The Enigmatic Quaco Cobbles, Upper Triassic, Canadian Maritimes: Deformation by Tectonics or Seismic Shock?

The most distinctive feature of the Upper Triassic (Carnian) Quaco Formation, coastal New Brunswick, Canada, is the pervasive occurrence of cm-scale circular to elliptical markings and indentations on the cobble surfaces. Also present on many of the cobbles are fractures that radiate from these indentations and spalled clast margins. The smooth-surfaced depressions that occur on most cobbles likely originated through pressure solution, potentially enhanced by a stage of tectonic compression during basin inversion. However, the radial fracturing of the clasts, grain fracturing within the clast fabric, and clast spalling could be explained as the result of concussion fracturing at points of clast contact during the passage of strong shock waves, such as those resulting from the Manicouagan impact. The open nature of some of the fractures and micro-faulting of the cobbles suggest that compressive tectonics enhanced previously formed fractures. At this time it is not possible to determine the origin of the fractures conclusively. Thus, the competing hypotheses of tectonics and seismic shock remain viable.