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Stratigraphic ConceptsDemonstrating equivalency (genetic/time)

between strata (rock units)• lithostratigraphy - organization of strata on the basis of

their lithologic characteristics• biostratigraphy - organization of strata on the basis of the

fossils they contain• magnetostratigraphy - organization of strata on the basis of

their magnetic characteristics• chemostratigraphy - organization of strata on the basis of

their isotopic characteristics• seismic stratigraphy - organization of strata on the basis of

their seismic characteristics

• chronostratigraphy - time relationships• sequence stratigraphy - depositional sequences, packages of

strata bounded by unconformities

San BenitoGravels

• Age?• Sedimentation

Rate?S.R.=∆d/∆t

• Episodic deposit.?– pause– erosion ∆d=?

∆t=?

Unconformity?

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Absolute vs. Relative Time• Absolute time - set within the framework of

geologic time– essential for reconstructing tectonic history,

paleoclimates, etc.• Relative time - time represented by the outcrop

– essential for computing accumulation rates, etc.time

depth

time

depth

∆t

∆d

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“Timing is everything”

• Chronology (stratigraphy) is essential forunderstanding all earth history & henceearth system processes,– tectonic– climatic– biologic (evolution).

Lithostratigraphy

LITHOSTRATIGRAPHIC UNITS -bodies of rocks distinguished on thebasis of observable lithologiccharacteristics– no connotation of age other than

the law of superposition– separated by contacts– Stratotype - type section (most

complete)

the study & organization of strata on the basis oflithologic characteristics

• lithology - type, color, mineral composition, andgrain size

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Fundamental Units*:

• Group - collection of formations• Formation- lithologically distinctive unit

that is large enough in scale to be mapable(single lithology or regular alternation oflithologies)– e.g. Monterey Formation

• Member - (subdivision of a formation)characteristics that distinguish it from otherparts of the formation– e.g. series of phosphatic-rich layers interfingering

with dolomites/cherts• Beds - subdivision of a member, smallest

unit*Only applied to land-based sedimentary

sections

Gro

up I

C

B

C

Lith

olog

y, m

embe

rs?

form

atio

n

Paleocene-Eocene Strata: Gulf & Atlantic Coast

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Paleocene-Eocene Strata: Gulf & Atlantic Coast

Wilson Lake

Clayton

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Paleocene-Eocene Strata: Gulf & Atlantic CoastClayton Wilson LakeGl-91

Butano SS

San Lorenzo

Monterey

Vaqueros

Santa Cruz MS

Purisma

Locatelli

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Contactsboundaries between units1. plane or irregular surfaces2. Conformable or unconformable

Site

126

3A -

13H

- S6

- 50

.5 c

m

Paleocene

Eocene

• Conformity (Conformable) -no physical evidence of non-deposition– Abrupt– Gradational

• progressive - gradation• intercalated - gradation is an

inter-bedded interval

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Contacts• Unconformity (unconformable) - break or hiatus in

deposition (erosional, non-deposition)current scour surface, or sub-aerial weathering surface, slump or

slide surfaces– Angular - younger sediments atop eroded surface of

tilted or folded rocks– Disconformity - parallel bedding planes, but erosional

surface (channeled, paleosols, lag-gravel deposit) -uplift, sea-level regression

• lithology may change– Paraconformity - same lithology above and below, non

deposition or dissolution.• Can only be recognized by other stratigraphic techniques

• Nonconformity - sedimentary / igneous or metamorphicrock

Angular UC

Disconformity

Nonconformity

?

?

?

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Recognition of Unconformities

• basal conglomerates• deeply weathered soils horizons• truncated bedding• clasts• burrowing or hardgrounds• channel deposits• truncated fossil ranges (several lineages)

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Lateral Contacts• pinch-out - progressive thinning of a bed• intertonguing - lateral splitting of units that

pinch out independently– shoreline migrating back and forth

• progressive lateral gradation

Stratigraphic completeness:hiatuses (diastems) - abundant in the rock record -

– more frequent in high energy environments• Sedimentation rates high, but episodic (erosion common)

• continental– non-deposition and erosion - uplift– episodic deposition - flooding

gap

gaptimedepth• shallow marine

– Erosion - regressions (local, global)– non-deposition, wave erosion– deposition during storms

• hemi-pelagic– slumps, turbidites represent episodic

deposition• pelagic

– sedimentation tends to be lower, but morecontinuous

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ODP Site 1207,northwest Pacific

• Drilling Objectives:– P/E Boundary– K/T Boundary– Aptian/Albian

ODP Site 1207, northwest Pacific

Age/Depth

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Facies Relationships in Space and TimePrograding Delta

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Vertical and Lateral Successions of Strata:

• conformable and unconformable contacts dividesedimentary rocks into vertical successions.Walther’s law - to be conformable, vertically

adjacent facies must reflect those facies whichoccur side by side

• facies - a body of rock with some consistentcharacteristic

• lithofacies - a consistent lithologic characteristicwithin a formation (shale facies, evaporite facies)

Walther’s Law

Deepening upward Shallowing upward

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Cyclic successionsrhythmic sedimentation

(repetitions of strata)• Temporal scales

annual to m.y.• All environments

pelagic - limestone/marldelta - repeated

coarsening upwardcycles

Paleocene Scalia Rosa, Dolomites, Italy

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Cycle Order (Scales)

Cycle OrderMajor sedimentary cycle durations as influenced by

eustatic sea level changes

Milankovitch glacioeustatic cycles0.01-0.25th

Milankovitch glacioeustatic cycles0.2-0.54th

Mid-ocean ridge spreadingchanges - volume

1-103rd

Mid-ocean ridge spreadingchanges - volume

10-1002nd

Tectonic: formation and breakupof supercontinents

200-4001st

CauseDuration (m.y.)Type

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Cyclic successions

Causal Mechanisms:• Autocyclic - internal to the

basinswitching delta lobe, storm

beds, floodplain– beds show limited lateral

continuity• Allocyclic - mainly external

to the basinclimate, sealevel, tectonic

movements in source area– beds may show extensive

lateral continuity

Flood Plain Deposits,Paleocene-Eocene Bighorn Basin, WY

U. Carboniferous, S Wales

Allocyclic Mechanisms

Milankovitch cycles -oscillations in earth's orbit

primary periods:• 19, 23 ky - precession of

the pole (wobble)• 41ky - obliquity (tilt)• 100, 410 ky - eccentricity

Perihelion -147x106kmAphelion - 152x106km

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Orbital cycles:Effect on Insolation• Eccentricity 95, 100, 120, 413 ky (2.3

m.y.)– Earth - sun distance 0.0 to 0.06Effect on insolation: ca. 0.7 W/m2

uniform across latitudes• Tilt 41 ky (29, 54 ky,1.25 m.y.)

– angle ~ 22.0-24.3°hotter summers / colder winters in both

hemispheresEffect on insolation: up to 17 W/m2 at high

latitudes• Precession 19, 23 ky

– wobble - gravitational pull of sun on earths’equatorial bulge

– elliptical precessionhot summers/cold winters in one hemisphere,

and cold summers/hot winters on the other.Effect on insolation: up to 40 W/m2

Ice-sheets (18 kya, present day)

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Sea-level Change, 150 kya to present

Sea Level

120 meters

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100 m

90 m

~10 km

Transgressions/RegressionsTransgressions - shoreline moves landwardRegressions - shoreline moves seaward3 Causes:

1. Sea level - rise and fall2. Tectonic - uplift /subsidence3. Sediment Supply

• eustatic changes– ice-volume (glacioeustatic), basin geometryGlobal signal

• relative changes– Local subsidence/uplift or sediment supplyRegional signal

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Sea level Rise

• Transgression– Sea level rise w/ no change in sediment supply

• Stationary– Sea level rise w/ balanced by sediment supply

• Regression– Sea level rise w/ large increase in sediment supply

• all three produce coastal onlap because sea level is rising

transgression

fining upward sequence

sandmudsilt

time lines

coastal onlap

• Standstill of sealevel - no coastal onlap, but top-lap– regression

coastal onlap

Sea level rising (high sediment supply)

erosional surface

regression

Top lap Regression

Sea level stationary

no onlap

• Rising sealevel - coastal onlap,– regression

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Lithofacies are time transgressive

Section A Section BSection C

Coast Basin

Sea level Rise

Transgression (Deepnening)Regression (Shallowing)

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Asymmetry of Transgressive andRegressive Cycles:

• Transgressive - classic finingupward?– Rare- fining upward less common

than coarsening upward• Rapid rise in SL - erosion/non-

deposit during transgressive phase– coastal and shallow marine deposits -

thin or non-existent– deposition mainly during regressive

phases– e.g. delta progradation -

Carboniferous Cyclothems

non-marine marinenon-marine marine

transgression

regression(rapid)

transgression

regression

Delta Progradation

Rapid transgression

coal

shale(marine)

limestoneshaly

limestone

shale (sandy)channel sand

disconfomity

Carboniferous (299 to 359 Mya)

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Carboniferous(299 to 359 Mya)

Carboniferous Coals, West Virginia

Cyclothems:Allocyclic vs. Autocyclic

Correlation of Lithostratigraphic Units• Lateral Tracing

• most direct method• only where strata are continuously

exposed• Lithologic Similarity and Stratigraphic

position• indirect method• correlation based on facies sequence• difficult to apply to cyclic successions

• Event Stratigraphy• Marker beds• ash (bentonites)

(e.g. Bishop Tuff, Long Valley Caldera; 740ky)

• lava flows

lithologicsimilarity

Key bed(ash)

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Correlation of Lithostratigraphic UnitsBishop Tuff• Long Valley Caldera• 740 ky

Lithologic Similarity and Stratigraphic position