Transcript Stratigraphy - Cal State LA

Stratigraphy
Rocks, Fossils, and Time
Introduction
• Stratigraphy crucial to Historical
• Helps us understand vertical and lateral
changes in rocks/environments with
time
• Helps us understand paleogeograpy e.g.
where the land was, where the ocean
was, where the mountains were, etc
What is Stratigraphy?
•A branch of geology concerned with composition, origin, age relationships,
and geographic extent of layered or stratified rocks
Valley of the Gods, Utah
Stratigraphy
•Stratigraphic section
–Tracing stratigraphic sections from outcrops leads to correlation of rock
units
–Sequence of sedimentary rocks on a bed-by-bed basis
–Demonstrates equivalency across an area
Sedimentary Facies
•Body of sediment/rock characterized by physical, chemical, and biological
attributes
•Sediment/rock makes it recognizable and different from laterally adjacent
rock
–Could be textures
–Could be fossils
–Could be composition
•Transgression – Sea Level rises, environments march landward
•Regression– Sea Level drops, environments march seaward
Lateral Changes in Stratigraphy
• Beds may change laterally in thickness or lithology
–Differences due to changes in sedimentation rate or depositional
environment
Lateral Changes
•Pinchout
•Intertonguing
•Lateral Gradation
Walther’s Law
•Those facies found in a conformable (no faults, unconformities,
folds) lateral sequence will replace one another vertically
Conformable Vertical Successions
Regression
Transgression
• Sea level ↑; shoreline moves inalnd
• Sea level ↓; shoreline moves seaward
• Facies are time transgressive
• Ages of facies vary from place to place
•Did sea level rise or
fall?
Transgressive Sequence
•Note time lines cut across units
Regressive Sequence
Transgressive and Regressive Sequence
•V-shaped pattern produced by migrating facies during
a transgression followed by a regression
–Three stratigraphic sections illustrate facies generated in
vertical succession at three different locations
Causes of Transgressions and Regressions
•Cold and warm climates
–Cold climate  glaciation
–Warm climate  ice melt
•Uplift and Subsidence
•Uplift  regression
•Subsidence 
transgression
•Seafloor spreading
•Fast spreading 
transgression
•Slow spreading 
regression
Fossils and Correlation
•William Smith 1769-1839
–Determined relative ages of rocks
using fossils and assemblages of
fossils
–Established Principle of Faunal
Succession
Principle of Fossil Succession
•Fossils succeed one another through time in a regular and determinable
order
•Because of this, any time period can be recognized by its fossil content
Fossil Assemblages
•Species have a time of origination
and a time of extinction
•Because of this, different times
have different fauna and flora
•If sedimentary rocks from different
places contain the same fossils, they
were deposited at the same time
–If fossil A range = 40 – 20 my
–If fossil B range = 25 – present
–What is age of rocks containing last
appearance of A & first appearance
of B.
Establishing Correlation of Units
•How do we establish correlation or equivalency of
units?