New Mexico Geological Society Annual Spring Meeting — Abstracts

Low temperature thermochronology and sonic log data are combined to assess the postOligocene unroofing history of the San Juan Basin. Preliminary apatite (U-Th)/He data indicates that rocks as shallow as 385 m were at temperatures above 70ºC until about 4-5 Ma and have since cooled. Apatite fission track (AFT) data suggests higher temperatures to the north with cooling below 110ºC occurring in the late Miocene. The San Juan Basin cooled from T>110 ºC due to erosional exhumation from <15 Ma. AFT ages in the San Juan Basin decrease toward the north.

Interval transit time, which is the reciprocal of sonic velocity, can be measured from sonic geophysical well logs and used to estimate relative amounts of exhumation across an area. This technique is very useful due to the ubiquity of sonic logs and the relative efficiency with which the data can be processed. This method can be used as a cross-check of erosion estimates determined from low temperature thermochronology data and can be used to evaluate exhumation versus heat flow variations across a basin. Interval transit time decreases exponentially 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 interval travel time relative to the calibrated compaction curve. The calibrated compaction curve for the Mancos Shale in the San Juan Basin was determined from over 100 well logs. The average interval transit time for Mancos Shale at depths shallower than 915 m ranges from about 75-110 μs/ft and shows a pronounced exponential decrease in interval travel time with depth. The maximum amount of relative exhumation recorded by the shale sonic log data is 1.2 km in the San Juan River valley; however, the typical amount of relative exhumation in the San Juan Basin is less than 1 km.