based on the vertical zonation of these and other fossils. Occasionally these zona-tions are refined when geologists or paleontologists find fossils slightly below (older) or above (younger) the previously known ranges, which is part of the testing and subsequent improvement of the fossil record that occurs every day.

Guide fossils were applied to mapping the distribution of sedimentary rocks in parts of Europe in the late eighteenth and early nineteenth centuries, well before Charles Darwin published any of his research on biological evolution. Since then, evolution and extinction are now recognized as primary factors that controlled the vertical distribution of fossils in the geologic record (Chapter 6 and 16). Bio-stratigraphy, the use of guide fossils in mapping rocks and interpreting their ages, was largely responsible for the establishment of a worldwide, standard geologic time scale (see Fig. 1.2), with its relative time divisions, such as eons, periods, and epochs, each represented by distinctive fossil assemblages. The time divisions are called time units, whereas the rocks that represent those times are chronostratigraphic units: for example, the Triassic Period is a time unit and the Triassic System is a chronostratigraphic unit. The rocks from the early part of the Triassic are called Lower Triassic, whereas their age is Early Triassic (which is older than both Middle and Late Triassic). Likewise, the Jurassic is divided into Early, Middle, and Late, but the Cretaceous is divided into Early and Late. Upper and lower also refer to all rock units, not just chronostratigraphic units. Earth-resource companies use the geologic time scale and such divisions in their exploration for fossil fuels (oil, gas, and coal) and minerals, and they have been well tested since the beginning of the Industrial Revolution.

The description of strata that contain distinctive sediments and fossils, which can lead to the recognition of mappable units that a geologist can readily identify in the field, are called formations. Formations are given formal names by geologists on the basis of the locality of a stratigraphic section that ideally is representative of the rock types found in the formation, called the type section. A formation name can be the place name for the type section followed by a generic formation designation, such as the aforementioned Upper Jurassic Morrison Formation, which is named after Morrison, Colorado (Chapter 3). These names can also reflect the main rock type in the formation, such as the Upper Devonian Chattanooga Shale, which is named after the location in Chattanooga, Tennessee, and the rock type. Formations can be subdivided into members or grouped together into groups. Guide fossils can help with identifying a formation, but the easy visual identification of a formation by its lithology is more important for geologists who want to describe its distribution on a geologic map.

As stated above, strata do not always equate with time units because the same type of sediment could have been deposited at different times when an environment left sediments as it migrated laterally. For example, think of how a lateral change in the position of the shoreline to landward environments (called a transgression) causes the sea to cover formerly dry land and progressively deposit marine sediments farther inland through time. This type of change, perhaps caused by a rise in sea level, makes the marine sediments time transgressive. Likewise, a lateral change in the position of the shoreline in the direction of seaward environments (regression), maybe through a drop in sea level, would cause continental sediments to overlap the formerly marine sediments. However, the continental sediments are still time transgressive because they were gradually deposited through time. Geologists keep this in mind, so if different species of dinosaurs (or any other fossils) are found in the same formation in widely separated places, the organisms they represent are not always assumed to have lived at the same time.

Application of scientific methods to the basic geologic principles of relative age dating involves testing each criterion for consistency, instead of accepting that any one of them fulfills the possible geologic interpretation. For example, if inclusions that definitely come from one bed are found in the bed directly underlying it, we can conclude that the sequence is overturned (upside-down), and hence a hypothesis about age relations of the strata based simply on superposition could be wrong. Similarly, a fossil that seems far too young for the previously known worldwide geologic range of the strata in which it was found may have been reworked, that is, exhumed from its older layer and reburied, only to re-emerge in this younger strata. As a result, the hypothesis that the fossil is an inclusion and is an indicator of some sort of previous erosion (transport of sediment) from its original entombing bed must be tested. The hypothesis that some dinosaurs lived in the Cenozoic Era, which was based on the occurrence of dinosaur bones in the oldest Cenozoic strata, has been falsified by evidence indicating that these fossils were reworked from underlying Mesozoic strata (Chapters 7 and 16).

Surfaces of non-deposition or erosion, called unconformities, are very important to recognize because they signify a gap in the time record; that is, time is not represented by rocks in the area of an unconformity. In some cases, the missing geologic record consists of millions of years (Fig. 4.4). Consequently, geologists and paleontologists are often depressed by unconformities and lament the many fossils lost to the cruel processes of erosion. These surfaces can be identified with the use of the principles of relative age dating and a few other criteria.

FIGURE 4.4 Labeled unconformity near Morrison, Colorado (adjacent to Red Rocks Amphitheatre), separating the Fountain Formation (Pennsylvanian Period, about 300 Ma) from a 1.7 billion-year-old Precambrian metamorphic rock (gneiss); the surface represents a lost record of about 1400 million years.

FIGURE 4.4 Labeled unconformity near Morrison, Colorado (adjacent to Red Rocks Amphitheatre), separating the Fountain Formation (Pennsylvanian Period, about 300 Ma) from a 1.7 billion-year-old Precambrian metamorphic rock (gneiss); the surface represents a lost record of about 1400 million years.

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