VALLEYS THAT NEVER WERE: TIME SURFACES VERSUS STRATIGRAPHIC SURFACES

We describe incised valley evolution 1) as observed in the landscape (synoptic valleys) and 2) as preserved as stratigraphy (stratigraphic valleys) in an experimental basin responding to changes in eustatic sea level (ESL) given passive margin style subsidence and constant conditions of sediment and water supply. Our data comes from an experiment (XES Run 02) conducted in the eXperimental EarthScape (XES) facility at the National Center for Earth-surface Dynamics (NCED), St. Anthony Falls Laboratory, 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 as well as fully controllable ESL. The observations that we present here are two-fold. First, we find that incised valleys preserved as stratigraphy (startigraphic valleys) in the basin are generally not equivalent to any incised valley that ever existed in the paleolandscape For example, preserved incised-valley structures are typically broader, with more gentle side slopes, than the actual topographic features they develop from. Also, because of the widening driven by deposition during both relative sea-level rise and fall, there is virtually no record left in the stratigraphy of terraces formed during falling relative sea level. We find that the process of filling an incised valley due to rising relative sea level is not a passive completely depositional process that tends to simply bury and thus preserve the original shape of the valley, but rather includes an aggressively erosional component that does much to reshape the original valley form. Second, because these stratigraphic valleys comprise highly composite erosional surfaces, formed during numerous widening and filling events, it follows that these same erosional unconformities are highly diachronous surfaces. Neither strike sections through preserved valley walls nor dip sections through valley floors comprise isochronous time lines.