Geology 102 Sedimentary Rocks Lecture 3
Facies, Transgressions and Regressions, Walther's Law

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.

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.

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

Transgressions and Regressions

  • Transgression = sea level rise
  • Regression - sea level drop

    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 area would have been exposed and dry.

    2. Rate of sea floor spreading - during times of rapid sea floor 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 due to isostatic uplift by up to 300 meters.

    Walther's Law

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

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

    Transgressive sequence

    Deeper water facies overlie shallow water facies.
    A "deepening upward" sequence.

    Transgressive sequence

    Regressive sequence

    Shallow water facies overlie deeper water facies.
    A "shallowing upward" sequence.

    Regressive sequence

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    This page created by Pamela J. W. Gore
    DeKalb College, Clarkston, GA
    pgore@gpc.edu

    Created September 29, 1995
    Last modified October 15, 1996