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

摘要

Subsidence is a major factor in the accumulation and architecture of natural basin fills. A recently built experimental facility (Exper-imental 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 var-iations 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 ana-lyzed. 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 precisely known changes of ab-solute base level, subsidence, and sedimentation. Relative base-level changes, which strongly varied in the basin owing to the sag ge-ometry of subsidence, seem to exert primary control on sedimen-tary 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 maxi-mum subsidence, suggesting that incision at the shoreline may be very sensitive to changes in relative base level. Once started, how-ever, 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 stratigraphicrecord of this sequence boundary is a single yet very subtle wide-spread unconformity that becomes conformable downstream, which is difficult to trace in stratigraphic cross section.In contrast, the incised valley that formed during the rapid base-level fall was relatively narrow, deep, and lengthened over time as deposits at the mouth of the valley were gradually exposed and incised through. Wholesale backfilling of the incised valley did not begin until the rapid base-level rise started. As a result, the rapid base-level change produced a more easily recognized incised valley in the stratigraphic record than did the slow base-level change.Potential reservoir development within the strata is evaluated by means of a gray-scale proxy for porosity. Four distinctive zones of enhanced reservoir quality occurred in the basin: the most prox-imal part of the basin; the upper part of growth-fault—bounded sed-imentary wedges; deep-water forced regressive systems tract com-posed of grainflow deposits; and transgressive systems tract formed during the rapid base-level rise. This distribution of relatively po-rous units suggests that, for a variety of reasons, rapid sea level cycles may produce the best reservoir units.