GEOPHYSICAL INVESTIGATION OF VARIABILITY IN GRANITE WEATHERING THICKNESS, LARAMIE RANGE, WYOMING

Granitic weathering is hydrologically important because fracture density, fracture orientation and thickness of weathering guide near-surface flow and thus affect surface water/groundwater interactions. Non-invasive geophysical methods such as electrical resistivity and seismic refraction have the potential to investigate the granitic weathering processes. In order to characterize the variability of the weathered layer in a single mountain range, data were collected within two small watersheds approximately 12 km apart within the Laramie Range in southeastern Wyoming. Our results show that the weathering profile can vary dramatically over relatively short distances, despite relatively uniform bedrock lithology. Seismic data collected on local outcrops indicate that intact granite has a velocity of about 4 km/s velocity near the surface; slower velocities in the upper few tens of meters indicate weathered and/or fractured bedrock. Seismic data collected in the northern watershed reveal a thick weathering profile where the unweathered bedrock occurs as much as 70 meters below the surface. The seismic data collected in the southern watershed reveal a much thinner weathering profile of ~10-15 m thickness. The resistivity profiles differ substantially; in the northern watershed resistivities are greater than 5000 ohm-m and in the southern watershed less 3000 ohm-m. Despite the differences in weathering thickness both data show linear conductive anomalies dipping into the subsurface, possibly indicative of large fractures that may control surface water infiltration. These studies will allow improved understanding of surface/groundwater interactions and of the controls on weathering thickness in granitic mountain settings.