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Week 6

Review of week 5o definition of facies

o geologist use of facies to build depositional systems

o differing uses of facies in practice

Topics for Week 6o sequence stratigraphy

Reading and reference:o Miall, Chapter 6, 6.1 to 6.4

o Class handouts: Embry, A. F., 2002; Catuneanu, O., 2002

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Facies Definition and Use

facies: a body of rock with specified characteristics.

A distinctive rock that forms under certain conditions

of sedimentation, reflecting a particular process or

environment (lithofacies, biofacies, seismic facies).

lithofacies: rock unit defined on the basis of

distinctive lithologic features such as composition,

grain size, sedimentary structures, bedding

characteristics. Each lithofacies represents an

individual depositional event.

lithofacies associations/assemblages: groups of

lithofacies characteristic of particular depositional

environments

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Facies Analysis within Basin Analysis

Goal is to get from facies analysis to a

paleogeographic synthesis depicting an interpretation

of the stratigraphic and geographic evolution of the

basin through time.

make use of large scale basin-fill patterns; these are

“depositional systems”

the difference between facies analysis and analysis of

depositional systems is one of scale

mapping is an essential tool to support the use of

facies analysis in interpretation of depositional

systems and sequences.

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Facies Model: Shoreface Zone

(Einsele, 2000, Fig 3.1)

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Scales of Sedimentological Analysis

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Summary: Facies Analysis

Facies: a distinctive rock that forms under certain

conditions of sedimentation, reflecting a particular

process or environment (Reading, 1986, p. 4)

Facies term can be applied by geologists in differing

contexts:o observation sense for rock product (sandstone facies)

o genetic sense for the products of a process (turbidite facies)

o environmental sense for the environment in which the rock or suite of

mixed rocks was formed (fluvial facies)

o tectonic setting or tectofacies (post-orogenic facies)

In Basin Analysis: facies description are essential for

interpreting depositional systems used in sequence

stratigraphy

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Sequence Stratigraphy: Objectives

Define sequence stratigraphy

Describe key concepts and terms

o What is a sequence?

o Why is describing basin fill in terms of sequences a useful

approach? Relation to seismic stratigraphy

o Development of sequence concept: eustasy, base level,

accomodation space

o Unconformities and their correlative conformities

o The terminology of sequence stratigraphy: bounding

surfaces; systems tracts

o Schematic diagrams illustrating use of concepts of

sequence stratigraphy

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Sequence Stratigraphy for Basin Analysis

(Catuneanu, 2002)

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Lithostratigraphy vs Sequence Stratigraphy

(Willis and Moslow, 1994))Triassic Halfway Formation, western Canada

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Sequence Stratigraphy

A field of study in which basin-filling sedimentary deposits,

called sequences, are interpreted in a framework of eustasy,

sedimentation and subsidence through time in order to

correlate strata and predict the stratigraphy of relatively

unknown areas. Sequences tend to show cyclicity of changes

in relative sea level and widespread unconformities, processes

of sedimentation and sources of sediments, climate and

tectonic activity over time.

Geometry of seismic reflectors very important in unraveling

sequence stratigraphy in a basin

Basinwide unconformities and correlative conformable

surfaces are the framework for describing sequences

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Definition

Sequence stratigraphy consists of the recognition and

correlation of changes in depositional trends in the

rock record. Such changes, which were generated by

the interplay of sedimentation and shifting base level,

are now recognized by sedimentological criteria and

geometrical relationships. (Embry, 2002)

Think 1) surfaces 2) systems tracts and 3) sequences

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Definitions: Sequence

Sequence

o A relatively conformable succession of genetically

related strata bounded by unconformities and their

correlative conformities (Vail et al, 1977)

o A sequence is composed of a succession of

genetically linked deposition systems (systems

tracts) and is interpreted to be deposited between

eustatic-fall inflection points (Posamentier, et al.,

1988).

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Definition: Base Level, Accommodation Space

Base Level:

o (Cross, 1991) a surface of equilibrium between erosion and

deposition

o (Schumm, 1993) an imaginary surface to which subaerial

erosion proceeds. It is effectively sea level, although rivers

erode slightly below it.

o sequence stratigraphy is concerned with base level

fluctuations at the shoreline

Accommodation space:

o (Jervey, 1988) the space made available for potential

sediment accumulation where in order for sediments to be

preserved, there must be space available below base level

(the level above which erosion will occur).

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Base Level

(Catuneanu, 2002)

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Eustasy

Global sea level and its variations. Changes in sea level can

result from movement of tectonic plates altering the volume

of ocean basins, or when changes in climate affect the

volume of water stored in glaciers and in polar icecaps.

Eustasy affects positions of shorelines and processes of

sedimentation, so interpretation of eustasy is an important

aspect of sequence stratigraphy.

(http://www.glossary.oilfield.slb.com)

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1st and 2nd Order Cycles of Sea Level Change

(Vail et al, 1977)

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Base Level: Eustasy, Tectonics, Sedimentation

(Catuneanu, 2002)

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Base Level Changes and Transgression/Regression

(Catuneanu, 2002)

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Scenarios for Relative Sea Level Rise

(Catuneanu, 2002)

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Scenarios for Relative Sea Level Fall

(Catuneanu, 2002)

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Response: Depositional Architecture

(from http://strata.geol.sc.edu)

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Transgression/Regression Cycle Response

MFS: maximum flooding surface (Catuneanu, 2002)

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Example Bounding Surfaces: Lowstand Shoreline

(Posamentier et al, 1992)

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Normal vs Forced Regression

(Posamentier et al, 1992)

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Rate of Base Level Change and Sedimentation

FR: forced regression

NR: normal regression(Catuneanu, 2002)

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Bounding Surfaces

Sequences and systems tracts are bounded by key

stratigraphic surfaces that signify specific events in

the depositional history of a basino maximum flooding surface, subaerial unconformity, …

o there are differing models of sequence stratigraphy that use different

bounding surfaces to define sequences

Sequences correspond to full stratigraphic cycles of

changing depositional trendso full cycle of systems tracts in response to base level change

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Reflector Geometry From Seismic Stratigraphy

(Mitchum et al, 1977)

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Surfaces of Sequence Stratigraphy

subaerial unconformity

shoreface ravinemento unconformable

o conformable

regressive surface of marine erosion

maximum regressive surface

maximum flooding surface

start of base level fall

start of base level rise

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Bounding Surfaces (Embry, 2002)

(Embry, 2002)

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Bounding Surfaces and Changing Base Level

(Embry, 2002)

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Defining Sequences: which boundary to use

Type 1 Sequence (Posamentier et al, 1988)o unconformity + start of base level fall

Type 2 Sequence (Posamentier et al, 1988)o unconformity + start of base level rise

Genetic Stratigraphic Sequence (Galloway, 1989)o maximum flooding surfaces

Transgressive-Regressive Sequence (Embry and

Johannessen, 1992)o unconformity + shoreface ravinement unconformable + maximum

regressive surface

Use depends on problem being investigated,

recognition of changes in rock record

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Sequences, Systems Tracts and Sea Level

(Miall, Fig 6.5)

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4 Different Definitions of Sequences

(Embry, 2002)

The main issue: what is the correlable conformity to the well

known subaerial unconformity

T-RS transgressive-

regressive sequence

T1DS type 1

depositional

sequence

T2DS type 2

depositional

sequence

GSS genetic

stratigraphic

sequence

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Definitions: Systems Tract

Systems Tract:o Genetically associated stratigraphic units that were deposited during

specific phases of the relative sea-level cycle (Posamentier, et al,

1999). These units are represented in the rock record as three-

dimensional facies assemblages. They are defined on the basis of

bounding surfaces, position within a sequence, and parasequence

stacking pattern (Van Wagoner et al., 1988).

o A linkage of contemporaneous depositional systems, forming the

subdivision of a sequence

o Interpreted based on stratal stacking patterns, position within the

sequence and types of bounding surfaces

o There are numerous systems tracts in use in sequence stratigraphy. It

is important to know how systems tracts are defined and the bounding

surfaces used.

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Sequence Stratigraphy: continued next week