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

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