A CHEMOSTRATIGRAPHIC ANALYSIS OF THE ORDOVICIAN AND SILURIAN BOUNDARY IN THE APPALACHIAN BASIN

The position of the Ordovician/Silurian boundary in the strata of the Appalachian basin had been questioned by Bergström and colleagues, with their discovery of elevated δC¹³ values (probable HICE) suggesting a Hirnantian age for the presumed Silurian, Manitoulin Fm. This study investigated this controversy through detailed fieldwork and analysis of δC¹³ values from bulk carbonate samples that were collected from outcrops and cores in Ontario, New York, and to determine a tentative age of debated Ordovician and Silurian units.

Fieldwork in southern Ontario and Ohio, led to new hypotheses about the depositional history of the Manitoulin Fm. and adjacent Whirlpool Sandstone. First, the marine sandstones of the upper Whirlpool grade laterally into the type, or upper, Manitoulin Fm found in southern Ontario through Manitoulin island, while the non-marine to marginal marine lower Whirlpool, which shows high δC¹³ values in basal samples at Niagara, may transition into the lower Manitoulin of the Bruce Peninsula, which contains the HICE. The hypotheses were first tested through the sampling of the Manitoulin Fm. near Hamilton, Ontario, which is apparently transitional into upper Whirlpool Fm. The HICE was not found in these samples, which is consistent with correlation into the upper Manitoulin Fm. farther northwest. Efforts were directed toward testing the complimentary hypothesis that the lower Whirlpool grades laterally in to the HICE bearing lower Manitoulin. Locations near Hornings Mill, Ontario, show upward gradation of lower Whirlpool sandstones and shale into the lower Manitoulin; this section was sampled for δC¹³ values.

A second hypothesis that in Ohio, the lower Manitoulin Fm/lower Whirlpool grades into a calcareous siltstone, termed the Centerville Fm., was tested by sampling within both outcrop and core for the presence/absence of an excursion. Documentation of a +1‰ δC¹³ excursion in the Centerville Fm. of West Union, Ohio is consistent with this hypothesis.

Based on preliminary observations, both the Manitoulin and Whirlpool are, at least in part, of potential Hirnantian age and appear to be intergradational. The results of this study could prove groundbreaking for Paleozoic research in the Appalachian basin since it would be evidence for Hirnantian strata above the widespread Cherokee Unconformity.