DISCRIMINATION OF METEORIC KARST BRECCIAS FROM TECTONO-THERMOBARIC BRECCIAS

Tectono-thermobaric breccias and associated hydrothermal dolomite reservoirs, such as those in the Trenton-Black River play, represent a major remaining resource in North America. Tectono-thermobaric breccias must be differentiated from paleokarst breccias for sound exploration and development decisions. Paleokarst breccias and collapsed meteoric caves are genetically related to sequence boundaries. Many have non-carbonate detrital matrix with vestiges of calcite speleothems. Perching on low-permeability strata is common. Modern meteoric caves are far more common in limestone than dolomite, typically <10 meters wide, and limited to areas of local topographic relief and discharge into a surface drainage system. Cavernous porosity is irregularly distributed and rarely more than 5% of the total rock volume. Ancient collapsed caves should show evidence for these characteristics.

Tectono-thermobaric breccias form where space is created in active fault zones. Thermobaric (high-pressure, high-temperature) fluids flow up the active faults, enlarge fractures and precipitate minerals such as saddle dolomite, calcite and sulfides between clasts. Breccias follow fault trends, can be up to hundreds of meters wide, and are commonly concentrated beneath sealing shales or argillaceous limestones. These breccias can occur in limestone or dolomite but are commonly associated with hydrothermal matrix dolomitization. High permeability and porosity can be preserved between partially cemented clasts and in linked vugs, fractures and matrix. Tectono-thermobaric breccias form mainly in previously unbrecciated strata, but they may serendipitously intersect earlier meteoric karst. Tectono-thermobaric brecciated reservoirs commonly occur around wrench faults identifiable on seismic data. These reservoirs commonly do not require structural closure, so many potential targets remain undrilled.

Tectono-thermobaric carbonate breccias also host many of the world’s sulfide ore deposits. Many brecciated reservoirs and ore deposits that have been previously interpreted as meteoric karst may in fact be tectono-thermobaric in origin.