The Earth Through Time, 8e

Eighth Edition
by Harold L. Levin

Chapter 5 - page 12

The Sedimentary Archives


The branch of geology that deals with the correlation of rock units from one area to another is known as stratigraphy.

There are three main types of correlation:

  1. Lithostratigraphic correlation - Matching up rock units on the basis of their lithology (composition, texture, color, etc.) and stratigraphic position.
  2. Biostratigraphic correlation - Matching up rock units on the basis of the fossils they contain.
  3. Chronostratigraphic correlation - Matching up rock units on the basis of age equivalence, as determined by radioactive dating methods or fossils.

Geologists can trace beds from one exposure to another. This is called lithostratigraphic correlation. Basically, correlation demonstrates the equivalency of rock units across an area. The sections being correlated are commonly miles apart. (Remember the Principle of Lateral Continuity.)

Basically, a correlation is a hypothesis that units in two widely separated sequences are equivalent. The more unique characteristics that two sections share, the greater the probability that the correlation is correct.

Demonstration of correlation
Demonstration of lithostratigraphic correlation of a limestone bed from one exposure to another.
Can you draw lines to correlate the other rock units?


There are two basic types of contacts between rock units, conformable and unconformable.

Conformable contacts between beds of sedimentary rocks may be either abrupt or gradational. Most abrupt contacts are bedding planes resulting from sudden minor changes in depositional conditions. Gradational contacts represent more gradual changes in depositional conditions. Conformable contacts indicate that no significant time gap or break in deposition has occurred.

Unconformable contacts (or unconformities) are surfaces which represent a gap in the geologic record, because of either erosion or nondeposition. The time represented by this gap can vary widely, ranging from millions of years to hundreds of millions of years (such as an erosional surface between Precambrian rocks and Recent sediments).

Types of unconformities:

A fourth type of unconformity is the paraconformity, separating two parallel units of sedimentary rock. There is no obvious evidence of erosion. A paraconformity is virtually indistinguishable from a sharp conformable contact. The fossils show that there is a considerable time gap represented by a paraconformity.

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Document created by: Pamela J. W. Gore
Georgia Perimeter College, Clarkston, GA

September 17, 2005