On the Deformation of the Funzie Conglomerate, Fetlar, Shetland

Abstract

The Funzie conglomerate in northeast Shetland is a very large body of deformed metaconglomerate of pre-Middle Old Red Sandstone age. Its elongated and flattened pebbles, which are mostly of quartzite and occasionally of marble, lie in a subgraywacke matrix. A plot based on the measurement of 548 quartzite pebbles shows that the shapes of the pebbles vary systematically over the area. A minimum value for the deformation of the conglomerate at any point is found by comparing the average pebble shape at that point with the average pebble shape in the area of least deformation. Compression in the plane normal to the elongation direction has forced the pebbles to elongate by flow (Einengung). It is shown that the consistency of direction of the pebble axes is related to the elongation of those axes and is due to differential flow of the matrix against the pebbles. During the deformation of the pebbles their constituent quartz grains, the clastic quartz grains in the matrix, and the calcite grains in the marble pebbles developed forms and orientations similar to those of the pebbles. At the same time, the quartz, calcite, and micaceous grains of the pebbles and the micaceous grains of the matrix all developed preferred lattice orientations which give partial girdles of the poles of the c-axes [or of (001) of mica] in the plane perpendicular to the elongation direction, while the clastic quartz grains in the matrix show no preferred lattice orientation. Later ruptural deformation gave rise to fracture cleavage in the quartzite pebbles, tension veins, and calcite twinning.