FAULT ZONE GEOMETRY AND STRATIGRAPHY AS CONTROLS ON FLUID NETWORK DEVELOPMENT: A CASE STUDY FROM THE SEVIER NORMAL FAULT ZONE, SOUTH-CENTRAL UTAH

As displacement increases within a segmented normal fault zone, stress fields around fault segments change, resulting in strain variations related to fault geometry. We investigated the NNE-striking, segmented Sevier normal fault in south-central Utah, where canyons expose 200 m of the Jurassic Navajo sandstone and the overlying Carmel Formation. In addition to standard field methods, we gathered data from inaccessible outcrops using unmanned-aerial-vehicle images combined with structure-from-motion (SfM) technology. We compared fractures within a transfer zone to those adjacent to a single fault segment outside the transfer zone. We used these data to consider how similar 3D fracture network characteristics would impact fluid flow within a subsurface fault system.

Where strain is accommodated by a single fault segment, fractures subparallel to the fault plane display high intensities within the footwall, decreasing rapidly with distance from the fault plane. In contrast, the fractured footwall of the transfer zone fault displays lower intensity values near the fault plane relative to the single-fault exposure, and we observed no decrease in intensity until fracturing terminates more than 150 m from the fault. Using SfM modeling to document the vertical fracture system, we observed that vertical fracture intensity changes abruptly, within both the cross-bedded units of the Navajo sandstone and within thinly-interbedded siltstone and limestone beds of the Carmel Formation. Within the Navajo sandstone, intensity changes are accommodated by fracture-branching events during upward propagation of fractures, usually at cross-bed set contacts; in the Carmel Formation, intensity changes appear dependent on mechanical stratigraphy, with fractures that rarely propagate across bed contacts.

We suggest that fracture intensity in transfer zones is lower but more widely distributed than where strain is accommodated by a single fault segment; in both cases, lateral fluid flow would be highest parallel to the fault strike. In addition, documented contrasts in vertical fracture intensity would result in significantly different rates of flow at different stratigraphic levels. Thus, both stratigraphy and fault geometry may exert fundamental controls on 3D fluid flow within subsurface normal fault systems.