PALEOFLUIDS WITHIN FAULT ZONES OF THE HARTFORD BASIN: EVIDENCE FROM FLUID INCLUSIONS

Common dip-slip to sinsitral oblique-slip brittle faults cut the Triassic-Jurassic rock section in the Hartford basin in central Connecticut. Fault strikes range from 345° - 030°, and dip 67°- 90° with slip lineations raking 70°-90° from the south. Displacements are indeterminate, most likely decimeters to meters. The faults are typically filled with blocky to euhedral quartz, calcite, and/or barite. Also present may be blocky to euhedral chalcopyrite, galena, and/or pyrite. Most faults also contain large (up to cm-scale) masses of black bitumen that pre-dates the sulfide minerals. Up to seven different stages of mineralization occur indicating multiple fault reactivation and fluid events.

Fluid inclusion microthermometry was used to investigate syn-deformational fluids along these faults. The fluid inclusions found in the quartz and calcite fault zone mineralization are primarily two-phase brine inclusions along with uncommon two-phase liquid hydrocarbon inclusions. The liquid hydrocarbon inclusions exhibit a range of fluorescence colors, from blue to blue-green. In addition, abundant black to brown single-phase bitumen inclusions fill healed microcracks and mineral interstices.

The aqueous inclusions in both calcite and quartz occur in two groups based on homogenization temperature. One group has homogenization values of 138 to 155 °C, with 7.9 to 11.7 wt. % NaCl equiv. salinity. With a pressure correction, these correspond to trapping temperatures of 150 to 200 °C, or 3.9 to 4.5 km depth assuming a 40 °C km^-1 geothermal gradient typical of rift basins. The other group has homogenization values of 70 to 118 °C, with three different salinity ranges of 1.0 to 4.0, 7.0, and 10.5 to 11.7 wt. % NaCl equiv. salinity. These inclusions were trapped at 73 to 158 °C or 1.3 to 3.5 km depth. The hydrocarbon inclusions homogenized at 100 to 125 °C.

The variation in homogenization temperatures and salinities may be the result of multiple episodes of fault motion, initially at a shallow depth, then at a deeper depth. Alternatively, the faults were active only when relatively shallow and had episodic fluid influx from multiple reservoirs due to seismic pumping, with high temperature fluids migrating along the faults from a deeper reservoir.