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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Variations of the
Sequence
Stratigraphic Model: Past Concepts,
Present Understanding,
and Future Directions
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Sequence
stratigraphic concepts, originally developed
on the basis of observations from seismic data largely from
passive margins, have recently had more widespread application
in a variety of other data bases including outcrop,
well
log
, and core, in a variety of different physiographic settings.
With these applications to higher resolution data bases have
come modifications and refinements of the
sequence
concepts.
A hierarchy of sequences within sequences is
observed as closer inspection reveals greater complexity.
The common thread linking the hierarchy is the occurrence
of sequences bounded by unconformities and composed of
systems tracts.
The working hypothesis upon which the sequence
concepts are based is that relative sea level change (i.e., a
function of eustatic, tectonic, and compactional effects)
results in changes in the capacity of a basin to accommodate
sediment, which, in turn, results in a succession of
sequences. The interplay between eustasy, tectonics, sediment
flux, and physiography yields a predictable geologic
response in carbonate, clastic, as
well
as mixed carbonate/clastic settings.
Sequence
-bounding unconformities subdivide
the rock record into genetically related packages
characterized by a continuum of deposition (albeit at
varying rates). The criteria for recognition of
sequence
boundaries can be varied within a given basin as
well
as
between basins. They include but are not restricted to (1) a
basinward shift of facies across a sharp bedding contact, (2)
onlapping stratal geometry, and (3) truncation of strata. The
key to the correct utilization of these concepts is to
recognize
sequence
stratigraphy as an approach or a tool
rather than a rigid template.
Observations from the upper Albian, Cretaceous, Viking Formation of the western Canadian sedimentary basin are presented to illustrate the stratigraphic expression of clastic depositional sequences on a ramp margin. In this setting, forced regressions and lowstand shorelines commonly occur, incised valleys sometimes occur, and submarine fans rarely occur, in response to fluctuations of relative sea level.
The base of the Viking Formation sometimes is characterized
by relatively coarse-grained sediments sharply
overlying fine-grained offshore muds and is interpreted as a
third-order sequence
boundary. Pebbles occasionally are
observed at this contact. Subsequently, a number of higher order
sequences within the lower to middle Viking are
observed and are characterized by the occurrence of forced
regressions and lowstand shorelines without associated
incised valleys. Within the upper Viking Formation,
significant incised valleys occur. The fill with associated
lowstand shorelines is complex and suggests a multistaged
fill history during an overall transgression. The upper
boundary of the Viking is a significant transgressive surface
interpreted to be a ravinement surface and is characterized
by relatively coarse-grained sediments sharply overlain by
fine-grained offshore muds. This surface is commonly
veneered by a pebble-rich, thin transgressive lag
(<1 m).
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