Abstract

A computer-based (digital) approach is used by the New Jersey Geological Survey for analyzing and characterizing the geological framework of fractured bedrock aquifers using outcrop data. The structural geologic data are managed and analyzed using custom personal-computer software. Outcrop locations are built as ARC/INFO Geographic Information Systems point coverages that are used to plot geo-referenced fracture orientations and fracture-spacing symbols. Fracture analysis is based on designating structural domains for faulted, folded, and regionally oriented panels of bedrock and conducting domain-overlap analysis. The regional fracture coverages are made available to the public in digital form to facilitate the evaluation of groundwater pollution sites using geographic information systems.

The study area covers parts of two tectonic fault blocks in the central part of the Mesozoic Newark basin in New Jersey. Fractures are grouped into bed-parallel and other (nonbedding) fracture sets for analyzing their distribution, orientation, spacing, and spatial variability relative to mapped faults and folds. Bed-parallel fractures are most frequently aligned subparallel to the faults bordering the basin on its northwestern margin. Nonmineralized nonbedding fractures display more variability and are frequently aligned subparallel to intrabasin faults. The strike of nonbedding fractures systematically varies across the region to reflect the strike of nearby faults; the fractures are less commonly oriented across bedding strike or at moderate angles to fault strike. Nonbedding fractures display the most variability in strike where bedding is gently warped and folded. Fold traces are frequently aligned approximately normal to fault strike. The most frequently mapped nonbedding fracture sets in the central part of the Newark basin have interfracture spacing from 4 to 20 cm. The distance between nonbedding fractures oriented subparallel to a major fault generally decreases toward the fault. A fracture density of > 50 nonbedding, subparallel fractures/meter is usually restricted to within 2 km of a major intrabasin fault in the Newark basin.