Sunday, September 18, 2005
Rim Gravels, Paleohydraulic Analysis, and the Flood/Post-flood boundary
Deposits Remaining from the Genesis Flood: Rim Gravels in Arizona
This is an interesting article. It discusses the gravel rocks in Arizona, and uses them to define the flood/post-flood boundary.
They conclude that the material for the gravels was eroded from the south and west, and that the "late Cenozoic" part of the geological column is the proper flood/post-flood boundary in this region. Discussing the heavy rounding of the quartzite rock, indicating massive amounts of water, and the percussive marks indicate that the transport was particularly energetic.
Using conservative numbers, they subjected the rocks to paleohydraulic analysis and came up with the following results:
Very interesting article, and this half-baked review of it certainly does not do it justice.
This is an interesting article. It discusses the gravel rocks in Arizona, and uses them to define the flood/post-flood boundary.
They conclude that the material for the gravels was eroded from the south and west, and that the "late Cenozoic" part of the geological column is the proper flood/post-flood boundary in this region. Discussing the heavy rounding of the quartzite rock, indicating massive amounts of water, and the percussive marks indicate that the transport was particularly energetic.
Using conservative numbers, they subjected the rocks to paleohydraulic analysis and came up with the following results:
- Estimated minimum depths range from 3.3 meters (11 feet) to 9.9 meters (32.5 feet). Actual depths may have been greater.
- Estimated minimum current speeds range from 11.5 m/s (26 mph) to 21.5 m/s (48 mph). Actual peak current speeds may have been greater. These are far in excess of the recommended maximum allowable current speed for channels excavated in hard rock, which is 3 to 4.5 m/s (6.7 to 10 mph) (Julien, 1995), indicating that very rapid erosion would have taken place. Peak current speeds in excess of 30 m/s (67 mph) may result in cavitation and extremely rapid destruction of rock masses (Holroyd, 1990a,b).
- Estimated discharge per meter width range from 38 to 198 m3/s per meter width (410 to 2,130 ft3/s per foot width). Actual peak unit discharge may have been greater. The estimated unit flows exceed historic peak flood unit flows for the Colorado River at Bright Angel. Unit discharge estimates indicate a very different environment of deposition for the Rim Gravels from current environments.
- Paleocurrents were supercritical (Fr>1.0). To reduce the Froude number to 1.0 (critical flow) would require a flow depth of 4.6 kilometers (2.86 miles)! Flow, therefore, was almost certainly rapid, not tranquil.
- Estimated minimum Reynolds numbers are near the boundary between laminar and transitional flow. If actual peak depths and current speeds exceed the minimums estimated here, Reynolds numbers would have been higher, and flow would have been turbulent.
Minimum paleocurrents would have been very energetic, capable of eroding hard rock, planing off obstructions, rounding clasts, and transporting large amounts of sediment.
Very interesting article, and this half-baked review of it certainly does not do it justice.
Permanent Link posted by crevo @ 8:40 AM 
