Geomorphology and sequence stratigraphy due to slow and rapid base-level changes in an experimental subsiding basin (XES 96-1)

Subsidence is a major factor in the accumulation and architecture of natura l basin fills. A recently built experimental facility (Experimental Earthsc ape Facility [XES]) at St. Anthony Falls Laboratory of the University of Mi nnesota incorporates, for the first time, a flexible subsiding floor in its design. Thus the experimental basin can model erosion and deposition assoc iated with independent variations in sediment supply, absolute base-level c hange, and rates and geometries of subsidence. The results of the first exp eriment 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 bas e-level cycles in a basin that has a sag geometry has been analyzed. A vide otape of the experiment and subsequent serial slicing of the dried strata i n the basin allow interpretation of the sequence development under conditio ns of precisely known changes of absolute base level, subsidence, and sedim entation. Relative base-level changes, which strongly varied in the basin o wing to the sag geometry of subsidence, seem to exert primary control on se dimentary patterns, although autocyclic changes were also important. Style of sequence boundaries differed between slow and fast base-level fall s. During the slow base-level fall, an incised valley developed once the sh oreline prograded out of the zone of maximum subsidence, suggesting that in cision at the shoreline may be very sensitive to changes in relative base l evel. Once started, however, the valley quickly widened, by knickpoint retr eat, into a broad, low-relief erosion surface that stretched across the ent ire basin. As erosion took place at the knickpoint, deposition occurred imm ediately downflow, so both the knickpoint and the upstream limit of deposit ion migrated landward together, producing a strong time-transgressive erosi on and onlap sequence. The stratigraphic record of this sequence boundary i s a single yet Very subtle widespread unconformity that becomes conformable downstream, which is difficult to trace in stratigraphic cross section. In contrast, the incised valley that formed during the rapid baselevel fall was relatively narrow, deep, and lengthened over time as deposits at the m outh of the valley were gradually exposed and incised through. Wholesale ba ckfilling of the incised valley did not begin until the rapid base-level ri se started. As a result, the rapid base-level, change produced a more easil y recognized incised valley in the stratigraphic record than did the slow b ase-level change. Potential reservoir development within the strata is evaluated by means of a gray-scale proxy for porosity. Four distinctive zones of enhanced reservo ir quality occurred in the basin: the most proximal part of the basin; the upper part of growth-fault-bounded sedimentary wedges; deep-water forced re gressive systems tract composed of grainflow deposits; and transgressive sy stems tract formed during the rapid base-level rise. This distribution of r elatively porous units suggests that, for a variety of reasons, rapid sea l evel cycles may produce the best reservoir units.