CHARACTERIZATION AND DEVELOPMENT OF FRACTURE PATTERNS IN DOMED SEDIMENTARY STRATA, PULPIT ARCH, HENRY MOUNTAINS, UTAH

Abstract

Fractures in sedimentary rock can be an important factor in influencing fluid migration in the subsurface. In high-porosity sandstones in the upper crust, deformation such as folding typically results in the production of deformation bands rather than joints or shear fractures. Deformation bands are zones of very low porosity and therefore tend to impede or redirect fluid rather than serve as preferential conduits like joints or shear fractures tend to. This project is designed to provide a rigorous characterization of deformation band patterns that developed during doming of high-porosity sandstone in the shallow crust. The Pulpit Arch, located in the Henry Mountains of southeastern Utah, is a dome of sedimentary rock located above an unexposed component lobe of the Mt. hillers laccolithic igneous center. The intrusion underlying the Pulpit Arch uplifted and folded the overlying strata, including the high-porosity Navajo Sandstone studied in this project, into a 3-dimensional dome with a diameter of roughly 2.5 km and an amplitude of approximately 200 m. Within the Navajo Sandstone, 690 deformation bands were characterized and mapped across 23 fracture stations in the Pulpit Arch and 4 in a nearby undeformed area. At every station a variety of characteristics were recorded, including fracture type, planarity, orientation, length, and termination style. Fracture intensity was calculated by measuring total fracture length per unit area for each fracture station. Hand samples were collected at each fracture station for thin section examination in the laboratory. Growth of the dome strongly influenced both the intensity and orientation of fractures in the Pulpit Arch. Deformation bands with orientations generally NE - SW and E - W were observed exclusively within the Pulpit Arch, while NW to SE trending fractures occurred in both areas. Average fracture intensity in the Pulpit Arch was calculated to be 0.30 m-1 and 0.03 m-1 in the control area. Porosity values in undeformed samples, deformed samples and deformation bands were calculated to be 13.1%, 22.5%, and 2.2%, respectively. These measurements agree with other published values for undeformed and deformed Navajo Sandstone, and deformation bands. Overall, considerable differences in fracture characteristics between the Pulpit Arch and control area were visible. However, fracture characteristics like intensity and orientation vary considerably across the Pulpit Arch. The Pulpit Arch is an extremely early stage dome (extension magnitudes are approximately 1%), and it is likely that the fracture system on the dome has not developed enough for clear fracture patterns to occur. Similar studies on more advanced domes would further understanding of how fracture systems evolve with progressive doming.