STRUCTURE-FROM-MOTION PHOTOGRAMMETRY YIELDS SHAPE, SHORTENING, AND RHEOLOGY AT THE WHALEBACK ANTICLINE (EAST-CENTRAL PENNSYLVANIA)

Bear Valley (Shamokin, PA) is on the southern limb of the Western Middle Anthracite Field Synclinorium in the Alleghanian Valley and Ridge Province. Lithostratigraphic heterogeneity of the Pennsylvanian Llewellyn Formation permitted strongly disharmonic folding at Bear Valley, with coal and shale “underclays” serving as detachment and accommodation horizons between folded sandstones. Strip mining for coal early in the last century exposed the surface of a several-meter-thick, folded sandstone horizon in exquisite detail. The Whaleback Anticline is the central of several folds of the horizon that form the floor and two walls of the Bear Valley mine. The Whaleback is separated from the overlying, disharmonically folded sanstone by 10 to 20 m of shale and coal. Given its position in a fold train, decoupled from the folded sandstone stratigraphically above, we identify the Whaleback as a buckle fold and describe it in the context of the abundant prior theoretical work on buckle folds.

We used structure-from-motion (SfM) photogrammetry to produce a digital surface model of the excavated Whaleback. Fold-hinge positions were identified by maxima in plane curvature, calculated from smoothed-splines fit to serial 2D profiles extracted from the SfM model. Wavelength and amplitude of the fold at each profile is determined from the distances between these hinge points. Measured in way, the Whaleback has a wavelength about 35 m and an amplitude about 10 m.

Treating the fold as single thin layer in a weak matrix, we use these values and an average of 4.5 m for the thickness of the sandstone to estimate rheological contrast at, and strain accommodated by, the Whaleback. We assume for these analyses that the rocks can be described as linear-viscous materials, and we neglect gravity, anisotropy within the layers, and the effect of other sandstones in the stratigraphic section. Given these first-order approximations, bulk shortening accommodated by the fold is about 50% and the viscosity ratio between the sandstone and coal-shale horizons is on the order of 100. This approach is used to estimate strain at different stages of folding, predictions that can be tested against field observations such as those presented in the poster by Montfort and Gray (this session).