AN INVESTIGATION INTO A FAULT DAMAGE ZONE IN THE TRIASSIC NEW HAVEN ARKOSE, SIMSBURY, CONNECTICUT

An outcrop of the Triassic New Haven Arkose in Simsbury, Connecticut contains a 10 m wide fault zone containing a series of closely-spaced normal faults striking ~194°. The outcrop is mostly composed of arkose sandstone and arkose siltstone that strike ~000° and dip ~15° east. Numerous small (<1m long) incipient normal faults define a damage zone extending ~ 60 m to the west and at least 10 m to the east of the fault zone. The incipient faults strike 195° to 222° and dip 40° to 80° to the west. In addition, there are numerous fractures that are sub-perpendicular to bedding and strike 180° to 190°. The two fracture sets that are found in these rocks may be related to two different deformation events. The incipient faults striking 195° to 222° are interpreted to be related to the ‘Drifting’ phase of the break-up of Pangea. The other fracture set striking 180° to 190° implies a ~N-S compressive stress and may be related to a transition to the ‘Shifting’ phase of the Pangea break-up.

The mechanical stratigraphy of the outcrop was determined to investigate the relationship to the fracturing and faulting. The hardness of the rock was tested with a Schmidt hammer as a proxy for rock strength. The gamma-ray response was measured with a RS-230 hand-held spectrometer. A 45-meter-long synthetic well log was created with a data spacing of 20 centimeters. The fine-grained siltstones and shales generally have a low hardness (<20 Schmidt Hammer units) and a generally high gamma-ray count (350-450). This is likely due to the higher clay mineral content that may be associated with uranium and thorium accumulation. In contrast, the coarse-grained rocks generally have a high hardness (>40 Schmidt Hammer units) and a generally low gamma-ray count (<350). Fracture spacing is both lithology and bed thickness dependent. Siltstones generally have a fracture spacing of 0.4 to 0.74 m, while sandstones have a spacing of 0.09 to 0.62m. thicker sandstone beds have a wider spacing than thinner sandstone beds. The incipient normal faults exhibit a distinct relationship to the mechanical layering where the segments of the fractures that pass through siltstone layers have a lower dip than those that pass through the sandstone layers. Understanding mechanical-stratigraphical relationships are important for characterizing subsurface groundwater and hydrocarbon fluid flow.