Outcrop Analog for Hydrothermal Dolomite Reservoirs, Mohawk Valley, New York

Slater, Brian¹, Richard Nyahay¹, Langhorne B. “Taury” Smith¹ (1) New York State Museum, Albany, NY

Field mapping and structural, petrographic and geochemical analysis of a brecciated dolomite outcrop in the Lower Ordovician Tribes Hill Formation of central New York suggest that it was produced by faulting and hydrothermal fluid flow. Comparison with 3-D seismic data and field maps suggest that the outcrop is a scaled analog for Trenton / Black River hydrothermal dolomite reservoirs of eastern North America. The dolomite occurs as a series of en echelon bodies in a host limestone. Dolomitization occurs in fault-bounded synclines or “sags” with common anticlines flanking the sags. Most porosity is related to vugs and open fractures with some matrix porosity. Fluid inclusions, stable isotopes, strontium isotopes and trace element data all support a high temperature origin for the dolomites from fluids that passed through basement rocks or immature siliciclastics prior to dolomitization. Cores drilled in and around the dolomite bodies show that the dolomite extends vertically to the base of the formation with some degree of facies control on dolomitization away from the faults. Trenches cut across the outcrop help relate offset along faults to the overall geometry of the dolomitized bodies. The faults appear to be associated with right-lateral oblique strike slip motion with a component of extension. Some faults change their sense of dip and motion as they cross the dolomitized feature which is to be expected in a wrench fault system. The exposure helps better understand highly complex hydrothermal dolomite reservoirs.