Effects of valley incision on the subsurface state of stress - Theory and application to the Rio Grande valley near Albuquerque, New Mexico

Basin-fill aquifer systems of the Cenozoic Santa Pe Group along the Rio Gra nde valley, including the Albuquerque Basin, appear to be over-consolidated partly as a consequence of Pleistocene down-cutting by the Rio Grande, whi ch 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 s olution for displacements and stresses in a compressible elastic half space bounded by periodic topography is used to illustrate the effects of topogr aphy on the subsurface state of stress under conditions of active (extensio nal), neutral, and passive (contractional) regional stress states, The mode l 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 estimat e the magnitude of overconsolidation due to Pleistocene incision of the Rio Grande valley in the Albuquerque area, Young's modulus for the sediments w as inferred from sonic logs and published elastic specific storage estimate s, Preliminary calculations based on this simple model suggest that the San ta Fe Group aquifer system beneath Albuquerque should be overconsolidated b y about 1 to 1.5 MPa directly beneath the river, suggesting that irrecovera ble virgin compaction should not begin until water level drawdowns exceed 1 00 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 subseque nt erosion of an unknown thickness of sediment, which is not incorporated i nto the model, so the 1 MPa estimate is likely to be conservative.