Geologic Time Scale

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Geologic time correlates rocks and time. The modern geologic time scale shown in Figure A1 was nearly complete by the end of the 19th century and was based on stratigraphic and fossil studies in northern Europe and the United States. The scale, developed before absolute dating techniques were discovered, is a relative geologic scale that provides a standard of reference for dating rocks throughout the world. It lists the succession of rock depositions that are recognized on and immediately beneath the Earth's surface. The standard stratigraphic column, based on fossil plant and animal assemblages from different European strata, is used to date fossils in strata from other parts of the Earth and is the foundation of the geologic time scale.

The application of radiometric dating techniques began early in the 20th century. The quantitative methods provided by these techniques had the potential for dating divisions of the geologic time scale and for estimating the age of the Earth itself. The age of the Earth now is estimated to be between 5 billion and 4.7 billion years and estimates of the duration of the geologic time scale divisions have been made.

Originally, geologic time scale divisions were based on the natural breaks in the stratigraphic column. The breaks were thought to have resulted from worldwide events of mountain building during which no sedimentation occurred and left gaps in the record of rocks, that is, an unconformity. It is now realized that mountain building events were not necessarily worldwide, but may be limited to a single continent or even part of a continent during one interval of geologic time.

The longest divisions of geologic time are the eras. Most geologic time scales recognize four eras, three of which have been named for the fossils in the associated strata. Thus, the Paleozoic Era refers to ‘‘ancient life,’’ the Mesozoic Era to ‘‘medieval life’’ and the Cenozoic Era to ‘‘modern life.’’ Rocks older than Paleozoic generally lack diagnostic fossils and are widely known as belonging to the Precambrian Era. This era included about 80 percent of Earth's history, that is, from nearly 5 billion years to 800 million or 700 million years ago.

The eras are divided into periods of time. Rock deposits that relate to or were formed during a certain period of time constitute a system of rocks. Periods are divided further into epochs of time and rock systems into series.

In northern Arizona nearly the entire stratigraphic column is exposed. Near the botton of the 1.6-km- (1 mi) deep Grand Canyon, a complex of deformed and metamorphosed rocks lies unconformably below the lowest Cambrian sedimentary rocks (Figure A2). The oldest strata in the canyon is the Vishnu Schist that contains intrusive granite and pegmatite dikes. These rocks have been called Archeozoic (early Precambrian). Radiometric dating of the granites indicates an intrusive age of between 1.8 billion and 1.6 billion years placing them in the early Proterozoic (Palmer, 1983). Lying uncomformably on the Vishnu Schist are strata of the Grand Canyon series, quartzite, limestone, conglomerate and shale. These sequences have been called Proterozoic and probably are late Precambrian (middle or late Proterozoic).

Approximately 910 m (3,000 ft) of Paleozoic sedimentary rock also are exposed in the Grand Canyon. These rocks extend from the Cambrian to the Permian, but rocks of the Ordovician and Silurian periods are missing. They either were never deposited or were deposited and worn away.

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FIGURE A1. Modern Geologic Time Scale (after D. L. Eicher, 1976)

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FIGURE A2. Stratigraphic Column of Northern Arizona (after D. Beasely and W. J. Breed, 1975)

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The Quaternary Period includes the Pleistocene and Holocene epochs. These epochs are of major importance in evaluating the genesis and distribution of modern soils since the environmental conditions that existed then may have directly influenced the soils. Conversely, the climatic conditions of earlier geologic time periods only indirectly influenced modern soils by affecting the nature of the soil parent material and the evolution of the Earth's physiography.

Glaciation of large portions of North America, northern Europe and other parts of the world is associated with the Pleistocene. The Pleistocene did not experience just one great ice age, but had alternations of glacials, or stadials, and stages of relatively greater warmth, interglacials, or interstadials. Table A outlines the classical glacial divisions of the Pleistocene for North America, four glacial and three interglacial ages. Similar divisions also were developed for European glaciation. But use of modern and refined observational and dating techniques has shown that glacial events were much more complex than the classical divisions indicate. Of particular significance is the investigation of the stratigraphic records of sea floors that has led to new views on the length and frequency of glacial and interglacial episodes. For example, van Dork ( 1976) suggested 21 glacial cycles during the last 2.3 million years, while Kent, Opdyke and Ewing ( 1971) favored 16 glacial phases during the last 2.5 million years. Although these researchers do not agree on the exact number of cycles, it is clear that their data provide a very different picture than the classical fourfold sequence of glaciation.

TABLE A. The Classical Subdivision of the Quaternary for North America (Modified from Schuchert and Dunbar, 1964).
Epoch Stages/Ages of Deposition Estimated Years Before Present
Holocene 10,000
Wisconsin glacial 125,000
Sangamon interglacial 250,000
Illinoian glacial 350,000
Pleistocene Yarmouth interglacial 650,000
Kansan glacial 750,000
Aftonian interglacial 950,000
Nebraskan glacial 1,000,000

The complexity of the nature of glaciated cycles and the important gaps in the knowledge of geologic time also have given rise to a considerable range of views of the duration and definition of the Pleistocene Epoch (Cooke, 1973). Some researchers have tended to put the boundary between the Pliocene and the Pleistocene at about 2 million years ago based on the appearance of some faunal genera and the disappearance of others (Glass et al, 1967; Zagwijn, 1974). Others place the boundary at between 3 million and 2.5 million years ago based on the marked appearance of mid-latitude glaciers as contrasted with those of the polar regions. Thus, the duration of the Pleistocene now is considered to be between 2 million and 3 million years. The classical view was that it lasted about 1 million years.

Researchers also have disagreed about the boundary between the Pleistocene and Holocene. Different authors have placed it between more than 20,000 and 4,000 years ago. A compromise time of 10,000 years ago for the beginning of the Holocene has been recommended. Using this time, the boundary stratotype (the earliest deposition of rocks different from those associated with the Pleistocene) in the vicinity of Goteborg, Sweden, was further recommended.(Fairbridge, 1983).

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