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FIGURE 4.2 Idealized diagram of basic field relations of rocks that can be used to determine relative ages, using original horizontality, superposition, lateral continuity, inclusions, cross-cutting relationships, and biologic succession. Phenomena are labeled from oldest (1) to youngest (19).

consolidated by compression from overlying sediments and cementation, the sediments become sedimentary rock. Sedimentary rock is further classified by its litho-logy, or rock type, which is based on the composition and texture of the sediment. Layers of sedimentary rock are called strata (plural of stratum) or beds. A series of strata in a particular area, stacked on top of one another, may be collectively called a stratigraphic sequence. If the strata in a stratigraphic sequence are tilted significantly beyond a horizontal plane, then an observer knows that the tilting happened after deposition of all of the sediments and their formation into sedimentary rock.

Superposition is the concept that each layer of sediment deposited on top of an underlying layer is relatively younger than the latter, assuming the strata have not been tilted far beyond their original horizontality. Superposition, along with original horizontality, may seem too inherently obvious, but Nicolaus Steno (Niels Stensen) of Denmark, in the seventeenth century, was the first person to actually articulate it (Chapter 3). To visualize this principle, think of the sediments at the bottom of the sequence as being deposited first, and the sediments at the top being deposited most recently. If the rate of layering was known, the total age of the strata could be calculated. However, this is not straightforward as, in reality, there is the possibility that the rate of deposition varied through time, that the strata was overturned, or that some layers were removed throughout the history of deposition, usually by erosion.

Related to the concept of superposition is lateral continuity, which describes how sedimentary layers will continue in lateral directions until they encounter some barrier that prevents their further spread, or they otherwise run out of sediment. If a laterally continuous layer is found in widely separated places, it may represent approximately the same time of deposition, which is an example of correlation. Laterally adjacent layers also can succeed one another vertically, which represents how changes in environments through time can cause one environment to overlap the other.

Cross-cutting relationships and inclusions have opposite significance in relative age dating. With cross-cutting relationships, a geologic feature cutting across another feature is the younger of the two, such as a fracture that cuts through all strata in a particular geologic section (Fig. 4.3). In contrast, inclusions, which are particles of a preexisting rock incorporated into sediment, must be older than the rock including them. British geologist Charles Lyell expressed these latter two principles in the early nineteenth century (Chapter 3).

All of these principles can determine relative-age dates of rocks without the use of fossils, but the combination of these principles with rocks containing identifiable guide fossils adds a powerful dimension to interpreting the geologic history of an area. Guide fossils are typically body fossils (but in some cases trace fossils) that are:

1 abundant;

2 easily identifiable;

3 geographically widespread;

4 vertically restricted in their range; and

5 likely to be deposited with sediment independently of the environment in which they lived.

In terms of biologic succession, their vertical zonation, also known as geologic range, may demonstrate when an organism first evolved (at the bottom of the zone) and when it went extinct (at the top of the zone) in that area. When these geologic ranges for different guide fossils are noticed and recorded both locally and worldwide, they can represent different times that are identifiable whenever these fossils are found, which is another aid to correlating strata.

For example, bones of Coelophysis, a small theropod, have been found only in strata below those containing bones of Allosaurus, a larger and different theropod, and Apatosaurus, a large sauropod (Chapters 9 and 10). Paleontologists thus conclude that the fossils of the overlying layers succeeded the underlying ones. The strata containing the remains of Coelophysis are from the Late Triassic, whereas the rocks bearing Allosaurus and Apatosaurus fossils are from the Late Jurassic, which is

FIGURE 4.3 Fault cross-cutting thick stratigraphie sequence of the Santa Elana Limestone (Late Cretaceous) in Big Bend National Park, Texas. Students for scale.

FIGURE 4.3 Fault cross-cutting thick stratigraphie sequence of the Santa Elana Limestone (Late Cretaceous) in Big Bend National Park, Texas. Students for scale.

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