BSCI392
9-5-07
Sedimentology - reconstructing the the environmental context

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What is a rock? A purely descriptive definition is that a rock is - A naturally occurring aggregate of minerals and other solid material. - Usually, there are several minerals in the aggregate, though some rocks may have only one. The other materials may include natural glasses, organic material, or fossils.

Geologists usually think of rocks in a second important way, however. Please memorize this and recite it like a mantra:

* A rock is a record of the environment in which it formed. *

The three major rock types:

The Rock Cycle: Consider the three basic rock types and how they form:

The material that makes up any rock might have a complex history.

Geologists describe this range of possible histories as the Rock Cycle. As the schematic shows, it actually encompasses many possible cycles.

Additional information

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: The chemical and mechanical breakdown of preexisting rocks.

  • 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. For larger fragments, it's just a matter of being dumped.

  • Diagenesis: Physical and chemical changes that result in the lithification of the sediment into a solid rock, and maybe other subsequent changes.

    Each of these processes leave their signature on the resulting rock, with the result that we can learn a great deal about:

    • The source rock from which sediments were weathered

    • How far and by what means they were transported

    • In what environment they were deposited

    • The physical changes that have occurred in the depositional environment since deposition.

    • And of course, what the fossils that they contain say about the history of life and the nature of the ancient environment in which the sediments were deposited.

    Before we can do this, we have to establish a vocabulary that we can use to talk about sedimentary rocks.

  • 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 a number of criteria that we needn't go into. You should be aware of thebasic breakdown of clastic sedimentary rocks based on clast size:

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

    Depositional Environments

    Don't forget that sedimentary rocks preserve both the fossil record and a record of the environment in which the fossil was deposited. The physical characteristics and geographical location determines the type of sediment that will normally be deposited. In broad strokes, geologists classify depositional environments as:

    The Geologic Time Scale

    We haven't always understood the significance of sedimentary rocks. Interestingly, fossils played a big part in the discovery of the Earth's age and how it could be inferred from the rock record. Indeed, the science of Biology didn't take shape until the discovery of geologic time (deep time).