Geomorphological conditions of gabbro weathering at Mount Megantic, Quebec
Chemical and mineralogical analyses of a deeply weathered gabbroic rock showed that smectite (nontronite) is the main secondary mineral in association with iron-rich chlorite. The granular disintegration of the rock is mainly induced by the alteration of ferromagnesian minerals. The occurrence of silica-rich minerals is explained by the confining conditions imposed by the depressed topography of the site; along steeper slopes gibbsite is found in thinner saprolites. Weathering, though still proceeding, must have started before the last glaciation in order to explain the thickness of the profile; this is also supported by geomorphological considerations. The succession of erosion and weathering stages during Pleistocene times contributed to the shaping of the gabbroic depression.