Hydromechanical Impacts of Pleistocene Glaciations on Pore Fluid Pressure Evolution, Rock Failure, and Brine Migration Within Sedimentary Basins and the Crystalline Basement

Abstract

The effects of Pleistocene glacial loading on rock failure, permeability increases, pore pressure evolution, and brine migration within two linked sedimentary basins were evaluated using a multiphysics control volume finite element model. We applied this model to an idealized cross section that extends across the continent of North America from the Hudson Bay to the Gulf of Mexico. Our analysis considered lithosphere geomechanical stress changes (σyy > 35 MPa) in response to 10 cycles of ice sheet loading. Hydrologic boundary conditions, lithosphere rheological properties, and aquifer/confining unit configuration were varied in a sensitivity study. We used a Coulomb Failure Stress change metric (ΔCFSp > 0.1 MPa) to increase permeability by a factor of 100 in some simulations. Results suggest that a buildup of anomalous pore pressures up to about 3 MPa occurred in confining units during periods of glaciations, but this had only a second‐order effect on triggering rock failure. In regions prone to failure, permeability increases during glaciations help to explain observations of brine flushing in sedimentary basin aquifers. During the Holocene to present day, deglaciation resulted in underpressure formation in confining units primarily along the northern margin of the northern basin. Holocene‐modern geomechanical stress fields were relatively small (<0.6 MPa). However, pore pressure increases associated with postglacial rebound, especially when a basal sedimentary basin aquifer is present, induced rock failure and seismicity up to 150 km beyond the terminus of the ice sheet. Sedimentary basin salinity patterns did not equilibrate after 10 simulated glacial cycles.

Reactivation of Non‐Optimally Orientated Faults Due to Glacially Induced Stresses

“…The pore pressure was assumed to be 0 MPa throughout the entire glacial cycle. However, it is known that pore pressure changes are induced during the growth and melting of ice sheets (e.g., Zhang et al., 2018). An additional increase of the pore pressure would elevate the stress magnitudes and more non‐optimally orientated faults could become unstable.…”

Models of overpressure build-up in shallow sediments by glacial deposition and glacial loading with respect to chimney formation

“…The blowout has been dated to between 12,500 years and 12,000 years BP, coinciding with the end of the last glaciation in North America. Several similar blowout structures have been mapped in the same area (Zhang et al 2018); therefore, the blowout structures appear to result from overpressure build-up produced by glacial loading (Neuzil 2012;Zhang et al 2018).…”

“…Several features of the chimneys are poorly understood. One important issue is their formation process, although chimneys and pipes can be explained by hydraulic fracturing of an impermeable caprock (Berndt 2005;Cartwright et al 2007;Løseth et al 2009;Kartens and Berndt 2015;Zhang et al 2018). Assuming that natural hydraulic fracturing is the process that generated the chimneys rooted in the Utsira Aquifer, then the aquifer must once have had a high overpressure.…”

“…There have been observations of blowout structures on the prairies of North and South Dakota, USA (Christiansen et al 1982;Zhang et al 2018). These structures, despite their different geological setting, could be the result of the same hydraulic fracturing process that occurred in the caprock of the Utsira Formation.…”

“…These two models have different sediment rheologies. The next four models (C to F) produce overpressure build-up in the Quaternary sediments by glacial loading, another process responsible for overpressure (Neuzil 2012;Person et al 2012;Zhang et al 2018). One of these models (C) has soil properties, which is the same sediment rheology as in model B.…”