Stratigraphic Record Help

By — McGraw-Hill Professional
Updated on Aug 31, 2011

Stratigraphic Record

The stratigraphic record is the overall picture of a series of facies in a region. Stratigraphy and structural studies of continental rocks have allowed geologists to piece together the principles and relationships of physical geology. Plate tectonics was finally figured out after decades of stratigraphic observations and careful measurements. Layer upon layer of sedimentary rock over millions of years and during all types of weather and environmental changes have provided geologists a road map of ancient times.

There are also breaks in the stratigraphic record. Since sedimentation happens at different rates all around the world, some places have a barely readable record. Wind, erosion, and other factors can wipe out sediment before it has a chance to gather, let alone harden into rock.

Unconformity is a good-sized gap or break in the stratigraphic record that shows that a part of the rock record is missing.

When geologists see an unconformity in the stratigraphic record, they know that part of the story is missing. There are several factors that can break the sedimentary rock record. These include the uplifting of landmasses, changes in sea level, and changes in climate that change streams, rivers, and glaciers.

At the ocean floor, strong currents move sediments and can cause underwater landslides of deep trenches and other large areas where sediments pile up. Sometimes these changes cover up other strata making a geologist’s job even trickier.

When there is a large break or gap in the rock record, geologists call the missing time period a hiatus .

A long-term gap in the sedimentary rock strata, affecting thousands and millions of years, is known as a hiatus.

No solid information on sedimentation, climate, or tectonics is obtainable for the time period represented by the hiatus. Geologists have to “wing it” and make assumptions based on their experience and knowledge of the surrounding area’s history.

Some unconformities are much shorter and temporary than those seen during a hiatus. These unconformities are the newsworthy events that television reporters and journalists look for. Sudden natural events are called diastems .

For example, when huge tropical storms wash away decades of beachfront habitats and collected sand, sedimentation is affected. When locally heavy rain swell rivers beyond their banks causing swift flooding and erosion for miles, it affects the crops that year, but not for the next one hundred or a thousand years.

Brief gaps or strata disruptions, caused by sudden events like flash floods or mud slides, are called diastems.

Four Types of Unconformities

There are four different types of unconformities. These are the angular unconformity , disconformity , paraconformity , and nonconformity . These missing puzzle pieces are found in a variety of different circumstances.

Figure 5-1 illustrates the different kinds of unconformities commonly found in the crust. The individual differences between the unconformities are described below:

  • angular unconformity has a break between older and younger strata with the one forced upward at an angle to the other,
  • disconformity has parallel strata layers with a rough surface break erosion,
  • nonconformity has stratified rocks that lie over igneous rock and metamorphic rocks with a rough break between them, and
  • paraconformity has parallel beds divided by an unconformity of only a different bedding plane between them.

Strata and Land Eras

Strata and Land Eras

Fig. 5-1. Unconformities are found in four types depending on angle and adjacent rock.

Practice problems of this concept can be fround at: Strata and Land Era Practice Test

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