FOLD CHRONOLOGY AND STRATIGRAPHIC RELATIONSHIPS - NEW INSIGHT, SE ADIRONDACK MOUNTAINS, NY

Large nappe folds of at least 250 square kilometers were formed between 1249 and 1155 Ma during the Shawinigan orogeny based on published age dates and are designated F1 folds for this region. Nappe cores consist of basement granitic gneisses, and all metasedimentary units are present in intervening F1 synforms. Igneous rocks of the mangerite-charnockite-granite (MCG) association intruded the terrane at about 1155 Ma (McLelland et al., 2004). F2 folds were superimposed on the nappes before 1080 Ma during the Ottawan orogeny which lasted from 1090 to 1030 Ma, thus dating F2 folding as between 1090-1080 Ma. Rocks of the MCG suite were folded in the northern part of the map area and overthrust in the southeastern part during the F2 event. F2 fold axial surface traces trend E-W in the northern part of the field area and are increasingly convex and overturned to the northwest as one progresses southeasterly. F3 folds must be inferred from the presence of basins and domes in the F2 folds. Stratigraphic conclusions such as the presence of a widespread major marble unit are supported by field mapping across a broad swath from the central Adirondacks to the eastern Adirondacks. Other aspects of the stratigraphy remain conjectural in that many factors, including intrafolial folds, sedimentary facies changes, tectonic thickening/thinning, and tectonic smearing into conformity complicate synthesis of a unifying concept. A major discrepancy that we have discovered is that the so-called “charnockite dome” in the Pharaoh Mountain quadrangle is actually a F2 antiform imposed on a nappe of basement granitic gneiss and a sill-like body of charnockite.