PSEUDOBEDDING, PRIMARY STRUCTURES, AND THRUST FAULTS IN THE GRANDFATHER MOUNTAIN FORMATION, NW NORTH CAROLINA, USA

The structure of the lower Grandfather Mountain Formation (GFMF) is typically considered to be an overturned pile of metasedimentary rocks with a locally interbedded suite of bimodal metarhyolite-metabasalt. Marker horizons that extend for kilometers are unknown. Local conglomerate beds are seemingly lenticular and do not provide continuous stratigraphic markers useful for working out the overall structure of the formation. A few primary structures do exist, most notably bedding, laminations, cross bedding, cross-laminations, and rare ripple marks. Thinly laminated units are common in the lower GFMF and consist of silt to fine sand-sized grains comprising mm thick laminae separated from similar overlying and underlying laminae by thinner phyllosilicate-rich laminae. Many of these thinly laminated units are, in fact, recovered mylonites formed along shear zones within the formation. The shear zones range in thickness from less than a centimeter to more than 1/2 meter thick. The rock appearance is metasedimentary and the shear zones cut relict sedimentary laminations, yet they yield conflicting facing directions for the sections in which they occur, especially where those sections contain unequivocal relict cross-beds. We have recognized 14 shear zones, but have had limited success tracing them along the modestly exposed SE flank of Grandfather Mountain. At Rough Ridge (Ship Rock) on the Blue Ridge Parkway, five thrust fault shear zones cut the section, separate overturned from right-side-up rocks, and define boundaries between isoclinally folded and less severely folded fault slices. Significant shortening as a consequence of the thrust faulting and folding here is seemingly greater than shortening in the “upper” GFMF exposed to the northwest, where folding is common, but thrust faults internal to the GFMF are not known to be significant. The presence of complex structure in this (lower) part of the GFMF section indicates that neither the thickness nor the overall structure of the GFMF is known.