Abstract

Organic microfossils (spore tetrads, acrit-archs, scolecodonts, chitinozoans) from six Late Ordovician through Early Silurian samples (Elkhorn Formation of the Richmond Group; lower and upper beds of Belfast Member of the Brassfield Formation) straddling the Ordovician-Silurian paraconformity provide evidence about water depth. The spore tetrads appear to have been derived from plants growing in environments no deeper than an intertidal, semiterrestrial or very shallow-marine if not a nonmarine position. Their concentration on both sides of the paraconformity indicates at least shallower depth conditions than are present in the overlying upper part of the Brassfield Formation with its Eocoelia and succeeding deeper water Pentamerus benthic marine life zone faunas. A progressive deepening of water in the Early Silurian away from the paraconformity is also indicated by the inverse relationship existing between abundant spores in the basal bed of the Belfast Member and abundant Acanthomorphitae acritarchs in the upper bed. The presence of Sphaeromorphitae acritarchs in all sixsamples indicates that those microplankton are restricted to very shallow water depths. Spores from the Elkhorn Formation may have been derived from plants living in Ordovician time or they have “leaked” into the Elkhorn from the Silurian. In the former event, these spores may represent the oldest vascular plant remains recognized in North America.

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