Introduction to Geology

Chapter 7 - Sedimentary Rocks & Processes

Sedimentary rocks are exposed throughout the world, covering about half of the exposed land on the earth surface. This "sedimentary cover" was deposited mostly in shallow seas or coastal environments in the last several hundred million years. They 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 stories of ancient environments and stories of changing landscapes through time.

1. List and describe types of sedimentary rocks
2. Explain how a sediment becomes a sedimentary rock
3. Describe the types of environments where sedimentary rocks are formed.
4. Describe sedimentary structures
5. Explain the significance of fossils in sedimentary rocks.

Keywords and Essential Concepts

1. List and describe types of sedimentary rocks

Sedimentary rocks are classified into two groups:

clastic—sediments or sedimentary rocks composed of fragments (detritus) derived from older rocks; examples: mud (mudstone), sand (sandstone), gravel (conglomerate).

non-clastic—sediments or sedimentary rocks composed of materials precipitated from water or produced by biological activity (skeletal material, respiration, and excretion); also called "chemical sedimentary rocks."

Rock Cycle Illustrated

1. Clastic sediments and sedimentary rocks

clastic grain size

Sediment	Rock	Occurrence
gravel	conglomerate	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.
			gravel Coyote Creek Morgan Hill, California	conglomerate Point Reyes Formation Point Reyes, California
sand	sandstone	Sand goes through degrees of refinement at it moves away from source areas. Sand deposits near mountain ranges will 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.
			sand Ano Nuevo State Beach San Mateo County, CA	sandstone from Santa Cruz Mountains
mud	mudstone and shale	Mud is a general term lumping together sediments consisting of a mix of clay, silt, and sand. Mud may be an unsorted mix of fine grain materials.
			Mud Tidal mudflats in Australia	Mudstone from Santa Cruz coast
clay	shale	clay minerals—any of various hydrated aluminum silicates that have a fine crystalline structure and are components of clay (sediment).
			Clay is used to make ceramic pottery.	sandstone and shale Loma Prieta Peak Santa Clara County, CA

Clastic Sedimentary Rocks: Rocks composed of grains of mineral and rock fragments derived from erosion of other rocks.

mudstone—a fine-grained sedimentary rock formed from the compaction and cementation (litification) of muddy sediments.

shale—a soft, finely stratified sedimentary rock that formed from consolidated mud or clay and can be split easily into fragile plates, such as along bedding plains.

sandstone—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.

arkose—a coarse-grained sandstone that is at least 25 percent feldspar. Arkose is associated with ancient alluvial sediments derived from granitic mountain ranges.

graywacke—a sedimentary rock consisting of a mix of angular fragments of quartz, feldspar, and mafic minerals set in a muddy base; a "dirty sandstone or mudstone, common in active continental margin regions such as the Coast Ranges of California.

conglomerate—a coarse-grained sedimentary rock composed 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.

breccia—a rock consisting of angular rock fragments cemented together. 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.


conglomerate	sedimentary and volcanic breccia	arkose and graywacke	river sand (angular, poor sorting)


river sand (angular, poor sorting)	beach sand (rounded, well sorted)	beach sand with microfossils	dune sand (frosted, rounded, well sorted)


classification of mudrocks	shale and mudstone	marine shale (Cretaceous Mancos Shale, Capital Reef National Park , Utah)	terrestrial shale (Triassic Chinle Formation, Petrified Forest National Park, Arizona)

2. 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 from precipitation directly from water or formed by accumulation of plant material and the skeletal remains of plankton, shells, bone, or other biological materials.

Non-clastic sediments and sedimentary rocks

bioaccumulation—the buildup of organic remains, such as coral reefs, peat, algae, plankton, and shell or bone beds.

planktonic ooze—slimy mud on the bottom of an ocean or lakebed formed from the accumulation of skeletal and organic remains of microscopic organisms.

lime mud—sediment composed of calcium carbonate (CaCO₃) derived from the skeletal remains of shelled organisms, coral, and calcareous algae; the sediment that with compaction and cementation (lithification) become limestone.

peat—an accumulation of partially decayed vegetation matter that has a brown, soil-like character typical of boggy, acid ground or swampy setting.

Non-clastic Sedimentary Rocks: rocks composed primarily of sediment precipitated from water or through organic processes (life and death of organisms, particularly planktonic skelletal material, shells, bone materials, and other organic remains.

limestone—a sedimentary rock consisting predominantly of calcium carbonate (CaCO₃) derived from the skeletal remains of marine marine microorganisms, shells and coral. Limestone is commonly used in the manufacture of lime for cement and used as building stone.

travertine—white or light-colored calcareous rock (CaCO₃) deposited from mineral springs; or a common name for freshwater limestone deposits. Cavern speleothems are typically consist of travertine.

tufa—calcareous and siliceous rock deposits of springs, lakes, or ground water.

coquina—a soft type of limestone composed almost entirely of compacted and cemented shell fragments, commonly found on upper beach ares in warm, humid climates.

chert—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.

flint—a hard (typically gray or brown) rock consisting of nearly pure silica (chert), occurring chiefly as nodules in chalk or limestone.

rock salt—a rock dominantly composed of sodium chloride (NaCl - the mineral halite). 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.

dolostone—A rock composed mostly of the mineral dolomite, typically a white, light gray or pink with a sugary crystalline texture. The mineral dolomite, CaMg(CO₃)₂, often is observed replacing calcite in limestone, particularly in ancient carbonate rocks.


Planktonic ooze forms chalk (calcareous) and chert (siliceous)	Reef erosion and carbonate accumulation forms limestone	Depositional environments in South Florida	Ooids and oolitic limestone


travertine and tufa	swamp deposits and peat	sabkha environment	salt pan, Death Valley

2. Explain how a sediment becomes a sedimentary rock

Sediments can become "lithified" into sedimentary rock once they've been deposited in a stable setting where burial, compaction, and cementation can take place. The processes, collectively called 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.

deposition—the process of sediments settling and accumulating from a moving fluid (wind, water, or ice).

diagenesis—the physical and chemical changes occurring during the conversion of sediment to sedimentary rock.

compaction—the process of gravitation consolidation of sediments, decreasing porosity and increasing hardness.

cementation—processes that harden sediments through the precipitation of minerals in pore spaces between grains of rock and mineral fragments, binding them together.

3. Describe the types of environments where sedimentary rocks are formed.

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.

sedimentary depositional environment—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.

sedimentary facies—the character of a rock expressed by its formation, composition, and fossil content (examples: beach facies, fluvial facies, lacustrine facies).


sedimentary environments	topography and bathymetry of the Monterey Bay region


clastic depositional environments in humid regions	depositional environments in arid regions	marine and coastal 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.

"high energy" depositional environments (coarse-grain sediments dominate)	"low energy" depositional environments (fine grained sediments dominate)
stream and river channels beaches offshore bars (above wave base) dunes submarine channels and canyons	river flood plains swamps lakes lagoons marshes below wave base offshore

High-energy sedimentary environments

beach and barrier island	dunes	coral reefs	river channels
Low-energy sedimentary environments

lake (lacustrine)	freshwater swamp	tidewater marsh	river floodplains

Sedimentary rock formations representing ancient sedimentary environments


Navajo Sandstone Jurassic ancient dune deposits Navajo Reservation, AZ	Chinle Shale Triassic ancient floodplain deposits Navajo Reservation, AZ	Capitan Limestone Permian ancient reef deposits Guadalupe National Park, Texas	Fountain Formation Pennsylvanian ancient alluvial fan deposits Garden of the Gods Colorado Springs, Colorado

Mesaverde Formation Cretaceous ancient swamp and river channel deposits Mesa Verde National Park, Utah	Chadron Formation Eocene ancient lake (lacustrine) deposits Bryce Canyon National Park, Utah	Santa Cruz Mudstone Miocene-Pliocene ancient continental shelf deposits Ano Nuevo State Park, CA	ribbon chert Franciscan Formation Jurassic deep ocean basin ooze deposit Marin Headlands, California

4. Describe sedimentary processes and sedimentary structures

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.
sorting—process by which sedimentary particles of similar size, shape, or density are selected and separated from associated but dissimilar particles by the agent of transportation (water and wind). roundness—the degree of abrasion of sedimentary particles; expressed as the radius of the average radius of curvature of the edges or corners to the radius of curvature of the maximum inscribed sphere.

laminae (lamination)—a layer of sediment or sedimentary rock only a small fraction of an inch (less than a centimeter) in thickness. 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—the smallest division of a geologic formation or stratigraphic rock series marked by well-defined divisional planes (bedding planes) separating it from layers above and below.


roundness of particles	sorting of particles	lamination	bedding

ripple marks—a series of small ridges produced in sand by water currents or by wind.

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).

desiccation cracks—mudcracks; irregular fracture formed by shrinkage of clay, silt, or mud under the drying effects of atmospheric conditions at the surface.

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 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.


ripple marks on sand dunes and in water deposits form from current flow (air or water)	cross bedding in ancient sand dune deposits Zion National Park, Utah	Formation of cross bedding caused by the migration of ripples or dunes	desiccation mud cracks Grand Canyon, Arizona


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.
	Turbidity currents scour canyons in the deep offshore environment and deposit sediments in the deep ocean	Appearance and example of graded bedding in sedimentary deposits	ancient graded bedding deposits exposed at Bean Hollow State Beach, California

5. Explain the significance of fossils in sedimentary rocks.

fossil—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—the processes that turn plant or animal remains to stone.

trace fossil—a fossil impression of a footprint, trail, burrow, or other trace of an animal rather than of the animal itself.

bioturbation—the stirring or mixing of sediment or soil by organisms, especially by burrowing, boring, crawling, feeding or other traces left by biological activity.


shell fossils in mudstone Santa Cruz, California	fossil wood Petrified Forest National Park, Arizona	dinosaur tracks Tuba City, Arizona	arthropod burrows are an example of bioturbation Tuba City, Arizona

How fossils form (or better, 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
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.

Selected resource:

Paleontology Portal (Berkeley Museum of Paleontology)

Quiz Questions

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3/15/13