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(Begin page 817)
AAPG Bulletin, V.
Geomorphology and sequence stratigraphy due to slow and rapid base-level
changes in an experimental subsiding basin (XES 96-1)
1Department of Geology and Geophysics, University of Wyoming,
Laramie, Wyoming, 82071; email: Heller@uwyo.edu
2Department of Geology and Geophysics, University of Minnesota, 310 Pillsbury Drive SE, Room 108, Minneapolis, Minnesota, 55455-0219; email: email@example.com
3Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, 82071
4Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, 82071; email: firstname.lastname@example.org
5Department of Geology and Geophysics, University of Wyoming, Laramie, Wyoming, 82071; email: email@example.com
Paul Heller's interest is in the interplay between sedimentation, tectonics, and sea level over a variety of time scales. His current projects involve basin modeling and field studies of marine-nonmarine interactions in Wyoming, Utah, and Spain. He received his Ph.D. from the University of Arizona in 1983. Since that time he has taught at the University of Wyoming, where he is presently professor of geology and geophysics.
Chris Paola is a professor of geology and geophysics at the University of Minnesota and associate director of the St. Anthony Falls Hydraulic Laboratory. He received his Sc.D. degree in 1983 from the Massachusetts Institute of Technology and Woods Hole Oceanographic Institution. His interests are in understanding physical sedimentary processes over scales from small (e.g., bed-form development), through intermediate (e.g., mechanism of river avulsion), to large (e.g., modeling controls of entire basin-filling sequences).
In-Gul Hwang received B.Sc. and M.Sc. degrees and a Ph.D. from the Seoul National University, Korea. In 1993 he joined the Korea Institute of Geology, Mining, and Materials (KIGAM), where he worked as a senior researcher on Cretaceous and Neogene sedimentary basins of southeast Korea. From 1997 to 1998 he undertook a postdoctoral fellowship at the University of Wyoming. His research interests are fieldwork-based sedimentology and sequence stratigraphy, especially on deltas and fan deltas, as well as fossil fuels.
Barbara E. John received her Ph.D. from the University of California, Santa Barbara (1986). She is a field-based geologist who studies lithospheric extension processes using geochemical, sedimentologic, isotopic, thermochronologic, microstructural, and geophysical studies to constrain the nature of extension processes (both magmatic and structural). She taught from 1987 until 1992 at Cambridge University (United Kingdom). She is now an associate professor in the Department of Geology and Geophysics at the University of Wyoming.
Ron Steel is Wold Professor of Energy and professor of geology and geophysics at the University of Wyoming. He graduated from the University of Glasgow in Scotland, where he obtained a Ph.D. His main interests at present are in clastic sedimentology, sequence stratigraphy, and reservoir architecture.
Funding for construction of the basin was provided by St. Anthony Falls Industrial Consortium. This study was funded by the National Science Foundation (EAR-9720416 to Heller; EAR-9725989 to Paola). We appreciate the input and reviews of many colleagues, including Frank Ethridge, David Mohrig, Colin North, Gary Parker, Lincoln Pratson, James Syvitski, John Swenson, and participants of the sedimentary seminar at the University of Wyoming.
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 × 1.6 × 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 precisely 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 (Begin page 818) record of this sequence boundary is 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 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 proximal part of the basin; the upper part of growth-fault-bounded sedimentary wedges; deep-water forced regressive systems tract composed of grainflow deposits; and transgressive systems tract formed during the rapid base-level rise. This distribution of relatively porous units suggests that, for a variety of reasons, rapid sea level cycles may produce the best reservoir units.
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