Stratigraphy

Concepts Related to Subdivision of the Rock Record

Basin Analysis

• Use stratigraphic methods to work out sequence and timing of deposition of rocks– usually sedimentary

• Synthesis of data from multiple disciplines– sedimentology to determine environment of deposition

– paleontology to get time

• Used to be main objective– Petroleum industry

Modern Stratigraphy

• Still focuses on synthesis • Has evolved to include many new techniques +

deep marine realm• many of the advances are driven by need to have

very precise age control

• Can resolve:» events in some section to the order of < 20 ky

» patterns of events within a basin (regional)

» patterns by correlating between basins (need more than 1 data point) (global)

Stratigraphy

• Initially used informally• Now, governed by

– North American Stratigraphic Code

– International Stratrigraphic Code

History of Stratigraphy

• Based on idea that strata (layers of sediment) are deposited one on top of another

• oldest at bottom and youngest at top, over long period of time (ky - my) (1000s to 1,000,000s of years)

• Relatively recent concept

History of Stratigraphy

• Greeks and Romans– Recognized world was ever changing and thought of it

as eternal

• Fall of Rome and Rise of Church– Developed idea of world based on literal interpretation

of bible• Attributed variations to Noah’s flood

– e.g., seashells high in Italian Appenines (near Florence)» Either Noah’s flood brought them or God’s mystery

History of Stratigraphy

• Renaissance and age of Reason– Began to move away from extreme views and

began to develop scientific principles– Da Vinci (1452- 1519)

• Suggested rivers had carried the shells down from the Alps

• Mix of mud and fossils turned to stone, uplifted

wrong, but significant in that it showed beginning of change in thought

Da Vinci’s Insight

• In 1500 C.E. Leonardo Da Vinci recognized that fossil shells in the layered rocks represented ancient marine life

• Observed that many fossil rich layers were separated by unfossiliferous layers thereby repudiating the concept of one flood

• Had the idea that seasonal events responsible

History of Stratigraphy

• Nicolaus Steno– Niels Stensen (1638-1686)

• famous for Stenos’ laws

– Worked near Tuscany• First to suggest IN PRINT that rocks enclosing fossils had at

one time been soft• Sharks’ teeth convinced him

– Noticed living sharks had teeth identical to ‘tonguestones’ – Petrified tongues of dragons and snakes

• Realized shark’s teeth were deposited in mud, which encased the teeth and then hardened

History of Stratigraphy

• Steno’s Laws– Original horizontality– Original continuity– Superposition

• Provides set of rules for organizing or ordering strata into relative ages

• Idea caught on but was derailed by the Neptunians

History of Stratigraphy

• Neptunians• Arduino

– Ordered same Tuscan rocks as Steno into 4 groups

– Primitive (Schists, basalts, granite cores of mountains)

– Secondary (ls, shale; fossiliferous sedimentary rocks)

– Tertiary fossiliferous sedimentary rocks found in hills• Not universally applicable!

History of Stratigraphy

– Werner • Rocks form from flood and then precipitated in flood

– (goofy, but not that unreasonable!)

» Primitive

» Transition

» Stratified

» Alluvial

History of Stratigraphy• Plutonists

– Fought for fiery, volcanic rock– Hutton

• Father of Geology! • Uniformitarianism

– No begninning and no end– Saw Earth as an engine – 1795- published the Theory of the Earth– “anti- church”; not easy to read

– Lyell• Took extreme view to defend plutonisms against neptunists• Actualism (uniform natural laws and processes)• Gradualism (uniform RATES of change)

History of Stratigraphy• William Smith

– Engineer; surveyor (late 1700’s)– Analyzed rocks exposed in mines, canals of England– Realized rock units had distinct fossils and could use them to

recognize rock units, and named them• Law (Principle of ) Faunal Succession

– Can divide rock record based on fossils

• Cuvier– 1769 – 1832– Also recognized faunal succession but didn’t use it like

Smith for correlation• (Focused on Paris Basin)

William Smith, Father of Stratigraphy

• The Map that Changed the World, Simon Winchester

• 1796 wrote “wonderful order and regularity with which nature has disposed of these singular productions [fossils] and assigned to each its class and peculiar stratum”

• 1815 Publication of the 1st geologic map of England intended for the development of canals, quarries and mines as well as natural resources

William Smith’s "Strata Identified by Organized Fossils"

• Soon after the first issue of his great geological map of England in 1815, William Smith published the Strata Identified by Organized Fossils.

• It was intended as a kind of geological users manual with illustrations to identify fossils.

• But Smith’s work went beyond the mere illustration of fossils. Smith had deciphered the hieroglyphics of nature-the distinctive inscriptions borne by the different strata. With the Strata Identified . . . and its colored plates in hand, anyone would be able to compare the plates with fossils collected in the field and immediately identify the strata from which they came.

• The strata once identified, their place in the orderly succession of the strata-which lay above and which lay below, as Smith had determined it - was then known. All this, Smith wrote, "without the necessity of deep reading, or the previous acquirement of difficult arts."

Georges Cuvier and Alexandre Brongniart- Paris Basin

• Development of geologic map of Paris Basin• The strata of the Paris Basin were close to

horizontal. • As of 1811, Cuvier and Brongniart employed

fossils but only in the few instances where more obvious evidences of sequence were absent.

• The title of their work was Géographie Minéralogique by which they meant the distribution of what Werner had called the "external" characteristics of the mineral and fossil contents, shapes, colors, and textures of the strata within the Paris basin.

• Today call this lithology. They determined the order of the strata from their superposition, their lithology and by tracing them across the basin

• Cuvier firmly established the fact of the extinction of past lifeforms

History of Stratigraphy

• Time Scale– A way of formally organizing rocks and time

• Artificial division of time• Evolved as workers began to apply scientific principles to the

study of rocks

• Smith, Cuvier, others– Named rocks– Rock names became basis for time scale

GEOLOGIC TIME SCALE

Subdivision of the Rock Record

• Lithostratigraphy – – Study of the physical relationship among rock

units; • No time connotation other than superposition• Physical properties and stratigraphic position relative

to other lithostratigraphic units

• Chronostratigraphy – • Integrated approach to establishing the time

relationships among geologic units

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 using 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

Stratigraphic Correlation

Types of Stratigraphy

• Each has separate role– Ultimate goal is integration and better

interpretation (more refined)

• Additional divisions of stratigraphy– Stratigraphic Code– Divided into sub-disciplines based on physical

limits or geologic age

Lithostratigraphy

• Definition– Study of the sequence and interpretation of

layered rock sequences, based on PHYSICAL CHARACTERISTICS (lithology) of the rocks

• Essentially, study of TIME

• Allows us to ORDER events!

Lithostratigraphy

• 2 uses of term lithology– Study and description of physical

characteristics of rocks • Especially hand specimen and outcrop

– e.g., investigate the lithology

– Term that refers to these physical characteristics

– e.g., lithology of a stratigraphy unit- such as sandstone

Lithostratigraphy

• Divide rocks on basis of lithology– Stratotype

• Designated type unit of readily accessible rocks in natural outcrops, mines or bore holes.

– Type section

• Formal designation

– Lithosome• Masses of rock with uniform character

– Dissolve away all other rock Sloss (1963)

• Informal designation

Lithostratigraphy

• Some generalizations about lithostratigraphic units– Sedimentary, extrusive igneous, meta-sedimentary,

meta-volcanic

– Generally conforms to law of superposition

– Commonly stratified (layered) and tabular in form

– Identification is based on observable rock characteristics

– Boundaries can be either sharp and clear, or gradational

Uses of Lithostratigraphy

• What lithostratigraphy can do for you– Place unit in geologic framework– Establish stratigraphic relationship with units

above and below– Aid in correlation between other

LITHOLOGIC units

• What it CAN’T do for you– Denote time

Units of Lithostratigraphy

• Lithostratigraphic classification and nomenclature– System for standardization

• Fundamental unit– Formation

• Lithologically distinctive stratigraphic unit that is large enough in scale to be mappable at the surface or traceable in the subsurface.

• May be just 1 lithosome or multiple

• Subdivision of formation

Lithostratigraphy

• Other formal lithostratigraphic units– Smaller than a formation

• Member– Subdivision of a formation

• Bed– Smallest formal lithostratigraphic unit

– Collections of formations• Group

– 2 or more adjacent formations that have some unifying lithological and/ or genetic features

• Supergroup– Groups of groups

» e.g., Newark Supergroup

Stratigraphic Contacts

• Contacts– Plane or irregular surfaces between different

types of rocks– Separate units

• Conformable

• Unconformable

Types of Contacts

• Conformable boundaries– Conformable strata form unbroken depositional sequences

• Layers are deposited by ~ uninterrupted deposition

– Abrupt or gradational• Abrupt• Sudden distinctive changes in lithology • Often, local change

– Gradational• Gradual change in depositional conditions with time progressive

gradual contact• One lithology grades into another

– e.g., ss becomes finer upsection until it becomes a siltstone

Types of Contacts

– Intercalated contact• Increasing number of interbeds of another lithology

– Laterally adjacent contacts– Pinch-outs – Inter-tonguing– Progressive lateral gradation

Geometry of Facies/Lithostratigraphic Relationships

Unconformable• Break in deposition

– Strata do not succeed 1 another as a continuous whole• Separated by an unconformity• Lack of continuity in deposition

• Unconformity: – Surface of erosion or non- deposition separating younger strata from older

rocks– Represents a significant hiatus

• Diastem– Minor interruption in depositional conditions

• Interpretation– Period of non-deposition, weathering or erosion– Such processes may occur sub-aerially (exposed to air) or sub-aqueously

(under water)

Unconformities

• Types of unconformities – Angular unconformity– Nonconformity– Disconformity

• Erosional surface between // units

– Paraconformity (obscure)- • // units with no obvious erosional surface b/n the units

• use obscure traits to identify it

Unconformity

Following Stratigraphic Code

Unconformities

• Hiatus– Total interval of geologic time represented by

missing strata • At a specific position along a stratigraphic sequence

• Don’t confuse syndepositional variations with post-depositional alteration of beds– Groundwater changes color (Fe) or resistance

(cementation)

Vertical/lateral variations in rock• Recognition of lateral variation

– Because rocks are not constant through space, you get a variation in lithology within one unit.

• Initially people didn’t recognize this-

– Thought Noah’s flood left layer cake layers of strata that extended to ends of Earth without change in lithology or thickness

• Began to trace beds – walked them – looked at how the rocks between the boundaries changed– realized fallacy of idea

– We retain layer cake to describe stacking pattern; not infinite nature

Walther’s Law of Correlation of Facies

• Relationship between vertical and lateral variations

• The fact that there is lateral variation in facies leads to vertical variation in facies

– Walther’s Law of Correlation of Facies

• Lateral variations are expressed in the vertical due to the succession of facies

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.”

Walther’s Law: Transgression-Regression

– Transgression• Landward movement of shoreline (progessive deepening)

– Stand on beach

– Over time, you would be under water as shoreline moved landward

– Regression• Seaward movement of shoreline

– (progessive shallowing)

– Results in lateral and vertical changes

Transgression • Geometric relationship of "graded, shore parallel facies

belts“– Fining Upwards Sequence:FUS

– More basin-ward facies overlie more landward facies• Compared to depositional systems models

Transgression and Regression

• Shallowing upwards, shoreline moves basinward through time--> Regression– sea level drop +/- uplift +/- sediment supply

• Progradation – 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: basinward shift in sedimentary

facies» Strata across lithologic boundaries NOT in accordance with Walther’s

law

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

Transgression - Regression

• What drives transgression/ regression?• can’t tell from this information!

– sea level change has so many components– relative = local– eustatic = global– sediment supply

» can drive a regression/ transgression

• ONLY KNOW that shoreline has shifted position– multiple factors responsible for sea- level change

– Say sea level rise or fall and you are WRONG!– Say transgression or regression!

Causes of Sea Level Change

Relative Change

Eustatic Change

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

Trans

gres

sion

and

Regre

ssio

n/

Relativ

e Sea

Lev

el

Preservation potential of rocks that are deposited

• Majority of sediments in fossil record – marine

• Most sub-aereal environments – erosional

• WHY?– Accommodation space!

• Space available controls accumulation– no place to put it, then no deposition– base level

» balance between erosion and deposition

• Base level of erosion– level below which erosion cannot occur ( = sea level)

• not strictly true! – you do get erosion below sl.

» storms = erosion on shelf» downcurtting = canyons on slopes» deep sea currents = erosion of sea floor

• base level of aggradation– level above which sediments cannot accumulate

permanently• most marine sediments dominate record• rise in sea level tends to result in increased preservation of seds

– high rate of erosion explains why there are “more gaps than record” in geol. record.

Correlation

• Lithostratigraphic Correlation– Match

• match lithologies

– Correlate• link units of similar lithology and stratigraphic position

• PROCEDURE• identify environments of deposition

– (use simple depositional model)

• draw lines of correlation between adjacent facies

• make interpretation- trans/ regression/ no change

Actualism and "Genetic Stratigraphy"

• Recognition of Uniformitarianism– the relationship between modern processes of

sedimentation and the rock record

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)

Diachronous Stratigraphic Units

Diachronous Stratigraphic Units

Basement