HR: 1340h
AN: T23C-0567    [Abstracts]
TI: Exhumation Estimates Derived from Sonic Well Log Data in the Colorado Plateau - Southern Rocky Mountain Region
AU: * Kelley, S A
EM: sakelley@ix.netcom.com
AF: New Mexico Tech, Department of Earth and Environmental Science, Socorro, NM 87801 United States
AB: Interval transit time, which is the reciprocal of sonic velocity, can be determined from sonic geophysical well logs and used to estimate exhumation. Interval transit time decreases with increasing burial depth according to a compaction curve calibrated for each rock unit examined. Because rock compaction is an irreversible process, rock units that are at a depth shallower than their maximum burial depth will have a lower-than-expected Δ interval travel time relative to the calibrated compaction curve. This technique is very useful because the sonic logs are ubiquitous and a large amount of data can be acquired and analyzed in a relatively short time, so that exhumation trends over a large area can be examined. Interval transit time on sonic logs generally decreases exponentially as a function of depth in Cretaceous shales in two Laramide basins, the Raton Basin and the San Juan Basin, in northern New Mexico and southern Colorado. The average interval transit time for the Mancos Shale in the San Juan Basin at depths shallower than 3000 feet (914 m) ranges from 75 to 105 μs/ft and shows a pronounced decrease with depth. In contrast, interval travel time averages about 70 to 80 μs/ft at depths greater than about 3000 feet and the average is insensitive to depth. The interval transit time of the Mancos Shale north of Township 30 N in New Mexico actually increases with increasing depth, suggesting that some mechanism other than simple compaction (over pressuring or a previously unrecognized facies change) controls the velocity in this area. The average interval transit time for the Pierre Shale in the Raton Basin ranges from 65 to 110 μs/ft and is generally correlated with depth. The calibrated compaction curve for the Late Cretaceous Pierre Shale in the Raton Basin, based on data from ten wells, is similar to the compaction curve for the Late Cretaceous Mancos Shale in the San Juan Basin, which is determined from approximately one hundred well logs. The amount of exhumation recorded by the shale sonic data in the Raton Basin is slightly higher (maximum exhumation of 1.5 km; typically less than 1.2 km) than that in the San Juan Basin (maximum exhumation 1.2 km, typically less than 1 km). Maximum exhumation is near the southern end of the basin in both areas.
DE: 8110 Continental tectonics: general (0905)
DE: 8175 Tectonics and landscape evolution
SC: Tectonophysics [T]
MN: Fall Meeting 2005