This page is the first step of a seismic
stratigraphy interpretation. Its objective
is to define the genetic reflection packages by the surfaces that
envelope seismic sequences and systems tracts. These bounding
discontinuities are identified on the basis of reflection termination patterns and
their continuity.
Boundaries are defined on a seismic line by identifying the termination of
seismic reflectors at the discontinuity surfaces. These terminations occur:
Belowa discontinuity and the definition of the upper sequence
boundary. Examples of this include:
o Toplap: termination of strata against an overlying surface,
representing the result of non-deposition and/or minor
erosion.
o Truncation: this implies the deposition of strata and their
subsequent tilting and removal along
an unconformity surface. This termination is the most
reliable top-discordant criterion of a sequence boundary.
Such truncation can also be caused by termination against
erosional surface, as for instance a channel.
Abovea discontinuity and the definition of the lower sequence
boundary:
o Onlap: A base-discordant relationship in which initially
horizontal strata progressively terminate against an initially
inclined surface, or in which initially inclined strata
terminate progressively updip against a surface of greater
initial inclination.
o Downlap: a relationship in which seismic reflections of
inclined strata terminate downdip against an inclined or
horizontal surface. Examples of downlap surfaces include
a top basin floor fan surface, a top slope fan surface, and
a maximum flooding surface.
Note: If onlap cannot be distinguished from downlap because of
subsequence deformation, the term baselap is used.
The lecture seen in the movie involves a geologic model that makes the
following assumptions::
Sea level position varied
Subsidence was constant
Sediment supply was constant
The sequence is divided by surfaces systems tracts. Each systems tract is
represented by a collection of the sediments of the associated
sedimentary depositional systems that were active during the different phases
of base level change. Thus systems tract sediments can be considered as
sedimentary units that were deposited synchronously and can be mapped as
being enclosed by continuous surfaces that extend from sub-aerial and to
sub-aqueous settings.
The systems tracts defined in order of deposition to form the
ideal sequence are:
Early phase lowstand systems tract
Late phase lowstand systems tract
Transgressive systems tract
Highstand systems tract
Early phase lowstand systems tract is associated with:
falling stage of relative sea level induced by eustasy falling rapidly
and/or tectonic uplift outpacing the rate of change in sea level
position
Fluvial incision up dip with formation of an unconformity or sequence
boundary and the focus of sediment input at the shoreline
Forced regressions induced by the lack
of accommodation producing stacking patterns of downward
stepping prograding clinoforms over the condensed section formed
during the previous transgressive and highstand systems tracts
Slope instability caused by the rapid deposition of sediment from the
fluvial systems
Basin floor fans formed from sediment transported from the shelf
margin when this fails under the weight of the rapid
sediment accumulation associated with the forced regression
Shelf margin and slope fans form when rates of sedimentation slows
and slope instability is reduced so sediment is not displaced so far
downslope
Onlap of sediments onto the prograding clinoforms below the shelf
break
The lower boundaries of the early phase lowstand systems tract are
the updip unconformity and the top of the downdip condensed
section. These surfaces form by different mechanisms and have
different time significance
The top of the downdip condensed section immediately underlies
the downlapping prograding clinoforms of the forced regression
The top of the early phase lowstand systems tract in theory is marked
by an initial onlap onto the often eroded surface of the
prograding clinoforms of the forced regression
Late phase lowstand systems tract is associated with:
A slow relative sea level rise is induced when eustasy begins to rise
slowly and/or tectonic uplift slows
Sediment is now outpaced by an increase in accommodation and in
response the sediment begins to onlap onto the basin margin
River profiles stabilize
Valleys backfill
Prograding lowstand clinoforms form and are capped by topset layers
that onlap, aggrade, become thicker upward and landward
Transgressive systems tract is associated with:
A rapid relative sea level rise above the shelf margin occurs
when eustasy begins to rise rapidly, exceeding the effects of any
tectonic uplift
Condensed sequences are often composed of sediment layers rich in
the tests of fauna that are no longer masked by
sediment accumulation because sedimentation rates are very slow
in response to the greater area of sea floor exposed to
sedimentation
Ravinement erosion surface formed when the transgressing sea
reworks either the prior sequence boundary or the sediments that
may have collected during the forced regression that may have
followed the generation of that sequence boundary.
Maximum flooding surface forms when the last fine-grained
widespread transgressive sediment collects before the high
stand builds out over it.
Highstand systems tract is associated with:
Slow rise of relative sea level followed by a slow fall; essentially a still
stand of base level when the slower rate eustatic change balances
that of tectonic motion
Sediment outpacing loss of accommodation
River profiles stabilize
River valleys are dispersed laterally in a position landward of the shelf
margin.
Prograding highstand clinoforms develop capped by
aggrading topsets that become thinner upward.
