APPLICATION OF QUICK-LOOK ANALYSIS TECHNIQUES FOR REFINING MAP AND CROSS-SECTIONAL INTERPRETATIONS THROUGH THE HUDSON VALLEY FOLD-THRUST BELT NEAR ROSENDALE, NEW YORK

Quick-look analysis techniques provide a means for checking geologic maps and associated cross sections for 2-D and 3-D geometric and kinematic consistency in regions with limited data. These techniques were recently applied to a 1:10,000-scale geologic map and six serial cross sections in the Rosendale Natural Cement District in the Hudson Valley of eastern New York State. Here, the deformed stratigraphic sequence involves a mechanically rigid strut of Siluro-Devonian clastic and carbonate sedimentary strata that is sandwiched between thick, relatively ductile units of Ordovician shale (below) and Middle Devonian shale (above). This rigid strut thins markedly along strike from south to north, producing along-strike changes in fold-thrust belt architecture. Our new analysis has led to revised interpretation of enigmatic structural relationships that have been difficult to resolve in the field because of poor-quality outcrop and lack of subsurface data. Major structural features such as the Binnewater-Hickory Bush Anticlinorium (north-central part of the map area) and the Helderberg Escarpment (eastern part of the map area) are interpreted to be uplifted by large wavelength and amplitude duplex structures that developed above a regional detachment in the underlying Ordovician strata. Faults underlying the Binnewater-Hickory Bush Anticlinorium are now interpreted as continuous features that splay from a major fault exposed in the Shawangunk Mountains south of Rondout Creek. North of Rondout Creek, smaller wavelength, second-order duplexes increasingly imbricate the Siluro-Devonian section between a regional Silurian floor thrust and a newly recognized regional roof thrust in the Devonian Kalkberg-New Scotland Formations. This structural transition occurs in response to the northward thinning of the Silurian units, particularly in synclinal areas between the larger first-order folds.