SIGNIFICANCE OF d13C, d18O, AND STRONTIUM CONTENT IN THE IDENTIFICATION OF SURFACES OF SUBAERIAL EXPOSURE IN ORDOVICIAN LIMESTONES OF THE NASHVILLE DOME, TENNESSEE, U.S.A

d¹³C, d¹⁸O, and Sr content of 250 samples from Ordovician limestones of the Nashville Dome, Tennessee suggest that sampling along multiple vertical transects produces a more representative isotopic analysis of a locality than single-sample strategies and will be more useful in identifying surfaces of subaerial exposure. In this study, each vertical transect consists of samples taken in 10 cm increments 1 m above and 2 m below the M4-M5 sequence boundary of Holland and Patzkowsky (1997, 1998). A total of 10 vertical transects were sampled, laterally spaced 2 m apart.

Variation between vertical transects suggests that dissolution of aragonite is not laterally uniform and implies that lateral replicates must be taken in order to characterize the isotopic composition of a horizon. Confidence limits determined for the mean d¹³C of each stratigraphic horizon indicate that negative excursions in d¹³C are statistically significant below sequence-stratigraphically defined subaerial exposure surfaces, such as the M4-M5 3rd order sequence boundary and peritidal-capped cycles. Variance in d¹³C is greatest below exposure surfaces, suggesting that meteoric alteration is not uniform along such surfaces. These data also suggest the presence of a third exposure surface that lacks any macroscopic evidence of subaerial exposure.

Analysis of oxygen isotopes have previously had limited success in identifying exposure surfaces. In this study, confidence limits on the mean oxygen isotope composition of each horizon indicate that d¹⁸O increases upward to both the parasequence boundary at -120 cm and the M4-M5 sequence boundary as a result of calcite precipitation from isotopically heavy water. d¹⁸O decreases upward toward the erosional surface located at -70 cm, which suggests a short period of precipitation of calcite from isotopically lighter water.

Weight percent Sr displays low mean values and low variance beneath the M4-M5 sequence boundary and the peritidal-capped cycle. Low variance in a diagentically altered section is expected because removal of Sr only requires the fluid be undersaturated with respect to aragonite and that a fluid pathway for migration exists. Low variance in Wt% Sr suggests that removal of Sr during meteoric diagenesis is relatively uniform.