Depositional EnvIronments And Facies As A Record Of Historical Geologic Conditions

The importance of the lateral variations in strata was not appreciated until 1838 when interfinger-

ing of two different lithologies with different fossil assemblages was described from a mountainside in Switzerland. The term sedimentary facies described the difference in lithology between the two interfin-gering rock units, and the two were interpreted as products of different environments of deposition, with the gradual change in the character of the rocks reflecting a gradual change in the aspects of the original environments, such as a transition from a sandy beach to a muddy offshore shelf. It is customary to name the facies of a rock unit by the dominant lithol-ogy of the unit, for instance, a mud-rich rock might be called a muddy facies.

The analysis of sedimentary facies is aimed primarily at interpreting the environment of deposition of the rocks, and therefore includes descriptions of both rock lithologies and the fossils contained in the rocks. An important aspect of the analysis and interpretation of sedimentary facies is using the principal of uniformitarianism, where features in old rocks are compared with modern environments to understand the origin of the old features.

Facies patterns are best understood by examining changes on regional scales, and can be very useful if not essential for reconstructing past environments. By examining many different facies patterns and determining the paleoenvironments, it is possible to understand the history of environments and life on Earth, one of the goals of historical geology.

Analysis of sedimentary facies shows that sea level is constantly moving up and down, and that the land surface is also moving up and down relative to sea level. Sinking of the land relative to sea level is called subsidence, whereas uplift refers to land rising relative to sea level. Some places in the world are currently subsiding, such as Venice, Italy; New Orleans, Louisiana; and the North Sea coast of Holland. The North Sea coast has been sinking for the past 10,000 years, since the last glacial advance, such that more and more ancient shorelines are located farther and farther offshore. Likewise, the coastal environment along the Mississippi Delta of southern Louisiana is rapidly subsiding, such that many square miles of land sink below sea level every year, and the shoreline is gradually moving inland toward the city of New Orleans. As this process of subsidence continues, the sedimentary environments of the shoreline, the shallow marine and deeper marine all move landward, with the deeper facies migrating on top of the shallow ones. This pattern is produced in what is known as a marine transgression, and can be caused by the land sinking relative to the sea, or by a global sea-level rise.

In contrast to a transgression, a regression of the sea occurs when sea level falls relative to the land, and the shoreline moves away from the present-

Sand and silt Shale Carbonate Sea facies facies facies level

Sand and silt Shale Carbonate Sea facies facies facies level

Facies Changes Geology

Cross section of beach and near-shore environment showing sedimentary facies change from subaerial dunes to beach to shallow marine to transitional to deep marine. Note how the rocks deposited in each facies interfinger with each other.

day coast. Like a transgression, a regression causes a continuous and gradual shift of the sedimentary environments, as well as the sedimentary and biologic products that form a wedge of material that overlies older deposits from the facies that previously existed in that location. In both transgressions and regressions the facies boundaries (and lithology changes in the rock record) form inclined surfaces, whereas time lines, corresponding to the seafloor surface at any time, may be subhorizontal surfaces, and cut across the lithologies as the paleoenvironments changed from beach to nearshore to offshore. Thus even though formations may be defined with attributes such as "near-shore sand" or "offshore mud," time lines cut across formation boundaries.

To interpret the history of an area one must often pick out characteristics of facies patterns to determine whether the sea level was rising in a transgression during deposition, or whether it was falling in a regression. Transgressive patterns are characterized by shrinking land areas, and typically transgressive rock packages are underlain by unconformity surfaces. They show a landward shift of facies through time, and become finer upward at any given location. In contrast regressive patterns that form from the uplift of the land or the fall of the sea show an enlargement of the land area, typically have an ero-sional unconformity at their top, show a seaward shift of facies with time, and are coarser upward at any given location.

Patterns of facies were studied extensively in the late 1800s by the German geologist Johannes Walther, who noted that facies tend to shift with time in the geological record, and that as the facies shift, adjacent environments succeed each other in the vertical sequence. This understanding led to the formation of "Walther's law," which states that the vertical progression of facies is the same as the corresponding lateral facies changes.

Understanding the local patterns of marine transgressions and regressions was a major accomplishment for geologists, but the next step, correlating the patterns from one shoreline or continent to establish a global pattern, was more difficult. In some cases it seemed as if one continent was rising or falling while others were not, and in other cases the geologic evidence suggested that sea levels were rising or falling at the same time in most locations across the world. These global sea-level rise or fall events are referred to as eustatic changes and may be caused by changes in the amount of water in the oceans from melting or freezing glaciers, or changes in the volume of the deep ocean basins by changing the volume of the midocean ridges through increased or decreased seafloor spreading. Continental collisions can also uplift large portions of the continents, effectively decreasing the amount of continental material in the oceans, expanding the volume of the ocean basins,

Transgression Regression

Sea level at time B Former sea level

Time-rock unit B Time-rock unit A

Sea level at time C Former sea levels

Sea level at time B Former sea level

Time-rock unit B Time-rock unit A

Sea level at time C Former sea levels

Time-rock unit C Time-rock unit B Time-rock unit A

Rising sea level causes the sedimentary facies to migrate shoreward and be deposited one on top of the other and move progressively shoreward. A regression shows the opposite direction of migration of facies. Note how a vertical profile through these sections would yield a sequence of facies that is the same as the horizontal sequence of facies on the surface and that the order of succession can be used to tell the difference between a regressive and a transgressive sequence.

A Disconformity

A Disconformity

Time Transgressive Angular Unconformity

1. Beds 1, 2 deposited

2. Erosion

3. Beds 5-7 deposited

Angular unconformity

Angular unconformity

Time Transgressive Angular Unconformity
IjP.

1. Beds 1-6 deposited

2. Beds 1-6 tilled

3. Erosion

4. Beds 9,10 deposited

C Angular nonconformity

C Angular nonconformity

1. Granite formed

2. Granite exposed by erosion

3. Beds 1-3 deposited

6 Infobase Publishing and causing global sea levels to fall. To determine whether sea-level changes are local or global, a well-correlated geological timescale is needed for each

Three types of unconformities, including disconformity, angular unconformity, and nonconformity continent, which, over time, has led to the establishment of global eustatic sea-level curves showing the heights of sea level with time.

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Responses

  • ute
    Why do facies interfinger laterally?
    3 years ago
  • Leyton
    How depositional environment related with facies analysis?
    10 months ago

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