New Mexico Geological Society Annual Spring Meeting — Abstracts

An understanding of the impact of fault zones on subsurface fluid flow is of importance in the construction of realistic groundwater flow models. While conceptual models and quantitative hydrogeologic data exist for fault zones in lithified sedimentary rock very little work has addressed faults in poorly lithified sediments. We are developing a hydrogeologic model of fault zones in poorly lithified sediments based on the Sand Hill Fault, by field mapping at a variety of scales, in-situ and laboratory permeability measurements, structural analysis, and statistical treatment of the data.

The Sand Hill Fault is a large displacement (up to 600m), basin-bounding growth fault in the Rio Grande rift west of Albuquerque, NM. The fault juxtaposes synrift sediments ofthe Oligocene-Miocene Middle and Pliocene-Pleistocene Upper Santa Fe Group. Previous work has shown that the width, structural complexity of fault zone architecture, and extent of fault zone cementation are qualitatively predictable based on the local stratigraphy. These parameters generally increase with increasing grain size of the host sediment.

This study focuses on determining the relationships between mappable fault zone structural units and fault zone hydrogeologic units identified by statistical and geostatistical measures of the permeability data. To place the permeability data in geologic context, structural study of the fault zone will emphasize identification ofactiveI deformation mechanisms and development of structures with displacement. Hydrologic, petrophysical and structural data will be combined with classification and regression analysis to identify the primary petrophysical factors and/or tectonic processes controlling permeability.

Preliminary data suggest that highly deformed fault zone structural units have distinct hydrologic signatures, with low means and variances of permeability. These units are characterized by penetrative planar and linear fabrics and grain-scale tectonic mixing. We are employing a petrographic image analysis system to quantify fabric development and petrophysical factors related to permeability.