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Abstract
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Chapter 4
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Controls on
Sequence Stacking and Fluvial to Shallow-Marine Architecture in a Foreland
Basin
Peter Schwans
Exxon Production Research
Company
Houston, Texas, U.S.A.
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ABSTRACT
Sequence stratigraphic patterns in a passive
continental margin setting are dependent on regional tectonic subsidence
rates related to the gradual thermal decay of the underlying rifted crust.
In contrast, regional subsidence rates in a ramp-type foreland basin first
increase and then decrease basinward of the thrust-fold belt. As a consequence,
stratal stacking patterns in a foreland basin are very different and determined
by a combination of tectonic events and eustatic forcing.
Analyses of the Cretaceous foreland basin
fill in west-central Utah indicate the existence of two structural and
stratigraphic zones that together define the foreland basin: (1) a zone
proximal to the thrust load with high tectonic subsidence and sediment
accommodation rates, and (2) a zone of reduced tectonic subsidence and
sediment accommodation rates that is located farther basinward or distal
to the thrust load. The two zones reflect the structural topography of
the basement underlying the tectonic loads and the associated subsidence
dynamics. Specifically, tectonic subsidence in the proximal zone was mainly
Airy-isostatic, due to the varied response of the basement blocks of the
underlying Proterozoic rifted margin. The resulting incipient Early Cretaceous
foreland basin was narrow and parallel to the thrust front; subsidence
and sediment accommodation rates within the basin were generally high.
This incipient foreland basin was structurally and depositionally disconnected
from remaining foreland areas located in central and eastern Utah, here
called the distal zone. Subsequent tectonic subsidence in the distal zone
was mainly flexural in nature, due to the  stable platform character of
the underlying basement. Airy-isostatic tectonic subsidence is thus superposed
on the flexural subsidence signal; together, these determine the classic
ramp character of the Cretaceous foreland basin.
The stratal patterns in the foreland basin
fill reflect the structural partitioning. Sequence boundaries, facies architecture,
and sequence stacking vary significantly within the proximal-to-distal
zone transect, here called accommodation profile. Unconformities in the
proximal zone are composites of
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