EXPERIMENTAL STRATIGRAPHY AND GEOMORPHOLOGY

Experiments can give a very clear view of how landscapes evolve and how landscape evolution is preserved in the subsurface as stratigraphy. Here we present experimental data that demonstrates why startigraphic unconformities, e.g. sequence boundaries and incised valleys, are not always equivalent to paleogeomorphic surfaces, i.e. a lot of what we see preserved in stratigraphy probably never existed in the paleolandscape.

The data we present comes from an experimental study of landscape and stratigraphic response to changes in eustatic sea level, given passive margin style subsidence and constant conditions of sediment and water supply. This experiment (XES Run 02) was conducted in the eXperimental EarthScape (XES) facility (Jurassic Tank) at the National Center for earth-Surface Dynamics (NCED) at St. Anthony Falls Laboratory at the University of Minnesota. The XES facility is a large (6 m x 3 m x 1.3 m) experimental basin with a programmable subsiding floor and fully controllable water and sediment discharge into the basin as well as fully controllable sea level.

We find that stratigraphic unconformities in the basin were formed at points of local minima in relative sea level and were shaped both during falling and rising sea level, producing surfaces that are strongly time transgressive. One result of this is that, stratigraphic valley depth, i.e. the valley depth most likely to be recorded in the statigraphic record, comprises a diachronous and composite surface that increases in depth with the downstream direction, a trend that is the opposite of that of the instantaneous valley depth visible in the landscape, which decreases downstream due to increasing rates of subsidence.