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

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

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

Sedimentary layers become progressively younger upward through a stratigraphic sequence.

Principle of Superposition

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

Tilting of Beds by Folding

Tilting of Beds by Faulting

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

Bed Termination by Faulting

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.

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

AngularUnconformity

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

Disconformities

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

Photo by W. W. Little

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

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.

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

Fossils are found in a consistent and predictable succession everywhere on earth.

Principle of Faunal (biological) Succession

Geologic Time Scale

Incomplete Record

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

Unconformities

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

Photo by W. W. Little

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.

• 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

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.

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.

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.

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.

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.

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.

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

Eustatic Cycles

Global sea-level fluctuation occurs on a variety of scales (orders) related to tectonic and climatic processes.

1st: ~50 to 200+ Ma2nd: ~5 to 50 Ma3rd: ~0.2 to 5 Ma4th: ~100 to 200 Ka5th: ~10 to 100 ka

Photo by W. W. Little

Nested EventsA 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

Walther’s LawIn 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

Photo by W. W. Little

Walther’s Law & Delta Progradation

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.

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.

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).