Damage Zone Structure and Structural Diagenesis Associated With Thrust Faults – Emery County, Utah

Abstract

Fault damage zones, associated with layered sedimentary rocks with varied mechanical properties, impact fluid flow in and around faults, including the potential filling and trapping of fluids in structures. The Cedar Mountain thrust, in Emery County, Utah is a north-south trending west dipping thrust fault, with an anastomosing damage zone associated with several synthetic thrust faults and splays. The fault is characterized by 50 m of throw and the damage zone thickness associated with the thrust varies along strike, from 10’s of meters at the main fault to 1-5 meters distributed around individual fault splays. These faults cut the Jurassic Entrada, Curtis, Summerville, and Morrison Formations. We evaluate the response of the thin mechanically-distinct beds versus thick mechanically-similar beds in the footwall and hanging wall of a well-exposed section of the fault. Detailed analysis of along-strike variability indicates a dominance of footwall folding and hanging wall cataclasis. Fine-grained thin-bedded calcite-cemented sandstone and siltstone of the footwall are folded into asymmetric, south-plunging, high frequency, tight folds, while the medium to fine-grained, medium-bedded sandstones of the hanging wall are dominated cataclasis, development of fault gouge, and breccias.

Outcrop studies and petrographic analysis of damage zone rocks indicate variation in deformation features including, calcite-cemented fault breccia, S-C foliation, grain rotation, and clay smear. We note the presence of mineralized fractures and bleached zones adjacent to many fractures in the footwall, as well as development extensional veins. Thin-section petrography indicates multiple fluid-flow events and reactivation of deformation features within the damage zone via structurally controlled permeability pathways. Increased fluid pressure, fluid flow and associated diagenesis along the fault plane could have facilitated movement on the fault and altered the mechanical properties of the rocks in the damage zone.

The Cedar Mountain thrust and associated damage zone provide an excellent natural analog for seismic scale offset with exposure of the sub-seismic deformation features. Understanding such mesoscale variability in deformation zones associated with thrust faults is crucial for the development of fault zone permeability and architecture models in order to understand the cross-fault sealing potential and trap filling.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018