PRELIMINARY STUDY OF FRACTURE CHARACTERIZATION IN OUTCROPS OF THE PENNSYLVANIAN JACKFORK FORMATION, LITTLE ROCK, ARKANSAS

Outcrop fracture characterization is essential for understanding subsurface fluid flow in groundwater aquifers and hydrocarbon reservoirs, as fractures in the subsurface often fall below the resolution of seismic-reflection data and above the resolution of borehole data. In this preliminary study, we conducted fieldwork to better understand the factors controlling the development of natural fractures within the Pennsylvanian Jackfork Formation in North Little Rock, Arkansas. Our study area, located at the northeastern end of the Ouachita Fold and Thrust Belt, includes outcrops of fractured interbedded shale and sandstone of the Jackfork Formation, accessible through abandoned mining sites and road cuts. Using linear scanline and window-sampling methods, we investigated the distribution, intensity, orientation, and origin of these natural fractures. At Site #1 (Pershing Boulevard), we observed two sets of fractures, consisting of dip-parallel, high-angle conjugate fractures striking primarily SSE and NNW. The linear scanline method at this site revealed a systematic increase in lateral fracture density towards a damaged zone where outcrops are poorly preserved. Additionally, we observed an increased number of fractures and slickened sides within the sandstone units of this formation. At Site #2 (Big Rock Quarry), in addition to the conjugate fractures striking SSE and WSW, we identified the development of high-angle, ESE-striking fractures. These ESE-striking fractures exhibit en échelon geometries and commonly develop above fold axes. Our preliminary study suggests that at least three factors influence the development of natural fractures in the Jackfork Formation in our study area: regional deformation related to the Ouachita orogeny, mechanical stratigraphy, and the presence of local faults and folds. Future work will incorporate photogrammetric and petrographic analyses to further characterize these fractures at outcrop and microscopic scales, respectively. The results of this study will provide a foundation for improved permeability prediction in the Jackfork Formation and other deformed deepwater sedimentary rocks worldwide.