Eustatic controls on sequence deposition:
conceptual framework
(Posamentier et al. 1988)
Topics:
eustacy vs relative sea level
rate of accommodation change
stratal patterns
equilibrium point
shoreline vs bayline
2-D transect with differential subsidence
onset/cessation of widespread fluvial deposition
stratal patterns & varying rates of eustatic rise/fall
1
Conceptual models
objective: conceptualize relationships between sealevel change & stratal patterns
 effects of changes in accommodation (space
where sedimentation can occur) on basin fill
stratigraphy
- generally applicable
- local factors must be incorporated
- predictions of…
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2
5 Assumptions of sequence strat. paradigm
1. constant subsidence at any single location
2. subsidence increases basinward
3. shelf/slope/basin margin
4. constant sediment supply
5. eustatic change is curvilinear
3
How can relative sea level (RSL) rise during a
eustatic stillstand?
RSL change as a function of eustasy & subsidence
eustasy = sea-surface relative to a fixed datum
RSL = sea-surface relative to a near-surface datum
- incorporates local subsidence/uplift (Fig. 8)
Does RSL = water depth?
water depth = RSL less accumulated sediment
How can RSL rise & water-depth decrease?
4
Accommodation = all space made available for
sediment to fill (see Fig. 7)
 function of eustatic fluctuations & subsidence
- new space added
- old space leftover
 bound by sea floor & base level
 graded stream profile (non-marine setting)
5
How does RSL (eustasy & subsidence) change effect
stratal patterns?
 RSL change determines dimension/location of
wedge-shaped space between sea surface & sea floor
(i.e., accommodation)
 parasequence stacking pattern depends of rate at
which space added & filled
- input at landward end
- amount of space available
- rate of change of new space added
6
What happens if sediment supply is sufficient to
aggrade to base level, and rate of addition of new
space slows?
 topset aggradation ____________
 topset bypass produces ____________
7
Assumption #5 eustatic change is curvilinear (see
Figure 9)
inflection point = greatest rate of eustatic change
@ F (falling) inflection points (see Fig. 11),
 rate of new shelf space added is least
 rate of aggradation ________________
i.e., thinnest topset beds (per unit time)
i.e., rate of progradation increased
 “shoreline” regression progressively more rapid
approaching the F
 BUT onlap progressively farther landward 
basinward shift of coastal onlap @ F
What kind of parasequence stacking patterns
created?
8
@ R (rising) inflection points (see Fig. 12),
 rate of new shelf space added is greatest
 maximum addition of new space
i.e., thickest topset bed
 minimum areal extent (for active depocenter)
i.e., basinward pinchout migrates landward
 onlap progressively farther landward
 maximum landward encroachment of condensed
section usually after the R
i.e., max. landward extent of condensed sections
What kind of parasequence stacking patterns
created?
9
One-dimensional model (see Figure 10)
rate of RSL change = rate of eustatic change - rate of
subsidence
if eustasy and datum subside at same rate, the rate of
RSL change is ...
if eustasy falls more slowly than datum, then RSL ...
if eustasy falls more rapidly than datum, then RSL ...
10
Two-dimensional model (see Figure 13)
 differential subsidence
 greatest accommodation on outer shelf
 least on inner shelf
At any one time, do different parts of the margin
experience the same rate of RSL change?
11
Given:
Eustatic-change rate (at any one time) applies to the
entire basin
But, subsidence rate is dependent upon location
We can define…
equilibrium point (ep) = point along basin profile
where rate of eustatic change = rate of subsidence
i.e., point where rate of RSL change is zero
“sliding” two-part subdivision of the basin during
eustatic change (position of subdivision migrates)
 seaward of ep,
rate of subsidence ___ rate of eustatic fall
 landward of ep,
rate of subsidence ___ rate of eustatic fall
12
ep migrates as rate of eustatic change varies
-> @ F inflection point,
- ep reaches maximum seaward position
-> @ R inflection point,
- ep reaches maximum landward position
13
response of sedimentation to an interval of slow
eustatic fall (fig. 16) (i.e., type 2 SB)
- from T1 - T4, rate of eustatic fall gradually
increases
- equilibrium point migrates basinward
bayline = line between fluvial* & coastal environs
* doesn’t include fluvial seds in the delta/coastal
plain
- does bayline = shoreline? (yes, if no bay/lagoon
exist…)
- does bayline = point of coastal onlap? (yes if no
fluvial sedimentation occurs)
- b/c of slow RSL rise prior to T4, the bayline
migrates landward
- as sediment fills shelfal accommodation (i.e.,
basinward of the bayline), the resulting stratal pattern
will show a gradual landward shift of coastal onlap
14
- rate of landward shift of the bayline decreases as
the rate of RSL rise at the bayline decreases
approaching T4
15
- after the equilibrium point reaches the bayline at
T4, the bayline migrates basinward with the
equilibrium point
- when the bayline migrates basinward across a
surface of low relief, subaerial new space is added
- lateral (basinward) shift of graded stream profiles
@ T4 - T6
- widespread fluvial aggradation as streams attempt
to maintain a graded profile
16
- fluvial deposition ceases when the equilibrium
point/bayline reaches its basinward-most position @
the F inflection point
- once fluvial deposition ceases, the point of coastal
onlap abruptly shifts basinward to the bayline
- deposition is once more restricted to the wedgeshaped space between the sea floor and the sea
surface (T6 to T8)
17