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Southern California Geology

Geologic Time

In order to appreciate the geologic setting and geologic history of southern California, the visitor needs to be familiar with how geologists classify and describe the vast time intervals of the geologic past.

Geologic events always have a time context:

 

  • Geologic events occur relative to each other chronologically . For example, "this sandstone formation and its fossil sea shells was deposited before this overlying limestone formation and its fossil shells, but both formations were deposited after granitic rocks beneath them cooled"

 

  • Geologic events occur during some portion of a continuous absolute time scale. For example, "this basalt flow moved across the Mojave Desert landscape 11.5 million years ago"

When placed in the context of geologic time, geologists are able to reconstruct geologic-history events like the following:

 

  • This sedimentary sequence accumulated during the early Miocene Epoch while this mountainous area to the northwest was being uplifted by thrust faults and eroded to provide sediment for the basin
  • This fault was active during the early Pleistocene Epoch, but then was abandoned as tectonic activity shifted to another fault during the late Pleistocene Epoch
  • This gold deposit formed during the late Cretaceous Period while enclosing granitic rocks were intruding early Paleozoic limestone bodies that host the deposits
  • Desertification of this part of the Mojave Desert began during this time period, before the mountains to the west were uplifted to create a regional rain shadow effect

In order to make statements like these, earth scientists need to do two things:

 

  • estimate the relative and absolute ages of geologic materials, geologic structures, and geologic events
  • establish a standardized time scale for splitting the geologic past into small parts that can be recognized globally

Over the last 150 years or so, geologists have made extraordinary progress in working out a global geologic time scale as well as determining the age of various geologic events such as the extinction period of the dinosaurs, the age of earliest faulting on the San Andreas Fault, and the time interval when marine conditions existed in what is now part of the onshore Los Angeles Basin. Figure 1 illustrates a simplified version of the state-of-the-art global time scale.

Simplified geologic-time chart

Figure 1. -Simplified geologic-time chart (taken from Geologic time scales at the National Park Service/USGS Geologic Basics website). The right-hand column is the total geologic-time scale; the left-hand column is the Phanerozoic Era expanded so that its finer time subdivisions can be illustrated. Time before present is indicated down the left side of each column. Note the vast difference in elapsed time between the Cenozoic, Mesozoic, and Paleozoic Eras (about 570 million years) and the elapsed time of the Precambrian (almost 4 billion years).

Where can I find information about Geologic Time and its Classification?

The Geologic Basics website maintained by the National Park Service and the U.S. Geological Survey has an excellent discussion of geologic time, including:

 

  • How earth scientists estimate the age of the earth , including a discussion of techniques for determining radiometric ages
  • A simplified geologic time scale , with links to proportional time scales that show the duration of the time-period intervals relative to each other. In the " Close-up" part of that page, click on any of the individual geologic pages (Triassic, for example) to obtain additional information about that specific age.

The University of California (Berkeley) Museum of Paleontology (UCMP) has several online resources that can help you understand and envision geologic time:

 

  • UCMP's interactive Geologic Time Machine clearly presents the major geologic-time intervals, their refined subdivisions, and some of the unique geologic and biologic events associated with each time interval. This can be a nifty adventure-enjoy it!
  • The Geological Society of America published a Geologic Time Chart as part of its Decade of North American Geology series; an online version of the GSA time chart is posted at the UCMP Time Machine website.
  • Also see UCMP's brief but helpful discussion about Geology and Time -a summary of how geologists came to realize that geologic events have succeeded one another over vast lengths of time
  • The UCMP link to Stratigraphy-Cenozoic Era gives an example of how geologists in Europe recognized the basis for this particular geologic time interval, and then extended (correlated) that interval around the world

The following general-interest publications by the U.S. Geological Survey help you understand how geologists measure geologic time in rocks:

 

Berggren and others (1995) recently compiled a series of studies that update and refine the geologic time scale and its use in correlating geologic events globally.

 

Berggren, W.A., Kent, D.V., Aubry, M-P., and Hardenbol, J., eds., 1995, Geochronology, time scales and global stratigraphic correlation: Tulsa, Oklahoma, Society of Economic Paleontologists and Mineralogists, Special Publication 54, 386 p.

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This site last updated September 3, 2004 (ps)

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