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Geomorphology And Sequence Stratigraphy Due To Slow And Rapid Base-level Changes In An Experimental Subsiding Basin (XES 96-1)

Paul L. Heller, Chris Paola, In-Gul Hwang, Barbara John, Ronald Steel

2001, American Association of Petroleum Geologists Bulletin, v.85, no. 5, p. 815-838

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1. ABSTRACT

2. HOW THE BASIN WORKS

3. ABSOLUTE & RELATIVE BASE-LEVEL vs. SHORELINE

4. MOVIES OF FAST VS. SLOW BASE-LEVEL CHANGES

5. STRATIGRAPHIC CROSS SECTIONS

6 DOWNLOADABLE TRUE-SCALE PHOTO of LINE +210

7 INTERPRETATION OF STRATIGRAPHY OF LINE +210

8. FOCUS ON FLOODING SURFACE AND GROWTH FAULTS

9. PROXY POROSITY OF LINE +210

10. CONCLUSIONS

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ABSTRACT-

An experimental basin built at St. Anthony Falls Laboratory of the University of Minnesota, is designed to model the erosional and depositional development of successions in basins associated with variations in sediment supply, 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 slow and then rapid base-level cycles in a basin with a sag geometry has been analyzed. Videotape of the experiment as well as subsequent serial slicing, photographing, and peeling 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. Of these the only controlling parameter varied was absolute base level.

The resultant fill developed under eight phases of imposed base-level change: (1) a period of no change (10 hrs.) allowing the shoreline to reach an equilibrium position; (2) a period of slow base-level fall (12 hrs.); followed by (3) a period of prolonged still stand or very slow rise (6 hrs.); (4) a period of slow base-level rise (12 hrs.) in turn followed by; (5) a period of no changes (6 hrs.) allowing shoreline to again reach an equilibrium position; (6) a period of rapid base-level fall (1.25 hrs.) followed immediately by; (7) an identical period of rapid base-level rise; and (8) a final equilibrium period (5.5 hrs.) after which the basin was drained, dried, sliced, and photographed.

Changes in development of the basinal stratigraphy occurred in response to imposed base-level changes. Absolute base level had less of an effect than the combined effect of base level and subsidence (relative base-level changes). Autocyclic changes in the sedimentary system developed over short time scales, took place in the fluvial, shoreline, and slope systems although no variation in any of the controlling parameters were imposed.

Sequence boundaries of differing style developed during slow and fast base-level falls. Incised valleys that formed during these times cut preexisting fluvial and deltaic deposits. During the slow fall, subsidence in the center of the basin was always great enough to cause a relative base-level rise. Incised valley development did not begin until the shoreline prograded out of the zone of relative base-level rise into the zone of base-level fall. However, once initiated, erosion cut rapidly headwards due to strong channelized flow and weak substrate. As erosion took place at the nick point, deposition occurred farther out in the basin, so that both the erosion front and the depositional fronts migrated landwards. This resulted in a single sequence boundary that had a time-transgressive history.

During the rapid base-level fall, incision began upstream and grew downstream over time. Sediment deposited at the delta front were gradually exposed and incised as the fall continued. Wholesale backfilling of the incised valley did not begin until the rapid base-level rise started. As a result, the incised valley grew longer by cutting both headward and downstream over time and the total duration of incised valley development was much longer, by two fold, than the incised valley that developed during the slow base-level fall.

Well-developed growth faults formed synchronous with the rapid base-level fall and during the final equilibrium period of no base-level change. The faults have a characteristic listric geometry, accommodated moderate slip, and apparently sole into coal horizons. They most likely formed due to gravity sliding down toward the basin, as with growth faults in delta slope deposits worldwide.

Reservoir development within the strata is evaluated by means of gray-scale proxy for porosity. Four distinctive zones of enhanced reservoir quality occurred in the basin including: 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, all but the first of which are sharply transitional into overlying impermeable deposits, suggests that for a variety of reasons rapid sea-level cycles may produce the best reservoir units.


HOW THE BASIN WORKS-

The experimental basin is the brainchild of Chris Paola at the St. Anthony Falls Laboratory of the University of Minnesota.

Link to Prototype Movie

< Click photo to see a Quicktime Movie (2400 Kb) showing operation of protoype basin.

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MOVIES OF SURFACE OF EXPERIMENTAL BASIN DURING RAPID AND SLOW BASE-LEVEL CHANGE -

A movie camera was set up over the experimental basin that took pictures at 5 minute intervals. These time lapse films allowed us to define shoreline position, and channel development throughout the experiment. The excerpts shown here show just the topographic development during the base level falls.

Link to Fast Fall Movie

< Click photo to see a Quicktime Movie (492 Kb) showing surface of experiment during the rapid fall of base level.

Link to Slow Fall movie

< Click photo to see a Quicktime Movie (1200 Kb) showing surface of experiment during the slow fall of base level.

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STRATIGRAPHIC CROSS SECTIONS -

Link to Experimental set up

< Click on diagram for related figures. Map view of basin showing location of three lines highlighted in movie below.

Link to slice movie

< Click photo to see a Quicktime Movie (1800 Kb) showing serial slicing of basin at 2.5 cm (1 inch) intervals. The animation provides a way of following key surfaces across the basin.

Link to large block diagram

< Click to Enlarge: This block diagram shows three-dimensional perspective of the basin fill. Source area is from the back wall. Image is graciously provided from Lincoln Pratson (Duke University).

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FOCUS ON GROWTH FAULTS & FLOODING SURFACE -

Link to Faults of XES

< Click on Photo to enlarge growth faults formed before and at the beginning of the rapid base-level fall

Link to Thicken Upwards cycles

< Click on Photo to enlarge thickening upward cycles formed during rapid transgression.

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