SIGNIFICANCE AND VARIATION IN d¹³C AND d¹⁸O ACROSS SURFACES OF SUBAERIAL EXPOSURE IN ORDOVICIAN LIMESTONES OF THE NASHVILLE DOME, TN

d¹³C and d¹⁸O compositions of 250 samples from Ordovician limestones of the Nashville Dome, Tennessee suggest that multiple vertical transect sampling produces the most representative isotopic composition of a locality and that sequence stratigraphic methodologies may misinterpret the extent of meteoric diagenesis. Each vertical transect consists of samples taken in 10cm increments 1m above and 2m below the M4-M5 sequence boundary. A total of 10 vertical transects were sampled, laterally spaced 2m apart.

Variation between vertical transects suggests that dissolution of aragonite is not laterally uniform and implies that lateral multiples must be taken in order to define 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. These data also interpret the presence of two additional exposure surfaces previously undefined by sequence stratigraphy.

Oxygen isotopes are commonly variable in their usefulness for identifying exposure surfaces. Confidence limits on the mean of the horizon indicate that d¹⁸O in this diagenetic system increases upward toward each exposure surface due to calcite precipitation from isotopically heavy meteoric waters.