Sizing up the sub-Tommotian unconformity in Siberia: Reply

Abstract

We appreciate Khomentovsky's review of his views on basal Cambrian correlations across the Siberian Platform but do not find his arguments compelling. Indeed, his Comment emphasizes the need for correlational tools beyond those offered by small shelly fossils (SSFs). In Siberia, the record of Nemakit-Daldynian and Tommotian SSP's is largely cumulative, with interpreted zonal boundaries based on first appearances rather than extinctions. In consequence, published zonal schemes are relatively insensitive to hiatuses within sections. They are sensitive to the environmental and taphonomic factors that govern local stratigraphic ranges of SSP taxa, and that is one reason why the three principal groups of Russian stratigraphers working on Proterozoic-Cambrian beds differ on intraplatform correlations (Khomentovsky and Karlova, 1993; Repina and Rozanov, 1992; Missarzhevsky, 1989). A second reason for the discrepancy is a misapprehension, repeated here by Khomentovsky, that stage boundaries are defined by a particular set of fossils rather than a boundary point in a stratotype section. We did not choose any subset of "basal Tommotian" SSFs as diagnostic for the initial stage boundary; we noted only that all principal taxa of the N. sunnaginicus zone were present when the first rocks above the boundary point accumulated in the stratotype region.