Transcript Transgressions and regressions: the key to Earth history

Transgressions and regressions:
the key to Earth history
Geology 103
The oceans were deeper
• Rock (principally marine
limestones and shales) and
fossil evidence indicates that
the ocean occupied the interior
of North America (as well as
other continents)
• Conclusion: sea level was
higher (change in the elevation
of the continent also played a
role)
• This is a sea level transgression
The oceans were shallower
Walther’s Law of Facies
• Facies = a rock or rocks
deposited in a
particular environment
• Johannes Walther
(1894) stated “A vertical
change in facies in a
sequence is reflected in
the horizontal change in
facies along a surface.”
Facies change during a transgression
• As sea level increases in
an area, the water
energy decreases,
allowing the deposition
of finer-grained
sediments in the same
area
• This is an example of
Walther’s Law
Facies change during regressions, too
So, at an outcrop, transgressive
sequences are distinguishable from
regressive sequences
Worldwide transgressive sequences are
found in the rock record
• Indicates that sea level
changes occur over the
millions of years
timescale, so correlated
to plate tectonic
changes and continent
orientations (especially
over polar regions)
• Why no regressive
sequences recorded?
Cratonic transgressive sequences are
related to orogenic events
• Transgressive
sequences seem to
coincide with periods of
significant mountain
building
• The mountains provide
the source of sediment
for the sequences