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Lines of Evidence
for Past Change

Lines of Evidence for Past Change:

Stratigraphy:
Timing Events in the Past

Sequence of Events

How can you interpret the events illustrated at the left?

Which is the oldest event? The youngest?

In order to sequence events, there are several principles to keep in mind:

  • Original horizontality
  • Superposition
  • Lateral continuity
  • Fossil succession

As well as these related concepts:

  • Cross-cutting
  • Unconformities

Original horizontality:

Initially, sedimentary rocks are lying flat.

Superposition:

The oldest layers are on the bottom; the younger on top.

Lateral Continuity:

 When sediments are initially deposited, they are continuous across the depositional area. If that continuity is disrupted by an event, the layers can be correlated to different parts of the basin. The two layers designated by the red arrows were initially continuous, but have since been offset.

Cross-cutting:

These are units that cut through other layers. They are younger than the host rock. The orange layer “C” is such an intrusion.

Unconformities:

 These are erosion surfaces, hiatuses, or gaps in time, such as the erosion surface shown by the red arrow.

Based on the previous information then, the order of the events in the slide from oldest to youngest are:

Layers in “d” are deposited. Fault “b” breaks and offsets the layers. Intrusion “c” occurs. Erosion of “d, b, and c” occur creating an unconformity. Units “e” are deposited and later eroded.

Fossil Succession

Fossils in sedimentary rocks can also be correlated – see yellow arrow. These layers can also be given an absolute date by bracketing them between igneous rocks dated radiometrically – see blue stars.

Parent and Daughter Nuclides

The chart below describes common isotopes used for determining the absolute age of igneous rocks.

All images courtesey
http://tornado.sfsu.edu/geosciences/classes/lwhite/time.htm

Additional Resources

A larger color copy of the events diagram.

A downloadable PDF version of the events diagram.

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