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Controls on Sequence Stacking and Fluvial to Shallow-Marine Architecture in a Foreland Basin

By
Peter Schwans
Peter Schwans
Exxon Production Research Company Houston, Texas, U.S.A.
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Published:
January 01, 1995

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 basin- ward 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 basin ward 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 merged, 3rd-order unconformities, whereas surfaces in the distal zone are composites of higher-frequency unconformities. Within the transect, uncon formity expression changes from correlative conformity and incised paleovalley on the ramp to angular unconformity approaching the thrust-fold belt. The component systems tracts of basin-fill sequences exhibit type-facies associations and a particular depositional architecture. For example, low-stand systems tracts (LST) in the distal zone consist of fluvial-dominated deltaic and estuarine-bay deposits within incised paleovalleys. Coeval fluvial strata in the proximal zone consist of amalgamated sheets of braided-stream deposits. The overlying highstand systems tract (HST) features aggradationally to progradationally stacked, wave-dominated shoreline parasequences and marine shales in the distal zone. Coeval fluvial strata in the proximal zone comprise thick intervals of overbank deposits with coals, splays, and well-defined single-channel geometries.

Despite the structural partitioning of the foreland basin margin, basin-fill strata display superposed eustatic cycles with frequencies that range from 0.1-0.2 m.y. (4th-order cycle) to 1-2 m.y. (3rd-order cycle). The higher frequency, 3rd- and 4th-order cycles are superposed on a 16.5 m.y. cycle, here called a transgressive-regressive cycle. The 16.5 m.y. cycle is interpreted as a regional tectonic cycle that formed in response to foreland basin subsidence, crustal relaxation, and rebound. The cycle determined the evolution of the Cretaceous foreland basin from an early, sediment-starved and underfilled basin with retrogradational to aggradational stacking of component 3rd-order sequences to a late and overfilled basin with progradationally stacked component sequences.

The influence of eustasy and tectonic subsidence on patterns in the foreland basin fill is defined through a foreland basin accommodation cycle. Times of high tectonic subsidence and higher-frequency eustatic change are characterized in the distal zone by minor basinward shifts in facies and changes in parasequence stacking. Coeval nonmarine strata in the proximal zone are dominated by overbank deposits. Basinward shifts are poorly defined and expressed by increases in channel clustering and lateral coalescence of single-channel geometries. Conversely, times of low tectonic subsidence and multiple, higher-frequency eustatic changes are characterized in the distal zone by significant changes in parasequence stacking. Widespread subaerial exposure and rapid shifts in facies on the ramp result in the formation of extensive incised valley systems. Coeval fluvial deposits in the proximal zone are dominated by coarse, braided-stream clastics.

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Contents

AAPG Memoir

Sequence Stratigraphy of Foreland Basin Deposits: Outcrop and Subsurface Examples from the Cretaceous of North America

J.C. Van Wagoner
J.C. Van Wagoner
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G.T. Bertram
G.T. Bertram
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American Association of Petroleum Geologists
Volume
64
ISBN electronic:
9781629810850
Publication date:
January 01, 1995

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