20b_G435.pps 1 Practical Sequence Stratigraphy Applications of Sequence Theory to Interpretation of the Stratigraphic Record.

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Practical Sequence Stratigraphy

Applications of Sequence Theory to Interpretation of the Stratigraphic

Record

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Some Definitions and Review of Key Concepts

• Depositional Environment– (Geographic) Place on the earth’s surface today (or in the geologic past) where sediment

is/was deposited

• Sedimentary Facies– A rock unit with distinct lithologic “aspect” produced by physical, biological, and

chemical processes in some depositional environment

• Facies Associations– spatial associations of genetically related sedimentary facies reflecting some

depositional environment in the stratigraphic record – Facies Sequences are distinct vertical successions of facies reflecting some depositional

environment in the stratigraphic record

• Depositional System– Three dimensional association of sedimentary facies representing the stratigraphic

record of a particular depositional environment– Basically a facies association

• Depositional Systems Tract– Three dimensional assemblage of sedimentary strata deposited in genetically related

depositional environments during a limited period of geological time, associated depositional systems.

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Some Definitions• Sequence Stratigraphy [Also see USC Sequence Stratigraphy Web]

– Sequence Stratigraphy is an approach to the study of time, space, and lithologic relationships in the stratigraphic record that uses cycles of relative sea level change and accommodation space in sedimentary basins to explain and predict stratal geometry. (This term is tied to the version of allostratigraphy devised by the Exxon Production Research Group)

– This approach emphasizes the recognition of key stratal surfaces, • their relationship to accommodation space

• their chronostratigraphic significance

• their predictive utility

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Some Definitions and Review of Key Concepts

• Allostratigraphy– Subdivision of the stratigraphic record otbo unconformities, their correlative

conformities, and other key stratal surfaces; – These surfaces are allogenic (generated by processes essentially external to the site of

sedimentation) and are chronostratigraphically (provide information on relative geologic age) significant

“In any given basin, or part of a basin, the effect on accommodationspace of base level changes will exert a fundamental control on stratigraphy. The effects of these changes can be used as a wayof analyzing the succession of sedimentary strata. The utility ofthe allostratigraphic approach is unquestionable regardless of ourinability to clearly establish the synchroneity of global eustaticchange with local, relative sea level change.”

Nichols, 1999

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Some Definitions• Depositional Sequence

– A succession of relatively conformable, genetically related strata bounded by unconformities (and their correlative conformities). A subdivision of the rock record.

– Depositional sequences are inferred to form through one cycle of relative sea level fall, rise and subsequent fall on a time frame between 1 to 10 my (third order cycles of relative sea level change)

– Depositional sequences have a predictable internal structure consisting of major key stratal surfaces and depositional systems tracts

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Some Definitions• Depositional Systems Tracts

– The components of depositional sequences formed during different portions of a cycle of relative sea level change representing genetically associated stratigraphic units

• Lowstand Systems Tract

• Transgressive Systems Tract

• Highstand Systems Tract

Internal “anatomy” of depositional systems tracts

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Some Definitions• Major Stratal Surfaces

– sequence boundaries (erosional unconformity; disconformity)

– transgressive surfaces (ravinement surfaces)

– maximum flooding surfaces (condensed section; paraconformities)

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Shelf-Break versus Ramp Clastics-Dominated Continental Margin

8

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variable

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Basic Depositional Sequence/Systems Tract Model for a Generic, Clastics-Dominated Continental Margin

10

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Scales of Sequence Stratigraphy

• Depositional Sequences were defined using seismic data (resolution 10’s to 100’s of meters): Low Resolution Sequence Stratigraphy

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High Resolution Sequence Stratigraphy

• Integration of Log, Core, and/or Outcrop scale observations: Parasequences– relatively conformable

successions of genetically related beds/bedsets bounded by marine flooding surfaces (and their correlative stratal surfaces)

– generally consist of upward coarsening/shoaling successions bounded by thin, upward deepening units

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Some Definitions• Parasequences

– genetically related, progradational, shoaling or shallowing upwards bed sets bounded at the top (and base) by a (smaller scale time and amplitude) marine flooding surface. A subdivision of the rock record.

– Shoaling upwards packages of sediment related to one idealized cycle of relative sea level rise and fall on a time frame between 0.1 to 1 my (fourth or fifth order {“higher”} order) cycles of relative sea level change or variations in sediment input due to autocyclic processes)

– Chronostratigraphic significance of flooding “stratal” surfaces, best represented in shore line to shelf successions

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Some Definitions• Internal stratigraphic Architecture and Stacking patterns

– Retrogradational stacking patterns

– Aggradational stacking patterns

– Progradational Stacking patterns

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Hierarchy of Sedimentary Sequences

Rise

Fall

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Sequence StratigraphySequence Stratigraphy First Principles and ControlsFirst Principles and Controls

• Implication of high resolution sequence concepts to aquifer/hydrocarbon reservoir correlation

Retrogradational System Progradational System

Van Wagoner, et al., 1990

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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