Sedimentary Rocks

© Pamela J. W. Gore, 1995-2004
Georgia Perimeter College

Tectonic Setting

Principle tectonic elements of a continent:

Geosynclines

Theory promoted by James Hall (1859) and D. Dana (1873) resulting from their work on the Appalachians. Sediments thicken at continental margins (to the east in the Appalachians).


Origin of Sedimentary Rocks

Depositional basin - a place where sediments accumulate

Sedimentary rocks may be:

The goal in Historical Geology is to try to interpret the conditions under which the sediments accumulate.

Source area - the land which weathers and erodes to form terrigenous sediments (extrabasinal).

Terrigenous sedimentary rocks are a product of:


Texture as an indicator of energy levels

Waves, currents, moving water = high energy

Quiet water = low energy

How do you determine energy levels?

  1. Grain size
    1. coarse sediments indicate high energy (sandstone, conglomerate)
    2. fine sediments indicate low energy (shale, micrite)
  2. Grain shape
    1. Roundness
      1. Roundness is the absence of sharp corners
      2. Roundness is due to abrasion in streams or waves
      3. Roundness takes both energy and time
      4. Large grains round faster than small grains - in a conglomerate, the larger grains are rounded, but the smaller grains may be angular

    2. Sphericity
      1. Sphericity is equal dimensions

    3. Chart of roundness vs. sphericity - examples of high and low roundness and high and low sphericity
  3. Textural maturity
    1. Three steps
      1. Winnowing or washing out of fines
      2. Sorting of grain sizes
        1. Good sorting implies consistent energy (washing)
        2. Poor sorting implies inconsistent energy (dumping)

      3. Rounding

    2. Muddy sediment is immature
    3. Poorly sorted sediment with no mud is submature
    4. Well sorted sediment with no mud is mature
    5. Well sorted and rounded sediment with no mud is supermature

  4. Textural inversions
    1. An unusual association of textures, such as rounded sand (implies high energy) in clay (implies low energy). Cannot be fit into the textural maturity classification above.

Sandstone composition

Sandstones are classified on the basis of the composition of their grains.

Three components are considered:

  1. Quartz grains
  2. Feldspar grains
  3. Rock fragment grains

Three major types of sandstone:

  1. Quartz sandstone (also called quartz arenite)
  2. Arkose
  3. Litharenite or lithic sandstone (commonly but imprecisely called graywacke)

Composition is an indicator of time in the basin of deposition (among other things).

  1. Mature
    1. Dominated by quartz (very stable sedimentary mineral)
  2. Not mature
    1. Not dominated by quartz
    2. Contains unstable mineral and rock fragments
Remember Bowen's Reaction Series and the Goldich Stability Series

Arkose

Each type of sandstone implies something about depositional history:

  1. Quartz sandstone implies a long time in the depositional basin.
  2. Arkose implies a short time in the depositional basin (because feldspar typically weathers quickly to clay). Also implies rapid erosion, arid climate, tectonic activity, steep slopes.
  3. Litharenite implies rapid erosion, temperate or arid (not humid) climate


SANDSTONE INTERPRETATION GUIDE

Sandstone textures and compositions may be used to interpret many things about the history of the sand, including source area lithology, paleoclimate, tectonic activity, processes acting in the depositional basin, and time duration in the basin. Remember that the source area is the land which is weathering and eroding to supply terrigenous debris to the depositional basin.

Source area lithology

Composition gives the key information (minerals or rock fragments present). Remember that quartz sandstone or quartz arenite is dominated by quartz grains; arkose is dominated by feldspar grains (usually potassium feldspar); and graywacke is dominated by rock fragment grains.

Paleoclimate

Paleoclimate refers to the climate which existed in the source area. We are particularly concerned with weathering rates here. Remember that in humid climates, feldspar weathers to clay by hydrolysis. Other minerals also weather to clay (with associated iron oxides), such as olivine, pyroxene, and amphibole.

Also remember the difference between weathering (BREAKDOWN of rock by hydrolysis, dissolution, oxidation, exfoliation, frost wedging, or freeze thaw), and erosion (TRANSPORTATION of particles).

Tectonic activity in the source area

We are basically classifying tectonic activity as "active" or "passive". For a good model, consider the west coast of the US as tectonically active - steep slopes, mountains close to the sea, lots of earthquakes, tectonic uplift, and volcanic activity. On the other hand, consider the east coast of the US as tectonically passive - broad, flat coastal plain, few or no earthquakes, no uplift, and no volcanic activity.

Tectonic activity also influences sorting, time duration in the depositional environment (and to some extent, compositional maturity). High tectonic activity might produce rapid dumping of sediments into the basin with little or no time for sorting. Low tectonic activity means little uplift, low erosion rates, and therefore little sediment supplied to the basin; what sediment that is there is likely to wash around for a long time and become well sorted and rounded, and grains other than quartz are likely to be destroyed (by abrasion or chemical weathering).

Processes acting in the depositional basin

This question refers to energy levels ("high" vs. "low") and consistency of energy. Texture gives the key information.

Grain size:

Sorting:

Time duration in the depositional environment

Both mineralogy and texture can be used to determine time in the depositinal environment.

A compositionally mature sediment (abundant quartz grains) suggests a long time in the depositional environment. Quartz is more resistant to abrasion than feldspar or rock fragments.

A compositionally immature sediment (abundant feldspar or rock fragment grains) suggests a short time in the depositional environment.

Textural maturity is also useful in interpreting time in the depositional environment. Immature or submature sediments probably spent only a short time in the basin before burial. Mature or supermature sediments were probably rolling around in the basin for a long time before burial. Roundness is a good clue to a long time in the depositional environment. Rounding of grains takes a long time; it is more likely in a tectonically passive situation. Desert sands are often well rounded because of the "sandblasting" process of wind transport. Hence, in an arid desert, it is possible to get a well-rounded (supermature) arkose.


Examples

For each of the following, interpret:

  1. Source area lithology (rock type from which it was derived)
  2. Paleoclimate (humid? arid?)
  3. Tectonic activity (high or low tectonic activity)
  4. Energy levels (high or low, consistent or inconsistent energy levels)
  5. Time (long or short time in depositional basin)
Interpret:

  1. Quartz sandstone, well sorted, well rounded
  2. Arkose, poorly sorted, poorly rounded
  3. Others


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© 1995, Pamela J.W. Gore
Georgia Perimeter College

Created September 24, 1995
Document last modified October 9, 1996
Typo corrected October 1, 1997
Modified February 3, 1999
Modified December 12, 2003

Links updated August 15, 2009