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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Death of the
Sequence
Boundary:
Applying Modern Concepts to the Cretaceous
Interior Seaway of North America
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Robert Sheriff Professor of Sequence
Stratigraphy
Department of Earth and Atmospheric Sciences
University of Houston
Houston, TX
The use of arbitrary boundaries in defining lithostratigraphic
units in the 1950s resulted in a confusing proliferation of
different names for lithofacies of the same age. Early versions
of sequence
stratigraphy also failed, because of insistence on
definitions using arbitrary vertical cutoffs. Seismic stratigraphy
fundamentally transformed the science of stratigraphy by
providing vastly superior images that allowed correlation of
genetically related chronostratigraphically
significant units. Reflection seismic data
thus provided the key technological breakthrough
that provided continuous crosssectional
views of stratigraphic basin fills
and fundamentally revitalized the science of
stratigraphy.
Sequence
stratigraphy solved the basic problem that was
genetically related, but different lithofacies were routinely
assigned to different lithostratigraphic units defined by arbitrary
vertical and horizontal cutoffs.
Sequence
stratigraphically
important lapout relationships can be observed in seismic data
and can be documented in continuous outcrops, such as in the
deserts of the Western Interior of North America and in closelyspaced
well
log
data sets. Finding good isochronous stratigraphic
datums, such as bentonites or condensed sections, is key. Not all
surfaces defined by lapout boundaries are readily identifiable in
1D sections, and in
well
logs lapout relationships must be
interpolated. This introduces uncertainty in correlation and
designation of sequences and systems tracts and their associated
surfaces.
The uncertainty in dating of fluvial terrace deposits is shown
by use of detailed facies architectural studies, combined
with Wheeler analysis
, as
well
as recent modeling and Quaternary
studies. These studies call into question the assumed
chronostratigraphic significance of many so-called
sequence
boundaries identified in the rocks of the Cretaceous Interior
Seaway of North America, such as the boundary between the
Blackhawk-Castlegate formations in Utah, and suggest that they
may have far higher diachroneity than has previously been
assumed. Although a glacio-eustatic origin for Cretaceous
sequences is still highly debated, modern glacio-eustatic falls of
sea-level are commonly prolonged and irregular, whereas rises are
typically very short lived.
Sequence
boundaries formed during
such prolonged falls may be less chronostratigraphically
significant than the transgressive surfaces
formed during rapid rises. As a consequence,
flooding surfaces are both theoretically
more significant and also have greater utility
as allostratigraphic boundaries.
Tectonic unconformities are also common in
the Cretaceous Western Interior. Tectonics produces differential
lithospheric deformation, which results in angular unconformities.
In the Cretaceous Interior Seaway of North America, such
unconformities may be expressed by marine erosion in basin
distal settings. Regional isochronous bentonite beds provide
useful regional marker beds that clearly illustrate angular
discordance. In the fluvial realm, such tectonic discontinuities are
indicated by changes in paleocurrent orientations as well
as by
provenance changes.
Although sequence
stratigraphy provides a powerful methodology
and theoretical framework for correlating and understanding the
evolution of stratigraphic successions in the context of changing
accommodation, allostratigraphy remains the only accepted
scheme for formal naming of stratigraphic units based on bounding
discontinuities. However, whatever type of
sequence
stratigraphy
or allostratigraphy one prefers, it is key in all cases to recognize
that
sequence
stratigraphy, at its heart, is the re-ordering, correlation,
and sometimes renaming of stratigraphic units on the basis of
bounding discontinuities and their correlative surfaces, as
opposed to the arbitrary lithofacies–oriented approach using broad
facies “shazams” or arbitrary cutoffs, such as is used in traditional
lithostratigraphy.