Relative age dating

The seventeenth century Danish naturalist Nicholas Steno3 was the first to recognize that:

1 in any vertical, stacked sequence of sedimentary rocks, the oldest rocks are found at the bottom and successively younger-aged rocks are found above, with the youngest occurring at the top (a observation now termed the "law" of superposition) and

2 all sedimentary rock sequences were originally horizontal (although subsequent geological events may have disrupted their original orientation in space).

That younger sediments are deposited upon older sediments seems clearly self-evident, and yet this obvious conclusion is the fundamental basis of all correlations of sedimentary strata in time. If a stratum lies above another (and the rocks have not been subsequently deformed by various geological processes), it is younger than the one below it (Figure 2.3). Ascertaining the relative ages of the two strata is termed relative dating: the type of dating that, while not providing age in years before present, provides the age of one stratum relative to another stratum. In historical terms, we might use the presence of record albums and a record player in a picture to infer that the picture was taken before there were compact discs (CDs) and CD players. Here, then, is part of the solution to dating dinosaur bone. Suppose that a stratum containing a dinosaur bone is

3 An interesting biography of Steno is Cutler A. 2003. The Seashell on the Mountaintop:A Story of Science, Sainthood and the Humble Genius who Discovered a New History of the Earth. Dutton, New York, 228pp.

Relative Age Dating
Figure 2.3. Superposition of strata in Petrified Forest National Park, Arizona, USA.Thick stacks of red mudstones were deposited by rivers 213 Ma to produce the succession of layers visible in this photograph.

sandwiched between two layers of volcanic ash. Ideally, an absolute age date could be obtained from each of the ash layers. We would know that the bone was younger than the lower layer, but older than the upper layer. Depending upon how much time separates the two layers, the bone between them can be dated with greater or lesser accuracy.

Datable ash layers thus truly fall out of the sky, but rarely because stratigraphers need them. More commonly, rocks for which absolute ages can be obtained are widely separated, not only in time, but also geographically. The challenge then is to correlate the strata of known absolute ages with those of unknown age. But how can one tell that two geographically separated deposits were deposited at the same time if absolute ages are unknown? In this, fortunately, stratigraphers are aided by one last, extremely important, tool: biostratigraphy.

Biostratigraphy Biostratigraphy is a method of relative dating that utilizes the presence of fossil organisms. It is based upon the idea that a particular time interval can be characterized by specific organisms, because different creatures lived at different times. For example, if one knows that dinosaurs lived from 228 to 65 Ma,4 then any rock containing a

4 We use the expression Ma, from the latin mille annos, to mean million of years.Thus 65 Ma is 65 million years ago.

dinosaur fragment must fall within that age range. The question is, how precise a date can it really give?

In fact, biostratigraphic correlation - the linking of geographically separated rocks based upon the fossils they contain - can be very precise. Although, like superposition, biostratigraphy cannot provide ages in years before present, the fact that many species of organisms have existed on earth for 1-2 million-year intervals enables them to be used as powerful dating tools. For example, Tyrannosaurus rex lived for only about 2 million years, from 67 to 65 Ma. Thus, if we found a Tyrannosaurus fossil (a good find, indeed), we would know that, no matter where that dinosaur was found, it would be correlative with T. rex-bearing sediments in North America that have been well dated at 67-65 Ma.

Eras and Periods and The oldest method of dating sediments is biostratigraphy. Leonardo da Epochs Oh My! Vinci observed marine shells far inland where the ocean clearly was not; he correctly deduced that where he stood had once been covered by an ocean waters. By the early 1800s, French anatomist Georges Cuvier, studying strata around Paris, noted that higher strata had a greater proportion of fossil shells with living counterparts than did lower strata. The increasingly modern aspect of the fauna is due to the fact that the highest rocks are closer in time to the present, and thus the faunas that they contain are the most like those of today.

Within a generation of Cuvier, a remarkable revolution had been wrought in geological thinking. The Phanerozoic (phaneros - light, meaning visible; zoo - life) time interval, representing that interval of earth history during which there have been organisms with skeletons or hard shells present, was established. Using biostratigraphy as a time indicator, a variety of rock outcrops in northwestern Europe was designated as type sections, or original locations, where a particular interval of time is represented. The names of the largest of the blocks of time within the Phanerozoic, the Eras, came from a description of the life contained within each. These Eras are, from oldest to youngest, the Paleozoic (paleo - ancient), the Mesozoic (meso - middle), and the Cenozoic (cenos - new). Within each are smaller subdivisions (still consisting of 10s of millions of years each) called Periods, and within these, in turn, are yet smaller subdivisions of time called Epochs (consisting of several millions of years each). Figure 2.4 shows the currently

Figure 2.4. The Mesozoic part of the geologic time scale. All ages (except that of the age of the earth, which is given in billions of years) given in millions of years before present. The Mesozoic constitutes only a rather tiny fraction of the expanse of earth time. If you compacted earth time into a single year; from January I (the formation of the earth) to December 31 (the past 100,000 years of which, by this way of measuring earth history, would occur in less than a day), then dinosaurs were on earth from about December I I to December 25. (Dates from Gradstein, F. M„ Agterberg, F. R, Ogg,J. C„ Hardenbol, J„ Van Veen, R,Thierry, J., and Huang, Z, 1995. ATriassic, Jurassic, and Cretaceous time scale. In Berggren.WA., Kent, D.V.,Aubry, M.-Rand Hardenbol, J. (eds.), Geochronology.Time Scales, and Global Stratigraphic Correlation. SEPM Special Publication no. 54, pp. 95-126.)

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Relative Age Dating

understood distribution in time of the different Periods within the Mesozoic Era, the time interval of special relevance in this book.

All intervals of time are hierarchically arranged, and so large blocks of time are sudivided into smaller blocks of time, which are in turn subdivided into even smaller blocks of time. This is convenient, because the age of a rock or fossil can be designated with only as much precision as is known to the investigator. For example, a dinosaur fossil might be 70 million years old, and thus it belongs to the interval called the "Maastrichtian." Alternatively, we might not know its exact age, and thus be able to identify it only as Cretaceous, the period of time of which the Maastrichtian is a part. The Cretaceous includes more time than the Maastrichtian and is thus a less-precise age designation.

Periods are also subdivided in other ways. "Early," "Middle," and "Late," mean oldest, middle, and youngest, respectively. Thus the Late Triassic, for example, represents the youngest years still encompassed within the Triassic; that is, that part of the Triassic that is closest in time to us. Remember we said that stratigraphers dichotomize rocks and time. Therefore, when we say "Upper Triassic," we are referring to the rock record deposited within the Late Triassic, even though the former is unlikely to record all of Late Triassic time, as we discussed earlier. Here Steno's law applies, and the uppermost rocks of the Triassic correspond to those that are youngest. Hence, "Lower," "Middle," and "Upper," designations refer to the rocks laided down during the "Early," "Middle," and Late" time intervals, respectively.

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