BRECCIA-FILLED FRACTURES ON WESTERN ADIRONDACK SUMMITS: RELICTS OF AN ORDOVICIAN PALEOSURFACE?

Breccia-filled fractures preserved in Middle Proterozoic gneiss on and near summits of mountains in the western Adirondacks are characterized by fine-grained, angular fragments of quartz and K-feldspar. Cryptocrystalline quartz and chlorite are typical cements, the latter easily weathering to Fe-hydroxide giving the breccia-filled fractures a rusty appearance.

The breccia-filled fractures range in width from 1 to 15 mm and are most abundant on the summit of Bald Mountain (of the Fulton Chain of Lakes), but have been observed on or near summits of Wakely, Woodhull, Bare, Mitchell Ponds, and Moose River Mountains and a few unnamed summits near Stillwater Reservoir and Middle Settlement Lake. Overall, the fractures strike in two dominate directions: N60W and N10W (n = 47). Breccia-filled fractures are rare, if present at all, in adjacent valley exposures of gneiss.

On the summit of Bald Mountain, about 70 m NE of the Rondaxe fire tower, a triangular-shaped, breccia-filled void, about 9 cm wide and 22 cm long, is preserved on the trail (at N43.73904; W74.91206). From this large, breccia-filled void was extracted a 13.5 cm long vertical drill core of quartz- and chlorite-cemented breccia comprising fine-grained angular fragments of quartz, microperthite, and pebble-size angular fragments of country rock (quartz-microperthite gneiss). The pebble-size fragments are matrix-supported, and a crude layering (with a dip of about 40 degrees) is preserved in the finer-grained fragments in the bottom 3 cm of the core.

Breccia-filled fractures on and near summits of mountains in the western Adirondacks could be tectonic in origin. However, their common occurrence on mountain summits coupled with the presence of a large breccia-filled void, suggest they may have formed instead by country rock weathering, spalling, and infilling of open fractures and void spaces in crystalline bedrock associated with the once present Middle Ordovician paleosurface. Layering at the core bottom may reflect fragment accumulation at the angle of repose. The quartz and chlorite cements are interpreted to have formed during later Paleozoic burial. This interpretation suggests that many mountain summits in the western Adirondacks are presently located in proximity to the former Middle Ordovician paleosurface.