Transcript Lateral Lithostratigraphic Changes - Cal State LA

Lateral Lithostratigraphic
Changes
I. INTRO
A. Beds laterally bounded
1. erosion
2. gradational
i. pinchout
ii. intertonguing
iii. lateral gradational
B. Pinch-out
1. unit thins laterally
2. small angle of
convergence
3. some
environments, rocks
thin rapidly
4. good stratigraphic
trap
Turbidite Pinchout
ftp.pet.hw.ac.uk/.../deep_water/research.htm
Pinchouts May Form Stratigraphic
Traps.
www.elsandcompany.com/looking.htm
Pinchout of turbidites filling
channels--Chile
clasticdetritus.com/.../
C. Intertonguing
1. sed body splits,
each w/ pinchout
2. intercalation of
units
3. Tongues = changes
in sed rates due to
tectonics or
changes in sea
level.
 Intertonguing
Relationships &
Designation of
formation
boundaries
D. Lateral Gradations
1. thinning
unnecessary
2. lateral lithologic
change
(mixed/continuous)
3. changes can
occur over miles
4. ss goes to sh =
shale out
II Causes of Changes
A Sedimentation
rate
B. Subsidence
rates
C. Above related
to:
1. Tectonics
2. Climate
SEDIMENTARY FACIES
I. INTRO
A. Environment =
lithologic/paleontologic control
1. same environment, not age
2. need to differentiate:
i. vertical changes (formations,
fossil zones)
ii. lateral changes in rocks)
 B. Gressley (1838)
1. classified rocks vertically
2. unsatisfied, coined facies
i. Latin = face, figure, appearance,
aspect, look, condition
3. Distinguish between facies and
depositional environment
4. Facies is product of environment
5. Dep environment = biological,
physical, chemical process
impacting sedimentation

Rocks and Facies
From Sediment Transport to Rocks - We have been talking about sediment transport and
structures. These are processes that influence sedimentary rocks. What we really need is to be able to
use our understanding of the processes to interpret ancient rocks when we can no longer see the
processes in action. As I mentioned in the first class, we can use the modern processes as a model for
interpreting past processes, which is the principle of Uniformitarianism. However, it is often very
different to see a process going on than it is to look at the ultimate deposited rock and interpret the
process. For example, with bed forms, the entire shape of the structure you see as it migrates is rarely
preserved. Instead, you only see a small part of it, if you get any sediment accumulation at all. Thus,
we can also start the interpretation from the rock end by describing the general characteristics of the
rocks and interpret flow from things like grain size, preserved cross stratification, and biogenic
components. Then we can see which environments are consistent with those characteristics.
Sedimentary Facies Analysis - This approach to interpretation is called sedimentary facies analysis.
A facies (Latin for aspect or appearance) is a body of rock (i.e. a sequence of beds, etc.) marked by a
particular combination of compositional, physical and biological structures that distinguish it from
bodies of rock above, below and adjacent to it. A sedimentary facies has a characteristic set of
properties that makes it distinctive, which the geologist defines.
Facies vs Environments - By grouping characteristics of the rocks into facies, the depositional
environments can be more easily compared and interpreted. It is important to remember that the
sedimentary environment is the combination of physical, chemical and biological processes that
influence sediment deposition, whereas sedimentary facies are the characteristics of the rocks after
deposition. It is the difference between a water flow speed of 20 cm/sec and high angle cross
stratification; the stratification is the result of high flow speed, but they are not the same.
C. Term not solely stratigraphic
1. metamorphic, garnet, western,
trilobite
2. muddled use
3. descriptive vs. inferred
environment
i. sandstone vs. fluvial
4. distinguish petrologic/faunal
characteristics
i. lithofacies vs. biofacies
III Walther’s Law (1983-1894)
A. Within a given sedimentary cycle, the
same succession of facies that occur
laterally is also present in a vertical
succession.
B. Current usage:
1. The sum of the lithologic and
paleontologic characteristics of a sed
deposits
i. sum of all the primary
characteristics
C. Applied to conformable succession
1. unconformities
2. faults

Walther's Law of Facies, named after
the geologist Johannes Walther, states
that the vertical succession of facies
reflects lateral changes in environment.
Conversely, it states that when a
depositional environment "migrates"
laterally, sediments of one depositional
environment come to lie on top of
another.[3] A classic example of this
law is the vertical stratigraphic
succession that typifies marine
trangressions and regressions.
However, the law is not applicable
where the contact between different
lithologies is non-conformable (i.e.
sedimentation was not continuous), or
in instances of rapid environmental
change where non-adjacent
environments may replace one another.
http://faculty.weber.edu/bdattilo//fossils/notes/facies.html
Transgression
Regression
Units Do Not Correspond to Same
Age
http://higheredbcs.wiley.com/legacy/college/levin/0471697435/chap_tut/chaps/chapter05-11.htm
Showing Pinchouts