GEOLOGIC TIME

Geologic Time

• Two ways to view geologic time:
• Relative time
• "older" vs. "younger"
• placed in order
• Absolute time
• "120 m.y."

Relative Time
Basic Rules

• Relative Time - places the rock record within a sequence of events
• Basic Rules
• Law of Superposition - in undeformed strata:
• lowest unit oldest
• units become younger upward
• Principle of Original Horizontality
• Strata generally deposited horizontally
• tilting post-dates deposition

Relative Time
Basic Rules (cont.)

• Principle of Original Horizontality
• Strata generally deposited horizontally
• tilting post-dates deposition
• Cross-cutting relations
• the geologic feature that cuts across or penetrates another rock body must be younger than the rock unit it penetrates
• Inclusions
• older than the rock in which they are included

Unconformities

• Unconformities
• When sedimentation is continuous without interruption, it is said to be conformable
• Gaps in time in the stratigraphic record are called unconformities
• May represent erosion surface or just a period of non-deposition

Types of Unconformities

• May represent erosion surface or just a period of non-deposition
• Two types of unconformities are:
• Angular unconformity

Types of Unconformities (cont.)

Relative Time Problem

Correlation and Stratigraphy

• While seas were flooding Texas and Colorado in the Cretaceous, was Europe flooded? (Implications?)
• Did the extinction of the dinosaurs in North America occur at the same time in Europe? (Implications?)
• To answer these questions, need to correlate sedimentary rocks in one area with those in another.

Stratigraphic Units

• First need to define units of strata
• Three classification schemes
• Rock Units
• defined based on observable characteristics of rock
• Fundamental rock unit is a formation
• mappable, lithologically distinct body of rock having recognizable boundries (contacts) with other formations
• formations subdivided into members or grouped into groups

Stratigraphic Units
Correlation and Time

Stratigraphic Units (cont.)

• Time units
• the Geologic Time Scale divides geologic time into time units -- Eras, Periods, Epochs
• Time-rock units
• classification of sedimentary layers deposited during a particular time interval (which corresponds to a time unit)

Stratigraphic Correlation

• Correlate By:
• lithology - type of layered sedimentary or igneous rock
• fossils - remains and traces of organisms
• first occurrence, last occurrence
• index fossils
• Principle of Faunal and Floral Succesion
• layer of strata has a particular fossil assemblage associated with it (faunal assemblage changes with time).

Stratigraphic Correlation

• Correlation by fossils