4/12/05. Lecture 10.
TIME - CONTINUED

INTRODUCTION

For most sedimentary rocks, there is no material that is datable (contains suitable parent-daughter elements). Sedimentary rocks, therefore, are commonly dated by 'bracketing' them between rocks whose absolute age has been determined.

BRACKETING

Consider the geologic cross section to the right. The numbered layers are sedimentary rocks; A and B are igneous intrusives; C is a lava flow with its 'feeder pipe'; U-U' is an unconformity (buried erosion surface).

The relative ages of events are as follows:

  1. deposition of layers 7 through 15+

  2. tilting of layers 7 through 15+

  3. intrusion of A

  4. erosion down to U-U'

  5. deposition of 83 through 88

  • intrusion of B some time after deposition of lower 85 and before extrusion of C
  1. extrusion of C

  2. deposition of 89-91

The sequence of events described above is illustrated schematically in the diagram to the right.

Note that two alternate sequences of events (histories) are shown.

  • In one history (1, 2, 3, 4, 5-a,
    6-a, 7, 8) , intrusion of B takes place after about three layers of sediment have been deposited.
  • In the second history (1, 2, 3, 4, 5-b,
    6-b, 7, 8), intrusion of B takes place after six layers of sediment have been deposited.
From the information available for the situation illustrated, there is no way of knowing which history is correct. In fact, an infinite number of alternate histories are possible, in which the number of layers deposited before B intrudes varies anywhere from 3 to 6. That is, the number of layers (or parts of layers) already deposited when B was intruded was at least about three, but additional material up to a total of six layers may have been present.

To get absolute ages for the sedimentary layers, the absolute ages of igneous rocks A, B, and C are determined by radiometric dating:
  • A = 100 million years (my)
  • b = 70 mya
  • C = 30 mya
The ages of the igneous rocks place limits on the ages of the sedimentary layers. That is, the ages of the sedimentary rocks are 'bracketed' between the ages of the igneous rocks:
  • layers 7 through 15 are older than A, thus older than 100 my.
  • layers 83 through lower 85 are younger than A but older than B, thus they are younger than 100 my but older than 70 my.
  • layers upper 85 through 88 are younger than A but older than C, thus they are younger than 100 my but older than 30 my.
  • layers 89 through 91 are younger than C, thus younger than 30 my.

As the ages of sedimentary layers in contact with dated igneous rocks are 'bracketed', their ages may be assigned to sedimentary layers with which they have been correlated by means of fossils or other methods.

C. OTHER ABSOLUTE AGE DATING TECHNIQUES

The United States Geological Survey provides simple descriptions of other techniques for determining absolute age.

THE SCIENCE-CREATIONIST CONTROVERSY

On the web, you will find a raging debate between geochronologists (geologists who determine absolute ages) and creationists and supporters of 'intelligent design' who deny the validity of absolute age dating techniques. For a discussion of the debate, click here.

THE GEOLOGIC TIME SCALE

Working over about the last two centuries, using the Laws of Superposition, Original Horizontality, Cross-Cutting Relationships, and Biotal Sucession, geologists have worked at determining the relative ages of sedimentary rocks. Long before the absolute ages of the rocks were known, it became convenient to create names for the times the rocks represented. So, in a manner similar to which the year is broken up into months, months into weeks, weeks into days, days into hours, etc., geologic time was divided into eons (longest), eras, periods, epochs, and ages (shortest) to create a Geologic Time Scale.

However, unlike months, weeks, days, etc., geologic time units of a given type (periods, for example), were not conceived as being of the same length. Rather, the divisions for the most part were made on what seemed to be natural breaks in the record, particularly the presence or absence of preserved life (fossils), or significant changes in preserved life assemblages.

As radiometric dating techniques were developed, absolute ages were assigned to the divisions. These absolute ages are in a continual process of refinement as uncertainties are increasingly narrowed down.

To see a recent version of the Geologic Time Scale, click here. Note that the image of the time scale can be enlarged by placing the cursor just to the right of the lower right corner of the diagram and clicking on the 'enlargment button' that will appear.

Another version of the time scale is shown on p. 36 of your text book.

To follow the arguments concerning what happened to the dinosaurs, there are certain parts of the Geologic Time Scale with which you need to be familiar. To the right is a 'skeletal' time scale. The terms and numbers you need to know are indicated in colors.

  • Precambrian - the rocks show little or
    no evidence of life
  • Paleozoic - 'ancient' life
  • Mesozoic - 'middle' life;
    the 'age of dinosaurs'
  • Cenozoic - 'modern' life;
    the 'age of mammals'
  • Cretaceous (K) - at its end,
    the last dinosaurs
  • Tertiary (T) - no more dinosaurs
  • Danian - the earliest part of the Tertiary
  • Maastrichtian - the last part of the Cretaceous
  • Quaternary - when modern people
    came into being
  • The Recent - the time since the end of the Great Ice Age
  • Permian - at its end, the largest mass extinction ever
  • 0 - when we live
  • 65 - the K-T boundary
  • 4600 - the age of the earth
The K-T boundary is shown by a heavy black line.

THE GEOMAGNETIC REVERSAL TIME SCALE

Geologists are always looking for new ways to correlate and determine the absolute ages of events in earth history. The record of magnetic polarity reversals preserved in earth materials in the form of remanent magnetism provides just such an opportunity.

Determination of the ages and polarity of young lava flows on land (up to about five million years old) was the first step in developing a geomagnetic reversal time scale. Absolute ages and polarity (normal or reversed) were determined for successive lava flows from a series of volcanos (only one volcano (A and detail B) is shown in the diagram). The ages were then plotted as points on a graph (C) representing time.

Once the points are plotted, dividing lines may be drawn to separate times of opposite polarity (polarity epochs or chrons). As more measurements are made and points plotted, the positions of the lines separating chrons may be refined, and new, brief chrons may be discovered. Compare Time Scales X and X'.

With the establishment of the Geomagnetic Reversal Time Scale based on examination of lava flows up to about five million years old (see A in the diagram to the right), the scale was extended based upon magnetic investigation of the ocean floor. As may be recalled, as oceanic lithosphere forms at mid-ocean ridges, the polarity of the prevailing earth's magnetic field is imprinted in the rocks. With the action of sea-floor spreading, a series of magnetic anomaly stripes (see B) are created whose widths depend on the duration of the polarity epoch (either 'normal' or 'reverse'). If the rate of sea-floor spreading along a particular mid-ocean ridge is constant, then the widths of the magnetic anomaly stripes in that ocean basin will be directly proportional to the duration of the polarity epochs. For example, if the rate of spreading is 5 cm per year, and the duration of the epoch is one million years, then the width of the corresponding magnetic anomaly stripe will be 5 million cm (50 km). If the duration is two million years, the width of the stripe will be 100 km, etc. Thus, assuming that the spreading rate is known and is constant, the age of the crust under any location on the ocean floor may (in theory) be obtained by multiplying the spreading rate by the distance from the mid-ocean ridge.

Remanent magnetism in sediment and sedimentary rock also can record the history of the earth's magnetic field. Wind blown dust, volcanic ash, meteoric dust continually fall to the sea floor. These materials are joined by the downward drift of the shells of trillions of deceased one celled organisms that inhabit the oceans. As such sediment accumulates on the sea floor, it incorporates evidence of the earth's magnetic polarity as remanent magnetism. Accordingly, a drill core of seafloor sediment (see B) examined for magnetism displays along its length evidence of times of alternately normal and reverse polarity. If the rate of sea floor sedimentation is constant, the lengths of the core sample showing successive normal and reverse zones should be proportional to the duration of polarity epochs on the polarity reversal time scale. Upon examination, they have been found to be proportional to durations of polarity epochs; they have also been found to be proportional to the widths of magnetic anomaly stripes.

EXAMINING ASSUMPTIONS

The usefulness of the Geomagnetic Time Scale depends importantly upon assumptions as to the rates and constancy of rates of sea floor spreading and sediment accumulation on the deep sea floor.

Determining spreading rate and rate-constancy:

  • As discussed, determination of the ages and polarity of lava flows on land permits a geomagnetic polarity reversal time scale to be established. If the widths of the anomaly stripes on the ocean floor are proportional to the duration of the polarity epochs, constancy of the spreading rate during the measured time span is supported, but not proven. (It is worth noting, however, that acceptance of the notion of sea floor spreading was spurred by the observed proportionality.)

  • Actual measurements of radiometric ages of dredged up oceanic crust at varying distances from the mid-ocean ridge allow determination of spreading rates. Where radiometric ages are not obtainable, sampling of fossiliferous sea floor sediment allows determination of the minimum age of underlying oceanic crust. For example, if the age of the oldest (bottommost) sediment is 10 million years, then by application of the Law of Superposition, the underlying crust must be at least 10 million years old. Many such ages (radiometric and fossil) have been determined in different locations at various distances from mid-ocean ridges and confirm constancy of the spreading rate.

  • As noted above, drill core lengths of deep sea sediment of alternately normal and reverse polarity have been found to be proportional to durations of polarity epochs as judged from lava flows on land; they have also been found to be proprotional to the widths of magnetic anomaly stripes. This proportionality supports the assumption of constancy of the rate of sediment accumulation on the deep sea floor and also the assumption of constancy of the rate of sea floor spreading.

  • In the 1980s, measurements made from earth-orbiting satellites on successive occasions (months and years apart) confirmed plate movement. Furthermore, calculations of the rate of plate movement (and therefore seafloor spreading) made from these measurements yielded results that were in high agreement with those derived from magnetic anomaly stripes and the Geomagnetic Reversal Time Scale.

In this fashion, using evidence from dated lava flows on land, widths of magnetic anomaly stripes on the ocean floor, and lengths of polarity epochs in deep sea sediment cores, a Geomagnetic Reversal Time Scale has been (and continues to be) constructed. The Time Scale now extends back about 155 million years and, according to Maurice A. Tivey of the Woods Hole Marine Magnetism Group, has the potential to be extended back about 175 million years.

Because the pattern of reversals on the Geomagnetic Reversal Time Scale is random, undated sequences of rocks or sediment that show sections of uninterrupted reversal patterns may be dated through correlation with the scale, following a 'matching procedure' similar to that employed in dating varves.

The Geomagnetic Reversal Time Scale may be useful even when the 'matching procedure' is not applicable by providing a 'check' on absolute age determined by other means. For example, consider an isolated sample that shows polarity, but no polarity reversals. Its absolute age cannot be obtained by correlation with the time scale. However, if the absolute age is determined by other means, such as radiometric dating, then the rock's polarity (normal or reversed) should agree with the polarity for materials of that same age that can be linked to the Time Scale. If they are opposite, then further investigation will be required.

THE GEOMAGNETIC REVERSAL TIME SCALE AND WALTER ALVAREZ AT GUBBIO

Walter Alvarez' field work at Gubbio originally had as its aim an investigation of the rotation and drift of the 'microplate' on which Gubbio is situated. That investigation was abandoned when it became apparent that there had been slippage between successive layers of rock that hopelessly blurred the record of plate movement. However, having examined the sedimentary rocks at Gubbio, he realized that he was looking at material that had originally been deposited on the deep sea floor and subsequently uplifted to be observable on land - a fortunate discovery since exposure of deep sea sediment on land is rare. Moreover, the layers contained measurable remanent magnetism and were highly fossiliferous.

At the time, 1972, the ability to check the pattern of ocean crust polarity reversals against reversals recorded in deep sea sediment cores was limited. Relatively shallow, young sediments, up to about five million years old, could be successfully retrieved and examined . However, drilling techniques had to be used to reach older, deeper layers, and vibration from the drill bit disturbed the loose, wet sediment and destroyed the polarity reversal record.

Alvarez realized, therefore, that at Gubbio, where the sediments were directly observable, he had a unique opportunity to study ancient sediments at or near the K-T boundary (roughly 65 million years old) and to help confirm and expand the Geomagnetic Time Scale. In the course of that investigation, he discovered peculiarities of the K-T boundary rocks that led to a literally earth-shattering hypothesis as to what happened to the dinosaurs. (Walter Alvarez, T-rex and the Crater of Doom, Vintage Books Edition, 1998, p. 18-42.)