The peripheral upwarp (forebulge) associated with early foreland basin evolution may be recorded as an unconformity at the base of the foreland basin fill. A forebulge unconformity represents a distinctive type of megasequence boundary. Forward modeling of foreland flexure illustrates the expected geometry of an unconformity cut by erosion of the forebulge during the early stages of foreland basin development: internal conformity, a zone of rapid increase in the depth of erosion, and an external zone of steady-state stratigraphic gap. This geometry is modified by the preexisting bathymetry and by eustatic sea level changes, which result in the formation of a composite unconformity. However, the geometry of the unconformity and the onlap of the overlying sediments are fundamentally different from those associated with a major eustatic sea level fall, and represent a distinctive assemblage of petroleum play components. Producing hydrocarbon accumulations are associated with both forebulge arching and truncational unconformity traps.
Modeling achieves an accurate simulation of the geometry and stratigraphic gap of the basal unconformity of the North Alpine foreland basin (Switzerland). Further support for the flexural forebulge interpretation is provided by facies patterns in the overlying sediments.
Suppression and smothering of forebulges during the late (Molasse) stage of foreland basin development, coupled with the cannibalization of the early (Flysch) stage stratigraphy by the advancing orogenic wedge, may limit the formation and recognition of forebulge unconformities.
Model results may be helpful in predicting the timing, duration, and amount of uplift associated with flexural forebulges, and thereby aid in assessing petroleum play potential, particularly the formation of traps and the development of enhanced porosity.