Eighth Edition
by Harold L. Levin

Chapter 5 - page 11

The Sedimentary Archives

Rock Units

A lithostratigraphic unit is defined as a body of sedimentary, extrusive igneous, metasedimentary, or metavolcanic strata which is distinguished on the basis of lithologic characteristics and stratigraphic position (position in the rock sequence).

The smallest lithostratigraphic rock unit is the bed.
A formation is a set of similar beds. Formations are the fundamental units of stratigraphy.

A formation is: Lithologically homogeneous (all beds are the same rock type or a distinctive set of interbedded rock types). Distinct and different from rock units above and below. Traceable from exposure to exposure, and of sufficient thickness to be mappable (formations are commonly hundreds of feet thick, but may be thinner or thicker). Named. Formations are usually named for some geographic locality where they are particularly well exposed. (This locality is referred to as the type section.) If the beds are dominated by a single rock type, this may appear in the name. (Also, to be valid, the name of a formation must be published in the geological literature.)

Examples of formation names:

Chattanooga Shale	Rome Formation
Shady Dolomite	Tapeats Sandstone
Fort Payne Chert	Green River Formation
Dakota Sandstone	Red Mountain Formation

A set of similar or related formations is called a group.
Groups also have names.

Subdivisions within formations are called members. Members also have names. A formation, however, does not have to contain members. Members may be designated to single out units of special interest or economic value, such as coal beds or volcanic ash layers.

Lithostratigraphic units in the Grand Canyon National Park.
For scale, the Kaibab Formation, at the rim of the canyon, is about 320 ft thick.
The Paleozoic sedimentary rock sequence in the canyon (shown here) ranges from 2,400 to 5,000 feet thick.

Boundaries of lithostratigraphic units are placed where the lithology (or rock type) changes. They may be placed at a distinct contact, or may be set arbitrarily within a zone of gradation.

Virtually all lithostratigraphic units are "time transgressive" or diachronous (meaning that they, or their contacts, cut across time lines). For example, a particular unit of early Cambrian sandstone in southern California and Nevada may be traced continuously to the northeast, but in Colorado, it is late Cambrian, or roughly 35 million years younger.

The lithostratigraphic terms (bed, member, formation, and group) refer to sedimentary, volcanic, metasedimentary, and metavolcanic rocks only. Intrusive and highly deformed and metamorphosed rocks are called lithodemic units. The fundamental lithodemic unit is the lithodeme (roughly equivalent to formation). The term "formation" should not be used for intrusive and metamorphic rock (according to the North American Stratigraphic Code of 1982).

Facies

The term facies refers to all of the characteristics of a particular rock unit. For example, you might refer to a "tan, cross-bedded oolitic limestone facies." The characteristics of the rock unit come from the depositional environment. Because these distinguishing characteristics are lithologic, it can further be designated as a lithofacies.

Every depositional environment puts its own distinctive imprint on the sediment, making a particular facies. Thus, a facies is a distinct kind of rock for that area or environment.

Facies (or lithofacies) developed in the marine environment, going progressively offshore include the sand and silt facies of the beach and nearshore environment, the shale facies in deeper, quieter water, and the carbonate facies, far from shore in warm shallow seas where there is little or no clastic, terrigenous input. In a swamp area on a delta, the shale and coal facies develops.

Each depositional environment grades laterally into other environments. We call this facies change when dealing with the rock record.

Different species of organisms may inhabit different areas in an environment. The different fossil assemblages in a uniform rock unit may be referred to as biofacies.

Facies and sea level changes

A sea level rise is called a transgression.

A transgression or sea level rise will produce a vertical sequence of facies representing progressively deeper water environments (a deepening-upward sequence). As a result, a transgressive sequence will have finer-grained facies overlying coarser-grained facies (fining-upward from sand at the bottom, and then to silt, and then to shale). This is sometimes referred to as an onlap sequence.

Transgressions can be caused by melting of polar ice caps, displacement of ocean water by undersea volcanism, or by localized sinking or subsidence of the land in coastal areas.

Sediment deposition during a transgression.

A sea level drop is called a regression.

A regression will produce a sequence of facies representing progressively shallower water environments (shallowing-upward sequence). As a result, a regressive sequence will have coarser-grained facies overlying finer-grained facies (coarsening-upward). This is sometimes called an offlap sequence.

Regressive or offlap sequence. Shallower water facies come to overlie deeper water facies.

Regression can be caused by a buildup of ice in the polar ice caps, or localized uplift of the land in coastal areas.

Transgressive sequence Deepening upward. Fining upward.	Regressive sequence Shallowing upward. Coarsening upward.

Walther's Law (or Principle)

Sedimentary environments that started out side-by-side will end up overlapping one another over time due to sea level change (transgressions and regressions).

The result is a vertical sequence of beds. The vertical sequence of facies mirrors the original lateral distribution of sedimentary environments.

Illustration of Walther's Law or Principle, which states that vertical facies changes correspond to lateral facies changes. A transgressive sequence is shown.

Sea Level Changes

Worldwide sea level change is known as eustatic sea level change.

Fluctuations in sea level are caused by things such as:

1. Changes in the size of the polar ice caps, due to climatic changes

1. Melting of ice caps leads to sea level rise (transgression). It has been calculated that complete melting of the Antarctic Ice Sheet would cause a sea level rise of 60 - 70 meters (200 feet).
2. Growth of ice caps leads to drop in sea level (regression). Calculations show that sea level was as much as 100 meters (300 feet) lower than at present at the height of the last Ice Age glaciation. Much of the continental shelf would have been exposed and dry.

2. Rate of seafloor spreading - During times of rapid seafloor spreading and submarine volcanism, the ocean ridge system is enlarged by the addition of lava, displacing water onto the edges of the continents (transgression).
3. Localized subsidence or uplift of the land - In the 8000 - 10,000 years since the melting of the last glacial ice sheet over North America, parts of Canada have risen by up to 300 meters due to isostatic uplift associated with removing the weight of the glacial ice sheet.
   Other areas are subsiding (or sinking), such as the Mississippi delta region.

Cyclic rises and drops of sea level related to events along the mid-ocean ridges are known as Vail Cycles (named for Peter R. Vail who demonstrated their use in lithostratigraphic studies).

In the past, eustatic sea level rise caused the flooding of vast areas of North America. These high sea level stands produced shallow inland seas that are referred to as epeiric or epicontinental seas.