IMAGE ANALYSIS AND 3D RECONSTRUCTION OF PILLOW STRUCTURES AND LAVA FLOW DIRECTION IN THE TALCOTT BASALT, HARTFORD BASIN, CONNECTICUT

The Lower Jurassic Talcott Formation is the oldest of three basaltic lava flows within the Hartford Basin in eastern North America associated with the fragmentation of Pangaea. The field area in Meriden, Connecticut, is a spectacular outcrop of pillow basalts (Skinner et. al., 2012) exposed in two vertical cross-sections, oriented nearly perpendicular to each other, allowing examination of the pillow geometries in 3D. A Gigapan EPIC Pro V mount and DSLR camera were used to collect 26 high resolution images and 4 composite panoramas along a N-S (~004°) cross-section about 170 m long, and a NW-SE (~300°) cross-section about 200 m long. After minor image adjustments for brightness and contrast, a total of ~1200 identifiable pillow shapes were traced onto layers in Inkscape. Each image layer with outlined pillows was processed using the software EllipseFit to calculate a moment-equivalent ellipse for each of the outlined pillow shapes, giving its orientation (ϕ), coordinates, maximum and minimum radii, and axial ratio R. For each image, the ellipses were plotted on equal-area fabric plots, and the mean ellipse calculated using shape matrix eigenvalues. A mean was then calculated from the images making up the two cross-sections. The mean R, ϕ values in the N-S cross-section are 1.19, 10.712°, compared to 1.93, 175.76° in the NW-SE cross-section. The higher R value for pillows in the NW-SE cross-section suggests that it is closer to the lava flow direction than the N-S section. To further quantify this, a 3D reconstruction was done using EllipseFit to calculate the best-fit ellipsoid from 26 images, which had sufficient variability in strike. In evaluating errors, images were discarded if they had a misfit residual of >20°. The resulting 23 images give a substantially reduced 95% confidence region on a Nadai-Hsu ellipsoid plot. The best-fit ellipsoid has principle axes of 1.4 : 1.1 : 0.7, and the long axis has a trend and plunge of 297°/1.8°, after correcting for bedding dip. This approximately east-to-west lava flow direction is consistent with the location of the Talcott Basalt feeder Higganum Dike to the east of the Hartford Basin, and with the paleogeographic interpretation of the basin with westward fluvial transport direction recorded in the underlying New Haven Arkose (Hubert et. al., 1992; Skinner et. al., 2012).