AN INVESTIGATION INTO THE MECHANICAL STRATIGRAPHY OF A PORTION OF THE TRIASSIC NEW HAVEN ARKOSE, SIMSBURY, CONNECTICUT

The relationship between facies, diagenesis, sedimentary cycles, and fracturing is the fundamental basis of mechanical stratigraphy. Understanding this relationship is important for characterizing subsurface fluid flow in terms of groundwater and hydrocarbons.

The mechanical stratigraphy of an outcrop of the Triassic New Haven Arkose in Simsbury, Connecticut was examined. The outcrop is mostly composed of arkose sandstone and arkose siltstone that strike ~000° and dip ~15° east. Locally, the outcrop has experienced significant weathering, and bore holes for blasting are evident along the face of the outcrop. Normal faults occur throughout the outcrop, and a fault with 3+ meter displacement cuts the measured section. 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 30-meter-long synthetic well log was created with a data spacing of 20 centimeters.

The fine-grained siltstones generally have a low hardness (<20 Schmidt Hammer units) and a generally high gamma-ray count (325-390). 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 (<300). Interestingly, the Schmidt Hammer and gamma-ray responses provide a much finer resolution of the lithology than can be determined simply by visual inspection. The part of the measured section cut by the normal fault appears to have a subdued gamma-ray response, which may be related to leaching by groundwater passing along the fault. In addition, weathering may play a factor in Schmidt Hammer response.

In general, natural fractures (joints) are much more common in the coarse-grained sandstones and nearly absent in the fine-grained siltstones. Most joints strike ~200° and dip ~5° west. Spacing in the sandstone ranges from 2 to 30 cm. Further work will done to characterize the joint distribution.