Effects of valley incision on the subsurface state of stress; theory and application to the Rio Grande Valley near Albuquerque, New Mexico

New Mexico Bureau of Mines & Mineral Resources, New Mexico Institute of Mining & Technology, Albuquerque, NM, United States

Basin-fill aquifer systems of the Cenozoic Santa Fe Group along the Rio Grande valley, including the Albuquerque Basin, appear to be over-consolidated partly as a consequence of Pleistocene downcutting by the Rio Grande, which carved a valley about 150 m deep within the broader Rio Grande rift, and partly as a consequence of tectonic exhumation. A first-order analytical solution for displacements and stresses in a compressible elastic half space bounded by periodic topography is used to illustrate the effects of topography on the subsurface state of stress under conditions of active (extensional), neutral, and passive (contractional) regional stress states. The model takes the regional state of stress into account by using a coefficient of lateral earth pressure, and a coefficient of k = 1/3 was used to simulate the state of stress in a rift basin. The solutions are then used to estimate the magnitude of overconsolidation due to Pleistocene incision of the Rio Grande valley in the Albuquerque area. Young's modulus for the sediments was inferred from sonic logs and published elastic specific storage estimates. Preliminary calculations based on this simple model suggest that the Santa Fe Group aquifer system beneath Albuquerque should be overconsolidated by about 1 to 1.5 MPa directly beneath the river, suggesting that irrecoverable virgin compaction should not begin until water level drawdowns exceed 100 to 150 m. The predicted degree of overconsolidation decreases with depth and laterally with distance from the valley center. In reality, the flanks of the basin are probably overconsolidated by tectonic uplift and subsequent erosion of an unknown thickness of sediment, which is not incorporated into the model, so the 1 MPa estimate is likely to be conservative.

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