Discussion: Upper Lower Cambrian depositional sequence in Avalonian New Brunswick

Abstract

The authors (E. Landing and S.R. Westrop) wish to abandon most of the local stratigraphic nomenclature of the Saint John Group in southern New Brunswick, in order to emphasize the broader tectonic significance of these lower Paleozoic platformal rocks throughout the Avalon terrane. Their approach, based in part on biostratigraphic data, is to use a somewhat arbitrary mixture of stratigraphic terms taken from widely separate type sections in New Brunswick and Newfoundland. Thus, the Ratcliffe Brook Group (a formation defined in New Brunswick), would include the Rencontre and Chapel Island formations (both defined in Newfoundland), whereas the Chamberlain's Brook Formation (defined in Newfoundland) would include the Fossil Brook Member (defined in New Brunswick). The Saint John Group, which clearly has historical precedence (Williams et al. 1985), is left out of their stratigraphic scheme. Since the establishment of a formal lithostratigraphic unit requires that its lower and upper boundaries be explicitly defined, it is difficult to see how this can be accomplished when the supposedly adjacent units are from type sections or reference sections located about 1000 krn apart. Even if separate sections contain similar successions, it is unlikely that lithological boundaries would be identical over large areas in such an episodically high-energy shelf environment. Difficulties in adequately describing formational boundaries are compounded in the case of the Hanford Brook Formation since, as the authors point out, it does not even occur in the Newfoundland type section where the underlying and overlying strata are defined. The two type sections are, therefore, not truly homotaxial, so extrapolation of Newfoundland terminology to New Brunswick may not be justified. The authors insist on placing the upper part of the Glen Falls Formation into the overlying Hanford Brook Formation since they claim a major unconformity exists within the former. Although such a revision is not unreasonable, it is not required. Stratigraphic nomenclature is supposed to be based on empirical lithological changes rather than interpretive depositional history, so the traditional lithological components of the Glen Falls Formation could be retained even if the claimed unconformity were proven to exist. In conclusion, the authors have made a significant contribution to the understanding of tectonic controls on the development of Paleozoic shelf facies covering the Avalon terrane. However, this could have been accomplished without resorting to wholesale name changes, the sole purpose of which is to create a general stratigraphic nomenclature to encompass what is surely a complex depositional setting containing many depocentres. A standard correlation chart retaining local terminology would be a more practical way to convey the reasoning behind the authors' tectono-stratigraphic synthesis. I believe that the composite stratigraphic column proposed by Landing and Westrop creates more problems than it solves and is unlikely to be adopted by others working in this part of the northern Appalachians.