Geol 370: Sedimentology and Stratigraphy Topic 19: Principles of Stratigraphy

1. 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 can be considered the history of past
geological events and adds the dimension of time to sedimentology.
Stratigraphy
Photo by W. W. Little

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

3. Principles of Stratigraphy
• Superposition
• Original Horizontality
• Lateral Continuity
• Crosscutting Relationships
• Inclusions
• Faunal (biological) Succession
• Incomplete record
• Base-level
• Accommodation
• Preservation Potential
• Cyclicity
• Walther’s Law
• Correlation

4. Sedimentary layers become progressively younger upward through a
stratigraphic sequence.
Principle of Superposition

5. At the time of deposition, sedimentary layers are horizontal. If they
are no longer horizontal, they have undergone some type of
deformation. The two most common types of deformation are folding
and faulting.
Principle of Original Horizontality

6. Tilting of Beds by Folding

7. Tilting of Beds by Faulting

8. At the time of deposition, sedimentary layers are continuous for long
distances. If they terminate abruptly, they have either undergone
deformation or they have been eroded. The two most common causes
of abrupt bed termination are faulting and unconformities.
Sedimentary layers can taper laterally into other sedimentary units.
Principle of Lateral Continuity

9. Bed Termination by Faulting

10. Bed Termination by Unconformity
An unconformity is a surface that represents an episode of missing
geologic time. During the formation of an unconformity, sediment is
either not deposited or it is deposited and subsequently removed by
erosion.

12. There are three types of unconformities based on the types of rocks
involved and their attitude with respect to each other.
Types of Unconformities
Disconformity (paraconformity) Nonconformity
Angular
Unconformity

13. Disconformities are unconformities where horizontal sedimentary layers
overly horizontal sedimentary layers. They represent episodes of sea
level change or simple uplift and subsidence.
Disconformities

14. Angular unconformities are unconformities where horizontal
sedimentary layers overly tilted sedimentary layers. They represent
episodes of deposition followed by deformation resulting in tilting and
then a second episode of deposition.
Angular Unconformities

15. Photo by W. W. Little

16. Nonconformities are unconformities where horizontal sedimentary layers
overly intrusive igneous rocks and metamorphic rocks. They represent
episodes of uplift and erosion followed by deposition.
Nonconformities

18. Principle of Cross-cutting
Relationships
A rock body that is cut (crossed) by another rock body, structure, or
unconformity is older than what is cutting it.

23. Fragments of one rock body that are contained within another must be
older than the body that contains them. Virtually every particle in
clastic sedimentary rock can be considered as an inclusion.
Principle of Inclusions

25. Fossils are found in a consistent and predictable succession
everywhere on earth.
Principle of Faunal (biological)
Succession

26. Geologic Time Scale

28. Incomplete Record
The rock record consists of punctuated depositional events, such as
floods, storms, and turbidity flows, separated by hiatuses.

29. Unconformities
Unconformities represent major gaps in the stratigraphic record and
typically separate systems tracts. Unconformities reflect significant
tectonic events or fluctuations in sea-level.

31. Diastems
Diastems represent minor gaps between similar depositional events,
such as floods, storms, and turbidity flows, that form a natural part
of a depositional system.

32. • Sea level is the ultimate base level.
• Lakes and reservoirs provide temporary base levels
• Changes in sea level can be eustatic or relative.
• Changes in base level create and destroy accommodation space
• Base-level is the primary factor determining erosion vs. deposition
Base level is the level below which erosion cannot occur and above
which deposition does not take place.
Base-level

33. Graded (Longitudinal) Profile
Stream gradient is related to water velocity and the load it can transport.
A change in any of these affects the other two. Each can be influenced
by factors outside of the stream system, such as base level fluctuations
(gradient), precipitation changes (discharge), and sediment character.

35. Absolute vs. Relative Base-level
Absolute (eustatic) base-level fluctuations occur on a global scale and
are controlled by major climatic or tectonic events, such as glacial
episodes and inflation/deflation along spreading centers. Relative base-
level fluctuations include both eustatic and local signals. Major local
factors include basin subsidence and sedimentation rates.

36. Accommodation
Sediment accumulation is limited by the amount of
space available below base-level. This is referred to
as accommodation space. The amount of
accommodation is determined by a combination of
eustatic sea-level fluctuation and basin subsidence
and can be either positive or negative. The rate and
direction of accommodation change has a strong
impact on the nature and geometry of sedimentary
packages.

37. Preservation Potential
Preservation potential refers to the likelihood of a sedimentary
deposit to both accumulate and to be saved as part of the
stratigraphic record. Preservation is affected by depositional
environment, position in the graded profile, tectonic setting, and
phase within a eustatic cycle.

38. 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.

39. Sediment Supply
Transgressions and regressions can be highly influenced by changes in
sediment supply, which, in turn, can be impacted by source area uplift
or denudation, changes in provenance, and climatic fluctuations.

40. Cyclicity (cyclothems/sequences)
It has long been recognized that stratigraphic successions occur as
cyclical packages. The term “cyclothem” was coined to describe
these successions in the Paleozoic of the US Midwest and later
applied to Cretaceous strata of the Western Interior Basin. The term
has now been replaced, for the most part, with the term “sequence.”
Midwest Paleozoic cyclothem
Cretacous Western Interior cycle
Stratigraphic sequences

41. Eustatic Cycles
Global sea-level fluctuation occurs on a variety of scales (orders)
related to tectonic and climatic processes.
1st
: ~50 to 200+ Ma
2nd
: ~5 to 50 Ma
3rd
: ~0.2 to 5 Ma
4th
: ~100 to 200 Ka
5th
: ~10 to 100 ka

42. 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).
• Laminations: Floods (floodplain), waves (beaches), tides (tidal flats), etc.
• Beds: Floods (channel), paleosols (floodplain), storms (marine), turbidites, etc.
• Bed sets: Base-level fluctuations, short-term climatic variations, tectonic movement
• Member/formation: Plate migration, long-term climate change, major base-level fluctuation
Bedding scale

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

44. Photo by W. W. Little
Walther’s Law & Delta Progradation

45. 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.

47. Terminology
• Sea-level rise and fall refers to the vertical movement of the sea
surface.
• Transgression and regression relate to the horizontal movement of
the shoreline.
• Progradation, aggradation, and retrogradation have to do with
lateral shoreline movement as related to filling of a basin.

48. Cyclical Stacking Patterns
(parasequence sets)
Sedimentary pulses are
progradational. However,
depending upon the overall
trend of relative base-level
movement, stacking patterns
(parasequence sets) can be
progradational (building
seaward), aggradational
(building vertically), or
retrogradational, (building
landward).