Geomorphology and sequence stratigraphy due to slow and rapid base-level changes in an experimental subsiding basin (XES96-1)

Subsidence is a major factor in the accumulation and architecture of natural basin fills. A recently built experimental facility (Experimental Earthscape Facility [XES]) at St. Anthony Falls Laboratory of the University of Minnesota incorporates, for the first time, a flexible subsiding floor in its design. Thus the experimental basin can model erosion and deposition associated with independent variations in sediment supply, absolute base-level change, and rates and geometries of subsidence. The results of the first experiment in a prototype basin (1 x 1.6 x 0.8 m) are described here, wherein the stratigraphic development associated with first slow and then rapid base-level cycles in a basin that has a sag geometry has been analyzed. A videotape of the experiment and subsequent serial slicing of the dried strata in the basin allow interpretation of the sequence development under conditions of presicely known changes of absolute base level, subsidence, and sedimentation . Relative base-level changes, which strongly varied in the basin owing to the sag geometry of subsidence, seem to exert primary control on sedimentary patterns, although autocyclic changes were also important. Style of sequence boundaries differed between slow and fast base-level falls. During the slow base-level fall, an incised valley developed once the shoreline prograded out of the zone of maximum subsidence, suggesting that incision at the shoreline may be very sensitive to changes in relative base level. Once started, however, the valley quickly widened, by knickpoint retreat, into a broad, low-relief erosion surface that stretched across the entire basin. As erosion took place at the knickpoint, deposition occurred immediately downflow, so both the knickpoint and the upstream limit of deposition migrated landward together, producing a strong time-transgressive erosion and onlap sequence. The stratigraphic of growth-fault bounded sedimentary wedges; and (4) transgressive systems tract formed during the rapid base-level rise. The distribution of relatively porous units, combined with their sharp transition into overlying coaly (equivalent to fine-grained) deposits, suggest the possibility that, for a variety of reasons, rapid sea level cycles may produce the best reservoir units. Several self-organized feature also developed in the basin. Particularly noteworthy are the timing and geometries of shoreline trajectories, unconformities, growth-fault development, and sediment partitioning during base-level changes, and a wide variety of autocyclic behaviors, all of which are seen in natural depositional systems. None of these features were caused by short-tern changes in any controlling parameters, and they indicate that even relatively large-scale cyclic stratigraphy need not be the direct result of external controls. The wealth of self-organizing features observed in the experiment suggests that the basin captures some of the complexity of natural basins. We do not pretend, however, that small-scale experimental models such as this capture all of the processes that occur in natural basins over long time scales. Nonetheless, it does provide us with an understanding of sedimentary response to a series of well-constrained, in dependently controlled variables. Thus the model provides a framework to help us look for and understand stratigraphic relationship seen in real-world basins.