Controls on Geochemical Expression of Subaerial Exposure in Ordovician Limestones from the Nashville Dome, Tennessee, U.s.a

FIG. 1.—Alternative explanations of strata exhibiting progradational followed by retrogradational stacking. In Interpretation 1, the switch in stacking patterns is interpreted as a combined type I sequence boundary (SB(I)) and transgressive surface (TS), underlain by the highstand systems tract (HST) and overlain by the transgressive systems tract (TST). In Interpretation II, the switch in stacking patterns marks only the transgressive surface and is overlain by the transgressive systems tract. A type II sequence boundary (SB(II)) lies somewhere below the transgressive surface and separates the shelf-margin systems tract (SMST) from the underlying highstand systems tract; the SB in this case is not manifested by any obvious change in lithofacies or stacking patterns. See Posamentier and James (1993) for an extended discussion of the relationships between type I and type II sequences. ABSTRACT: Stable-isotope and trace-element data from 301 samples in six stratigraphic sections largely support previous inferences of subaerial exposure at third-order sequence boundaries in Mohawkian and Cincinnatian limestones of the Nashville Dome. They also reveal the presence of previously unrecognized surfaces of subaerial exposure. For example, evidence of subaerial exposure at the tops of parasequences suggests that at least some of the latter are better interpreted as high-frequency sequences. Clusters of surfaces of subaerial exposure near some previously interpreted sequence boundaries suggest that the latter may represent sequence boundary zones. Some surfaces of subaerial exposure are at neither recognized sequence boundaries nor parasequence boundaries and may represent missed beats in which sealevel fluctuations are preserved geochemically but not recognized on stratigraphic criteria. Recognition of these unexpected surfaces of subaerial exposure may in part depend on the close spacing of samples used in this study. Different sequence boundaries exhibit different degrees of geochemical alteration, and more extensive alteration appears to characterize sequence boundaries of greater temporal significance. Overprinting of exposure surfaces is controlled by accommodation rate, in that meteoric alteration (as suggested by low C isotope composition and Sr concentration) is seemingly pervasive in an interval of slow accommodation but not in an interval of rapid accommodation. d18O values show little relationship to exposure surfaces in these strata, so that Sr–d13C plots are more useful than d18O–d13C plots in recognizing meteoric diagenesis in aggregated data. In this data set, d13C excursions interpreted to result from pedogenic processes are at most 2.4 ‰ and extend at most only 1.5 meters below exposure surfaces, less than the extent of such signatures in younger carbonates. Such minima are best developed in paleo-lowland and paleo-coastal settings, and they are essentially unrecognizable in paleoinland settings where more section was removed by lowstand and transgressive erosion.