9 seq strat 1

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A genetic approach to large-scale stratigraphic problems

Sequence Stratigraphy 1

GEOL 755, Spring 2011

1Tuesday, April 5, 2011

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Resources at the DeLamare Library:

Van Wagoner, et al, 1990, “Siliciclastic Sequence Stratigraphy in Well Logs, Cores, and Outcrops”, AAPG Methods in Exploration Series No. 7.

Chapter 1 in Loucks and Sarg, 1992, “Carbonate Sequence Stratigraphy”, AAPG Memoir #57


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resources on line:

two university sites seem to be the best and include tutorials– Univ. of South Carolina’s site at: http://

strata.geol.sc.edu/index.html– University of Georgiaʼs site at: http://

www.uga.edu/~strata/sequence/seqStrat.html

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http://strata.geol.sc.edu/index.html




http://www.uga.edu/~strata/sequence/seqStrat.html






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the “depositional sequence”

term has a long history of use, – e.g. Larry Sloss’ landmark North American

“sequence” scheme redefined as a seismic stratigraphic unit by

Exxon Production Research (now ExxonMobil) in 1970’s– 4th order scale (~500,000 yr)– Sloss sequences promoted to “supersequences”


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the Sloss supersequences (2nd order)

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“sequence” presently defined:

“a conformable succession of genetically related strata, bounded at base and top by unconformities or their correlative conformable surfaces”– Van Wagoner et. al., 1990

note* this supersedes and preempts all other uses


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precedent: allostratigraphy

North American Stratigraphic Code (1983): “An allostratigraphic unit is a mappable stratiform body of sedimentary rock that is defined and identified on the basis of its bounding disconformities”– laterally and vertically variable– a geomorphic surface can be a boundary

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implicit in the definition (1) -

deposition of a sequence occurs during a time of– generation of accommodation space

» keep up vs. catch up?

– delivery of sediment into a “depositional system”

» to be known as a “systems tract”


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implicit in the definition (2) -

sequence deposition ends... – when sediment delivery stops, because ...– accommodation space moves elsewhere

area is then in hiatus until things change again -

creates a boundary unconformity/disconformity that is an isochron


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the link to chronostratigraphy

• a) seismic section,• x = distance (all)• y = depth

• b) chronostrat. correlation, • y = linear time

• c) sea-level driver,• y = linear time

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Wheeler diagram


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Wheeler diagram exercise

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The process: sea level change drives accommodation space

s.l. rise and s.l. fall - control of sedimentation– where– how much


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when introduced, this was controversial! … why?

1. originally based on seismic sections– depositional systems tracts were not

documented with rocks– geophysicists were trying to do

sedimentology? (how dare they!) 2. authors tried to make the case that they

could use the technique to develop a global sea-level curve.


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Why did the scheme come from seismic stratigraphy?

it’s a matter of scale!– seismic sections are thick– seismic sections are long

You can see big relationships

dept

h

distance14Tuesday, April 5, 2011

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What are we looking at?“raw” data

distance

2-w

ay ti

me “Variable area” plot

- fill in half the wave forms to show acoustic “events” more clearly

(difficult to see acoustic events)


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visualizing scale … (Miall, 1984)


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consensus now?

1. The genetic concept is valid, and field work has shown it works based on outcrop data.

2. The global sea-level curve is still controversial, but most of us think that the fundamental idea of sequence stratigraphy is sound without global correlation aspect.


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Cycles and sequences


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notice ordering inconsistencies

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Boggs textbook


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Where do “sequences” fit? “parasequences”?


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Compare scales Boggs, from Einsele et al.


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“sequence” defined again:

“a conformable succession of genetically related strata, bounded at base and top by unconformities or their correlative conformable surfaces”

Why do sequences form?


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Why do sequences form?

In response to processes that make accommodation space– sea-level rise (relative or eustatic)– subsidence


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facies shift with sea-level (review)

24Chris Kendall24Tuesday, April 5, 2011

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sedimentary record of sea-level is complex, even when the signal is simple

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Sea level

deposition

Sea level

deposition

where is the strand line?• sed. supply• real s.l. change

progradation• = shallowing up• = s.l. fall

★ retrogradation• = deepening up


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normal regression

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After Catuneanu, 2002After Catuneanu, 2002

prograding facies as sea level rises


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forced regression erosion and reworking as sea-level falls

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transgression

flooding and reworking, facies shift landward

28After Catuneanu, 2002After Catuneanu, 200228Tuesday, April 5, 2011

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some terminology

how/where do sequence boundaries end?

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After Catuneanu, 2002After Catuneanu, 2002


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time when accommodation space is being generated

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when does deposition occur? (subsidence always down, s.l. up and down)

subsidence

time

relatives.l.


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time when accommodation space is being generated31

more possibilities: (subsidence always down, s.l. up and down)

subsidence

time

relatives.l.


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time when accommodation space is being provided32

more possibilities: (subsidence always down, s.l. up or down)

relatives.l.

subsidence

time


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compare tectonic subsidence and orbitally forced s.l. rise rates passive margin (thermal) subsidence ~ 2mm/100yrs cont. interior basins ~ 1mm/100yrs foreland basins - 1mm/70yrs strike-slip basins ~ 3mm/10yrs orbitally forced s.l. rise ~ 2mm/5yrs

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*1) note that orbital forcing assumes significant ice formation to control ocean volume.2) the range of orbitally forced s.l. change is 10s of meters, while tectonic range may be kilometers.


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building the sequence: the“systems tract” concept

sediments deposited as a depositional system in response to a sea level position– low stand systems tract (LST)– transgressive systems tract (TST)– high stand systems tract (HST)

*note that little or no deposition is expected during sea-level fall in this scheme.


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sequences are built from systems tracts

lowstand systems tract (LST) lies on basal boundary, mainly in distal areas during proximal erosion

transgressive systems tract (TST) forms during s.l. rise, shifts progressively inland

highstand systems tract (HST) completes sequence, fills in proximal areas to sea level, beach moves seaward


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relating the sedimentation to sea-level

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review

resume


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two, stacked sequences

HST TST LST

older HST

Van Wagoner et al, 1990

incised valley fill


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two kinds of sequences:

Type 1 - subsidence less than s.l. drop rates– boundaries are erosional unconformities

Type 2 - subsidence exceeds s.l. drop rates– boundaries are not erosional, – deposition continues even during s.l. drop


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two type 1 sequences (again)

HST TST LST

older HST



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Sequence boundary definitions

Type 1 boundary - unconformity with– erosion associated with rejuvenation of

streams, incising older sediments– basinward shift in facies (deeper) related to

base-level shift– downward/outward shift in coastal onlap


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two, type 2 sequences- what are the differences?

HST TST SMST*

older HST


*SMST - shelf margin systems tract (no LST for type 2 systems)


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Sequence boundary definitions

Type 2 boundary - like #1 except– lacks significant erosion– lacks lowstand reworking


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what are the “lighting bolts”?parasequences

the higher-order responses to base-level changes

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sequence boundaryparasequence boundary


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parasequence defined

a relatively conformable succession of genetically related beds, bounded by a marine flooding surface or a correlative surface

an outcrop-scale concept! landward shift in facies

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examples of parasequence cycles

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onshore

offshoreSteven Holland

floodingsurface

floodingsurface


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parasequence key ideas: a single cycle of response to creation of

accommodation space (4th or 5th order) usually (but not necessarily) ~1-20 meters thick shallowing-up character predictable vertical facies associations

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what about sediment delivery rate?D = delivery rateA = accommodation rate

D/A > 1


D/A < 1

D/A = 1


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applications of sequence stratigraphy

local/regional chrono-correlation based on depositional facies

world-wide correlation based on global eustasy?


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getting to more realistic correlation:


Datum = top of sandstone

Datum = wellhead elevation


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chronocorrelation shows true lateral relationships

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Datum = top of sandstone

Datum = time

Van Wagoner et al, 199050Tuesday, April 5, 2011

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9 seq strat 1

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