129-2 Strain Accommodation within the San Rafael Monocline, Utah: Reconnaissance Investigation of Deformation within the Jurassic Navajo Sandstone During Contractional Fault-propagation Folding

Abstract:

By documenting deformation mechanisms associated with contractional fault-propagation folds, researchers can better predict fluid flow in subsurface systems. As a fold forms in front of a propagating fault, deformation bands, shear fractures, pressure solution features, and minor faults initiate, propagate, and interact within sedimentary layers, affecting potential flow paths.

In this study, we investigate the steepest dipping (>60 degrees) forelimb exposure of the Jurassic Navajo Sandstone within the San Rafael monocline. The Laramide-age monocline, the result of folding of sedimentary strata in front of a steeply west dipping basement fault system, is arguably the best exposed fault-propagation fold in the world. Previous researchers used trishear kinematic modeling to explain 2D cross-sectional fold evolution and strain accommodation within the fold; we situate our investigation in the context of their results but expand our analysis to 3D, parallel to the fold axis.

To gather data, we used imagery from an unmanned aerial vehicle (UAV), field-based structural measurements, and thin-section analysis from canyon exposures to document changes in the type, orientation, and distribution of deformation through the monocline. The UAV imagery was used to construct 3D virtual outcrop models of rock exposures. We used these models and field data to map the monocline and create a conceptual model that explains how porous sandstone deforms in response to the evolving stresses associated with fault-propagation folding.

We suggest that deformation bands, low (<1 m) displacement faults, and conjugate faulting dominate the accommodation of strain within the steepest (~60 degree dip) exposures of strata within the forelimb. Structural data indicate that faults form parallel to cross-bed set contacts and at boundaries between individual cross-beds. In addition, at several excellent exposures of low-displacement faults and deformation band networks, we document conjugate fault sets with kinematics and orientations consistent with a sub-horizontal sigma-1 oriented WNW-ESE, approximately perpendicular to the NE-trending monocline axis. Low-angle (<30 degree) deformation band networks commonly bound high-angle (>50 degree) ladder bands, which abut these low-angle structures, consistent with published research results.

We suggest that a diverse set of mechanisms accommodate fault-propagation folding in porous sandstone, with most structures likely reducing porosity and therefore fluid-flow potential. Future research will focus on providing better three-dimensional spatial constraints on how these mechanisms interact and could affect fluid flow within folded strata.

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