Geophysical investigations in the Colorado Delta Region

Abstract

The combined approach of gravity and seismic refraction techniques was used to determine depths of the Cenozoic section and fault patterns of the Colorado delta region. Bouguer gravity anomalies range from −10 to −88 mgal with respect to the International Ellipsoid, and the trend of isogal contours is northwesterly. In the center of the basin, depths to basement estimated from gravity data are less than known minimum depths determined from seismic refraction profiles. This discrepancy suggests density complexities within the deeper parts of the stratigraphic section, an interpretation that is supported by sparse measurements from well samples; local isostatic compensation may also contribute to the discrepancy but is not thought to be the major cause. Measured seismic velocities ranged from 5650 to 20,000 ft/sec, and several consistent velocity zones were widespread above the pre-Tertiary basement; indicated depths to basement varied from 2200 feet to at least 15,400 feet, the greatest depth being near the international border. Many members of the San Andreas fault system are well delineated by gravity and seismic data, including the Elsinore and San Jacinto faults and a fault beneath the Sand Hills. Of these, the San Jacinto fault appears to be the most continuous through the delta region, but all appear to be en echelon to the trend of the San Andreas fault system as a whole.