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Stratigraphy

Concepts Related to Subdivision of the Rock

Record

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Inextricably Related Issues in Geology

(that must be distinguished!)• Material Units: The Rock Record

– Have physical extent and consist of tangible material

– The material record of events that have occurred during the passage of geological time

• Geological Time: The Abstraction of Time Since the Advent of the Earth– Measured in years before present– Divided into discrete intervals

• Ma; millions of years before present• Ga; billions of years before present

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Subdivision of the Rock Record

• Lithostratigraphy – – study of the physical relationship

among rock units; • no time connotation other than

superposition• Otbo physical properties and stratigraphic

position relative to other lithostrat units

• Chronostratigraphy – • integrated approach to establishing the

time relationships among geologic units

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Chronostratigraphy:Integrated Approach to Establishing the

Time Relationships Among Geologic Units

• Biostratigraphy– Study of the fossil record with emphasis on faunal

succession to establish relative time relationships– The correlation web

• Magnetostratigraphy– Study of the magnetic properties of rock units for the

purpose of correlation otbo magnetic polarity reversals

• Allostratigraphy– Study of rock units defined by unconformities and other

features generated by base level change

• Geochronology– various techniques, especially isotope geochemistry, to

establish the absolute age of rock units

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M o r e L a t e r

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Stratigraphic Correlation

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Lithostratigraphy

• Formations, Members, Groups, etc.– Material Units and "Classical Layer Cake Stratigraphy“– catastrophism: continuous layering = time equivalence – NO!

• Further organization and subdivision of the rock record otbo– Relative age

• Superposition, unconformities, cross cutting relationships, included fragments

– Original Horizontality– Lateral Continuity

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Objective Subdivision of the Stratigraphic Record Into Distinct Lithostratigraphic Units

• Formations – with a type section, geographic or lithologic name,

and definition based on• limited and distinctive lithologic variability• consistent stratigraphic context• “extensive” map distribution in the surface or subsurface

• Groups and Supergroups– Are formations lumped otbo stratigraphic

association

• Members and beds – Subdivisions of Formations – lithostrat units with less areal extent defined as it

is useful

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Actualism and "Genetic Stratigraphy"

• Recognition of Uniformitarianism– the relationship between modern

processes of sedimentation and the rock record

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Actualism and "Genetic Stratigraphy"

• Sediments with distinct lithologic aspect are deposited in only limited areas in a given time period.

• This results from the limited lateral extent of contiguous depositional environments.

• Distinct depositional environments migrate in space, through time so that lithofacies units are inherently diachronous (variable in age throughout)

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Diachronous Stratigraphic Units

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Diachronous Stratigraphic Units

Basement

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Actualism and "Genetic Stratigraphy"

• Sedimentary (litho)Facies: – lithologic aspect; a result of depositional process not age.

• Lithologically distinct sediment bodies deposited synchronously in adjacent sites (distinct lithofacies) must be separated across some kind of boundary.

• Such contemporaneous and physically associated strata are said to be in Facies Relationship.

• Lithologic aspect in space is not directly equitable to time but rather depositional process at that place at some time.

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Geometry of Facies/Lithostratigraphic Relationships

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Actualism and "Genetic Stratigraphy"

• Depositional Processes -----> Geological Product

Sedimentary Depositional Environments Sedimentary Lithofacies

• Defined by – Lithology– Sedimentary structures (physical, chemical, deformational,

etc)– Geometry: bedding scale and outcrop scale, vertical trends– Paleocurrent indicators– Fossils

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Walther's Law of Correlation of Facies

• “Only those lithofacies which are a product of sedimentary environments found adjacent to one another in the modern can be occur superimposed in continuous, uninterrupted stratigraphic succession.”

Also, lith units in conformable vertical relationship were in lateral relationship during deposition

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Autostratigraphic Variation Autocyclicity

• Lithologic variability, within distinct geomorphic settings, in space and time may be the result of variations in depositional conditions inherent to that depositional environment----->

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External Controls on Stratigraphic

Architecture: Allocyclicity • Variation in lithologic character due to

regional scale variations in Base Level– level at the earth's surface above which

permanent sediment accumulation does not occur

– Closely related to local sea level• Basin Subsidence (tectonic)• Eustacy (global sea level)• Sediment input (climate and

topography/tectonics)

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Unconformity

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Transgression and Regression Geometric relationship of

idealized "graded, shore parallel facies belts“• deepening upwards, shoreline moves

landward through time: Transgression

– Basin Subsidence, +/- Sea Level Rise, +/- Sediment Starvation (With Subsidence)

– May (rarely) leave a distinct material record--->• Marine flooding surfaces• Ravinement surfaces• Landward shift in sedimentary facies up section

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Transgression • Geometric relationship of idealized "graded,

shore parallel facies belts“– Fining Upwards Sequence: FUS– More basin-ward facies overlie more landward

facies• Compared to depositional systems models

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Transgression and Regression

• Shallowing upwards, shoreline moves basinward through time

Regression– sea level drop, +/- uplift, +/- sediment supply

• Progradation; Normal Regression – excess sediment supply relative to accommodation space

• Forced Regression: Relative sea level drop and formation of erosion surfaces: Unconformity (surface of subaerial exposure)

– Soils; kaolinitized, clay-rich layers– Angular discordance with underlying units (disconformity)– Plant remains, rooted zones– Non-genetic stratal relationships: Abrupt Basinward Shift In Sedimentary Facies

Upsection– Strata across lithologic boundaries NOT in accordance with

Walther’s law

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Regression• Geometric relationship of "graded, shore

parallel facies belts“– Coarsening Upwards Sequence: CUS– More landward facies overlie more basin-ward

facies• Compared to depositional systems models

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Causes of Base Level Change

Relative Change

Eustatic Change

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Sea Level Cycles• 1st Order Cycles

– 100’s my– 100’s of meters

• 2nd Order Cycles– 10’s my– 100’s of meters

• 3rd Order Cycles– 1-10 my– 10’s of meters

Falling

Rising

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Transgression/Regression vs. Relative Sea Level

• Relative Sea Level Curves

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