Geology 370: Sedimentology and Stratigraphy Topic 1: Introduction to Sedimentology and Stratigraphy

1. Photo by W. W. Little
Topic 1: Introduction to
Sedimentology and Stratigraphy

2. Photo by W. W. Little
What do you see?

3. Sedimentology vs. Stratigraphy
Sedimentology involves all factors
that go into the accumulation of
sediment at a specific locality under
a relatively uniform set of
conditions.
Stratigraphy records changes in
sedimentology through time as a
vertical succession.
Photo by W. W. Little
Photo by W. W. Little
Photo by W. W. Little

4. Photo by W. W. Little
Sediment Production,
Preservation, and Alteration
A sedimentary rock is the cumulative
product of six distinct steps:
• Weathering
• Erosion
• Transport
• Deposition
• Lithification
• Diagenesis

5. Environmental Interpretation
The primary objectives of sedimentology are to:
• Determine the conditions (flow characteristics, water chemistry, climate,
topography, sediment source, etc) under which a package of sediment was deposited
• Determine how one package of sediment relates to other, time-equivalent packages
Modern analog
Depositional model
Big picture
Rock record
Photo by W. W. Little
Photo by W. W. Little

6. Scientists make observations of an event or an object and then try to explain those
observations by organizing them into a logical system. As rock bodies can cover
extensive areas and represent vast periods of time, much of geological research
cannot be done as controlled experiments in a formal laboratory setting. In these
situations, we rely on the assumption that chemical and physical laws are constant.
That is, the processes operating today are the same as those that operated in the
past. For instance, since water flows down hill today, it must have done so in the
past. Therefore, if we identify a body of rocks that exhibit characteristics similar to
those found in modern geological environments, we assume that they must have
formed in a similar manner.
Assumes Uniformitarianism
Ancient river channelsModern river channels
Photo by W. W. LittlePhotographer unknown

7. Three Levels of Interpretation
Depositional Environment (facies)
Depositional System (facies assemblage)
Systems Tracts
A facies is a body of sediment/rock
with characteristics (lithology, texture,
structures, fossils) formed under a
unique set of physical, chemical, and
biological conditions representing a
specific depositional environment.
A facies assemblage is a body of
sediment/rock composed of
contemporaneous, genetically-related
facies that form part of a depositional
system. Often defined as a formation.
A systems tract is a series of laterally-
equivalent bodies of sediment/rock
composed of contemporaneous, facies
assemblages.
Photo by W. W. Little Photo by W. W. Little
Photo by W. W. Little Photo by W. W. Little
Photo by NASA

8. Three Levels of Models
Depositional System
Systems Tract
Circles denote
specific environments
A group of environments makes a
depositional system. A group of
depositional systems forms a
systems tract.

9. Stratigraphy is the study of temporal relationships in sedimentary
rock bodies and reflects changes in the balance between rates at
which space is produced and filled.
Stratigraphy
Photo by W. W. Little

10. Photo by W. W. Little
Sediment Preservation Potential
• To be preserved, sediment must be deposited within a basin.
• A basin is space located below base-level.
• Space available for sediment accumulation is referred to as
accommodation space.
• Accommodation space is created/destroyed by relative base-
level fluctuations.
• Base-level fluctuations are caused by changes eustatic sea-level
or uplift/subsidence of the basin floor.
• The nature of stratigraphic packages is controlled by a balance
between the rate at which is space is created and filled.

11. Photo by W. W. Little
Stratigraphy Records History
The stratigraphic record preserves details pertaining to major
geologic events, such as:
• Mountain-building events (orogenies)
• Eustatically-driven transgressive/regressive cycles (sequences)
• Climatic changes

12. Floodplain
Levee
Point Bar
Photographer unknown
Walther’s Law
In a conformable stratigraphic succession, facies that are now in
vertical contact represent environments that were once laterally
adjacent.
Photo by W. W. Little

13. Photo by W. W. Little
Nested Events
A rock body is often composed of laminae (formed in
seconds) within lamination sets within beds within bed
sets within members within formations within groups
within sequences (formed over millions of years).

14. Photo by W. W. Little
Correlation
One of the principle jobs of a stratigrapher is to correlate (or
match) equivalent stratigraphic units from one area to another.
This has led to a number of different branches of stratigraphy.
Some of the formalized types of stratigraphy are:
• Lithostratigraphy
• Biostratigraphy
• Allostratigraphy (sequence stratigraphy)
• Chronostratigraphy
• Pedostratigraphy
• Magnetostratigraphy
Rules governing the naming and use of stratigraphic units are found
in the North American and international codes of stratigraphic
nomenclature.

15. Geologic Time Scale