IS EASTERN LAKE ERIE A FAMENNIAN NON-DEPOSITIONAL FEATURE?

The Great Lakes of North America occupy bedrock depressions that are thought to have formed by glacial action. Lake Ontario, for example, occupies such a bowl. Several of its glacially removed beds are viewed as northward extensions of beds truncated at the Niagara Escarpment, particularly where that cuesta runs west from Rochester, NY, to Hamilton, ON, Canada. The eastern end of Lake Erie tells a different story. Its last chapters are glacial, but the stratigraphic evidence indicates an absence or near-absence of Catskill Delta deposition during the early Famennian.

The primary evidence is the northwest thinning of the Canadaway Group interval from the base of the Dunkirk Formation to the base of the Laona Formation in southwestern New York State. Well log and outcrop data show this shale-dominated interval thinning to the northwest—i.e., toward the lake—at about 7 m/km. Subintervals within this interval exhibit commensurable thinning in a similar direction. Such thinning is evident at locations inland from the lake where the entire interval remains intact and ranges up to 300 m thick. Thinning of the interval and its subintervals can be projected northwestward to the lake plain where the interval is partly missing and to the footprint of the lake where the projected interval and subintervals pinch out. Does this provide evidence of depositional absence? Or absence of depositional evidence? Complete certainty is not possible; however, as a matter of geometry, multi-layered beds that thin northwestward toward a common pinchout zone are readily explained by northwest-thinning deposition, not by erosional truncation.

Prior work shows analogous northwest thinning along the Lake Erie shore in Pennsylvania and northeast Ohio. This merits additional attention, as do recent work on the Canadian side of the lake, assessment of possible correlation with the Algonquin Arch, and local work on depositional facies, geochemical fingerprinting, radiometric dating, fossil evidence, etc. Pending such additional work, the evidence favors an interpretation that the eastern end of Lake Erie occupies a bedrock low hundreds of millions of years old, subsequently deepened by glacial action but originally a Famennian non-depositional or minimally depositional feature.