Northeastern Section - 49th Annual Meeting (23–25 March)

Paper No. 10
Presentation Time: 1:30 PM-5:35 PM


HECKLAU, Samuel, Geology, St. Lawrence University, 23 Romoda Drive, Canton, NY 13617, MACKENZIE, Kevin, Registrars Office, St. Lawrence University, 23 Romoda Drive, Canton, NY 13617 and CHIARENZELLI, Jeff, Department of Geology, St. Lawrence University, Canton, NY 13617,

In late August of 2011 Tropical Storm Irene produced record rainfalls and widespread flooding damage along Johns Brook near Keene, New York and along the length of the Ausable River into which it flows. The heavy rain triggered numerous landslides in the Adirondack High Peaks Region. These long (1-2 km), linear areas of bare rock provide unprecedented and continuous exposure through the core of Marcy anorthosite massif. Among these, the slide along Bennies Brook (Bennies Root Canal Slide) on the north facing slope of Lower Wolfjaw Mountain is noteworthy for its diversity of igneous rocks and structural features. Coarse-grained massif anorthosite, by far the predominant rock type, is cross-cut by numerous variable dikes ranging in composition from leucogranitic to mafic. Of these only jotunitic and gabbroic rocks are of volumetric significance comprising bodies up to 20-30 meter across. Jotunitic rocks contain increasing amounts of andesine xenocrysts towards contacts with the anorthosite. Igneous textures are faithfully preserved in anorthositic and mafic rocks. Small, thin (10-20 cm), Neoproterozoic(?) basaltic dikes are ubiquitous. Garnet is a widespread metamorphic phase often forming small one-grain thick rims on oxides and pyroxenes in the anorthosite; however, in ductile shear zones it is often the predominant phase and has undergone intense ductile deformation. Most shear zones are spatially associated with heterogeneities in the anorthosite, including igneous contacts. Gabbroic pegmatites are often strongly sheared. The trend of larger ductile shear zones is 110-130o; similar to the Piseco Lake Shear Zone well to the south. Several large brittle faults containing closely spaced fractures, fault breccia, and extensive alteration occur and also share an E-W orientation as well as conjugate orientations. This work underscores the geologic diversity of the Marcy Anorthosite Massif and supports previous interpretations emphasizing the contemporaneity and spatial association of rocks of the AMCG suite. In addition, it notes the preferential formation of garnet in high-grade ductile shear zones of E-W orientation which are likely of Shawinigan age.