GEOCHEMICAL EVIDENCE OF SUBAERIAL EXPOSURE AT PARASEQUENCE BOUNDARIES IN UPPER ORDOVICIAN LIMESTONES FROM THE NASHVILLE DOME, TENNESSEE, U.S.A

Measurement of d¹³C and Sr concentration reveals evidence of subaerial exposure below at least two parasequence tops, as well as at one third-order sequence boundary, in the Upper Ordovician Ridley Limestone of the Nashville Dome. The evidence of subaerial exposure at the parasequence tops is unexpected because these surfaces are bounded entirely by subtidal facies and they display no macroscopic evidence of subaerial exposure, such as microkarst. Samples were collected at 10 cm intervals above and below surfaces of interest. Micritic portions of each field sample were drilled and subjected to geochemical analysis. d¹³C values immediately below the sequence boundary are as much 0.6 ‰ lower, and Sr concentrations are typically 0.010 wt. % less, than those of overlying limestones. d¹³C values below parasequence tops are as much 0.4 ‰ lower, and Sr concentrations are 0.005 to 0.010 wt. % less, than those of overlying limestones. Although geochemical evidence for subaerial exposure is not as pronounced at parasequence tops as at the sequence boundary, the data suggest that subaerial exposure occurred before deposition of overlying strata at all three boundaries. The less pronounced geochemical signal at parasequence tops can be explained by shorter time of exposure than that at sequence boundaries. Low d¹³C values below exposure surfaces in post-Silurian strata have been interpreted to reflect photosynthesis on the exposed surface; the low d¹³C values below these Upper Ordovician surfaces may also be attributed to photosynthetic activity. Recognition of these previously unidentified surfaces of subaerial exposure in shallow subtidal sediments implies that subaerial exposure in shallow water carbonates occurs more frequently than would be predicted based on the occurrence of sequence boundaries and peritidal facies. Surfaces conventionally identified as parasequence tops may in some cases reflect higher-order sequence boundaries.