ELEMENTAL AND ISOTOPIC CHEMOSTRATIGRAPHY OF THE LOUISVILLE COMPOSITE SECTION, SILURIAN OF KENTUCKY

Chemostratigraphy has recently become a widely applied tool for attempting chronostratigraphic correlation in successions with little other information available. For example, carbon isotope chemostratigraphy has been used to stich together the history of Snowball Earth from rare and widely spaced sedimentary packages. In industry, chemostratigraphy has been used to link basinal mudrock intervals, which often lack unequivocal sequence stratigraphic geometries.

In this study, we examine to what extent elemental ratios (Mg/Ca, Sr/Ca, Fe/Ca, and Mn/Ca) can reduce uncertainty around carbon isotope chemostratigraphy. Handsamples were collected from the Silurian (Wenlock) Louisville Composite Section along Interstate 71 near Louisville, Kentucky, USA, which is approximately 27m thick. Density of sampling yielded a temporal resolution of approximately 50 ky. Bulk samples were drilled from sixty hand samples of dolomite and limestone and were analyzed for elemental ratios via ICP-MS. All elemental ratios are given in mmol/mol.

Mg/Ca generally increases up section from 800 to 975, but it becomes more variable above 17m. Sr/Ca is initially 0.3, but decreases sharply to about 0.1. From 16-27m, Sr/Ca gradually increases from 0.1 to 0.3. Fe/Ca starts at 40-60 for the basal 10m, but then it drops into the range 4-20 for the remainder of the section. Mn/Ca decreases more or less constantly from 3 to 1 over the section.

With increasingly more negative δ¹⁸O, Mg/Ca decreases from about 975 at -3‰ δ¹⁸O to 800 at -4.5‰. Sr/Ca are relatively constant (0.3-0.4) over this interval. Fe/Ca is quite variable, but Mn/Ca decreases from 2.4 to 0.3.

The increase in δ¹⁸O, Sr/Ca, Fe/Ca and Mn/Ca and decrease in Mg/Ca down section is consistent with contamination by burial diagenetic cements. Undiluted by these burial phases, the early Wenlock’s δ¹⁸C excursion would have been even more positive. Thus it is important to consider how diagenesis can affect the carbonate δ¹⁸C when performing chemostratigraphic correlation.