Spatial variation of fracture development during folding of a silty sandstone, West Virginia

Abstract

Understanding the evolution of fracture systems in response to progressive folding is crucial for developing structural and hydrogeologic models of these systems. Depending on their specific characteristics, fractures, such as joints and veins can either enhance or detract from permeability within a lithologic layer. Correctly quantifying the specific character such as density of fractures, connectivity, composition, and orientations in geologic media is key to fully understanding subsurface fluid resources. This project aims to add to the discussion on field methodology and findings relevant to fracture characterization by analyzing a sedimentary unit fractured during basin development and folding as a consequence of the Paleozoic Alleghanian orogeny. The Appalachian Basin is a 2000-kilometer long retroarc basin with millions of cubic kilometers of Paleozoic sediment stored in the geologic record. At the westernmost extent of major regional Alleghanian-age deformation in the basin, the structural setting provides an opportunity to investigate how mechanical strength and regional stresses interact to form fractures in sedimentary strata. Deformation of the Chemung Formation, an Upper Devonian mixed siliciclastic unit, was investigated over a 40 km cross section across westernmost Maryland and northeastern West Virginia. The cross section extends through variably folded Paleozoic strata across three fault-bounded structural domains. In total, measurements were taken at 27 individual locations for fractures, lithologic properties, in-situ mechanical strength, and bedding thickness. Additionally, 8 stations were selected for thin section study. All fractures observed in the field were quantified and described using a rapid orientation measurement method which allows for simultaneous collection of quantity and orientation data for a specific bedding unit. In the field measurements of layer mechanical strength were taken using a Schmidt hammer. Stratigraphic data collection included bedding orientation, layer thickness, and qualitative analysis of local folding. Fracture density was later calculated by determining the number of fractures (joints) per square meter of measured beds. The directionality and intensity of Alleghanian folding strongly influences the specific fracture populations present across the study area. The main structural zones studied are, with increasing strain, the Allegheny Plateau, Broadtop Synclinorium, and the Wills Mountain Anticlinorium. The structural zones are separated by significant, kilometer-scale fault and fold systems. Throughout the study area, higher intensity folding generally results in higher fracture density and more strongly clustered joint orientations compared to a relatively low-strain control zone selected for this study. This study further supports previous workers’ conclusions that mechanical layer thickness and structural setting are key controls on fracture density. Across the Chemung Formation, joint densities ranged from 5 to 50 per square meter. In general, joint and vein density increases to the southeast- consistent with regional strain studies- toward the core of the fold and thrust belt, although outcrop observations suggest the Wills Mountain Anticlinorium, the most inboard manifestation of orogenic forcing has been more heavily fractured than the passively folded Broadtop Anticlinorium to the southeast. As folding increases, joint orientations become more strongly controlled by the orientation of the regional scale fold axes. This study also found that meter and smaller-scale folds had minimal impact on local fracture characteristics.