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Allostratigraphy/Sequence Stratigraphy• Subdivision of the stratigraphic record otbo

bounding discontinuities • Bounding discontinuities define Depositional

Sequences– relatively conformable successions of genetically

related strata bounded by unconformity or correlative conformities

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Utility of Depositional Sequences

• If bounding discontinuities are produced by isochronous events (at least basin-wide) then these material (or non-material) features can be used for correlation since they are isochronous.

• How are sequence bounding unconformities recognized and correlated in different places?

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Discontinuities in the Stratigraphic Record that Define Allostratigraphic Units

• Stratigraphic contacts– Concordant:

– Strata is parallel on either side of contact

• Continuous deposition: – Gradational, intercalated, abrupt

• Diastem: – Small scale, obscure discontinuity in

sediment accumulation caused by an hiatus (time interval without deposition)

• Paraconformity: – No physical evidence of diastem but with significant geological time missing

• Disconformity: – Bedding is parallel on either side of an irregular surface produced by (subaerial) exposure and

erosion

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Aggradation, Progradation, and Retrogradation

• Stacking patterns of sedimentary basin fill due to relative sea level and sediment input influence on accommodation– Aggradation– Progradation– Retrogradation

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Aggradation

• Sediment input ~ accommodation– Not common

for long periods

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Progradation/Regression

• Sediment input > accommodation– Facies

prograde and shift offshore

• Regression

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Forced Regression

• Rapid Relative Sea Level Fall– “Negative

accommodation”– Facies shift

basinward– Erosion occurs in

landward areas

• Regression and Unconformity Surface

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Transgression/Retrogradation

• Relative Sea Level Rise– Accommodation

created in excess of sediment input

– Facies shift landward

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Effects of Changing Accommodation on the

Stratigraphic Record• Transgression/Regression

– Water depth/shoreline changes interpreted from vertical changes in grain size (CUS/FUS) or sedimentary facies in local sections

• Onlap/Offlap– Landward/basinward shift

in shoreline due to relative sea level change

– The fundamental geometric aspect of the sedimentary record upon which relative sea level change is interpreted

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Wheeler DiagramsTime/Space relationships of

Unconformity• Space/Space

Stratigraphic Cross Section

• Time/Space Chronostratigraphic Chart

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Allostratigraphy and Lithostratigraphy

• Lithostratigraphy maps sedimentary rocks solely on the basis of their lithology and does not necessarily consider that these rocks may have accumulated over different periods of time (diachronous lithostratigraphic units)– “Lithostratigraphy” often ignores significant breaks in the

sedimentary section, including those caused by unconformities, omission surfaces, ravinement surfaces, and flooding surfaces

Dunbar and Rodger, 1957;copied directly from Prothero and Schwab, 1996

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Allostratigraphy and Lithostratigraphy

• Allostratigraphy maps rock units on the basis of the timing of their accumulation– Allostratigraphy (sequence stratigraphy) uses a framework

based on surfaces of erosion and non-deposition (sequence boundaries), and flooding (transgressive surfaces and/or maximum flooding surfaces [mfs]) that can be recognized in 2-D and 3-D seismic, well log data, and outcrops

Dunbar and Rodger, 1957;copied directly from Prothero and Schwab, 1996

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In Fichter and Poche, 2001

After Mitchum and Van Wagoner, 1990

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Cycles of Relative Sea Level

Change

Fichter and Poche, 2001

Nichols, 1999

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Depositional Sequences• Relatively Conformable• Genetically Related (abide

by Walther's Law)• Bounded by Unconformity

or Correlative Conformity– isochronous with respect

to unconformity

• Depositional Sequences are Chronostratigraphic Units – the defining Sequence Boundary

defines older (below) and younger (above) strata

• Chronostratigraphic Units – geological time significant

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Depositional Systems Tracts• Use of Depositional Sequence concepts to

predict stratal geometry– Three dimensional

assemblage of sedimentary facies genetically linked by their origin in related depositional environments

– Basically Waltherian World

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Low Stand Systems Tract• Forced Regression• Subaerial exposure and formation of a

Sequence Boundary• Offlap • Basinward shift in facies belts

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Transgressive Systems Tract• Retrogradation • Flooding of the shelf and onlap• Landward shift in facies belts

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Maximum Flooding SurfaceCondensed Section

• Between TST and HST (highstand systems tract)• Represented by a change from retrogradation to

aggradation/progradation• The surface or thin succession characterized by evidence

for slow rates of sedimentation

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Highstand Systems Tract

• Progradation (accommodation < sediment input)

• Regressive successions• Onlap and offlap/down-lap

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Major Sequence Stratigraphic Major Sequence Stratigraphic and Chronostratigraphic and Chronostratigraphic

SurfacesSurfaces

Kendall, USC Sequence Stratigraphy www site

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Stratigraphic Expression of Relative

Sea Level Cycles• The Sequence Model

– A tool for predicting stratigraphic architecture

– Methodology for basin wide correlation

– May be used for interregional correlation through recognition of Eustatic Cycles

Long Term C

ycles

Short T

erm C

ycle

s