Basic Principles

 

Lecture : Basic Principles (Review)

 

I. Fundamentals-generally local relationships Geologic Time: Basics    
    A. First Principle-Principle of Superposition (Nicholas Steno)

    B. Second Principle-Principle of Original Horizontality

    C. Third Principle-Principle of original lateral continuity

    D. Intrusive relationships based on igneous rocks

    E. Crosscutting relationships-based on faults or unconformities

    F. Principle of components-included fragments (sed. or other) are older
               than the matrix in which they are included

    G. These principles were used to establish relative ages of most bodies
               of rocks that adjacent to one another at the earth's surface

II. Fossils-William Smith, British surveyor
 

                A. Used to correlate over greater distances

                B. Established fossil succession as a viable technique

        C. This was not based on evolution; just on vertical distribution of
            occurrences; established the idea of fossil ranges

III. Geologic Time-Time units evolved in Europe during the 19th century Geologic Time: Major Divisions of Geologic Time

    A. Relative Time Geologic Time: Relative Time Scale

        1. Eons-Archean, Proterozoic, and Phanerozoic

        2. Eras-Paleozoic, Mesozoic, and Cenozoic

        3. Periods-subdivisions of Eras

IV. Correlation and Dating of Rocks

    A. Stratigraphic sections-represent a local or regional vertical sequence

    B. Primarily based on fossil occurrences

    C. Time scale based on fossils is a relative time scale

    D. index or guide fossils

        1. easily distinguished

        2. occur over a large geographic area and can be used for
                    correlation

        3. occur in many kinds of sedimentary rocks

        4. are restricted to narrow stratigraphic intervals

V. Radioactivity and Absolute Ages   Radiometric dating

    A. Henri Bequerel, 1895, discovers radioactivity

    B. radiometric dating-based on the natural occurrence of six or so
               naturally radioactive elements; used to supplement and establish certain
               constraints on the Geologic Time Scale

VI. Fossils vs Radioactivity; the accuracy of correlation; radio dating errors may be greater than errors using index fossils

VII. Time-Parallel Surfaces in Rocks-hypothetical plastic sheet layer

    A. Key or marker bed
        1. volcanic ash

       2. glacial tills

       3. certain evaporites

    B. Shifting Depositional Boundaries

      1. facies-represent a local environment

      2. facies changes-represent lateral changes in depositional environments

      3. transgression vs regression-Vail (1977) and later Haq sea level curves .

VIII. Plate Tectonics-Unifying theory of geology; very important in the early
       history of Florida Understanding plate motions [This Dynamic Earth, USGS]  Plate Tectonics 

IX. Magnetic Stratigraphy-normal vs reversed intervals in the Cenozoic and
          Mesozoic; average about 500,000 years

X. The Units of Stratigraphy-
   "Stratigraphy is the triumph of terminology over common sense."(Paul Krynine);
   consists of lithostratigraphic, chronostratigraphic, and biostratigraphic units.

    A. Lithostratigraphic Units

        1. formations-mappable unit; 3 dimensional rock units; usually have
          geographically associated names.

            a. basic rock unit; generally tabular in 3-D

                (1) Common geologic features (continuity, contacts, key beds, etc.)

                (2) Mass properties (chemical and physical composition)

                (3) Contained peculiarities (fossils, pebbles, heavy minerals, trace
          elements, etc.)

          b. should be known from a Type section-a particular locality where the
          upper and lower boundaries are defined (Hawthorne, Ocala, etc.)
 

          c. similar, distinct lithologic composition (including alternating rock types)

          d. mappable unit

          e. thickness is not a factor, but too thick and too thin should be avoided

          f. similar in vertical relations to other formations

          g. Designation is without regard for time relationships; boundaries may or
          may not be time-parallel; often are transected obliquely.

           h.  Units may not be the same age in all geographic areas

    B. Chronostratigraphic Units (Time Stratigraphic Units)

        1. actual rock successions bounded by theoretically uniform time planes

        2. independent of lithology

        3. a division solely for the purpose of time (age) correlation

        4. places rocks in their proper context of earth history

        5. this theory often is modified in practice by stratigraphers

   C. Biostratigraphic Units

        1. the basic unit; nearly time parallel units; approximates a chronostratigraphic
          classification

         2. rock strata characterized by its content of fossils; certain fossil assemblage
          with no regard to lithology

            a. fossils are penecontemporaneous with deposition

            b. ranges of fossils may differ from boundaries of formations; may cross
          formation boundaries
 

       3. biostratigraphic zones are the principal basis for establishing worldwide   systems (Cambrian, Ordovician, etc.; each system is represented by a type section)

       4. Systems are further subdivided:

            a. Erathem

            b. Series

            c. Stages
X. Time Units (Geologic Time Scale)

XI. Glacial Stratigraphic Units

    A. based on stages

    B. Fla. nomenclature is out of date with recent ideas on multiple sea level stands
          and karstic unloading of Florida peninsula

     C. Fla. marine terraces

          1. Maximum of 7 recognized

          2. characterized by elevations

          3. related to glacial stages
               a. Aftonian
               b. Yarmouth
               c. Sangamon
               d. Wisconsin
                e. Recent