FRACTURING IN THE LOCKATONG FORMATION, STOCKTON, NEW JERSEY

The Field Geology class at the Richard Stockton College of New Jersey mapped joints and fractures in the Lockatong formation exposed in Wickecheoke Creek at Stockton, New Jersey. The dominant joint system strikes N30°E; an approximately orthogonal system strikes N60°W. In addition, the western part of the area has a third joint set striking approximately East-West, but with more variation than in the N30°E set.

The joints appear to be Mode I (tensile) joints for the following reasons: 1) Many of them, particularly in the N30°E set, exhibit plumose structures; 2) Joints terminate at lithologic boundaries; and 3) Younger joints truncate against older ones. The N30°E set is the oldest; both the N60°W set and the East-West set are younger, but the age relationships between the latter two is unclear.

Spacing (the distance between nearly parallel joints) is generally related to grain size in the rocks, with larger spacing in coarser grained rocks. Some of the finer siltstones have very closely spaced joints. Shaley layers have minimal jointing; they are characterized more by irregular fractures. None of the joint set was observed to cross a shaley layer.

Within the shaley layers are arcuate fractures with well-developed slickenlines. The arcuate fractures occur in almost any orientation, and the slickenlines are consistent with normal motion. We interpret these features to indicate that the shaley layers were softer than the siltstones and underwent compaction and lateral extension, consistent with the apparent normal faults. These weaker layers also served as boundaries to limit the propagation of the joints.