PRELIMINARY FINITE STRAIN AND FABRIC ANALYSIS OF CONGLOMERATE FROM THE SHAWANGUNK FORMATION; NORTHERN APPALACHIAN FOLD-THRUST BELT, HUDSON VALLEY, NEW YORK

Recent mapping suggests the silica-cemented, quartz pebble conglomerate of the Silurian Shawangunk Fm acted as a mechanically rigid strut and strongly influenced the structural architecture of the Appalachian fold-thrust belt in southeastern NY. Indeed, the northward thinning and pinch-out of the Shawangunk Fm near Rosendale corresponds with a decrease in thrust fault spacing and the scale of fault-related folds. Excellent exposures of the Shawangunk Fm across this transition provide opportunities to map macro-scale strain gradients in a wide range of fold-thrust structures. However, the nature of grain-scale deformation remains largely unresolved. Like most competent units within the sub-metamorphic portions of fold-thrust belts, the Shawangunk Fm likely developed only slight penetrative strains. Such low-strain rocks present a challenge for finite strain analysis techniques, because the effects of primary fabrics associated with deposition and compaction make it difficult to resolve tectonic strains.

To determine feasibility of a regional study to resolve low magnitude strains from primary fabrics in the Shawangunk Fm, three mutually perpendicular 50 x 75mm thin sections were prepared from seven oriented samples. The new computer program ELLIPSEFIT was used to digitize a series of polygons from grain outlines in photomicrographs of the thin sections. ELLIPSEFIT then calculated moment-equivalent ellipses from these polygons. The benefits of the ellipse-moment polygon fitting technique include allowing the use of irregular grains and significantly reducing operator bias. A hyperbolic vector mean calculated from the ellipses with a bootstrap error estimate was then used to un-strain the data, which were contoured on polar hyperboloidal projections to examine for fabric modes. Strain ellipsoid axial ratios were calculated from the three section-ellipses for each sample and plotted on Flinn and Hossack-type graphs. Strain axial orientations were plotted on a spherical equal-area projection using the program ORIENT. Although involved in fold-thrust deformation, preliminary results indicate the Shawangunk Fm accumulated only very low strains and that the long axes of strain ellipsoids do not consistently parallel the regional structural grain defined by the trends of fold axes and cleavage.