THE ROLE OF BRITTLE, DISTRIBUTED DEFORMATION ZONES IN THE WATER SUPPLY OF THE TURKEY CREEK WATERSHED FRACTURED CRYSTALLINE BEDROCK AQUIFER SYSTEM, COLORADO, USA

Brittle, distributed deformation zones (DDZs) in the Precambrian crystalline bedrock of the Colorado Rocky Mountain Front Range have mapped widths over 1 kilometer, individual trace lengths of 10’s of kilometers, and displacements of a few 100’s of meters. Possible Precambrian ancestry and episodic reactivation have accommodated all modes of crustal deformation. Because of their significant size, extreme fracture intensity relative to their protoliths, high relative potential porosity and permeability the DDZs are likely among the most significant hydrogeologic elements in the Turkey Creek Watershed impacting ground water recharge, storage, contaminant transport, surface water interaction, and possibly inter-watershed flow.

Despite the degree of deformation there are remarkably few internal structures that appear to have accommodated motion. The predominant fault zone architectural element is extreme and pervasive brittle fracturing. Most internal fractures are truncated against neighboring fractures, have trace lengths of only a few meters, and show no evidence of shear. Other internal features include small faults, minor breccia zones, discontinuous stringers of clay-rich gouge, sulfide alteration, Liesegang bands, and few macroscopic veins.

Field characterization and discrete fracture network (DFN) modeling of outcrop scale potential porosity and potential permeability in the DDZs and their protoliths have been prompted by the growing population west of metropolitan Denver and concerns regarding the potential mining of the limited groundwater resource. Inclusive of aperture uncertainties potential porosity model results range from 0.0016% to 0.51% in non-faulted rocks and 0.021% to 2.7% in DDZs. Estimates of potential permeability for 100mm aperture cases range from 2.9x10^-13m² to 6.6x10^-14m² with directional permeabilities that are nearly isotropic to anisotropic (k_max/k_min from 2x to 10x) in the non-faulted rocks. DDZ model results range from 1.2x10^-11m² to 5.9x10^-12m² with directional permeabilities of 2x to 6x. However, these hydraulic properties are not static with depth, thus continued modeling will address the relative changes in DNF models due to in-situ overburden stress which likely further limits the quantity of the resource.