CHARACTERIZATION OF A PREVIOUSLY UNMAPPED SUBSURFACE FAULT ZONE IN CENTRAL NEW JERSEY

The fractured bedrock study area is underlain by the Livingston Member of the Passaic Formation, part of the Newark Supergroup. With the exception outcrops along a nearby brook, bedrock in the study area is covered by till, outwash deposits, and the terminal moraine of the Wisconsinan glaciation; there is no surface expression of faulting. Initial evidence of a fault zone came from environmental studies of volatile organic contaminant plumes at a site in the study area and included plume geometry, bedrock topography, rock-core stratigraphy, down-hole geophysical data and hydrogeology. The fault zone acts as a preferential pathway for impacted groundwater, and different segments provide a source of groundwater recharge to and a sink for discharge from the overlying buried valley aquifer units. The convergence of commingled groundwater plumes into the systematic network of steeply-dipping en echelon extensional fractures and bedding-parallel fractures aligned subparallel to the fault zone created the need to better understand the structures and contaminant transport in this fractured aquifer.

A portion of the fault zone has an estimated lateral offset of approximately 800 feet and a vertical offset of about 60 feet based on marker beds in cores. Investigators unofficially named the buried structure the Farmhaven Brook Fault Zone and presented it at the 2006 GSA conference (Bond et. al. 2006 https://gsa.confex.com/gsa/2006AM/finalprogram/abstract_115143.htm). Preliminary estimates of orientation and bedding offsets suggests this structure, together with the nearby faults documented in the Menlo Copper Mine, indicate that the Farmhaven Brook Fault Zone is likely in the same category as intrabasinal-mapped faults such as the New Brunswick Fault and Westons Mill Pond Fault Systems. Work presented in this project includes a total of six seismic refraction traverses totaling 7,900-feet taken across the fault zone using an airless jackhammer to generate seismic waves. The resulting wave velocities were measured using an aligned array of geophones, spaced at 20-foot intervals and connected to a seismograph, and subsequently modeled. New (December 2013) bedrock wells with downhole geophysical data are being installed in key locations adjacent the fault zone and will provide additional evidence of the fault zone.