Sedimentary Rocks: Typically boring and ugly compared to igneous or metamorphic, but they have great utility because they speak to us about the history of the Earth's surface, where life lives. Sedimentary rock - rock composed of the transported remains of pre-existing rocks, i.e. sediment.

Sediment: material derived from the weathering of preexisting rock.

General life history of sediment: In order to make a sedimentary rock, four things need to happen:

  • Weathering: we discussed previously

  • Transport: running water, gravity, ice, wind. Material may be transported as fragments or in solution.

  • Deposition: The process of depositing the sediment. For materials in solution, this involves precipitation.

  • Cementation: The binding of grains through the precipitation of mineral cements (or, sometimes, through chemical reactions between the grains, themselves, that cause them to bind).
    Each of these processes leave their signature on the resulting rock, with the result that we can learn a great deal about: Before we can do this, we have to establish a vocabulary that we can use to talk about sedimentary rocks. This is the major thrust of this lecture.

    Classification: Sediment may be particles such as gravel or sand, the remains of plants and animals, or chemicals in solution. This encompasses a great variety of rock types. We try to make this variety manageable by classifying sedimentary rocks into the following types:

    Of course, within each type, grains and other materials can be described further according to: Clastic or detrital sediments: - made up of broken fragments of material. Sometimes you hear the term siliciclastic in connection with clastic rocks because their most common components, quartz & feldspar are silica (SiO2) containing minerals. There are three distinct issues that we need to be aware of in describing clastic rocks:

    Clast size:

    When a claystone fractures along numerous thin laminae, we call it a shale. Note that mudrocks tend to be much more vulnerable to weathering than sandstone.
    Rounding:

    As a poorly rounded clast is transported, its jagged edges get worn down, yielding a spheroidal well rounded fragment. The manner and amount of rounding tells us about how transport occurred and how long it lasted. There is one rock name to remember in connection with this:
    Remember volcanic breccias? A clastic rock made of pebble sized clasts or larger that are angular and poorly rounded is also called a breccia.

    Sorting:

    In a deposit of sediment, there is always a range of clast size. If that range is narrow, we say the deposit is well sorted. If it is wide, we call it poorly sorted. Sorting is very revealing about methods of transport. For instance, wind blown sediments are always very well sorted. Sediments transported by glaciers are very poorly transported.

    Thus, a full description of a sedimentary rock might be something like, "A well sorted, well rounded, quartz sandstone."

    Diagenesis: The chemical alteration of sedimentary rock after its deposition. Let's flesh out this concept, now that we have a basic sedimentological vocabulary:

    Chemical sedimentary rocks: Also called evaporites These precipitates may have the texture of a crystalline crust or may form around small particles that have a clastic texture, i.e. they resemble clastic rocks.

    Crystalline chemical sedimentary rocks:

    "Pseudoclastic" chemical sedimentary rocks:
    Biochemical sedimentary rocks (AKA biogenic) - clasts are remains of organisms.


    One last headache - concretions: Quite often, because of local variations in groundwater chemistry, a small region (usually pea to watermelon sized) will precipitate a harder or less soluble cement than the surrounding area during cementation. These regions for nodules called concretions. Quite often, these remain intact after the surrounding rock has weathered away. It's not uncommon for naive people to mistake these for exotic items like fossils or meteorites, or alien artifacts.


    Key concepts and vocabulary:

  • Clast size categories:
  • Clastic rock clast-size names
  • Diagenesis
  • Common cements
  • Chemical sedimentary rocks
  • Common mineral precipitates
  • Chemical sedimentary rock types
  • Pseudoclastic rocks
  • Biochemical sedimentary rocks
  • Common minerals
  • Chert
  • Carbonized organismal remains
  • Concretions