Luncheon – September 18, 2009

Title: Dynamic Topography and the Formation of the Cretaceous Western Interior Basin

Speaker: Dag Nummedal, Colorado Energy Research Institute, Colorado School of Mines, Golden, CO

Date: September 18, 2009

Publication: The Outcrop, September 2009, 18-19

It is well known that deep earth processes can have a profound influence on sedimentary basin evolution, but progress has been slow in quantifying this relationship and differentiating their role from purely crustal processes of basin formation. Dynamic topography is generated by mantle flow that has been induced by lateral density differences. With amplitudes of up to 1 km, dynamic topography occurs over both oceans and continents, including intracratonic areas, and active and passive margins. Since there is no clear demarcation between topography induced by crustal thickness variations, lithospheric stretching, and mantle anomalies, the extraction of dynamic topography was slow to emerge. Nevertheless, the empirical evidence linking anomalous vertical motions to dynamic topography, especially in large petroleum bearing sedimentary basins, has become overwhelming. The Cretaceous Western Interior Basin (WIB) of the U.S. is probably one of the best examples.

Recent work has combined inverse (here meaning ‘backward in time’) modeling of the history of the Fara I Ion plate (the Caltech geodynamics group of Mike Gurnis et al.) and our own (Nummedal, Liu and Pang) reconstruction of the development of the Cretaceous WIB based on backstripping and decompaction of the well-dated shallow marine succession. The main finding of these combined studies is that the WIB migrated eastward together with the crest of the subducted slab during the Late Cretaceous. This is one of the first (the first?) documentation of continental sedimentary basins migrating laterally over time relative to the crust. From about 95 Ma to 75 Ma the depocenters of the WIB migrated from western Utah to beyond the Denver basin (and equivalent distances in Wyoming). Moreover, the subsidence in Colorado is less than expected (and less than in Wyoming), which implies a lower density of the part of the Farallon slab beneath Colorado. It was probably a subducted oceanic plateau. The Farallon plate has continued eastward since the Cretaceous and underlies the U.S. east cost today, where it currently accounts for a negative dynamic topography of about 100 m.

The U.S. Cretaceous WIB has long been considered a foreland basin created by flexural loading by the Sevier thrust belt, although Cross and Pilger recognized earlier that there had to be an additional “hidden load” to account for its width. Subsequent studies demonstrated that the flexural load exists only within a narrow band directly in front of the thrust belt, about 120 to 180 km wide, although the basin is wider than this by a factor of five. These recent studies demonstrate the dynamic origin of this much wider subsidence belt.

The uplift of the Laramide ranges at the close of the Cretaceous reflects another key component of dynamic subsidence – as contrasted with most other basin-forming mechanisms – namely its transient nature. Once the slab moves on and/or sinks the buoyancy pull disappears, the region is re-occupied by the asthenosphere, and the surface starts to rebound. In the case of the Rockies this episode is of course the uplift of the Laramide ranges. The emergence of this tectonic style shows up in the subsurface at about 80 Ma, even though the uplift of subaerial topography appeared later.