COMPARISON OF EXHUMATION ESTIMATED FROM SONIC LOG VELOCITIES IN CRETACEOUS SHALE IN THE SAN JUAN AND RATON BASINS, NEW MEXICO AND COLORADO

The interval transit time (Δt) recorded on sonic geophysical well logs through a shale interval decreases exponentially with increasing burial depth. Rock units that are brought to a depth shallower than their maximum burial depth by exhumation will have a lower-than-expected Δt relative to a calibrated compaction curve. In this study we measure Δt as a function of depth in Cretaceous shales in two Laramide basins, one on the High Plains of northeastern New Mexico and southeastern Colorado (Raton Basin) and one on the Colorado Plateau (San Juan Basin). The modern drainage through the Raton Basin is well integrated to the Gulf of Mexico, while modern drainage across the San Juan Basin is well integrated to the Colorado River system in the northern and western parts of the basin and moderately integrated to the Rio Grande system in the southern and eastern parts of the basin. The average interval transit time for the Mancos Shale in the San Juan Basin at depths shallower than 4500 feet (1370 m) ranges from 75 to 105 µs/ft and shows a pronounced decrease in Δt with depth. In contrast, Δt averages about 70 to 80 µs/ft at depths greater than about 4500 feet and the average is insensitive to depth. 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 was 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). In both areas, maximum exhumation is near the southern end of the basin. The exhumation amounts and patterns are consistent with independent geologic constraints (e.g., preservation of the Oligocene-Miocene Chuska Sandstone in the San Juan Basin) and low temperature thermochronology.