Ench week06
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Transcript of Ench week06
1
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