lithogenous sediments form through the processes of weathering and erosion of materials exposed on land and along coastlines (coastlines are described in Chapter 16). Lithogenous sediments consist of solid fragments of inorganic or organic material that come from the weathering of rock and soil erosion, and are carried and deposited by wind, water, or ice. Most lithogenic sediments eventually are deposited along the margins of ocean basins (Figure 10-4) However, there are many deposits that form on continents in a variety of sedimentary environments (discussed below).

Biogenous sediments are composed of the remains of living organisms, including microscopic phytoplankton (plants) and microscopic zooplankton (animals), terrestrial and aquatic plants, shells of invertebrates, and vertebrate material (teeth, bone), and associated organic residues. Coal, oil, and gas are derived from biogenous sediments. Biogenic sediments represent regional massive deposits associated with modern and ancient carbonate reef systems (such as the Australian Barrier Reef, South Florida and the Bahamas, the Yucatan and Caribbean, atolls in the Pacific, and many other locations.

Hydrogenous sediments are sediments directly precipitated from water. Examples include rocks called evaporites formed by the evaporation of salt bearing water (seawater or briny freshwater). Salt (NaCl) and Gypsum (CaSO4) are Manganese nodules form on the ocean bed (mostly in the deep Pacific) from the slow precipitation of metal oxides in the absence of other kinds of sediments.

Cosmogenous sediments originated from outer space. Scientists have used satellites to estimate how much material enters the earth's atmosphere, with current estimates from satellite data suggesting about 100 to 300 tons (mostly cosmic dust) hits earth each day. This is just a tiny fraction of the sediments generated on earth each day. However, early in the history of the Solar System Earth and other planets, moons, comets and asteroids formed from the gravitational accumulation of extraterrestrial material, but by 4.5 million years ago, most of this cosmogenic accumulation had significantly diminished. However, cosmogenic materials including iron-nickel and stony meteorites can be found. Extraterrestrial impacts have changed life on earth repeatedly, including the mass extinction at the end of the Mesozoic Era associated with the extinction of dinosaurs and many other forms of life on land and in the oceans. Tektites are silica glass generated by extraterrestrial impacts.

Appalachian Basin and Atlantic Margin
Fig. 10-4. Continental margins
are places where large quantities of lithogenous sediments accumulate. In many places around the world the thickness of sediments eroded from continental landmasses and volcanic chains and deposited in the adjacent ocean basin can be many miles thick!
10.5

Clastic vs. Non-Clastic Classification of Sediments and Rocks

Sediments and sedimentary rocks are usually classified as clastic and non-clastic varieties.

Clastic sediments and sedimentary rocks

The word clastic is also commonly used to describe sediments or sedimentary rocks composed of fragments (or detritus) derived from older rocks. The word clast means rock fragment; the word is derived from the Greek word klastos which means "broken." Gravel, sand, and silt are examples of clastic sediments. Lithogenic sediments (described above) are mostly clastic sediments. A classification of clastic sediments and sedimentary rocks is illustrated in Figure 10-5 and is discussed in detail below.

Non-clastic sediments and sedimentary rocks

The term non-clastic refers to sediments or sedimentary rocks composed of materials produced by biological activity (skeletal material, respiration, and excretion) and sediments precipitated from water (the later are also called chemical sedimentary rocks). Biogenous and hydrogenous sediments described above are non-clastic sediments. A classification of non-clastic sediments and sedimentary rocks is illustrated in Figure 10-6 and is discussed in detail below.
Classification of sedimentary rocks
Fig. 10-5. Classification of clastic sediments and sedimentary rocks
Non-clastic sediments
Fig. 10-6. Classification of non-clastic sediments and sedimentary rocks
10.6

Clastic Sediments and Clastic Sedimentary Rocks

Clastic Sedimentary Rocks: Rocks composed of grains of mineral and rock fragments derived from erosion of other rocks. Three general groups are coarse-grained, sand-size grained, and fine-grained (mudrocks).

Coarse-grained sediments and sedimentary rocks


Gravel is rock particles that have been moved by moving water. Gravel usually consists of a mix of the more durable and most abundant rock types in the sediment source areas (Figure 10-7). Gravel deposits typically occur along stream valleys close to mountainous source areas and along rocky coastlines with high wave action.

Conglomerate is a sedimentary rock composed of cemented gravel. It consists of rounded to subangular fragments (larger than 2 mm in diameter) set in a fine-grained matrix of sand or silt, and commonly cemented by calcium carbonate, iron oxide, silica, or hardened clay; the consolidated equivalent to gravel (Figures 10-8 and 10-9). A sedimentary form of breccia is a rock consisting of angular rock fragments cemented together (Figure 10-10). Breccia can be found forming in sedimentary settings close to areas of high relief, such as on alluvial fans, along mountain stream valleys, material ejected from volcanoes, shattered rock in fault zones, areas of mass wasting, cavern collapse and landsliding. The composition reflects the rocks and regolith in the area where it forms.

Sand-size-grained sediments and sedimentary rocks

Sand goes through degrees of refinement at it moves away from source areas. Sand deposits near mountain ranges may be enriched in feldspars. Volcanic regions may produce sand enriched in dark minerals. "Mature" sand that has traveled long distances in streams, blown by wind, or worked by waves will be enriched in quartz and individual grains will be very well rounded and well sorted (see below). Large sand deposit accumulate along stream valleys, on beaches, barrier islands, and offshore bars, and in dune fields in coastal areas and in desert environments (Figure 10-11).

Sandstone
is a sedimentary rock formed by the consolidation and compaction of sand and held together by a natural cement, such as silica, calcite, and iron-oxide minerals (Figure 10-12). Most sandstone is dominated by the mineral quartz. However, two other types of sandstone include arkose and graywacke (Figure 10-13). Arkose is a coarse-grained sandstone that is at least 25 percent feldspar. Arkose is associated with ancient alluvial sediments derived from granitic mountain ranges (example: Figure 10-14). Graywacke is a sedimentary rock consisting of a mix of angular fragments of quartz, feldspar, and mafic minerals set in a muddy base; it is a "dirty" dark brown or gray sandstone or silty mudstone, common in active continental margin regions such as the Coast Ranges of California.

Fine-grained sediments and sedimentary rocks (Mudrocks)

Mud
is a general term lumping together sediments consisting of a mix of clay, silt, and may contain sand. Mud is usually an unsorted mix of fine grain materials. Mud accumulates in quiet water settings separated from where coarser materials have settled out elsewhere (Figure 10-15). Most soil is mud. Mud-rich accumulations are common in river delta regions, swampy coastal regions, tidal flats, and in lake and deep water settings.

Mudstone is a fine-grained sedimentary rock formed from the compaction and cementation (litification) of muddy sediments rich in silt (but may include percentages of fine sand and clay).

Shale is a soft, finely stratified sedimentary rock that formed from consolidated mud rich in clay minerals and can be split easily into fragile plates, such as along bedding plains (Figure 10-18). Shale forms from the compaction of sediment dominated by clays.

Clays are composed of any microscopic mineral particles. Most dust is clay sized particles. However, there are several types of clay minerals. Clay minerals are any of various hydrated aluminum silicates that have a fine crystalline structure and are components of clay (sediment). Clay minerals form from the weathering of feldspars and other silicate minerals and are the dominant sediment found on earth.

Some types of clay minerals can soak up and hold water, and release it when it dries out. These are called expanding clays and are a real problem for home foundations and infrastructure in some parts of the country. When it gets wet it expands. While wet they are slick and have a greasy feel, and sticks to everything. When it dries out, it shrinks and turns hard as rock, and develops a dried, gray popcorn-like texture. Many-a-rancher can tell stories of getting their truck stuck in gumbo as it is often called!
Gravel bar along Coyote Creek in Morgan Hill, California
Fig. 10-7.
A gravel deposit exposed in a dry stream bed during a dry season.
conglomerate
Fig. 10-8.
Conglomerate formed from an ancient stream gravel deposit.
breccia
Fig. 10-9. Conglomerate
typically has stream-rounded clasts
breccia
Fig. 10-10.
Sedimentary and volcanic breccia have angular clasts.
Quartz sand beach
Fig. 10-11. Sand
is winnowed (sorted) and accumulates on a beach by wave action
Outcrop of Navajo Sandstone near Tuba City, Arizona
Fig. 10-12. Sandstone
outcrops exposed in Utah's Canyonlands National Park
arkose and graywacke
Fig. 10-13.
Three kinds of sandstone: arkose, quartz sandstone, and graywacke
Flatirons of Boulder Colorado
Fig. 10-14.
The Flatirons of Boulder, Colorado are large outcrops of arkose.
classification of mudrocks
Fig. 10-15. Classification of 'mudrocks'
Mudrocks are classified similar to soils (discussed in Chapter 8) by the percentage of content of 3 components: sand silt and clay (with the 3 equivalent rocks being sandstone, siltstone, and shale)
Clay used in pottery
Fig. 10-16.
Certain kinds of Clays are used to make ceramic pottery. Clay is made up of clay minerals.
Austrailian mudflats
Fig. 10-17. Mud
accumulates in quiet-water environments as illustrated with these tidal flats.
  shale and mudstone Shale exposed in Capitol Reef National Park Ash beds in John Day Chinle Shale
Fig. 10-18. Comparison of shale and mudstone. Shale tends to be flaky and splits into thin layers. Mudstone tends to form more massive layers. Fig. 10-19. Shale (blue gray) and mudstone (brown) outcrops in Utah's Capitol Reef National Park. Marine shales tend to be shades of blue, green, and gray.
Fig. 10-20. Volcanic ash is typically silt that weathers into clays, forming shale (such at these brightly colored sedimentary beds near John Day, Oregon). Fig. 10-21. Terrestrial shale (Triassic Chinle Formation, Petrified Forest National Park, Arizona). Terrestrial deposits are typically shades of red, orange, and yellow.
10.7
5. Non-clastic sediments and non-clastic sedimentary rocks
Non-clastic sediments include materials that are not formed by the weathering and erosion of rock, but include sediments formed by precipitation directly from water or formed by accumulation of organic materials. These organic residues form through organic processes (life and death of organisms), particularly microscopic planktonic skeletal material, shells, bone materials, and other organic remains.

Bioaccumulation is the buildup of organic remains, such as deposits associated with coral reefs, peat bogs, shell or bone beds, and algae and planktonic ooze.
Planktonic ooze is slimy mud sediment on the bottom of an ocean or lakebed formed from the accumulation of skeletal and organic remains of microscopic organisms (Figure 10-22). Calcareous microplankton deposits can become chalk (such as exposed in the White Cliffs of Dove). Siliceous microplankton deposits can become chert (see below).

Limey sediments and limestone

Lime mud
is sediment composed of calcium carbonate (CaCO3) derived from the skeletal remains of shelled organisms, coral, and calcareous algae and plankton. Large amounts of lime mud is created by waves battering reefs and material being chewed up and excreted by reef-living organisms (Figures 10-23 and 10-24). The sediment sediments with compaction and cementation (lithification) become limestone (see below).

Limestone is a sedimentary rock consisting predominantly of calcium carbonate (CaCO3) derived from the skeletal remains of marine microorganisms, including shells and coral) and eroded and transported sediments associated with reef environments in shallow, warm tropical marine waters (Figure 10-24). Limestone is commonly used in the manufacture of lime for cement and used as building stone.

Marine Limestones: Most limestone exposed throughout the United States formed in ancient shallow marine seaways that flooded portions of the continent in the geologic past. Some limestones preserve large quantities of fossil material as crushed up shells or even old reef communities are sometimes preserved in nearly intact orientation of the corals and other calcareous organisms.
Chalk is a soft, fine-grained, white to grayish variety of limestone that is composed of the calcareous skeletal remains of microscopic marine organisms including coccoliths and foraminifera. Some of the purest varieties can have up to 99 percent calcium carbonate (see Figure 10-22).
Coquina is a name for a soft type of limestone composed almost entirely of compacted and cemented shell fragments, commonly found on upper beach areas in warm, humid climates. Some fossiliferous limestones can be called coquina (Figure 10-26).
Oolitic limestone is composed of ooids. Ooids are small spheroidal, "coated" (layered) "pearl-like" sedimentary grains (<2 mm in diameter), usually composed of calcium carbonate, but also sometimes iron- or phosphate-based minerals. Ooids usually form by agitation in turbulent waters in shallow tropical setting (such as around the Bahamas or in the Persian Gulf). The Empire State Building is made from ancient oolitic limestone mined in Bedford, Indiana (Figure 10-27).

Freshwater limestones: Most limestone forms from marine sediments, but three varieties of freshwater limestone include tufa, travertine and caliche.
tufa—calcareous and siliceous rock deposits of springs, lakes, or ground water, commonly found around dry lake regions (such as the Trona Towers near Death Valley, CA [used in lots of TV commercials], Figure 10-28).
travertine—white or light-colored calcareous rock (CaCO3) deposited from mineral springs; or a common name for freshwater limestone deposits (Figures 11-28 and 11-29). Speleothems (stalactites, stalagmites, etc.) in limestone caverns consist of travertine.
caliche—a hardened zone in soils and surficial deposits found in semiarid regions where of calcium carbonate and possibly other carbonates, clay minerals, or crystalline salts such as sodium chloride or sodium nitrate impregnated the pore spaces in the sediment or soil. Figure 10-30 shows stream terraces capped with caliche layers. Caliche is abundant through the US West.

Dolostone
is a rock composed mostly of the mineral dolomite, typically a white, light gray or pink with a sugary crystalline texture. The mineral dolomite, with a chemical formula of CaMg(CO3)2, often is observed replacing calcite in limestone, particularly in ancient carbonate rocks. The more ancient the rock, the more likely it has been exposed to magnesium-rich brines that can react with calcite to form dolomite. Most Paleozoic-age rock formations around the world are enriched in dolomite (example, Figure 10-31).

Siliceous non-clastic sedimentary rocks

Chert is a hard, dense sedimentary rock, consisting chiefly of interlocking microscopic crystals of quartz and may contain opal. It has a conchoidal fracture and may occur in a variety of colors. Most chert forms from recrystallization of siliceous microplankton remains (siliceous ooze eventually looses its water content, recrystallizes and turns into chert, Figure 10-32).

Flint is a hard (typically gray or brown) rock consisting of nearly pure silica. Flint often occurs as nodules in chalk or limestone. Flint is a variety of chert; both were used by prehistoric peoples around the world to make arrow and spear points (Figure 10-33).

Carbonaceous sediments and sedimentary rocks

Peat is an accumulation of partially decayed vegetation matter that has a brown, soil-like character typically found in of boggy, acid ground or swampy settings. With litification peat will become coal (Figures 10-34 and 10-35). The formation of coal, oil, and gas are discussed in more detail in Chapter 11).

Evaporites (Salts)

An evaporite is a rock composed of salt minerals left behind by the evaporation of salty water.

rock salt—a rock dominantly composed of sodium chloride (NaCl - the mineral halite is what rock salt is composed of; Figure 10-36). Rock salt is an evaporite formed in restricted basins with an inflow of seawater located in an arid environmental setting.

gypsum—a mineral composed of hydrous calcium sulfate (CaSO4-2H2O); an evaporite mineral used in the manufacture of plaster. Gypsum is deposited by concentrated seawater and by evaporation of freshwater in arid regions. Crystals of gypsum are common in soils in arid regions. If gypsum looses its water content, it is called anhydrite.

Salts are precipitated when sea water (or briny lake water) is concentrated by evaporation. Shorelines along the oceans in hot arid regions of the world are places where salt, gypsum and anhydrite are being deposited today. Examples include around the Persian Gulf and the Great Salt Lake in Utah (Figures 10-38 and 10-39).
limestone
Fig. 10-22. Planktonic ooze
forms chalk (calcareous) and chert (siliceous)
limestone
Fig. 10-23.
Skeletal remains of calcareous reef organisms erode and accumulate over time and forms limestone
Florida depositional environments
Fig. 10-24. Limey
(shallow and warm) depositional environments in South Florida
Limestone with bryozoan fossils
Fig. 10-25. Fossiliferous limestone
. This sample from Ohio is loaded with ancient coral like fossil bryozoans.
Brachiopodal limestone
Fig. 10-26. Fossiliferous limestone
. This layer from Ohio is a coquina loaded with ancient brachiopod shells.
oolites
Fig. 10-27. Ooids
and oolitic limestone. A variety from Bedford, Indiana was used in the construction of the Empire State Building, NYC
tufa
Fig. 10-28. Travertine
and Tufa form from spring water releasing and depositing calcium carbonate
Speleothems: stalagtites, stalagmites, and columns
Fig. 10-29. Travertine speleothems
(stalactites, stalagmites, and columns); Lehman Cave, Nevada
caliche
Fig. 10-30.
Caliche is a surface soil layer that is tightly cemented by calcium carbonate that commonly forms in desert regions.
Dolomites of northern Italy
Fig. 10-31.
The Dolomites are scenic mountains in northern Italy's Alps and are composed of dolostone.
ribbon chert
Fig. 10-32.
Layers of chert exposed in the Marin Headlands, California
Arrowheads made of flint
Fig. 10-33.
Chert and flint arrowheads and spear points from southwestern Ohio.
organic deposits
Fig. 10-34.
Organic sediments that accumulate in isolated forested swamps (or bogs) can become peat. With time and burial it can become coal
bedding
Fig. 10-35. Coal seams
are ancient forest swamp deposits. These are exposed
in Mesa Verde National Park, Colorado
Halite
Fig. 10-36.
Rock salt (mineral is halite)
anhydrite
Fig. 10-37.
Anhydrite gypsum.
evaporites
Fig. 10-38.
A sabkha is a desert coastal environment is where salts, including halite and gypsum, are commonly deposited.
salt pan, Death Valley
Fig. 10-39.
Polygons of salt crystals (halite) being deposited on a salt pan on a dry lake bed in Death Valley National Park, California
10.8

Deposition and Sedimentary Depositional Environments

Deposition is the process of sediments settling and accumulating from a moving fluid (wind, water, or ice). Sediments will erode anywhere where forces associated with currents are strong enough to dislodge and move sediment particles. The "heavier" (both larger and/or denser) the particle, the more energy it take to move. Water is perhaps the primary mover of materials on earth. Wind typical can only move small particles of dust-to-sand-sized particles. Water can move particles of any practically any size if the current is strong enough. Moving glacier ice (though limited to mountainous regions and high latitude regions during Ice Ages) can move materials including house-sized blocks of rock. In general, as a fluid speeds up materials will erode. As the fluid slows down materials will settle out and be deposited. Over time, sediments will accumulate and possibly become buried, becoming permanently deposited until possibly exposed to surface erosion at some later period in geologic time. Places where sediments accumulated are called depositional environments. Depositional environments occur in many different geologic settings on land and under water (examples are illustrated in Figure 10-40).

Sedimentary rocks typically preserve numerous clues as to how and where the original sediments were deposited. Over time, regional climates may change, sea level may rise and fall, or tectonic activity, volcanism, or regional erosional patterns may change. As a result, sedimentary rocks exposed in a region may preserve a variety of sedimentary rocks formed in a mix of sedimentary depositional environments during different periods of geologic history.

A sedimentary depositional environment includes the combination of physical, chemical and biological processes associated with the deposition of a particular type of sediment. The characteristics of ancient sedimentary depositional environments are often preserved in sedimentary rocks.

The term sedimentary facies is used to described the character of a sedimentary rocks expressed by their formation, composition, and fossil content (examples: beach facies, fluvial facies, lacustrine facies).
Sedimentary environments
Fig. 10-40. Sedimentary depositional environments occur on land and under water.
Monterey Bay
Fig. 10-41. Topography and bathymetry of the Monterey Bay region
10.9

Energy in Depositional Environments

Sedimentary deposits may preserve aspects of the overall energy conditions of an environmental setting. High energy environments include locations where water is flowing (streams or waves are crashing) but may be locations where coarse sediments (gravel and sand) may selectively settle out, but currents winnow out and remove the finer and lighter materials (silt, clay, organic particles) which will be carried away and deposited in a possibly distant low energy environment. Clastic environments Desert environments Carbonate depositional environments
Fig. 10-42. Clastic depositional environments in humid regions and marginal marine settings. Fig. 10-43. Depositional environments in arid regions include alluvial fans, dry lake beds, and dunes. Fig. 10-44. Marine and coastal depositional environments associated with non-clastic sediments and rocks.
10.10

How does kinetic energy in different environmental settings impact how and where sediments accumulate that can become sedimentary rocks?

High-Energy and Low Energy Depositional Environments: Flowing water is the dominant force causing erosion and deposition on Earth (with human mining and construction activity rapidly closing on that claim!). The faster the water moves, the higher the energy in a physical setting. As flowing water increases in speed, the more it may become turbulent, increasing its ability to lift and move particles. Fast moving water can carry materials of different particles ranging from boulders and gravel to finer materials (sand, silt, and clays). Flowing water also sorts sediments by size and density. High-energy environments include river channels, beach and shallow offshore environments (Figures 9-45 to 9-48). Flowing water may let larger materials settle and be deposited while finer materials are carried away and deposited in quieter settings or (low-energy environments) (Figures 9-49 to 9-52).

Different sedimentary environments have different energy characteristics that may change from time to time. The forces of energy in a stream will increase as the volume of water increases, such as during flood. For most of a year, a stream will may be a calm environment, that that changes during a flood, or during flood season. The same is true of beach and offshore bar environments. As wave energy increases, the greater the amount of energy translates into shoreline erosion and the moving of sediments to quieter and deeper offshore settings. Wave action separates sand from courser and finer fractions, building up or eroding beaches with changing conditions. A beach or offshore region can remain basically calm, relatively low energy for years until a hurricane comes along, and the setting becomes high energy. One big storm event can move more sediments in a few days that might have moved for decades or even centuries. For example, Hurricane Camille did this to the coast of Alabama and Mississippi in 1969.

high energy depositional environments (coarse-grain sediments dominate). Examples include: stream and river channels, beaches, dunes, offshore bars (above wave base)submarine channels and canyons where strong ocean currents persist. Weather (climate) and wave energy are variable factors in high-energy environmental settings. Sediments are constantly being deposited or eroded in these settings.

low energy depositional environments (fine grained sediments dominate): Examples include: river flood plains, swamps, lakes, lagoons, marshes, and offshore below wave base. Slow-moving currents prevent coarse-grained sediment from migrating into in low-energy depositional environments. Fine materials can be carried long distances before they can settle out in the absence of waves and currents.
High Energy Depositional Environments
river
Fig. 10-45.
River channels
beach environment
Fig. 10-46.
Beaches
dunes
Fig. 10-47.
dunes
reef
Fig. 10-48.
Coral reefs
Low Energy Depositional Environments
lake environment
Fig. 10-49.
Lake (lacustrine)
swamp
Fig. 10-50.
Swamp
floodplain
Fig. 10-51.
River floodplains
Elkhorn Slough estuary
Fig. 10-52.
Tidewater marsh
10.11

Sedimentary Rock Formations

The term rock formation is the primary unit of stratigraphy, consisting of a succession of strata useful for mapping or description. A rock formation typical consists of a unique lithology (rock type) that has a relatively defined geologic age and is considered "mapable" (meaning that it occurs throughout area or region, both on the surface and in the subsurface). Rock formations can be of igneous, sedimentary, or metamorphic origin. Sedimentary rock formations preserve information about sedimentary facies they preserve (discussed above).

Examples of sedimentary rock formations: each of the rock formations illustrated below formed in a corresponding ancient sedimentary depositional environment.
Navajo Sandstone Chinle Formation El Capitan Limestone Garden of the Gods, Colorado
Fig. 10-53. Ancient dune deposits preserved in Navajo Sandstone Formation
(Jurassic age) on the
Navajo Reservation, Arizona
Fig. 10-54. Ancient river floodplain deposits preserved in the Chinle Shale Formation (Triassic age) on the Navajo Reservation, Arizona Fig. 10-55. Ancient reef deposits preserved in the Capitan Limestone Formation (Permian age) in Guadalupe National Park, West Texas Fig. 10-56. Ancient alluvial fan deposits preserved in the Fountain Formation
(Pennsylvanian age) in
Garden of the Gods, Colorado
Delmar Dog Beach Bryce Canyon National Park Santa Cruz Mudstone at Wilder Ranch State Park ribbon chert, Marin Headlands, California
Fig. 10-57. Ancient beach and coastal dune deposits exposed in the Del Mar Formation (Eocene age) at the Del Mar Dog Beach, San Diego County, California Fig. 10-58. Ancient lake (lacustrine) deposits preserved in the Chadron Formation (Eocene age) in
Bryce Canyon National Park, Utah
Fig. 10-59. Ancient continental shelf deposits preserved in the Santa Cruz Mudstone Formation (Miocene-Pliocene age) in Santa Cruz, CA Fig. 10-60. Ancient deep ocean siliceous ooze deposits preserved as ribbon chert in the Franciscan Formation (Jurassic age), Santa Cruz Mountains, CA
1012

Sedimentary Processes and Sedimentary Structures

Unique characteristics of sedimentary deposits

Sediments preserve other characteristics that may tell information about the environment where they occur. Sediment particle shapes, degree of sorting, bedding characteristics are typically unique to different geologic settings.

Rounding of sediment grains

When particles are moved by running water they become rounded (roundness is illustrated in Figure 10-61). The corners hit first and are worn down. The sharp edges are also pounded. The particles may become round boulders or pebbles. Bits of sand move with them. As the water slows the largest particles drop out first, making deposits of round boulders and pebbles called conglomerate. The smaller particles are swept away downstream (unless they are trapped between or beneath the large particles).
Roundness of grains
Fig. 10-61. Roundness of sediment grains: The farther a particle is moved, the more rounded and spherical it should become. Angular particles tend to be deposited close to their source, they become more rounded the farther they travel downstream. Grains of beach sand are typically well rounded. Dune sand is typically even more rounded and better sorted (Image from Powers, 1959).
10.13

Sorting

This sediment moving process of running water sorts particles by size and to a lesser degree by shape. This is called sorting (illustrated in Figure 10-62). Sediments exposed to longer transport or exposure to currents and waves tend to be more sorted by shape and size.
Sorting
Fig. 10-62. Sorting of sedimentary particles.
The amount of sorting depending on the conditions and amount of time at which the stream currents or ocean waves works on the particles. Obviously particles of the same mineral that are more rounded and sorted have traveled further. The sediments sorting, roundness, and sphericity could act as a clue to following either modern or ancient alluvial rocks to their ultimate source. For example, very well sorted and rounded materials may suggest a source from an older sedimentary rock rather than from freshly exposed igneous rocks. Sand from rivers and stream are very different from sands associated with beach and sand-dune deposits (see Figures 9-63 to 9-67).
10.14

Examples of different kinds of sand under magnification

sand from an upland stream is rich in feldspars River sand rich in quartz fragments Beach sand rich in quartz grains Beach sand rich in microfossils Dune sand
Fig. 10-63. Sand from a mountain stream may be rich in poorly sorted and angular grains of feldspars, quartz, and other minerals. Fig. 10-64. With erosional transport over long distances, river sand will become enriched in quartz as feldspars decay. Fig. 10-65. Beach sand is enriched in well rounded and well sorted quartz grains. Fine materials are winnowed out. Fig. 10-66. Beach sand in many tropical settings may be enriched in shell material, including microfossils. Fig. 10-67. Wind-blown dune sand is typically very well sorted and very well rounded, polished to frosted grains of mostly quartz.

9. Lamination and bedding

Sediments are deposited in layers ranging from paper-thin sheets to massive beds tens to hundreds of feet thick! A laminae (or lamination) is a layer of sediment or sedimentary rock layer only a small fraction of an inch (less than a centimeter) in thickness (see Figure 10-68). Thin lamination is typically associated with fine-grained sediments deposited in quiet or slack-water environments, such as in a lake basin or offshore below the influence of waves and strong currents. Bedding is the smallest division of a sedimentary rock formation or stratigraphic rock series marked by well-defined divisional planes (bedding planes) separating it from layers above and below (see Figure 10-69). A sedimentary rock formation commonly is made up of a series of "beds" (strata) of similar origin.
lamination bedding, Bisti National Monument, NM
Fig. 10-68. Lamination in shale. Each laminae my be an annual cycle of deposition. Fig. 10-69. Bedding exposed in a canyon carved in flat-lying sedimentary rocks.
10.15

10. Sedimentary structures preserved in bedding

Sedimentary deposits (including sediments and sedimentary rocks) commonly preserve evidence of how they were deposited. Anyone who has been to the beach or a sand dune area have seen ripple marks created by the movement of sand under the influence of wind or water. Listed below are examples of sedimentary structures preserved in bedding of ancient sedimentary rocks. The processes that created them are the same that are occurring today.

Ripple marks
—a series of small ridges produced in sand by water currents or by wind (Figure 10-70).

Cross bedding
—inclined sedimentary structures in a horizontal unit of rock. These tilted structures are deposits from bedforms such as ripples and dunes, and they indicate that the depositional environment contained a flowing fluid (typically, water or wind) (Figure 10-71 and 9-72).

Desiccation cracks
—mudcracks; irregular fracture formed by shrinkage of clay, silt, or mud under the drying effects of atmospheric conditions at the surface (Figure 10-73).

Graded bedding
—bed is one characterized by a systematic change in grain or clast size from the base of the bed to the top. Large fragments tend to settle out fastest from a slowing turbulent flow.

Turbidity flows
—a turbid, dense current of sediments in suspension moving along downslope and along the bottom of a ocean or lake. As turbidity flows slow down, they drop their coarse sediment fractions first, and finer and finer sediments as the currents diminish, resulting in graded bedding.
bedding cross bedding Migration of ripples, dunes, and sandwaves dessication cracks
Fig. 10-70. Ripple marks on sand dune sand in water deposits form from current flow (air or water). Fig. 10-71. Cross bedding in ancient sand dune deposits
Zion National Park, Utah
Fig. 10-72. Formation of cross bedding caused by the migration of ripples or dunes Fig. 10-73. Desiccation mud cracks
Grand Canyon, Arizona
Examples of greaded bedding
Fig. 10-74. Appearance and example of graded bedding in sedimentary deposits. Graded beds will "fine-upward" as currents slow down. They may "coarsen upward" if the energy of the depositing flow (current) increases.
Turbidity currents and graded bedding
Fig. 10-75. Turbidity currents flow down slope under water under the influence of gravity. At peak flow, turbidity currents will scour the seabed, but as flow slows and stops, coarse sediments are deposited first, and finer material last.
Monerey Canyon
Fig. 10-76. Turbidity currents scour canyons in the deep offshore environment and deposit sediments in the deep ocean
graded bedding
Fig. 10-77. Ancient graded bedding deposits exposed at
Bean Hollow State Beach, California. Each layer of sandstone (light colored) with a shale bed (dark colored) is a deposit from a turbidity flow on a deep sea fan offshore.
10.16

11. What the significance of fossils in sedimentary rocks?

Paleontology is the study of "ancient dead things." Paleontologists study fossils and the sediments they were preserved in.

A fossil is a remnant or trace of an organism of a some earlier geologic age, such as a skeleton or leaf imprint, embedded and preserved in the earth's crust.

Fossilization
involves all the processes that turn plant or animal remains to stone.

Not all sedimentary rocks preserve fossils.
Most animal and plant remains are destroyed by a variety of processes before they can be preserved. However, some "non fossil" evidence is commonly preserved, called "trace fossils."

Trace fossils
are "fossil impressions" of footprints, trails, burrows, scratch marks, root traces, or other trace of animals or plants rather than of the organism itself. Trace fossils (also called itchnofossils) are sedimentary structures, like ripple marks and desiccation cracks described above. For example, throughput the western United States, there are many examples of dinosaur trackways. These trace fossils can often provide abundant information about how the ancient creatures interacted with the environment.

Bioturbation
is the stirring or mixing of sediment or soil by organisms, especially by burrowing, boring, crawling, feeding or other traces left by biological activity. Most sedimentary rocks preserve evidence of bioturbation. As animals dig through the sediments they are typically searching for food or seeking shelter from predators. Over time, burrowing organisms churn up sediments and consume most organic remains in sediment.

Paleoecology is the science of studying the relationship of organisms to the sedimentary depositional environments in which their remains have been preserved. Paleoecologists study modern climate settings to compare with the evidence of ancient environments preserved in sedimentary facies.
shell fossils petrified wood dinosaur tracks fossil burrows
Fig. 10-78. Shell fossils in mudstone. Every rock (and fossil) tells a story of its existence in the past. (Sample from Santa Cruz, California) Fig. 10-79. Fossil wood
Petrified Forest National Park, Arizona. Not only is the wood preserved, but a rich history of information is preserved in the sediments.
Fig. 10-80. Dinosaur tracks
near Tuba City, Arizona. Many sedimentary deposits preserve trackways that reveal information about ancient animal behavior.
Fig. 10-81. Crustacean (shrimp) burrows preserved in sandstone are examples of bioturbation. This example is from ancient coastal dune deposits (Tuba City, Arizona)
10.17

What is significant about a fossil?

The importance of a fossil is to reveal information about:
• What was the life habits of the organism—how it lived, what role it had within a community of other organisms, how and why it died.
• What was the geologic age and origin of the sedimentary deposits in which it was discovered—what type of environment did the organism the live in?
• How was the organism preserved, and why? How did it get to its location, and what happened to it along its journey there?
• What geologic events were occurring in the region at the time it was deposited , and what happened over time after the fossil formed.
• How does it compare to life habits of similar organisms that exist today? Is it an ancestral form of other known organism?

Paleontologists may spend their entire careers collecting and examining fossils in a area or region, comparing fossils in one area with others around the world. The largest employer of paleontologist is the petroleum industry where most work involved studying microfossils (fossil plankton and shell fragments) derived from well cores. Many work for universities and both government and non-governmental organizations studying climate history and climate change, or as curators in museums. There are hundreds of museum collections around the world, supporting the interests of the curious public and preserving precious collections for future generations to study.
10.18

How fossils form (or better, how do they survive destruction)?

1. survive being eaten (at least partially)
2. survive transport to a site of preservation
3. survive burial in sediments or volcanic materials
4. survive bioturbation (chewed up by invertebrates)
5. survive bacterial decay and dissolution
6. survive compaction
7. survive chemical changes associated with lithification
8. survive uplift, weathering and erosional exposure
9. discovered and identified (to have educational value)
10. studied, reported, and curated in a museum research collection.

Step 10 is extremely important! If you find a fossil you think is important... Tell someone who knows and cares about its possible significance. The best thing to do if it is rare or unusually well preserved is to donate it to a museum. If possible write an article in a published journal about it! Note that many of the most important fossils have been originally found by amateur fossil hunters and collectors. However, it is important to have knowledge about what you are looking for, and to share what you have found! That is the nature of science!
Chapter 10 Quiz Questions
http://gotbooks.miracosta.edu/earth_science/chapter10.html 7/20/2017